Video dac rise time control and supply noise suppression

Abstract

Adverse visual effects on a video display caused by switching operations in a digital-to-analog converter of the video subsystem and by parasitic impedance loading the DAC are minimized. Current cells associated with the DAC generate discrete currents for the video display. Each current cell has a current source for providing a current continuously and first and second switching mechanisms. The first switching mechanism is actuated by a select signal for switching the current to a current sink having a dummy resistance R.sub.d, and the second switching mechanism is actuated by an nselect signal for switching the current to the video display. The select and nselect signals, are generated from input data. A first feedback loop combines the select signal with the data to derive the nselect signal so that the nselect signal is generated after the select signal decreases to a predefined threshold, preferably zero. A second feedback loop combines the nselect signal with the input data to derive the select signal so that the select signal is generated after the nselect signal decreases to the predefined threshold, preferably zero. Accordingly, the current output from the current source is stabilized by more precise feedback-controlled switching of the first and second switching mechanisms so that the adverse visual effects on the video display are eliminated.