The Stability Analysis of a Biopotential Measurement System Equipped With Driven-Right-Leg and Shield-Driver Circuits

One of the major problems encountered in biopotential recording is electromagnetic interference (EMI). The major source of EMI, in bioelectric measurement systems, is mains power, which is mainly coupled to the system through stray capacitances. The interference reduction is usually implemented using driven-right-leg (DRL) and shield-driver circuits, which are both feedback circuits. A variety of other auxiliary circuits may also be used for safety, ESD protection, and radio-frequency EMI filtering. Therefore, the system has great potential to become unstable. The general equivalent circuit of the bioelectric measurement system, which models the parasitic capacitive couplings between mains power and the system, is well known. The stability analysis of such systems is performed based on the simplified model, which is extracted from the general model by neglecting many electrical elements. In this paper, a simple and powerful approach is proposed to define the loop gain of the bioelectric measurement system explicitly and directly from the general model. Due to the nature of the proposed method, it can be implemented directly using computer-aided design software, to calculate the loop gain of the general model. Thus, the design process is improved significantly and many complex situations can be analyzed more easily. In particular, the proposed approach shows its advantages when the power line interference is coupled to the system through various capacitive paths. Another contribution of this paper is the stability analysis of isolated amplifier system, which is equipped with transconductance DRL and shield-driver circuits.