AD9243_15 [ADI]
Complete 14-Bit, 3.0 MSPS Monolithic A/D Converter;
| 型号： | AD9243_15 |
| 厂家： | 
ADI |
| 描述： | Complete 14-Bit, 3.0 MSPS Monolithic A/D Converter |
| 文件： | 总24页 (文件大小：403K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Complete 14-Bit, 3.0 MSPS
Monolithic A/D Converter
a
AD9243
FEATURES
FUNCTIONAL BLOCK DIAGRAM
Monolithic 14-Bit, 3 MSPS A/D Converter
Low Power Dissipation: 110 mW
Single +5 V Supply
Integral Nonlinearity Error: 2.5 LSB
Differential Nonlinearity Error: 0.6 LSB
Input Referred Noise: 0.36 LSB
Complete: On-Chip Sample-and-Hold Amplifier and
Voltage Reference
Signal-to-Noise and Distortion Ratio: 79.0 dB
Spurious-Free Dynamic Range: 91.0 dB
Out-of-Range Indicator
Straight Binary Output Data
44-Lead MQFP
AVDD
DVDD
DRVDD
CLK
SHA
VINA
VINB
MDAC1
GAIN = 16
MDAC3
GAIN = 8
MDAC2
GAIN = 8
5
4
4
CML
CAPT
CAPB
A/D
A/D
A/D
A/D
5
4
4
4
DIGITAL CORRECTION LOGIC
14
VREF
OTR
OUTPUT BUFFERS
SENSE
BIT 1
(MSB)
1V
MODE
SELECT
BIT 14
(LSB)
AD9243
REFCOM
AVSS
DVSS
DRVSS
PRODUCT DESCRIPTION
PRODUCT HIGHLIGHTS
The AD9243 is a 3 MSPS, single supply, 14-bit analog-to-
digital converter (ADC). It combines a low cost, high speed
CMOS process and a novel architecture to achieve the resolution
and speed of existing hybrid implementations at a fraction of the
power consumption and cost. It is a complete, monolithic ADC
with an on-chip, high performance, low noise sample-and-hold
amplifier and programmable voltage reference. An external refer-
ence can also be chosen to suit the dc accuracy and temperature
drift requirements of the application. The device uses a multistage
differential pipelined architecture with digital output error correc-
tion logic to guarantee no missing codes over the full operating
temperature range.
The AD9243 offers a complete single-chip sampling 14-bit,
analog-to-digital conversion function in a 44-lead Metric Quad
Flatpack.
Low Power and Single Supply
The AD9243 consumes only 110 mW on a single +5 V power
supply.
Excellent DC Performance Over Temperature
The AD9243 provides no missing codes, and excellent tempera-
ture drift performance over the full operating temperature range.
Excellent AC Performance and Low Noise
The AD9243 provides nearly 13 ENOB performance and has an
input referred noise of 0.36 LSB rms.
The input of the AD9243 is highly flexible, allowing for easy
interfacing to imaging, communications, medical, and data-
acquisition systems. A truly differential input structure allows
for both single-ended and differential input interfaces of varying
input spans. The sample-and-hold amplifier (SHA) is equally
suited for both multiplexed systems that switch full-scale voltage
levels in successive channels as well as sampling single-channel
inputs at frequencies up to and beyond the Nyquist rate. Also,
the AD9243 performs well in communication systems employ-
ing Direct-IF Down Conversion since the SHA in the differen-
tial input mode can achieve excellent dynamic performance well
beyond its specified Nyquist frequency of 1.5 MHz.
Flexible Analog Input Range
The versatile onboard sample-and-hold (SHA) can be configured
for either single ended or differential inputs of varying input spans.
Flexible Digital Outputs
The digital outputs can be configured to interface with +3 V and
+5 V CMOS logic families.
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 (OTR) signal indicates an
overflow condition which can be used with the most significant
bit to determine low or high overflow.
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
© Analog Devices, Inc., 1998
AD9243–SPECIFICATIONS
(AVDD = +5 V, DVDD = +5 V, DRVDD = +5 V, fSAMPLE = 3 MSPS, VREF = 2.5 V, VINB = 2.5 V, TMIN to TMAX unless
DC SPECIFICATIONS otherwise noted)
Parameter
AD9243
Units
RESOLUTION
14
3
Bits min
MHz min
MAX CONVERSION RATE
INPUT REFERRED NOISE
V
REF = 1 V
0.9
0.36
LSB rms typ
LSB rms typ
VREF = 2.5 V
ACCURACY
Integral Nonlinearity (INL)
Differential Nonlinearity (DNL)
±2.5
±0.6
±1.0
±2.5
±0.7
14
0.3
1.5
0.75
LSB typ
LSB typ
LSB max
LSB typ
INL1
DNL1
LSB typ
No Missing Codes
Zero Error (@ +25°C)
Gain Error (@ +25°C)2
Gain Error (@ +25°C)3
Bits Guaranteed
% FSR max
% FSR max
% FSR max
TEMPERATURE DRIFT
Zero Error
3.0
20.0
5.0
ppm/°C typ
ppm/°C typ
ppm/°C typ
Gain Error2
Gain Error3
POWER SUPPLY REJECTION
0.1
% FSR max
ANALOG INPUT
Input Span (with VREF = 1.0 V)
Input Span (with VREF = 2.5 V)
Input (VINA or VINB) Range
2
5
0
V p-p min
V p-p max
V min
AVDD
16
V max
pF typ
Input Capacitance
INTERNAL VOLTAGE REFERENCE
Output Voltage (1 V Mode)
Output Voltage Tolerance (1 V Mode)
Output Voltage (2.5 V Mode)
Output Voltage Tolerance (2.5 V Mode)
Load Regulation4
1
Volts typ
mV max
Volts typ
mV max
mV max
±14
2.5
±35
2.0
REFERENCE INPUT RESISTANCE
5
kΩ typ
POWER SUPPLIES
Supply Voltages
AVDD
+5
+5
+5
V (±5% AVDD Operating)
V (±5% DVDD Operating)
V (±5% DRVDD Operating)
DVDD
DRVDD
Supply Current
IAVDD
IDRVDD
23.0
1.0
5.0
mA max (20 mA typ)
mA max (0.5 mA typ)
mA max (3.5 mA typ)
IDVDD
POWER CONSUMPTION
110
145
mW typ
mW max
NOTES
1VREF =1 V.
2Including internal reference.
3Excluding internal reference.
4Load regulation with 1 mA load current (in addition to that required by the AD9243).
Specification subject to change without notice.
–2–
REV. A
AD9243
(AVDD = +5 V, DVDD= +5 V, DRVDD = +5 V, fSAMPLE = 3 MSPS, VREF = 2.5 V, AIN = –0.5 dBFS, AC Coupled/
Differential Input, TMIN to TMAX unless otherwise noted)
AC SPECIFICATIONS
Parameter
AD9243
Units
SIGNAL-TO-NOISE AND DISTORTION RATIO (S/N+D)
fINPUT = 500 kHz
75.0
79.0
77.0
dB min
dB typ
dB typ
fINPUT = 1.5 MHz
EFFECTIVE NUMBER OF BITS (ENOB)
f
INPUT = 500 kHz
12.3
12.8
12.5
Bits min
Bits typ
Bits typ
fINPUT = 1.5 MHz
SIGNAL-TO-NOISE RATIO (SNR)
f
INPUT = 500 kHz
76.0
80.0
79.0
dB min
dB typ
dB typ
fINPUT = 1.5 MHz
TOTAL HARMONIC DISTORTION (THD)
f
INPUT = 500 kHz
–78.0
–87.0
–82.0
dB max
dB typ
dB typ
fINPUT = 1.5 MHz
SPURIOUS FREE DYNAMIC RANGE
f
INPUT = 500 kHz
91.0
84.0
dB typ
dB typ
fINPUT = 1.5 MHz
DYNAMIC PERFORMANCE
Full Power Bandwidth
Small Signal Bandwidth
Aperture Delay
40
40
1
MHz typ
MHz typ
ns typ
Aperture Jitter
Acquisition to Full-Scale Step (0.0025%)
Overvoltage Recovery Time
4
80
167
ps rms typ
ns typ
ns typ
Specifications subject to change without notice.
