What is input impedance in an instrumentation amplifier, and why is a high input impedance desirable?

• A. Input impedance is the impedance seen by the source; high input impedance minimizes source loading and preserves signal amplitude.
• B. Input impedance is the impedance on the output stage.
• C. Input impedance is the impedance of the power supply.
• D. Input impedance is the impedance of casing.

Answer: (A)

Input impedance is the impedance the signal source effectively sees when it is connected to the amplifier’s inputs. In an instrumentation amplifier, having a high input impedance means the amplifier draws very little current from the source, so the source voltage isn’t loaded down. This preserves the signal amplitude and integrity, which is crucial for accurate measurement, especially with high-impedance sensors or long cables that introduce their own resistance and capacitance. Instrumentation amplifiers usually achieve this with input buffering, giving a very large impedance at the input. So the best answer is that input impedance is the impedance seen by the source, and a high value minimizes loading to keep the signal intact.