Electrical – No output reading from Op Amp on Oscilloscope

circuit analysisfunction generatormeasurementoperational-amplifieroscilloscope

So I have an inverting op amp (gain = -1) wired up, and it works with a DC signal. I checked the DC signal by measuring the input and output voltages with a multimeter which were 5V and -5V, respectively.

Now I hooked up a function generator to the input signal. The input is now 5V peak-to-peak at 1kHz. The input shows up fine on the oscilloscope, but that is because it is practically connected to the source of the generator. When I measure the output, nothing shows up on the oscilloscope (just a little bit of noise). I tried connecting the output to the virtual ground and then measure the output with the oscilloscope, but that didn't do anything.

The op amp I am using is TL051CP.

How am I not measuring it correctly?

Solution

So I guess the problem was my signal was grounded and that messed up the output. I just removed the ground from the circuit, and it is working fine. I don't know why it works when the signal is not grounded.

I believe I was shorting the circuit with the oscilloscope, which caused this to happen.

Best Answer

There are two problems that could be causing grief, the first is the mode of the oscilloscope, the second is the ground of the oscilloscope.

The first thing to do is validate your equipment, which you already did kind of. Make sure the oscilloscope is in DC mode, and measure a DC voltage. The probe may be capacitance coupled (in AC mode) which would explain that you can measure a 5V 1kHz square wave but not a DC signal.

Measure a battery or power supply rail to make sure the oscilloscope is measuring correctly. Once you have measured DC signals then return to the op amp and make sure your oscilloscope ground is the same of the op amp.

Edit: On most scopes the ground is connected to mains ground and not isolated, so if you connect a ground point to something that isn't ground (or a virtual ground that isn't isolated) then a short to earth ground will be created), check the point that you are grounding and make sure it's really ground. Another way to do this is to measure the voltage between the point you are connecting it to and the oscilloscope ground with a DMM. If it measures something more than a few 10's of mV you may have a problem.

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