ECG Acquisition Hardware

For a full review please follow the link: http://www.mit.edu/~gari/ecgbook/ch2.pdf

###Single Channel Architecture#

Fluctuations in electrical potential between electrodes on the skin's surface will form a weak lead signal that must be amplified using an using an optically isolated instrumentation amplifier. In general, there will also be a third electrode for each differential signal to act as a ground. The lead signal is then passed through a highpass filter, a second amplification stage, then a lowpass filter, and finally sampled by an analogue-to-digital converter.

###Isolation and Protection#

Any circuit that uses electrical power and comes in human contact must have the board designed into an isolated and nonisolated segment to prevent current leakage to the patient. The direct power source feeding the nonisolated board can be transferred to the isolated segment using DC-to-DC converters. Similarly, information is transferred from the patient (isolated) side of the board to the nonisolated side via opto-isolators. They are placed between the isolation gap and powered by the DC-to-DC converter output on the isolated side, and by the main power source on the nonisolated side. Note that high level resistors should be placed between each input signal (and ground) for static/defibrillation protection. Furthermore, a current limiting resistor at the output is required in case the op-amps fail. Also note that optical isolation in the early stage of amplification can introduce significant noise and thus often preferable to isolate directly after digitizing the signal.