An instrumentation amplifier, or in-amp, is not an op-amp. For the customer that doesn’t work with in-amps every day, here is a quick refresher. An in-amp is a precision amplifier that measures the difference between its two inputs while rejecting any signals common to both inputs. The output voltage is measured relative to a reference pin, which is typically connected to ground at the load. In order to measure small differential signals, in-amps have gain. The gain is applied to the differential signal only and it is most often programmed by the value of a single external resistor. Typical signals measured with in-amps include biological signals like ECG and very small signals from sensors, such as load cells. Because these signal sources often have output impedances of several kilo-ohms and more, it is important that the instrumentation amplifier have a very high input impedance. Also because the in-amp often drives later stages that have a lower input impedance, such as an analog-to-digital converter, it must also have low output impedance. Finally, in-amps are optimized to measure very small signals, so they must have even smaller errors. Especially at high gain. Therefore in-amps are designed to have low offset voltages, low noise, and very high common mode rejection ratio.