# Grounding oscilloscope probe effects

I have built an active band-pass filter on a breadboard with the use of an MC33078p op amp. Gain is 32 on paper.

When I apply the AC signal to the circuit, 18.1mV P-P and 660kHz, the oscilloscope measures a voltage of 1.22V at the output but with zero frequency. When the earth probe is not connected, I measure a frequency of between 30 and 60 Hz.

Is there any obvious reason for this?

As well as the frequency problem, the output voltage doesn’t seem to be inverted, it’s a fairly basic circuit and don’t believe I’ve connected anything wrong.

Any ideas?

• A circuit diagram and your measurement setup would be helpful in answering your question. What do you mean by zero frequency? Zero frequency is defined as DC. Your frequency of 30 to 60 Hz is probably AC mains pickup (especially with no ground conneted). – Barry Feb 21 '18 at 22:24
• @Barry I have added a circuit diagram picture. Thanks for replying. The zero frequency refers to the output of the op amp (pin 1 for MC33078p) and I’m completely unsure as to why it’s happening. Anything you can think of? Thanks again – Jandy12 Feb 21 '18 at 22:43
• You are well into cutoff at 660khz and should not see much. 1nF is ~250 ohms so the output would be 1mv or so. Run it through a simulator. And removing the scope ground will get you meaningless results, like a lot of 60hz line noise. – lakeweb Feb 21 '18 at 23:25
• @lakeweb the supply is meant to imitate a received frequency signal from a radar receiver (intended to measure speed of moving object) which I tested to be around this using an oscilloscope that’s why it’s so high. Thanks so much for the pointers – Jandy12 Feb 21 '18 at 23:36
• Your op-amp (-) input is 150 K/4.7K ohms. That makes it very sensitive to local noise. – Sparky256 Feb 22 '18 at 0:03

## 1 Answer

Using this op amp as an active filters is not really feasible at this frequency. Active filter theory relies on the op amp to be close to ideal at the frequency of interest. At your frequency of 660 Khz, the op amp exhibits a phase shift of -100 degrees and the output voltage swing is also greatly attenuated. Best approach is to build a passive network to build your filter function, then add a gain stage with a high speed op amp. See the charts from the op amp data sheet: