A 20-bit ±40-mV Range Read-Out IC With 50-nV Offset and 0.04% Gain Error for Bridge Transducers

Abstract

This paper presents a 20-b read-out IC with ±40-mV full-scale range that
is intended for use with bridge transducers. It consists of a
current-feedback instrumentation amplifier (CFIA) followed by a
switched-capacitor incremental ΔΣ ADC. The CFIA's offset and 1/
f
noise are mitigated by chopping, while its gain accuracy and gain drift
are improved by applying dynamic element matching to its input and
feedback transconductors. Their mismatch is reduced by a digitally
assisted correction loop, which further reduces the CFIA's gain drift.
Finally, bulk-biasing and impedance-balancing techniques are used to
reduce the common-mode dependency of these transconductors, which would
otherwise limit the achievable gain accuracy. The combination of these
techniques enables the read-out IC to achieve 140-dB CMRR, a worst-case
gain error of 0.04% over a 0-2.5 V common-mode range, a maximum gain
drift of 0.7 ppm/°C and an INL of 5 ppm. After applying nested-chopping,
the read-out IC achieves 50-nV offset, 6-nV/°C offset drift, a thermal
noise floor of 16.2 nV/√Hz and a 0.1-mHz 1/
f
noise corner. Implemented in a 0.7-μm CMOS technology, the prototype read-out IC consumes 270 μA from a 5-V supply.