AD7356_08 [ADI]
Differential Input, Dual, Simultaneous Sampling, 5 MSPS, 12-Bit, SAR ADC; 差分输入，双通道，同步采样， 5 MSPS ， 12位SAR ADC
| 型号： | AD7356_08 |
| 厂家： | 
ADI |
| 描述： | Differential Input, Dual, Simultaneous Sampling, 5 MSPS, 12-Bit, SAR ADC |
| 文件： | 总20页 (文件大小：525K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Differential Input, Dual, Simultaneous
Sampling, 5 MSPS, 12-Bit, SAR ADC
AD7356
FUNCTIONAL BLOCK DIAGRAM
FEATURES
V
V
DRIVE
DD
Dual 12-bit SAR ADC
Simultaneous sampling
AD7356
Throughput rate: 5 MSPS per channel
Specified for VDD at 2.5 V
No conversion latency
V
V
INA+
12-BIT
SUCCESSIVE
APPROXIMATION
ADC
T/H
SDATA
INA–
A
REF
A
Power dissipation: 36 mW at 5 MSPS
On-chip reference: 2.048 V 0.25%, 6 ppm/°C
Dual conversion with read
High speed serial interface: SPI-/QSPI™-/MICROWIRE™-/DSP-
compatible
BUF
SCLK
CS
CONTROL
LOGIC
REF
BUF
−40°C to +125°C operation
Available in a 16-lead TSSOP
REF
V
B
12-BIT
SUCCESSIVE
APPROXIMATION
ADC
SDATA
B
INB+
INB–
T/H
V
AGND
AGND
REFGND
DGND
Figure 1.
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD73561 is a dual, 12-bit, high speed, low power, successive
approximation ADC that operates from a single 2.5 V power
supply and features throughput rates up to 5 MSPS. The part
contains two ADCs, each preceded by a low noise, wide band-
width track-and-hold circuit that can handle input frequencies
in excess of 110 MHz.
1. Two Complete ADC Functions.
These functions allow simultaneous sampling and
conversion of two channels. The conversion result of both
channels is simultaneously available on separate data lines
or in succession on one data line if only one serial port is
available.
2. High Throughput with Low Power Consumption.
The AD7356 offers a 5 MSPS throughput rate with 36 mW
power consumption.
The conversion process and data acquisition use standard
control inputs allowing for easy interfacing to microprocessors
CS
or DSPs. The input signal is sampled on the falling edge of
;
3. No Conversion Latency.
a conversion is also initiated at this point. The conversion time
is determined by the SCLK frequency.
The AD7356 features two standard successive approx-
imation ADCs with accurate control of the sampling
The AD7356 uses advanced design techniques to achieve very
low power dissipation at high throughput rates. With a 2.5 V
supply and a 5 MSPS throughput rate, the part consumes typically
14 mA. The part also offers a flexible power/throughput rate
management option.
CS
instant via a
input and, once off, conversion control.
Table 1. Related Devices
Generic
AD7352
AD7266
AD7866
AD7366
AD7367
Resolution Throughput Analog Input
12-bit
12-bit
12-bit
12-bit
14-bit
3 MSPS
2 MSPS
1 MSPS
1 MSPS
1 MSPS
Differential
Differential/single ended
Single-ended
Single-ended bipolar
Single-ended bipolar
The analog input range for the part is the differential common
mode
VREF/2. The AD7356 has an on-chip 2.048 V reference
that can be overdriven when an external reference is preferred.
The AD7356 is available in a 16-lead thin shrink small outline
package (TSSOP).
1 Protected by U.S. Patent No. 6,681,332.
Rev. 0
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 that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registeredtrademarks arethe property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Fax: 781.461.3113
www.analog.com
©2008 Analog Devices, Inc. All rights reserved.
AD7356
TABLE OF CONTENTS
Features .............................................................................................. 1
Analog Inputs ............................................................................. 13
Driving Differential Inputs ....................................................... 14
ADC Transfer Function............................................................. 14
Modes of Operation ....................................................................... 15
Normal Mode.............................................................................. 15
Partial Power-Down Mode ....................................................... 15
Full Power-Down Mode ............................................................ 16
Power-Up Times......................................................................... 17
Power vs. Throughput Rate....................................................... 17
Serial Interface ................................................................................ 18
Application Hints ........................................................................... 19
Grounding and Layout .............................................................. 19
Evaluating the AD7356 Performance...................................... 19
Outline Dimensions....................................................................... 20
Ordering Guide .......................................................................... 20
Functional Block Diagram .............................................................. 1
General Description......................................................................... 1
Product Highlights ........................................................................... 1
Revision History ............................................................................... 2
Specifications..................................................................................... 3
Timing Specifications .................................................................. 5
Absolute Maximum Ratings............................................................ 6
ESD Caution.................................................................................. 6
Pin Configuration and Function Descriptions............................. 7
Typical Performance Characteristics ............................................. 8
Terminology .................................................................................... 10
Theory of Operation ...................................................................... 12
Circuit Information.................................................................... 12
Converter Operation.................................................................. 12
Analog Input Structure.............................................................. 12
REVISION HISTORY
10/08—Revision 0: Initial Version
Rev. 0 | Page 2 of 20
AD7356
SPECIFICATIONS
VDD = 2.5 V 10%, VDRIVE = 2.25 V to 3.6 V, internal reference = 2.048 V, fSCLK = 80 MHz, fSAMPLE = 5 MSPS, TA = TMIN to TMAX1, unless
otherwise noted.
Table 2.
