AD9225_技术文档

Complete 12-Bit, 25 MSPS

Monolithic A/D Converter

a

AD9225

FUNCTIONAL BLOCK DIAGRAM

FEATURES

Monolithic 12-Bit, 25 MSPS A/D Converter

Low Power Dissipation: 280 mW

Single +5 V Supply

No Missing Codes Guaranteed

Differential Nonlinearity Error: ؎0.4 LSB

Complete On-Chip Sample-and-Hold Amplifier and

Voltage Reference

CLK

AVDD

DRVDD

SHA

VINA

VINB

MDAC1

GAIN = 16

MDAC2

GAIN = 4

MDAC3

GAIN = 4

5

3

A/D

CAPT

CAPB

5

3

4

3

DIGITAL CORRECTION LOGIC

Signal-to-Noise and Distortion Ratio: 71 dB

Spurious-Free Dynamic Range: –85 dB

Out-of-Range Indicator

Straight Binary Output Data

28-Lead SOIC

12

VREF

OUTPUT BUFFERS

OTR

SENSE

BIT 1

(MSB)

1V

MODE

SELECT

BIT 12

(LSB)

AD9225

28-Lead SSOP

REFCOM

DRVSS

CML

AVSS

Compatible with 3 V Logic

A single clock input is used to control all internal conversion

cycles. The digital output data is presented in straight binary

output format. An out-of-range signal indicates an overflow

condition that can be used with the most significant bit to deter-

mine low or high overflow.

PRODUCT DESCRIPTION

The AD9225 is a monolithic, single supply, 12-bit, 25 MSPS,

analog-to-digital converter with an on-chip, high performance

sample-and-hold amplifier and voltage reference. The AD9225

uses a multistage differential pipelined architecture with output

error correction logic to provide 12-bit accuracy at 25 MSPS

data rates, and guarantees no missing codes over the full operat-

ing temperature range.

PRODUCT HIGHLIGHTS

The AD9225 is fabricated on a very cost effective CMOS

process. High-speed precision analog circuits are now combined

with high density logic circuits.

The AD9225 combines a low cost high speed CMOS process

and a novel architecture to achieve the resolution and speed of

existing bipolar implementations at a fraction of the power

consumption and cost.

The AD9225 offers a complete single-chip sampling 12-bit,

25 MSPS analog-to-digital conversion function in 28-lead SOIC

and SSOP packages.

The input of the AD9225 allows for easy interfacing to both

imaging and communications systems. With a truly differential

input structure, the user can select a variety of input ranges and

offsets including single-ended applications. The dynamic perfor-

mance is excellent.

Low Power—The AD9225 at 280 mW consumes a fraction of

the power of presently available in existing monolithic solutions.

On-Board Sample-and-Hold (SHA)—The versatile SHA

input can be configured for either single-ended or differential

inputs.

The sample-and-hold (SHA) amplifier is well suited for both

multiplexed systems that switch full-scale voltage levels in suc-

cessive channels and sampling single-channel inputs at frequen-

cies up to and well beyond the Nyquist rate.

Out of Range (OTR)—The OTR output bit indicates when

the input signal is beyond the AD9225’s input range.

Single Supply—The AD9225 uses a single +5 V power supply

simplifying system power supply design. It also features a sepa-

rate digital driven supply line to accommodate 3 V and 5 V logic

families.

The AD9225’s wideband input combined with the power and

cost savings over previously available monolithics, is suitable for

applications in communications, imaging and medical ultrasound.

The AD9225 has an onboard programmable reference. An

external reference can also be chosen to suit the dc accuracy and

temperature drift requirements of the application.

Pin Compatibility—The AD9225 is pin compatible with the

AD9220, AD9221, AD9223 and AD9224 ADCs.

REV. A

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781/329-4700

Fax: 781/326-8703

World Wide Web Site: http://www.analog.com

AD9225–SPECIFICATIONS

(AVDD = +5 V, DRVDD = +5 V, f_SAMPLE= 25 MSPS, VREF = 2.0 V, VINB = 2.5 V dc, T_MINto T_MAXunless otherwise noted)

DC SPECIFICATIONS

Parameter

Min

12

Typ

Max

Units

Bits

RESOLUTION

MAX CONVERSION RATE

25

MHz

INPUT REFERRED NOISE

VREF = 1.0 V

VREF = 2.0 V

0.35

0.17

LSB rms

ACCURACY

Integral Nonlinearity (INL)

Differential Nonlinearity (DNL)

No Missing Codes

Zero Error (@ +25°C)

Gain Error (@ +25°C)¹

Gain Error (@ +25°C)²

±1.0

±0.4

12

±0.3

±0.5

±0.4

±2.5

±1.0

LSB

Bits Guaranteed

% FSR

±0.6

±2.2

±1.7

TEMPERATURE DRIFT

Zero Error

±2

±26

±0.4

ppm/°C

Gain Error¹

Gain Error²

POWER SUPPLY REJECTION

AVDD (+5 V ± 0.25 V)

±0.1

±0.35

% FSR

ANALOG INPUT

Input Span

2

4

0

V p-p

V

Input (VINA or VINB) Range

AVDD

10

V

pF

Input Capacitance

INTERNAL VOLTAGE REFERENCE

Output Voltage (1 V Mode)

Output Voltage Tolerance (1 V Mode)

Output Voltage (2.0 V Mode)

Output Voltage Tolerance (2.0 V Mode)

Output Current (Available for External Loads)

Load Regulation³

1.0

±5

2.0

±10

1.0

V

mV

V

mV

mA

mV

±17

±35

3.4

REFERENCE INPUT RESISTANCE

8

kΩ

POWER SUPPLIES

Supply Voltages

AVDD

4.75

2.85

5

5.25

V (±5% AVDD Operating)

V (±5% DRVDD Operating)

DRVDD

Supply Currents

IAVDD

IDRVDD

65

2.0

72.5

4.0

mA

POWER CONSUMPTION

External Reference

280

335

290

345

310

373

mW (VREF = 1 V)

mW (VREF = 2 V)

mW (VREF = 1 V)

mW (VREF = 2 V)

Internal Reference

NOTES

¹Includes internal voltage reference error.

²Excludes internal voltage reference error.

³Load regulation with 1 mA load current (in addition to that required by the AD9225).

Specifications subject to change without notice.

–2–

REV. A

AD9225

AC SPECIFICATIONS

Parameter

(AVDD = +5 V, DRVDD = +5 V, f_SAMPLE= 25 MSPS, VREF= 2.0 V, T_MINto T_MAX, Differential Input unless otherwise noted)

Min

Typ

Max

Units

SIGNAL-TO-NOISE AND DISTORTION RATIO (S/N+D)

f_INPUT= 2.5 MHz

f_INPUT= 10 MHz

67.4

66.7

70.7

69.6

dB

SIGNAL-TO-NOISE RATIO (SNR)

f_INPUT= 2.5 MHz

f_INPUT= 10 MHz

69.0

68.2

71

70

dB

TOTAL HARMONIC DISTORTION (THD)

f_INPUT= 2.5 MHz

f_INPUT= 10 MHz

–82

–81

–72

–71.5

dB

SPURIOUS FREE DYNAMIC RANGE

f_INPUT= 2.5 MHz

73

72.5

–85

–83

105

1

dB

MHz

ns

f

_INPUT= 10 MHz

Full Power Bandwidth

Small Signal Bandwidth

Aperture Delay

Aperture Jitter

Acquisition to Full-Scale Step

1

10

ps rms

ns

Specifications subject to change without notice.

(AVDD = +5 V, DRVDD = +5 V, unless otherwise noted)

DIGITAL SPECIFICATIONS

Parameters

Symbol

Min

Typ

Max

Units

LOGIC INPUTS

High Level Input Voltage

Low Level Input Voltage

High Level Input Current (V_IN= DRVDD)

Low Level Input Current (V_IN= 0 V)

Input Capacitance

V_IH

V_IL

I_IH

I_IL

C_IN

+3.5

V

µA

pF

+1.0

+10

–10

5

LOGIC OUTPUTS

High Level Output Voltage (I_OH= 50 µA)

High Level Output Voltage (I_OH= 0.5 mA)

Low Level Output Voltage (I_OL= 1.6 mA)

Low Level Output Voltage (I_OL= 50 µA)

Output Capacitance

V_OH

V_OL

C_OUT

+4.5

+2.4

V

+0.4

+0.1

5

pF

LOGIC OUTPUTS (with DRVDD = 3 V)

High Level Output Voltage (I_OH= 50 µA)

High Level Output Voltage (I_OH= 0.5 mA)

Low Level Output Voltage (I_OL= 1.6 mA)

Low Level Output Voltage (I_OL= 50 µA)

V_OH

V_OL

+2.95

+2.80

V

+0.4

+0.05

Specifications subject to change without notice

.

(T_MINto T_MAXwith AVDD = +5 V, DRVDD = +5 V, C_L= 20 pF)

SWITCHING SPECIFICATIONS

Parameters

Symbol

Min

Typ

Max

Units

Clock Period¹

CLOCK Pulsewidth High

CLOCK Pulsewidth Low

Output Delay

t_C

40

18

13

ns

t_CH

t_CL

t_OD

Pipeline Delay (Latency)

3

Clock Cycles

NOTES

¹The clock period may be extended to 1 ms without degradation in specified performance @ +25°C.

Specifications subject to change without notice.

REV. A

–3–

AD9225

PIN CONNECTION

28-Lead SOIC and SSOP

S1

S2

ANALOG

INPUT

S4

t_C

S3

t_CH

t_CL

1

2

28

27

26

25

24

23

22

21

20

19

18

17

16

15

CLK

(LSB) BIT 12

BIT 11

BIT 10

BIT 9

DRVDD

DRVSS

AVDD

INPUT

CLOCK

3

t_OD

4

AVSS

DATA

OUTPUT

5

VINB

DATA 1

6

BIT 8

VINA

AD9225

TOP VIEW

(Not to Scale)

Figure 1. Timing Diagram

7

BIT 7

CML

8

BIT 6

CAPT

ABSOLUTE MAXIMUM RATINGS*

With

9

BIT 5

CAPB

10

11

12

13

14

BIT 4

REFCOM (AVSS)

VREF

BIT 3

Pin Name

Respect to Min

Max

Units

BIT 2

SENSE

AVSS

AVDD

DRVDD

AVSS

AVDD

REFCOM

CLK

AVSS

–0.3

–6.5

–0.3

+6.5

+0.3

+6.5

+0.3

V

°C

(MSB) BIT 1

OTR

DRVSS

DRVDD

AVSS

AVDD

PIN FUNCTION DESCRIPTIONS

AVSS

AVDD + 0.3

DRVDD + 0.3

AVDD + 0.3

AVDD+ 0.3

+150

Digital Outputs DRVSS

Number

VINA, VINB

VREF

SENSE

AVSS

Name

Description

1

2

3–12

13

14

15, 26

16, 25

17

18

19

CLK

Clock Input Pin

Least Significant Data Bit (LSB)

Data Output Bit

BIT 12

BIT 11–2

BIT 1

CAPB, CAPT

Junction Temperature

Storage Temperature

Most Significant Data Bit (MSB)

–65

+150

+300

OTR

Out of Range

Lead Temperature (10 sec)

AVDD

AVSS

SENSE

VREF

+5 V Analog Supply

Analog Ground

Reference Select

*Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent damage to the device. This is a stress rating only; functional operation of the

device at these or any other conditions above those indicated in the operational

sections of this specification is not implied. Exposure to absolute maximum

ratings for extended periods may affect device reliability.

