A 16-Bit, 6 MSPS SAR ADC System with Low Power Input Drivers and Reference...

Circuit Note
CN-0307
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
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AD7625
16-Bit, 6 MSPS, PulSAR, Differential ADC
ADA4897-1/
ADA4897-2
Low Power, Low Noise, Single/Dual
Amplifier
Ultralow Noise XFET Voltage Reference
with Current Sink and Source Capability
2.7 V, 800 µA, 80 MHz Single/Dual Rail-toRail I/O Amplifier
ADR434
AD8031/
AD8032
A 16-Bit, 6 MSPS SAR ADC System with Low Power Input Drivers and Reference
Optimized for Multiplexed Applications
EVALUATION AND DESIGN SUPPORT
distortion [THD] = −110 dBc) at low power. The circuit is ideal
for high performance multiplexed data acquisition systems, such as
portable digital x-ray systems and security scanners, because the
SAR architecture can sample without the latency or pipeline delay
typically incurred with pipeline ADCs. The 6 MSPS sampling rate
allows fast sampling of multiple channels, and the ADC has true
16-bit dc linearity performance and a serial low voltage differential
signaling (LVDS) interface for low pin count and low digital noise.
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The circuit in Figure 1 is a 16-bit, 6 MSPS, successive approximation
(SAR) analog-to-digital converter (ADC) and differential-todifferential driver combination optimized for low noise (signal-tonoise ratio [SNR] = 88.6 dB) and low distortion (total harmonic
+5V
+4.096V
+7V
+4.096V
AD8031
ADR434
+5V
VDD +7V/+5V
0.1µF
VIN+
590Ω
20Ω
VCM
2.7nF
ADA4897-1
0.1µF
VREF = 4.096V
+2.5V
VCM = VREF ÷ 2
= 2.048V
49.9Ω
REFIN REF VDD1 VDD2 VIO
0.1µF
CNV+/
CNV–
100Ω
VIN+
VCM = 2.048V
IN+
–2V/0V VSS
VIN–
D+/D–
100Ω
DCO+/DCO–
100Ω
GND
AD7625
VDD +7V/+5V
0.1µF
VIN–
VCM = VREF ÷ 2
= 2.048V
49.9Ω
590Ω
0.1µF
VCM
IN–
20Ω
GND
2.7nF
ADA4897-1 0.1µF
VCM
CLK+/CLK–
100Ω
+2.048V
+5V
AD8031
11130-001
–2V/0V VSS
Figure 1. The ADA4897-1 Driving the AD7625 (All Connections and Decoupling Not Shown)
Rev. 0
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CN-0307
Circuit Note
The driver uses two low noise (1 nV/√Hz) ADA4897-1 op amps
that maintain the dynamic performance of the AD7625 ADC at
low power levels (3 mA per amplifier). The fast settling time
(45 ns to 0.1%) of ADA4897-1 makes them ideal for multiplexed
applications.
This combination offers industry-leading dynamic performance
at low power in a small board area with the AD7625 in a 5 mm ×
5 mm, 32-lead LFCSP; the ADA4897-1 in an 8-lead SOIC; and
the AD8031 in a 5-lead SOT-23 package.
CIRCUIT DESCRIPTION
The ADA4897-1 has low distortion (−93 dB spurious-free dynamic
range [SFDR] at 1 MHz), a fast settling time (36 ns to 0.1%), and
high bandwidth (230 MHz, −3 dB, G = 1). Both ADA4897-1
drivers are configured with a gain of 1. The single-pole 2.95 MHz
low-pass RC filter, using a 20 Ω resistor and 2.7 nF capacitor, is
placed between each of the drivers and the ADC. This filter limits
the output noise of the op amp at the inputs of the AD7625 and
provides some attenuation of the out-of-band harmonics.
The ADR434 is available in either an 8-lead MSOP or an 8-lead,
narrow SOIC package. An AD8031 op amp isolates the ADR434
output from the reference input of the AD7625 and provides
low impedance and fast settling to the transient current on the
REF input.
