INA821 datasheet(30/51 Pages) TI2

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

R1 value is 100 kΩ 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).

Use Equation 5 to calculate R2 given VD = ±400 mV, VIN = ±10 V, and R1 = 100 kΩ.

R + R

V = V

= 4.167 kW

(5)

The value obtained from Equation 5 is not a standard 0.1% value, so 4.17 kΩ is selected. R1 and R2 also use

0.1% tolerance resistors to minimize error.

Use Equation 6 to calculate the ideal gain of the instrumentation amplifier.

OUT

REF

G =

= 5.75

4.8 V

2.5 V

400 mV

(6)

Equation 7 calculates the gain-setting resistor value using the INA819 gain equation (Equation 1).

50 k

10.5 k

5.75

(7)

Use a standard 0.1% resistor value of 10.5 kΩ for this design.

9.2.1.3 Application Curves

Figure 9-6 and Figure 9-7 show typical characteristic curves for the circuit in Figure 9-5.

-10

-5

Input Voltage (V)

Figure 9-6. PLC Output Voltage vs Input Voltage

C001

-20

-10

Input Current (mA)

C001

Figure 9-7. PLC Output Voltage vs Input Current

INA819

SBOS959D – DECEMBER 2018 – REVISED APRIL 2022

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INA821 Datasheet(PDF) 30 Page - Texas Instruments

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