Input Coupling Capacitor Precharging

When the input coupling switch is set to GND, the input signal is connected to ground through the input coupling capacitor in series with a 1-Mi2 resistor to form a pre-charging network. This network allows the input coupling capacitor to charge to the average dc-voltage level of the signal applied to the probe. Thus, any large voltage transients that may accidentally be generated will not be applied to the amplifier input when the input coupling switch is moved from GND to AC. The precharging network also provides a measure of protection to the external circuitry by reducing the current levels that can be drawn from the external circuitry during capacitor charging.

The following procedure should be used whenever the probe tip is connected to a signal source having a different dc level than that previously applied, especially if the dc-level difference is more than 10 times the VOLTS/DIV switch setting:

1. Set the AC-GIMD-DC switch to GND before connecting the probe tip to a signal source.

2. Insert the probe tip into the oscilloscope GND connector.

1ST OR LEFT VERTICAL GRATICULE LINE

CENTER VERTICAL GRATICULE LINE

11TH OR RIGHT VERTICAL GRATICULE LINE

1ST OR LEFT VERTICAL GRATICULE LINE

RISE AND FALL TIME MEASUREMENT PERCENTAGE MARKERS

RISE AND FALL TIME MEASUREMENT PERCENTAGE MARKERS

CENTER HORIZONTAL GRATICULE LINE

3. Wait several seconds for the input coupling capacitor to discharge.

6. Set the AC-GND-DC switch to AC. The display will remain on the screen, and the ac component of the signal can be measured in the normal manner.

4. Connect the probe tip to the signal source.

5. Wait several seconds for the input coupling capacitor to charge.

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