9.2.2 Detailed Design Procedure

There are two modes of operation for the circuit shown in Figure 62: current input and voltage input. This design requires R₁ >> R₂ >> R₃. Given this relationship, the current input mode transfer function is given by Equation 2.

Equation 2.

where

• G represents the gain of the instrumentation amplifier

The transfer function for the voltage input mode is shown by Equation 3.

Equation 3.

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).

Equation 4 can be used to calculate R₂ given V_D = ±400 mV, V_IN = ±10 V, and R₁ = 100 kΩ.

Equation 4.

The value obtained from Equation 4 is not a standard 0.1% value, so 4.12 kΩ is selected. R₁ and R₂ also use 0.1% tolerance resistors to minimize error.

The ideal gain of the instrumentation amplifier is calculated with Equation 5.

Equation 5.

Using the INA827 gain equation, the gain-setting resistor value is calculated as shown by Equation 6.

Equation 6.

107 kΩ is a standard 0.1% resistor value that can be used in this design. Finally, the output RC filter components are selected to have a –3-dB cutoff frequency of 1 MHz.