Use the RES11A in conjunction with a dual-channel operational amplifier to implement a discrete instrumentation amplifier (INA). The ratiometric matching between the two resistor dividers improves CMRR performance for the circuit when compared to a similar implementation using unmatched discrete resistors, and results in better over-temperature and over-aging gain drift characteristics. INAs are often used instead of difference amplifiers when high input impedance and low bias currents are needed, such as when measuring bridge sensors.

Discrete INAs are often configured as a differential-input differential-output circuit; see Figure 8-8. While not shown, if needed, use an additional discrete difference amplifier stage (requiring a second RES11A and another op-amp channel) to convert the differential output voltage to a single-ended voltage (for example, when driving a single-ended ADC). This extra stage also adds an additional offset and provides additional gain, effectively mimicking the common three-amplifier INA architecture.

Figure 8-8 Differential-Input, Differential-Output Instrumentation Amplifier Using the RES11A

One benefit of the discrete approach with RES11A, when compared to a common three-amp INA such as the INA821, is that the resistors between the inverting pins of the amplifiers have the same temperature coefficient as the feedback resistors. Therefore, all of the resistors that establish the common-mode and differential gains drift together, so the circuit gain error is extremely steady across temperature. In comparison, the INA821 and similar devices rely on an external gain-setting resistor that is inherently unmatched to the internal, laser-trimmed feedback resistors. As a result, the drift performance of the INA821 is directly correlated with how precise and low-drift of an external resistor is used to set the gain, so for best performance a relatively expensive low-drift resistor is typically required. The RES11A resistors are all well matched and have comparable temperature coefficients, and the circuits shown with RES11A do not require additional external resistors; therefore, the discrete approach avoids this problem.

Less commonly, a discrete INA is able to be implemented as a differential-input, single-ended output circuit; see Figure 8-9. This topology maintains high input impedances, allows an offset to be applied, and gives a single-ended output without requiring a third amplifier channel. Drive the offset with a low-impedance source, such as a reference buffer. When designing a discrete INA, carefully consider the output swing and input common-mode range limitations of the amplifiers used in the circuit design process.

Figure 8-9 Differential-Input, Single-Ended Output Instrumentation Amplifier Using the RES11A