Measuring a negative input on unipolar supply ADC

Pseudo differential takes a full positive swing on the +IN input whether it is unipolar or bipolar. If it is bipolar the -IN needs to be fed the nominal dc offset of the signal fed to +IN.

This is an advantage on analogue systems using a mid-rail generator (such as a voltage reference and op-amp buffer). If the voltage reference moves up or down a little, it doesn't affect the ADC code value produced because that offset movement will affect both input signals together.

For a unipolar input where -IN is 0V and a mid-rail generator is used, if the mid-rail generator shifts a little, then that is reflected as a dc error in the converted result.

I tend to use unipolar configurations (even when a mid-rail generator is used) because there appears to me more choice in ADCs AND I always choose really good references PLUS, in a lot of signal processing I do, I don't care about (say) a 10mV offset end-to-end.

For example, the input signal is a single ended sin function with range from 0 to 3V and a DC bias of 1.5V. Should we connect the IN– to 1.5V or to GND?

• If the dc bias of 1.5 volts and very stable it doesn't matter which you use
• If you don't care too much about small dc offset variations then use either
• If you do care about offset variations then use bipolar pseudo differential

...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).