A Novel Approach to Noise Shaping in Digital Pixels for Infrared Imagers using Over-Integration

Digital readout integrated circuits (DROICs) for small pitch infrared focal plane arrays (IR-FPAs) suffer from low in-pixel resolution owing to the limited pixel real estate. To this end, a new technique to improve the resolution of pulse frequency modulation (PFM) based pixels, using quantization charge noise shaping, is presented. Multiple integration operations are performed in a single frame and the quantization error from each integration phase is retained and effectively induced into the next integration phase. The result is a high pass noise transfer function (NTF) equivalent to what is obtained in a first order sigma delta modulator. Along with a theoretical analysis of the technique, a prototype based on a single pixel and a 2
^nd
order decimation filter is developed to demonstrate the performance of the proposed technique. With an in-pixel circuitry generating 5 bits, a resolution of 11 bits (65 dB) is achieved with an over-integration ratio of 64. With a 100 Hz frame rate and 64 integration operations per frame, a readout noise of 1100e- is measured at full-well fill from the test pixel fabricated in a 90nm CMOS process.