Re: [SI-LIST] : Measuring noise with Differntial probe

Laurence Michaels (lmichael@techmail.gdc.com)
Thu, 9 Sep 1999 08:59:52 -0400

Tadashi ARAI wrote:
>
> Hello SI-list members,
>
> 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 ?
>
> //// /// // / / / / / / / / / /
> Tadashi Arai//Platform Developing Dept.,Desktop Prd Div. Fujitsu Limited
> arap@paso.fujitsu.co.jp TEL:+81-42-370-7617 Inagi-shi, Tokyo, Japan
> / / / / / / / / / / / // ///
How different is 'slightly different'? 10%? 1%?

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.

Regards,

-- 
Laurence Michaels
Et tu, Microsoft?

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