0
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ Don't forget this op-amp has bjt outputs so the maximum output swing is within 2.5 volts of Vcc and Vee. These are very high quality op-amps, but do not have rail-to-rail outputs. Do you have bypass caps (100nF) on the power rails to ground to prevent oscillation? \$\endgroup\$ – Sparky256 Aug 13 '18 at 22:57
  • \$\begingroup\$ I found the issue to be that the signal is grounded and that messed up the output. When I un-connect the ground from the circuit it works fine. Do you know why? \$\endgroup\$ – Alex M Aug 13 '18 at 23:01
  • \$\begingroup\$ "When I un-connect the ground from the circuit it works fine. Do you know why?" - Umm. How about because when you ground the signal you force it to zero? Have you truly never heard the term "short circuit"? \$\endgroup\$ – WhatRoughBeast Aug 14 '18 at 4:13
4
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ I found the issue to be that the signal is grounded and that messed up the output. When I un-connect the signal ground from the circuit it works fine. Do you know why? \$\endgroup\$ – Alex M Aug 13 '18 at 23:01
  • \$\begingroup\$ I edited the answer \$\endgroup\$ – Voltage Spike Aug 13 '18 at 23:06
  • \$\begingroup\$ Ah! So I guess I was shorting the circuit with the oscilloscope! Thanks! \$\endgroup\$ – Alex M Aug 13 '18 at 23:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.