MOS transistor digital-to-analog converter

Abstract

A monolithic, low power, digital-to-analog converter (DAC) circuit which uses an efficient transistor element to perform both switching and resistive current division functions simultaneously. This allows a R-2R type ladder network to be built using only conventional MOS transistors which can both switch and accurately divide current among the branches of the ladder network, without the need for separate resistors. The lower parts count and requirement for MOS transistors only, without the need for separate resistors, makes this circuit very compatible with low cost monolithic implementation. The DAC of this patent is useful in an application requiring the multiplication of two analog signals, where one of the signals is presented as a digital word. In this application, a Gilbert multiplier circuit is used to multiply the two signals, Vdig and Vsig, where Vdig represents the binary-weighted discrete levels from the DAC and Vsig is a continuous analog signal. Additionally, in order to obtain linear multiplication over a wide range, the Gilbert multiplier requires the use of a predistortion circuit in conjunction with the Vdig signal coming from the DAC to compensate for the logarithmic current-voltage transfer function of this circuit.

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