23.7 A 130dB CMRR Instrumentation Amplifier with Common-Mode Replication

Abstract

Interfacing with high-impedance sensors, such as dry-contacted electrodes and accelerometers requires high CMRR with sufficient input impedance concurrently. From the system point of view, the total CMRR (TCMRR) is determined by the intrinsic CMRR (ICMRR) of the front-end amplifier as well as the imbalance of source impedance; the latter has to be accommodated by large input common-mode (CM) impedance (ZIN, CM). Widely adopted CMRR enhancement techniques, i.e., chopping, common-mode feedback (CMFB), common-mode feedforward, improve ICMRR only [1]–[3]. Positive feedback improves the input differential-mode (DM) impedance (ZIN, DM) effectively [4]. However, it is the input CM impedance that contributes to the TCMRR, which has not been commonly noted. Single-ended amplifiers, e.g., active electrodes (AE) [3], [5], and traditional 3-opamp instrumentation amplifiers do not suffer from the confusion, where the ZIN, CM and ZIN, DM are inherently the same and are enhanced by the same mechanism for higher TCMRR. On the other hand, dedicated single-ended stages require extra design effort as well as power consumption. Pre-charge techniques improve ZIN, CM and ZIN, DM concurrently at low frequencies by employing two additional chopped buffers [6]. Power-supply regulation enhances both CMRR and (ZIN, CM). However, feedback regulation of the power supply is often sophisticated and power-consuming [7].