[SI-LIST] Re: How to Measure Ground Noise

  • From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 6 Apr 2005 13:17:43 -0400

Doug Smith (www.dsmith.org or www.emcesd.com)  has done work and written
some articles about making scope measurements with paper-clip loops or a
shorted scope probe like yours.

Ideally, your probe isn't picking up any voltage from the board, but the
probe loop makes it into a magnetic probe.

However, since the scope itself is probably grounded (through the AC mains)
too, connecting the shorted probe leads to "ground" or anything on your PC
board causes a handful of interesting things to happen, including some
rather large ground loops that you might not want to think about.  In truth,
these loops are probably there all the time even when you're probing
"normal" signals, so some of the effects are things you ought to concern
yourself with all the time anyway.

I've heard someone suggest a way to measure "ground noise" on a PCB, is to
float the scope (isolate it from AC mains ground), connect the scope probe
pin to "ground" on the board, and the scope's "ground" lead to a DC power
rail.  In other words, connect it backwards between GND and PWR.  (I don't
understand the reasoning behind doing that, however.)

Ask yourself what you want to measure.  What is "ground noise", anyway?  It
makes no sense to even think about the "ground noise" at a single point in a
circuit.  Voltages are always relative to something!  A single IC has no
"ground noise" unless the IC has multiple GND pins.  Presumably, what you
want to check is how much this ground TP differs from that ground TP over
there.  If you think you are measuring "ground noise" on a circuit relative
to your lab's system ground, think about the frequency range over which that
assumption applies in a non-transmission line environment.

Regards,
Andy

------------------------------------------------------------------
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
  

Other related posts: