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• 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.
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.
• What if the DUT's ground reference isn't at the same voltage as the ground loop connection? For
instance, if your circuit is powered by a bipolar power supply used to produce +10 volts and -10
volts, and then your circuit uses the -10 volt rail as its ground voltage, but there exists a ground loop
through MAINS earth ground to the power supply's 0 volt output, then effectively the ground on your
DUT is actually -10 volts relative to the host PC. Connecting Logic will immediately short out the
DUT power supply and potentially damage all devices present in the loop.
Testing Multiple DUTs
In some cases, you need to record signals from multiple devices under test simultaneously with the same
logic analyzer. This can be done safely, but it is important to review how both devices are powered before
connecting the logic analyzer.
Since the channels in each Saleae logic analyzer are not electrically isolated from each other, all devices
will need to share a common ground. If the devices under test already share a common ground, then
you're all set. However, if they are floating relative each other (meaning their grounds are not directly or
indirectly connected to each other), or if there is a common mode voltage setup between them, then the
additional steps below are required:
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