Perhaps the differential probe is loading the signal resistively, and
the FET probe is loading capacitively. This difference in loading may
be what you are seeing.
I've seen differential probes with 50 ohm input impedance, and FET
probes with megohm (1000000 Ohm) impedances.
If you are using a low impedance differential probe on the power supply,
then the additional load may (depending on the actual supply) decrease
or increase the noise generated. Some older switching supplies have a
required minimum load for stability.
Remember, 3.3V through 50 ohms = 70 mA, which may be a significant load,
especially if this is a one-IC test board.
-- Laurence Michaels Et tu, Microsoft?
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