INA821 datasheet(34/52 Pages) TI

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The following equation shows the transfer function for the voltage input mode:

VOUT=VDIFF×G= – VIN × R2R2+R1

(7)

where

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

• VIN is the input voltage to the PLC.

The voltages on the output pins of the INA851 follow the relationships in Equation 8 and Equation 9.

VOUT+= VDIFF×G2 + VOCM

(8)

VOUT−=−VDIFF×G2 + VOCM

(9)

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. A 20‑Ω value is selected for R3 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 10 to calculate R2 given VDIFF = ±400 mV, VIN = ±10 V, and R1 = 100 kΩ.

VDIFF=VIN× R2R1+R2 R2=R1×VDIFF

VIN−VDIFF=4.167 kΩ

(10)

The value obtained from Equation 10 is not a standard 0.1% value; therefore, 4.17 kΩ is selected. R1 and R2

also use 0.1% tolerance resistors to minimize error.

Use Equation 11 to calculate the gain of the instrumentation amplifier.

G=VOUTVDIFF= 4.95 V400 mV=12.375 V/V

(11)

Equation 12 calculates the gain-setting resistor value using the INA851 gain equation for GOUT = 1 V/V (Equation

1).

RG=6 kΩG−1= 6 kΩ

12.375−1=527.473 Ω

(12)

Use a standard 0.1% resistor value of 530 Ω for this design.

The ADS8920B is selected because of the differential input, 1-MSPS sampling rate, and integrated reference

buffer. Implement the antialiasing R-C-R filter using two 47.4-Ω resistors, a COG or NPO-type 510-pF differential

capacitor, and two ceramic 51-pF common-mode capacitors. The REF5050 is selected to create a 5-V reference

voltage for the ADC. Use well-matched precision resistors to create a voltage divider that generates a stable

2.5-V VOCM reference. Connect the VCLAMP+ and VCLAMP− pins of the INA851 to the supplies of the ADC

to protect against overdrive damage in the event of a fault. Consider implementing a TVS diode from the ADC

supply to GND for additional protection, and include 100-nF decoupling capacitors between the amplifier and

ADC supplies and GND.

9.2.1.3 Application Curves

Figure 9-4 and Figure 9-5 show typical characteristic curves for the circuit in Figure 9-3.

INA851

SBOS999A – MARCH 2022 – REVISED OCTOBER 2022

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