Beyond Measure: Instrumentation Amplifiers

In the first article about measurement systems we looked at sensors as a way to bring data into a measurement system. I explained that a sensor measures physical quantities which are turned into a voltage with a variable conversion element such as a resistor bridge. There will always be noise in any system, and an operational amplifier (op-amp) can be used to remove some of that noise. The example we considered used an op-amp in a differential configuration that removes any disturbance signal that is common to both inputs of the op-amp.

But that single application of an op-amp is just skimming the surface of the process of bringing a real-world measurement of a physical quantity into a digital system. Often, you’ll need to do more work on the signal before it’s ready for sampling with a digital-to-analog converter. Signal conditioning with amplifiers is a deep and rich topic, so let me make it clear that that this article will not cover every aspect of designing and implementing a measurement system. Instead, I’m aiming to get you started without getting too technical and math-y. Let’s just relax and ponder amplifiers without getting lost in detail. Doesn’t that sound nice?

Taking a step back, let’s look at some basic op-amp configurations. The fundamental op-amp circuits are designed to perform mathematical operations. (I don’t need to draw any more attention to what just happened, do I?) This means we can use an op-amp to add, subtract, multiply, or divide and this might seem like a simplification of an op-amp but there are some complex calculations that can be done with these basic operations.

Using an op-amp, we can take several voltages and add them together to get a summation of multiple inputs. We can then take that sum and divide it by the number of inputs we added together to get an average value of all the inputs. And all within the same op-amp circuit, we can amplify the signal with a desired gain by choosing feedback resistor values appropriately.

Another use for an op-amp is a voltage follower. In the voltage follower configuration we have a gain of 1 (unity gain) so we get out the same voltage that we put in. This may seem like a waste of an op-amp but it’s crucial when we want to isolate one part of a circuit from another. A typical use case would be in between the resistive conditioning parts of a circuit and the bit that measures the conditioned signal. The high impedance of a voltage follower (or buffer) is what reduces the loading of a circuit that is being measured. A voltage follower can be thought of as a multiply by 1 operation, which coincidentally is a beautiful segue into an inverter configuration. An inverting buffer is a voltage follower that has reversed polarity, or you might think of it as a multiply by -1.

By this point we have all either used an op-amp or are at least understand how they are used. The LM386 audio amplifier in an Altoids tin or cigar box is a rite of passage where I come from. However, the instrumentation amplifier (in-amp) tends to be thing of mystery. Let’s get over that barrier to entry and learn what an in-amp is and how to use it, because when you’re interested in precision measurement, in-amps are where it’s at.

We saw that the op-amp is a device that allows us to do mathematical operations on voltages, in-amps take the basic operations and build on them. If you read a datasheet or application note you might come across what is referred to as a three op-amp in-amp design. Which is exactly what it sounds like: an in-amp made up of three op-amps. This might look intimidating but if we take it one piece at a time it’s nothing we haven’t seen before; the basic idea is to use two op-amps to buffer the input signals and a third to cancel out the common-mode noise.

Three op-amp in-amp

Amps A1 and A2 are configured as non-inverting with gain. The amp A3 is configured as a difference amplifier using amp A1 as the inverting input and A2 as the non-inverting input. As you can see from the diagram, you can tweak the gain on the input stages as needed. I’ll leave you alone with the red equation included in the image if you just need to get your math on.

So that’s it? The mysterious instrumentation amplifier is just three op-amps? Yes and no. When you build one yourself, it is. But when you buy a dedicated in-amp IC, you’re getting these op-amps with the gain resistors pre-set and matched. Moreover, the op-amps themselves are made on exactly the same process — indeed on the same piece of silicon. Because everything is on the same chip, temperature differences between different parts are also likely to be minute. In short, you’re getting the same circuit made to tolerances that would be expensive, or fiddly, to replicate with individual parts.

Of course, if you want to experiment around with instrumentation amps and see how they can improve your signal path, there’s no substitute for building your own. But when you have settled on a design, you’ll probably want to grab one off the shelf. Either way, the in-amp can help your tiny and weak sensor voltage blossom into something suitable for the next stage in our signal-conditioning saga: analog to digital conversion. And that’s where we will pick up next time. Stay tuned.

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