Patent ID: 6891487

Claim:
A method for calibrating a switched capacitor array in a SAR data converter, which capacitor array includes a plurality of primary capacitors each having a common node plate interfaced to a common node and a switched plate interfaced to a switch that is operable to be switched between first and second reference voltages, comprising the steps of: providing a comparator having an input connected to the common node and a reference input connected to a comparator reference node for receiving a comparator reference voltage; in a first calibration step for calibrating a select one of the primary capacitors: providing a reference capacitor with a common plate connected to the common node and a switched plate interfaced to a switch that is operable to be switched between the first and second reference voltages, connecting the switched plate of the select primary capacitor to the first reference voltage, connecting the switched plate of the other capacitors and the reference capacitor to the second reference voltage, and driving voltage to the common node and the comparator reference node to dispose a first voltage thereon; in a second calibration step: allowing the common node to float, connecting the switched plate of the select primary capacitor to the second reference voltage, connecting the switched plate of the reference capacitor to the first reference voltage, and comparing the voltage on the common node to the first voltage on the comparator reference node with the comparator to determine if the voltage on the common node is greater than the first voltage; providing a plurality of trim capacitors; and if, in the second calibration step, it was determined that the voltage on the common node was greater than the first voltage, then disposing one of the trim capacitors in parallel with the select one of the primary capacitors and then repeating the first and second calibrating steps.