Input Span Select (Reference I/O)

REFCOM Reference Common

(AVSS)

20

21

22

23

24

27

28

CAPB

CAPT

CML

VINA

VINB

DRVSS

DRVDD

Noise Reduction Pin

Common-Mode Level (Midsupply)

Analog Input Pin (+)

Analog Input Pin (–)

Digital Output Driver Ground

+3 V to +5 V Digital Output

Driver Supply

ORDERING GUIDE

Package Descriptions

Model

Temperature Range

Package Options

AD9225AR

AD9225ARS

AD9225-EB

–40°C to +85°C

28-Lead Wide Body Small Outline

28-Lead Shrink Small Outline

Evaluation Board

R-28

RS-28

CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily

accumulate on the human body and test equipment and can discharge without detection.

Although the AD9225 features proprietary ESD protection circuitry, permanent damage may

occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD

precautions are recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

–4–

REV. A

AD9225

DEFINITIONS OF SPECIFICATION

INTEGRAL NONLINEARITY (INL)

APERTURE DELAY

Aperture delay is a measure of the sample-and-hold amplifier

(SHA) performance and is measured from the rising edge of the

clock input to when the input signal is held for conversion.

INL refers to the deviation of each individual code from a line

drawn from “negative full scale” through “positive full scale.”

The point used as “negative full scale” occurs 1/2 LSB before

the first code transition. “Positive full scale” is defined as a level

1 1/2 LSB beyond the last code transition. The deviation is

measured from the middle of each particular code to the true

straight line.

SIGNAL-TO-NOISE AND DISTORTION (S/N+D, SINAD)

RATIO

S/N+D is the ratio of the rms value of the measured input signal

to the rms sum of all other spectral components below the

Nyquist frequency, including harmonics but excluding dc. The

value for S/N+D is expressed in decibels.

DIFFERENTIAL NONLINEARITY (DNL, NO MISSING

CODES)

An ideal ADC exhibits code transitions that are exactly 1 LSB

apart. DNL is the deviation from this ideal value. Guaranteed

no missing codes to 12-bit resolution indicates that all 4096

codes, respectively, must be present over all operating ranges.

EFFECTIVE NUMBER OF BITS (ENOB)

For a sine wave, SINAD can be expressed in terms of the num-

ber of bits. Using the following formula,

N = (SINAD – 1.76)/6.02

it is possible to get a measure of performance expressed as N,

the effective number of bits.

ZERO ERROR

The major carry transition should occur for an analog value

1/2 LSB below VINA = VINB. Zero error is defined as the

deviation of the actual transition from that point.

Thus, effective number of bits for a device for sine wave inputs

at a given input frequency can be calculated directly from its

measured SINAD.

GAIN ERROR

The first code transition should occur at an analog value

1/2 LSB above negative full scale. The last transition should

occur at an analog value 1 1/2 LSB below the nominal full scale.

Gain error is the deviation of the actual difference between first

and last code transitions and the ideal difference between first

and last code transitions.

TOTAL HARMONIC DISTORTION (THD)

THD is the ratio of the rms sum of the first six harmonic com-

ponents to the rms value of the measured input signal and is

expressed as a percentage or in decibels.

SIGNAL-TO-NOISE RATIO (SNR)

SNR is the ratio of the rms value of the measured input signal to

the rms sum of all other spectral components below the Nyquist

frequency, excluding the first six harmonics and dc. The value

for SNR is expressed in decibels.

TEMPERATURE DRIFT

The temperature drift for zero error and gain error specifies the

maximum change from the initial (+25°C) value to the value at

T_MINor T_MAX

.

SPURIOUS FREE DYNAMIC RANGE (SFDR)

SFDR is the difference in dB between the rms amplitude of the

input signal and the peak spurious signal.

POWER SUPPLY REJECTION

The specification shows the maximum change in full scale from

the value with the supply at the minimum limit to the value with

the supply at its maximum limit.

APERTURE JITTER

Aperture jitter is the variation in aperture delay for successive

samples and is manifested as noise on the input to the A/D.

REV. A

–5–

AD9225

Typical Performance Characteristics

(AVDD, DRVDD = +5 V, f_S= 25 MHz (50% Duty Cycle) unless otherwise noted)

2.00

1.50

1.00

0.75

1.00

0.50

0.25

0

0.00

؊0.50

؊1.00

؊1.50

؊2.00

–0.25

–0.50

–0.75

–1.00

0

511

1022

1533

2044 2555

3066

3577 4095

0

511

1022

1533 2044

Title

2555

3066

3577

4095

TITLE

Figure 2. Typical DNL

Figure 5. Typical INL

75

70

65

60

55

50

70

65

60

55

50

45

؊0.5dB INT 1V

؊6dB INT 1V

؊0.5dB 2V INT REF

؊6dB 2V INT REF

؊20dB INT 1V

؊20dB 2V INT REF

10

1

10

1

FREQUENCY – MHz

Figure 3. SINAD vs. Input Frequency (Input Span = 4.0 V p-p,

Figure 6. SINAD vs. Input Frequency (Input Span = 2 V p-p

Differential Input)

V_CM= 2.5 V Differential Input)

–60

–65

–60

؊20dB INT 1V

–65

؊20.0dB 2V INT REF

–70

–75

–80

–85

–70

–75

؊6dB INT 1V

؊6.0dB 2V INT REF

؊0.5dB 2V INT REF

–80

؊0.5dB INT 1V

–85

10

1

10

1

FREQUENCY – MHz

Figure 4. THD vs. Input Frequency (Input Span = 4.0 V p-p,

V_CM= 2.5 V Differential Input)

Figure 7. THD vs. Input Frequency (Input Span = 2 V p-p,

V_CM= 2.5 V Differential Input)

–6–

REV. A

AD9225

30

35

40

45

50

55

60

65

70

75

–70

–75

–80

–85

–90

SNR INT 2V REF

SINAD INT 2V REF

INT 1V REF

INT 2V REF

10

FREQUENCY – MHz

–40

–35

–30

–25

–20

–15

– dB

–10

–5

0

5

A

IN

Figure 11. THD vs. Sample Rate, (A_IN= –0.5 dB, V_CM= 2.5 V,

Input Span = 4.0 V p-p Differential Input)

Figure 8. SNR/SINAD vs. A_IN(Input Amplitude)

(F_IN= 12.5 MHz, Input Span = 4.0 V p-p, V_CM= 2.5 V

Differential Input)

75

–65

؊0.5dB 2V INT REF

؊20.0dB 2V INT REF

70

–70

؊6.0dB 2V INT REF

65

–75

؊6.0dB 2V INT REF

60

–80

؊0.5dB 2V INT REF

55

–85

؊20.0dB 2V INT REF

50

–90

1

10

1

FREQUENCY – MHz

Figure 12. SNR vs. Input Frequency (Input

Span = 4.0 V p-p, V_CM= 2.5 V Single-Ended

Input)

Figure 9. THD vs. Input Frequency (Input Span =

4.0 V p-p, V_CM= 2.5 V Single-Ended Input)

246447

3299

N – 1

4206

N + 1

N

B

IN

Figure 10. “Grounded-Input” Histogram (Input Span =

40 V p-p)

REV. A

–7–

AD9225

difference of the voltages applied at the VINA and VINB input

pins. Therefore, the equation,

INTRODUCTION

The AD9225 is a high performance, complete single-supply 12-

bit ADC. The analog input range of the AD9225 is highly flex-

ible allowing for both single-ended or differential inputs of

varying amplitudes that can be ac or dc coupled.

V

_CORE= VINA – VINB

(1)

defines the output of the differential input stage and provides

the input to the A/D core.

It utilizes a four-stage pipeline architecture with a wideband

input sample-and-hold amplifier (SHA) implemented on a cost-

effective CMOS process. Each stage of the pipeline, excluding

the last stage, consists of a low resolution flash A/D connected

to a switched capacitor DAC and interstage residue amplifier

(MDAC). The residue amplifier amplifies the difference be-

tween the reconstructed DAC output and the flash input for the

next stage in the pipeline. One bit of redundancy is used in each

of the stages to facilitate digital correction of flash errors. The

last stage simply consists of a flash A/D.

The voltage, V_CORE, must satisfy the condition,

–VREF ≤ V_CORE≤ VREF

(2)

where VREF is the voltage at the VREF pin.

While an infinite combination of VINA and VINB inputs exist

that satisfy Equation 2, there is an additional limitation placed

on the inputs by the power supply voltages of the AD9225. The

power supplies bound the valid operating range for VINA and

VINB. The condition,

AVSS – 0.3 V < VINA < AVDD + 0.3 V

AVSS – 0.3 V < VINB < AVDD + 0.3 V

The pipeline architecture allows a greater throughput rate at the

expense of pipeline delay or latency. This means that while the

converter is capable of capturing a new input sample every clock

cycle, it actually takes three clock cycles for the conversion to be

fully processed and appear at the output. This latency is not a

concern in most applications. The digital output, together with

the out-of-range indicator (OTR), is latched into an output

buffer to drive the output pins. The output drivers of the

AD9225 can be configured to interface with +5 V or +3.3 V

logic families.

(3)

where AVSS is nominally 0 V and AVDD is nominally +5 V,

defines this requirement. The range of valid inputs for VINA

and VINB is any combination that satisfies both Equations 2

and 3.

For additional information showing the relationship between

VINA, VINB, VREF and the digital output of the AD9225, see

Table IV.

Refer to Table I and Table II at the end of this section for a

summary of both the various analog input and reference

configurations.

The AD9225 uses both edges of the clock in its internal timing

circuitry (see Figure 1 and specification page for exact timing

requirements). The A/D samples the analog input on the rising

edge of the clock input. During the clock low time (between the

falling edge and rising edge of the clock), the input SHA is in

the sample mode; during the clock high time it is in hold. Sys-

tem disturbances just prior to the rising edge of the clock and/or

excessive clock jitter may cause the input SHA to acquire the

wrong value, and should be minimized.

ANALOG INPUT OPERATION

Figure 14 shows the equivalent analog input of the AD9225

which consists of a differential sample-and-hold amplifier

(SHA). The differential input structure of the SHA is highly

flexible, allowing the devices to be easily configured for either a

differential or single-ended input. The dc offset, or common-

mode voltage, of the input(s) can be set to accommodate either

single-supply or dual-supply systems. Also, note that the analog

inputs, VINA and VINB, are interchangeable with the exception

that reversing the inputs to the VINA and VINB pins results in a

polarity inversion.