Parameter
Min
Typ
Max
Unit
Test Conditions/Comments
DYNAMIC PERFORMANCE
Signal-to-Noise Ratio (SNR)2
Signal-to-(Noise and Distortion) (SINAD)2
Total Harmonic Distortion (THD)2
Spurious Free Dynamic Range (SFDR)2
Intermodulation Distortion (IMD)2
Second-Order Terms
Third-Order Terms
fIN = 1 MHz sine wave
70
69.5
71.5
71
dB
dB
dB
dB
−84
−85
−77.5
−78.5
fa = 1 MHz + 50 kHz, fb = 1 MHz − 50 KHz
−84
−76
−100
−100
dB
dB
dB
dB
ADC-to-ADC Isolation2
CMRR2
fIN = 1 MHz, fNOISE = 100 kHz to 2.5 MHz
fNOISE = 100 kHz to 2.5 MHz
SAMPLE AND HOLD
Aperture Delay
Aperture Delay Match
Aperture Jitter
3.5
40
ns
ps
ps
16
Full Power Bandwidth
@ 3 dB
@ 0.1 dB
110
77
MHz
MHz
DC ACCURACY
Resolution
12
Bits
LSB
LSB
LSB
LSB
LSB
LSB
LSB
LSB
Integral Nonlinearity (INL)2
Differential Nonlinearity (DNL)2
Positive Full-Scale Error2
Positive Full-Scale Error Match2
Midscale Error2
0.5
0.5
1
1
0.99
Guaranteed no missed codes to 12 bits
6
2
8
+5
2
0/+11
Midscale Error Match2
Negative Full-Scale Error2
Negative Full-Scale Error Match2
ANALOG INPUT
8
6
8
1
2
Fully Differential Input Range (VIN+ and VIN−)
VCM VREF/2
V
V
VCM = common-mode voltage, VIN+ and
VIN− must remain within GND and VDD
The voltage around which VIN+ and VIN− are
centered
Common-Mode Voltage Range
0.5
1.9
5
DC Leakage Current
Input Capacitance
0.5
32
8
μA
pF
pF
When in track mode
When in hold mode
REFERENCE INPUT/OUTPUT
VREF Input Voltage Range
VREF Input Current
2.048 + 0.1
VDD
V
mA
V
0.3
0.45
2.058
2.053
20
When in reference overdrive mode
2.048 V 0.5% max @ VDD = 2.5 V 5%
2.048 V 0.25% max @ VDD = 2.5 V 5% and 25°C
VREF Output Voltage
2.038
2.043
V
VREF Temperature Coefficient
VREF Long Term Stability
VREF Thermal Hysteresis2
VREF Noise
6
ppm/°C
ppm
ppm
μV rms
Ω
100
50
60
1
For 1000 hours
VREF Output Impedance
Rev. 0 | Page 3 of 20
AD7356
Parameter
Min
Typ
Max
Unit
Test Conditions/Comments
LOGIC INPUTS
Input High Voltage (VINH
)
0.6 × VDRIVE
V
Input Low Voltage (VINL
Input Current (IIN))
Input Capacitance (CIN)
LOGIC OUTPUTS
)
0.3 × VDRIVE
1
V
μA
pF
VIN = 0 V or VDRIVE
3
Output High Voltage (VOH)
Output Low Voltage (VOL)
Floating-State Leakage Current
Floating-State Output Capacitance
Output Coding
VDRIVE − 0.2
V
V
μA
pF
0.2
1
5.5
Straight binary
CONVERSION RATE
Conversion Time
t2 + 13 × tSCLK
ns
ns
Track-and-Hold Acquisition Time2
30
5
Full-scale step input, settling to 0.5 LSBs
Throughput Rate
MSPS
POWER REQUIREMENTS3
VDD
2.25
2.25
2.75
3.6
V
V
Nominal VDD = 2.5 V
VDRIVE
4
ITOTAL
Digital inputs = 0 V or VDRIVE
Normal Mode (Operational)
Normal Mode (Static)
Partial Power-Down Mode
Full Power-Down Mode
14
6
3.5
5
20
7.8
4.5
40
90
mA
mA
mA
μA
SCLK on or off
SCLK on or off
SCLK on or off, −40°C to +85°C
SCLK on or off, 85°C to 125°C
μA
Power Dissipation
Normal Mode (Operational)
Normal Mode (Static)
Partial Power-Down Mode
Full Power-Down Mode
36
16
9.5
16
59
mW
mW
mW
μW
21.5
11.5
110
250
SCLK on or off
SCLK on or off
SCLK on or off, −40°C to +85°C
SCLK on or off, 85°C to 125°C
μW
1 Temperature ranges are as follows: Y Grade: −40°C to +125°C; B Grade: −40°C to +85°C.
2 See the Terminology section.
3 Current and power typical specifications are based on results with VDD = 2.5 V and VDRIVE = 3.0 V.
4 ITOTAL is the total current flowing in VDD and VDRIVE
.
Rev. 0 | Page 4 of 20
AD7356
TIMING SPECIFICATIONS
VDD = 2.5 V 10%, VDRIVE = 2.25 V to 3.6 V, internal reference = 2.048 V, TA = TMAX to TMIN1, unless otherwise noted.
Table 3.
Parameter
Limit at TMIN, TMAX
Unit
Description
fSCLK
50
80
kHz min
MHz max
ns max
ns min
ns min
ns max
tCONVERT
tQUIET
t2
t2 + 13 × tSCLK
tSCLK = 1/fSCLK
5
5
6
Minimum time between end of serial read and next falling edge of CS
CS to SCLK setup time
2
t3
Delay from CS until SDATAA and SDATAB are three-state disabled
Data access time after SCLK falling edge
1.8 V ≤ VDRIVE < 2.25 V
2.25 V ≤ VDRIVE < 2.75 V
2.75 V ≤ VDRIVE < 3.3 V
3.3 V ≤ VDRIVE ≤ 3.6 V
SCLK low pulse width
SCLK high pulse width
SCLK to data valid hold time
CS rising edge to SDATAA, SDATAB high impedance
CS rising edge to falling edge pulse width
SCLK falling edge to SDATAA, SDATAB high impedance
SCLK falling edge to SDATAA, SDATAB high impedance
2, 3
t4
12.5
11
9.5
9
5
5
3.5
9.5
5
ns max
ns max
ns max
ns max
ns min
ns min
ns min
ns max
ns min
ns min
ns max
t5
t6
2
t7
2
t8
t9
2
t10
4.5
9.5
1 Temperature ranges are as follows: Y Grade: −40°C to +125°C; B Grade: −40°C to +85°C.
2 Specified with a load capacitance of 10 pF on SDATAA and SDATAB.
3 The time required for the output to cross 0.4 V or 2.4 V.
Rev. 0 | Page 5 of 20
AD7356
ABSOLUTE MAXIMUM RATINGS
Table 4.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent 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
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
Parameter
Rating
VDD to AGND, DGND, REFGND
VDRIVE to AGND, DGND, REFGND
VDD to VDRIVE
−0.3 V to +3 V
−0.3 V to +5 V
−5 V to +3 V
AGND to DGND to REFGND
Analog Input Voltages1 to AGND
Digital Input Voltages2 to DGND
Digital Output Voltages3 to DGND
Input Current to Any Pin Except Supply Pins4
Operating Temperature Range
Y Grade
−0.3 V to +0.3 V
−0.3 V to VDD + 0.3 V
−0.3 V to VDRIVE + 0.3 V
−0.3 V to VDRIVE + 0.3 V
10 mA
ESD CAUTION
−40°C to +125°C
−40°C to +85°C
−65°C to +150°C
150°C
B Grade
Storage Temperature Range
Junction Temperature
TSSOP
θJA Thermal Impedance
θJC Thermal Impedance
Lead Temperature, Soldering
Reflow Temperature (10 sec to 30 sec)
ESD
143°C/W
45°C/W
255°C
1.5 kV
1 Analog input voltages are VINA+, VINA−, VINB+, VINB−, REFA, and REFB.
2
CS
Digital input voltages are and SCLK.