DIGITAL SPECIFICATIONS (AVDD = +5 V, DVDD= +5 V, TMIN to TMAX unless otherwise noted)
Parameters
Symbol
AD9243
Units
LOGIC INPUTS
High Level Input Voltage
Low Level Input Voltage
High Level Input Current (VIN = DVDD)
Low Level Input Current (VIN = 0 V)
Input Capacitance
VIH
VIL
IIH
IIL
CIN
+3.5
+1.0
±10
±10
5
V min
V max
µA max
µA max
pF typ
LOGIC OUTPUTS (with DRVDD = 5 V)
High Level Output Voltage (IOH = 50 µA)
High Level Output Voltage (IOH = 0.5 mA)
Low Level Output Voltage (IOL = 1.6 mA)
Low Level Output Voltage (IOL = 50 µA)
Output Capacitance
VOH
VOH
VOL
VOL
COUT
+4.5
+2.4
+0.4
+0.1
5
V min
V min
V max
V max
pF typ
LOGIC OUTPUTS (with DRVDD = 3 V)
High Level Output Voltage (IOH = 50 µA)
Low Level Output Voltage (IOL = 50 µA)
VOH
VOL
+2.4
+0.7
V min
V max
Specifications subject to change without notice.
–3–
REV. A
AD9243
(TMIN to TMAX with AVDD = +5 V, DVDD = +5 V, DRVDD = +5 V, CL = 20 pF)
SWITCHING SPECIFICATIONS
Parameters
Symbol
AD9243
Units
Clock Period1
CLOCK Pulsewidth High
CLOCK Pulsewidth Low
Output Delay
tC
333
150
150
8
ns min
ns min
ns min
ns min
tCH
tCL
tOD
13
19
3
ns typ
ns max
Clock Cycles
Pipeline Delay (Latency)
NOTES
1The clock period may be extended to 1 ms without degradation in specified performance @ +25°C.
Specifications subject to change without notice.
THERMAL CHARACTERISTICS
Thermal Resistance
44-Lead MQFP
S1
S2
ANALOG
INPUT
S4
tC
S3
tCH
tCL
θJA = 53.2°C/W
θJC = 19°C/W
INPUT
CLOCK
tOD
ORDERING GUIDE
DATA
DATA 1
OUTPUT
Temperature
Range
Package
Description
Package
Option*
Model
Figure 1. Timing Diagram
AD9243AS
AD9243EB
–40°C to +85°C
Evaluation Board
44-Lead MQFP
S-44
ABSOLUTE MAXIMUM RATINGS*
*S = Metric Quad Flatpack.
With
Respect
to
Parameter
Min Max
Units
PIN CONNECTIONS
AVDD
DVDD
AVSS
AVDD
DRVDD
DRVSS
REFCOM
CLK
Digital Outputs
VINA, VINB
VREF
AVSS
DVSS
DVSS
DVDD –6.5 +6.5
DRVSS –0.3 +6.5
AVSS
AVSS
DVSS
–0.3 +6.5
–0.3 +6.5
–0.3 +0.3
V
V
V
V
V
V
V
V
44 43 42 41 40 39 38 37 36 35 34
–0.3 +0.3
–0.3 +0.3
–0.3 DVDD + 0.3
DVSS
AVSS
1
2
3
33
32
REFCOM
PIN 1
IDENTIFIER
VREF
SENSE
NC
31
DVDD
AVDD
DRVSS
DRVDD
CLK
DRVSS –0.3 DRVDD + 0.3 V
4
5
30
29
AVSS
AVSS
AVSS
AVSS
–0.3 AVDD + 0.3
–0.3 AVDD + 0.3
–0.3 AVDD + 0.3
–0.3 AVDD + 0.3
+150
V
V
V
V
°C
°C
AVSS
AVDD
AD9243
TOP VIEW
(Not to Scale)
6
7
28
SENSE
27 NC
CAPB, CAPT
Junction Temperature
Storage Temperature
Lead Temperature
(10 sec)
NC
26
NC
8
9
NC
25 OTR
–65 +150
10
11
BIT 1 (MSB)
NC
24
(LSB) BIT 14
23 BIT 2
+300
°C
12 13 14 15 16 17 18 19 20 21 22
*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 effect device reliability.
NC = NO CONNECT
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 AD9243 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
AD9243
PIN DESCRIPTION
Description
overvoltage (50% greater than full-scale range), measured from
the time the overvoltage signal reenters the converter’s range.
Pin
Number
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
Name
1
DVSS
AVSS
DVDD
AVDD
DRVSS
Digital Ground
Analog Ground
+5 V Digital Supply
+5 V Analog Supply
2, 29
3
4, 28
5
6
7
TMIN or TMAX
.
POWER SUPPLY REJECTION
Digital Output Driver Ground
Digital Output Driver Supply
Clock Input Pin
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.
DRVDD
CLK
8–10
11
NC
BIT 14
No Connect
Least Significant Data Bit (LSB)
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.
12–23
24
25
BIT 13–BIT 2 Data Output Bits
BIT 1
OTR
Most Significant Data Bit (MSB)
Out of Range
No Connect
APERTURE DELAY
26, 27, 30 NC
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.
31
32
33
SENSE
VREF
REFCOM
Reference Select
Reference I/O
Reference Common
No Connect
34, 35, 38 NC
40, 43, 44
36
37
39
41
42
SIGNAL-TO-NOISE AND DISTORTION (S/N+D, SINAD)
RATIO
S/N+D is the ratio of the rms value of the measured input sig-
nal 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.
CAPB
Noise Reduction Pin
Noise Reduction Pin
Common-Mode Level (Midsupply)
Analog Input Pin (+)
Analog Input Pin (–)
CAPT
CML
VINA
VINB
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,
DEFINITIONS OF SPECIFICATION
INTEGRAL NONLINEARITY (INL)
N = (SINAD – 1.76)/6.02
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.
it is possible to get a measure of performance expressed as N,
the effective number of bits.
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.
TOTAL HARMONIC DISTORTION (THD)
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 14-bit resolution indicates that all 16384
codes, respectively, must be present over all operating ranges.
THD is the ratio of the rms sum of the first six harmonic
components 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.
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.
SPURIOUS FREE DYNAMIC RANGE (SFDR)
SFDR is the difference in dB between the rms amplitude of the
input signal and the peak spurious signal.
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 differ-
ence between first and last code transitions.
TWO-TONE SFDR
The ratio of the rms value of either input tone to the rms value
of the peak spurious component. The peak spurious component
may or may not be an IMD product. May be reported in dBc
(i.e., degrades as signal level is lowered), or in dBFS (always
related back to converter full scale).
OVERVOLTAGE RECOVERY TIME
Overvoltage recovery time is defined as that amount of time
required for the ADC to achieve a specified accuracy after an
–5–
REV. A
AD9243
(AVDD = +5 V, DVDD = +5 V, DRVDD = +5 V, fSAMPLE
Typical Differential AC Characterization Curves/Plots = 3.00 MSPS, TA = +25؇C, Differential Input)
1
0
90
85
80
75
70
65
60
55
50
14.7
13.8
13.0
12.2
11.3
10.5
9.7
–40
–50
–60
–70
–80
–90
–100
–15
–30
–0.5dBFS
–6.0dBFS
–45
–20.0dBFS
–60
–75
4
9
3
7
2
8
6
–90
–6.0dBFS
–0.5dBFS
5
–105
–120
–135
–150
–20.0dBFS
8.8
8.0
10M 20M
0
1.5
100k
1M
INPUT FREQUENCY – Hz
20M
10M
100k
1M
INPUT FREQUENCY – Hz
FREQUENCY – MHz
Figure 2. SINAD vs. Input Frequency
(Input Span = 5 V, VCM = 2.5 V)
Figure 3. THD vs. Input Frequency
(Input Span = 5 V, VCM = 2.5 V)
Figure 4. Typical FFT, fIN = 500 kHz
(Input Span = 5 V, VCM = 2.5 V)
1
0
90
85
80
75
70
65
60
55
50
14.7
13.8
13.0
12.2
11.3
10.5
9.7
–40
–50
–60
–15
–30
–45
–60
–75
–0.5dBFS
–6.0dBFS
–20.0dBFS
–70
9
4
–90
–105
–120
–135
–150
7
5
8
6
2
–6.0dBFS
–80
3
–0.5dBFS
–90
–20.0dBFS
8.8
–100
100k
8.0
10M 20M
0
1.5
1M
10M 20M
100k
1M
INPUT FREQUENCY – Hz
FREQUENCY – MHz
INPUT FREQUENCY – Hz
Figure 5. SINAD vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
Figure 6. THD vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
Figure 7. Typical FFT, fIN = 1.50 MHz
(Input Span = 2 V, VCM = 2.5 V)
110
–60
–65
–70
110
dBFS – 5V
105
5V SPAN - dBFS
100
100
dBFS – 2V
90
95
2V SPAN - dBFS
90
–75
80
5V SPAN
–80
85
70
60
50
40
dBc – 5V
80
–85
5V SPAN - dBc
2V SPAN
75
–90
–95
70
65
60
2V SPAN - dBc
dBc – 2V
–100
–60
–50 –40
–30
AIN – dBFS
–20
–10
0
–40 –35 –30 –25 –20 –15 –10 –5
INPUT POWER LEVEL (F = F ) – dBFS
0
0.1
1
5
SAMPLE RATE – MSPS
1
2
Figure 10. Dual Tone SFDR
(f1 = 0.95 MHz, f2 = 1.04 MHz,
Figure 8. THD vs. Sample Rate
(fIN = 1.5 MHz, AIN = –0.5 dBFS,
Figure 9. Single Tone SFDR
(fIN = 1.5 MHz, VCM = 2.5 V)
VCM = 2.5 V)
VCM = 2.5 V)
–6–
REV. A
AD9243
(AVDD = +5 V, DVDD = +5 V, DRVDD = +5 V, fSAMPLE = 3.00 MSPS, TA = +25؇C,
Single-Ended Input)
Other Characterization Curves/Plots
1.0
0.8
2.5
2.0
4,343,995
0.6
1.5
0.4
1.0
0.2
0.0
0.5
–0.2
–0.4
–0.6
–0.8
–1.0
0.0
–0.5
–1.0
–1.5
439,383
N–1
356,972
N+1
N
0
16383
0
16383
CODE
CODE
CODE
Figure 12. Typical DNL
(Input Span = 5 V)
Figure 13. “Grounded-Input”
Histogram (Input Span = 5 V)
Figure 11. Typical INL
(Input Span = 5 V)
–40
–50
–60
–70
–80
–90
–100
20
30
40
50
60
70
80
90
90
85
80
75
70
65
60
55
50
14.7
13.8
13.0
12.2
11.3
10.5
9.7
–20dBFS
–0.5dBFS
–6.0dBFS
–6.0dBFS
–0.5dBFS
8.8
–20.0dBFS
8.0
10M
0.1
1
10
100
100k
1M
10M
100k
1M
INPUT FREQUENCY – Hz
FREQUENCY – MHz
INPUT FREQUENCY – Hz
Figure 14. SINAD vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
Figure 15. THD vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
Figure 16. CMR vs. Input Frequency
(Input Span = 2 V, VCM = 2.5 V)
0.01
0.008
0.006
0.004
0.002
0
–40
–50
90
85
80
75
70
65
60
55
50
14.7
13.8
13.0
12.2
11.3
10.5
9.7
–0.5dBFS
–6.0dBFS
–60
–20dBFS
–70
–0.5dBFS
–0.002
–0.004
–0.006
–0.008
–0.01
–80
–6.0dBFS
–20.0dBFS
–90
8.8
–100
100k
8.0
10M
–60 –40 –20
0
20 40 60 80 100 120 140
1M
10M
100k
1M
INPUT FREQUENCY – Hz
TEMPERATURE – ؇C
INPUT FREQUENCY – Hz
Figure 19. Typical Voltage Reference
Error vs. Temperature
Figure 17. SINAD vs. Input Frequency
(Input Span = 5 V, VCM = 2.5 V)
Figure 18. THD vs. Input Frequency
(Input Span = 5 V, VCM = 2.5 V)
–7–
REV. A
AD9243
INTRODUCTION
Therefore, the equation,
VCORE = VINA – VINB
The AD9243 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 differ-
ence between 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 er-
rors. The last stage simply consists of a flash A/D.
(1)
defines the output of the differential input stage and provides the
input to the A/D core.
The voltage, VCORE, must satisfy the condition,
–VREF ≤ VCORE ≤ 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 AD9243. The
power supplies bound the valid operating range for VINA and
VINB. The condition,
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 can be con-
figured to interface with +5 V or +3.3 V logic families.
AVSS – 0.3 V < VINA < AVDD + 0.3 V
AVSS – 0.3 V < VINB < AVDD + 0.3 V
(3)
where AVSS is nominally 0 V and AVDD is nominally +5 V,
defines this requirement. Thus, the range of valid inputs for
VINA and VINB is any combination that satisfies both Equa-
tions 2 and 3.
The AD9243 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 the hold
mode. System 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.
For additional information showing the relationship between
VINA, VINB, VREF and the digital output of the AD9243, see
Table IV.
Refer to Table I and Table II for a summary of the various
analog input and reference configurations.
ANALOG INPUT OPERATION
Figure 21 shows the equivalent analog input of the AD9243
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 differen-
tial 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 20, a simplified model of the AD9243, highlights the rela-
tionship 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 minimum
input voltage to the A/D core is automatically defined to be –VREF.
AD9243
+V
C
H
VINA
REF
14
Q
V
S2
CORE
A/D
CORE
+
C
PIN
C
Q
S
S1
C
PAR
VINA
VINB
–V
VINB
REF
Q
C
Q
H1
S
S1
–
C
C
PIN
PAR
Q
Figure 20. AD9243 Equivalent Functional Input Circuit
S2
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 difference
of the voltages applied at the VINA and VINB input pins.
C
H
Figure 21. AD9243 Simplified Input Circuit
–8–
REV. A
AD9243
The input SHA of the AD9243 is optimized to meet the perfor-
mance requirements for some of the most demanding commu-
nication, imaging, and data acquisition applications while
maintaining low power dissipation. Figure 22 is a graph of the
full-power bandwidth of the AD9243, typically 40 MHz. Note
that the small signal bandwidth is the same as the full-power
bandwidth. The settling time response to a full-scale stepped
input is shown in Figure 23 and is typically 80 ns to 0.0025%.
The low input referred noise of 0.36 LSB’s rms is displayed via
a grounded histogram and is shown in Figure 13.
tion in THD performance as the input frequency increases.
