What is the ideal characteristic for input impedance in an instrumentation setting?

In instrumentation settings, very high input impedance is ideal because it allows for accurate measurement of small signals without loading the circuit being measured. High input impedance ensures that the measurement device, such as an amplifier or sensor, draws minimal current from the signal source. This characteristic is crucial in avoiding distortion of the signal and ensuring that the measurement reflects the true behavior of the system being studied.

When the input impedance is very high, it prevents the measurement device from affecting the voltage levels of the signal source, which is especially important in applications involving biological signals or low-level electrical signals. A device with lower input impedance could introduce significant error by altering the output of the source, leading to inaccurate readings.

In contrast, other options suggest characteristics that would not support accurate measurements in sensitive instrumentation. For instance, less than the common rejection ratio or a low input impedance could significantly compromise the quality of the measurements, leading to higher signal distortion and loss of fidelity.