Why is my oscilloscope showing noise when I short both terminals of the probe and connecting them to any metal surface?

Question

My understanding that when the terminals of an oscilloscope probe are shorted, the voltage level shown should be around zero.

Today, in my lab, I shorted both terminals and then connected the probe to a few of metal exposures on equipment laying around, whereby I saw some unnerving noise on the scope.

Pictures are attached below:

Probe terminals shorted

Voltage reading on the scope

Probe terminals shorted AND connected to a metal surface

Voltage reading on the scope. Noise is visible

I have tried disconnecting the equipment from the mains, but the noise still remains visible on the scope.

What could be the reason?

Answer 1

This is due to imperfect common-mode rejection of the oscilloscope's input circuitry.

You initially short the probe tip and, there is a little bit of noise. This is to be expected because, all amplification systems are imperfect. Then, you connect the shorted probe tip to an alternative earth point. That alternative earth point might have several tens or hundreds of millivolts AC signal on it with respect to the oscilloscope's own ground and, you see a signal.

That is also to be expected.

If you used a differential probe (instead of an unbalanced scope probe) then it's much more likely that the waveform seen on the scope will be like your first scope image i.e. the probe shorted and not connected to anything.

Of course, the short makes a loop antenna and you can easily pick up mV to volts when testing such things as switch mode power supplies but, in you example, I believe that what you see is due to imperfect common-mode rejection of the scope's input circuitry.

Answer 2

Generally, the "short" you form with the clip forms a loop antenna which readily picks up magnetic interference when in proximity.

To confirm/exclude this: If you go close to the object without touching it, you would still detect the signal if it is due to magnetic pickup of the clip loop.

In contrast, if the signal appears only when in contact, a grounding current flows through the scope lead's outer contact and the finite impedance of this lead causes a detectable voltage drop that you are seeing. Even if the object you touch is itself earthed, it will still develop RF potentials, because of the ground loop formed between it and the scope.

Answer 3

Like Andy aka said, the issue is the lack of common mode rejection of the measurement setup. To some extent this is to be expected when not using a differential probe, but single-ended probes work well for many practical tasks.

If the input was shorted with 0 ohm impedance, the oscilloscope would see 0 volts input. But it isn't - all of the cables in the system have significant inductance:

^{simulate this circuit – Schematic created using CircuitLab}

The cable inductance picks up noise and causes voltage drop. In the coaxial pair, the inner and outer conductor are closely coupled and noise voltage mostly cancels out. However, the few centimeters of the ground clip wire already presents a significant unbalanced inductance.

When the noise source is floating, there is not much voltage drop over this inductance. But when the noise is coupled through mains cabling, current will flow through the probe cable outer conductor. At the oscilloscope end, the outer terminal is connected directly to the power supply ground. Thus it presents a low impedance to any ground-referred noise source, and the noise voltage at oscilloscope ground terminal is small - most of the voltage is lost in the cable inductance.

That sounds like a good thing, but the positive terminal presents a high (typically 1 Mohm) impedance. The noise coupled there is not attenuated, and is detected by the input amplifier. Cancellation doesn't occur because the common ground causes attenuation at the negative terminal.

The source of the high frequency noise is probably some mains-powered device nearby. For me, it has usually been cheap LED lighting. Try disconnecting devices and see if affects the detected noise.

Switching to a shorter ground clip cable on the oscilloscope or to a ground spring will also give a big improvement. Here is a comparison I did with 10 MHz 20Vp-p square wave from a signal generator. White line is with 10 cm long ground clip cable, and yellow line is with ground spring (both shorted to the probe tip, like in original post):