ADuCM355 ADC negative input AGND

Hi,

For measuring single ended sensor voltage,

You may choose N input as ADCVBIAS_CAP, and use the following transfer function:

For PGA gain = 1,

VIN = (VREF × (((ADCDAT − 0x8000)/215))) + ADCVBIAS_CAP 

For PGA gain = 1.5,

VIN = (VREF/1.5 × (((ADCDAT − 0x8000)/215) × 1.835/1.82)) + ADCVBIAS_CAP

For PGA gain = 2, 4, or 9,

VIN = (VREF/PGA_GAIN × (((ADCDAT − 0x8000)/215))) + ADCVBIAS_CAP

where: VREF = 1.82 V (typical).

ADCDAT is the ADC conversion result.

ADCVBIAS_CAP = 1.11 V (typical).

PGA_GAIN is the PGA gain setting minus one. This gain is set by ADCCON, Bits[18:16].

I recognize that from the datasheet, however it does not produce accurate results.

Perhaps it's because the voltage being measured is about 0.82v, so it's less than

the ADCVBIAS_CAP voltage.

You may choose VZERO0 instead of ADCVBIAS_CAP and set it to low value by setting DACIN6 of LPDACDAT0 to low value.

Also, ADCVBIAS_CAP may be replaced by VZERO0 in the equation.

I would still prefer to use AGND for the negative input. I will connect one of the analog inputs to ground

for the next board revision, but for now, I'm using the TEMPN selection as AGND. I have added a function

to measure the tiny offset and compensate for it.