Electronic – Measuring a potentiometer with noisy power

...as far as i have read about them (also their data sheets), if an ADC has both single and differential ended inputs, we can use it as a single ended input ADC OR we can use it as a differential ended input ADC. Which means we cannot use it as both single and differential input at the same time.

This is not always true. For example, I have used ADS1015 on a couple of projects recently. On this chip, whenever you switch the channel being read you also have the option to switch between single-ended and differential measurement. (This is not an endorsement of this chip for your project. Just an example of a chip that doesn't have the limitation you thought was universal)

Furthermore, even if you had a device that had to be configured as single-ended or differential for all channels at the same time, nothing prevents you from using ground as one of the inputs to a differential channel. So you could just configure it as differential and move on with your design. The only thing you'd lose is the opportunity to use the 4th input pin for some other purpose.

Another option, if you plan to use external signal conditioning, you can do differential to single-ended conversion in the signal conditioning circuit, and your ADC will never know the signals are anything but single-ended. This essentially makes the amplifiers shown in your diagrams to be external devices rather than internal to the ADC chip (and add some filtering in their feedback networks to reduce noise).

Neither the manual nor the datasheet are particularly clear about powering the sensor. It's quite possible that at some voltages, it doesn't need the full 25mA. But without finding that information in the datasheet, then even if the sensor seemed to work with lower current, we'd never be sure it was going to be reliable.

So you need to find a way of providing the 25mA at between 10 and 30V. Do not try to control the voltage of your DC power supply with a pot/divider, they can only regulate voltage at negligible load, if you want to regulate the voltage you should use a suitable voltage regulator IC. But unless I am misunderstanding the datasheet, you don't need to regulate it at all, 12V is between 10 and 30, so you can just connect it up.

Introducing a second ground is unlikely to be a problem, for two reasons:

• Many AC/DC converters one can buy are isolated anyway. Double check yours, but it probably doesn't introduce a ground.
• The manual states that the sensor input and output are isolated, so it wouldn't be a problem if they used different grounds.