Detecting negative voltages with SAADC

[Noah]

Hi! I am going to work on a project that uses a specific sensor which produces voltage outputs between 250 mV and -250 mV. Can someone help me with ideas to detect the negative voltages with the nRF52810 SAADC? Are there SAADC options I can set via software related to offset, or would this best be handled with a hardware/electronics solution? I cannot lose resolution of the sensor, the full range must still be 500 mV (i.e. I cannot map the 250 mV to -250 mV output to land with a 250 mV to 0 mV range). 

Any advice would be super helpful! Thanks

[hmolesworth]

Typically you can use a flying capacitor to level shift from a range outside the ADC to a range within the range of the ADC, by using analogue switches. A c/o (changeover) switches the lower end of the capacitor from the negative signal (perhaps its -ve supply) to (typically) nRF52832 Gnd and a second c/o switches from the positive signal to the SAADC analogue input pin. This shifts the sensor sampled voltage on the flying capacitor from +-250mV to +500mV above nRF52 Gnd.

Without a schematic I can't give more details, but this works very well when shifting small signals sitting on top of +ve or -ve pedestal voltages, which can in some cases be quite large (the pedestal in this case is -250mV, quite small). A quad sp or dual c/o analogue switch works well, with supply voltages taken from the sensor rails.