Did you ever find a solution to this?
The technical reference manual says on page 579, under register SENS_SAR_START_FORCE_REG:
SENS_SAR2_EN_TEST SAR2_EN_TEST is active only when reg_sar2_dig_force = 0. (R/W)
The reg_sar2_dig_force is not defined any where, I assume they mean SENS_SAR2_DIG_FORCE in SENS_SAR_READ_CTRL2_REG.
I have checked my code and this bit an is set to 0, I also did some experiments on selection other AD channels, but did not find a cure.
Section 28,3.2 mentions:
• One channel for internal voltage vdd33, two for pa_pkdet (available on selected controllers)
But the manual does not state that it should be possible to get a AD reading of the 3,3 V. Mine read 0.
I miss information on where the ADC reference is connected to. Normally this should be clear in the datasheet, but I did not find this information. As I can't see any AD reference on the pin out, I assume they have an internal AD ref. probably some build in reference or simply the 3V3 supply voltage somewhere on die where voltage stability is good. In case the reference is tied to 3V3 supply voltage, the AD conversion will always result in a reading of 4095, so it will not be possible to measure the 3V3 voltage, meaning that the en_test selection can only be used for some internal testing (Although I don't see the meaning in this)