AD8422ACPZ-R7 datasheet(26/28 Pages) AD

26 / 28 page

Data Sheet

AD8422

APPLICATIONS INFORMATION

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Rev. C | 26 of 28

PRECISION BRIDGE CONDITIONING

With its high CMRR, low drift, and rail-to-rail output, the AD8422

is an excellent choice for conditioning a signal from a Wheatstone

bridge. With appropriate supply voltages, the gain and reference

pin voltage can be adjusted to match the full-scale bridge output

to any desired output range, such as 0 V to 5 V. Figure 64 shows

a circuit to convert a bridge signal into a 4 mA to 20 mA output

using the AD8276 low power, precision difference amplifier, and

the ADA4096-2 low power, rail-to-rail input and output, overvoltage

protected op amp. With high precision bridge circuits, care must be

taken to compensate offsets and temperature errors. For example,

if the voltage at the REF pin is used to compensate for the bridge

offset, ensure that the AD8422 is within its operating range for

the maximum expected offset. If the zero-adjust potentiometer is

excluded, connect the positive op amp input to the center of the

24.9 kΩ, 10.7 kΩ divider, which is at 1.5 V. If lower supply voltages

are used for the AD8276 and the ADA4096-2, ensure that the

desired output voltage of the AD8276 is within its output range,

and VL is within the input and output range of the ADA4096-2. The

transistor must have sufficient breakdown voltage and IC. Low cost

transistors, such as the BC847 or 2N5210, are recommended.

PROCESS CONTROL ANALOG INPUT

In process control systems such as programmable logic controllers

(PLC) and distributed control systems (DCS), analog variables

typically occur in just a few standard voltage or current ranges,

including 4 mA to 20 mA and ±10 V. Variables within these input

ranges must often be gained or attenuated and level shifted to

match a specific ADC input range such as 0 V to 5 V. The circuit in

Figure 65 shows one way this can be done with a single AD8422.

Low power, overvoltage protection, and high precision make the

AD8422 a good match for process control applications, and high

input impedance, low bias current, and low current noise allow

significant source resistance with minimum additional errors.

Figure 64. Bridge Circuit with 4 mA to 20 mA Output

Figure 65. Process Control Analog Input

AD8422ACPZ-R7 Datasheet(PDF) 26 Page - Analog Devices