Circuit Note
CN-0042
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
Apply these product pairings quickly and with confidence.
For more information and/or support call 1-800-AnalogD
(1-800-262-5643) or visit www.analog.com/circuit.
AD7366/
AD7367
Bipolar Input, Dual 12-/14-Bit, 2-Channel,
SAR ADC
AD8021
Low Noise, High Speed Amplifier
Driving the AD7366/AD7367 Bipolar SAR ADC in Low-Distortion DC-Coupled
Applications
CIRCUIT FUNCTION AND BENEFITS
The AD7366/AD7367 are fabricated on the Analog Devices,
Inc., industrial CMOS process (iCMOS), which is a technology
platform combining the advantages of low and high voltage
CMOS. The input circuits of the AD7366/AD7367 operate on
VDD and VSS voltages of ±12 V nominal, while the rest of the
ADC operates on an AVCC, a DVCC, and a VDRIVE of +5 V. The
iCMOS process, therefore, allows the AD7366/AD7367 to
accept high voltage bipolar signals in addition to reducing
power consumption and package size.
The circuit described in this document provides single-ended,
low-distortion sampling of an industrial level, dc-coupled
signal. The driver circuit shown in Figure 1 is optimized for
applications requiring best distortion performance. Maximum
AD7366/AD7367 performance is achieved by providing
adequate settling time and low impedance in the circuit.
CIRCUIT DESCRIPTION
The AD7366 and the AD7367 are, respectively, 12-bit and
14-bit, 1 MSPS, 2-channel, simultaneous sampling SAR ADCs.
These devices have a total of four analog multiplexed inputs
(two per channel), which operate in single-ended mode. The
analog input range on the AD7366/AD7367 is programmable
and can support ±10 V, ±5 V, and 0 V to 10 V using the internal
2.5 V reference. An analog input range of ±12 V requires a 3 V
external reference.
In applications where the signal source has high impedance,
analog input signals should be buffered before being applied to
the inputs of the AD7366/AD7367 because large source
impedances significantly affect the ac performance of the ADC.
The choice of the op amp used to drive the inputs is a function
of the particular application and depends on the analog input
voltage range selected. The driver amplifier must be able to
settle for a full-scale step to a 14-bit level (0.0061%) for the
AD7367 or a 12-bit level (0.024%) for the AD7366 in less than
the specified acquisition time of 140 ns.
+
+12V
+5V
+
10µF
+5V/+2.5V
AGND
+
500Ω
CCOMP = 10pF
0.1µF
AD8021
VA1
–5V/–2.5V
+
1kΩ
VDD DVCC AVCC VDRIVE
AD7366/
AD7367*
1kΩ
15pF
VB1
VSS
+
0.1µF
–12V
*ADDITIONAL PINS OMITTED FOR CLARITY.
AGND
DGND
08481-001
10µF
Figure 1. Typical Connection Diagram with the AD8021 for Driving the Analog Inputs of the AD7366/AD7367
(Simplified Schematic, Decoupling and All Connections Not Shown)
Rev. A
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each circuit, and their function and performance have been tested and verified in a lab environment
at room temperature. However, youare solely responsible for testing the circuit and determining its
suitability and applicability for your use and application. Accordingly, in no event shall Analog
Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to
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Fax: 781.461.3113 ©2008-2009 Analog Devices, Inc. All rights reserved.
CN-0042
Circuit Note
The AD8021 high performance voltage feedback op amp is an
ideal choice as a single-ended input buffer/driver for the
AD7366/AD7367 due to its exceptionally high performance,
high speed, low noise, and low distortion performance. It also
meets the above stated requirement when operating in singleended mode. Figure 1 shows the configuration of the AD7366/
AD7367 with the AD8021 in a single-ended configuration. The
AD8021 needs an external compensating NPO type capacitor
(CCOMP), as indicated in Figure 1. The AD8021 is connected in
the noninverting mode with a gain of 2. The AD7366/AD7367
programmable bipolar input voltage ranges (referenced to the
input of the AD8021) are ±5 V and ±2.5 V.
The circuit must be constructed on a multilayer PC board with
a large area ground plane. Proper layout, grounding, and
decoupling techniques must be used to achieve optimum
performance (see MT-031 Tutorial, MT-101 Tutorial, and the
AD7366/AD7367 evaluation board layout).
COMMON VARIATIONS
The AD8022 is a suitable replacement for the AD8021 in high
frequency applications where a dual version is required. For
lower frequency applications, recommended op amps are the
AD797, AD845, and AD8610.
LEARN MORE
MT-031 Tutorial, Grounding Data Converters and Solving
the Mystery of "AGND" and "DGND." Analog Devices.
MT-036 Tutorial, Op Amp Output Phase-Reversal and Input
Over-Voltage Protection. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Data Sheets and Evaluation Boards
AD7366 Data Sheet.
AD7367 Data Sheet.
AD8021 Data Sheet.
OP177 Data Sheet.
AD7366/AD7367 Evaluation Board.
REVISION HISTORY
09/09—Rev. 0 to Rev. A
Updated Format .................................................................. Universal
10/08—Revision 0: Initial Version
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CN08481-0-9/09(A)
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