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What can a designer do to measure voltages near the supplies? The most straightforward option (if it can be done) is simply to increase the supply voltage. Another option is to decrease the instrumentation amplifier gain. Decreasing the gain reduces the voltage on the internal nodes and makes it more likely they will not saturate. If a gain of one had been used, rather than four in the previous example, there would have been no issues. Users could then use a gain stage after the instrumentation amplifier to get the correct gain. Note that in addition to requiring the extra expense, power, and space of an additional gain stage, this solution typically will have a slightly worse accuracy, since instrumentation amplifiers typically have better accuracy when they are used in a higher gain. Finally, designers may be able to switch to a different instrumentation amplifier that may have a more forgiving internal structure for an application. In the example, the AD8231 was chosen, a part that uses internal rail to rail op amps and a straightforward architecture. In reality most instrumentation amplifiers do not use rail to rail structures and may have additional voltage shifts. Modern instrumentation amplifier datasheets publish a graph of the typical performance curves showing where their internal nodes rail. The application’s common mode voltage and output voltage should be set so they stay inside the hexagon in the plot.
