FEATURES
FUNCTIONAL BLOCK DIAGRAM
VDD
Dual 14-bit SAR ADC
Simultaneous sampling
Throughput rate: 4.2 MSPS per channel
Specified for a VDD of 2.5 V
Power dissipation: 36 mW at 4.2 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
−40°C to +125°C Y grade operation, −40°C to +85°C B grade
operation
16-lead TSSOP and 18-lead LFCSP packages
Qualified for automotive applications
VDRIVE
AD7357
VINA+
14-BIT
SUCCESSIVE
APPROXIMATION
ADC
T/H
VINA–
REFA
BUF
SCLK
CONTROL
LOGIC
REF
CS
BUF
REFB
VINB+
14-BIT
SUCCESSIVE
APPROXIMATION
ADC
T/H
VINB–
APPLICATIONS
AGND
AGND
REFGND
SDATAB
DGND
Figure 1.
Automotive radar
Data acquisition systems
Motion control
I and Q demodulation
RFID readers
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD7357 is a dual, 14-bit, high speed, low power, successive
approximation analog-to-digital converter (ADC) that operates
from a single 2.5 V power supply and features throughput rates
up to 4.2 MSPS. The device contains two ADCs, each preceded
by a low noise, wide bandwidth track-and-hold circuit that can
handle input frequencies in excess of 110 MHz.
1.
1
The conversion process and data acquisition use standard control
inputs allowing for easy interfacing to microprocessors or
digital signal processors (DSPs). The input signal is sampled on
the falling edge of CS; a conversion is also initiated at this point.
The conversion time is determined by the SCLK frequency.
The AD7357 uses advanced design techniques to achieve very
low power dissipation at high throughput rates. With a 2.5 V
supply and a 4.2 MSPS throughput rate, the device consumes
14 mA typically. The device also offers flexible power and
throughput rate management options.
The analog input range for the device is the differential common
mode ±VREF/2. The AD7357 has an on-chip 2.048 V reference
that can be overdriven when an external reference is preferred.
The AD7357 is available in a 16-lead thin shrink small outline
package (TSSOP) and an 18-lead lead frame chip scale package
(LFCSP).
1
SDATAA
07757-001
Data Sheet
Differential Input, Dual, Simultaneous
Sampling, 4.2 MSPS, 14-Bit, SAR ADC
AD7357
2.
3.
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.
High Throughput with Low Power Consumption.
The AD7357 offers a 4.2 MSPS throughput rate with 36 mW
power consumption.
Simultaneous Sampling.
The device features two standard successive approximation
ADCs with accurate control of the sampling instant via a CS
input and once off conversion control.
Table 1. Related Devices
Generic
AD7356
AD7352
AD7266
Resolution
12-bit
12-bit
12-bit
Throughput
5 MSPS
3 MSPS
2 MSPS
AD7866
AD7366
AD7367
12-bit
12-bit
14-bit
1 MSPS
1 MSPS
1 MSPS
Analog Input
Differential
Differential
Differential/singleended
Single-ended
Single-ended bipolar
Single-ended bipolar
Protected by U.S. Patent No. 6,681,332.
Rev. E
Document Feedback
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 registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 ©2009–2015 Analog Devices, Inc. All rights reserved.
Technical Support
www.analog.com
AD7357
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Driving Differential Inputs ....................................................... 16
Applications ....................................................................................... 1
Voltage Reference ....................................................................... 17
Functional Block Diagram .............................................................. 1
ADC Transfer Function ............................................................. 17
General Description ......................................................................... 1
Modes of Operation ....................................................................... 18
Product Highlights ........................................................................... 1
Normal Mode .............................................................................. 18
Revision History ............................................................................... 2
Partial Power-Down Mode ....................................................... 18
Specifications..................................................................................... 3
Full Power-Down Mode ............................................................ 19
Timing Specifications .................................................................. 6
Power-Up Times ......................................................................... 20
Absolute Maximum Ratings ............................................................ 7
Power vs. Throughput Rate ....................................................... 20
ESD Caution .................................................................................. 7
Serial Interface ................................................................................ 21
Pin Configurations and Function Descriptions ........................... 8
Application Suggestions................................................................. 22
Typical Performance Characteristics ........................................... 10
Grounding and Layout .............................................................. 22
Terminology .................................................................................... 12
Evaluating the AD7357 Performance ...................................... 22
Theory of Operation ...................................................................... 14
Outline Dimensions ....................................................................... 23
Circuit Information .................................................................... 14
Ordering Guide .......................................................................... 24
Converter Operation .................................................................. 14
Automotive Products ................................................................. 24
Analog Input Structure .............................................................. 14
Analog Inputs .............................................................................. 16
REVISION HISTORY
12/15—Rev. D to Rev. E
Changes to Figure 3 and Table 5 ..................................................... 8
10/15—Rev. C to Rev. D
Changes to Figure 20 ...................................................................... 16
6/15—Rev. B to Rev. C
Added 18-Lead LFCSP....................................................... Universal
Change to Features Section ............................................................. 1
Changes to Table 2 ............................................................................ 5
Added Figure 3, Renumbered Sequentially .................................. 8
Change to Circuit Information Section ....................................... 13
Updated Outline Dimensions ....................................................... 22
Changes to Ordering Guide .......................................................... 22
2/11—Rev. 0 to Rev. A
Changes to Features and Applications Sections ............................1
Changes to Table 2.............................................................................3
Added AD7357WY Temperature Range to Endnote 1 in Table 3....5
Changes to SDATAB, SDATAA Pin Description ............................7
Changes to Figure 20 and Figure 22 ............................................ 14
Changes to Figure 31...................................................................... 18
Changes to Ordering Guide .......................................................... 20
Added Automotive Products Section .......................................... 20
4/09—Revision 0: Initial Version
8/11—Rev. A to Rev. B
Changes to Midscale Error Match Parameter, Table 2 ................ 3
Changes to Figure 21 and Figure 22............................................. 14
Added Voltage Reference Section ......................................................... 14
Rev. E | Page 2 of 24
Data Sheet
AD7357
SPECIFICATIONS
VDD = 2.5 ± 10% V, VDRIVE = 2.25 V to 3.6 V, internal reference = 2.048 V, fSCLK = 80 MHz, fSAMPLE = 4.2 MSPS, TA = TMIN to TMAX1, unless
otherwise noted.
Table 2.
