[SI-LIST] Re: Why did my measurement work?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 23 Apr 2012 20:03:17 -0700
Michael Faraday.
Steve.
On 4/23/2012 7:36 PM, Iain Waugh wrote:
> 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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