Similarly, note how the THD performance at lower frequencies
becomes less sensitive to the common-mode voltage. As the
input frequency approaches dc, the distortion will be domi-
nated by static nonlinearities such as INL and DNL. It is
important to note that these dc static nonlinearities are inde-
pendent of any RON modulation.
–50
0
–60
V
= 1.0V
CM
–3
–70
–80
–90
–6
V
= 2.5V
CM
–9
0.1
1
FREQUENCY – MHz
10
Figure 24. AD9243 THD vs. Frequency for VCM = 2.5 V and
1.0 V (AIN = –0.5 dB, Input Span = 2.0 V p-p)
–12
1
10
100
FREQUENCY – MHz
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 RON modulation and its effects on
distortion.
Figure 22. Full-Power Bandwidth
16000
12000
8000
4000
0
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., 5 V input span) and matched
input impedance for VINA and VINB. Note that only a slight
degradation in dc linearity performance exists between the
2 V and 5 V input span as specified in the AD9243 “DC
SPECIFICATIONS.”
0
10
20
30
40
50
60
70
80
Referring to Figure 21, the differential SHA is implemented
using a switched-capacitor topology. Hence, its input imped-
ance and its subsequent effects on the input drive source should
be understood to maximize the converter’s performance. The
combination of the pin capacitance, CPIN, parasitic capacitance
CPAR, and the sampling capacitance, CS, is typically less than
16 pF. When the SHA goes into track mode, the input source
must charge or discharge the voltage stored on CS to the new
input voltage. This action of charging and discharging CS which
is approximately 4 pF, averaged over a period of time and for a
given sampling frequency, FS, makes the input impedance ap-
pear to have a benign resistive component (i.e., 83 kΩ at FS =
3.0 MSPS). However, if this action is analyzed within a sam-
pling period (i.e., T = <1/FS), the input impedance is dynamic
due to the instantaneous requirement of charging and discharg-
ing CS. A series resistor inserted between the input drive source
and the SHA input as shown in Figure 25 provides the effective
isolation.
SETTLING TIME – ns
Figure 23. Settling Time
The SHA’s optimum distortion performance for a differential or
single-ended input is achieved under the following two condi-
tions: (1) the common-mode voltage is centered around mid
supply (i.e., AVDD/2 or approximately 2.5 V) and (2) the input
signal voltage span of the SHA is set at its lowest (i.e., 2 V input
span). This is due to the sampling switches, QS1, being CMOS
switches whose RON resistance is very low but has some signal
dependency which causes frequency dependent ac distortion
while the SHA is in the track mode. The RON resistance of a
CMOS switch is typically lowest at its midsupply but increases
symmetrically as the input signal approaches either AVDD or
AVSS. A lower input signal voltage span centered at midsupply
reduces the degree of RON modulation.
Figure 24 compares the AD9243’s THD vs. frequency perfor-
mance for a 2 V input span with a common-mode voltage of
1 V and 2.5 V. Note the difference in the amount of degrada-
–9–
REV. A
AD9243
V
The noise or small-signal bandwidth of the AD9243 is the same
as its full-power bandwidth. For noise sensitive applications, the
excessive bandwidth may be detrimental and the addition of a
series resistor and/or shunt capacitor can help limit the wide-
band 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 AD9243 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 resis-
tance may be selected in combination with the SHA’s nominal
16 pF of input capacitance to set the filter’s 3 dB cutoff frequency.
CC
AD9243
R
*
S
VINA
R
*
S
VINB
V
EE
VREF
10F
0.1F
SENSE
REFCOM
*OPTIONAL SERIES RESISTOR
Figure 25. Series Resistor Isolates Switched-Capacitor
SHA Input from Op Amp. Matching Resistors Improve
SNR Performance
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 AD9243, 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, CH, further reduc-
ing current transients seen at the op amp’s output.
The optimum size of this resistor is dependent on several factors
which include the AD9243 sampling rate, the selected op amp,
and the particular application. In most applications, a 30 Ω to
50 Ω resistor is sufficient. However, some applications may re-
quire a larger resistor value to reduce the noise bandwidth or
possibly limit the fault current in an overvoltage condition.
Other applications may require a larger resistor value as part of
an anti-aliasing filter. In any case, since the THD performance
is dependent on the series resistance and the above mentioned
factors, optimizing this resistor value for a given application is
encouraged.
The effect of this increased capacitive load on the op amp driv-
ing the AD9243 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.
A slight improvement in SNR performance and dc offset perfor-
mance is achieved by matching the input resistance connected
to VINA and VINB. The degree of improvement is dependent on
the resistor value and the sampling rate. For series resistor
values greater than 100 Ω, the use of a matching resistor is
encouraged.
Table I. Analog Input Configuration Summary
Input
Connection
Single-Ended
Input
Coupling Span (V)
Input Range (V)
Figure
#
32, 33
VINA1
0 to 2
VINB1
Comments
DC
2
1
Best for stepped input response applications, suboptimum
THD and noise performance, requires ±5 V op amp.
2 × VREF
0 to
2 × VREF
VREF
32, 33
Same as above but with improved noise performance due to
increase in dynamic range. Headroom/settling time require-
ments of ±5 op amp should be evaluated.
5
0 to 5
2.5
2.5
32, 33
39
Optimum noise performance, excellent THD performance. Requires
op amp with VCC > +5 V due to insufficient headroom @ 5 V.
2 × VREF
2.5 – VREF
to
2.5 + VREF
Optimum THD performance with VREF = 1, noise performance
improves while THD performance degrades as VREF increases
to 2.5 V. Single supply operation (i.e., +5 V) for many op amps.
Single-Ended
Differential
AC
2 or
2 × VREF
0 to 1 or
0 to 2 × VREF
1 or VREF
34
Suboptimum ac performance due to input common-mode
level not biased at optimum midsupply level (i.e., 2.5 V).
5
0 to 5
2.5
2.5
34
35
Optimum noise performance, excellent THD performance.
2 × VREF
2.5 – VREF
to
2.5 + VREF
Flexible input range, Optimum THD performance with
VREF = 1. Noise performance improves while THD perfor-
mance degrades as VREF increases to 2.5 V.
AC or
DC
2
2 to 3
3 to 2
29–31
Optimum full-scale THD and SFDR performance well be-
yond the A/Ds Nyquist frequency.
2 × VREF
2.5 – VREF/2 2.5 + VREF/2 29–31
to to
2.5 + VREF/2 2.5 – VREF/2
Same as 2 V to 3 V input range with the exception that full-scale
THD and SFDR performance can be traded off for better noise
performance.
5
1.75 to 3.25 3.25 to 1.75
29–31
Widest dynamic range (i.e., ENOBs) due to Optimum Noise
performance.
NOTE
1VINA and VINB can be interchanged if signal inversion is required.
–10–
REV. A
AD9243
REFERENCE OPERATION
2.5 V. If the SENSE pin is tied to the VREF pin via a short or
resistor, the switch is connected to the SENSE pin. A short will
provide a VREF of 1.0 V while an external resistor network will
provide an alternative VREF between 1.0 V and 2.5 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.
The AD9243 contains an onboard bandgap reference that pro-
vides a pin-strappable option to generate either a 1 V or 2.5 V
output. With the addition of two external resistors, the user can
generate reference voltages other than 1 V and 2.5 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 AD9243 reference
configurations.
The actual reference voltages used by the internal circuitry of
the AD9243 appear 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 27 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.
