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RC voltage on Scope

Posted: Sun Feb 24, 2008 12:32 pm
I am trying to measure ac voltage in a simple RC circuit, (one resistor and on capacitor in series), with an oscilloscope. The book says to use two probes and use the A minus B mode. My scope does not have this. It has channel 1 and channel 2 buttons and in between them is a add button how can measure the voltage across the components, capacitor or resistor. Help.
Thanks Brad

Posted: Sun Feb 24, 2008 1:29 pm
by Robert Reed
Can you invert channel B. This would give you subtraction in the scopes 'add' mode.

Posted: Sun Feb 24, 2008 8:17 pm
by Dean Huster
Other than some wild differential amplifier plug-ins for a mainframe, I've never seen a scope that has a dedicated "A-B" function, let alone a portable like most folks use. As Robert noted, the differential mode of a scope is the "ADD" mode with Channel 2 inverted. In the "ADD" mode without Channel 2 inverted, it's an "A+B" function; with Channel 2 inverted, it becomes an "A-B" function. All the differential mode does for you is allow you to connect both probes across signal points much like a multimeter can do.


Posted: Mon Feb 25, 2008 4:53 pm
Thanks, that worked.

Posted: Mon Feb 25, 2008 5:10 pm
by Dean Huster
Oh, and I might add that if you have a lot of common-mode signal or noise that's burying the signal you want to see, the differential mode ("A-B") could be your ticket to a better waveform observation.