Digital noise stems from the switching transients of digital circuits. In all vision chips digital signals are present at least to scan the outputs of the array out of the chip. There may also be some digital circuits for performing on-chip processing. The effect of digital noise on analog circuits is related to the distance between the two circuits. For small distances there is a linear relationship between the distance and the amount of digital noise [Masui 92, Su et al. 93, Kerns et al. 96, Verghese et al. 96]. As the distance increases the noise remains almost constant. This has been associated with the noise coupling through the bulk substrate. There can also be some direct capacitive or resistive coupling between the switching signals and the nodes in the analog circuits. There are some techniques to partially reduce the effect of digital noise [Makie-Fukuda et al. 95, Basedau and Qiuting 95].
Most of the studies performed on modeling and characterizing digital noise has been focused on the separate analog and digital modules. However, in vision chips there is a direct coupling between at least the digital scanning signals and the analog biasing or read out lines. If the only digital circuit on the chip is the scanning circuitry, one can reduce the effect of digital noise by letting the signals to settle after every digital transition. In motion detection chips, however, this is not feasible. Motion detection vision chips operating in continuous mode are more susceptible to digital noise, as the circuits usually should operate in subthreshold and also be very sensitive to be able to detect very low temporal contrast images [Moini et al. 96]. In these circuits the scanning signals should be carefully designed to have slow transitions.