An Energy-Efficient CMOS Biophotometry Sensor With Incremental DT-∑Δ ADC Conversion

This paper presents a high-sensitivity incremental discrete time ∑Δ analog-to-digital converter (ADC) with variable clock and on-chip decimation for biophotometry sensing. A digital correlated double sampling (CDS) scheme is merged with the decimation filter for providing energy- and area-efficient mean to suppress low-frequency noise. Chopper modulation and differential sensing using a dummy photodiode along with charge transfer switches are used as well to allow for the detection of ultra low-input low fluorescence light, down to a few femttowatt, by suppressing the low-frequency noise of the ∑Δ modulator and the dark current of the photodetector. A variable clock scheme is used across the sensor operating phases (namely the reset conversion phase and the signal conversion phase) to decrease the biosensor conversion time. To decrease illumination time and save the excitation light source energy, the biosensor uses a short sensing duty cycle of 1.2 ms (12%) for a sampling period of 10 ms. The proposed optoelectronic biosensor is implemented in a 0.18-μm CMOS technology, consuming 56 μW from a 3.3/1.8-V supply voltage, while achieving a high signal-to-noise and distortion ratio of 101 dB and a minimum detectable current of rms , within conversion time of 1.2 ms. The proposed biosensor presents a FOM of 0.73 pJ/conv., which is among the best reported performances compared to previous solutions.