Measurement Tutorial
AC
VOLTAGE
AND
CURRENT
MEASUR&ENT
ERROR
IN
mA
=
(Displayed current)
X
-
Es
-
Eb
ERROR
IN
PERCENT
=
-
X
100
EXAMPLE:
Displayed current
=
1460 mA
Es
=
15V (measured with 8840A in VDC function)
Eb
=
0.4V (measured with 884OA as described below)
t
0.4V
Error in percent
=
-
X
100
=
2.67%
15V
Error in mA
=
(1460 mA)
X
0.4V
=
40 mA
15V
-
0.4V
8840A
Rb
Error in percent
:
Es
=
Source voltage (measured)
Eb
=
Burden voltage (measured)
RL=
Load
+
source resistance
Rb
=
8840A internal resistance
13V
Error in mA
=
(1460 rnA\
X
0.4V
=
40
mA
\
J
2A INPUT LO INPUT
-
Es
RL
411-
To get the correct current, add the error in mA to the displayed current:
Correct current
=
1460 mA
+
40 mA
=
1500 mA
NOTE: MEASURING BURDEN VOLTAGE
The 8840A allows you to measure burden voltage (Eb) directly. Leaving the
2A
and LO INPUT leads in place for
current measurement, select the VDC function. Then, connect a third lead to the
HI
INPUT terminal and with it
measure voltage at the 2A terminal.
Figure
45.
Burden Voltage Error Calculation
increase for more "pointed" waveforms as shown in Figure
4-7.
The
8840A
has a full-scale crest factor limit of 3.0 for the
20V and
700V
ranges, and 6.0 for the other ranges. For
full-scale input signals with a crest factor above these
limits, dynamic range limitations can begin to cause large
errors. However,
as
Figure 4-7 shows, signals with a crest
factor above 3.0 are unusual.
If you don't know the crest factor of a particular waveform
but wish to know if it falls within the crest factor limit of
the 8840A7 measure the signal with both the 8840A
and
an
ac-coupled oscilloscope.
If
the rms reading on the
8840A
is 113 or more of the waveform's zero-to-peak voltage, the
crest factor is 3.0 or less.
4-19.
AC-Coupled
AC
Measurements
Input signals are ac-coupled in the ac functions. One
of
the
advantages of ac coupling is that ac measurements can
be
made on power supply outputs,. phone lines, etc. Ripple
measurements, for instance, cannot
be
made with dc coup
ling. Remember, however, that when the 8840A measures
signals with the ac functions, the reading on the display
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