Parameter
DYNAMIC PERFORMANCE
Signal-to-Noise Ratio (SNR)
Signal-to-Noise and Distortion
(SINAD)2
Min
AD7357B/AD7357Y
Typ
Max
74.5
76.5
74
76
Min
74.5
74
74
AD7357WY
Typ
Max
76.5
76
73.5
Total Harmonic Distortion (THD)2
−83
−80
−83
Spurious Free Dynamic Range
(SFDR)
−85
−82
−85
−80
−79
−82
−81
Unit
Test Conditions/
Comments
fIN = 500 kHz sine wave
dB
dB
dB
−40°C to +25°C only
dB
dB
−40°C to +25°C only
dB
dB
Intermodulation Distortion (IMD)2
Second-Order Terms
Third-Order Terms
ADC to ADC Isolation2
CMRR2
SAMPLE-AND-HOLD
Aperture Delay
Aperture Delay Match
Aperture Jitter
Full Power Bandwidth
At 3 dB
At 0.1 dB
DC ACCURACY
Resolution
Integral Nonlinearity (INL)2
Differential Nonlinearity (DNL)2
−86
−79
−100
−86
−79
−100
dB
dB
dB
−100
−100
dB
3.5
40
16
16
ns
ps
ps
110
77
110
77
MHz
MHz
14
14
±2
±0.5
Positive Full-Scale Error2
Positive Full-Scale Error Match2
Midscale Error2
Midscale Error Match2
Negative Full-Scale Error2
Negative Full-Scale Error Match2
ANALOG INPUT
Fully Differential Input Range
(VIN+ and VIN−)
Common-Mode Voltage Range
DC Leakage Current
Input Capacitance
3.5
40
0.5
±3
±0.99
Bits
LSB
LSB
±20
±20
0/35
±12
±20
±20
±20
±20
0/38
±15
±20
±20
LSB
LSB
LSB
LSB
LSB
LSB
VCM ±
VREF/2
VCM ±
VREF/2
V
1.6
V
±5
μA
pF
pF
±3
±0.99
1.6
±0.5
32
8
±2
±0.5
0.5
±5
Rev. E | Page 3 of 24
±0.5
32
8
−40°C to +25°C only
−40°C to +25°C only
fa = 1 MHz + 50 kHz,
fb = 1 MHz − 50 kHz
fIN = 1 MHz, fNOISE =
100 kHz to 2.5 MHz
fNOISE = 100 kHz to 2.5 MHz
At 0.1 dB
Guaranteed no missed
codes to 14 bits
VCM = common-mode
voltage; VIN+ and VIN− must
remain within GND
and VDD
The voltage around
which VIN+ and VIN− are
centered
When in track mode
When in hold mode
AD7357
Parameter
REFERENCE INPUT/OUTPUT
VREF Input Voltage Range
Data Sheet
Min
AD7357B/AD7357Y
Typ
Max
2.048 + 0.1
VREF Input Current
VREF Output Voltage
VREF Temperature Coefficient
VREF Long Term Stability
VREF Thermal Hysteresis
VREF Noise
VREF Output Impedance
LOGIC INPUTS
Input Voltage
High, VINH
Low, VINL
Input Current, IIN
Input Capacitance, CIN
LOGIC OUTPUTS
Output Voltage
High, VOH
Low, VOL
Floating State Leakage Current
Floating State Output
Capacitance
Output Coding
CONVERSION RATE
Conversion Time
Track-and-Hold Acquisition Time2
Throughput Rate
POWER REQUIREMENTS
VDD
VDRIVE3
ITOTAL4
Normal Mode
Operational
Static
Power-Down Mode
Partial
Full
VDD
0.3
AD7357WY
Typ
Max
Min
2.048 +
0.1
0.45
0.3
Unit
VDD
V
0.45
mA
2.038
2.058
2.038
2.058
V
2.043
2.053
2.043
2.053
V
20
ppm/°C
ppm
ppm
μV rms
Ω
6
100
50
60
1
20
0.6 × VDRIVE
6
100
50
60
1
0.6 × VDRIVE
0.3 × VDRIVE
0.3 ×
VDRIVE
±1
±1
3
3
VDRIVE − 0.2
VDRIVE − 0.2
0.2
±1
0.2
±1
5.5
5.5
Straight binary
When in reference
overdrive mode
±2.048 V ± 0.5%
maximum at
VDD = 2.5 V ± 5%
2.048 V ± 0.25%
maximum at
VDD = 2.5 V ± 5% and 25°C
For 1000 hours
V
V
μA
pF
VIN = 0 V or VDRIVE
V
V
μA
pF
Straight binary
t2 + 15.5 ×
tSCLK
ns
t2 + 15.5 ×
tSCLK
33
4.2
2.25
2.25
Test Conditions/
Comments
2.75
3.6
2.25
2.25
33
4.2
ns
MSPS
Full-scale step input
2.75
3.6
V
V
Nominal VDD = 2.5 V
Digital inputs = 0 V or
VDRIVE
14
6
20
7.6
14
6
20
7.6
mA
mA
SCLK on or off
3.5
5
4.5
40
3.5
5
4.5
40
mA
μA
SCLK on or off
SCLK on or off
Rev. E | Page 4 of 24
Data Sheet
Parameter
Power Dissipation
Normal Mode
Operational
Static
Power-Down Mode
Partial
Full
AD7357
Min
AD7357B/AD7357Y
Typ
Max
Min
AD7357WY
Typ
Max
Unit
Test Conditions/
Comments
36
16
59
21
36
16
59
21
mW
mW
SCLK on or off
9.5
16
11.5
110
9.5
16
11.5
110
mW
µW
SCLK on or off
SCLK on or off
Temperature ranges are as follows: AD7357Y: −40°C to +125°C; AD7357B (TSSOP): −40°C to +85°C; AD7357B (LFCSP): −40°C to +125°C; AD7357WY: −40°C to +125°C.
See the Terminology section.
3
The interface is functional with VDRIVE voltages down to 1.8 V. In this condition, the SCLK speed may need to be slowed down. See the access and hold times in the
Timing Specifications section.
4
ITOTAL is the total current flowing in VDD and VDRIVE.
1
2
Rev. E | Page 5 of 24
AD7357
Data Sheet
TIMING SPECIFICATIONS
VDD = 2.5 V ± 10%, VDRIVE = 2.25 V to 3.6 V, internal reference = 2.048 V, TA = TMAX to TMIN 1, unless otherwise noted.
Table 3.