Figure 26 shows a simplified model of the internal voltage
reference of the AD9243. 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,
Full-Scale Input Span = 2 × VREF
AD9243
TO
A/D
5k⍀
CAPT
5k⍀
A2
0.1F
5k⍀
CAPB
CAPT
5k⍀
AD9243
10F
0.1F
DISABLE
LOGIC
A2
0.1F
CAPB
VREF
A1
1V
7.5k⍀
Figure 27. 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 and
VREF to REFCOM and the capacitive decoupling network
removed. The external voltages applied to CAPT and CAPB
must be 2.5 V + Input Span/4 and 2.5 V – Input Span/4 respec-
tively in which the input span can be varied between 2 V and 5 V.
Note that those samples within the pipeline A/D during any
reference transition will be corrupted and should be discarded.
SENSE
DISABLE
A1
5k⍀
LOGIC
REFCOM
Figure 26. Equivalent Reference Circuit
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 REFCOM, the switch is con-
nected to the internal resistor network thus providing a VREF of
Table II. Reference Configuration Summary
Input Span (VINA–VINB)
Reference
Operating Mode
(V p-p)
Required VREF (V)
Connect
To
INTERNAL
INTERNAL
INTERNAL
2
5
1
2.5
SENSE
SENSE
R1
VREF
REFCOM
VREF AND SENSE
SENSE AND REFCOM
2 ≤ SPAN ≤ 5 AND
SPAN = 2 × VREF
1 ≤ VREF ≤ 2.5 AND
VREF = (1 + R1/R2)
R2
EXTERNAL
(NONDYNAMIC)
2 ≤ SPAN ≤ 5
2 ≤ SPAN ≤ 5
1 ≤ VREF ≤ 2.5
SENSE
VREF
AVDD
EXT. REF.
EXTERNAL
(DYNAMIC)
CAPT and CAPB
Externally Driven
SENSE
VREF
EXT. REF.
EXT. REF.
AVDD
REFCOM
CAPT
CAPB
–11–
REV. A
AD9243
DRIVING THE ANALOG INPUTS
INTRODUCTION
AVDD
D2
V
CC
R
R
S1
S2
The AD9243 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.
1N4148
30⍀
20⍀
AD9243
D1
1N4148
V
EE
Figure 28. Simple Clamping Circuit
DIFFERENTIAL MODE OF OPERATION
Since not all applications have a signal preconditioned for
differential operation, there is often a need to perform a single-
ended-to-differential conversion. A single-ended-to-differential
conversion can be realized with an RF transformer or a dual op
amp differential driver. The optimum method depends on
whether the application requires the input signal to be ac or dc
coupled to AD9243.
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 may be
appropriate for many 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 AD9243.
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.
AC Coupling via an RF Transformer
In applications that do not need to be dc coupled, an RF trans-
former with a center tap is the best method to generate differen-
tial inputs for the AD9243. It provides all the benefits of
operating the A/D in the differential mode without contributing
additional noise or distortion. An RF transformer has the added
benefit of providing electrical isolation between the signal source
and the A/D.
Alternatively, the differential mode of operation provides the
best THD and SFDR performance over a wide frequency range.
A transformer coupled differential input should be considered
for the most demanding spectral-based applications which allow
ac coupling (e.g., Direct IF to Digital Conversion). The dc-
coupled differential mode of operation also provides an enhance-
ment in distortion and noise performance at higher input spans.
Furthermore, it allows the AD9243 to be configured for a 5 V
span using op amps specified for +5 V or ±5 V operation.
Figure 29 shows the schematic of the suggested transformer
circuit. The circuit uses a Mini-Circuits RF transformer, model
#T4-6T, 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 centertap of the transformer provides a convenient means
of level shifting the input signal to a desired common-mode
voltage. Optimum performance can be realized when the centertap
is tied to CML of the AD9243 which is the common-mode bias
level of the internal SHA.
Single-ended operation requires that VINA be ac or dc coupled
to the input signal source while VINB of the AD9243 be biased
to the appropriate voltage corresponding to a midscale code
transition. Note that signal inversion may be easily accom-
plished by transposing VINA and VINB.
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
AD9243 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 as shown in Figure 16.
VINA
50⍀
CML
0.1F
200⍀
AD9243
VINB
MINI-CIRCUITS
T4-6T
Figure 29. Transformer Coupled Input
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. Hence, selecting a transformer with a higher
impedance ratio (i.e., Mini-Circuits T16-6T with a 1:16 imped-
ance ratio) effectively “steps up” the signal level, further reduc-
ing the driving requirements of the signal source.
As is typical of most CMOS devices, exceeding the supply limits
will turn on internal parasitic diodes resulting in transient cur-
rents within the device. Figure 28 shows a simple means of
clamping a dc coupled input with the addition of two series
resistors and two diodes. 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.
–12–
REV. A
AD9243
390⍀
DC Coupling with Op Amps
AV
Applications that require dc coupling can also benefit by driving
the AD9243 differentially. Since the signal swing requirements
of each input is reduced by a factor of two in the differential
mode, the AD9243 can be configured for a 5 V input span in a
+5 V or ±5 V system. This allows various high performance op
amps specified for +5 V and ±5 V operation to be configured in
various differential driver topologies. The optimum op amp
driver topology depends on whether the common-mode voltage
of the single-ended-input signal requires level-shifting.
DD
V
–VIN
AD8047
CML
390⍀
390⍀
390⍀
33⍀
VINA
390⍀
V
IN
AV
DD
390⍀
AD9243
VINB
390⍀
390⍀
390⍀
V
+VIN
CML
AD8047
33⍀
Figure 30 shows a cross-coupled differential driver circuit best
suited for systems in which the common-mode signal of the
input is already biased to approximately midsupply (i.e., 2.5 V).
The common-mode voltage of the differential output is set by
the voltage applied to the “+” input of A2. The closed loop
gain of this symmetrical driver can be easily set by RIN and RF.
For more insight into the operation of this cross-coupled driver,
please refer to the AD8042 data sheet.
2.5k⍀
390⍀
0.1F
100⍀
1F
CML
0.1F
OP113
Figure 31. Differential Driver with Level-Shifting
R
F
1k⍀
SINGLE-ENDED MODE OF OPERATION
The AD9243 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 de-
grade 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.
R
IN
V
+VIN
CML
1k⍀
AD8042
V
33⍀
IN
VINA
1k⍀
1k⍀
1k⍀
C *
F
AD9243
1k⍀
V
–VIN
CML
33⍀
VINB
CML
AV /2
AD8042
DD
DC COUPLING AND INTERFACE ISSUES
Many applications require the analog input signal to be dc
coupled to the AD9243. An operational amplifier can be con-
figured 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.
0.1F
*OPTIONAL NOISE/BAND LIMITING CAPACITOR
Figure 30. Cross-Coupled Differential Driver
The driver circuit shown in Figure 31 is best suited for systems
in which the bipolar input signal is referenced to AGND and
requires proper level shifting. This driver circuit provides the
ability to level-shift the input signal to within the common-
mode range of the AD9243. The two op amps are configured as
matched difference amplifiers with the input signal applied to
opposing inputs to provide the differential output. The common-
mode offset voltage is applied to the noninverting resistor net-
work which provides the proper level shifting. The circuit also
employs optional diodes and pull-up resistors which may help
improve the op amps’ distortion performance by reducing their
headroom requirements. Rail-to-rail output amplifiers like the
AD8042 have sufficient headroom and thus do not require
these optional components.
Many of the new high performance op amps are specified for
only ±5 V operation and have limited input/output swing capa-
bilities. Hence, the selected input range of the AD9243 should
be sensitive to 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 inher-
ently limit its output swing to within the power supply rails.
Rail-to-rail output amplifiers such as the AD8041 allow the
AD9243 to be configured with larger input spans which im-
proves the noise performance.
