Tektronix 475A - Applications and Measurements; Preliminary; Signal Ground; Input Coupling Capacitor Precharging

APPLICATIONS
AND
MEASUREMENTS
Refer
to
Basic
Oscilloscope
Displays
as
needed
to
obtain
correct
displays.
PRELIMINARY
CAUTION |
NON Ol EA all Mh.
Avoid cross-grounding because instrument damage
may occur.
Signal Ground
The most reliable measurements are made when the
oscilloscope ground is connected to the ground of the unit
under test by the ground strap on the signal probe. Also a
© ground lead can be connected to the ground banana jack
on the oscilloscope chassis to establish acommon ground
with the unit under test.
input Coupling Capacitor Precharging
In the GND position, the input signal is connected to
ground through a one-megohm resistor to form a
precharging 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 accidentally generated will not be
applied to the amplifier input. The precharge network also
provides a measure of protection to the external circuitry
bv 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, especiativ if the de
level difference is more than 10 times the VOLTS/
setting.
1. Before connecting the probe tip toa signal: source
set the Input Coupling switch to GND.
2. Touch the probe tip to oscilloscope chassis gre und
Wait several seconds for the Input Coupling capacitor'ti
discharge.
3. Connect the probe tip to the signal source.
4. Wait several seconds for the Input Coupling
capacitor to charge.
5. Set the Input Coupling Switch to AC. The display
will remain on the screen so the ac component of the
signal can be measured in the normal manner.
28
475A/DM44
Operators
©.