Measuring voltage without a common ground
Your circuit is incomplete. There is no reference between your ground symbol and the power supply. You need this or the circuit will be floating relative to the supply rails and will drift. Connecting it to the mid-point of a pair of potential divider resistors from the PSU might be good enough.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. Conventional schematic layout is signal running left to right and voltages arranged with positive to top and negative to bottom. Current generally flows top to bottom.
R1, R2 and C1 form a ground reference relative to the power supply.