The patent badge is an abbreviated version of the USPTO patent document. The patent badge does contain a link to the full patent document.
The patent badge is an abbreviated version of the USPTO patent document. The patent badge covers the following: Patent number, Date patent was issued, Date patent was filed, Title of the patent, Applicant, Inventor, Assignee, Attorney firm, Primary examiner, Assistant examiner, CPCs, and Abstract. The patent badge does contain a link to the full patent document (in Adobe Acrobat format, aka pdf). To download or print any patent click here.
Patent No.:
Date of Patent:
Aug. 25, 1998
Filed:
Jul. 19, 1996
Edison Fong, Sunnyvale, CA (US);
National Semiconductor Corporation, Santa Clara, CA (US);
Abstract
An operational amplifier in a bias voltage generator of a MOS current summing digital to analog converter corrects deviations in output current due to variations in drain to source voltages in current slaves caused by differing output resistances and supply voltages. Matching of MOS current sources uses an operational amplifier feedback circuit to create a controlled turn-on reference voltage used for biasing selected differential current paths so as to eliminate drain to source voltage variations in precisely ratioed current slave MOS transistors. One transistor of each differential current pair is enabled by a corresponding switch coupled to the turn-on reference voltage produced by the operational amplifier. In the preferred embodiment, the switches are CMOS transmission gates enabled by the binary digital input and its complement. Low voltage (3 volts) operation is achieved by having minimum number of stacked transistors between power supply voltages. Reliable current matching allows converter resolution of 10 bits. Due to cascode switching action controlling alternative differential current paths, output current is independent of output resistance. The operational amplifier samples the current bias reference used to bias the binary ratioed slaves and the drain of a current mirror and drives a feedback transistor gate and the enabled differential current paths with the turn-on reference voltage so that the precisely ratioed current slaves have the same drain to source voltage as the reference current transistor. The reference voltages produced are thereby independent of supply voltages and process.