Patent ID: 6331830
Filing Date: 2001-12-18
Classification: H03M

Abstract:
A self-trimming current source for a switched current source digital-to-analog converter (DAC), comprising:a fixed current source, comprising:a first field-effect transistor (FET) having its gate connected to a first bias voltage, its source connected to a first junction, and its drain providing the output of said self-trimming current source, said first FET saturated by said first bias voltage such that a known voltage is established at said first junction,a second FET complementary to said first FET, having its gate connected to a second bias voltage and its source connected to said first junction such that said known voltage establishes said second FET's gate-source voltage and thereby a first output current at said output, the drain terminals of said first and second FETs each providing a high impedance to the respective circuits to which they are connected,a variable current source connected to provide a current to said first junction and thereby a second output current at said output in response to a control signal received at an input, the total output current of said self-trimming current source being the sum of said first and second output currents,a first switch responsive to a calibrate signal which connects the drain of said second FET to a measurement circuit when said calibrate signal is in a first state and to a circuit common point when said calibrate signal is in a second state,a measurement circuit which produces an output representative of the magnitude of said total output current when said calibrate signal is in said first state, anda correction circuit which receives said current measurement circuit output at an input and provides said control signal to said variable current source at an output, said correction circuit arranged to vary said control signal such that said total output current is made equal to a predetermined value within a predetermined tolerance,said self-trimming current source being one of a plurality of current sources making up a switched current source DAC.