1. About the ADS1118 ADC converter
I'm using the
ADS1118 ADC converter from Texas instruments to measure a voltage. You can find the datasheet here:
The converter has four analog inputs. You can configure them to be four independent single-ended inputs, or two differential inputs.
2. Single-ended mode
If configured as single-ended inputs, my setup could look like this (simplified):
3. Differential mode
In differential mode, I can measure
Vsig better because I can compensate for the resistance (and possible noise pickup) from the cable going to the unit producing
I expect to see
AIN1 signals like this:
What bothers me is
AIN0 being so close to
GND (Obviously, it actually is
GND measured from a little distance). In my experience with OPAMPS, input signals should always stay away from the positive or negative rails. Some rail-to-rail OPAMPS can deal with such inputs, but even then you usually should keep at least a few
OPAMPS perform best if you keep the inputs close to mid-rail. In other words, you keep the common mode of your differential input pair close to mid-rail.
4. What says the datasheet?
I've searched in the
ADS1118 datasheet, but couldn't find any figure relating the signal's closeness to the
Vdd rails to reading errors.
The only figure that could be useful is this one:
But the figure isn't entirely clear to me. It looks like the x-axis represents the differential input, being for example
Vdiff = AIN1 - AIN0. If that's the case, then this figure is useless for my case, because it doesn't say anything about the common mode of this differential signal.
5. What if it doesn't matter?
What if the common mode of the differential signal doesn't matter? Then the following two cases should give exactly the same results:
I feel like the situation on the right (SITUATION TWO) will give much better readings than the situation on the left (SITUATION ONE). But I can't find anywhere proof of that in the datasheet.
6. My question
So my question is:
Will SITUATION TWO from the last figure give better results (lower error) compared to SITUATION ONE?
If yes, can the error difference be quantified (some formula/graph)?
Where do you find this info in the datasheet?
Thank you very much ^_^