MSP430FG438: About the DAC Calibration

Mike,

regarding you questions:

1. When you mean DAC calibration you refer to the implemented offset calibration described in user guide chapter "31.2.5 DAC12 Output Amplifier Offset Calibration" correct? So by what value it differs from cycle to cycle.

2. I think a and b represent the parameters for the linear curve shown below the table. While a represents the offset of the y axis it is finally the offset error while b is multiplied with x and represents rise rate of the DAC conversion curve and therefore it represents the GAIN error. In the end it is a simple linear curve.

3. The offset calibration can be done automatically as described in user guide before DAC measurement. The gain error has to be either calibrated by customer or taken as worst case from datasheet with 3.5% from FSR.

With respect to difference in calibration please take note of the last marked sentence from users guide. If device configuration is different between cal cycles e.g. CPU running for one cycle while off for another cycle it would explain differences. The point is that the device loading has an influence on supply voltages which again impacts analog performance.

The DAC12 has the capability to calibrate the offset voltage of the output
amplifier. Setting the DAC12CALON bit initiates the offset calibration. The
calibration should complete before using the DAC12. When the calibration is
complete, the DAC12CALON bit is automatically reset. The DAC12AMPx bits
should be configured before calibration. For best calibration results, port and
CPU activity should be minimized during calibration.