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

Douglas C. Smith (doug@dsmith.org)
Thu, 09 Sep 1999 08:13:23 -0700

Hi Tedashi and the rest of the group,

I thought I would reply to all because of the general interest in this
type of measurement.

You might not want to believe either! The first question is exactly
which probes are you using. If the probes have an input capacitance of
one or two pf (or more), then you can get resonance with the
measurement loop inductance and are limited to very short connections.
In fact many FET input probes, single ended or differential, with 1 to
2 pf input capacitance when used with a standard ground lead that is
sometimes included will give less accurate peak readings than regular
probes. The reason for this is the resonant frequency of the loop goes
up inversly as the square root of the capacitance, but so does the Q
of the circuit. So if you go from 10 pf to 2 pf, the resonant
frequency goes from a typical 100 MHz to about 220 Mhz, but when you
get there, the Q is 2.236 times greater (with a standard ground lead
of 4 to 6 inches) leading often to a 50% to 100% overshoot. The
problem is with power-ground noise you don't know what the waveshape
should be and may not be aware of the problem.

I would recommend using a "balanced coaxial probe" made from a 180
degree combiner and two 50 ohm coaxial cables with shields in contact
most of their length. At the probe tip end, terminate in 50 ohms with
a tip resistor. If a Mini-circuits ZFS-CJ-2-1 combiner and a 764 Ohm
tip resistor is used you have a 50:1 probe. On most scopes that gives
a max sensitivity of 50 mV/div. If you can't see your noise on that
scale, you probably do not have a problem.

I have published information on this structure in various forums, go
to my webpage at http://emcesd.com and click on "published papers."

Hope this helps, I could keep on writing the rest of the day on this
topic, but the information above should get you going.

Let me know the models of the probes you are using and I may be able
to give you more detailed information on the particular measurement
you are making.

Doug

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
> / / / / / / / / / / / // ///
>
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