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

´

´

´

RG = 10.5 NŸ

R3 =

20

Ÿ

R2 = 4.17 NŸ

R1 = 100 NŸ

INA828

VOUT

±10 V

±20 mA

2.5 V ± 2.3 V

+VS

-VS

REF

Copyright © 2017, Texas Instruments Incorporated

15 V

REF5025

1 F

VOUT

NR

VIN

GND

1 F

1 F

15 V

-15 V

-IN

+IN

RG

RG

OUT

26

INA828

SBOS792A – AUGUST 2017 – REVISED JANUARY 2018

www.ti.com

Product Folder Links: INA828

Submit Documentation Feedback

Copyright © 2017–2018, Texas Instruments Incorporated

8.4 Typical Application

Figure 68 shows a three-terminal programmable-logic controller (PLC) design for the INA828. 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). Many PLCs typically have these input and output ranges.

Figure 68. PLC Input (±10 V, 4 mA to 20 mA)

8.4.1 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

8.4.2 Detailed Design Procedure

There are two modes of operation for the circuit shown in Figure 68: current input and voltage input. This design

requires R1 >> R2 >> R3. Given this relationship, Equation 3 calculates the current input mode transfer function.

where

•

G represents the gain of the instrumentation amplifier

•

VD represents the differential voltage at the INA828 inputs

•

VREF is the voltage at the INA828 REF pin

•

IIN is the input current

(3)

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

where

•

VIN is the input voltage

(4)

R1 sets the input impedance of the voltage input mode. The minimum typical input impedance is 100 kΩ. 100 kΩ

is selected for R1 because increasing the R1 value also increases noise. The value of R3 must be extremely

small compared to R1 and R2. 20 Ω for R3 is selected because that resistance value is much smaller than R1 and

yields an input voltage of ±400 mV when operated in current mode (±20 mA).