Parameter
fSCLK
tCONVERT
tQUIET
t2
t3 2
t42, 3
t5
t6
t7 2
t8
t9
t102
Latency
Limit at TMIN , TMAX
500
80
t2 + 15.5 × tSCLK
5
5
6
Unit
kHz min
MHz max
ns min
ns min
ns min
ns max
12.5
11
9.5
9
5
5
ns max
ns max
ns max
ns max
ns min
ns min
3.5
3
9.5
5
4.5
9.5
ns min
ns min
ns max
ns min
ns min
ns max
1 conversion latency
Description
tSCLK = 1/fSCLK
Minimum time between end of serial read and next falling edge of CS
CS to SCLK setup time
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
1.8 V ≤ VDRIVE < 2.75 V
2.75 V ≤ VDRIVE ≤ 3.6 V
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
Temperature ranges are as follows: AD7357Y: −40°C to +125°C; AD7357B (TSSOP): −40°C to +85°C; AD7357B (LFCSP): −40°C to +125°C; AD7357WY: −40°C to +125°C.
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.
1
2
Rev. E | Page 6 of 24
Data Sheet
AD7357
ABSOLUTE MAXIMUM RATINGS
Table 4.
Parameter
VDD to AGND, DGND, REFGND
VDRIVE to AGND, DGND, REFGND
VDD to VDRIVE
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 Supplies4
Operating Temperature Range
AD7357Y
AD7357B (TSSOP)
AD7357B (LFCSP)
AD7357WY
Storage Temperature Range
Junction Temperature
TSSOP Package
θJA Thermal Impedance
θJC Thermal Impedance
LFCSP Package
θJA Thermal Impedance
θJC Thermal Impedance
Lead Temperature, Soldering
Reflow Temperature (10 sec to 30 sec)
ESD
Rating
−0.3 V to +3 V
−0.3 V to +5 V
−5 V to +3 V
−0.3 V to +0.3 V
−0.3 V to VDD + 0.3 V
−0.3 V to VDRIVE + 0.3V
−0.3 V to VDRIVE + 0.3 V
±10 mA
Stresses at or above those listed under Absolute Maximum
Ratings may cause permanent damage to the product. This is a
stress rating only; functional operation of the product at these
or any other conditions above those indicated in the operational
section of this specification is not implied. Operation beyond
the maximum operating conditions for extended periods may
affect product reliability.
ESD CAUTION
−40°C to +125°C
−40°C to +85°C
−40°C to +125°C
−40°C to +125°C
−65°C to +150°C
150°C
143°C/W
45°C/W
44°C/W
22°C/W
255°C
2 kV
1
Analog input voltages are VINA+, VINA−, VINB+, VINB−, REFA, and REFB.
Digital input voltages are CS and SCLK.
3
Digital output voltages are SDATAA and SDATAB.
4
Transient currents of up to 100 mA do not cause SCR latch-up.
2
Rev. E | Page 7 of 24
AD7357
Data Sheet
14 VINA–
16 REFGND
15 REFA
17 AGND
18 REFB
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
13 VINA+
VINB– 1
AD7357
VINB+ 2
13
SDATAB
AGND 5
12
DGND
REFB 6
11
AGND
VINB–
7
10
CS
VINB+
8
9
VDD
NOTES
1. NIC = NO INTERNAL CONNECTION.
2. THE EXPOSED PAD IS NOT CONNECTED INTERNALLY.
FOR INCREASED RELIABILITY OF THE SOLDER
JOINTS AND FOR MAXIMUM THERMAL CAPABILITY,
SOLDER THE EXPOSED PAD TO THE PRINTED
CIRCUIT BOARD (PCB).
Figure 2. Pin Configuration, TSSOP
07757-103
SDATAA
TOP VIEW
(Not to Scale)
SDATAA 9
AD7357
REFGND 4
SDATAB 8
SCLK
DGND 7
15
14
VINA–
11 VDRIVE
10 SCLK
VDD 4
07757-002
2
REFA 3
TOP VIEW
(Not to Scale)
NIC 3
VDRIVE
CS 5
16
AGND 6
VINA+ 1
12 NIC
Figure 3. Pin Configuration, LFCSP
Table 5. Pin Function Descriptions
TSSOP
1, 2
3, 6
Pin No.
LFCSP
13, 14
15, 18
Mnemonic
VINA+, VINA−
REFA, REFB
4
16
REFGND
5, 11
6, 17
AGND
7, 8
9
1, 2
4
VINB−, VINB+
VDD
10
5
CS
12
7
DGND
13, 14
8, 9
SDATAB,
SDATAA
15
10
SCLK
Description
Analog Inputs of ADC A. These analog inputs form a fully differential pair.
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 μF capacitor. Provided that the
output is buffered, take the on-chip reference from these pins and apply it 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.
Reference Ground. This is the ground reference point for the reference circuitry on the
AD7357. Refer any external reference signal to this REFGND voltage. Decoupling capacitors
must be placed between this pin and the REFA and REFB pins.
Analog Ground. This is the ground reference point for all analog circuitry on the AD7357.
Refer all analog input signals to this AGND voltage. The AGND and DGND voltages must
ideally be at the same potential and must not be more than 0.3 V apart, even on a transient basis.
Analog Inputs of ADC B. These analog inputs form a fully differential pair.
Power Supply Input. The VDD range for the AD7357 is 2.5 V ± 10%. Decouple the supply to
AGND with a 0.1 μF capacitor and a 10 μF tantalum capacitor.
Chip Select. Active low, logic input. This input provides the dual function of initiating
conversions on the AD7357 and framing the serial data transfer.
Digital Ground. This is the ground reference point for all digital circuitry on the AD7357.
Connect this pin to the DGND plane of a system. The DGND and AGND voltages must ideally
be at the same potential and must not be more than 0.3 V apart, even on a transient basis.
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. 16 SCLK falling edges are required to
access the 14 bits of data from the AD7357. 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 14 bits of conversion data. The data is provided MSB first. If CS
is held low for 18 SCLK cycles rather than 16, then two trailing zeros appear after the 14 bits of
data. If CS is held low for an additional 18 SCLK cycles on either SDATAA or SDATAB , the data
from the other ADC follows on the SDATAx pins. This allows data from a simultaneous
conversion on both ADCs to gather in serial format on either SDATAA or SDATAB.
Serial Clock. Logic input. A serial clock input provides the SCLK for accessing the data from
the AD7357. This clock is also used as the clock source for the conversion process.
Rev. E | Page 8 of 24
Data Sheet
TSSOP
16
Pin No.
