A quiet digitally assisted auto-zero-stabilized voltage buffer with 0.6pA input current and offset

Abstract

The readout of high impedance sensors and sampled voltage references [1] requires amplifiers with both low offset and low input current. Chopper amplifiers can achieve low offset, but the switching of their input chopper gives rise to significant input current (40 to 110pA) [2-4]. Auto-zero (AZ) amplifiers require less input switching, but exhibit more voltage noise. However, ping-pong amplifiers continuously swap two auto-zeroed input stages, leading to more switching [5,7]. In this work, an AZ stabilized topology is proposed, in which a single amplifier is always present in the signal path. Only one input switch is required, resulting in an input current of 0.6pA (max), a 66x improvement on the state-of-the art [4]. Furthermore, a digitally assisted offset-reduction scheme reduces its low-frequency (LF) noise to the theoretical V5x limit. It also achieves a state-of-the-art maximum offset of 0.6μΥ.