A 4.5 GΩ-Input Impedance Chopper Amplifier With Embedded DC-Servo and Ripple Reduction Loops for Impedance Boosting to Sub-Hz

摘要

Achieving a high input impedance Zin is important for the instrumentation amplifier (IA) performing neural and biopotential signal sensing. Previously, impedance boosting techniques have been used with a positive feedback loop (PFL) driven by the output or auxiliary (AUX) path driven by the input. We propose that the conventional approach to DC-servo loop (DSL), which blocks the electrode offset (EOS), does not fully enable the function of a PFL or AUX path. This results in either limited impedance boosting or reduction of a low-frequency Z(in) in the sub-Hz band. To solve this problem and achieve Z(in) boosting down to sub-Hz, we propose a new approach to DSL for a capacitively-coupled chopper instrumentation amplifier (CCIA). The proposed DSL is realized using a 30-nA opamp, and the AUX path is designed using a 450-nA duty-cycled buffer. The CCIA achieves a gain of 40 dB and is implemented using a 0.18 mu m CMOS process with an area of 0.19 mm(2). The measured results show that Zin is increased by 184 times up to 4.6 G Omega at 0.01 Hz. Compared to the CCIA using conventional DSL, Zin is improved by 7.8 times. An input-referred noise of 2.1 mu Vrms is achieved at 2.14 mu W.