How to sense a control voltage without pulling it down?
Try this. Only U1 is really required. The other two op-amps are only conditionally necessary.







simulate this circuit &ndash; Schematic created using CircuitLab

The reason I quartered the input signal then re-amplified by two intead of just halving it was becase since I could not figure out how to add an offset using a unity-gain voltage follower or else I would have done that for U1 and just stepped down by half.

Instead, I had to use the non-inverting amp circuit for U1, but the non-inverting amp must always have a gain greater than 1 so I had to make up for it by stepping down the signal more than actually required at R2/R1.

The reason I halved it should be obvious...to fit the 10V peak to peak swing of -5V to +5V into the 5V peak to peak swing allowable by 0V to 5V. I'm assuming your Arduino accepts 5V at the ADC. If it accepts 3.3V max then you have to adjust the stepdown and the gain to accommodate.

If the input impedance is too low without U3, you may be able to do without U3 by increasing R2 and R1 to something like 75K and 25K, respectively. I really feel that input impedance was never your problem though. It was that you were feeding negative supply voltages to op-amps or circuits that could not handle them.

Oh yeah, if you have an oscilloscope, you should scope the user control output to see what it is normally, and what it is with the circuit connected to it. That would let you know what's going on almost immediately.