V

= V

G + V

=

G + V

OUT V

D

REF

-

-

REF

´

´

V

IN

´

R

2

R + R

1

2

V

= V

G + V

= (I

R )

G + V

OUT I

D

REF

IN

3

-

-

REF

´

´

´

20

Ÿ

INA821

±10 V

±20 mA

2.5 V ± 2.3 V

+VS

-VS

REF

Copyright © 2017, Texas Instruments Incorporated

15 V

REF5025

1 F

NR

GND

1 F

1 F

15 V

-15 V

-IN

+IN

RG

RG

OUT

31

INA821

www.ti.com

SBOS893A – AUGUST 2018 – REVISED DECEMBER 2018

Product Folder Links: INA821

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Copyright © 2018, Texas Instruments Incorporated

9.2 Typical Application

Figure 69 shows a three-pin programmable-logic controller (PLC) design for the INA821. This PLC reference

design accepts inputs of ±10 V or ±20 mA. The output is a single-ended voltage of 2.5 V ±2.3 V (or 200 mV to

4.8 V). Typically, PLCs have these input and output ranges.

Figure 69. PLC Input (±10 V, 4 mA to 20 mA)

9.2.1 Design Requirements

For this application, the design requirements are:

•

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 k

Ω

•

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

•

Output range within 0 V to 5 V

9.2.2 Detailed Design Procedure

There are two modes of operation for the circuit shown in Figure 69: current input and voltage input. This design

where

•

G represents the gain of the instrumentation amplifier

•

•

•

(3)

Equation 4 shows the transfer function for the voltage input mode.

where

•

(4)

±400 mV when operating in current mode (±20 mA).