Fluke 8020A - 3-20. VOLTAGE MEASUREMENTS; 3-22. CURRENT MEASUREMENT

TO
DIGITAL
CONTROL
LOGIC
.IZ I INTEG. 1 READ
-
.- -
10,000
I I 1
I I I
COUNTER OUTPUT
-4
I I I
166.7 mS 5 00 1000 1500 2000
TEN LINE
CYCLES FOR
OPTIMUM
k COMMON MODE
AND NORMAL
MODE REJECTION
Figure 3 - 2. Dual Slope AID Converter
One is a reference voltage and the other IS an unknown dc
voltage within the range of
- 0.2 to +0.2V dc. If the
function being measured is other than a dc voltage within
the
f 0.2V range, it must be scaled and/ or conditioned
before being presented to the
a/d converted. For
example, higher dc levels must be divided; ac inputs must
be divided, rectified, and filtered; and resistance,
conductance and current inputs must be scaled and
converted to dc voltage levels. The following paragraphs
describe
the input signal conditioners used for each of the
8020A measurement functions.
3
- 20. VOLTAGE MEASUREMENTS
3
- 2 1.
Both the ac and dc voltage ranges use an over
-
voltage - protected, 10 Mfl input divider as shown in
Figure 3
- 3A. Under normal conditions, assuming a dc
input level on the proper range, the divider output is a
- 0.2 to +0.2V dc signal, and is an exact (power - of - 10)
ratio of the input signal. If the VAC function is selected,
the divider output is ac coupled to an active full
- wave
rectifier whose dc output is calibrated to equal the rms
level of the ac inputs. The conditioned signal for the
selected function (V ac or V dc) is then passed through a
filter before being presented to the
a/d converter as the
unknown input.
3
- 22. CURRENT MEASUREMENT
3
- 23. Current measurements are made using a fuse
protected, switchable, four
- terminal current shunt (0.1 fl,
lfl, 100 or loon) to perform the current - to - voltage
conversion required by the
a/d converter. See Figure
3
- 3B. The voltage (1-R) drops produced across the
selected shunt may be either ac or dc depending upon
the selected function,
mA AC or mA DC. If the input
current is dc and the dc function is selected, the
1-R drop
is passed through a low
- pass filter, and presented as the
unknown input to the
a/d converter. However, if the