Bench Talk

What’s the Difference between Instrumentation and Precision Amplifiers?

Every now and then our little group of engineers gets into philosophical arguments that go off into the weeds of extreme detail. A Dilbert comic catches this perfectly when a group of engineers argues about how to share two pizzas in a meeting. In our case, it started when I wanted to know the difference between an Instrumentation Amplifier (in-amp) and a Precision Amplifier. I know that an in-amp is a precision amp, but there must be something more if you have to rename it.

Why is an instrumentation amplifier called that? Someone said that it was a differential amp. Another said it had industrial temps. But I have seen precision amps with industrial temperature ratings. Time to do some research.

Analog Devices says, “An in-amp is a precision closed-loop gain block.” Evidently an in-amp can only be programmed for gain, and merely over a specific range, whereas a regular op amp can be made to function in many different ways. ADI goes on to say that a difference amplifier “is not a true in-amp” (at least not by the criteria that they state.) Furthermore, if you are going to make a difference, aka subtracting amplifier, then you would need to precisely match the resistors in order to have an effective common mode rejection ratio (CMRR). Recall that CMRR is a measure of how well a difference amplifier rejects unwanted signals that are common to both input legs of the op amp. The higher the CMRR value (usually measured in dB), the better. So even if you hand-picked a couple of 1% precision resistors, each resistor in the lot could vary by 1% in either direction, and you could end up with 2% total difference if you were very unlucky. (Yes, you could get 2% if your spec says +/-1%. Be careful, because some specsmanship might be involved to mislead those who would think that 1% means 0.5% deviation in either direction. There is also the reaction in precision with respect to temperature change and random drift in accuracy from use and sheer age to consider. But these are details for another day.)

Figure 1: Op Amp Subtractor or Difference Amplifier, courtesy ADI’s MT-061 Tutorial.

The solution is a monolithic amplifier with integrated laser-precision resistors. And “precision amplifier” can be defined thus.
Precision amplifiers are known for low noise, precision off-set, zero drift over time, and so forth. You can have a precision op amp that is made with exacting accuracy, to perform oodles of transfer functions with a more accurate result. An Instrumentation amplifier is also a precision amplifier; it only provides a set function of gain over a specific range. There are a ba-zillion things you can do with operational amplifiers.

This is the perfect venue and opportunity for you to recreationally argue the point or add insight. Use the comments section and tell me what you think.