Digital-to-analog converter with on-board unity gain inverting amplifier

Abstract

The invention provides a monolithic Y-bit resistive-ladder type digital-to-analog converter (DAC) having a unity gain inverting operational amplifier as an input buffer to the resistive ladder segment of the DAC. The reference voltage is applied to the input buffer amplifier. Optional bipolar operation is provided by applying a non-inverted reference voltage to the output of the resistive ladder segment of the DAC through a scaled resistance. Analog ground current cancellation is provided by a secondary X-bit R-2R ladder (where X Y) with the non-inverted reference voltage applied to it. The secondary bit ladder is switched in parallel with the top X bits of the main ladder, thereby supplying or sinking roughly the same amount of current as the X most significant bits of the main resistive ladder, but with opposite sense. The effect on the DAC output of the resistance and temperature coefficient of the switches used in the R-2R ladder and elsewhere in the circuitry is minimized by selecting switches appropriately scaled to provide resistances whose sum effect is to cancel each other out. (Particularly, a FET switch biased permanently on is provided at the input of the unity gain inverting operational amplifier.) Since the DAC is monolithic, the switches have approximately equal temperature coefficients such that the effect of the switches is consistent at all temperatures.