[SI-LIST] Re: How to Measure Ground Noise
- From: <ji-wei_du@xxxxxxxxxxx>
- To: <doug@xxxxxxxxxx>, <gary_pratt@xxxxxxxxxxx>
- Date: Mon, 16 May 2005 22:32:17 +0800
Hiell team :=20
I am wondering whether it's possible to make the ground noise with =
"Scope+ a current probe rather than a voltage probe". In fact , I have =
been recommended this methods to several of my friends . And the =
concerns with a current probe is "bandwidth" . It should be no =
problems if the current probe bandwidth is enough , or else , we need to =
look for alternative solutions .=20
Regards
Jiwei Du
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx =
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Doug Smith
Sent: Saturday, April 09, 2005 12:39 AM
To: gary_pratt@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: How to Measure Ground Noise
Hi Gary and the group,
The scope chassis is large enough to be "grounded" at frequencies over=20
a few MHz even if it were battery powered and completely floating. It=20
is not possible to isolate the scope except at DC and power line=20
frequencies, it it were no antenna would work and we all would not be=20
in this line of work!
I agree that the current Pyng is seeing is probably environmental. If=20
you leave the probe shorted and lift is 1 mm from the circuit and the=20
scope display does not change much, then the loop of the probe itself=20
is picking up a magnetic field. I have only seen this be the source of=20
the problem once. Usually ground current on the probe is the problem.
By the way, the equipment to which the probe is attached does not need=20
to be grounded for this to happen. A few pF of capacitance back to the=20
scope chassis will allow tens of milliamperes to flow if driven by a=20
few Volts at tens of MHz. A series resonant circuit is formed by the=20
probe shield inductance back to the scope and the capacitance between=20
the scope and the EUT. I demonstrate this live using a small battery=20
powered oscillator powered by a few AA batteries in my seminar. Even=20
this generates 2 to 4 Volts drop across a 6 inch ground lead of the=20
scope probe.
Doug
Pratt, Gary wrote:
> I suspect you are actually measuring the common mode ground current
> coupling of the scope (plus the radiated energy passing through the =
loop
> of the probe ground wire). See if your results change if you float =
the
> scope power with a good medical isolation transformer (at your own
> risk), or wrap the power cord many times through a good torroid, or =
wrap
> the ground cable around the tip of the scope probe (to reduce the loop
> area). =3D20
>=20
> If you see a dramatic change, you will know your measurement reflects
> more of whats going on in your measurement environment than what is
> going on in your PCB. =3D20
>=20
> Gary
>=20
>=20
>=20
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx =
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of ospyng
> Sent: Wednesday, April 06, 2005 1:23 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] How to Measure Ground Noise
>=20
>=20
> 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.
>=20
> is this the correct where? and Why?=3D20
> 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?
>=20
> thanks
> pyng
>=20
>=20
>=20
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-------------------------------------------------------
___ _ Doug Smith
\ / ) P.O. Box 1457
=3D=3D=3D=3D=3D=3D=3D=3D=3D Los Gatos, CA 95031-1457
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