[SI-LIST] Re: Measuring Power and Ground Plane Noise
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: viru@xxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 29 Aug 2002 15:46:52 -0700 (PDT)
If you use a single ended probe (like the coax I mentioned)
then the shield of the probe is put at the same potential
as the ground plane (hopefully zero volts in this case).
If the ground plane is at some elevated voltage then you might want to
use a differential probe.
However the voltage you measure should be the same.
A single ended probe measures the difference between A and B
where B is assumed to be zero.
A differential probe measures the difference between A and B
where B could be any value.
In the case where B = zero volts then they are equivalent.
-Ray
>
>
>Ray, will it make a difference if i use a single ended or a differential probe
to do such measurements..??
>thanks in advance
>Virendra
> Ray Anderson wrote:
>
>The measurement of noise on power planes is a relative
>measurement. A noise voltage is measured _relative_ to
>some reference.
>
>You want to measure the noise on plane "A" relative to a
>reference point on plane "B" at the same XY coordinates.
>(i.e., don't measure the voltage at a point on the left side
>of the board with your ground connection on the right side
>of the board) The ground connection should be short and direct.
>
>You can't just measure a voltage on a plane relative to some
>fictitious (spice node 0) ground. It should be measured relative
>to a local ground. Also note that you might have 10 volts of
>a 100 kHz Hz AC signal (hopefully not) between your planes and say
>the chassis, but at the same time you might only have 100 mV noise
>between the ground plane and the Vdd plane. It is that 100 mV of
>differential noise that will effect the operation of your circuit
>from an SI perspective. however the larger common mode noise certainly
>may be an EMI/EMC concern. (The common mode noise would be the same
>on both planes, hence a zero differential potential).
>
>Typically I like to make power plane noise measurements using
>small diameter flexible semirigid coax connected from an oscilloscope
>or spectrum analyzer to soldered down connections on the board.
>Select a decoupling capacitor site, remove the capacitor and
>solder the shield and center conductor directly to the mounting
>pads on the PCB. (In the case of the spectrum analyzer be sure
>to observe the DC voltage limitations of your instrument).
>
>-Ray Anderson
>Sun Microsystems Inc.
>
>>
>>How does one properly measure power plane or ground plane noise? I have a
>>case where I believe there is 66 MHz noise on the planes. I want to know
>>how much noise is on each plane.
>>
>>Both measurements require a reference point that you must assume to be
>>stable, or perhaps that is considered stable by the measurement. So I
>>measure across a decoupling capacitor and that shows some noise on the power
>>plane. But how do I get a good measurement of the ground noise? And if
>>there is noise on the ground plane, how do I get that noise out of the power
>>plane measurement?
>>
>>Thanks for any help.
>>
>> - Paul
>
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