ADC differential output

GUNTOJU ANAND KUMAR said:

configured ADC0 SS3 in Differential mode  by using channel 1.

Perhaps not - does not "differential mode" demand the use of a "proper, channel pair?"    (Ans: yes - thus your single channel usage is doomed)   Upon "re-read" you may have implied, but did not write, "channel pair #1."    That's acceptable.

Your title, "ADC differential output" may not be as clear as you thought.    There is no "conventional" output from an ADC - analog channel data is captured - and multiple ADC program examples detail how that ADC data may be then stored - usually w/in data variables.

Further - you note "negative voltage" input.    If such voltage is wrt (with respect to) ground you are violating the MCUs ADC input specification - are you not?    I believe that this vendor's ADC - when placed into differential (input) mode - will generate data at/around, "mid-ADC scale" when the differential input voltage is 0V - and will transition then both "above & below" that mid-scale result as the differential voltage drives positive or negative - "with respect to those 2 analog channels, only."    

Usually this constrains your differential analog signal voltage to 1/2 of the ADC's maximum input voltage spec - and the voltage upon either differential input pair pin must never "go negative" wrt ground!   (i.e. the difference in voltage between these 2 analog pins (differential pair) may span both negative & positive - but ONLY in reference to each other - and both voltages must (always) remain above ground and w/in the ADC's maximum input voltage rating)

I've wondered what the use of this differential input is.

It's got essentially zero common mode range so that rules out a whole class of inputs where the signal is riding on an offset, you're going to need a differential op-amp in any case.

It needs a rather large signal (+/- 1V6) so that rules out small signal applications, you're going to want some like an instrumentation amp anyway.

Any bi-polar input will either need to be level-shifted or if it is floating have its reference somehow pinned to mid-range.

Anything remote you are going to need some signal conditioning and protection. An op-amp is probably desirable. Even w/o the op-amp why wouldn't a single ended configuration be just as effective?

I'm left with sources that are bi-polar with natural midscale references that are mounted on board but I don't understand why it's useful to use the differential configuration rather than the normal single ended configuration in that case.

Robert

Robert Adsett said:

I've wondered what the use of this differential input is.  

Make that "two" in some wonderment, friend Robert.    In time - vendor's Amit may reveal, "that which is behind the curtain."

Poster's usage sheds no clue - "terminology & description light (strange those)" fails miserably here.

We do employ an AD8251 IN Amps to (dynamically) "gain-up" the measure of several critical charge pump voltages - resident upon our 80 Amp, battery powered, BLDC Controllers.    And - as you note - we inject into "single ended" ADC channels - not differential.

Of course there do exist Differential Amps which provide symmetrical yet opposite polarity output levels - which usually - but not always - may be "tweaked" into riding at the ADC's mid-point input voltage...