Lab Instrumentation — question 10 of 10
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You're debugging a USB-powered peripheral board that's earth-grounded through its metal enclosure via the USB cable's shield back to your laptop. You're powering the board's separate analog section from a bench supply whose chassis is also earth-grounded through the building's mains wiring. You clip your oscilloscope's probe ground lead onto a ground test point on the board to check a signal. The instant the clip makes contact, there's a small spark and your laptop's USB port shuts down. What just happened, and how would you set up the measurement to avoid it next time?