A time error model for time-based PWM pixel with correlated double sample in the circumstance of nonlinear response

• A mathematical model of time error in time-based PWM pixel with CDS is established.
• The influence of nonlinear response and noises on imaging is studied.
• The relationship between relative error and the reference voltage of the pixel-level comparator is studied.
• There is an optimal choice of reference voltage to decrease the error of imaging.

A time error mathematical model is proposed to study the impact of nonlinear response and noises on imaging of time-based Pulse Width Modulation (PWM) pixel with the Correlated Double Sampling (CDS). The errors introduced by nonlinearity and certain noises, including fixed pattern noise (FPN), reset noise, shot noise and input-referred noise of pixel-level comparator, are studied. The relative error is impacted not only by the noise voltage, but also the comparator reference voltages, Vrefh and Vrefl. In the 0.18 μm CMOS process, the simulation result shows that the noises which have the largest impact on imaging are FPN and shot noise. The higher the Vrefh and Vrefl are, the smaller the error is introduced by FPN. The larger the difference between Vrefh and Vrefl is, the better the signal-to-noise ratio (SNR) is. The influence of reference voltage and clock frequency on dynamic range, and the trade-off between dynamic range and speed are also analyzed in this paper. Finally, the rules of reference voltage selection are discussed. The results can serve as a guideline to the design of the time-based PWM pixel with CDS.