ANALOG INPUT AND REFERENCE OVERVIEW

Figure 13 is a simplified model of the AD9225. It highlights the

relationship between the analog inputs, VINA, VINB, and the

reference voltage, VREF. Like the voltage applied to the top of

the resistor ladder in a flash A/D converter, the value VREF

defines the maximum input voltage to the A/D core. The mini-

mum input voltage to the A/D core is automatically defined to

be –VREF.

C

H

Q

S2

+

C

PAR

C

Q

S

S1

VINA

VINB

AD9225

VINA

+VREF

Q

C

Q

H1

S

S1

V

12

–

CORE

C

A/D

PAR

Q

S2

CORE

C

–VREF

H

VINB

Figure 14. Simplified Input Circuit

Figure 13. Equivalent Functional Input Circuit

The AD9225 has a wide input range. The input peaks may be

moved to AVDD or AVSS before performance is compromised.

This allows for much greater flexibility when selecting single-

ended drive schemes. Op amps and ac coupling clamps can be

set to available reference levels rather than be dictated to what

the ADC “needs.”

The addition of a differential input structure gives the user an

additional level of flexibility that is not possible with traditional

flash converters. The input stage allows the user to easily config-

ure the inputs for either single-ended operation or differential

operation. The A/D’s input structure allows the dc offset of the

input signal to be varied independently of the input span of

the converter. Specifically, the input to the A/D core is the

–8–

REV. A

AD9225

V

CC

Due to the high degree of symmetry within the SHA topology, a

significant improvement in distortion performance for differen-

tial input signals with frequencies up to and beyond Nyquist can

be realized. This inherent symmetry provides excellent cancella-

tion of both common-mode distortion and noise. Also, the

required input signal voltage span is reduced by a half which

further reduces the degree of R_ONmodulation and its effects on

distortion.

AD9225

R

S

VINA

R

S

VINB

V

EE

VREF

10␮F

0.1␮F

SENSE

REFCOM

The optimum noise and dc linearity performance for either

differential or single-ended inputs is achieved with the largest

input signal voltage span (i.e., 4 V input span) and matched

input impedance for VINA and VINB. Only a slight degrada-

tion in dc linearity performance exists between the 2 V and

4 V input spans.

Figure 15. Series Resistor Isolates Switched-Capacitor

SHA Input from Op Amp. Matching Resistors Improve

SNR Performance.

The optimum size of this resistor is dependent on several factors

which include the ADC sampling rate, the selected op amp, and

the particular application. In most applications, a 30 Ω to 100 Ω

resistor is sufficient. However, some applications may require a

larger resistor value to reduce the noise bandwidth or possibly

limit the fault current in an overvoltage condition. Other appli-

cations may require a larger resistor value as part of an antialias-

ing filter. In any case, since the THD performance is dependent

on the series resistance and the above mentioned factors, opti-

mizing this resistor value for a given application is encouraged.

Referring to Figure 14, the differential SHA is implemented

using a switched-capacitor topology. Its input impedance and

its switching effects on the input drive source should be consid-

ered in order to maximize the converter’s performance. The

combination of the pin capacitance, C_PIN, parasitic capacitance

C

_PAR, and the sampling capacitance, C_S, is typically less than

5 pF. When the SHA goes into track mode, the input source

must charge or discharge the voltage stored on C_Sto the new

input voltage. This action of charging and discharging C_S, aver-

aged over a period of time and for a given sampling frequency,

F_S, makes the input impedance appear to have a benign resistive

component. However, if this action is analyzed within a sam-

pling period (i.e., T = 1/F_S), the input impedance is dynamic

and hence certain precautions on the input drive source should be

observed.

The source impedance driving VINA and VINB should be

matched. Failure to provide that matching will result in degra-

dation of the AD9225’s superb SNR, THD and SFDR.

For noise sensitive applications, the very high bandwidth of the

AD9225 may be detrimental and the addition of a series resistor

and/or shunt capacitor can help limit the wideband noise at the

A/D’s input by forming a low-pass filter. Note, however, that

the combination of this series resistance with the equivalent

input capacitance of the AD9225 should be evaluated for those

time domain applications that are sensitive to the input signal’s

absolute settling time. In applications where harmonic distortion

is not a primary concern, the series resistance may be selected in

combination with the SHA’s nominal 10 pF of input capaci-

tance to set the filter’s 3 dB cutoff frequency.

The resistive component to the input impedance can be com-

puted by calculating the average charge that gets drawn by C_H

from the input drive source. It can be shown that if C_Sis

allowed to fully charge up to the input voltage before switches

Q_S1are opened, then the average current into the input is the

same as if there were a resistor of 1/(C_SF_S) ohms connected

between the inputs. This means that the input impedance is

inversely proportional to the converter’s sample rate. Since C_Sis

only 5 pF, this resistive component is typically much larger than

that of the drive source (i.e., 8 kΩ at F_S= 25 MSPS).

A better method of reducing the noise bandwidth, while possi-

bly establishing a real pole for an antialiasing filter, is to add

some additional shunt capacitance between the input (i.e.,

VINA and/or VINB) and analog ground. Since this additional

shunt capacitance combines with the equivalent input capaci-

tance of the AD9225, a lower series resistance can be selected to

establish the filter’s cutoff frequency while not degrading the

distortion performance of the device. The shunt capacitance

also acts like a charge reservoir, sinking or sourcing the addi-

tional charge required by the hold capacitor, C_H, further reduc-

ing current transients seen at the op amp’s output.

The SHA’s input impedance over a sampling period appears as

a dynamic input impedance to the input drive source. When the

SHA goes into the track mode, the input source should ideally

provide the charging current through R_ONof switch Q_S1in an

exponential manner. The requirement of exponential charging

means that the most common input source, an op amp, must

exhibit a source impedance that is both low and resistive up to

and beyond the sampling frequency.

The output impedance of an op amp can be modeled with a

series inductor and resistor. When a capacitive load is switched

onto the output of the op amp, the output will momentarily

drop due to its effective output impedance. As the output recov-

ers, ringing may occur. To remedy the situation, a series resistor

can be inserted between the op amp and the SHA input as

shown in Figure 15. The series resistance helps isolate the op

amp from the switched-capacitor load.

The effect of this increased capacitive load on the op amp driv-

ing the AD9225 should be evaluated. To optimize performance

when noise is the primary consideration, increase the shunt

capacitance as much as the transient response of the input signal

will allow. Increasing the capacitance too much may adversely

affect the op amp’s settling time, frequency response, and dis-

tortion performance.

REV. A

–9–

AD9225

REFERENCE OPERATION

The actual reference voltages used by the internal circuitry of

the AD9225 appears on the CAPT and CAPB pins. For proper

operation when using the internal or an external reference, it is

necessary to add a capacitor network to decouple these pins.

Figure 17 shows the recommended decoupling network. This

capacitive network performs the following three functions: (1)

along with the reference amplifier, A2, it provides a low source

impedance over a large frequency range to drive the A/D inter-

nal circuitry, (2) it provides the necessary compensation for A2,

and (3) it bandlimits the noise contribution from the reference.

The turn-on time of the reference voltage appearing between

CAPT and CAPB is approximately 15 ms and should be evalu-

ated in any power-down mode of operation.

The AD9225 contains an onboard bandgap reference that pro-

vides a pin strappable option to generate either a 1 V or 2 V

output. With the addition of two external resistors, the user can

generate reference voltages other than 1 V and 2 V. Another

alternative is to use an external reference for designs requiring

enhanced accuracy and/or drift performance. See Table II for a

summary of the pin-strapping options for the AD9225 reference

configurations.

Figure 16 shows a simplified model of the internal voltage refer-

ence of the AD9225. A pin strappable reference amplifier

buffers a 1 V fixed reference. The output from the reference

amplifier, A1, appears on the VREF pin. The voltage on the

VREF pin determines the full-scale input span of the A/D. This

input span equals,

0.1␮F

CAPT

Full-Scale Input Span = 2 × VREF

10␮F

0.1␮F

AD9225

CAPB

The voltage appearing at the VREF pin as well as the state of

the internal reference amplifier, A1, are determined by the volt-

age appearing at the SENSE pin. The logic circuitry contains

two comparators which monitor the voltage at the SENSE pin.

The comparator with the lowest set point (approximately 0.3 V)

controls the position of the switch within the feedback path of

A1. If the SENSE pin is tied to AVSS (AGND), the switch is

connected to the internal resistor network thus providing a

VREF of 2.0 V. If the SENSE pin is tied to the VREF pin via a

short or resistor, the switch will connect to the SENSE pin. This

short will provide a VREF of 1.0 V. An external resistor network

will provide an alternative VREF between 1.0 V and 2.0 V. The

other comparator controls internal circuitry which will disable

the reference amplifier if the SENSE pin is tied AVDD. Dis-

abling the reference amplifier allows the VREF pin to be driven

by an external voltage reference.

0.1␮F

Figure 17. Recommended CAPT/CAPB Decoupling Network

The A/D’s input span may be varied dynamically by changing

the differential reference voltage appearing across CAPT and

CAPB symmetrically around 2.5 V (i.e., midsupply). To change

the reference at speeds beyond the capabilities of A2, it will be

necessary to drive CAPT and CAPB with two high speed, low

noise amplifiers. In this case, both internal amplifiers (i.e., A1

and A2) must be disabled by connecting SENSE to AVDD,

connecting VREF to AVSS and removing the capacitive decou-

pling network. The external voltages applied to CAPT and

CAPB must be 2.0 V + Input Span/4 and 2.0 V – Input Span/4

respectively in which the input span can be varied between 2 V

and 4 V. Note that those samples within the pipeline A/D dur-

ing any reference transition will be corrupted and should be

discarded.

AD9225

TO

A/D

5k⍀

DRIVING THE ANALOG INPUTS

The AD9225 has a highly flexible input structure allowing it to

interface with single ended or differential input interface cir-

cuitry. The applications shown in sections “Driving the Analog

Inputs” and “Reference Configurations” along with the infor-

mation presented in “Input and Reference Overview” of this

data sheet, give examples of both single-ended and differential

operation. Refer to Tables I and II for a list of the different

possible input and reference configurations and their associated

figures in the data sheet.

CAPT

5k⍀

A2

5k⍀

CAPB

5k⍀

DISABLE

LOGIC

A2

VREF

A1

1V

6.25k⍀

The optimum mode of operation, analog input range, and asso-

ciated interface circuitry will be determined by the particular

applications performance requirements as well as power supply

options. For example, a dc coupled single-ended input would be

appropriate for most data acquisition and imaging applications.

Also, many communication applications which require a dc

coupled input for proper demodulation can take advantage of

the excellent single-ended distortion performance of the AD9225.

The input span should be configured such that the system’s

performance objectives and the headroom requirements of the

driving op amp are simultaneously met.