3 Digital output voltages are SDATAA and SDATAB.
4 Transient currents of up to 100 mA do not cause SCR latch-up.
Rev. 0 | Page 6 of 20
AD7356
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
V
V
V
DRIVE
INA+
SCLK
SDATA
SDATA
DGND
AGND
CS
INA–
REF
A
A
B
AD7356
TOP VIEW
REFGND
AGND
(Not to Scale)
REF
B
INB–
INB+
V
V
V
DD
Figure 2. Pin Configuration
Table 5. Pin Function Descriptions
Pin No. Mnemonic Description
Analog Inputs of ADC A. These analog inputs form a fully differential pair.
1, 2
3, 6
VINA+, VINA−
REFA, REFB
Reference Decoupling Capacitor Pins. Decoupling capacitors are connected between these pins and the
REFGND pin to decouple the reference buffer for each respective ADC. It is recommended to decouple each
reference pin with a 10 ꢀF capacitor. Provided the output is buffered, the on-chip reference can be taken from
these pins and applied externally to the rest of the system. The nominal internal reference voltage is 2.048 V
and appears at these pins. These pins can also be overdriven by an external reference. The input voltage range
for the external reference is 2.048 V + 100 mV to VDD.
4
REFGND
AGND
Reference Ground. This is the ground reference point for the reference circuitry on the AD7356. Refer any
external reference signal to this REFGND voltage. Decoupling capacitors must be placed between this pin and
the REFA and REFB pins. Connect the REFGND pin to the AGND plane of a system.
Analog Ground. This is the ground reference point for all analog circuitry on the AD7356. All analog input
signals should be referred to this AGND voltage. The AGND and DGND voltages should ideally be at the same
potential and must not be more than 0.3 V apart, even on a transient basis.
5, 11
7, 8
9
VINB−, VINB+
VDD
Analog Inputs of ADC B. These analog inputs form a fully differential pair.
Power Supply Input. The VDD range for the AD7356 is 2.5 V 10%. Decouple the supply to AGND with a 0.1 ꢀF
capacitor in parallel with a 10 ꢀF tantalum capacitor.
10
12
CS
Chip Select. Active low logic input. This input provides the dual functions of initiating conversions on the
AD7356 and framing the serial data transfer.
Digital Ground. This is the ground reference point for all digital circuitry on the AD7356. Connect this pin to
the DGND plane of a system. The DGND and AGND voltages should ideally be at the same potential and must
not be more than 0.3 V apart, even on a transient basis.
DGND
13, 14
SDATAB, SDATAA Serial Data Outputs. The data output is supplied to each pin as a serial data stream. The bits are clocked out
on the falling edge of the SCLK input. To access the 12 bits of data from the AD7356, 14 SCLK falling edges are
required. The data simultaneously appears on both data output pins from the simultaneous conversions of
both ADCs. The data stream consists of two leading zeros followed by the 12 bits of conversion data. The data
is provided MSB first. If CS is held low for 16 SCLK cycles rather than 14 on the AD7356, then two trailing zeros
appear after the 12 bits of data. If CS is held low for a further 16 SCLK cycles on either SDATAA or SDATAB, the
data from the other ADC follows on the SDATA pins. This allows data from a simultaneous conversion on both
ADCs to be gathered in serial format on either SDATAA or SDATAB.
15
16
SCLK
Serial Clock. Logic input. A serial clock input provides the serial clock for accessing the data from the AD7356.
This clock is also used as the clock source for the conversion process.
Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the interface operates.
The voltage at this pin may be different than the voltage at VDD. The VDRIVE supply should be decoupled to
DGND with a 0.1 ꢀF capacitor in parallel with a 10 ꢀF tantalum capacitor.
VDRIVE
Rev. 0 | Page 7 of 20
AD7356
TYPICAL PERFORMANCE CHARACTERISTICS
0
16,384 POINT FFT
fSAMPLE = 5MSPS
60,000
50,000
fIN = 1MHz
–20
SNR = 71.8dB
SINAD = 71.6dB
THD = –83.5dB
–40
40,000
30,000
20,000
–60
–80
–100
–120
10,000
0
93 HITS
2046
20 HITS
2048
2044
2045
2047
2049
2050
0
250 500 750 1000 1250 1500 1750 2000 2249 2499
FREQUENCY (kHz)
CODE
Figure 6. Histogram of Codes for 65,000 Samples
Figure 3. Typical FFT
1.0
0.8
73
72
71
70
69
68
67
66
65
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
0
500
1000 1500 2000 2500 3000 3500 4000
CODE
0
1000
2000
3000
4000
5000
ANALOG INPUT FREQUENCY (kHz)
Figure 7. SNR vs. Analog Input Frequency
Figure 4. Typical DNL Error
–60
–65
–70
–75
–80
–85
–90
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
0
5
10
15
20
25
0
500
1000 1500 2000 2500 3000 3500 4000
CODE
SUPPLY RIPPLE FREQUENCY (MHz)
Figure 8. PSRR vs. Supply Ripple Frequency with No Supply Decoupling
Figure 5. Typical INL Error
Rev. 0 | Page 8 of 20
AD7356
11
10
9
2.0482
2.0480
2.0478
2.0476
2.0474
2.0472
2.0470
2.0468
2.0466
2.0464
2.0462
2.0460
+125°C
+85°C
+25°C
–40°C
8
7
6
5
1.8
2.0
2.2
2.4
2.6
V
2.8
(V)
3.0
3.2
3.4
3.6
0
500
1000
1500
2000
2500
3000
CURRENT LOAD (µA)
DRIVE
Figure 9. VREF vs. Reference Output Current Drive
Figure 12. Access Time vs. VDRIVE
9
8
7
6
5
4
1.0
0.8
0.6
0.4
+125°C
+85°C
+25°C
–40°C
INL MAX
0.2
0
DNL MAX
INL MIN
–0.2
–0.4
–0.6
–0.8
–1.0
DNL MIN
1.8
2.0
2.2
2.4
2.6
2.8
(V)
3.0
3.2
3.4
3.6
0
10
20
30
40
50
60
70
80
V
DRIVE
SCLK FREQUENCY (MHz)
Figure 10. Linearity Error vs. SCLK Frequency
Figure 13. Hold Time vs. VDRIVE
1.0
0.6
DNL MAX
INL MAX
0.2
–0.2
–0.6
–1.0
INL MIN
DNL MIN
2.10
2.15
2.20
2.25
2.30
2.35
(V)
2.40
2.45
2.50
EXTERNAL V
REF
Figure 11. Linearity Error vs. External VREF
Rev. 0 | Page 9 of 20
AD7356
TERMINOLOGY
Power Supply Rejection Ratio (PSRR)
Integral Nonlinearity (INL)
PSRR is defined as the ratio of the power in the ADC output at
full-scale frequency, f, to the power of a 100 mV p-p sine wave
applied to the ADC VDD supply of frequency, fS. The frequency
of the input varies from 5 kHz to 25 MHz.
