Grounding

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Leonardo
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Joined: Tue Jan 14, 2020 6:08 pm

Grounding

Post by Leonardo » Tue Jan 14, 2020 6:14 pm

Hi, I bought a PicoScope 2204A. The manual warns me not to connect the probe ground to anything with a significant voltage difference. Could you please specify "significant" and give an example where such an difference could happen? I plan to use the PicoScope with a desktop PC, which should be connected to the ground of my home, like all other devices plugged in. So there should be no difference, right?

Gerry
PICO STAFF
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Re: Grounding

Post by Gerry » Wed Jan 15, 2020 1:16 pm

Hi Leonardo,

There are 2 significant voltages that you should consider here.

The first and most important significant voltage would be one that would be high enough above the ground reference voltage (for the building) so as to damage the PicoScope. In this case you want to be very careful when you are working with high voltages, where you are thinking of attaching the tip of the probe to a voltage much larger than the input voltage range of the PicoScope, and the ground of the probe to a point on the Design Under Test (DUT) that is above zero volts, e.g. so that the voltage between the probe tips can be reduced to a level that can be measured using the input range. This could create a potentially dangerous situation for both the PicoScope and the user. The easiest way to avoid this would be to use a Differential Probe (see here: https://www.picotech.com/accessories/active-oscilloscope-probes) whenever you need to work with high voltage DUT's.

The second significant voltage would be one large enough to corrupt the measurement data and is a little more complicated to explain. One of your measurement goals should be to capture a signal with the minimum amount of noise and interference, so that you can measure it with the largest possible dynamic range. Any noise or induced unwanted signals will restrict your measurement placing a lower limit on the smallest change in signal level that you would be able to see. The power supply will already have noise and interference, and there may be more of it in the environment in which you will perform measurements. As the level of noise and interference that you see will be proportional to any difference in voltage between the ground tip of your Probe and the return ground of the building your aim should be to minimize this.

To put this into perspective, for building wiring, you can have differences of a few volts between spurs of different ring mains, down to a few millivolts between spurs on the same ring. If the leakage currents of equipment being powered are typical (i.e. very low) and the equipment is not interconnected, these supply voltage differences should not be significant enough to cause any problems, but once you start connecting equipment you need to be aware of how you are changing the return paths, so that you don't create ground loops with common impedance coupling, where too much current is flowing through the same shared resistance. This would result in increasing the voltage difference between the Probe tip and the Building ground return reference, and corrupting your measurements with too much noise and unwanted signals, mains hum, etc.

So, to sum up, in your case there will be no significant difference in voltage, regarding safe use of the PicoScope, and there should be no issue regarding local mains induced interference, (as long as you are not creating ground loops with significant return current).

Regards,

Gerry
Gerry
Technical Specialist

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