circuit to measure positive & negative voltage + Vref resolution
This does not directly answer your question but may give you an idea for a different solution.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. The simplest offset adjust.

If you can live with reduced resolution on the ADC the circuit of Figure 1 may be acceptable.

Vin       Vout
-2 V      +1.5 V
 0 V      +2.5 V
+2 V      +3.5 V


It reduces the 5 V span to only 2 V with the resistor arrangement shown.

I think the best span you can get with this arrangement will be by reducing R1 to 2k. With this ratio the ADC starts at 0 V.

Vin       Vout
-2 V       0 V
 0 V      +1.428 V
+2 V      +2.857 V


You can scale the resistors to suit. The ratio is what is important.