Correlated double sampling with up/down counter

An image sensing device having correlated double sampling circuitry, said image device comprising:

• a plurality of imager cells arranged in rows and columns, with the imager cells of any given column being coupled to a column data line of that column, and each imager cell being operable to selectively provide a first signal on an associated column data line indicative of a reset level during a first sampling interval and to provide a second signal on the associated column data line during a second sampling interval indicative of an amount of light incident upon that imager cell;
• a unidirectional counter operative to count in one of an up or down direction during each of said first and second sampling intervals;
• a digital to analog converter coupled to an output of said unidirectional counter, for converting an output thereof to a time varying reference signals which ramps during each of said first and second sampling intervals;
• a plurality of comparators, each having a first input coupled to an associated one of said column data lines for receiving said first and second signals in the first and second sampling intervals, respectively, and a second input coupled to receive said time varying reference signal;
• and
• a plurality of up/down counters, each operable to count in a first direction during said first sampling interval and in an opposite direction during said second sampling interval, each of said up/down counters being responsive to an output of one of said comparators to stop counting during each of the first and second time sampling when the amplitude of the time varying reference signal substantially equals the amplitude of the respective first or second signal, whereby each said up/down counter provides an output representing a subtraction of one of said first or second signals from the other to complete a correlated double sampling operation.

A method for performing correlated double sampling, comprising the steps of:

• digitally counting in a first direction during a first sampling interval;
• comparing a first signal to a time varying reference signal during the first sampling interval and stopping said counting when the first signal and the time varying reference signals are substantially equal to produce a first count;
• digitally counting, from said first count, in an opposite direction to said first direction during a second sampling interval;
• and
• comparing a second signal to the time varying reference signal during the second sampling interval and ceasing to count when the second signal substantially equals the reference signal, to thereby produce a second count which represents a subtraction of one of said first and second signals from the other.