PIC24FJ 16-bit ADC Diff. Channel

[nilsener]

PIC24FJ64GC006

Dear,

I want to use the Diff.-Channels CH0+ and CH0- from the 16-Bit ADC of the PIC24FJ64GC006 in a new project. The question is, what is the range of the result of the AD-Conversion (Vref+ = Vdd, Vref- = Vss)?

I think it can be one of this possibilities:

1. range is 0x0000 to 0xFFFF for CH0+ >= CH0- and 0x0000 for CH0+ < CH0- (unipolar operation)
or
2. range is 0x4000 to 0xFFFF for CH0+ >= CH0- and 0x4000 to 0x0000 for CH0+ < CH0- (bipolar operation)
or
3. range is 0x0000 to 0x7FFF for CH0+ >= CH0- and 0x8000 (two'2 complement) to 0x0000 with MSB = sign for CH0+ < CH0- (bipolar operation)

Finally the question is, if the ADC can operate bipolar with CH0+ > CH0- and CH0+ < CH0- or not.

What do you think? Thanks for any help!

Nils

[Ttelmah]

First get a copy of DS30687a. This is vital.....

It's not as simple as a 16bit ADC giving a 16bit result. You can program the result of the oversampling to be rounded to 8, 16 or 24bits (or 32bits, unrounded). This ADC is _noisy_ (this is inherent in SD converters), so you take a minimum of 16* oversampling (up to 1024*).
The result is always a 2's complement (signed) 32bit value. So actually 0x80000000 to 0x7FFFFFFF.
The value is quite inaccurate as well, so you need to apply gain correction.
Yes all of the differential ADC's generate -ve results when IP+ is below IP-. However none is 'bipolar' in terms of allowing an input to operate below the supply rails. The minimum voltage for the Vref-, is the lower supply rail, and the maximum voltage for Vref+, is the positive supply rail. The minimum and maximum values for the inputs are Vref- to Vref+ (however they are rated to handle up to the supply rails).