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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

Can you invert channel B. This would give you subtraction in the scopes 'add' mode.

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.

Dean

Thanks, that worked.
Brad

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.

Dean