Necessity of an indifferent electrode connection (‘right leg’) - its use in reducing AC interference

1. The right leg electrode is not a signal sensing electrode, it is simply an earth connection to reduce interference

2. However, earthing the patient with a RL electrode is undesirable for

a) Safety reasons

b) Intensifies potential gradients on patient’s surface

i) By allowing the whole amplifier to ‘float’ at the same potential as the patient, these gradients are minimised so that the common mode voltages are reduced and therefore less difficult to reject

3. In a differential amplifier, the AC current common to both input electrodes is returned via the RL electrode

4. The effect is to cancel the incoming common mode voltage & permit the amplifier to look at only the differential signal existing between the two limb leads

5. A high CMRR means that the amplifier strongly attenuates signals that are common to both inputs, but amplifies the difference between the two signals

a) The efficiency of common mode rejection is also related to the amplitude of the common, in-phase signal to be rejected