INL is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The
endpoints of the transfer function are zero scale (1 LSB below
the first code transition) and full scale (1 LSB above the last
code transition).
PSRR (dB) = 10 log(Pf/PfS)
Differential Nonlinearity (DNL)
where:
DNL is the difference between the measured and the ideal
1 LSB change between any two adjacent codes in the ADC.
Pf is the power at frequency, f, in the ADC output.
PfS is the power at frequency, fS, in the ADC output.
Negative Full-Scale Error
Common-Mode Rejection Ratio (CMRR)
Negative full-scale error is the deviation of the first code
transition (00 … 000) to (00 … 001) from the ideal (that is,
−VREF + 0.5 LSB) after the midscale error has been adjusted out.
CMRR is defined as the ratio of the power in the ADC output
at full-scale frequency, f, to the power of a 100 mV p-p sine
wave applied to the common-mode voltage of V
IN+ and VIN−
of frequency, fS.
Negative Full-Scale Error Match
Negative full-scale error match is the difference in negative full-
CMRR (dB) = 10 log(Pf/PfS)
scale error between the two ADCs.
where:
Midscale Error
Pf is the power at frequency (f) in the ADC output.
Midscale error is the deviation of the midscale code transition
(011 … 111) to (100 … 000) from the ideal (that is, 0 V).
PfS is the power at frequency (fS) in the ADC output.
Track-and-Hold Acquisition Time
Midscale Error Match
Midscale error match is the difference in midscale error
between the two ADCs.
The track-and-hold amplifier returns to track mode at the end
of a conversion. The track-and-hold acquisition time is the time
required for the output of the track-and-hold amplifier to reach
its final value, within 0.5 LSB, after the end of a conversion.
Positive Full-Scale Error
Positive full-scale error is the deviation of the last code
transition (111 … 110) to (111 … 111) from the ideal (that is,
Signal-to-(Noise and Distortion) Ratio (SINAD)
SINAD is the measured ratio of signal-to-(noise and distortion)
at the output of the ADC. The signal is the rms amplitude of the
fundamental. Noise is the sum of all nonfundamental signals up
to half the sampling frequency (fS/2), excluding dc. The ratio is
dependent on the number of quantization levels in the digitiza-
tion process; the more levels, the smaller the quantization noise.
V
REF − 1.5 LSB) after the midscale error has been adjusted out.
Positive Full-Scale Error Match
Positive full-scale error match is the difference in positive full-
scale error between the two ADCs.
ADC-to-ADC Isolation
The theoretical SINAD for an ideal N-bit converter with a sine
wave input is given by
ADC-to-ADC isolation is a measure of the level of crosstalk
between ADC A and ADC B. It is measured by applying a full-
scale 1 MHz sine wave signal to one of the two ADCs and
applying a full-scale signal of variable frequency to the other
ADC. The ADC-to-ADC isolation is defined as the ratio of the
power of the 1 MHz signal on the converted ADC to the power
of the noise signal on the other ADC that appears in the FFT.
The noise frequency on the unselected channel varies from
100 kHz to 2.5 MHz.
SINAD = (6.02 N + 1.76) dB
Thus, for a 12-bit converter, SINAD is 74 dB and for a 14-bit
converter, SINAD is 86 dB.
Rev. 0 | Page 10 of 20
AD7356
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7356, it is defined as
The AD7356 is tested using the CCIF standard where two input
frequencies near the top end of the input bandwidth are used.
In this case, the second-order terms are usually distanced in
frequency from the original sine waves and the third-order
terms are usually at a frequency close to the input frequencies.
As a result, the second- and third-order terms are specified
separately. The calculation of the intermodulation distortion
is as per the THD specification, where it is the ratio of the rms
sum of the individual distortion products to the rms amplitude
of the sum of the fundamentals expressed in decibels.
2
2
2
2
2
V2 +V3 +V4 +V5 +V6
THD dB = −20 log
( )
V1
where:
V1 is the rms amplitude of the fundamental.
V2, V3, V4, V5, and V6 are the rms amplitudes of the second
through the sixth harmonics.
Thermal Hysteresis
Spurious Free Dynamic Range (SFDR)
Thermal hysteresis is defined as the absolute maximum change
of reference output voltage after the device is cycled through
temperature from either
SFDR is the ratio of the rms value of the next largest component
in the ADC output spectrum (up to fS/2 and excluding dc) to
the rms value of the fundamental. Normally, the value of this
specification is determined by the largest harmonic in the
spectrum, but for ADCs where the harmonics are buried in
the noise floor, it is a noise peak.
T_HYS+ = +25°C to TMAX to +25°C
T_HYS– = +25°C to TMIN to +25°C
Thermal hysteresis is expressed in ppm using the following
equation:
Intermodulation Distortion (IMD)
With inputs consisting of sine waves at two frequencies, fa
and fb, any active device with nonlinearities creates distortion
products at sum and difference frequencies of mfa nfb where
m, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms
are those for which neither m nor n is equal to zero. For example,
the second-order terms include (fa + fb) and (fa − fb), while the
third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and
(fa − 2fb).
V
REF (25°C)−VREF (T _ HYS)
VHYS (ppm) =
× 106
V
REF (25°C)
where:
REF(25°C) is VREF at 25°C.
VREF(T_HYS) is the maximum change of VREF at T_HYS+
V
or T_HYS–.
Rev. 0 | Page 11 of 20
AD7356
THEORY OF OPERATION
When the ADC starts a conversion (see Figure 15), SW3 opens
and SW1 and SW2 move to Position B, causing the comparator
to become unbalanced. Both inputs are disconnected once the
conversion begins. The control logic and charge redistribution
DACs are used to add and subtract fixed amounts of charge
from the sampling capacitor arrays to bring the comparator
back into a balanced condition. When the comparator is
rebalanced, the conversion is complete. The control logic
generates the ADC output code. The output impedances of
the sources driving the VIN+ and VIN− pins must be matched;
otherwise, the two inputs may have different settling times,
resulting in errors.
