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

Abstract

Achieving a high input impedance Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in</sub> in the sub-Hz band. To solve this problem and achieve Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in</sub> 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 μm CMOS process with an area of 0.19 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The measured results show that Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in</sub> is increased by 184 times up to 4.6 GQ at 0.01 Hz. Compared to the CCIA using conventional DSL, Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in</sub> is improved by 7.8 times. An input-referred noise of 2.1 μVrms is achieved at 2.14 μW.