Then matched resistors are added around the op amp, which creates a difference amplifier. Like an instrumentation amplifier, a difference amplifier is a subtractor. Unlike an instrumentation amplifier, a difference amplifier has fairly low input impedance, which prevents it from measuring sensors with high output impedance. Also, unlike an instrumentation amplifier, the difference amplifier has a tradeoff between gain and input impedance. High gain instrumentation amplifiers typically have low input impedance.