A novel active AC coupled input stage of instrumentation amplifier for bioelectrical applications

Abstract

For recording bioelectrical signals using Instrumentation Amplifiers (INA), capacitive AC coupling of the differential inputs is necessary for two major reasons, providing DC electrical isolation of the subject to avoid hazards of tissue burn, and to block DC or very low frequency skin-electrode contact potentials. The latter, when met, allows high admissible differential gain of the INA, which in turn, is expected to give a desirable high Common Mode Rejection Ratio (CMRR). However, the use of conventional passive RC high pass filters at the input greatly reduces CMRR. This work intends to design an improved AC coupled input circuitry for INA giving high CMRR and high input impedance. A differential RC high-pass input circuit has been developed using an active element. It creates an effective high resistance for the common mode current but provides a low resistance path to input bias currents for normal operation of the INA. Between 1 Hz and 100 Hz, CMRR of the DC coupled INA (LT1167) was 115 dB, using a well matched traditional RC coupling, 55 to 95 dB, while it was 110 dB using the improved active AC coupled circuit. This improvement was practically demonstrated through Electrocardiogram measurements from a human subject. A novel active AC coupling circuitry has been proposed and developed giving high CMRR, very close to the maximum of an INA, which is non-varying within the frequency band of interest of Bioelectrical amplifiers. The developed input circuit will improve all bioelectrical measurements in the future.