Discussion
The input impedance of a biomedical amplifier should be
Answer: C
The input impedance should be very high (in the MΩ range).
The electrode-skin interface has a source impedance from a few kilohms up to hundreds of kilohms, and it differs between electrodes. The amplifier's input impedance forms a voltage divider with it. Only when the input impedance is far larger, megohms or more, does almost all of the biopotential appear at the input, and do mismatches between electrodes stop turning common-mode noise into a differential error.
A very low or moderate (1-10 kΩ) input impedance would load the source and lose most of the signal. Matching the tissue impedance gives maximum power transfer, but a biopotential amplifier needs to measure voltage, not draw power.
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