SENSE

DISABLE

6.25k⍀

LOGIC

A1

REFCOM

Figure 16. Equivalent Reference Circuit

–10–

REV. A

AD9225

Table I. Analog Input Configuration Summary

Input

Connection

Input

Coupling Span (V)

Input Range (V)

VINA¹VINB¹

Figure

#

Comments

Single-Ended

DC

2

0 to 2

1

19, 20

Best for stepped input response applications, requires ±5 V op amp.

2 × VREF

0 to

2 × VREF

VREF

Same as above but with improved noise performance due to

increase in dynamic range. Headroom/settling time requirements

of ±5 op amp should be evaluated.

4

0 to 4

2.0

19, 20

30

Optimum noise performance, excellent THD performance, often

requires low distortion op amp with VCC > +5 V due to its head-

room issues.

2 × VREF

2.0 – VREF

to

Optimum THD performance with VREF = 1. Single supply

operation (i.e., +5 V) for many op amps.

2.0 + VREF

Single-Ended

AC

2 or

2 × VREF

0 to 1 or

0 to 2 × VREF

1 or VREF

2.0

21, 22

22

4

0.5 to 4.5

Optimum noise performance, excellent THD performance, ability

to use ±5 V op amp.

2 × VREF

2.0 – VREF

to

2.0

21

Flexible input range, optimum THD performance with VREF = 1.

Ability to use either +5 V or ±5 V op amp.

2.0 + VREF

Differential

(via Transformer)

or Amplifier

AC/DC

2

2 to 3

3 to 2

23, 24

Optimum full-scale THD and SFDR performance well beyond

the A/Ds Nyquist frequency. Preferred mode for undersampling

applications.

2 × VREF

2.0 – VREF/2 2.0 + VREF/2 23, 24

to to

2.0 + VREF/2 2.0 – VREF/2

Same as above with the exception that full-scale THD and SFDR

performance can be traded off for better noise performance.

4.0

2.0 to 3.0

3.0 to 2.0

23, 24

Optimum noise performance.

NOTE

¹VINA and VINB can be interchanged if signal inversion is required.

Table II. Reference Configuration Summary

Input Span (VINA–VINB)

Reference

Operating Mode

(V p-p)

Required VREF (V)

Connect

To

INTERNAL

2

4

1

2

SENSE

R1

VREF

REFCOM

VREF AND SENSE

SENSE AND REFCOM

2 ≤ SPAN ≤ 4 AND

SPAN = 2

1 ≤ VREF ≤ 2.0 AND

VREF = (1 + R1/R2)

×

VREF

R2

EXTERNAL

(NONDYNAMIC)

2 ≤ SPAN ≤ 4

1 ≤ VREF ≤ 2.0

SENSE

VREF

AVDD

EXT. REF.

EXTERNAL

(DYNAMIC)

CAPT and CAPB

Externally Driven

SENSE

VREF

EXT. REF.

AVDD

AVSS

CAPT

CAPB

REV. A

–11–

AD9225

Single-ended operation is often limited by the availability of

driving op amps. Very low distortion op amps that provide great

performance out to the Nyquist frequency of the converter are

hard to find. Compounding the problem, for dc-coupled, single-

ended applications, is the inability of many high performance

amplifiers to maintain low distortions as their outputs approach

their positive output voltage limit (i.e., 1 dB compression point).

For this reason, it is recommended that applications requiring high

performance dc coupling use the single-ended to differential

circuit shown in Figure 23.

Differential modes of operation (ac- or dc-coupled input) pro-

vide the best THD and SFDR performance over a wide fre-

quency range. Differential operation should be considered for the

most demanding spectral-based applications (e.g., direct IF-to-

digital conversion). See Figures 23, 24 and section on Differen-

tial Mode of Operation. Differential input characterization was

performed for this data sheet using the configuration shown in

Figure 24.

Single-ended operation requires that VINA be ac or dc coupled

to the input signal source while VINB of the AD9225 be biased

to the appropriate voltage corresponding to a midscale code

transition. Note that signal inversion may be easily accomplished

by transposing VINA and VINB. Most of the single-ended specifi-

cations for the AD9225 are characterized using Figure 21 cir-

cuitry with input spans of 4 V and 2 V as well as VINB = 2.5 V.

DC COUPLING AND INTERFACE ISSUES

Many applications require the analog input signal to be dc coupled

to the AD9225. An operational amplifier can be configured to

rescale and level shift the input signal so that it is compatible

with the selected input range of the A/D. The input range to the

A/D should be selected on the basis of system performance

objectives as well as the analog power supply availability since

this will place certain constraints on the op amp selection.

Differential operation requires that VINA and VINB be simulta-

neously driven with two equal signals that are in and out of

phase versions of the input signal. Differential operation of the

AD9225 offers the following benefits: (1) Signal swings are

smaller and therefore linearity requirements placed on the input

signal source may be easier to achieve, (2) Signal swings are

smaller and therefore may allow the use of op amps which may

otherwise have been constrained by headroom limitations, (3)

Differential operation minimizes even-order harmonic products,

and (4) Differential operation offers noise immunity based on

the device’s common-mode rejection.

Many of the new high performance op amps are specified for

only ±5 V operation and have limited input/output swing capa-

bilities. The selected input range of the AD9225 should be

considered with the headroom requirements of the particular op

amp to prevent clipping of the signal. Also, since the output of a

dual supply amplifier can swing below –0.3 V, clamping its

output should be considered in some applications.

In some applications, it may be advantageous to use an op amp

specified for single supply +5 V operation since it will inherently

limit its output swing to within the power supply rails. Amplifiers

like the AD8041, AD8011 and are useful for this purpose but

their low bandwidths will limit the AD9225’s performance. High

performance amplifiers such as the AD9631, AD9632, AD8056

or AD8055 allow the AD9225 to be configured for larger

input spans which will improve the ADC’s noise performance.

As is typical of most IC devices, exceeding the supply limits will

turn on internal parasitic diodes resulting in transient currents

within the device. Figure 18 shows a simple means of clamping

an ac or dc coupled single ended input with the addition of two

series resistors and two diodes. An optional capacitor is shown

for ac coupled applications. Note that a larger series resistor

could be used to limit the fault current through D1 and D2 but

should be evaluated since it can cause a degradation in overall

performance. A similar clamping circuit could also be used for

each input if a differential input signal is being applied. The

diodes might cause nonlinearities in the signal. Careful evalua-

tion should be performed on the diodes used.

Op amp circuits using a noninverting and inverting topology are

discussed in the next section. Although not shown, the nonin-

verting and inverting topologies can be easily configured as part

of an antialiasing filter by using a Sallen-Key or Multiple-Feed-

back topology. An additional R-C network can be inserted be-

tween the op amp’s output and the AD9225 input to provide a

filter pole.

OPTIONAL

AC COUPLING

CAPACITOR

AVDD

D2

V

CC

R

30⍀

R

S1

S2

Simple Op Amp Buffer

1N4148

20⍀

AD9225

In the simplest case, the input signal to the AD9225 will already

be biased at levels in accordance with the selected input range.

It is simply necessary to provide an adequately low source im-

pedance for the VINA and VINB analog pins of the A/D. Figure

19 shows the recommended configuration a single-ended drive

using an op amp. In this case, the op amp is shown in a nonin-

verting unity gain configuration driving the VINA pin. The

internal reference drives the VINB pin. Note that the addi-

tion of a small series resistor of 30 Ω to 50 Ω connected to

VINA and VINB will be beneficial in nearly all cases. Refer to

section “Analog Input Operation” a discussion on resistor selec-

tion. Figure 19 shows the proper connection for a 0 V to 4 V

input range. Alternative single ended ranges of 0 V to 2 × VREF

can also be realized with the proper configuration of VREF

(refer to the section “Using the Internal Reference”). Headroom

limitations of the op amp must always be considered.

D1

1N4148

V

EE

Figure 18. Simple Clamping Circuit

SINGLE-ENDED MODE OF OPERATION

The AD9225 can be configured for single ended operation using

dc or ac coupling. In either case, the input of the A/D must be

driven from an operational amplifier that will not degrade the

A/D’s performance. Because the A/D operates from a single

supply, it will be necessary to level shift ground-based bipolar

signals to comply with its input requirements. Both dc and ac

coupling provide this necessary function, but each method results

in different interface issues which may influence the system

design and performance.

–12–

REV. A

AD9225

+V

U1

–V

maintaining a low impedance over a wide frequency range. The

combination of the capacitor and the resistor form a high-pass

filter with a high-pass –3 dB frequency determined by the equation,

4V

0V

AD9225

R

S

VINA

R

S

VINB

f

_{–3 dB}= 1/(2 × π × R × (C1 + C2))

2.0V

10␮F

VREF

The low impedance VREF voltage source biases both the VINB

input and provides the bias voltage for the VINA input. Figure

21 shows the VREF configured for 2.0 V thus the input range of

the A/D is 0 V to 4 V. Other input ranges could be selected by

changing VREF.

0.1␮F

SENSE

Figure 19. Single-Ended AD9225 Op Amp Drive Circuit

Op Amp with DC Level Shifting

C1

Figure 20 shows a dc-coupled level shifting circuit employing an

op amp, A1, to sum the input signal with the desired dc set.

Configuring the op amp in the inverting mode with the given

resistor values results in an ac signal gain of –1. If the signal

inversion is undesirable, interchange the VINA and VINB con-

nections to reestablish the original signal polarity. The dc volt-

age at VREF sets the common-mode voltage of the AD9225.

For example, when VREF = 2.0 V, the input level from the op

amp will also be centered around 2.0 V. The use of ratio matched,

thin-film resistor networks will minimize gain and offset errors.

Also, an optional pull-up resistor, RP, may be used to reduce

the output load on VREF to less than its 1 mA maximum.

+V +V

10␮F

+5V

AD9631

4.5

2.5

0.5

+2V

0V

–2V

R

AD9225

VINA

R

V

S

IN

C2

0.1␮F

–5V

S

VINB

R

0.1␮F

10␮F

Figure 21. AC-Coupled Input

Alternative AC Interface

Figure 22 shows a flexible ac coupled circuit which can be con-

figured for different input spans. Since the common mode volt-

age of VINA and VINB are biased to midsupply independent of

VREF, VREF can be pin strapped or reconfigured to achieve

input spans between 2 V and 4 V p-p. The AD9225’s CMRR

along with the symmetrical coupling R-C networks will reject

both power supply variations and noise. The resistors, R, estab-

lish the common-mode voltage. They may have a high value

(e.g., 5 kΩ) to minimize power consumption and establish a low

cutoff frequency. The capacitors, C1 and C2, are typically a

0.1 µF ceramic and 10 µF tantalum capacitor in parallel to

achieve a low cutoff frequency while maintaining a low imped-

ance over a wide frequency range. R_Sisolates the buffer ampli-

fier from the A/D input. The optimum performance is achieved

when VINA and VINB are driven via symmetrical networks.

