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Hot on the trail of an elusive current fluctuation

Ron Davison
7/29/2010 11:09 PM EDT

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peralta_mike
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re: Hot on the trail of an elusive current fluctuation
peralta_mike   8/5/2010 5:27:09 PM
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Clever Idea ! Thanks. - Mike Peralta

Liyu
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re: Hot on the trail of an elusive current fluctuation
Liyu   8/4/2010 7:49:13 PM
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The simplest way to measure a small AC signal from a large DC signal is to set the probe to be AC coupled, so the DC signal will not be displayed on the scope. This is what we do when measuring the output ripple of a DC supply. My question: is the described method can provide more accurate AC measurement?

hithesh
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re: Hot on the trail of an elusive current fluctuation
hithesh   8/1/2010 4:50:43 AM
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I get the basic idea. But a crude ckt diagram would be useful.

WKetel
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re: Hot on the trail of an elusive current fluctuation
WKetel   7/30/2010 10:02:40 PM
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A very good solution, indeed. But you did need to consider the stability of the smaller current source, because any variation there would be magnified 200 times on your scope waveform. But it shows us just how important the fundamental concepts can be.

mixed_signal
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re: Hot on the trail of an elusive current fluctuation
mixed_signal   7/30/2010 6:45:23 PM
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Nice summary. Canceling large offsets is good technique in general for using small signals to correct noise in large ones, or as here to reduce a required measurement range. This is the first time I've seen a magnetic situation for this, though. I checked around and it turns out that Tektronics mentions this technique in their probe manual, online: http://www.sy-yq.com/tek/pdf/tcp202.pdf. They call the offset current a 'bucking current.' By the way, an ideal current source has infinite output impedance, not a low output impedance.

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