44.3.2 Manual Input Offset Voltage Calibration

The OPAxOFFSET register can be used to recalibrate or adjust the input offset voltage from the factory calibrated value. This can be accomplished by using the DAC module or an external constant voltage source, in conjunction with the ADC module. The OPA input offset voltage can be recalibrated using the following steps:

  1. Set the OFCSEL bits to ‘b01 to select OPAxOFFSET register value as the input offset source.
  2. Configure the DAC module to be used as a constant voltage reference connected to the noninverting input (OPAxIN+) of the OPA module.
    • An external constant voltage reference can be used instead of the DAC, by directly connecting the external source to one of the noninverting input pins
  3. Configure the OPA module to operate in Unity Gain mode by setting the UG bit.
    • Use the PCH bits to connect the calibration source (either the DAC module or an external voltage reference) to the noninverting input (OPAxIN+) of the OPA
    • If using an external voltage reference, the PSS bits must also be used to select which OPAxIN+ pin will be connected to the noninverting input
  4. Perform an ADC conversion to measure the voltage of the selected calibration source. The value read by the ADC during this conversion will serve as the calibration target.
  5. Use the ADC Positive Channel Selection (ADPCH) register to select the OPA output (OPAxOUT) and then measure the output voltage of the OPA module using the ADC.
  6. The difference between the measured value of the calibration target and the measured value of the OPA output can be used to determine the value needed to calibrate the OPA input offset voltage using the OPAxOFFSET register.
Note: The OPAxOFFSET register stores an unsigned value which can be use to optimize both positive and negative offset voltages.