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 > > > > - ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu