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Circuit Note
CN-0039
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/circuits.
Devices Connected/Referenced
AD7266
Differential/Single-Ended, 12-Bit,
3-Channel SAR ADC
AD8022 Dual High Speed, Low Noise Op Amp
OP177 Ultra-Precision Operational Amplifier
AD7266 SAR ADC in DC-Coupled Differential and Single-Ended Applications
Rev. A
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engineers. Standard engineering practices have been employed in the design and construction of
each circuit, and thei
r function and performance have been tested and verified in a lab environment
at room temperature. However, you are 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
any cause whatsoever connected to the use of any “Circuit from the Lab ”. (Continued on last page)
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Tel: 781.329.4700
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Fax: 781.461.3113 ©2009 Analog Devices, Inc. All rights reserved.
CIRCUIT FUNCTION AND BENEFITS
The driver circuits shown in this circuit note are optimized for
dc-coupled applications requiring low distortion and low noise
performance. This drive circuit ensures that the maximum
AD7266 performance is achieved by providing adequate settling
time, low distortion, and low output impedance.
CIRCUIT DESCRIPTION
In applications where the signal source has high impedance, it is
recommended to buffer the analog input signal before applying
it to the switched capacitor inputs of the AD7266. This isolates
the source from the transient currents which appear at the input
of the ADC. A dual op amp pair can be used to directly couple a
differential signal to one of the analog input pairs of the AD7266.
The AD8022 is an ideal choice for the dual op amp and has low
power (4 mA/amp), low noise (2.7 nV/√Hz @ 100 kHz), and
low distortion (110 dB SFDR @ 200 kHz). The AD7266 has a
specified minimum acquisition time of 90 ns with a supply voltage
o f 5 V. This is the time from when the part enters track mode
until the next conversion is initiated. The op amp selected must
have adequate settling time to meet the acquisition time require-
ments of the AD7266 and achieve the specified performance.
20kΩ 0.47µF
R5
221Ω
1.67V
2.5V
+2.5V
–2.5V
27Ω
27Ω
–12V
+12V
GND
VIN
2.5V
3.75V
1.25V
2.5V
3.75V
1.25V
D
CAP
A/D
CAP
B
V
REF
= 2.5V
V
A1
AD7266
1
AD8022
AD8022
V
A2
R1
442Ω
R2
221Ω
0.47µF
1
ADDITIONAL PINS OMITTED FOR CLARITY.
221Ω
221Ω
+12V
–12V
10kΩ
A
08448-001
R3
R4
OP177
1/2
1/2
Figure 1. AD8022 DC-Coupled Circuit to Convert a Bipolar Single-Ended Signal into a Unipolar Differential Signal
(Simplified Schematic: Decoupling and All Connections Not Shown)
Verzeichnis
- ・ Blockdiagramm on Seite 1 Seite 2
- ・ Anwendungsbereich on Seite 1