LFCSP
11
Not applicable
Not applicable
3, 12
AD7357
Mnemonic
VDRIVE
NIC
EPAD
Description
Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the
interface operates. This pin is decoupled to DGND. The voltage at this pin may be different
than at VDD.
No Internal Connection. These pins are not connected internally.
Exposed Pad. The exposed pad is not connected internally. For increased reliability of the
solder joints and for maximum thermal capability, solder the exposed pad to the printed
circuit board (PCB).
Rev. E | Page 9 of 24
AD7357
Data Sheet
TYPICAL PERFORMANCE CHARACTERISTICS
0
20,000
–20
–40
NUMBER OF OCCURRENCES
16,384 POINT FFT
fSAMPLE = 4.2MSPS
fIN = 1MHz
SINAD = 76.8dB
THD = –84.5dB
–80
–100
–120
10,000
5000
–140
0
250
500
750 1000 1250 1500
FREQUENCY (kHz)
1750
2000
07757-003
32 HITS
07757-006
dB
–60
15,000
0
8188
Figure 4. Typical FFT
8189
8190
8191
8192
CODE
8193
8194
8195
Figure 7. Histogram of Codes
1.0
79
0.8
77
75
0.4
0.2
SNR (dB)
DNL ERROR (LSB)
0.6
0
–0.2
–0.4
73
71
69
–0.6
0
4000
8000
12,000
16,000
CODE
65
07757-004
–1.0
07757-007
67
–0.8
0
Figure 5. Typical DNL
2
3
4
ANALOG INPUT FREQUENCY (MHz)
5
Figure 8. SNR vs. Analog Input Frequency
1.5
–60
1.0
–65
PSSR (dB)
0.5
0
–70
–75
–0.5
–1.5
0
4000
8000
CODE
Figure 6. Typical INL
12,000
16,000
07757-008
–80
–1.0
–85
07757-005
INL ERROR (LSB)
1
0
5
10
15
20
SUPPLY RIPPLE FREQUENCY (MHz)
25
Figure 9. PSRR vs. Supply Ripple Frequency with No Supply Decoupling
Rev. E | Page 10 of 24
Data Sheet
AD7357
2.0482
10
2.0480
2.0478
+125°C
+85°C
+25°C
–40°C
9
ACCESS TIME (ns)
2.0476
VREF (V)
2.0474
2.0472
2.0470
2.0468
8
7
2.0466
07757-009
2.0462
2.0460
0
500
1000
1500
2000
CURRENT LOAD (µA)
2500
5
3000
07757-012
6
2.0464
1.8
2.0
Figure 10. VREF vs. Reference Output Current Drive
3.0
3.2
INL MAX
3.4
3.6
+125°C
+85°C
+25°C
–40°C
7
0.5
HOLD TIME (ns)
1.0
DNL MAX
0
DNL MIN
–0.5
6
5
–1.0
0
10
20
30
40
50
60
SCLK FREQUENCY (MHz)
07757-010
INL MIN
70
4
80
Figure 11. Linearity Error vs. SCLK Frequency
INL MAX
1.0
DNL MAX
0
DNL MIN
–0.5
INL MIN
–1.5
2.10
07757-011
–1.0
2.15
2.20
2.25
2.30
2.35
EXTERNAL VREF (V)
2.40
1.8
2.0
2.2
2.4
2.6
2.8
VDRIVE (V)
3.0
Figure 14. Hold Time vs. VDRIVE
1.5
0.5
07757-113
ERROR (LSB)
2.6
2.8
VDRIVE (V)
8
1.5
ERROR (LSB)
2.4
Figure 13. Access Time vs. VDRIVE
2.0
–1.5
2.2
2.45
2.50
Figure 12. Linearity Error vs. External VREF
Rev. E | Page 11 of 24
3.2
3.4
3.6
AD7357
Data Sheet
TERMINOLOGY
Integral Nonlinearity (INL)
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).
Common-Mode Rejection Ratio (CMRR)
CMRR is 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 VIN+ and VIN− of frequency, fS, as
follows:
Differential Nonlinearity (DNL)
DNL is the difference between the measured and the ideal
1 LSB change between any two adjacent codes in the ADC.
where:
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
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.
Track-and-Hold Acquisition Time
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 ±1 LSB, after the end of conversion.
Negative Full-Scale Error Match
Negative full-scale error match is the difference in negative fullscale error between the two ADCs.
Midscale Error
Midscale error is the deviation of the midscale code transition
(011 … 111) to (100 … 000) from the ideal (that is, 0 V).
Midscale Error Match
Midscale error match is the difference in midscale error
between the two ADCs.
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, VREF − 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 fullscale error between the two ADCs.
ADC to ADC Isolation
ADC to ADC isolation is a measure of the level of crosstalk
between ADC A and ADC B. It is measured by applying a fullscale 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 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.
Power Supply Rejection Ratio (PSRR)
PSRR is 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 the frequency, fS. The frequency of the
input varies from 5 kHz to 25 MHz.
CMRR (dB) = 10 log(Pf/PfS)
Signal-to-(Noise + Distortion) Ratio (SINAD)
SINAD is the measured ratio of signal-to-(noise + 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 digitization
process; the more levels, the smaller the quantization noise.
The theoretical SINAD for an ideal N-bit converter with a sine
wave input is given by
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.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the fundamental.
For the AD7357, it is defined as
THD dB 20 log
V 2 2 V 3 2 V 4 2 V 5 2 V6 2
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.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise 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.
PSRR (dB) = 10 log(Pf/PfS)
where:
Pf is the power at frequency, f, in the ADC output.
PfS is the power at frequency, fS, in the ADC output.
Rev. E | Page 12 of 24
Data Sheet
AD7357
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 are 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).
The AD7357 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, while 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 (see Table 2), 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 (dB).
Thermal Hysteresis
Thermal hysteresis is the absolute maximum change of the
reference output voltage after the device is cycled through
temperature from either
T_HYS+ = 25°C to TMAX to 25°C
T_HYS− = 25°C to TMIN to 25°C
It is expressed in ppm using the following equation:
VHYS (ppm) =
VREF (25°C) − VREF (T _ HYS)
× 106
VREF (25°C)
where:
VREF(25°C) is VREF at 25°C.
VREF(T_HYS) is the maximum change of VREF at T_HYS+
or T_HYS−.
Rev. E | Page 13 of 24
AD7357
Data Sheet
THEORY OF OPERATION
The AD7357 is a high speed, dual, 14-bit, single-supply, successive
approximation ADC. The device operates from a 2.5 V power
supply and features throughput rates up to 4.2 MSPS.
