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9.2 Typical Applications

9.2.1 Three-Pin Programmable Logic Controller (PLC)

Figure 9-3 shows a three-pin programmable-logic controller (PLC) design for the INA851. This PLC reference

design accepts inputs of ±10 V or ±20 mA. The output is a differential voltage of ±4.95 V with a VOCM of 2.5 V (or

25 mV to 4.975 V on OUT+ and OUT−) to be measured by the ADS8920B SAR ADC.

530

4.17 k

100 k

INA851

±10 V

±20 mA

= ± 4.95 V

VS+

VCLAMP+

15 V

IN+

OUT+

OUT

AINP

AINM

51 pF

510 pF

VCLAMP

VOCM

ADS8920B

REFIN

GND

RVDD

REF5050

VOUT

VIN

GND

10 F

5 k

0.1

1 F

5.3 V

47.4

REFM

10 μF

1 k

100 nF

FDA_IN+

FDA_IN

100 pF

Figure 9-3. PLC Input with INA851 (±10 V, 4 mA to 20 mA)

9.2.1.1 Design Requirements

For this application, the design requirements are as follows:

• 4-mA to 20-mA input with less than 20-Ω burden

• ±20-mA input with less than 20-Ω burden

• ±10-V input with impedance of approximately 100 kΩ

• Maximum 4-mA to 20-mA or ±20-mA burden voltage equal to ±0.4 V

• Output range within 0 V to 5 V

9.2.1.2 Detailed Design Procedure

There are two modes of operation for the circuit shown in Figure 9-3: current input and voltage input. This

design requires R1 >> R2 >> R3. Given this relationship, the following equation calculates the current input mode

transfer function.

VOUT=VDIFF×G= − IIN×R3

(6)

where

• VOUT represents the differential voltage at the INA851 outputs in current input mode.

• VDIFF represents the differential voltage at the INA851 inputs.

• G represents the total gain of the INA851

• IIN is the input current to the PLC.

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INA851

SBOS999A – MARCH 2022 – REVISED OCTOBER 2022

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