I have been working on using an op-amp (non-inverting) to control some small transistors from an arduino. To simplify the circuit I connected the negative power supply of the op-amp to ground and positive to about ten volts. This works fine but for input below a certain level when I would expect op amp voltage output to stay low it goes back up high.
opamp.bmp (638 KB)
If the op-amp is not designed for single ended use, you are probably seeing some sort of common-mode problem. Use a single supply op-amp, or use a voltage divider to provide a ground reference at Vcc/2.
In addition to common mode voltage range, many op amps are not designed to have their inputs at voltages very close to the power supply voltages (in your case ground and the positive supply). For example, the inputs of the popular LM324 will not work properly if they are within 1.5 V of Vcc, but can be connected to ground. Also the output voltage swing may have significant limitations . You need to check the data sheet of your op amp for the input/output voltage ranges, or buy a "rail to rail" op amp.
Thanks for your help. It is working so far with a voltage divider, and a ground for the op amp and arduino and another ground for the rest of the circuit.
Older opamps in particular had a tendancy for the output polarity to reverse
if the inputs were driven out of common-mode range or even just over-driven.
The term for this is "phase reversal" - its always re-assuring to see "no phase-reversal"
in an op-amp datasheet.
