INA821 datasheet(30/45 Pages) TI1

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C001

-20

-10

Input Current (mA)

C001

-10

-5

Input Voltage (V)

49.4 k

10.4 k

5.75

OUT

REF

G =

= 5.75

4.8 V

2.5 V

400 mV

R + R

V = V

= 4.167 kW

INA821

SBOS893D – AUGUST 2018 – REVISED JUNE 2020

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Typical Application (continued)

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. The value of R3 is 20 Ω because that resistance value is smaller than R1 and yields an

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

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

(5)

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

tolerance resistors to minimize error.

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

(6)

Equation 7 calculates the gain-setting resistor value using the INA821 gain equation (Equation 1).

(7)

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

9.2.3 Application Curves

Figure 70 and Figure 71 show typical characteristic curves for the circuit in Figure 69.

Figure 70. PLC Output Voltage vs Input Voltage

Figure 71. PLC Output Voltage vs Input Current

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