Question

What is the amplitude of a typical ECG signal is as measured directly from skin surface electrodes? How will this answer guide your amplifier design? What features can you incorporate in your measurement device to reduce noise (especially at 60 Hz)?

Answer 1

The ECG signal classify the heart activity into three categories, the P-wave represents the electrical activity of the atrium (depolarization), QRS-wave represents the electrical activity of the ventricles (depolarization), T-wave represents electrical activity of the ventricles again but during filling with blood (repolarization). The typical maximum amplitude in an ECG waveform is 2.5 mV to 3 mV.

The amplifier should be designed in such a way that it provides very high input resistance, very high voltage gain and very low output resistance. The instrumentation amplifier designed using 3 op-amps (may be IC 741) is an ideal choice to provide these three features mentioned. The advantage of using instrumentation amplifier is that the gain of the amplifier can be controlled only by changing a single resistor value. For more information please refer to Instrumentation Amplifier.

The frequency of typical ECG is between 0.5 Hz to 80 Hz, so it is therefore highly necessary to incorporate circuitry that eliminate/reduce power line interference. To reduce the noise at 60 Hz, you can basically incorporate a notch filter at the center frequency of 60 Hz before amplification. Here, you have to choices to consider, a passive filter and an active filter. Passive filter provides only filtering but no gain but an active filter using an op-amp provides both filtering and amplification. The possible methods to reduce are using an IIR notch filter, FIR notch filter, adaptive filtering, or using wavelet transform. Out of these using wavelet transform may be an efficient way to reduce the power line interference from ECG signal.