Circuit Note
CN-0043
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.
AD7991
4-Channel, 12-Bit ADC
AD8599
Ultralow Distortion, Ultralow Noise Dual Op
Amp
AD780
Ultrahigh Precision Band Gap Voltage
Reference
Using the AD8599 Op Amp as an Ultralow Distortion Driver for the AD7991
4-Channel, 12-Bit ADC
CIRCUIT FUNCTION AND BENEFITS
an I2 C-compatible serial interface and is offered in an 8-lead
SOT-23 package.
The circuit described in this document provides an ultralow
distortion driver circuit for the AD7991 12-bit, 4-channel ADC,
which is designed to achieve optimum ac and dc performance.
The circuit uses the ultralow distortion, ultralow noise AD8599
dual-supply op amp and ultrahigh precision AD780 band gap
voltage reference to ensure that the maximum AD7991
performance is achieved by providing a low impedance driver
with adequate settling time and a highly accurate reference
voltage. The AD8599 is a dual operational amplifier that
operates with supplies from ±4.5 V to ±18 V. The AD7991 has
CIRCUIT DESCRIPTION
It is always recommended to buffer analog input signals before
applying them to ADCs with switched capacitor inputs such as
the AD7991. This is particularly important in applications
where the signal source has high source impedance and where
low distortion and high signal-to-noise-ratio is important. The
circuit illustrated in Figure 1 shows how the AD8599, an ideal
choice for high accuracy designs, can be used to buffer the
analog input channels.
+5V
+
10µF
RP
AD8599
–5V
VIN0
1µF
+2.5V
+
SDA
VIN2*
SCL
2-WIRE SERIAL
INTERFACE
MICROCONTROLLER/
MICROPROCESSOR
VIN3/VREF
GND
+5V
VDD
VIN1*
RP
AD7991
AD780
*CONNECTED TO AD8599 OP AMPS SIMILARLY TO VIN0
08482-001
ANALOG
INPUT
1/2
5V SUPPLY
+
0.1µF
Figure 1. AD7991 ADC with the AD8599 Low-Distortion Driver and AD780 Ultrahigh Precision Reference
(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
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CN-0043
Circuit Note
The AD7991 can be operated as a 4-channel input device using
VDD as a reference (the input voltage range is 0 V to VDD) or as a
3-channel input device with the fourth channel used as an
external reference input, VREF (the input range is 0 V to VREF).
These options are programmable via the I 2C-compatible
interface. The AD780 is a 2.5 V/3 V ultrahigh precision, band
gap voltage reference and is recommended for use with the
AD7991. A 1 μF decoupling capacitor is recommended on the
VIN3/VREF signal for best performance.
LEARN MORE
Care must be taken to ensure that the analog input signal to the
ADC does not exceed the supply rails by more than 300 mV. If
the signal does exceed this level, the internal ESD protection
diodes become forward-biased and start conducting current
into the substrate. Each diode can conduct a maximum current
of 10 mA without causing irreversible damage to the part. The
MT-036 Tutorial discusses methods to protect the input circuits
of op amps and ADCs against such damage.
Data Sheets and Evaluation Boards
In addition, 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
AD7991 evaluation board layout).
MT-031 Tutorial, Grounding Data Converters and Solving
the Mystery of "AGND" and "DGND." Analog Devices.
MT-035 Tutorial, Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
MT-036 Tutorial, Op Amp Output Phase-Reversal and Input
Over-Voltage Protection. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
AD7991 Data Sheet.
AD8599 Data Sheet.
AD8605 Data Sheet.
AD780 Data Sheet.
AD7991 Evaluation Board.
REVISION HISTORY
09/09—Rev. 0 to Rev. A
Updated Format .................................................................. Universal
10/08—Revision 0: Initial Version
COMMON VARIATIONS
If single-supply op amp operation is required for buffering the
input signal, the AD8605 is a suitable choice. Note that the
output of the AD8605 operating on a single +5 V supply can
only go to approximately 20 mV above ground; therefore, the
AD7991 input range from 0 V to +20 mV cannot be exercised
(see MT-035 Tutorial).
The AD7991 can accept a reference input voltage from 1.2 V to
VDD; therefore, different voltage reference sources can be used.
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©2008-2009 Analog Devices, Inc. All rights reserved. Trademarks and
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CN08482-0-9/09(A)
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