Signal Acquisition in ECG Machines

ECG signals are changed from the microvolt to the millivolt range. Due to this small range, the signals measured need to be amplified in order to be better interpreted. Typical biopotential amplifiers have high input impedance and are designed for safety first.
The output impedance of the amplifier should be very low to drive any external load with minimal distortion. Again, due to the small size of the signal, the gain should be large. Typically a gain of over 1000 is implemented in biopotential amplifier circuits. 
The amplifiers should have a high common mode rejection ratio to eliminate large offset signals. 
Finally, most biopotential amplifiers are differential. Differential amplifiers are used to make sure that noise from the inputs are not amplified thus yielding a higher integrity signal Differential amplifiers with such characteristics are difficult to find. Thus combinations of differential amplifiers are used to construct what is called an instrumentation amplifier. A basic three-op-amp instrumentation amplifier is shown below
                      

There are two stages for the instrumentation amplifier that help make it meet the characteristics of an ideal biopotential amplifier. The first stage is the input stage of the amplifier followed by the gain stage.

The input stage ideally supplies no common mode gain thus eliminating common mode noise. The three op-amps give the input stage high input impedance and the configuration gives a gain of 

 Gd = (V3-V4)/(V1-V2) = (2R2+R1)/R1

 The input stage also buffers the gain stage. Finally, the outputs of the input stage are the inputs of gain stage. The gain stage has low impedance and supplies a differential gain. Overall the amplifier amplifies only the differential component with a gain of 

Gd=(2R2+R1)/R1 * R4/R3 

 and provides a high common mode rejection ratio

OnlineResources http://www.egr.msu.edu/classes/ece480/capstone/spring13/group03/documents/ElectrocardiographyCircuitDesign.pdf