Dense offset calibration circuitry and method
AssigneeCrystal Semiconductor Corporation
InventorTimothy J. DuPuis
Life of this patent
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Abstract
An audio mixer circuit on an integrated circuit chip performs a calibration operation on power up which calibrates out most of the offset voltages of the operational amplifiers used in the mixer. The calibration logic includes a shared calibrate circuit which provides timing signals to each operational amplifier and its associated calibration circuitry. The calibration operation is performed by digitally controlling and changing the bias current into each of the operational amplifiers until the offset voltage is compensated. A class A flip-flop circuit is used in the digital counter of the calibration circuitry to drive a current digital-to-analog converter.
Claims7
A method for compensating for the offset voltage of a plurality of operational amplifiers comprising the steps of:
- a) providing a calibrate signal from a shared calibrate circuit to couple both inputs of each of the operational amplifiers to a reference voltage;
- b) providing control signals from said shared calibrate circuit to a plurality of calibration circuits, each coupled to one of the operational amplifiers in order to inject a bias current into at least one node of each of the operational amplifiers such that the offset voltages of the operational amplifiers are changed as a result of said injected current into said at least one node;
- and
- c) repeating step b) for different current values injected in a stepwise manner until an output of each of the operational amplifiers changes.
Apparatus for compensating for the offset voltage of a plurality of operational amplifiers comprising:
- a) a shared calibrate circuit having a calibrate signal output coupled to each of the operational amplifiers, each of the operational amplifiers coupling both inputs to a reference voltage in response to receiving said calibrate signal;
- b) a plurality of calibration circuits coupled to said shared calibrate circuit and to one of said plurality of operational amplifiers, each of the calibration circuits, in response to a control signal from said shared calibrate circuit, injecting a bias current into at least one node of an operational amplifier such that the offset voltages of the operational amplifiers are changed as a result of said injected current into said at least one node;
- and
- c) wherein the bias current injected is changed in a stepwise manner until an output of each of the operational amplifiers changes.
Dependencies are inferred from the claim text.