The dual driver requires only 54 mW, and when added to the ADC
power of 135 mW, and the reference and reference buffer power of
12 mW, yields a total power of only 201 mW for the entire circuit.
The circuit uses supplies of +7 V and −2 V for the input of the
ADA4897-1 drivers to minimize power dissipation and to achieve
the optimum system distortion performance. The ADA4897-1
output stage is rail-to-rail and swings between 150 mV and 4.85 V
when operating on a single 5 V supply. However, the additional
2 V headroom at each end of the range provides lower distortion.
Figure 2 shows the ac performance of the circuit using +7 V and
−2 V supplies for the input stage. The SNR = 88.6 dB, THD =
−110.7 dB, with a 20 kHz input signal 0.6 dB below full scale
(93% full scale).
11130-002
The common-mode voltage at the output of the ADA4897-1 is
set by buffering the VCM output voltage (nominally 2.048 V) of
the AD7625 using the AD8031 configured as a unity-gain buffer.
The common-mode bias voltage is applied to the inputs through
the 590 Ω series resistors. The AD8031 is ideal for driving the
common-mode voltage because of its low output impedance
and fast settling from transient currents.
The AD7625 achieves industry breakthrough dynamic
performance of 92 dB SNR at 6 MSPS with a 16-bit (1 LSB)
integral nonlinearity (INL) performance using an LVDS interface.
The ADR434 voltage reference (4.096 V) is a low noise, high
accuracy XFET reference with low temperature drift. It can
source up to 30 mA of output current and sink up to 20 mA.
Figure 2. AD7625 and ADA4897-1 in Dual Supply Operation (+7 V, −2 V), SNR = 88.6 dB, THD = −110.7 dB, Fundamental Amplitude = −0.6 dB of Full Scale
Rev. 0 | Page 2 of 5
CN-0307
11130-003
Circuit Note
Figure 3. AD7625 and ADA4897-1 in Single Supply Operation (5 V), SNR = 86.7 dB, THD = −101.1 dB, Fundamental Amplitude = −1.55 dB of Full Scale
Figure 3 shows the ac performance of the circuit using a single 5 V
supply for the input stage. The SNR = 86.7 dB, THD = −101.1 dB,
with a 20 kHz input signal 1.55 dB below full scale (84% full scale).
Another attractive 4.096 V reference is the ADR4540 low dropout
(>300 mV) high accuracy reference that allows operation on a
5 V supply.
The data shows an approximate 1.9 dB degradation in SNR and
a 9.6 dB degradation in THD due to reducing the supply voltage
from −2 V, +7 V to 0 V, + 5V.
The ADA4897-1 and AD8031 single op amps can be replaced with
their dual versions (ADA4897-2 and AD8032, respectively) if
desired.
The single supply configuration is useful for the users who do
not have dual supplies in their system but still must achieve
high performance.
For high input frequencies up to 3 MHz, the ADA4899-1
(15 mA/amp) is the recommended driving amplifier.
COMMON VARIATIONS
The AD7625 has an integrated internal reference as well as two
provisions for external references if system requirements dictate.
The reference voltage can be generated by applying the ADR3412
reference (1.2 V) output to the REFIN pin, which is amplified
internally by the on-chip reference buffer to the correct ADC
reference value of 4.096 V. The ADR3412 can be supplied by the
same 5 V analog rail used for the AD7625 and also make use of
the on-chip reference buffer.
Alternatively, a 4.096 V external reference, such as the ADR434 or
ADR444, can be connected to the unbuffered REF input of the
ADC using a buffer amplifier such as the AD8031 as shown in
Figure 1. This approach is common for multichannel applications
where the system reference is shared by several ADCs.
The ADR434 and ADR444 configurations also excel for single
channel applications where a low reference temperature coefficient
(3 ppm/°C maximum for ADR434B and ADR444B) is required.
The 7 V rail used to supply the ADA4897-1 op amps can also
supply the VIN supply pin of the ADR434 or ADR444.
The ADA4938-1 (37 mA/amp) is excellent for signals up to
10 MHz and can also be used as a single-ended-to-differential
converter.
The performance of this or any high speed circuit is highly
dependent on proper printed circuit board (PCB) layout.
