A duty-cycle controlled variable-gain instrumentation amplifier applied for two-electrode ECG measurement

An instrumentation amplifier with variable gain suitable for biopotential signal acquisition is presented. The architecture proposed is based on the superregenerative receiver created by Armstrong back in the 1920's. It allows controlling the gain by the duty-cycle of a digital clock. This feature reduces the complexity for implementing an automatic gain control circuit, since no digital-to-analog converter is needed in the interface of the gain control. The implemented instrumentation amplifier provides a variable gain of 40.2 dB with a 0.5 dB gain-step resolution, plus a fixed gain of 28.2 dB, giving a total gain range of 68.2 dB dB. It was evaluated by acquiring ECG signals using only two electrodes installed on the body of a volunteer. Results show that the amplifier acquires the ECG signal and that it is capable of adapting its gain to a specified voltage amplitude for delivering it to an A/D converter.