CIRCUIT INFORMATION
The AD7356 is a high speed, dual, 12-bit, single-supply, succes-
sive approximation analog-to-digital converter (ADC). The part
operates from a 2.5 V power supply and features throughput
rates of up to 5 MSPS.
The AD7356 contains two on-chip differential track-and-hold
amplifiers, two successive approximation ADCs, and a serial
interface with two separate data output pins. The part is housed
in a 16-lead TSSOP, offering the user considerable space-saving
advantages over alternative solutions.
The serial clock input accesses data from the part but also
provides the clock source for each successive approximation
ADC. The AD7356 has an on-chip 2.048 V reference. If an
external reference is desired the internal reference can be
overdriven with a reference value ranging from (2.048 V +
100 mV) to VDD. If the internal reference is to be used elsewhere
in the system, then the reference output needs to be buffered
first. The differential analog input range for the AD7356 is
VCM VREF/2.
CAPACITIVE
DAC
COMPARATOR
C
C
B
A
S
V
V
IN+
SW1
SW2
CONTROL
LOGIC
SW3
S
A
B
IN–
V
REF
CAPACITIVE
DAC
The AD7356 features power-down options to allow power
saving between conversions. The power-down feature is
implemented via the standard serial interface, as described
in the Modes of Operation section.
Figure 15. ADC Conversion Phase
ANALOG INPUT STRUCTURE
Figure 16 shows the equivalent circuit of the analog input structure
of the AD7356. The four diodes provide ESD protection for the
analog inputs. Care must be taken to ensure that the analog input
signals never exceed the supply rails by more than 300 mV.
This causes these diodes to become forward biased and start
conducting into the substrate. These diodes can conduct up
to 10 mA without causing irreversible damage to the part.
CONVERTER OPERATION
The AD7356 has two successive approximation ADCs, each
based around two capacitive DACs. Figure 14 and Figure 15
show simplified schematics of one of these ADCs in acquisition
and conversion phase. The ADC comprises a control logic, a
SAR, and two capacitive DACs. In Figure 14 (the acquisition
phase), SW3 is closed, SW1 and SW2 are in Position A, the
comparator is held in a balanced condition, and the sampling
capacitor arrays acquire the differential signal on the input.
The C1 capacitors in Figure 16 are typically 8 pF and can
primarily be attributed to pin capacitance. The R1 resistors
are lumped components made up of the on resistance of the
switches. The value of these resistors is typically about 30 Ω.
The C2 capacitors are the sampling capacitors of the ADC
with a capacitance of 32 pF typically.
CAPACITIVE
DAC
COMPARATOR
C
C
B
A
S
V
V
IN+
SW1
SW2
V
DD
CONTROL
LOGIC
SW3
S
A
B
IN–
D
D
C2
R1
V
IN+
V
REF
C1
CAPACITIVE
DAC
Figure 14. ADC Acquisition Phase
V
DD
D
D
C2
R1
V
IN–
C1
Figure 16. Equivalent Analog Input Circuit,
Conversion Phase–Switches Open,
Track Phase—Switches Closed
Rev. 0 | Page 12 of 20
AD7356
For ac applications, removing high frequency components from
the analog input signal is recommended by the use of an RC
low-pass filter on the analog input pins. In applications where
harmonic distortion and signal-to-noise ratio are critical, the
analog input should be driven from a low impedance source.
Large source impedances significantly affect the ac perfor-
mance of the ADC and may necessitate the use of an input
buffer amplifier. The choice of the op amp is a function of the
particular application.
ANALOG INPUTS
Differential signals have some benefits over single-ended
signals, including noise immunity based on the devices
common-mode rejection and improvements in distortion
performance. Figure 19 defines the fully differential input
of the AD7356.
V
p-p
V
IN+
REF
AD7356*
COMMON-MODE
VOLTAGE
When no amplifier is used to drive the analog input, limit
the source impedance to low values. The maximum source
impedance depends on the amount of THD that can be
tolerated. THD increases as the source impedance increases
and performance degrades. Figure 17 shows a graph of the
THD vs. the analog input signal frequency for different source
impedances.
V
p-p
V
REF
IN–
*
ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 19. Differential Input Definition
The amplitude of the differential signal is the difference
between the signals applied to the VIN+ and VIN− pins in
each differential pair (VIN+ − VIN−). VIN+ and VIN− should be
simultaneously driven by two signals each of amplitude (VREF
that are 180° out of phase. This amplitude of the differential
–65
)
–67
–69
–71
signal is, therefore, –VREF to +VREF peak-to -peak regardless of
the common mode (CM).
–73
100Ω
CM is the average of the two signals and is, therefore, the
voltage on which the two inputs are centered.
–75
–77
50Ω
CM = (VIN+ + VIN−)/2
–79
33Ω
–81
This results in the span of each input being CM
VREF/2. This
–83
voltage has to be set up externally. When setting up the CM,
ensure that VIN+ and VIN− remain within GND/VDD. When
a conversion takes place, CM is rejected, resulting in a virtually
noise-free signal of amplitude, –VREF to +VREF, corresponding
to the digital codes of 0 to 4095 for the AD7356.
10Ω
–85
–87
100
200
1000
1500
2000
2500
FREQUENCY (kHz)
Figure 17. THD vs. Analog Input Signal Frequency for Various Source
Impedances
Figure 18 shows a graph of the THD vs. the analog input
frequency while sampling at 5 MSPS. In this case, the source
impedance is 33 Ω.
–66
–70
–74
–78
–82
–86
–90
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
ANALOG INPUT FREQUENCY (kHz)
Figure 18. THD vs. Analog Input Frequency
Rev. 0 | Page 13 of 20
AD7356
The voltage applied to Point A sets up the common-mode
voltage. In both diagrams, it is connected in some way to the
reference. The AD8022 is a suitable dual op amp that could be
used in this configuration to provide differential drive to the
AD7356.
DRIVING DIFFERENTIAL INPUTS
Differential operation requires VIN+ and VIN− to be driven
simultaneously with two equal signals that are 180° out of phase.
Because not all applications have a signal preconditioned for
differential operation, there is often a need to perform a single-
ended-to-differential conversion.
2.048V
2 × V
REF
p-p
220Ω
V+
1.024V
0V
440Ω
Differential Amplifier
V
REF
27Ω
V
IN+
GND
An ideal method of applying differential drive to the AD7356
is to use a differential amplifier such as the AD8138. This part
can be used as a single-ended-to-differential amplifier or as a
differential-to-differential amplifier. The AD8138 also provides
common-mode level shifting. Figure 20 shows how the AD8138
can be used as a single-ended-to-differential amplifier. The
positive and negative outputs of the AD8138 are connected to
the respective inputs on the ADC via a pair of series resistors
to minimize the effects of switched capacitance on the front
end of the ADC. The architecture of the AD8138 results in
outputs that are very highly balanced over a wide frequency
range without requiring tightly matched external components.
