35.3.2.8 Offset Calibration

The input offset voltage of an op amp can be calibrated by using the DAC or an external constant-voltage source in conjunction with the ADC.

After a reset, the production measured calibration value is loaded from fuses into the OpAmp n Calibration (OPnCAL) register.

To calibrate an op amp with a different value than the production measured value, perform the following steps:
  • Use the DAC or the external I/O pin connected to the op amp non-inverting (+) input as a constant-voltage reference. If the DAC is used, it and its output buffer must be enabled.
  • Configure the op amp with:
    • MUXPOS in the OPnINMUX register set to the selected calibration source
    • MUXNEG in the OPnINMUX register set to OPnOUT, so the op amp will function as a voltage follower
  • Use the ADC to measure the voltage of the selected calibration source. This value becomes the calibration target.
  • Configure the ADC input multiplexer to select the output from OPn, then use the ADC to measure this voltage
  • Use the difference between the two ADC measurements to determine how much the calibration register (OPnCAL) should be adjusted. The calibration step size is found in the Electrical Characteristics section.