FreeLists / si-list / [SI-LIST] Re: How to Measure Ground Noise

[Ray Anderson <ray.anderson@xxxxxxxxxx>]

Pyng-

Aside from the excellent comments you have received so far, one other 
thing to consider is that the measurement of "ground noise" (i.e. the 
noise at some particular ground node with respect to some reference 
node) only makes sense if the measurements are close together time-wise.

As an example of a "ground noise" measurement that really doesn't make 
sense assume you have a PCB whose dimensions are12 inches x 12 inches. 
If you put your reference probe at the point where the power supply 
connects to the board on one edge and then measure the 'ground noise' 
with respect to that point at some other location 10  inches away, the 
value you measure will be pretty useless since there will be 10 inches 
worth of propagation delay between your reference point and your 
measurement point. At DC and low-frequencies this is OK, but at high 
frequencies the time delay is significant. You would be measuring the 
noise with many hundreds of picoseconds of delay between the sampling 
points. Yes you will get a number, but what does it mean???

Of course if you follow the sage advice of others about minimizing the 
loop inductance of your measurement setup then this won't be an issue 
since your probe locations will be very close together.

-Ray

- 
Raymond Anderson
Senior Signal Integrity Staff Engineer
Product Technology Dept.
Package Engineering Group
Xilinx Inc.




ospyng wrote:

>hi,
>I was taught to measure ground noise, by having the ground of the
>probe connect to ground TP of the board and the tip of the probe
>measuring the same TP.
>
>is this the correct where? and Why? 
>is it because of the ground cable (3 inch) cause a loop, therefore the
>reference and the tip of the probe has a different voltage?
>
>thanks
>pyng
>
>
>  
>

-


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