Incorrect voltage readings through ADC
I don't have the reputation to post this a comment so I've had to 'answer'.

Is the ADC Vdd really connected as shown in the schematic? If so it doesn't have a current path from the power rail and so doesn't have power.

It looks as though C3 and C4 are intended to provide decoupling. Capacitors performing this function should be connected between the positive and negative supply rails directly (hopefully near to the IC that they are providing decoupling for).

The Vdd pin of U3 should be connected directly to the positive rail and not through C3 or C4.

Sorry if I've misread your schematic and gone off on a tangent, but it should really be a lot clearer than it is.

(While we're at it, is there a difference between the 'ground' connected to the negative supply of OA1 and the negative rail used in the rest of the circuit?)