The Ins and Outs of In Amps

While designing a data-acquisition system for a government lab, my colleagues and I added an instrumentation amplifier to each of the system's 16 analog inputs. The buyer offered hazy specifications about the low-level signals the lab wanted to measure, so an instrumentation amplifier (in-amp or INA) provided a way to overcome signal problems that might arise later.

Not all data-acquisition systems require an in-amp, but because they amplify small signal differences from transducers and remove large common-mode ac and dc signals in-amps can come in handy. Think of an in-amp as a "gain block" that amplifies signals by an amount you determine. That may sound like a basic op-amp circuit, but an in-amp differs in several ways. First, an in-amp requires only one resistor to establish its gain. Op-amps typically require several carefully matched resistances and resistance ratios. Second, an in-amp exhibits a high common-mode ac/dc rejection ratio, which exceeds that available in most op-amps. Third, an in-amp provides a high input-impedance load, so it works well with many sensors that produce low-level signals that could include an ac or dc component.

The basic in-amp building block comprises three op-amps as shown in the schematic diagram above. In the strain-gauge bridge circuit shown here, one side of the bridge connects to Input 1 and the other side connects to Input 2. As the resistance of the strain gauge changes, the bridge circuit produces a small voltage difference between Inputs 1 and 2. But both inputs also receive a part of the bridge's excitation voltage, V+. Thus, if Input 1 has an applied voltage of 2.000V, Input 2 might have an applied voltage of 2.015V. The 0.015V difference between the two inputs carries the useful strain information. To start, the in-amp precisely and equally amplifies the Input 1 and 2 signals. (In this example, a single external resistor sets the same gain for both amplifiers).

Next, the two amplified signals go to a circuit that subtracts one signal from the other to remove the bridge circuit's 2.000V dc signal common to both inputs. Engineers refer to that as the common-mode voltage because it appears in common at both inputs. An in-amp also can remove common-mode ac signals, perhaps picked up from surrounding circuits, to present a clean signal to an ADC. Note the in-amp converted a differential signal to a single-ended signal. In-amps provide a reference input, so if the need arises you can offset the in-amp's output.

In addition to accurately amplifying low-level signals, monolithic in-amps have other characteristics such as low input bias and low-noise operation. Some in-amps provide fixed gains that users hard wire or set under computer control. So, when you plan your next measurement system, in-amps should get a close look.