AD8422ACPZ-R7 datasheet(25/27 Pages) AD

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Data Sheet

AD8422

Rev. B | Page 25 of 27

APPLICATIONS INFORMATION

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.

+5V

REF

SENSE

+24V

+IN

10.7kΩ

24.9kΩ

124Ω

1OPTIONAL

ZERO

ADJUST

–IN

+IN

–IN

VOUT_FS = ±15mV

IOUT = 4mA TO 20mA

V = 0.5V TO 2.5V

RG = 301Ω

G = 66.8V/V

VOUT

AD8422

ADA4096-2

AD8276

ADA4096-2

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

+15V

–15V

2.5V

REF

+IN

42.2kΩ

0V TO 10V, ±10V

TERMINAL

BLOCK

0V TO 5V, ±5V

0V TO 1V, ±1V

4mA TO 20mA,

0mA TO 20mA

±20mA

34kΩ

8.45kΩ

1kΩ

49.9Ω

–IN

AD8422

RG = 13.2kΩ

G = 2.5V/V

VOUT = 2.5V ±2.5V

Figure 65. Process Control Analog Input

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