op-amp outputs supply voltage on GND Signal
Looks like you driving the output tri-state (open circuit) rather than pulling it to ground.  You can add a resistor from the non-inverting input to ground or change your firmware. When it's open the op-amp input will float up or down, predictably if it's a bipolar op-amp to one supply or the other, and unpredictably if it's a MOS input op-amp. For example, an LM324 input will float upward because the bias current flows out of the op-amp input. 

You should also be using a "single supply" (or rail-to-rail I/O) op-amp - the output should swing to near ground and the input common mode range should include the negative rail. 

Edit: The MC33708 is not 'single supply', as you know. Some op-amps (not sure about the MC33708, I've used them, but have not knowingly violated the input common mode range that way) may behave the way you describe. LM358 is the the same pinout and should work okay.