A 9-b charge-to-digital converter for integrated image sensors

Charge-to-digital conversion offers advantages over conventional charge readout techniques because it performs digitization directly in the charge domain. The approach consolidates hardware, reduces power and weight, and eliminates many sources of noise and nonlinearity. This paper introduces an architecture for a charge-to-digital converter (CDC) that is tailored toward a charge-coupled device (CCD) implementation. New methods of generating charge, sensing charge, and comparing charge packets are described that improve conversion accuracy. Factors limiting device performance are discussed. Measured results are presented for two prototype CDCs. The first, using buried channel CCDs, is optimized for resolution. It achieves 56 dB spurious free dynamic range (SFDR) at a 2 MHz sampling rate and operates from 5 V. The second, using surface channel CCDs, is optimized for power and speed. It achieves 49 dB SFDR at a 15 MHz sampling rate and consumes 13 mW power at its maximum sampling rate of 22 MHz.