The f_{–3 dB}point can be approximated by the equation,

500⍀*

+V

CC

0.1␮F

NC

1

+VREF

–VREF

500⍀*

7

0V

2

3

DC

R

S

6

A1

VINA

R **

500⍀*

0.1␮F

P

5

+V

4

500⍀*

AD9225

NC

R

S

VREF

VINB

*OPTIONAL RESISTOR NETWORK-OHMTEK ORNA500D

**OPTIONAL PULL-UP RESISTOR WHEN USING INTERNAL REFERENCE

Figure 20. Single-Ended Input with DC-Coupled Level Shift

AC COUPLING AND INTERFACE ISSUES

1

For applications where ac coupling is appropriate, the op amp’s

output can be easily level-shifted via a coupling capacitor. This

has the advantage of allowing the op amp’s common-mode level

f _–3dB

=

2 π ×6K +(C1+C2)

to be symmetrically biased to its midsupply level (i.e., (V_CC

+

C2

0.1␮F

V_EE)/2). Op amps that operate symmetrically with respect to

their power supplies typically provide the best ac performance as

well as greatest input/output span. Various high speed/perfor-

mance amplifiers which are restricted to +5 V/–5 V operation

and/or specified for +5 V single-supply operation can be easily

configured for the 4 V or 2 V input span of the AD9225. Note

that differential transformer coupling, which is another form of

ac coupling, should be considered for optimum ac performance.

AD9225

R

S

V

IN

VINA

C1

1k⍀

10␮F

VCM

VINB

C3

0.1␮F

R

S

C1

10␮F

C2

0.1␮F

Simple AC Interface

Figure 21 shows a typical example of an ac-coupled, single-

ended configuration. The bias voltage shifts the bipolar, ground-

referenced input signal to approximately AVDD/2. The value

for C1 and C2 will depend on the size of the resistor, R. The

capacitors, C1 and C2, are a 0.1 µF ceramic and 10 µF tanta-

lum capacitor in parallel to achieve a low cutoff frequency while

Figure 22. AC-Coupled Input-Flexible Input Span,

_CM= 2.5 V

V

REV. A

–13–

AD9225

OP AMP SELECTION GUIDE

500⍀

Op amp selection for the AD9225 is highly dependent on a

particular application. In general, the performance requirements

of any given application can be characterized by either time

domain or frequency domain parameters. In either case, one

should carefully select an op amp which preserves the perfor-

mance of the A/D. This task becomes challenging when one

considers the AD9225’s high performance capabilities coupled

with other extraneous system level requirements such as power

consumption and cost.

500⍀

50⍀

VREF

VINA

0V

500⍀

AD9225

500⍀

+V

VINB

REF

R*

500⍀

0.1␮F

10␮F

*OPTIONAL

The ability to select the optimal op amp may be further compli-

cated by either limited power supply availability and/or limited

acceptable supplies for a desired op amp. Newer, high perfor-

mance op amps typically have input and output range limita-

tions in accordance with their lower supply voltages. As a result,

some op amps will be more appropriate in systems where ac-

coupling is allowable. When dc-coupling is required, op amps

without headroom constraints such as rail-to-rail op amps or

ones where larger supplies can be used should be considered.

The following section describes some op amps currently avail-

able from Analog Devices. The system designer is always

encouraged to contact the factory or local sales office to be

updated on Analog Devices latest amplifier product offerings.

Highlights of the areas where the op amps excel and where they

may limit the performance of the AD9225 is also included.

Figure 23. Direct Coupled Drive Circuit with AD8056

Dual Op Amp

The circuit shown in Figure 23 is an ideal method of applying a

differential dc drive to the AD9225. We have used this configu-

ration to drive the AD9225 from 2 V to 4 V spans at frequencies

approaching Nyquist with performance numbers matching those

shown on pages 2 and 3 of this data sheet (gathered through a

transformer). The dc input is shifted to a dc point swinging

symmetrically about the reference voltage. The optional resistor

will provide additional current if more reference drive is required.

The driver circuit shown in Figure 23 is optimized for dc cou-

pling applications requiring optimum distortion performance.

This differential op amp driver circuit is configured to convert

and level shift a 2 V p-p single-ended, ground referenced signal

to a 4 V p-p differential signal centered at the VREF level of the

ADC. The circuit is based on two op amps which are configured

as matched unity gain difference amplifiers. The single-ended

input signal is applied to opposing inputs of the difference am-

plifiers, thus providing differential drive. The common-mode

offset voltage is applied to the noninverting resistor leg of each

difference amplifier providing the required offset voltage. The

common-mode offset can be varied over a wide span without

any serious degradation in distortion performance as shown in

Figures 25a and 25b, thus providing some flexibility in improv-

ing output compression distortion from some ±5 V op amps

with limited positive voltage swing.

AD8055: f_{–3 dB}= 300 MHz.

Low cost. Best used for driving single-ended ac

coupled configuration.

Limit: THD is compromised when output is not

swinging about 0 V.

AD8056: Dual Version of above amp.

Perfect for single-ended to differential configuration

(see Figure 23). Harmonics cancel each other in

differential drive, making this amplifier highly recom-

mended for a single-ended input signal source. Handles

input signals past the 20 MHz Nyquist frequency.

AD9631: f_{–3 dB}= 250 MHz.

Moderate cost.

Good for single-ended drive applications when signal

is anywhere between 0 V and 3 V.

Limits: THD is compromised above 8 MHz.

To protect the AD9225 from an undervoltage fault condition

from op amps specified for ±5 V operation, two diodes to AGND

can be inserted between each op amp output and the AD9225

inputs. The AD9225 will inherently be protected against any

overvoltage condition if the op amps share the same positive

power supply (i.e., AVDD) as the AD9225. Note, the gain

accuracy and common-mode rejection of each difference ampli-

fier in this driver circuit can be enhanced by using a matched

thin-film resistor network (i.e., Ohmtek ORNA5000F) for the

op amps. Recall, the AD9225’s small signal bandwidth is

105 MHz, hence any noise falling within the baseband band-

width of the AD9225 will degrade its overall noise performance.

DIFFERENTIAL MODE OF OPERATION

Since not all applications have a signal preconditioned for differ-

ential operation, there is often a need to perform a single-ended-

to-differential conversion. In systems that do not need to be dc

coupled, an RF transformer with a center tap is the best method

to generate differential inputs for the AD9225. It provides all

the benefits of operating the A/D in the differential mode

without contributing additional noise or distortion. An RF

transformer also has the added benefit of providing electrical

isolation between the signal source and the A/D.

The noise performance of each unity gain differential driver

circuit is limited by its inherent noise gain of two. For unity gain

op amps ONLY, the noise gain can be reduced from two to one

beyond the input signals passband by adding a shunt capacitor,

C_F, across each op amp’s feedback resistor. This will essentially

establish a low-pass filter, which reduces the noise gain to one

beyond the filter’s f_{–3 dB}while simultaneously bandlimiting the

input signal to f_{–3 dB}. Note, the pole established by this filter can

also be used as the real pole of an antialiasing filter.

An improvement in THD and SFDR performance can be realized

by operating the AD9225 in the differential mode. The perfor-

mance enhancement between the differential and single-ended

mode is most noteworthy as the input frequency approaches

and goes beyond the Nyquist frequency (i.e., f_IN> F_S/2).

–14–

REV. A

AD9225

Figure 24 shows the schematic of the suggested transformer

circuit. The circuit uses a minicircuits RF transformer, model

#T4-1T, which has an impedance ratio of four (turns ratio of

2). The schematic assumes that the signal source has a 50 Ω

source impedance. The 1:4 impedance ratio requires the 200 Ω

secondary termination for optimum power transfer and VSWR.

The center tap of the transformer provides a convenient means

of level-shifting the input signal to a desired common-mode

voltage.

–76

–78

–80

–82

–84

–86

F

= 10MHz

= 2.5MHz

IN

F

R

IN

S

33⍀

VINA

CML

49.9⍀

0.1␮F

200⍀

AD9225

0

1

2

3

4

5

COMMON-MODE VOLTAGE – V

VINB

MINICIRCUITS

T4-1T

R

33⍀

S

Figure 25a. Common-Mode Voltage vs. THD

(A_IN= 2 V Differential)

Figure 24. Transformer Coupled Input

This (Figure 24) configuration was used to gather the differen-

tial data on pages 2 and 3.

–76

–78

–80

Transformers with other turns ratios may also be selected to

optimize the performance of a given application. For example, a

given input signal source or amplifier may realize an improve-

ment in distortion performance at reduced output power levels

and signal swings. For example, selecting a transformer with a

higher impedance ratio (e.g., Minicircuits T16-6T with a 1:16

impedance ratio) effectively “steps up” the signal level thus

further reducing the driving requirements of the signal source.

F

= 10MHz

IN

–82

–84

–86

F

= 2.5MHz

IN

Referring to Figure 24, a series resistors, R_S, and shunt capaci-

tor, C_S, were inserted between the AD9225 and the secondary

of the transformer. The value of 33 Ω was selected to specifi-

cally optimize both the THD and SNR performance of the A/D.

R_Sand C_Shelp provide a low-pass filter to block high frequency

noise.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

COMMON-MODE VOLTAGE – V

Figure 25b. Common-Mode Voltage vs. THD

(A_IN= 4 V Differential)

The AD9225 can be easily configured for either a 2 V p-p input

span or 4.0 V p-p input span by setting the internal reference

(see Table II). Other input spans can be realized with two exter-

nal gain setting resistors as shown in Figure 28 of this data

sheet. Figures 25a and 25b demonstrate how both spans of the

AD9225 achieve the high degree of linearity and SFDR over a

wide range of amplitudes required by the most demanding com-

munication applications.

0.0

–10.0

–20.0

–30.0

–40.0

–50.0

–60.0

–70.0

–80.0

–90.0

–100.0

FUND

F

= 2.5MHz

= 25MHz

IN

S

Figures 25a and 25b demonstrate the flexibility of common-

mode voltage (transformer center tap) with respect to THD.

2ND

4TH

3RD

–110.0

–119.7

2.0E+6

4.0E+6

6.0E+6

8.0E+6 10.0E+6

12.5E+6

Figure 25c. Single Tone Frequency Domain Plot

Common-Mode Voltage = 2.5 V (A_IN= 4 V

Differential)

REV. A

–15–

AD9225

REFERENCE CONFIGURATIONS

This reference configuration could also be used for a differential

input in which VINA and VINB are driven via a transformer as

shown in Figure 24. In this case, the common-mode voltage,

VCM , is set at midsupply by connecting the transformer’s

center tap to CML of the AD9225. VREF can be configured

for 1.0 V or 2.0 V by connecting SENSE to either VREF or

REFCOM respectively. Note that the valid input range for

each of the differential inputs is one half of the single-ended

input and thus becomes V_CM– VREF/2 to V_CM+ VREF/2.

The figures associated with this section on internal and external

reference operation do not show recommended matching series

resistors for VINA and VINB for the purpose of simplicity.