V–
220Ω
2.048V
1.024V
0V
AD7356*
220Ω
V+
27Ω
REF /REF
V
A
B
IN–
A
V–
10kΩ
10µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 21. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
into a Differential Signal
2.048V
2 × V
REF
p-p
220Ω
V+
1.024V
0V
C 1
F
440Ω
2.048V
1.024V
0V
GND
27Ω
R 1
F
V
IN+
R
R
*
*
V–
220Ω
S
S
R
1
G
V
IN+
2.048V
1.024V
0V
V
AD7356*
OCM
220Ω
+2.048V
GND
–2.048V
AD8138
AD7356
220Ω
51Ω
R
2
G
V+
V
REF /REF
A B
IN–
27Ω
REF /REF
B
2.048V
1.024V
0V
V
R 2
F
A
IN–
A
V–
C 2
F
10kΩ
10µF
10kΩ
10kΩ
20kΩ
10µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
*MOUNT AS CLOSE TO THE AD7356 AS POSSIBLE
Figure 22. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
a Differential Unipolar Signal
AND ENSURE THAT HIGH PRECISION R RESISTORS ARE USED.
S
R
R
– 33Ω; R 1 = R 1 = R 2 = 499Ω; C 1 = C 2 = 39pF;
G F F F F
2 = 523Ω
S
G
ADC TRANSFER FUNCTION
Figure 20. Using the AD8138 as a Single-Ended-to-Differential Amplifier
The output coding for the AD7356 is straight binary. The
designed code transitions occur at successive LSB values
(1 LSB, 2 LSBs, and so on). The LSB size is (2 × VREF)/4096.
The ideal transfer characteristic of the AD7356 is shown in
Figure 23.
If the analog inputs source being used has zero impedance, all
four resistors (RG1, RG2, RF1, and RF2) should be the same value
as each other. If the source has a 50 ꢀ impedance and a 50 ꢀ
termination, for example, increase the value of RG2 by 25 ꢀ to
balance this parallel impedance on the input and thus ensure
that both the positive and negative analog inputs have the
same gain. The outputs of the amplifier are perfectly matched
balanced differential outputs of identical amplitude, and are
exactly 180° out of phase.
111 ... 111
111 ... 110
111 ... 101
Op Amp Pair
An op amp pair can be used to directly couple a differential signal
to one of the analog input pairs of the AD7356. The circuit
configurations shown in Figure 21 and Figure 22 show how
an op amp pair can be used to convert a single-ended signal
into a differential signal for a bipolar and unipolar input signal,
respectively.
000 ... 010
000 ... 001
000 ... 000
–V
+ 1 LSB
+V
– 1 LSB
REF
REF
–V
+ 0.5 LSB
+V
– 1.5 LSB
REF
REF
ANALOG INPUT
Figure 23. AD7356 Ideal Transfer Characteristic
Rev. 0 | Page 14 of 20
AD7356
MODES OF OPERATION
The mode of operation of the AD7356 is selected by controlling
CS
When a data transfer is complete and SDATAA and SDATAB
have returned to three-state, another conversion can be initiated
CS
the logic state of the
signal during a conversion. There are
after the quiet time, tQUIET, has elapsed by bringing
low again
three possible modes of operation: normal mode, partial power-
down mode, and full power-down mode. After a conversion has
(assuming the required acquisition time has been allowed).
CS
is initiated, the point at which
is pulled high determines
PARTIAL POWER-DOWN MODE
which power-down mode, if any, the device enters. Similarly, if
Partial power-down mode is intended for use in applications in
which slower throughput rates are required. Either the ADC
is powered down between each conversion or a series of
conversions can be performed at a high throughput rate and
the ADC is then powered down between these bursts of several
conversions. It is recommended that the AD7356 not remain
in partial power-down mode for longer than 100 μs. When
the AD7356 is in partial power-down, all analog circuitry is
powered down except for the on-chip reference and reference
buffers.
CS
already in a power-down mode, can control whether the device
returns to normal operation or remains in a power-down mode.
These modes of operation are designed to provide flexible
power management options. These options can be chosen to
optimize the power dissipation/throughput rate ratio for the
differing application requirements.
NORMAL MODE
Normal mode is intended for applications needing the fastest
throughput rates because the user does not have to worry about
any power-up times because the AD7356 remains fully powered
at all times. Figure 24 shows the general diagram of the
operation of the AD7356 in normal mode.
To enter partial power-down mode, the conversion process
CS
must be interrupted by bringing
second falling edge of SCLK and before the 10th falling edge of
CS
high any time after the
SCLK, as shown in Figure 25. When
in this window of SCLKs, the part enters partial power-down,
CS
has been brought high
CS
1
10
14
the conversion that was initiated by the falling edge of
terminated, and SDATAA and SDATAB go back into three-state.
CS
is
SCLK
If
is brought high before the second SCLK falling edge, the
SDATA
SDATA
A
B
LEADING ZEROS + CONVERSION RESULT
part remains in normal mode and does not power down. This
Figure 24. Normal Mode Operation
CS
avoids accidental power-down due to glitches on the
line.
CS
The conversion is initiated on the falling edge of , as described
in the Serial Interface section. To ensure that the part remains
CS
1
2
10
14
CS
fully powered up at all times,
must remain low until at least
SCLK
CS
10 SCLK falling edges have elapsed after the falling edge of
.
th
CS
If
is brought high any time after the 10 SCLK falling edge
THREE-STATE
SDATA
SDATA
A
B
but before the 14th SCLK falling edge, the part remains powered
up; however, the conversion is terminated and SDATAA and
SDATAB go back into three-state. To complete the conversion
and access the conversion result for the AD7356, 14 serial clock
cycles are required. The SDATA lines do not return to three-
state after 14 SCLK cycles have elapsed but instead do so when
Figure 25. Entering Partial Power-Down Mode
To exit this mode of operation and power up the AD7356 again,
perform a dummy conversion. On the falling edge of , the
device begins to power up, and continues to power up as long
is held low until after the falling edge of the 10 SCLK.
The device is fully powered up after approximately 200 ns have
elapsed (or one full conversion) and valid data results from the
next conversion, as shown in Figure 26. If
before the second falling edge of SCLK, the AD7356 again goes
into partial power-down. This avoids accidental power-up due
CS
th
CS
as
CS
CS
is brought high again. If
is left low for another two SCLK
CS
cycles, two trailing zeros are clocked out after the data. If
is
left low for a further 14 SCLK cycles, the result for the other
ADC on board is also accessed on the same SDATA line (see
Figure 31 and the Serial Interface section).
