AC, DC and GND

Selecting AC, DC or GND

The CRO has two of these selection switches (one for each input channel ). They are used to select the characteristics of the input signal that we want to view on the CRO display. The selector switch has three positions, A.C. D.C. and GND, all of which are detailed below:

When the A.C-GND-D.C selector is in the A.C. position the CRO input signal is said to be A.C. coupled.

A.C coupling means that only A.C. signals are allowed to pass from the input terminal through to the measuring devices within the CRO. This is achieved by passing the input signal through a large capacitor which "blocks" any D.C. component of the incoming signal.

Pro's And Cons of the A.C. Position
*Advantages*	*Disadvantages*
Can be used to view A.C. ripple in a D.C. signal as it allows us to focus only on the ripple itself at any magnification without the signal being moved off the display by a large D.C. offset. (e.g 7V D.C. with 20mV ripple)	This method distorts A.C. signals that have a low frequency. This is due to the charging and discharging of the large capacitor that is used to effect A.C. coupling. An example of this can be found by clicking here.

When the GND position is selected the input terminal is completely isolated from the display. The trace on the display becomes a horizontal line that corresponds to an input that is grounded.

This is only useful when the zero position of the trace requires adjusting using the Trace Vertical Position Control. More information concerning the adjustment of the vertical position of the trace can be found by following the previous link and checking the link to the display.

The D.C. position is the "fool proof" setting to have the input signal set to. This is because when this setting is used the input signal is able to pass directly to the display unchanged.

The only problem with this arrangement is that there may be occasions where the D.C. component of a signal would be a hinderance to the accurate measurement of the A.C. component of the same signal. An example can be found by following this link.

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Revised: Friday, 17 July 1998