‘Balanced input’- need for this facility in biological amplifiers (EEG; ECG)

1. Single input amplifier

a) Measures the potential within one electrode with reference to ground

b) Subject to gross interference from electrostatic (stray capacitance) and electromagnetic (AC 50 Hz power line noise) sources

c) Biological signal would be lost in electrical noise

2. Differential amplifiers — Balanced input

a) Measures the difference in potential between 2 electrodes

b) Advantage of this system is that any interference that is common to both input terminals (eg: 50 Hz mains) is eliminated as only the difference between the terminals is amplified in the differential amplifier [feature called common mode rejection]