CS
is brought high
Once 32 SCLK cycles have elapsed, the SDATA line returns to
CS
CS
to glitches on the
line. Although the device may begin to
CS
three-state on the 32nd SCLK falling edge. If
prior to this, the SDATA line returns to three-state at that point.
CS
is brought high
power up on the falling edge of , it powers down again on the
rising edge of . If the AD7356 is already in partial power-down
is brought high between the second and 10
falling edges of SCLK, the device enters full power-down mode.
CS
Thus,
may idle low after 32 SCLK cycles until it is brought
th
CS
mode and
high again sometime prior to the next conversion. The bus still
returns to three-state upon completion of the dual result read.
Rev. 0 | Page 15 of 20
AD7356
To reach full power-down, the next conversion cycle must be
FULL POWER-DOWN MODE
CS
interrupted in the same way, as shown in Figure 27. When
brought high in this window of SCLKs, the part fully powers
down.
is
Full power-down mode is intended for use in applications
where throughput rates slower than those in partial power-
down mode are required because power-up from a full power-
down takes substantially longer than that from a partial power-
down. This mode is more suited to applications in which a
series of conversions performed at a relatively high throughput
rate are followed by a long period of inactivity and, thus, power-
down. When the AD7356 is in full power-down mode, all
analog circuitry is powered down including the on-chip
reference and reference buffers. Full power-down mode is
entered in a similar way as partial power-down mode, except
that the timing sequence shown in Figure 25 must be executed
twice. The conversion process must be interrupted in a similar
Note that it is not necessary to complete the 14 or 16 SCLKs
CS
once
has been brought high to enter a power-down mode.
To exit full power-down mode and power-up the AD7356,
perform a dummy conversion, similar to powering up from
CS
partial power-down. On the falling edge of , the device begins
CS
to power up as long as
is held low until after the falling edge
of the 10th SCLK. The required power-up time must elapse
before a conversion can be initiated, as shown in Figure 28.
CS
fashion by bringing
high anywhere after the second falling
edge of SCLK and before the 10th falling edge of SCLK. The
device enters partial power-down mode at this point.
THE PART IS FULLY POWERED UP;
SEE THE POWER-UP TIMES SECTION.
THE PART BEGINS
TO POWER UP.
tPOWER-UP1
CS
1
10
14
1
14
SCLK
SDATA
SDATA
A
B
INVALID DATA
VALID DATA
Figure 26. Exiting Partial Power-Down Mode
THE PART ENTERS
PARTIAL POWER-DOWN MODE.
THE PART BEGINS
TO POWER UP.
THE PART ENTERS
FULL POWER-DOWN MODE.
CS
1
2
10
14
1
2
10
14
SCLK
SDATA
SDATA
THREE-STATE
THREE-STATE
A
B
INVALID DATA
INVALID DATA
Figure 27. Entering Full Power-Down Mode
THE PART IS FULLY POWERED UP;
SEE THE POWER-UP TIMES SECTION.
THE PART BEGINS
TO POWER UP.
tPOWER-UP2
CS
10
14
1
14
1
SCLK
SDATA
SDATA
A
B
INVALID DATA
VALID DATA
Figure 28. Exiting Full Power-Down Mode
Rev. 0 | Page 16 of 20
AD7356
Alternatively, if the part is to be placed into full power-down
mode when the supplies are applied, three dummy cycles must
POWER-UP TIMES
The AD7356 has two power-down modes: partial power-down
and full power-down, which are described in detail in the Normal
Mode, Partial Power-Down Mode, and Full Power-Down Mode
sections. This section deals with the power-up time required
when coming out of any of these modes. Note that the recom-
mended decoupling capacitors must be in place on the REFA
and REFB pins for the power-up times to apply.
CS
be initiated. The first dummy cycle must hold
low until after
the 10th SCLK falling edge; the second and third dummy cycles
place the part into full power-down mode (see Figure 27 and
the Modes of Operation section).
POWER vs. THROUGHPUT RATE
The power consumption of the AD7356 varies with the
throughput rate. When using very slow throughput rates
and as fast an SCLK frequency as possible, the various power-
down options can be used to make significant power savings.
However, the AD7356 quiescent current is low enough that
even without using the power-down options, there is a noticeable
variation in power consumption with sampling rate. This is true
whether a fixed SCLK value is used or it is scaled with the
sampling rate. Figure 29 shows a plot of power vs. throughput rate
when operating in normal mode for a fixed maximum SCLK
frequency and a SCLK frequency that scales with the sampling
rate. The internal reference was used for Figure 29.
To power up from partial power-down mode, one dummy cycle
is required. The device is fully powered up after approximately
CS
200 ns have elapsed from the falling edge of . When the
partial power-up time has elapsed, the ADC is fully powered
up, and the input signal is acquired properly. The quiet time,
tQUIET, must still be allowed from the point where the bus goes
back into three-state after the dummy conversion to the next
CS
falling edge of
To power up from full power-down mode, approximately 6 ms
CS
.
should be allowed from the falling edge of , shown in
Figure 28 as tPOWER-UP2
.
Note that during power-up from partial power-down mode, the
track-and-hold, which is in hold mode while the part is powered
down, returns to track mode after the first SCLK edge that the
38
34
30
CS
part receives after the falling edge of
.
When power supplies are first applied to the AD7356, the ADC
can power up in either of the power-down modes or in normal
mode. Because of this, it is best to allow a dummy cycle to elapse
to ensure that the part is fully powered up before attempting a
valid conversion. Likewise, if the part is to be kept in partial
power-down mode immediately after the supplies are applied,
26
80MHz SCLK
22
VARIABLE SCLK
18
14
10
then two dummy cycles must be initiated. The first dummy
th
CS
cycle must hold
low until after the 10 SCLK falling edge; in
0
1000
2000
3000
4000
5000
THROUGHPUT (kSPS)
CS
the second cycle,
must be brought high between the second
and 10th SCLK falling edges (see Figure 25).
Figure 29. Power vs. Throughput Rate
Rev. 0 | Page 17 of 20
AD7356
SERIAL INTERFACE
Figure 30 shows the detailed timing diagram for serial interfacing
to the AD7356. The serial clock provides the conversion clock
and controls the transfer of information from the AD7356
during conversion.
SDATA line in use goes back into three-state on the 32nd SCLK
CS
falling edge or the rising edge of , whichever occurs first.
A minimum of 14 serial clock cycles is required to perform
the conversion process and to access data from one conversion
CS
The
signal initiates the data transfer and conversion process.