–13–
REV. A
AD9243
If the application requires the largest single-ended input range
(i.e., 0 V to 5 V) of the AD9243, the op amp will require larger
supplies to drive it. Various high speed amplifiers in the “Op
Amp Selection Guide” of this data sheet can be selected to
accommodate a wide range of supply options. Once again,
clamping the output of the amplifier should be considered for
these applications. Alternatively, a single-ended to differential
op amp driver circuit using the AD8042 could be used to
achieve the 5 V input span while operating from a single +5 V
supply.
500⍀*
0.1F
NC
+V
CC
+VREF
–VREF
500⍀*
7
0V
DC
2
3
1
6
5
R
R
S
VINA
A1
R **
500⍀*
0.1F
P
AVDD
4
NC
AD9243
500⍀*
S
VINB
VREF
Two dc coupled op amp circuits using a noninverting and inverting
topology are discussed below. 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, respectively. An additional R-C network can be
inserted between the op amp’s output and the AD9243 input to
provide a real pole.
*OPTIONAL RESISTOR NETWORK-OHMTEK ORNA500D
**OPTIONAL PULL-UP RESISTOR WHEN USING INTERNAL REFERENCE
NC = NO CONNECT
Figure 33. Single-Ended Input With DC-Coupled Level Shift
AC COUPLING AND INTERFACE ISSUES
For applications where ac coupling is appropriate, the op amp’s
output can be easily level shifted to the common-mode voltage,
VCM, of the AD9243 via a coupling capacitor. This has the
advantage of allowing the op amps common-mode level to be
symmetrically biased to its midsupply level (i.e., (VCC + VEE)/2).
Op amps which operate symmetrically with respect to their
power supplies typically provide the best ac performance as well
as greatest input/output span. Hence, 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 5 V or 2 V input span of the AD9243, respec-
tively. The best ac distortion performance is achieved when the
A/D is configured for a 2 V input span and common-mode
voltage of 2.5 V. Note that differential transformer coupling,
which is another form of ac coupling, should be considered for
optimum ac performance.
Simple Op Amp Buffer
In the simplest case, the input signal to the AD9243 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 input pins of the A/D.
Figure 32 shows the recommended configuration for a single-
ended drive using an op amp. In this case, the op amp is shown
in a noninverting unity gain configuration driving the VINA pin.
The internal reference drives the VINB pin. Note that the
addition 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” for a discussion on resistor
selection. Figure 32 shows the proper connection for a 0 V to 5 V
input range. Alternative single ended input 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”).
Simple AC Interface
Figure 34 shows a typical example of an ac-coupled, single-
ended configuration. The bias voltage shifts the bipolar, ground-
referenced input signal to approximately VREF. The value for
C1 and C2 will depend on the size of the resistor, R. The ca-
pacitors, 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 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,
+V
5V
0V
AD9243
VINA
R
S
U1
–V
R
S
VINB
2.5V
10F
VREF
0.1F
SENSE
Figure 32. Single-Ended AD9243 Op Amp Drive Circuit
f
–3 dB = 1/(2 × π × R × (C1 + C2))
Op Amp with DC Level Shifting
Figure 33 shows a dc-coupled level shifting circuit employing an
op amp, A1, to sum the input signal with the desired dc offset.
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 AD9243. For
example, when VREF = 2.5 V, the output level from the op amp
will also be centered around 2.5 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 ±1 mA.
The low impedance VREF voltage source biases both the VINB
input and provides the bias voltage for the VINA input. Figure
34 shows the VREF configured for 2.5 V. Thus the input range
C1
+5V
AD9243
+VREF
0V
–VREF
C2
R
R
S
V
IN
VINA
R
S
–5V
VINB
VREF
SENSE
C2
C1
Figure 34. AC-Coupled Input
–14–
REV. A
AD9243
of the A/D is 0 V to 5 V. Other input ranges could be selected
by changing VREF but the A/D’s distortion performance will
degrade slightly as the input common-mode voltage deviates
from its optimum level of 2.5 V.
AD812:
Dual, 145 MHz Unity GBW, Single-Supply Cur-
rent Feedback, +5 V to ±15 V Supplies
Best Applications: Differential and/or Low Imped-
ance Input Drivers
Limits: THD above 1 MHz
Alternative AC Interface
Figure 35 shows a flexible ac coupled circuit which can be con-
figured for different input spans. Since the common-mode
voltage of VINA and VINB are biased to midsupply indepen-
dent of VREF, VREF can be pin-strapped or reconfigured to
achieve input spans between 2 V and 5 V p-p. The AD9243’s
CMRR along with the symmetrical coupling R-C networks will
reject both power supply variations and noise. The resistors, R,
establish 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, C1and 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 impedance over
a wide frequency range. RS isolates the buffer amplifier from the
A/D input. The optimum performance is achieved when VINA
and VINB are driven via symmetrical networks. The high pass
f–3 dB point can be approximated by the equation,
AD8011: f–3 dB = 300 MHz, +5 V or ±5 V Supplies, Current
Feedback
Best Applications: Single-Supply, AC/DC-Coupled,
Good AC Specs, Low Noise, Low Power (5 mW)
Limits: THD above 5 MHz, Usable Input/Output
Range
AD8013: Triple, f–3 dB = 230 MHz, +5 V or ±5 V supplies,
Current Feedback, Disable Function
Best Applications: 3:1 Multiplexer, Good AC Specs
Limits: THD above 5 MHz, Input Range
AD9631: 220 MHz Unity GBW, 16 ns Settling to 0.01%,
±5 V Supplies
Best Applications: Best AC Specs, Low Noise,
AC-Coupled
Limits: Usable Input/Output Range, Power
Consumption
f
–3 dB = 1/(2 × π × R/2 × (C1 + C2))
AD8047: 130 MHz Unity GBW, 30 ns Settling to 0.01%,
±5 V Supplies
+5V
+5V
Best Applications: Good AC Specs, Low Noise,
AC-Coupled
Limits: THD > 5 MHz, Usable Input Range
AD9243
R
R
C1
C2
R
V
S
IN
VINA
AD8041: Rail-to-Rail, 160 MHz Unity GBW, 55 ns Settling
to 0.01%, +5 V Supply, 26 mW
–5V
R
R
S
+5V
VINB
Best Applications: Low Power, Single-Supply Sys-
tems, DC-Coupled, Large Input Range
C1
C2
R
Limits: Noise with 2 V Input Range
AD8042: Dual AD8041
Figure 35. AC-Coupled Input-Flexible Input Span,
CM = 2.5 V
Best Applications: Differential and/or Low Imped-
ance Input Drivers
V
Limits: Noise with 2 V Input Range
OP AMP SELECTION GUIDE
Op amp selection for the AD9243 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 AD9243’s high performance capabilities coupled
with other external system level requirements such as power
consumption and cost.
REFERENCE CONFIGURATIONS
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, Introduction” for a discussion of
this topic. Also, the figures do not show the decoupling network asso-
ciated with the CAPT and CAPB pins. Please refer to the section “Ref-
erence Operation” for a discussion of the internal reference circuitry
and the recommended decoupling network shown in Figure 27.
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 performance
op amps typically have input and output range limitations 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 available 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 AD9243 are also included.
USING THE INTERNAL REFERENCE
Single-Ended Input with 0 to 2
؋
VREF Range Figure 36 shows how to connect the AD9243 for a 0 V to 2 V or
0 V to 5 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 38 and
connecting VREF to VINB.
In either 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. When VINA is ≤ 0 V, the digital output
will be 0000 Hex; when VINA is ≥ 2 × VREF, the digital output
will be 3FFF Hex.
–15–
REV. A
AD9243
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 2.5
and the resultant VREF output is 2.5 V. Thus, the valid input
range becomes 0 V to 5 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.
