I found these two nice ADC that should accept input voltages more negative than the ground: BB-ADS1220 (https://www.olimex.com/Products/Breadboarding/BB-ADS1220/open-source-hardware) and BB-ADS1262 (https://www.olimex.com/Products/Breadboarding/BB-ADS1262/open-source-hardware).
Checking the schematic in PDF of the ADS1262 I realized that AVSS (the negative voltage from the power supply, needed for measuring voltages below GND) is connected to GNDA, and GNDA is connected to GNDD (double checked with KiCAD), which makes measuring negative voltages impossible. Is this correct?
Interestingly, this does not happen in ADS1220, so it seems that I can measure voltages below GND using that one, is this right?
Thanks!
Better get BB-ADS1220, you have more freedom about the powering. AVSS can go down to the absolute minimum of -2.8V.
In the BB-ADS1262 - it will be hard to measure negative voltages due to the design. Even if you disconnects the two grounds by removing resistor R1, we still create the voltages 3.3V->5V.