Design of Ultralow-Power Sensor Readout Circuits Through Adaptive Biasing for Gas Monitoring With Chemiresistive Sensors

This article presents two adaptive biasing techniques for comparators to achieve ultralow-power and high dynamic range in readout circuits for resistive gas sensors. The first technique is based on an on-demand biasing that generates a bias current up to
$3 \mu \text{A}$
when the differential input voltage is close to zero, while minimizing the bias current when a comparison is not imminent. The second technique takes advantage of the correlation between sensor current and delay requirements to bias the comparator with the appropriate current for each situation. These techniques were applied to two readout circuits, demonstrating a reduction of the energy per measurement to less than a fifth for the largest input resistance in comparison to fixed biasing without affecting SNR. The input range is above five decades in both cases, from 2.4
$\text{k}\Omega $
up to 1
$\text{G}\Omega $
and from 1
$\text{k}\Omega $
up to 270
$\text{M}\Omega $
. The signal to noise ratio ranges from 55 dB up to 70 dB which provides enough margin to interface a wide range of gas sensors. Lastly, one of the designs was successfully employed to measure varying concentrations of ammonia with a polypyrrole (PPy) gas sensor.