Identifying If The Dut Is Isolated From The Pc; Using An Isolated Wall Adapter To Power The Dut - Saleae Logic User Manual

•
If you see a voltage that indicates a noticeable current, then there is a ground loop between devices
and you should not connect the grounds together.
•
If you see an insignificant voltage across the resistor, then either there is no ground loop or there is
a ground loop, but both grounds are at the same reference. It is safe to connect the logic analyzer.

Identifying if the DUT is Isolated from the PC

The DUT's local ground is isolated from the host PC when one of the following is true:
•
The DUT is battery-powered and has no other electrical connections to the host PC or devices
powered from MAINS power.
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The DUT is powered from an isolated power supply that does NOT short MAINS earth ground to the
output ground. Bench top supplies with a separate green earth ground terminal do this. USB wall
adapters also do this. Common AC power adapters (chargers, "wall warts") with 2-prong plugs are
also isolated. Most power supplies do have transformers that can provide isolation if implemented
properly.
•
The Host PC is a laptop running from a battery or is plugged into an instrumentation isolation
transformer. Note that normal isolation transformers connect earth ground for human safety
reasons.
Warning: When working in an electrically isolated state, keep in mind that floating grounds can be
dangerous to the operator. When operating with equipment with a floating ground, please review and
follow appropriate safety measures.

Using an Isolated Wall Adapter to Power the DUT

Using isolated wall adapters such as USB wall adapters to power the DUT will isolate its ground from
MAINS ground, although that does not always eliminate ground loops. For example, if the DUT was
connected to the same computer that the Logic device is connected to, then a ground loop is formed.
Is the Logic Device Safe to Use in the Presence of Ground Loops?
Yes, it is completely safe to use the Logic device as long as both grounds are at the same voltage level
and as long as you only connect the Logic ground to the ground of the DUT.
Isolation: There Is a Ground Loop and a Difference in Potential Between Grounds
What do you do if the grounds are related but not at the same potential? You will need to fully isolate the
DUT from the PC. You can do this on either side of the logic analyzer.
To isolate the digital input side of the logic analyzer, you can consider using the
Kit
To isolate the USB 2.0 high-speed connection, you can consider using the
Isolator. We recently purchased this isolator and will be performing an evaluation soon. This is the first
SI84XXISO Evaluation
Intona High Speed USB
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