[SI-LIST] Re: Why did my measurement work?
- From: "A. Ingraham" <a.ingraham@xxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 24 Apr 2012 12:20:22 -0400
> 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.
This method of making differential measurements is never perfect,
partly because of differences between the two probes and their scope
channels. With the probes floating, both probes picked up no signal
so the difference between two nothings was still nothing. When
connected to "ground", they each had signal, which might cancel if
everything was perfectly matched across all frequencies.
> In the grounded tip case, how did the spikes appear on the 'scope when
> they should have been eliminated by the ground lead?
By connecting the shorted scope probe to the board's "ground", you are
adding another path for current to take, through the probe leads back
to the scope and power ground. This current induces the spiky signal
you could see.
Doug Smith probably has some notes about this on his website (www.emcesd.com).
Andy
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