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 have different settling
times, resulting in errors.
The AD7357 contains two on-chip differential track-and-hold
amplifiers, two successive approximation ADCs, and a serial
interface with two separate data output pins. The device is housed
in a 16-lead TSSOP package or an 18-lead LFCSP package, offering
the user considerable space-saving advantages over alternative
solutions.
The serial clock input accesses data from the device, but also
provides the clock source for each successive approximation ADC.
The AD7357 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,
the reference output needs to be buffered first. The differential
analog input range for the AD7357 is VCM ± VREF/2.
The AD7357 features power-down options to allow power saving
between conversions. The power-down options are implemented
via the standard serial interface, as described in the Modes of
Operation section.
CONVERTER OPERATION
The AD7357 has two successive approximation ADCs, each based
around two capacitive DACs. Figure 15 and Figure 16 show
simplified schematics of one of these ADCs in acquisition and
conversion phases, respectively. The ADC comprises control logic,
an SAR, and two capacitive DACs. In Figure 15 (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 may acquire the differential signal on the input.
CAPACITIVE
DAC
VIN–
COMPARATOR
CS
B
VIN+
A SW1
A
SW2
CONTROL
LOGIC
SW3
CS
B
VREF
CAPACITIVE
DAC
07757-014
CIRCUIT INFORMATION
Figure 16. ADC Conversion Phase
ANALOG INPUT STRUCTURE
Figure 17 shows the equivalent circuit of the analog input structure of the AD7357. The four diodes provide ESD protection for
the analog inputs. Take care to ensure that the analog input signals
never exceed the supply rails by more than 300 mV. Exceeding the
limit 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 device.
The C1 capacitors in Figure 17 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 ADC sampling capacitors with a capacitance of 32 pF
typically.
VDD
D
VIN+
C1
R1 C2
D
CAPACITIVE
DAC
CS
B
VIN+
VIN–
COMPARATOR
VDD
A SW1
A
SW2
CS
CONTROL
LOGIC
SW3
D
VIN–
C1
CAPACITIVE
DAC
07757-013
VREF
D
07757-015
B
R1 C2
Figure 17. Equivalent Analog Input Circuit
Conversion Phase—Switches Open, Track Phase—Switches Closed
Figure 15. ADC Acquisition Phase
When the ADC starts a conversion (see Figure 16), SW3 opens
and SW1 and SW2 move to Position B, causing the comparator
to become unbalanced. Both inputs are disconnected when the
conversion begins. The control logic and charge redistribution
DACs 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.
Rev. E | Page 14 of 24
Data Sheet
AD7357
–66.0
–70.0
–74.0
–65
–67
–69
–71
THD (dB)
–73
–75
–77
–79
–81
–83
07757-017
–85
–89
100
200
1000
1500
2000
ANALOG INPUT FREQUENCY (kHz)
–82.0
–86.0
–90.0
0
1000
2000
3000
4000
ANALOG INPUT FREQUENCY (kHz)
Figure 19. THD vs. Analog Input Frequency
10Ω
33Ω
50Ω
100Ω
–87
–78.0
07757–118
When no amplifier drives the analog input, limit the source
impedance to low values. The maximum source impedance
depends on the amount of THD that can be tolerated. The THD
increases as the source impedance increases and performance
degrades. Figure 18 shows a graph of the THD vs. the analog
input signal frequency for various source impedances.
Figure 19 shows a graph of the THD vs. the analog input
frequency while sampling at 4.2 MSPS. In this case, the source
impedance is 33 Ω.
THD (dB)
For ac applications, it is recommended to remove high frequency
components from the analog input signal 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 must be driven from a low impedance source. Large
source impedances significantly affect the ac performance of the
ADC and may necessitate the use of an input buffer amplifier. The
choice of the operational amplifier is a function of the particular
application.
2500
Figure 18. THD vs. Analog Input Frequency for Various Source Impedances
Rev. E | Page 15 of 24
5000
AD7357
Data Sheet
ANALOG INPUTS
DRIVING DIFFERENTIAL INPUTS
Differential signals have some benefits over single-ended signals,
including noise immunity based on the common-mode rejection
of the device and improvements in distortion performance.
Figure 20 defines the fully differential input of the AD7357.
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 singleended to differential conversion.
VREF p-p
Differential Amplifier
AD7357*
VREF p-p
An ideal method of applying differential drive to the AD7357 is
to use a differential amplifier such as the AD8138. This device
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 21 shows how the AD8138
is 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.
VIN–
07757-034
PINS OMITTED FOR CLARITY.
Figure 20. 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− are simultaneously driven by
two signals each of amplitude VREF that are 180° out of phase.
This amplitude of the differential signal is, therefore, −VREF to
+VREF peak-to-peak regardless of the common mode (CM).
The architecture of the AD8138 results in outputs that are very
highly balanced over a wide frequency range without requiring
tightly matched external components.
CM is the average of the two signals and is, therefore, the voltage
on which the two inputs are centered.
If the source for the analog inputs used has zero impedance, all
four resistors (RG1, RG2, RF1, and RF2) must be the same. If the
source has a 50 Ω impedance and a 50 Ω termination, for example,
increase the value of RG2 by 25 Ω to balance this parallel impedance
on the input and thus ensures 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.
CM = (VIN+ + VIN−)/2
This results in the span of each input being CM ± VREF/2. This
voltage must be set up externally. When setting up the CM,
ensure that that VIN+ and VIN− remain within GND/VDD. When a
conversion occurs, CM is rejected, resulting in a virtually noise
free signal of amplitude −VREF to +VREF corresponding to the digital
codes of 0 to 16,383.
CF1
2.048V
1.024V
0V
RF1
RG1
VOCM
+2.048V
GND
–2.048V
RG2
+1.024V
+5V
AD8138
–5V
RF2
CF2
10kΩ
10µF
RS*
+2.5V +2.5V TO +3.6V
VINx+
VDD
VDRIVE
AD7357
RS*
VINx–
REFA/REFB
2.048V
1.024V
0V
AGND AGND
+5V
OP177
+2.048V
10µF
10kΩ
–5V
*MOUNT AS CLOSE TO THE AD7352 AS POSSIBLE.
RS = 33Ω; RG1 = RG2 = RF1 = RF2 = 499Ω; C F1 = CF2 = 39pF.
Figure 21. Using the AD8138 as a Single-Ended to Differential Amplifier
Rev. E | Page 16 of 24
07757-131
COMMON
MODE
VOLTAGE
*ADDITIONAL
VIN+
Data Sheet
AD7357
Operational Amplifier Pair
VOLTAGE REFERENCE
An operational amplifier pair is used to directly couple a
differential signal to one of the analog input pairs of the AD7357.
