The wanted signal
We can expect the heart's peak-to-peak potential between the electrodes to be 1 mV - 5 mV 1 in the frequency range of 0.01 Hz - 300 Hz. An amplification of 620 V/V by using an 620k resistor to program gain would in worst case produce a 0.62 V peak-to-peak analogue signal and in best case one of 3.1 V. Not too inconvenient for a digital system of 3.3 V. However, with a gain of 620, our desired signal will meet tough competition from any noise, imbalance or interference. Let’s take a look at them.
Source impedance imbalance
The outermost layer of the skin has the highest impedance component in the signal path and varies greatly. The natural grease in the skin will also cause impedance variances. Differences in the impedances between the skin and the amplifier inputs causes the body's common mode potential to be seen as a differential potential and this will be amplified. Then there is the input bias currents for the amplifier that passes all impedance in the signal path.
The skin should
therefore be cleaned with alcohol and gently with sandpaper before the electrodes
are attached, to reduce most of the conversion from common mode to
differential mode signals.
This type of interference occurs when the electrodes are moved relative to the electrolyte (the gel underneath the electrode), and thus causes impedance imbalance as described above. Simply taping the electrode wires to the body can prevent much of the interference.
Muscles in our arms and legs, as our heart, generate biopotential when they move. Placing the electrodes at the wrist and the ankles means that the signal from the heart will pass the muscles in our arms and legs and pick up their interference. By placing the electrodes on our abdomen and thorax, we bypass much of the interference from our limbs, but some will still be present due to other muscle groups in proximity.
This type of noise can be observed by placing an electrode on both the wrist and thorax and swapping the electrode lead while lifting a weight. See BPM Biosignals' video for a good visualisation of this. Muscle movement causing interference. Source: BPM Biosignals.
Magnetic field interference
Magnetic fields will induce an electromotive force into any conductive loop. The wires connecting the user to the ECG device form loops that will pick up magnetic fields produced by the currents in the mains wiring. The voltage present at the input of the device will be proportional to the field strength and loop area. Keeping distance to mains wiring won’t be practical, but twisting the electrode wires and therefore reducing the loop area can be.
Electric field interference
For ECG systems, capacitive coupling to the mains wiring in walls, floor and ceiling can be a major source of interference. 50Hz common mode signals couple into the skin, through the electrodes, leads and into the amplifier input. Through differences in the impedance of the signal path, these may also be converted into differential mode signals. The instrumentation amplifier attenuates a great deal of the common mode signal, while an additional circuitry called the Right Leg Drive fights to stop the common mode signals before it enters the signal path.
Common mode interference from the mains line coupling onto the system converting to differential mode interference. Source: Texas Instruments