I have a voltage to sample that will only range between about 2.5 to 3.5 volts total span. So I thought that rather than using a single ended ADC on either a 0-5V range or 0-2.56V range, it would make more sense to try using a differential ADC for increased resolution. Is there any way to physically connect one of the differential ADC inputs to the internal 2.56V ADC reference on chip? There is no mention of this capability in the datasheet, but I just thought I would ask if anyone knew if this was possible anyway. Or must I use a voltage regulator at 2.5 or 3V as one of my differential inputs to difference against the voltage being sampled? Also, the datasheet states that when using a gain of either 1x or 10x on the differential ADC that the resolution will only be 8 bits... why is this? I was trying to go to the differential ADC because I thought the 10-bit single ended ADC's was too coarse... but going to differential drops your resolution to 8-bit?@? Also, the datasheet states that there is a "2.7 - Vcc differential ADC voltage range" with no further explanation. What does this mean? It seems to contradict a later equation given for calculating the differential ADC output of:
"ADC = (Vpos - Vneg) * 512 / Vref"
which has a range of values from -512 to +512 counts... or 1024 counts of total resolution... which is 10-bit... which seems to contradict the earlier statement in the datasheet that differential ADC is only 8-bit?
It is amazing to me how confusingly written the datasheets always are... If anyone would like to help interpret these statements for me I would be very appreciative...
1) What does the 2.7V - Vcc diff voltage range imply?
2) Is the diff resolution 8-bit or 10-bit?
3) Can I tap into the internal 2.56V ref as one of my differential inputs?