A reflected-light receiver with 2 nA sensitivity using CDS and separate low-pass filtering of the input signal at each clock-phase

Abstract

The design of an integrated receiver for reflected light using correlated double sampling (CDS) is presented. CDS has the disadvantage that input signals of high frequencies are folded back to the base band caused by sampling the continuous input signal. However, the circuit presented here uses a new method to limit the bandwidth of the CDS input signal. Two filter-capacitors, one for each clock phase, are periodically connected to the input of the CDS circuit. As the input signal is low-pass filtered by the ohmic output resistance of the optical sensor in conjuction with the filter-capacitors, aliasing can be avoided. Measurements of a test circuit demonstrate the effectiveness of this principle.