The circuit configurations shown in Figure 22 and Figure 23
show how an operational amplifier pair can be used to convert a
single-ended signal into a differential signal for a bipolar and
unipolar input signal, respectively. The voltage applied to Point A
sets up the common-mode voltage. In both diagrams, Point A is
connected in some way to the reference. The AD8022 is a suitable
dual operational amplifier that is used in this configuration to
provide differential drive to the AD7357.
The AD7357 allows the choice of a very low temperature drift
internal voltage reference or an external reference. The internal
2.048 V reference of the AD7357 provides excellent performance
and can be used in almost all applications.
VREF p-p
VREF
2
220Ω
220Ω
V+
27Ω
When the internal reference is used, the reference voltage is
present on the REFA and REFB pins. Decouple these pins to
REFGND with 10 μF capacitors. The internal reference voltage
can be used elsewhere in the system, provided it is buffered
externally.
The REFA and REFB pins can also be overdriven with an external
voltage reference if desired. The applied reference voltage can
range from 2.048 V + 100 mV to VDD. A common choice is to
use an external 2.5 V reference such as the ADR441 or ADR431.
2.048V
1.024V
0V
VIN+
GND
V–
220Ω
220Ω
2.048V
1.024V
0V
V+
A
V–
27Ω
ADC TRANSFER FUNCTION
AD7357*
VIN–
The output coding for the AD7357 is straight binary. The designed
code transitions occur at successive LSB values (such as 1 LSB
or 2 LSB). The LSB size is (2 × VREF)/16,384. The ideal transfer
characteristic of the AD7357 is shown in Figure 24.
REFA/REFB
10kΩ
10µF
07757-132
10kΩ
*ADDITIONAL PINS OMITTED FOR CLARITY.
440Ω
GND
220Ω
V+
27Ω
2.048V
1.024V
0V
VIN+
V–
220Ω
220Ω
V+
A
V–
27Ω
000 ... 010
000 ... 001
000 ... 000
AD7357*
–VREF + 1 LSB
VIN–
–VREF + 0.5 LSB
REFA/REFB
10kΩ
+VREF – 1 LSB
+VREF – 1.5 LSB
ANALOG INPUT
20kΩ
Figure 24. Deal Transfer Characteristic
10µF
07757-133
220Ω
2.048V
1.024V
0V
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 23. Dual Operational Amplifier Circuit to Convert a Single-Ended
Bipolar Signal into a Differential Unipolar Signal
Rev. E | Page 17 of 24
07757-023
2 × VREF p-p
ADC CODE
Figure 22. Dual Operational Amplifier Circuit to Convert a Single-Ended
Unipolar Signal into a Differential Signal
111 ... 111
111 ... 110
111 ... 101
AD7357
Data Sheet
MODES OF OPERATION
The AD7357 mode of operation is selected by controlling the
logic state of the CS signal during a conversion. There are three
possible modes of operation: normal mode, partial power-down
mode, and full power-down mode. After a conversion is initiated,
the point at which CS is pulled high determines which powerdown mode, if any, the device enters. Similarly, if already in a
power-down mode, CS 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 and throughput rate ratio for the differing
application requirements.
NORMAL MODE
Normal mode is intended for applications needing the fastest
throughput rates. The user does not need to worry about any
power-up times because the AD7357 remains fully powered at
all times. Figure 25 shows the general diagram of the operation
of the AD7357 in this mode.
CS
10
14
LEADING ZEROS + CONVERSION RESULT
07757-018
SDATAA
SDATAB
PARTIAL POWER-DOWN MODE
This mode is intended for use in applications where 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 for a relatively long duration between these
bursts of several conversions. When the AD7357 is in partial
power-down mode, all analog circuitry is powered down except
for the on-chip reference and reference buffers.
To enter partial power-down mode, interrupt the conversion
process by bringing CS high anywhere after the second falling
edge of SCLK and before the 10th falling edge of SCLK, as shown in
Figure 26. When CS is brought high in this window of SCLKs,
the device enters partial power-down mode, the conversion that
was initiated by the falling edge of CS is terminated, and SDATAA
and SDATAB go back into three-state. If CS is brought high before
the second SCLK falling edge, the device remains in normal mode
and does not power down. This avoids accidental power-down
due to glitches on the CS line.
Figure 25. Normal Mode Operation
CS
The conversion is initiated on the falling edge of CS, as described
in the Serial Interface section. To ensure that the device remains
fully powered up at all times, CS must remain low until at least
10 SCLK falling edges have elapsed after the falling edge of CS.
If CS is brought high any time after the 10th SCLK falling edge
but before the 16th SCLK falling edge, the device remains powered
up, but the conversion is terminated and SDATAA and SDATAB
go back into three-state. To complete the conversion and access
the conversion result for the AD7357, 16 serial clock cycles are
required. SDATA lines do not return to three-state after 16 SCLK
cycles have elapsed, but instead do so when CS is brought high
again. If CS is left low for another 2 SCLK cycles, two trailing
zeros are clocked out after the data. If CS is left low for a further
16 SCLK cycles, the result for the other ADC on board is also
accessed on the same SDATA line as shown in Figure 32 (see
the Serial Interface section).
When 32 SCLK cycles have elapsed, the SDATA line returns to
three-state on the 32nd SCLK falling edge. If CS is brought high
prior to this, the SDATA line returns to three-state at that point.
Thus, CS may idle low after 32 SCLK cycles until it is brought
high again sometime prior to the next conversion, if so desired,
because the bus still returns to three-state upon completion of
the dual result read.
1
2
10
14
SCLK
SDATAA
SDATAB
THREE-STATE
07757-019
1
SCLK
When a data transfer is complete and SDATAA and SDATAB have
returned to three-state, another conversion can be initiated after
the quiet time, tQUIET, has elapsed by bringing CS low again
(assuming the required acquisition time has been allowed).
Figure 26. Entering Partial Power-Down Mode
To exit this mode of operation and to power up the AD7357 again,
perform a dummy conversion. On the falling edge of CS, the device
begins to power up and continues to power up as long as CS is
held low until after the falling edge of the 10th SCLK. The device
is fully powered up after approximately 200 ns elapses (or one
full conversion), and valid data results from the next conversion, as
shown in Figure 27. If CS is brought high before the second falling
edge of SCLK, the AD7357 again goes into partial power-down
mode. This avoids accidental power-up due to glitches on the
CS line. Although the device may begin to power up on the falling
edge of CS, it powers down again on the rising edge of CS. If the
AD7357 is already in partial power-down mode and CS is brought
high between the second and 10th falling edges of SCLK, the device
enters full power-down mode.
