Where and how are the ECG signals measured?

EGG waveforms from skin pads require a huge amount of amplification to be a wide enough voltage range to be easily sampled and measured by an A/D converter.

ECG waveforms from the body are very much influenced by the power line frequency in the near environment due to low level fields and light. If I recall correctly, from work over 12 years ago, filters with rejection like -80 dB are required on the analog signals at 1X and 2X the power line frequency.

Patient safety for making ECG measurements at the skin requires a lot of isolation. This isolation is often provided by some type of AC coupled transformer or capacitance coupler. The demand for front end signal amplification at the skin leads normally requires the coupling of a certain amount of power across the isolation barrier to provide a DC supply for the amplifiers. ECG signal bandwidth requirements will often require the coupling channel to operate at frequencies of 30 to 100 times the highest ECG waveform frequency. Inclusion of this additional frequency into the isolation coupling channel requires there to be an accurate notch filter at that frequency.

Finally there is a need to properly bandpass the ECG signal so that what is presented to the A/D converter is a true representation of the actual signal being measured at the skin pads. This bandpass filter needs a very well defined behavior and steep cutoff at its upper corner frequency to prevent anti aliasing of the signal frequencies at the A/D input.

If you are concerned about several things that may be required for successful ECG measurements, which include offset correction, baseline averaging and automatic gain control, you will find that a 12-bit A/D converter is necessary to give you the starting point for a enough resolution to work with.

And finally, lest we not forget, be aware that skin ECG measurements are often done making measurements from 5, 6 and as many as 12 sensing channels. This requires mux and selection circuitry at the front end before the isolation channel and as such may complicate the front end design by a significant amount.

Michael Karas