We are running our F28069 with a 90 MHz clock, and using the internal 3.3 V reference for the ADC. ADC readings, as viewed with the debugger, are consistently low with small ACQPS values like 6, 8 or 10. They improve with higher values like 50. And are best with values of 100 or more.
With ACQPS = 6, when I tie the input to one of the channels to GND the ADC reads 0 solidly. When I tie it to 3.3 V, it reads 4081-4087.
With ACQPS = 200, GND reads 0-3, 3.3 V reads 4095 solid.
We have waited the required 1 ms after powering up the ADC, we run Device_cal() before using the ADC, and we perform the needed dummy ADC reads before each set of conversions.
Any suggestions?
Thanks. -JD