HP 400D - Set the RANGE

Section TI
Paragraphs 3-10 to 3-16
=— POSSIBLE GROUND LOOP +
T.O. 33A1-12-349-1
INSERT UNGROUNDED
ADAPTER TO BREAK
o T USE
GROUND LOOP eae
ONE SIDE OF
POWER LINE IS
ALSO GROUNDED
Figure 3-3. Test Setup for Avoiding Ground Loop
3-10. Since the voltmeter meter deflection is propor-
tional to the average value of the input waveform, it
is not adversely affected by moderate levels of random
noise. The effect that noise has on the accuracy of the
meter reading depends upon the waveform of the noise
and upon the signal-to-noise ratio. A square wave has
the greatest effect, a sine wave intermediate effect, and
"white" noise has the least effect on the meter reading.
3-11. If the noise signal is a 50% duty cycle square wave
and the signal-to-noise ratio is 10:1 (between peak
voltages), the error will be about 1% of the meter
reading. If the noise signal is "white" noise and the
signal-to-noise ratio 10:1, the error is negligible.
3-12. LOW-LEVEL MEASUREMENTS AND GROUND
CURRENTS.
3-13. When the voltmeter is used to measure signal
levels below a few millivolts, ground currents in the
meter test leads can cause an error in meter reading.
Such currents are created when two or more ground
connections are made between the instruments of a
test setup and/or between the instruments and the power
line ground. Two ground connections complete an
electrical circuit (ground loop) for the voltages which
are generated across all instrument chassis by stray
fields, particularly the fields of transformers. These
ground currents can be minimized by disconnecting
the ground lead in the power cord from either the
voltmeter or the signal source being measured, at the
power outlet as shown in figure 3-3, and by making
sure that in the test setup no other ground loop is
formed that can cause a ground current to flow in the
voltmeter test leads. Although the resultant voltage
developed across a test lead is in the order of micro-
volis, it is enough to cause noticeable errors in
measurements of a few millivolts. The presence of
ground currents can sometimes be determined by
simply changing the grounds for the instruments in the
3-2
setup and watching for a change in meter reading. If
changing the ground system causes a change in meter
reading, ground currents are present.
3-14. MEASUREMENT OF VOLTAGE.
3-15. The meter has two VOLTS scales, 0 to 1 and
0 to 3. When the RANGE switch is set to .001, .01,
-1, 1, 10, or 100 VOLTS, read the 0 to 1 scale. When
the RANGE switch is set to .003, .03, .3, 3, 30, or 300
VOLTS, read the 0 to 3 scale.
The lower (black) signal INPUT and OUT-
PUT terminals and the instrument case are
connected tothe power system ground when
the instrument is used witha standard three-
terminal (grounding) receptacle. Connect
only ground-potential circuits to the black
INPUT and OUTPUT terminals.
3-16. Operate the instrument as follows:
a. Connect the voltmeter to the a-c power source.
b. Turn the Power switch ON and allow a warmup
period of approximately five minutes.
c. Disconnect any external equipment from the OUT-
PUT terminals.
d. Set the RANGE switch to the VOLTS range which
will read the voltage to be measured at mid-scale or
above. If in doubt, select a higher VOLTS range.
e. Connect the voltage to be measured to the INPUT
terminals.
00102-2