A CMOS imager includes an array of active pixel sensors, wherein each
pixel is associated with a respective column in the array. The imager
also includes multiple circuits for reading out values of pixels from the
active sensor array. Each readout circuit can be associated with a
respective pair of columns in the array and can include first and second
sample-and-hold circuits. The first and second sample-and-hold circuits
are associated, respectively, with first and second columns of pixels in
the array. Each readout circuit also includes an operational
amplifier-based charge sensing circuit that selectively provides an
amplified differential output signal based on signals sampled either by
the first sample-and-hold circuit or the second sample-and-hold circuit.
The readout circuit also has an analog-to-digital converter for
converting the differential output to a corresponding digital signal
using a successive approximation technique. Use of the readout circuit
can increase the parallel structure of the overall chip, thereby reducing
the bandwidth which each readout circuit must be capable of handling.