Please refer to section “Driving the Analog Inputs, Introduc-

tion” for a discussion of this topic. Also, the figures do not show

the decoupling network associated with the CAPT and CAPB

pins. Please refer to the section “Reference Operation” for a

discussion of the internal reference circuitry and the recom-

mended decoupling network shown in Figure 17.

3.5V

VINA

USING THE INTERNAL REFERENCE

Single-Ended Input with 0 to 2

؋

VREF Range

Figure 26 shows how to connect the AD9225 for a 0 V to 2 V or

0 V to 4 V input range via pin strapping the SENSE pin. An

intermediate input range of 0 to 2 × VREF can be established

using the resistor programmable configuration in Figure 28.

1.5V

VCM

AD9225

VINB

1V

VREF

10␮F

0.1␮F

SENSE

REFCOM

In either case, both the midscale voltage and input span are

directly dependent on the value of VREF. More specifically, the

midscale voltage is equal to VREF while the input span is equal

to 2 × VREF. Thus, the valid input range extends from 0 to 2 ×

VREF. When VINA is ≤ 0 V, the digital output will be 000 Hex;

when VINA is ≥ 2 × VREF, the digital output will be FFF Hex.

Figure 27. Internal Reference—2 V p-p Input Span,

V_CM= 2.5 V

Resistor Programmable Reference

Figure 28 shows an example of how to generate a reference

voltage other than 1.0 V or 2.0 V with the addition of two exter-

nal resistors and a bypass capacitor. Use the equation,

Shorting the VREF pin directly to the SENSE pin places the

internal reference amplifier in unity-gain mode and the resultant

VREF output is 1 V. Therefore, the valid input range is 0 V to

2 V. However, shorting the SENSE pin directly to the REFCOM

pin configures the internal reference amplifier for a gain of 2.0

and the resultant VREF output is 2.0 V. Thus, the valid input

range becomes 0 V to 4 V. The VREF pin should be bypassed

to the REFCOM pin with a 10 µF tantalum capacitor in parallel

with a low-inductance 0.1 µF ceramic capacitor.

VREF = 1 V × (1 + R1/R2)

to determine appropriate values for R1 and R2. These resistors

should be in the 2 kΩ to 100 kΩ range. For the example shown,

R1 equals 2.5 kΩ and R2 equals 5 kΩ. From the equation above,

the resultant reference voltage on the VREF pin is 1.5 V. This

sets the input span to be 3 V p-p. To assure stability, place a

0.1 µF ceramic capacitor in parallel with R1.

The midscale voltage can be set to VREF by connecting VINB

to VREF to provide an input span of 0 to 2 × VREF. Alterna-

tively, the midscale voltage can be set to 2.5 V by connecting

VINB to a low impedance 2.5 V source. For the example

shown, the valid input single-ended range for VINA is 1 V to

4 V since VINB is set to an external, low impedance 2.5 V source.

The VREF pin should be bypassed to the REFCOM pin with a

10 µF tantalum capacitor in parallel with a low inductance

0.1 µF ceramic capacitor.

2

؋

VREF

VINA

0V

VINB

10␮F

0.1␮F

VREF

SHORT FOR 0V TO 2V

INPUT SPAN

AD9225

SENSE

SHORT FOR 0V TO 4V

INPUT SPAN

REFCOM

4V

VINA

Figure 26. Internal Reference—2 V p-p Input Span,

V_CM= 1 V, or 4 V p-p Input Span

1V

AD9225

2.5V

VINB

1.5V

VREF

C1

0.1␮F

R1

2.5k⍀

0.1␮F

10␮F

Figure 27 shows the single-ended configuration that gives good

dynamic performance (SINAD, SFDR). To optimize dynamic

specifications, center the common mode voltage of the analog

input at approximately by 2.5 V by connecting VINB to a low

impedance 2.5 V source. As described above, shorting the VREF

pin directly to the SENSE pin results in a 1 V reference voltage

and a 2 V p-p input span. The valid range for input signals is

1.5 V to 3.5 V. The VREF pin should be bypassed to the REF-

COM pin with a 10 µF tantalum capacitor in parallel with a

low-inductance 0.1 µF ceramic capacitor.

SENSE

R2

5k⍀

REFCOM

Figure 28. Resistor Programmable Reference +3 V p-p

Input Span, V_CM= 2.5 V

–16–

REV. A

AD9225

USING AN EXTERNAL REFERENCE

mum of a 10 µF capacitor in parallel with a 0.1 µF low inductance

Using an external reference may enhance the dc performance

of the AD9225 by improving drift and accuracy. Figures 29 and

30 show examples of how to use an external reference with the

A/D. Table III is a list of suitable voltage references from Ana-

log Devices. To use an external reference, the user must disable

the internal reference amplifier and drive the VREF pin. Con-

necting the SENSE pin to AVDD disables the internal reference

amplifier.

ceramic capacitor decouple the reference output to ground.

2

؋

REF

VINA

0V

+5V

VINB

VREF

0.1␮F

10␮F

0.1␮F

AD9225

VREF

0.1␮F

Table III. Suitable Voltage References

Initial

+5V

SENSE

Output

Voltage

Drift

(ppm/؇C)

Accuracy

% (max)

Operating

Current

Figure 30. Input Range = 0 V to 2 × VREF

DIGITAL INPUTS AND OUTPUTS

Digital Outputs

Internal

AD589

AD1580 1.225

REF191 2.048

1.00

1.235

26

1.4

1 mA

50 µA

45 µA

1 mA

10–100

50–100

5–25

26

1.2–2.8

0.08–0.8

0.1–0.5

1.4

The AD9225 output data is presented in positive true straight

binary for all input ranges. Table IV indicates the output data

formats for various input ranges regardless of the selected input

range. A twos complement output data format can be created by

inverting the MSB.

Internal

2.0

The AD9225 contains an internal reference buffer, A2 (see

Figure 16), that simplifies the drive requirements of an external

reference. The external reference must be able to drive about

5 kΩ (±20%) load. Note that the bandwidth of the reference

buffer is deliberately left small to minimize the reference noise

contribution. As a result, it is not possible to change the refer-

ence voltage rapidly in this mode.

Table IV. Output Data Format

I

nput (V)

Condition (V)

Digital Output

OTR

VINA–VINB

< – VREF

= – VREF

= 0

= + VREF – 1 LSB 1111 1111 1111

≥ + VREF 1111 1111 1111

0000 0000 0000

1000 0000 0000

1

0

1

2.5V+VREF

2.5V

2.5V–VREF

VINA

VINB

AD9225

2.5V

REF

+5V

0.1␮F

+FS –1 1/2 LSB

22␮F

0.1␮F

OTR DATA OUTPUTS

R1

R2

1111 1111 1111

1111 1111 1110

OTR

1

0

A1

VREF

0.1␮F

–FS+1/2 LSB

SENSE

+5V

0000 0000 0001

0000 0000 0000

0

1

Figure 29. External Reference

–FS

–FS –1/2 LSB

+FS

Variable Input Span with V_CM= 2.5 V

+FS –1/2 LSB

Figure 29 shows an example of the AD9225 configured for an

input span of 2 × VREF centered at 2.5 V. An external 2.5 V

reference drives the VINB pin thus setting the common-mode

voltage at 2.5 V. The input span can be independently set by a

voltage divider consisting of R1 and R2 which generates the

VREF signal. A1 buffers this resistor network and drives

VREF. Choose this op amp based on accuracy requirements. It

is essential that a minimum of a 10 µF capacitor in parallel with

a 0.1 µF low inductance ceramic capacitor decouple A1’s output

to ground.

Figure 31. Output Data Format

Out Of Range (OTR)

An out-of-range condition exists when the analog input voltage

is beyond the input range of the converter. OTR is a digital out

put that is updated along with the data output corresponding to

the particular sampled analog input voltage. Hence, OTR has

the same pipeline delay (latency) as the digital data. It is LOW

when the analog input voltage is within the analog input range.

It is HIGH when the analog input voltage exceeds the input

range as shown in Figure 32. OTR will remain HIGH until the

analog input returns within the input range and another conver-

sion is completed. By logical ANDing OTR with the MSB and

its complement, overrange high or underrange low conditions

can be detected. Table V is a truth table for the over/underrange

circuit in Figure 33 which uses NAND gates. Systems requiring

programmable gain conditioning of the AD9225 input signal

can immediately detect an out-of-range condition, thus elimi-

nating gain selection iterations. Also, OTR can be used for

digital offset and gain calibration.

Single-Ended Input with 0 to 2

؋

VREF Range

Figure 30 shows an example of an external reference driving

both VINB and VREF. In this case, both the common-mode

voltage and input span are directly dependent on the value of

VREF. More specifically, the common-mode voltage is equal to

VREF while the input span is equal to 2 × VREF. Thus, the

valid input range extends from 0 to 2 × VREF. For example, if

the REF191, a 2.048 V external reference was selected, the valid

input range extends from 0 to 4.096 V. In this case, 1 LSB of

the AD9225 corresponds to 1 mV. It is essential that a mini-

REV. A

–17–

AD9225

Table V. Out-of-Range Truth Table

The AD9225 has a clock tolerance of 5% at 25 MHz. One way

to obtain a 50% duty cycle clock is to divide down a clock of

higher frequency, as shown in Figure 33. This configuration will

also decrease the jitter of the source clock.

OTR

MSB

Analog Input Is

0

1

0

1

0

1

In Range

Underrange

Overrange

+5V

R

D

Q

MSB

OTR

MSB

OVER = “1”

25MHz

50MHz

Q

S

UNDER = “1”

+5V

Figure 33. Divide-by-Two Clock Circuit

Figure 32. Overrange or Underrange Logic

In this case a 50 MHz clock is divided by two to produce the

25 MHz clock input for the AD9225. In this configuration, the

duty cycle of the 50 MHz clock is irrelevant.

Digital Output Driver Considerations (DRVDD)

The AD9225 output drivers can be configured to interface with

+5 V or 3.3 V logic families by setting DRVDD to +5 V or 3.3 V

respectively. The output drivers are sized to provide sufficient

output current to drive a wide variety of logic families. However,

large drive currents tend to cause glitches on the supplies and

may affect SINAD performance. Applications requiring the

ADC to drive large capacitive loads or large fanout may require

additional decoupling capacitors on DRVDD. In extreme cases,

external buffers or latches may be required.

The input circuitry for the CLOCK pin is designed to accom-

modate CMOS inputs. The quality of the logic input, particu-

larly the rising edge, is critical in realizing the best possible jitter

performance of the part: the faster the rising edge, the better the

jitter performance.

As a result, careful selection of the logic family for the clock

driver, as well as the fanout and capacitive load on the clock

line, is important. Jitter-induced errors become more predomi-

nant at higher frequency, large amplitude inputs, where the

input slew rate is greatest.

