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

Douglas C. Smith (doug@dsmith.org)
Thu, 09 Sep 1999 21:50:42 -0700

Hi All,

In reference to Laurence's reply below, a probe may say it has a
megohm input resistance but that does not hold much above the audio
band. Above 160 kHz 1 pf input capacitance will dominate the input
impedance (Xc < R). Almost all probes with a high input resistance in
reality have a capacitive input impedance.

The limitations on the measurement loop size for a probe with a
resistive input impedance (say 1000 ohms) are that the loop be small
with respect to a wavelength and that it doesn't pick up stray
magnetic fields. A loop with a capacitive input impedance must have
the measurement loop much smaller otherwise resonance will occur
between the loop inductance and the probe capacitance.

Doug

Laurence Michaels wrote:
>
> 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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