This includes, but is not limited to, power supply bypassing,
controlled impedance lines (where required), component
placement, signal routing, and power and ground planes. (See
MT-031 Tutorial, MT-101 Tutorial, and the article A Practical
Guide to High-Speed Printed-Circuit-Board Layout for more
detailed information regarding PCB layout.)
CIRCUIT EVALUATION AND TEST
The EVAL-AD7625EDZ evaluation board was developed to
evaluate and test the AD7625 ADC. To test the circuit shown in
Figure 1, the two ADA4899-1 op amps (U13, U14) were replaced
with two ADA4897-1 op amps.
A detailed schematic and user instructions are available in the
EVAL-AD7625EDZ documentation. This documentation
describes how to run the ac tests described in this circuit note
Note that the +7 V and −2 V supplies for the input amplifiers are
connected to the EVAL-AD7625EDZ board from the external
dual power supply.
Rev. 0 | Page 3 of 5
CN-0307
Circuit Note
A functional block diagram of the test setup is shown in Figure 4,
and a photograph of the evaluation board is shown in Figure 5.
•
Equipment Needed
•
The following equipment is required to test the circuit:
POWER
SUPPLY
–2V
•
•
POWER
SUPPLY
AGILENT
E3630A
+7V
+7V @ 2A
ADA4897-1
IN+
SIGNAL
GENERATOR
AD7625
CED
PC
(USB)
IN–
AUDIO PRECISION
SYS-2702
ADA4897-1
EVAL-AD7625/AD7626EDZ
EVAL-CED1Z
(MODIFIED)
Figure 4. Functional Diagram of Test Setup
11130-005
•
The EVAL-AD7625EDZ modified evaluation board
(includes software and 7 V dc wall wart power supply)
The EVAL-CED1Z converter evaluation and demonstration
platform board
11130-004
•
A low distortion signal generator, such as the Agilent 81150A
or Audio Precision SYS2702
A PC with a USB 2.0 port running Windows® XP, Windows
Vista, or Windows 7 (32-Bit or 64-bit)
A 7 V dc wall wart (included with evaluation board)
External +7 V and −2 V dc supplies at 50 mA.
Figure 5. Modified EVAL-AD7625EDZ Board Connected to EVAL-CED1Z Board
Rev. 0 | Page 4 of 5
Circuit Note
CN-0307
LEARN MORE
Data Sheets and Evaluation Boards
CN-0307 Design Support Package:
http://www.analog.com/CN0307-DesignSupport
AD7625 Data Sheet
Ardizzoni, John, and Jonathan Pearson, High Speed Differential
ADC Driver Design Considerations, Application Note
AN-1026, Analog Devices, Inc.
ADA4897-1 Data Sheet
AD7625 Evaluation Board, EVAL-AD7625EDZ
Ardizzoni, John. “A Practical Guide to High-Speed PrintedCircuit-Board Layout,” Analog Dialogue 39-09, September
2005.
AN-742 Application Note, Frequency Domain Response of
Switched Capacitor ADCs. Analog Devices.
ADA4897-2 Data Sheet
AD8031 Data Sheet
AD8032 Data Sheet
ADR434 Datasheet
REVISION HISTORY
AN-827 Application Note, A Resonant Approach to Interfacing
Amplifiers to Switched-Capacitor ADCs. Analog Devices.
11/12—Revision 0: Initial Version
Kester, Walt. 2006. High Speed System Applications. Analog
Devices. Chapter 2, “Optimizing Data Converter Interfaces.”
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND.” Analog Devices.
MT-073 Tutorial, High Speed Variable Gain Amplifiers. Analog
Devices.
MT-074 Tutorial, Differential Drivers for Precision ADCs,
Analog Devices.
MT-075 Tutorial, Differential Drivers for High Speed ADCs
Overview, Analog Devices.
MT-076 Tutorial, Differential Driver Analysis, Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Analog Devices DiffAmpCalculator™ Design Tool
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©2012 Analog Devices, Inc. All rights reserved. Trademarks and
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CN11130-0-10/12(0)
Rev. 0 | Page 5 of 5
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