One technique that is commonly used to reduce 50 and 60 Hz noise in an ECG system is called right leg drive. The purpose of this circuit is not only for noise reduction, it is also used to provide a common mode reference. If a design uses a single supply of 5 V, the common mode reference will often be biased at mid-supply. The overall goal of this is to establish a common mode mid-point, which in this case is done with the gain resistor split on the front end INA, amplifies it, and feeds it back such that the overall noise seen at the input of the instrumentation amplifier is reduced. Therefore the amplifier has to reject to less noise through its common mode rejection ratio.