Clock Input and Considerations

The AD9225 internal timing uses the two edges of the clock

input to generate a variety of internal timing signals. The clock

input must meet or exceed the minimum specified pulsewidth

high and low (t_CHand t_CL) specifications for the given A/D as

defined in the Switching Specifications at the beginning of the

data sheet to meet the rated performance specifications. For

example, the clock input to the AD9225 operating at 25 MSPS

may have a duty cycle between 45% to 55% to meet this timing

requirement since the minimum specified t_CHand t_CLis 18 ns.

For low clock rates, the duty cycle may deviate from this range

to the extent that both t_CHand t_CLare satisfied.

Most of the power dissipated by the AD9225 is from the analog

power supplies. However, lower clock speeds will reduce digital

current. Figure 34 shows the relationship between power and

clock rate.

380

360

340

2V

All high speed high resolution A/Ds are sensitive to the quality

of the clock input. The degradation in SNR at a given full-scale

input frequency (f_IN) due to only aperture jitter (t_A) can be cal-

culated with the following equation:

320

300

280

260

240

220

200

180

INTERNAL

REFERENCE

1

SNR = 20 log₁₀

2π f _INt_A

1V

INTERNAL

REFERENCE

In the equation, the rms aperture jitter, t_A, represents the root-

sum square of all the jitter sources which include the clock in-

put, analog input signal, and A/D aperture jitter specification.

Undersampling applications are particularly sensitive to jitter.

0

5

10

15

20

25

30

35

SAMPLE RATE

Clock input should be treated as an analog signal in cases where

aperture jitter may affect the dynamic range of the AD9225.

Power supplies for clock drivers should be separated from the

A/D output driver supplies to avoid modulating the clock signal

with digital noise. Low jitter crystal controlled oscillators make

the best clock sources. If the clock is generated from another

type of source (by gating, dividing, or other method), it should

be retimed by the original clock at the last step.

Figure 34. Power Consumption vs. Clock Rate

Direct IF Down Conversion Using the AD9225

Sampling IF signals above an ADC’s baseband region (i.e., dc

to F_S/2) is becoming increasingly popular in communication

applications. This process is often referred to as Direct IF Down

Conversion or Undersampling. There are several potential ben-

efits in using the ADC to alias (i.e., or mix) down a narrowband

or wideband IF signal. First and foremost is the elimination of a

complete mixer stage with its associated baseband amplifiers

and filters, reducing cost and power dissipation. Second is the

ability to apply various DSP techniques to perform such func-

tions as filtering, channel selection, quadrature demodulation,

data reduction, detection, etc. A detailed discussion on using

The clock input is referred to as the analog supply. Its logic

threshold is AVDD/2. If the clock is being generated by 3 V

logic, it will have to be level shifted into 5 V CMOS logic levels.

This can also be accomplished by ac-coupling and level-shifting

the clock signal.

–18–

REV. A

AD9225

this technique in digital receivers can be found in Analog

Devices Application Notes AN-301 and AN-302.

“kick back” currents associated with the AD9225’s switched

capacitor input stage.

In Direct IF Down Conversion applications, one exploits the

inherent sampling process of an ADC in which an IF signal

lying outside the baseband region can be aliased back into the

baseband region in a similar manner that a mixer will down-

convert an IF signal. Similar to the mixer topology, an image

rejection filter is required to limit other potential interfering

signals from also aliasing back into the ADC’s baseband region.

A trade-off exists between the complexity of this image rejection

filter and the ADC’s sample rate as well as dynamic range.

The distortion and noise performance of an ADC at the given

IF frequency is of particular concern when evaluating an ADC

for a narrowband IF sampling application. Both single tone and

dual tone SFDR vs. amplitude are very useful in assessing an

ADC’s dynamic and static nonlinearities. SNR vs. amplitude

performance at the given IF is useful in assessing the ADC’s

noise performance and noise contribution due to aperture jitter.

In any application, one is advised to test several units of the

same device under the same conditions to evaluate the given

applications sensitivity to that particular device.

The AD9225 is well suited for various IF sampling applications.

The AD9225’s low distortion input SHA has a full-power band-

width extending beyond 130 MHz thus encompassing many

popular IF frequencies. A DNL of ±0.4 LSB (typ) combined

with low thermal input referred noise allows the AD9225 in the

2 V span to provide 69 dB of SNR for a baseband input sine

wave. Also, its low aperture jitter of 0.8 ps rms ensures mini-

mum SNR degradation at higher IF frequencies. In fact, the

AD9225 is capable of still maintaining 68 dB of SNR at an IF of

71 MHz with a 2 V input span. Note, although the AD9225 can

yield a 1 dB to 2 dB improvement in SNR when configured for

the larger 4 V span, the 2 V span achieves the optimum full-

scale distortion performance at these higher input frequencies.

Also, the 2 V span reduces the performance requirements of the

input driver circuitry (i.e., IP3) and thus may also be more

attractive from a system implementation perspective.

Figures 36–39 combine the dual tone SFDR as well as single

tone SFDR and SNR performances at IF frequencies of 35 MHz,

45 MHz, 70 MHz, and 85 MHz. Note, the SFDR vs. amplitude

data is referenced to dBFS while the single tone SNR data is

referenced to dBc. The performance characteristics in these

figures are representative of the AD9225 without any preceding

gain stage. The AD9225 was operated in the differential mode

(via transformer) with a 2 V span and a sample rate between

28 MSPS and 36 MSPS. The analog supply (AVDD) and the

digital supply (DRVDD) were set to +5 V and 3.3. V respectively.

100

SFDR

SINGLE-TONE

95

(dBFS)

90

85

Figure 35 shows a simplified schematic of the AD9225 config-

ured in an IF sampling application. To reduce the complexity of

the digital demodulator in many quadrature demodulation ap-

plications, the IF frequency and/or sample rate are strategically

selected such that the bandlimited IF signal aliases back into the

center of the ADC’s baseband region (i.e., F_S/4). This demodu-

lation technique typically reduces the complexity of the post

digital demodulator ASIC which follows the ADC.

SFDR

DUAL-TONE

(dBFS)

80

75

70

65

60

55

50

SNR

SINGLE-TONE

(dBc)

HIGH

0

–15

–10

–5

LINEARITY OPTIONAL

A

– dBFS

IN

RF

BANDPASS

FILTER

SAW

FILTER

AMPLIFIER

AD9225

VINA

MINICIRCUITS

FROM

MIXER

Figure 36. IF Undersampling at 35 MHz (F₁= 34.63 MHz,

F₂= 35.43 MHz, CLOCK = 20 MHz)

20⍀

200⍀

T4-6T

RF2317

RF2312

VINB

CML

20⍀

100

0.1␮F

SFDR

SINGLE-TONE

VREF

95

SENSE

(dBFS)

0.1␮F

90

10␮F

REFCOM

85

SFDR

DUAL-TONE

(dBFS)

80

75

70

65

60

55

50

Figure 35. Example of AD9225 IF Sampling Circuit

To maximize its distortion performance, the AD9225 is config-

ured in the differential mode with a 2 V span using a transformer.

The center-tap of the transformer is biased at midsupply via the

CML output of the AD9225. Preceding the AD9225 and trans-

former is an optional bandpass filter as well as a gain stage. A

low Q passive bandpass filter can be inserted to reduce out-

of-band distortion and noise which lies within the AD9225’s

130 MHz bandwidth. A large gain stage(s) is often required to

compensate for the high insertion losses of a SAW filter used for

channel selection and image rejection. The gain stage will also

provide adequate isolation for the SAW filter from the charge

SNR

SINGLE-TONE

(dBc)

0

–15

–10

–5

A

– dBFS

IN

Figure 37. IF Undersampling at 45 MHz (F₁= 44.81 MHz,

F₂= 45.23 MHz, CLOCK = 20 MHz)

REV. A

–19–

AD9225

directly under the AD9225. A solid ground plane under the A/D

is acceptable if the power and ground return currents are care-

fully managed. Alternatively, the ground plane under the A/D

may contain serrations to steer currents in predictable directions

where cross coupling between analog and digital would other-

wise be unavoidable. The AD9225/AD9225EB ground layout,

shown in Figure 47, depicts the serrated type of arrangement.

100

95

90

85

80

75

70

65

60

55

SFDR

SINGLE-TONE

(dBFS)

SFDR

DUAL-TONE

(dBFS)

The board is primarily built over a common ground plane. It has a

“slit” to route currents near the clock driver. Figure 40 illustrates

a general scheme of ground and power implementation, in and

around the AD9225.

SNR

SINGLE-TONE

(dBc)

LOGIC

SUPPLY

50

–15

AVDD

DVDD

0

–10

–5

A

– dBFS

IN

A

D

Figure 38. IF Undersampling at 70 MHz (F₁= 69.50 MHz,

F₂= 70.11 MHz, CLOCK = 25 MHz)

ADC

IC

DIGITAL

LOGIC

ICs

C

STRAY

ANALOG

CIRCUITS

DIGITAL

CIRCUITS

100

95

V

IN

B

A

STRAY

I

A

90

85

80

75

70

65

60

55

50

D

SFDR

SINGLE-TONE

(dBFS)

GND

D

AVSS

DVSS

= ANALOG

= DIGITAL

A

⌬V

A

D

SFDR

DUAL-TONE

(dBFS)

Figure 40. Ground and Power Consideration

Analog and Digital Driver Supply Decoupling

SNR

SINGLE-TONE

(dBc)

The AD9225 features separate analog and driver supply and

ground pins, helping to minimize digital corruption of sensitive

analog signals. In general, AVDD, the analog supply, should be

decoupled to AVSS, the analog common, as close to the chip as

physically possible. Figure 41 shows the recommended decou-

pling for the analog supplies; 0.1 µF ceramic chip and 10 µF

tantalum capacitors should provide adequately low impedance

over a wide frequency range. Note that the AVDD and AVSS

pins are co-located on the AD9225 to simplify the layout of the

decoupling capacitors and provide the shortest possible PCB

trace lengths. The AD9225/AD9225EB power plane layout,

shown in Figure 48 depicts a typical arrangement using a multi-

layer PCB.

0

–15

–10

–5

A

– dBFS

IN

Figure 39. IF Undersampling at 85 MHz (F₁= 84.81 MHz,

F₂= 85.23 MHz, CLOCK = 20 MHz)

GROUNDING AND DECOUPLING

Analog and Digital Grounding

Proper grounding is essential in any high speed, high resolution

system. Multilayer printed circuit boards (PCBs) are recom-

mended to provide optimal grounding and power schemes. The

use of ground and power planes offers distinct advantages:

1. The minimization of the loop area encompassed by a signal

and its return path.

AVDD

10␮F

AD9225

0.1␮F

AVSS

2. The minimization of the impedance associated with ground

and power paths.

Figure 41. Analog Supply Decoupling

3. The inherent distributed capacitor formed by the power

plane, PCB insulation and ground plane.

The CML is an internal analog bias point used internally by the

AD9225. This pin must be decoupled with at least a 0.1 µF

capacitor as shown in Figure 42. The dc level of CML is ap-

proximately AVDD/2. This voltage should be buffered if it is to

be used for any external biasing.