Rev. E | Page 18 of 24
Data Sheet
AD7357
To reach full power-down, the next conversion cycle must be
interrupted in the same way, as shown in Figure 28. When CS
has been brought high in this window of SCLKs, the device
completely powers down.
FULL POWER-DOWN MODE
This mode is intended for use in applications where throughput
rates slower than those in the partial power-down mode are
required, as power-up from a full power-down takes substantially
longer than from a partial power-down. This mode is more
suited to applications where 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 AD7357 is in full
power-down, all analog circuitry is powered down. Full powerdown is entered in a way that is similar to partial power-down,
except that the timing sequence shown in Figure 26 must be
executed twice. The conversion process must be interrupted in a
similar fashion by bringing CS 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.
Note that it is not necessary to complete the 16 SCLK pulses
after CS has been brought high to enter a power-down mode.
To exit full power-down mode and power up the AD7357, perform
a dummy conversion, such as powering up from partial powerdown. On the falling edge of CS, the device begins to power up, as
long as CS 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 29.
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
SDATAA
SDATAB
INVALID DATA
07757-020
SCLK
VALID DATA
Figure 27. Exiting Partial Power-Down Mode
THE PART ENTERS
PARTIAL POWER DOWN.
THE PART BEGINS
TO POWER UP.
THE PART ENTERS
FULL POWER DOWN.
CS
1
2
10
14
1
2
10
THREE-STATE
SDATAA
SDATAB
14
THREE-STATE
INVALID DATA
INVALID DATA
07757-021
SCLK
Figure 28. Entering Full Power-Down Mode
THE PART BEGINS
TO POWER UP.
THE PART IS FULLY POWERED UP,
SEE POWER-UP TIMES SECTION.
tPOWER-UP2
CS
SDATAA
SDATAB
10
1
14
INVALID DATA
14
1
VALID DATA
Figure 29. Exiting Full Power-Down Mode
Rev. E | Page 19 of 24
07757-022
SCLK
AD7357
Data Sheet
To power up from partial power-down mode, one dummy cycle
is required. The device is fully powered up after approximately
200 ns from the falling edge of CS has elapsed. As soon as 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 falling
edge of CS.
To power up from full power-down, allow approximately 6 ms
from the falling edge of CS, shown in Figure 29 as tPOWER-UP2.
Note that during power-up from partial power-down mode, the
track-and-hold, which is in hold mode while the device is powered
down, returns to track mode after the first SCLK edge that the
device receives after the falling edge of CS.
POWER vs. THROUGHPUT RATE
The power consumption of the AD7357 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 AD7357 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 if it is scaled with the sampling rate.
Figure 30 shows a plot of power vs. throughput rate when operating
in normal mode for a fixed maximum SCLK frequency and an
SCLK frequency that scales with the sampling rate. The internal
reference is used for Figure 30.
When power supplies are first applied to the AD7357, the ADC
powers 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 device is fully powered up before attempting a valid
conversion. Likewise, if the device is kept in partial power-down
mode immediately after the supplies are applied, then initiate
two dummy cycles. The first dummy cycle must hold CS low until
after the 10th SCLK falling edge; in the second cycle, CS must be
brought high between the second and 10th SCLK falling edges
(see Figure 26).
38
34
30
26
80MHz SCLK
22
VARIABLE SCLK
18
14
10
07757-129
The AD7357 has two power-down modes: partial power-down
and full power-down. They are described in detail in the Partial
Power-Down Mode section and the Full Power-Down Mode
section. This section describes the power-up time required when
coming out of either of these modes. Note that the power-up
times apply with the recommended decoupling capacitors in place
on the REFA and REFB pins.
Alternatively, if the device is placed into full power-down mode
when the supplies are applied, three dummy cycles must be
initiated. The first dummy cycle must hold CS low until after
the 10th SCLK falling edge; the second and third dummy cycles
place the device into full power-down mode (see Figure 28 and
the Modes of Operation section).
TOTAL POWER (mW)
POWER-UP TIMES
0
1000
2000
3000
THROUGHPUT RATE (kSPS)
Figure 30. Total Power vs. Throughput Rate
Rev. E | Page 20 of 24
4000
Data Sheet
AD7357
SERIAL INTERFACE
Figure 31 shows the detailed timing diagram for serial interfacing
to the AD7357. The serial clock (SCLK) provides the conversion
clock and controls the transfer of information from the AD7357
during conversion. There is a single sample delay in the result
that is clocked out from the AD7357.
Likewise, if CS is held low for an additional 16 SCLK cycles on
SDATAA, the data from the conversion on ADC A is output on
SDATAB (see Figure 32). In this case, the SDATA line in use
goes back into three-state on the 32nd SCLK falling edge or the
rising edge of CS, whichever occurs first.
The CS signal initiates the data transfer and conversion process.
The falling edge of CS puts the track-and-hold into hold mode.
At this point, the analog input is sampled and the bus is taken
out of three-state. The conversion is also initiated and requires a
minimum of 16 SCLKs to complete. When 16 SCLK falling
edges have elapsed, the track-and-hold goes back into track on
the next SCLK rising edge, as shown in Figure 31 at Point B. On
the rising edge of CS, the conversion is terminated and SDATAA
and SDATAB go back into three-state. If CS is not brought high but
is, instead, held low for an additional 16 SCLK cycles on SDATAA,
the data from the conversion on ADC B is output on SDATAA.
A minimum of 16 SCLK cycles are required to perform the
conversion process and to access data from one conversion on
either data line of the AD7357. Note that the data accessed on
SDATAA and SDATAB is the result of the previous conversion. CS
going low provides the 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 clocks out the second leading zero.
