On ground loop while connecting a Logic Analyzer

[mindentropy]

Hi,

While reading through safety page of Saleae Logic Analyzer I found the common ways a ground loop can exist in this page:

https://support.saleae.com/user-guide/safety-and-warranty#common-ways-a-ground-loop-can-exist

In there I find the two points:

1.Other USB devices (such as programmers) are connected to the DUT, or the DUT itself is plugged into the USB port on your computer. In addition to the logic analyzer's ground connection, the DUT's ground is also connected to the PC's ground through another USB port.

2. Non-isolated power supplies Most AC power supplies with 3-prong plugs will short the MAINS earth ground pin to the power supply ground output. That includes your PC's ground. If your DUT is powered from a 3-prong wall power supply and your PC is also plugged in, that will form another ground path. Keep in mind that if you're using a laptop that's not plugged in, even an attached external monitor or printer will create a ground loop.

I have a DUT which is connected to my laptop USB port and also my Saleae LA. Does this mean that both of these will be connected to the common ground of the PC? If yes, would this mean that a ground loop is formed? Will my laptop provide an isolation due to the battery connected? If I have connected the DUT to a PC would it have a common ground then?


In the section on Common ways damages can occur here -> https://support.saleae.com/user-guide/safety-and-warranty#common-ways-damage-can-occur

When connecting or disconnecting probes, one of the ground probes from the logic analyzer is accidentally brushed against a power supply pin on the DUT, such as +5V. If there are no other ground paths between the DUT and the PC, nothing will happen. However, if there is a ground path, then current will flow from that voltage supply through the logic analyzer's ground pin, through the USB cable and the host PC, and then through the secondary ground connection—either MAIN earth ground or another USB port, back to the ground on the DUT. Basically, that is the same as shorting out the voltage supply on your DUT, but it uses the logic analyzer and your host PC as the short circuit, which could damage all components in the loop.

Why nothing happens "If there are no other ground paths between the DUT and the PC"? Is it because there is no return path from the DUT power supply to the LA connected to the PC and finally to the ground of another PC?

[aliarifat794]

If you have a DUT connected to both your laptop's USB port and the Saleae Logic Analyzer (LA), then yes, both devices would typically share a common ground through the laptop's power system.

[mindentropy]

In the below statement:

When connecting or disconnecting probes, one of the ground probes from the logic analyzer is accidentally brushed against a power supply pin on the DUT, such as +5V.

My another confusion is won't this also cause a short if the DUT is powered by an external power supply and take out the DUT and the LA? Won't this also cause damage to the USB ports where the Saleae LA is connected?

[m k]

Energy through a brushed connection is many times pretty low.
It can usually damage only the source of that 5V.
If wire is very thin it can only damage its self.

[Retirednerd2020]

I often use a USB isolator for circuits I have concern about.  My personal policy is always use (USB/SERIAL) isolation if I'm dealing with DUT with circuits over 5V.  I don't count on the power supply isolation of a laptop for equipment or personal safety.  This may be a bit overkill but isolators don't cost much and generally don't get in the way.

[JustMeHere]

If you're nervous, run the laptop from battery when testing.

[mindentropy]

Energy through a brushed connection is many times pretty low.
It can usually damage only the source of that 5V.
If wire is very thin it can only damage its self.

If it is powered by an external source with Over Current Protection, wouldn't that be much better safety?

[m k]

Of course.
But keep in mind that you can short everything else also, and those others may be much less tolerant.
An output of a single OpAmp can be one, but it can be internally protected also.
So better learn a habit that everything stay put when power is on.

USB is a hot plug type connector, it can be disconnected anytime.
If you think a Windows message telling that it's now safe to disconnect, it's different.
Old Windows versions used delayed write style, so it's a software based message.