The inner workings of the three-op-amp INA

September 03, 2014 // By Bonnie Baker, Texas Instruments
Instrumentation amplifiers (INA) can sometimes throw you a curve ball for even the easiest applications. One would use an INA to amplify a small differential signal to a usable voltage level in preparation for a following analogue-to-digital converter (ADC).

The standard varieties of INAs come with two or three internal operational amplifiers (op amps). In this article, we look at the three-op amp variety (Figure 1).

Figure 1: A three-op-amp instrumentation amplifier using the INA333.

The three op amps in Figure 1 are A1, A2 and A3. Several variables influence the successful operation of this INA. These variables are VDD, VSS, VREF, RG, VCM, and VDIFF. This seems like a lot of balls to juggle, but the 'hidden' nodes that warrant special attention are the A1 and A2 output nodes: VOA1 and VOA2. A1 and A2 in combination with RG implement the gain of this INA, while A3 converts the differential signal into a single-ended output.