The 14-bit result then follows with the final bit in the data transfer
valid on the 16th falling edge, having been clocked out on the
previous (15th) 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. The first rising edge of SCLK
after the CS falling edge has the second leading zero provided and
the 15th rising SCLK edge has DB0 provided.
tACQUISITION
CS
t9
tCONVERT
t6
1
3
2
B
4
t3
5
DB11
DB12
16
t5
t7
t4
SDATAA
0
0
DB13
SDATAB THREESTATE
2 LEADING ZEROS
15
DB10
DB2
t8
DB1
DB0
tQUIET
0
THREE-STATE
07757-024
t2
SCLK
Figure 31. Serial Interface Timing Diagram
CS
t6
t2
SCLK
2
1
3
4
5
15
17
16
18
31
32
t5
THREESTATE
0
0
2 LEADING
ZEROS
t4
DB13 A
DB12 A
DB11 A
t7
DB0 A
0
0
DB13 B
DB12 B
2 ZEROS
Figure 32. Reading Data from Both ADCs on One SDATA Line with 32 SCLKs
Rev. E | Page 21 of 24
DB1B
DB0 B
THREESTATE
07757-025
t3
SDATAA
AD7357
Data Sheet
APPLICATION SUGGESTIONS
GROUNDING AND LAYOUT
The analog and digital supplies to the AD7357 are independent
and separately pinned out to minimize coupling between the
analog and digital sections of the device. The PCB that houses
the AD7357 must 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 easily separated ground planes.
To provide optimum shielding for ground planes, a minimum
etch technique is generally best. Sink the two AGND pins of the
AD7357 in the AGND plane. Join the digital and analog ground
plans in only one place. If the AD7357 is in a system where multiple
devices require an AGND and DGND connection, still make the
connection at one point only. Establish a star ground point as
close as possible to the ground pins on the AD7357.
Avoid running digital lines under the device because this couples
noise onto the die. Allow the analog ground planes to run under
the AD7357 to avoid noise coupling. The power supply lines to
the AD7357 must 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 feedthrough
within the board, run traces on opposite sides of the board at
right angles to each other. A microstrip technique is the best
method, but it is not always possible with a double-sided board.
In this technique, the component side of the board is dedicated
to ground planes, while signals are placed on the solder side.
Good decoupling is important; decouple all supplies with 10 μF
tantalum capacitors parallel with 0.1 μF capacitors to GND. To
achieve the best results from these decoupling components,
place them as close as possible to the device, ideally right up
against the device. The 0.1 μF capacitor must have low effective
series resistance (ESR) and effective series inductance (ESI),
such as the common ceramic types or surface-mount types.
These low ESR and ESI capacitors provide a low impedance
path to ground at high frequencies to handle transient currents
due to logic switching.
EVALUATING THE AD7357 PERFORMANCE
The recommended layout for the AD7357 is outlined in evaluation
board documentation. The evaluation board package includes a
fully assembled and tested evaluation board, documentation, 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 AD7357 evaluation board (as well as
many other evaluation boards ending in the ED designator from
Analog Devices, Inc.) to demonstrate and evaluate the ac and dc
performance of the AD7357.
The software allows the user to perform ac (fast Fourier transform)
and dc (linearity) tests on the AD7357. The software and documentation are on a CD that is shipped with the evaluation board.
Rev. E | Page 22 of 24
Data Sheet
AD7357
OUTLINE DIMENSIONS
5.10
5.00
4.90
16
9
4.50
4.40
4.30
6.40
BSC
1
8
PIN 1
1.20
MAX
0.15
0.05
0.65
BSC
0.30
0.19
COPLANARITY
0.10
0.20
0.09
SEATING
PLANE
0.75
0.60
0.45
8°
0°
COMPLIANT TO JEDEC STANDARDS MO-153-AB
Figure 33. 16-Lead Thin Shrink Small Outline Package [TSSOP]
(RU-16)
Dimensions shown in millimeters
3.70
3.60
3.45
5.00 BSC
PIN 1
INDICATOR
(R 0.20)
0.65
BSC
18
14
13
1
4.00 BSC
EXPOSED
PAD
10
TOP VIEW
PKG-003275
0.80
0.75
0.70
SEATING
PLANE
SIDE VIEW
0.35
0.30
0.25
0.50
0.40
0.30
0.20 REF
0.05 MAX
0.02 NOM
4
5
9
BOTTOM VIEW
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
Figure 34. 18-Lead Lead Frame Chip Scale Package [LFCSP_WQ]
5mm × 4 mm Body, Very Very Thin Quad
(CP-18-1)
Dimensions shown in millimeters
Rev. E | Page 23 of 24
2.70
2.60
2.45
05-18-2015-B
INDEX
AREA
AD7357
Data Sheet
ORDERING GUIDE
Model 1, 2, 3, 4
AD7357BCPZ
AD7357BCPZ-RL
AD7357BRUZ
AD7357BRUZ-500RL7
AD7357BRUZ-RL
AD7357YRUZ
AD7357YRUZ-500RL7
AD7357YRUZ-RL
AD7357WYRUZ
AD7357WYRUZ-RL
EVAL-AD7357EDZ
EVAL-CED1Z
Temperature Range
−40°C to +125°C
−40°C to +125°C
−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
−40°C to +125°C
−40°C to +125°C
Package Description
18-Lead Lead Frame Chip Scale Package [LFCSP_WQ]
18-Lead Lead Frame Chip Scale Package [LFCSP_WQ]
16-Lead Thin Shrink Small Outline Package [TSSOP]
16-Lead Thin Shrink Small Outline Package [TSSOP]
16-Lead Thin Shrink Small Outline Package [TSSOP]
16-Lead Thin Shrink Small Outline Package [TSSOP]
16-Lead Thin Shrink Small Outline Package [TSSOP]
16-Lead Thin Shrink Small Outline Package [TSSOP]
16-Lead Thin Shrink Small Outline Package [TSSOP]
16-Lead Thin Shrink Small Outline Package [TSSOP]
Evaluation Board
Converter Evaluation and Development Board
Package Option
CP-18-1
CP-18-1
RU-16
RU-16
RU-16
RU-16
RU-16
RU-16
RU-16
RU-16
Z = RoHS Compliant Part.
W = Qualified for Automotive Applications.
3
The EVAL-AD7357EDZ can be used as a standalone evaluation board or in conjunction with the EVAL-CED1Z board for evaluation/demonstration purposes.
4
The EVAL-CED1Z is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the ED designator.
1
2
AUTOMOTIVE PRODUCTS
The AD7357W models are available with controlled manufacturing to support the quality and reliability requirements of automotive
applications. Note that these automotive models may have specifications that differ from the commercial models; therefore, designers
should review the Specifications section of this data sheet carefully. Only the automotive grade products shown are available for use in
automotive applications. Contact your local Analog Devices account representative for specific product ordering information and to
obtain the specific Automotive Reliability reports for these models.
©2009–2015 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D07757-0-12/15(E)
Rev. E | Page 24 of 24
工商网监
湘ICP备2023018690号
