What are the advantages of a three op-amp instrumentation amplifier?

An instrumentation amplifier (INA) built using three operational amplifiers (op amps) can provide performance advantages in terms of high input impedance, low output impedance, easily adjustable gain, excellent common-mode rejection ratio (CMRR), high accuracy, and high open-loop gain.

This article examines the basic design of INAs and their performance benefits, then discusses some application considerations, and closes with a review of possible performance disadvantages of INAs.

The combination of high CMRR and high accuracy makes INAs well-suited for applications with small error budgets, such as test equipment, motor controllers, and patient monitoring systems. While an INA is often shown schematically like a standard op-amp, it is usually composed of three op-amps: one to buffer each input, one to provide the output, and one to support impedance matching (Figure 1).

Figure 1. Basic three op-amp instrumentation amplifier schematic. (Image: Wikipedia)

As shown above, an INA is a type of differential amplifier. The input buffer amplifiers eliminate the need for input impedance matching. An INA can be constructed using three discrete op amps, but the three op amps are often integrated into a single IC. This saves space, simplifies the design process, and can also improve CMRR.

Key differentiators of three op-amp INAs include:

Precise gain control. The gain can be easily and precisely controlled by changing the value of R_gain without modification to the rest of the circuit.

The input stage’s non-inverting configuration causes high input impedance. This can minimize signal sources’ loading, which is especially important with high-output impedance devices like sensors.

Low output impedance improves the efficiency of driving attached circuitry.

High CMRR since the basic design inherently rejects common mode noise signals, helping to ensure amplification of only the desired differential signal.

Application considerations

Several application considerations are necessary to maximize the advantages of three-op-amp INA. For example, the required gain, bandwidth, CMRR, and input common-mode voltage range are critical specifications to support specific application needs.

To support high-accuracy applications, an INA with low offset voltage and good gain stability should be selected. Most INAs require a dual supply voltage, typically between ±5V and ±15V. The power supply voltage can impact the input common-mode voltage range and limit the output voltage swing.

Resistor choices are important. The resistor pairs R1, R2, and R3 need precise matching to ensure the best performance. Mismatching degrades CMRR performance. In addition, internal resistors can contribute to thermal noise, limiting the ability to amplify tiny signals.

Possible challenges

There are several possible challenges when using three op-amp INAs that can impact accuracy and measurement reliability, including:

The restricted common-mode range can be a problem in applications with large common-mode signals. It can result in distortion or inaccurate readings caused by saturation of the input op-amps.

Compared with simpler designs, the additional amplification stages can result in longer signal delays.

High-gain applications for measuring small signal levels can have problems with bandwidth and frequency response, as an amplifier’s bandwidth decreases as the gain increases.

Three op amp INAs can suffer stability issues due to suboptimal component selection, especially in high-gain designs.

Since they are designed for low signal levels, these INAs can be overloaded by input signals that are too large, resulting in saturation or distortion.

CMMR is a crucial specification and indicates how well the INA can reject unwanted common-mode signals while amplifying only the differential signal between the inputs. A high CMMR means the INA can accurately measure small differential signals in the presence of large common-mode noise.

An ideal INA has infinite CMMR, meaning that if both inputs have a common voltage fluctuation, V_cm in Figure 2, while remaining constant relative to each other, that common voltage will have no impact on the output. A high CMMR ensures that only the differential signal is amplified.

Figure 2. In an ideal three-op amp INA, Vcm on VIN+ and V_IN- would perfectly cancel and result in an output of zero. (Image: Microchip)

Summary

Three op-amp INAs have performance advantages, including high input impedance, low output impedance, easily adjustable gain, excellent CMRR, high accuracy, and high open-loop gain. However, to achieve the best performance, designers must consider application-specific limitations and possible design challenges.