These characteristics result in both a reduction of electromag-

netic interference (EMI) and an overall improvement in

performance.

It is important to design a layout that prevents noise from cou-

pling onto the input signal. Digital signals should not be run in

parallel with input signal traces and should be routed away from

the input circuitry. While the AD9225 features separate analog

and driver ground pins, it should be treated as an analog com-

ponent. The AVSS and DRVSS pins must be joined together

CML

0.1␮F

AD9225

Figure 42. CML Decoupling

–20–

REV. A

AD9225

The digital activity on the AD9225 chip falls into two general

categories: correction logic, and output drivers. The internal

correction logic draws relatively small surges of current, mainly

during the clock transitions. The output drivers draw large

current impulses while the output bits are changing. The size

and duration of these currents are a function of the load on the

output bits: large capacitive loads are to be avoided. Note, the

internal correction logic of the AD9225 is referenced to AVDD

while the output drivers are referenced to DRVDD.

DRVDD

DRVSS

10␮F

AD9225

0.1␮F

Figure 43. Digital Supply Decoupling

A complete decoupling scheme will also include large tantalum

or electrolytic capacitors on the PCB to reduce low frequency

ripple to negligible levels. Refer to the AD9225/AD9225EB

schematic and layouts in Figures 44–50 for more information

regarding the placement of decoupling capacitors.

The decoupling shown in Figure 43, 0.1 µF ceramic chip capaci-

tors and a 10 µF tantalum are appropriate for a reasonable

capacitive load on the digital outputs (typically 20 pF on each

pin). Applications involving greater digital loads should consider

increasing the digital decoupling proportionally, and/or using

external buffers/latches.

REV. A

–21–

AD9225

U5

REF43

6

TP38

R34

50⍀

1

2

1

2

TP34

1

L2

FBEAD

J4

VOUT

VCC

VIN

GND

DUTAVDD

1

2

JP22

JP23

JP24

JP25

1

2

AVDDIN1

2

1

C18

1

2

C30

2

+

1

2

AVDD

JP26

1

2

P3

C39

10␮F

10V

1

4

0.1␮F

1

2

R31

1

+

0.001␮F

1

TP31

1

TP30

R25

2.49k⍀

2

C47

C22

10␮F

10V

+

820⍀

C52

0.1␮F

JP21

22␮F

20V

1

2

1

C36

0.1␮F

JP19

1

AGND

1

2

1

2

C57

0.1␮F

R3

+

2

C28

1

+

10k⍀

1

C2

2

1

2

0.1␮F

2

1

C29

0.1␮F

2

AVDD

1

10␮F

10V

2

R26

4.99k⍀

2

7

3

2

14

13

12

11

10

9

8

7

6

5

R4

15

IN

AVDD2

AVSS2

SENSE

VREF

OTR

D11

D10

D9

OTR

D11

D10

D9

D8

D7

D6

D5

D4

D3

2

8

R29

3

1

10k⍀

1

16

17

18

19

20

21

22

23

24

25

26

27

28

JP20

+V

U4

AD187

1

1k⍀

2

C21

10␮F

10V

+

1

2

N2

OUT

C35

0.1␮F

6

2

1

2

Q1

2N2222

2

R28

50⍀

N1

1

2

C27

0.1␮F

2

REFCOM

CAPB

D8

D7

D6

D5

D4

D3

D2

D1

–V

IN

4

1

2

R27

4.99k⍀

1

C34

0.1␮F

1

2

1

2

1

C20

10␮F

10V

CAPT

CML

VINA

VINB

AVSS1

AVDD1

DRVSS

DRVDD

C26

C33

U3

1

2

1

+

0.1␮F

VEE

R30

316⍀

1

2

C19

10␮F

10V

1

C31

0.1␮F

2

+

4

3

2

1

2

1

C32

D2

D1

D0

CLK

0.1␮F

TP37

L3

FBEAD

TP29

1

TP37

D0

R32

50⍀

VCCIN

AGND

1

2

CLK

1

VCC

1

P4

2

J3

1

+

1

2

C48

1

2

C24

0.1␮F

JP27

AD9225

2

C53

0.1␮F

22␮F

2

1

TP27

DRVDD

20V

2

JP18

R21

200⍀

1

TP33

1

DUTDRVDD

1

2

1

2

P4

C1

10␮F

10V

+

C37

0.1␮F

C40

0.001␮F

1

2

1

C42

15pF

R23

200⍀

TP28

1

R22

200⍀

TP32

1

2

DUTAUDD

2

T1

4

5

6

2

1

+

J1

1

2

1

2

C23

10␮F

10V

C41

0.001␮F

C38

0.1␮F

2

3

1

2

1

2

1

2

C46

15pF

C45

15pF

C44

15pF

C43

15pF

R24

50⍀

2

1

2

C25

0.1␮F

2

T4-6T

TP36

L6

FBEAD

L4

FBEAD

L1

TP40

TP5

1

DUTAVDDIN

2

FBEAD

1

2

VEEIN

1

2

DVDDIN

DGND

1

2

P6

DUTAVDD

3

P4

VEE

1

P2

DVDD

1

+

1

+

2

1

C58

1

C49

22␮F

20V

1

2

C59

1

2

22␮F

20V

AGND

C6

C54

0.1␮F

+

C14

0.1␮F

22␮F

2

20V

2

1

C12

0.1␮F

2

P2

TP6

U9

1

2

JP10

JP4

2

R17 22⍀

10

11

1

TP25

TP24

TP23

TP22

TP21

TP20

TP19

TP26

TP18

TP17

TP16

TP15

1

2

1

2

1

P1

2

P1

C4

TP7

L7404

U9

JP1

2

R5 22⍀

10V 10␮F

JP11

JP12

8

9

1

2

1

2

+ 1

3

5

7

9

4

R6 22⍀

TP8

JP5

2

U9

1

L7404

6

5

6

1

R7 22⍀

1

20

10

1

G1

G2

VCC

GND

8

L7404

DRVDD

L5

FBEAD

19

U9

R8 22⍀

JP16

DRVDDIN

DGND

1

2

1

13

12

3

U6

1

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

P5

B

A

D11

2

18

17

16

15

14

13

12

11

74541

A1

A2

A3

Y1

Y2

Y3

Y4

Y5

Y6

Y7

Y8

2

1

+

R9 22⍀

1

2

3

4

5

6

7

8

9

C51

22␮F

20V

L7404

1

2

C56

0.1␮F

11 P1

13 P1

D10

D9

D8

D7

D6

D5

R10 22⍀

2

DVDD

1

+

1

2

P5

A4

A5

A6

A7

A8

C7

C15

0.1␮F

10␮F

R11 22⍀

10V

TP39

1

2

R35

50⍀

15

17

19

21

23

25

27

29

31

33

35

37

39

P1

R12 22⍀

1

2

1

U1

REF43

U9

1

2

J5

DECOUPLING

C11

0.1␮F

2

R13 22⍀

1

2

VOUT

VIN

GND

AVDD

2

1

R14 22⍀

C8

1

2

1

2

1

2

C9

+

3

+

C16

0.1␮F

C17

0.1␮F

1

2

10␮F

10V

10␮F

10V

JP3

JP2

C5

10V 10␮F

2

1

2

R15 22⍀

1

20

1

2

+

1

2

G1

G2

VCC

GND

2

1

19

10

R19

4k⍀

R2

R18

5k⍀

1k⍀

2

1

2

18

D4

D3

D2

D1

D0

A1

Y1

Y2

Y3

Y4

Y5

Y6

Y7

Y8

CW

CCV

3

4

5

6

7

8

9

JP17

17

16

15

14

13

12

11

2

A2

A3

A4

A5

A6

A7

A8

3

1

U7

A

B

2

74541

C13

0.1␮F

1

TP4

U8

JP9

1

3

1

2

3

1

2

4

9

8

J2

CLK

OTR

1

R16

2

L7404

JP14

L7404

1

2

R1

TP14

TP13

TP12

TP11

22⍀

2

JP15

3

1

2

1

50⍀

1

B

A

B

A

2

1

JP32

U9

2

1

TP9

1

U9

1

TP10

1

U8

2

3

4

JP13

1

16

1

2

5

6

VCC

CLR

JP28

JP30

15

14

13

12

11

10

R20

2

3

4

5

6

7

8

TP1

L7404

RCO

QA

1

2

CLK

A

B

C

D

U8

22⍀

L7404

JP6

2

11

1

2

10

JP29

U2

1

2

AVDD

QB

QC

QD

1

TP2

74LS161

1

U8

C3

10␮F

10V

+

L7404

C10

DVDD

JP7

1

2

13

12

1

2

1

+

0.1␮F

1

2

C50

2

C55

0.1␮F

ENT

JP31

ENP

GND

TP3

1

10␮F

U8

L7404

2

9

1

U8

JP8

10V

2

LOAD

1

2

1

2

DECOUPLING

R33

1k⍀

2

1

L7404

DVDD

Figure 44. Evaluation Board Schematic

–22–

REV. A

AD9225

Figure 48. Evaluation Board Solder Side Layout

(Not to Scale)

Figure 45. Evaluation Board Component Side Layout

(Not to Scale)

Figure 49. Evaluation Board Power Plane Layout

Figure 46. Evaluation Board Ground Plane Layout

(Not to Scale)

Figure 50. Evaluation Board Solder Side Silkscreen

(Not to Scale)

Figure 47. Evaluation Board Component Side Silkscreen

(Not to Scale)

REV. A

–23–

AD9225

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

28-Lead Wide Body Small Outline IC

(R-28)

0.7125 (18.10)

0.6969 (17.70)

28

15

14

1

PIN 1

0.1043 (2.65)

0.0926 (2.35)

0.0291 (0.74)

0.0098 (0.25)

x 45؇

0.0500 (1.27)

0.0157 (0.40)

8؇

0؇

0.0500

(1.27)

BSC

0.0192 (0.49)

0.0138 (0.35)

0.0118 (0.30)

0.0040 (0.10)

SEATING

PLANE

0.0125 (0.32)

0.0091 (0.23)

28-Lead Shrink Small Outline IC

(RS-28)

0.407 (10.34)

0.397 (10.08)

28

15

14

1

0.07 (1.79)

0.066 (1.67)

0.078 (1.98) PIN 1

0.068 (1.73)

0.03 (0.762)

0.022 (0.558)

8°

0°

0.0256

(0.65)

BSC

0.015 (0.38)

0.010 (0.25)

0.008 (0.203)

0.002 (0.050)

SEATING

PLANE

0.009 (0.229)

0.005 (0.127)

–24–

REV. A


型号：	AD9225
厂家：	ADI
描述：	Complete 12-Bit, 25 MSPS Monolithic A/D Converter 完整的12位， 25 MSPS单芯片A / D转换器转换器
文件：	总24页 (文件大小：324K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

AD9225 [ADI]

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