Single supply Op-amp?

[Ocelot]

Can you use an op-amp with a single positive power supply (instead of dual) for a input signal that is both positive and negative if you only care about the positive portion of the signal?

For example, what happens if you setup an op-amp with a single supply and then send in a sine wave as the input? Will you only see the positive portion of the waveform?

[jwatte]

The input signal reference (positive and negative) may or may not be referenced the the supply. For some instrumentation amps, you can have the +/- pair be hundreds of volts away from the supply level.
Thus, for the use case you suggest, you have to check the data sheet to figure out how negative the input is allowed to go.

Another thing to do, if you don't need DC measurement, is to build a resistor bridge to get the "+" side centered, and then drive that "+" side from the input signal using a decoupling capacitor.

[waltr]

It really depends on which Op-amp you use. Some will get stuck on the rail (latch up) if the input goes to close or beyond the rail Voltage. Others will reverse the output Voltage. All tend to take extra time to recovery for this 'over Voltage' condition before acting like an op-amp again.

It is usually best to provide input protection before the op-amp. Rectified diode and/or diode clamps to prevent a negative input Voltage are typical methods.