CS
CS
on either data line of the AD7356.
falling low provides the
The falling edge of
puts the track and hold into hold mode,
leading zero to be read in by the microcontroller or DSP. The
remaining data is then clocked out by subsequent SCLK falling
edges, beginning with a second leading zero. Thus, the first
falling clock edge on the serial clock has the leading zero
provided and also clocks out the second leading zero. The
12-bit result then follows with the final bit in the data transfer
and is valid on the 14th falling edge (having been clocked out on
the previous (13th) falling edge). In applications with a slower
SCLK, it may be possible to read in data on each SCLK rising
edge depending on the SCLK frequency. With a slower SCLK,
at which point the analog input is sampled and the bus is taken
out of three-state. The conversion is also initiated at this point
and requires a minimum of 14 SCLKs to complete. Once
13 SCLK falling edges have elapsed, the track and hold goes
back into track on the next SCLK rising edge, as shown in
Figure 30 at Point B. If a 16-bit data transfer is used on the
AD7356, then two trailing zeros appear after the final LSB.
CS
On the rising edge of , the conversion is terminated and
CS
SDATAA and SDATAB go back into three-state. If
is not
CS
the first rising edge, of SCLK after the
falling edge has the
brought high, but is instead held low for an additional 14
SCLK cycles, the data from the conversion on ADC B is output
on SDATAA (see Figure 31). Likewise, the data from the
conversion on ADC A is output on SDATAB. In this case, the
second leading zero provided, and the 13th rising SCLK edge
has DB0 provided.
tACQUISITION
CS
t9
tCONVERT
t2
t6
B
SCLK
3
4
5
1
2
13
t5
tQUIET
t8
t7
t3
t4
DB9
SDATA
SDATA
A
B
0
0
DB11
DB10
DB8
DB2
DB1
DB0
THREE-STATE
THREE-
STATE
2 LEADING ZEROS
Figure 30. Serial Interface Timing Diagram
CS
t6
t2
SCLK
3
4
5
1
2
14
15
16
17
32
t5
t10
ZERO
2 TRAILING ZEROS
t3
t4
DB9
t7
DB11
DB10
DB11
B
0
0
ZERO
ZERO
ZERO
ZERO
ZERO
SDATA
A
A
A
A
THREE-
STATE
THREE-
STATE
2 LEADING
ZEROS
2 TRAILING ZEROS
2 LEADING ZEROS
Figure 31. Reading Data from Both ADCs on One SDATA Line with 32 SCLKs
Rev. 0 | Page 18 of 20
AD7356
APPLICATION HINTS
board should run at right angles to each other. A microstrip
GROUNDING AND LAYOUT
technique is the best method but is not always possible with a
double sided board. In this technique, the component side of
the board is dedicated to ground planes and signals are placed
on the solder side.
The analog and digital supplies to the AD7356 are independent
and separately pinned out to minimize coupling between the
analog and digital sections of the device. The printed circuit
board (PCB) that houses the AD7356 should be designed so
that the analog and digital sections are separated and confined
to certain areas of the board. This design facilitates the use of
ground planes that can be easily separated.
Good decoupling is important; decouple all supplies with 10 μF
tantalum capacitors in parallel with 0.1 μF capacitors to GND.
To achieve the best results from these decoupling components,
they must be placed as close as possible to the device, ideally
right up against the device. The 0.1 μF capacitor, (including the
common ceramic types or surface-mount types) should have
low effective series resistance (ESR) and effective series induc-
tance (ESI). These low ESR and ESI capacitors provide a low
impedance path to ground at high frequencies to handle
transient currents due to logic switching.
To provide optimum shielding for ground planes, a minimum
etch technique is generally best. The two AGND pins of the
AD7356 should be sunk in the AGND plane. The REFGND
pin should also be sunk in the AGND plane. Digital and analog
ground planes should be joined in only one place. If the
AD7356 is in a system in which multiple devices require an
AGND and DGND connection, the connection should still
be made at one point only, a star ground point that should
be established as close as possible to the ground pins on the
AD7356.
EVALUATING THE AD7356 PERFORMANCE
The recommended layout for the AD7356 is outlined in the
evaluation board documentation. The evaluation board package
includes a fully assembled and tested evaluation board, docu-
mentation, and software for controlling the board from the PC
via the converter evaluation and development board (CED).
The CED can be used in conjunction with the AD7356 eval-
uation board (as well as many other evaluation boards ending
in the ED designator from Analog Devices, Inc.) to demonstrate/
evaluate the ac and dc performance of the AD7356.
Avoid running digital lines under the device because this
couples noise onto the die. Allow the analog ground planes
to run under the AD7356 to avoid noise coupling. The power
supply lines to the AD7356 should use as large a trace as possible
to provide low impedance paths and reduce the effects of
glitches on the power supply line.
To avoid radiating noise to other sections of the board, shield
fast switching signals such as clocks, with digital ground; and
never run clock signals near the analog inputs. Avoid crossover
of digital and analog signals. To reduce the effects of feed-
through within the board, traces on opposite sides of the
The software allows the user to perform ac (fast Fourier transform)
and dc (linearity) tests on the AD7356. The software and docu-
mentation are on a CD shipped with the evaluation board.
Rev. 0 | Page 19 of 20
AD7356
OUTLINE DIMENSIONS
5.10
5.00
4.90
16
9
8
4.50
4.40
4.30
6.40
BSC
1
PIN 1
1.20
MAX
0.15
0.05
0.20
0.09
0.75
0.60
0.45
8°
0°
0.30
0.19
0.65
BSC
SEATING
PLANE
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-153-AB
Figure 32. 16-Lead Thin Shrink Small Outline Package [TSSOP]
(RU-16)
Dimensions shown in millimeters
ORDERING GUIDE
Model
Temperature Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +125°C
−40°C to +125°C
−40°C to +125°C
Package Description
16-Lead TSSOP
16-Lead TSSOP
16-Lead TSSOP
16-Lead TSSOP
16-Lead TSSOP
16-Lead TSSOP
Evaluation Board
Package Option
RU-16
RU-16
RU-16
RU-16
AD7356BRUZ1
AD7356BRUZ-500RL71
AD7356BRUZ-RL1
AD7356YRUZ1
AD7356YRUZ-500RL71
AD7356YRUZ-RL1
EVAL-AD7356EDZ1, 2
EVAL-CED1Z1, 3
RU-16
RU-16
Converter Evaluation and Development Board
1 Z = RoHS Compliant Part.
2 This evaluation board can be used as a standalone evaluation board or in conjunction with the EVAL-CED1Z board for evaluation/demonstration purposes.
3 This evaluation board is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the ED designator.
©2008 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D06505-0-10/08(0)
Rev. 0 | Page 20 of 20
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