A dual channel 20 bit current-input A/D converter for photo-sensor applications

A dual channel 20 bit current-input A/D converter is implemented in a 0.6 /spl mu/m CMOS process with a single 5 V supply for use in direct photo-sensor digitization. Continuous charge integration is achieved through the use of a double switched capacitor integrator at each channel input. The front end provides a programmable full-scale input charge range from 50 pC to 1000 pC while also converting the input current to a voltage output. The voltage output of the switched integrator is then digitized by a high dynamic range delta-sigma converter that multiplexes between the two input channels. This current-input A/D converter achieves 3 ppm RMS noise and 1 ppm linearity with low level inputs.