[SI-LIST] Re: Why did my measurement work?
- From: Sen Velmurugan <sen.velmurugan@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 26 Apr 2012 22:14:55 -0700
Hi Iain
When you touch down the probe leads, there is current flow
1. To/From the Board GND to the input amplifier GND
2. Board GND to input attenuator of the probe to the input amplifier GND
3. The i/p amplifier measure the voltage across the i/p impedance seen deep
inside the scope at i/p amplifier.
The reason is scope i/p GND is not exactly at 0.00000000V, the board GND w.r.t
scope GND is not at 0.000V
Now you can create an equivalent circuit model involving all the parasitic, use
loop current and node voltage analysis to know what the scope is "seeing".
Assuming scope is measuring all w.r.t to its zero voltage,
what it is displaying is GND noise w.r.t scope GND not w.r.t your board GND ;-)
Thanks
Sen
Von:si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Iain Waugh
Gesendet: Dienstag, 24. April 2012 04:36
An:si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Why did my measurement work?
Hi all,
I was debugging a colleague's design last week; the symptom was spikes of noise
in sensitive ADC readings. The problem was tracked down to the layout of a
switch mode PSU, where the local GND net was not kept away from the system GND.
Every time the MOSFET switched, the GND of the board would jump up or down.
The solution is to fix the layout, but my question comes from how I tracked it
down.
I was using a 500 MHz LeCroy 'scope, probing about looking for noise with
standard 500MHz passive probes. The 'scope was AC coupled and at 20mv/div with
2 extra bits of 'eres' (a running average function that increases bit depth at
the expense of bandwidth).
I tried looking for radiated noise by looping the flying ground lead to the tip
of the probe and moving the loop over various parts of the board. That found
nothing.
Then I tried subtracting the difference from 2 probes with their ground leads
connected to each other (about 3" long) and left floating. This showed the
noise spikes when I placed the probes in different locations, as you would
expect it to. What I didn't expect was that the spikes still showed up when
the probes' tips were touching each other AND touching the GND net.
I tried again with just 1 probe with its GND lead clipped to the probe tip.
Once the grounded tip was placed on the board's GND net, I could see the spikes.
In the 2 probe differential measurement case, why did the spikes appear when
the probes' tips were shorted? It only happened when the shorted tips were in
contact with the board, not in the air.
In the grounded tip case, how did the spikes appear on the 'scope when they
should have been eliminated by the ground lead?
Best regards,
Iain Waugh
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