Resistor Programmable Reference
Figure 38 shows an example of how to generate a reference
voltage other than 1 V or 2.5 V with the addition of two exter-
nal resistors and a bypass capacitor. Use the equation,
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 stabil-
ity, place a 0.1 µF ceramic capacitor in parallel with R1.
2xVREF
VINA
0V
VINB
The common-mode voltage can be set to VREF by connecting
VINB to VREF to provide an input span of 0 to 2 × VREF.
Alternatively, the common-mode 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 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 induc-
tance 0.1 µF ceramic capacitor.
10F
0.1F
VREF
AD9243
SHORT FOR 0V TO 2V
INPUT SPAN
SENSE
SHORT FOR 0V TO 5V
INPUT SPAN
REFCOM
Figure 36. Internal Reference (2 V p-p Input Span,
CM = 1 V, or 5 V p-p Input Span, VCM = 2.5 V)
V
4V
VINA
Single-Ended or Differential Input, VCM = 2.5 V
1V
Figure 37 shows the single-ended configuration that gives the
best SINAD performance. To optimize dynamic specifications,
center the common-mode voltage of the analog input at
approximately by 2.5 V by connecting VINB to VREF, a low-
impedance 2.5 V source. As described above, shorting the
SENSE pin directly to the REFCOM pin results in a 2.5 V
reference voltage and a 5 V p-p input span. The valid range
for input signals is 0 V to 5 V. The VREF pin should be by-
passed to the REFCOM pin with a 10 µF tantalum capacitor in
parallel with a low inductance 0.1 µF ceramic capacitor.
VINB
2.5V
1.5V
C1
VREF
R1
2.5k⍀
0.1F
10F
AD9243
0.1F
SENSE
R2
5k⍀
REFCOM
Figure 38. Resistor Programmable Reference (3 V p-p
Input Span, VCM = 2.5 V)
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 29. In this case, the common-mode voltage,
VCM, is set at midsupply by connecting the transformers center
tap to CML of the AD9243. VREF can be configured for 1 V
or 2.5 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 VCM – VREF/2 to VCM + VREF/2.
USING AN EXTERNAL REFERENCE
Using an external reference may enhance the dc performance of
the AD9243 by improving drift and accuracy. Figures 39
through 41 show examples of how to use an external reference
with the A/D. Table III is a list of suitable voltage references
from Analog Devices. To use an external reference, the user
must disable the internal reference amplifier and drive the VREF
pin. Connecting the SENSE pin to AVDD disables the inter-
nal reference amplifier.
5V
VINA
0V
Table III. Suitable Voltage References
VINB
AD9243
2.5V
VREF
Initial
Operating
Current
(A)
0.1F
10F
SENSE
Output
Voltage
Drift
Accuracy
% (max)
(ppm/؇C)
REFCOM
Internal
AD589
AD1580 1.225
REF191 2.048
Internal
REF192 2.50
REF43
1.00
1.235
26
1.4
N/A
50
50
45
N/A
45
10–100
50–100
5–25
26
5–25
10–25
3–7
1.2–2.8
0.08–0.8
0.1–0.5
1.4
0.08–0.4
0.06–0.1
0.04–0.2
Figure 37. Internal Reference—5 V p-p Input Span,
CM = 2.5 V
V
2.50
2.50
2.50
600
1000
AD780
–16–
REV. A
AD9243
The AD9243 contains an internal reference buffer, A2 (see
Figure 26), that simplifies the drive requirements of an external
reference. The external reference must be able to drive a ≈5 kΩ
(±20%) load. Note that the bandwidth of the reference buffer is
deliberately left small to minimize the reference noise contribu-
tion. As a result, it is not possible to change the reference volt-
age rapidly in this mode without the removal of the CAPT/
CAPB Decoupling Network, and driving these pins directly.
Low Cost/Power Reference
The external reference circuit shown in Figure 41 uses a low
cost 1.225 V external reference (e.g., AD580 or AD1580) along
with an op amp and transistor. The 2N2222 transistor acts in
conjunction with 1/2 of an OP282 to provide a very low imped-
ance drive for VINB. The selected op amp need not be a high
speed op amp and may be selected based on cost, power, and
accuracy.
Variable Input Span with VCM = 2.5 V
3.75V
Figure 39 shows an example of the AD9243 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 the reference
output to ground.
VINA
+5V
1.25V
820⍀
1k⍀
0.1F
VINB
1k⍀
10F
2N2222
0.1F
AD9243
1k⍀
316⍀
1/2
OP282
7.5k⍀
1.225V
+5V
+5V
VREF
10F
0.1F
AD1580
SENSE
2.5V+VREF
VINA
2.5V
Figure 41. External Reference Using the AD1580 and Low
Impedance Buffer
2.5V–VREF
2.5V
REF
VINB
+5V
0.1F
0.1F
22F
R1
R2
DIGITAL INPUTS AND OUTPUTS
Digital Outputs
AD9243
A1
VREF
The AD9243 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.
0.1F
SENSE
+5V
Figure 39. External Reference, VCM = 2.5 V (2.5 V on VINB,
Resistor Divider to Make VREF)
Table IV. Output Data Format
Single-Ended Input with 0 to 2
؋
VREF Range Input (V)
Condition (V)
Digital Output
OTR
Figure 40 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 external reference was selected, the valid
input range extends from 0 V to 4.096 V. In this case, 1 LSB of
the AD9243 corresponds to 0.250 mV. It is essential that a
minimum of a 10 µF capacitor in parallel with a 0.1 µF low induc-
tance ceramic capacitor decouple the reference output to ground.
VINA –VINB < – VREF
VINA –VINB = – VREF
VINA –VINB = 0
00 0000 0000 0000 1
00 0000 0000 0000 0
10 0000 0000 0000 0
VINA –VINB = + VREF – 1 LSB 11 1111 1111 1111 0
VINA –VINB ≥ + VREF
11 1111 1111 1111 1
+FS –1 1/2 LSB
OTR DATA OUTPUTS
OTR
1
0
0
111111 1111 1111
111111 1111 1111
111111 1111 1110
–FS +1/2 LSB
2xREF
VINA
0
0
1
000000 0000 0001
000000 0000 0000
000000 0000 0000
0V
VREF
+5V
VINB
–FS
+FS
10F
0.1F
0.1F
0.1F
AD9243
–FS –1/2 LSB
+FS –1/2 LSB
VREF
Figure 42. Output Data Format
Out Of Range (OTR)
SENSE
+5V
An out-of-range condition exists when the analog input voltage
is beyond the input range of the converter. OTR is a digital
output 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 42. OTR will remain HIGH
Figure 40. Input Range = 0 V to 2 × VREF
–17–
REV. A
AD9243
until the analog input returns within the input range and an-
other conversion 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 43 which uses NAND gates. Sys-
tems requiring programmable gain conditioning of the AD9243
input signal can immediately detect an out-of-range condition,
thus eliminating gain selection iterations. Also, OTR can be
used for digital offset and gain calibration.
For clock rates below 3 MSPS, the duty cycle may deviate from
this range to the extent that both tCH and tCL are satisfied.
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 (fIN) due to only aperture jitter (tA) can be
calculated with the following equation:
SNR = 20 log10 [1/(2 π fIN tA)]
In the equation, the rms aperture jitter, tA, 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.
For example, if a 1.5 MHz full-scale sine wave is sampled by an
A/D with a total rms jitter of 15 ps, the SNR performance of the
A/D will be limited to 77 dB. Undersampling applications are
particularly sensitive to jitter.
Table V. Out-of-Range Truth Table
OTR
MSB
Analog Input Is
0
0
1
1
0
1
0
1
In Range
In Range
Underrange
Overrange
The clock input should be treated as an analog signal in cases
where aperture jitter may affect the dynamic range of the
AD9243. As such, 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.
MSB
OTR
MSB
OVER = “1”
UNDER = “1”
Figure 43. Overrange or Underrange Logic
Most of the power dissipated by the AD9243 is from the analog
power supply. However, lower clock speeds will reduce digital
current slightly. Figure 44 shows the relationship between power
and clock rate.
Digital Output Driver Considerations (DRVDD)
The AD9243 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 AD9243 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 requir-
ing the AD9243 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.
125
120
115
110
5V p-p
Clock Input and Considerations
105
The AD9243 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 (tCH and tCL) 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 AD9243 operating at 3 MSPS
may have a duty cycle between 45% to 55% to meet this timing
requirement since the minimum specified tCH and tCL is 150 ns.
2V p-p
100
95
90
0
1
2
3
4
5
6
CLOCK FREQUENCY – MHz
Figure 44. AD9243 Power Consumption vs. Clock
Frequency
–18–
REV. A
AD9243
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:
adequately low impedance over a wide frequency range. Note
that the AVDD and AVSS pins are co-located on the AD9243
to simplify the layout of the decoupling capacitors and provide
the shortest possible PCB trace lengths. The AD9243/EB power
plane layout, shown in Figure 55 depicts a typical arrangement
using a multilayer PCB.
1. The minimization of the loop area encompassed by a signal
and its return path.
AVDD
2. The minimization of the impedance associated with ground
and power paths.
0.1F
AVSS
AD9243
3. The inherent distributed capacitor formed by the power
plane, PCB insulation, and ground plane.
AVDD
0.1F
These characteristics result in both a reduction of electro-
magnetic interference (EMI) and an overall improvement in
performance.
AVSS
It is important to design a layout that prevents noise from coupling
onto the input signal. Digital signals should not be run in paral-
lel with input signal traces and should be routed away from the
input circuitry. While the AD9243 features separate analog and
digital ground pins, it should be treated as an analog compo-
nent. The AVSS, DVSS and DRVSS pins must be joined together
directly under the AD9243. A solid ground plane under the A/D is
acceptable if the power and ground return currents are managed
carefully. 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 AD9243/EB ground layout, shown in
Figure 54, depicts the serrated type of arrangement. The analog
and digital grounds are connected by a jumper below the A/D.
Figure 46. Analog Supply Decoupling
The CML is an internal analog bias point used internally by the
AD9243. This pin must be decoupled with at least a 0.1 µF
capacitor as shown in Figure 47. 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.
CML
0.1F
AD9243
Figure 47. CML Decoupling
The digital activity on the AD9243 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 that
the internal correction logic of the AD9243 is referenced DVDD
while the output drivers are referenced to DRVDD.
Analog and Digital Supply Decoupling
The AD9243 features separate analog and digital supply and
ground pins, helping to minimize digital corruption of sensitive
analog signals.
120
DVDD
100
The decoupling shown in Figure 48, a 0.1 µF ceramic chip
capacitor, is 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.
80
60
AVDD
DVDD
DVSS
DRVDD
0.1F
0.1F
40
AD9243
1
10
100
1000
DRVSS
FREQUENCY – kHz
Figure 45. AD9243 PSSR vs. Frequency
Figure 48. Digital Supply Decoupling
Figure 45 shows the power supply rejection ratio vs. frequency
for a 200 mV p-p ripple applied to both AVDD and DVDD.
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 AD9243/EB schematic
and layouts in Figures 51–55 for more information regarding the
placement of decoupling capacitors.
In general, AVDD, the analog supply, should be decoupled to
AVSS, the analog common, as close to the chip as physically
possible. Figure 46 shows the recommended decoupling for the
analog supplies; 0.1 µF ceramic chip capacitors should provide
–19–
REV. A
AD9243
APPLICATIONS
0
–15
1
DIRECT IF DOWN CONVERSION USING THE AD9243
As previously noted, the AD9243’s performance in the differen-
tial mode of operation extends well beyond its baseband region
and into several Nyquist zone regions. Hence, the AD9243 may
be well suited as a mix down converter in both narrow and
wideband applications. Various IF frequencies exist over the
frequency range in which the AD9243 maintains excellent dy-
namic performance (e.g., refer to Figure 5 and 6). The IF sig-
nal will be aliased to the ADC’s baseband region due to the
sampling process in a similar manner that a mixer will down
convert an IF signal. For signals in various Nyquist zones, the
following equation may be used to determine the final frequency
after aliasing.
–30
–45
–60
–75
2
–90
9
4
7
–105
–120
–135
–150
0
1.25
FREQUENCY – MHz
f1 NYQUIST = fSIGNAL
f2 NYQUIST = fSAMPLE – fSIGNAL
f3 NYQUIST = abs (fSAMPLE – fSIGNAL
f4 NYQUIST = 2 × fSAMPLE – fSIGNAL
Figure 49. IF Sampling a 10.7 MHz Input Using the
AD9243 (VCM = 2.5 V, Input Span = 2 V p-p)
)
f
5 NYQUIST = abs (2 × fSAMPLE – fSIGNAL)
110
There are several potential benefits in using the ADC to alias
(i.e., mix) down a narrowband or wideband IF signal. First and
foremost is the elimination of a complete mixer stage with its
associated amplifiers and filters, reducing cost and power dissi-
pation. Second is the ability to apply various DSP techniques to
perform such functions as filtering, channel selection, quadra-
ture demodulation, data reduction, and detection.
100
SFDR – dBFS
90
80
70
SFDR – dBc
One common example is the digitization of a 10.7 MHz IF using a
low jitter 2.5 MHz sample clock. Using the equation above for
the fifth Nyquist zone, the resultant frequency after sampling is
700 kHz. Figure 49 shows the typical performance of the
AD9243 operating under these conditions. Figure 50 demon-
strates how the AD9243 is still able to maintain a high degree of
linearity and SFDR over a wide amplitude.
60
50
40
–60
–50
–40
–30
AIN – dBFS
–20
–10
0
Figure 50. AD9243 Differential Input SNR/SFDR vs.
Input Amplitude (AIN) @ 10.7 MHz
–20–
REV. A
AD9243
A V S S 1
D V D D
A V S S 2
A V D D 1
A V D D 2
D R V D D
D V S S
D R V S S
S J 5
S J 4
S J 3
S J 2
S J 1
J G 1
Figure 51. Evaluation Board Schematic
–21–
REV. A
AD9243
Figure 52. Evaluation Board Component Side Layout (Not to Scale)
Figure 53. Evaluation Board Solder Side Layout (Not to Scale)
–22–
REV. A
AD9243
Figure 54. Evaluation Board Ground Plane Layout (Not to Scale)
Figure 55. Evaluation Board Power Plane Layout (Not to Scale)
–23–
REV. A
AD9243
OUTLINE DIMENSIONS
Dimensions shown in mm and (inches).
44-Lead Metric Quad Flatpack (MQFP)
(S-44)
13.45 (0.530)
12.95 (0.510)
2.45 (0.096)
10.1 (0.398)
MAX
9.90 (0.390)
1.03 (0.041)
0.73 (0.029)
0؇
MIN
44
34
1
33
SEATING
PLANE
PIN 1
IDENTIFIER
8.45 (0.333)
8.3 (0.327)
TOP VIEW
(PINS DOWN)
11
23
0.25 (0.01)
MIN
12
22
0.23 (0.009)
0.13 (0.005)
0.45 (0.018)
0.3 (0.012)
0.8 (0.031)
BSC
2.1 (0.083)
1.95 (0.077)
–24–
REV. A
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