SBOS893D August 2018 – June 2020 INA821
There are two modes of operation for the circuit shown in Figure 69: current input and voltage input. This design requires R1 >> R2 >> R3. Given this relationship, Equation 3 calculates the current input mode transfer function.
Equation 4 shows the transfer function for the voltage input mode.
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.
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.
Use a standard 0.1% resistor value of 10.5 kΩ for this design.