Differential measurements with two separate probes are limited to a
bandwidth of a few MHz. Much above that and a host of effects come
into play that limit CM rejection to 10 dB or less, and you probably
have a large measurement loop caused by the separate probes.
Doug
"Ingraham, Andrew" wrote:
>
> >When I measure PWR-GND noise of high-frequency(~100MHz) IC, I have two
> >options.
> >One is using the standard Active (FET) probes, the other is using a
> >differential probe.
> >
> >The results of two measurement are slightly different.
> >
> >Which must I believe ?
>
> Believe neither. Any hi-freq. measurement should be suspect. Every probe
> will affect the signal and its display in some way.
>
> Are you probing the same exact two points, and are both probe leads VERY
> short? Probing a signal and ground pin that are only an inch apart from
> each other, can introduce sizable errors.
>
> The differential measurement *ought* to be better, *if* the diff. probe is
> sufficiently accurate and wide bandwidth, as it avoids the ground loop.
>
> Try doing a differential measurement with two FET probes, and see how it
> differs from the other two. Try various combinations to get a feel for how
> these changes affect what you measure.
>
> Regards,
> Andy Ingraham
>
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