Fluke 8021B Instruction Manual page 44

8021
3-18.
input Signal
Conditioners
3-19.
The
a/ci
converter requires
two
externally supplied input voltages to
complete
a
measurement
cycle.
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
±0.2
range,
it
must be scaled
and
/or conditioned before being presented
to
the
a/d
converter.
For example, higher dc
levels
must
be divided;
ac inputs
must be
divided,
rectified,
and
filtered;
and resistance
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
802 IB measurement functions.
The descriptions
are
illustrated
by Figure 3-3.
3-20.
VOLTAGE
MEASUREMENT
3-21.
Both the
ac
and dc
voltage
ranges use
an
over-voitage-protected, 10
Mil
input
divider as
shown
in
View A. Under normal conditions,
assuming a dc
input level
on
the
proper range, the divider-
output
is
a -0.2 to
A0.2V dc signal
and
is
an
exact (power-ol-
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
input.
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 (0.1ft, 1ft,
10ft.
or
100ft) to
perform
the current-to-voltage
conversion
required
by the
a/d converter.
See
View
B.
The voltage
(1
R) drop 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
thea/d converter.
However,
if
the
input current
is
ac
and the
AC
function
is
selected, the
1
R
drop
is
rectified
by the ac
converter before going to the
low-pass
filter,
in
either
event the
a/d
converter receives a
dc
inputvoltagc proportional
to the
current
passing
through
the selected shunt.
3-24.
RESISTANCE
MEASUREMENTS
3-25.
Resistance
measurements are
mde
using a ratio
technique
as
shown
in
Figure 3-3C
When
the
ft
function
is
selected,
a simple series circuit
is
formed by the internal
reterence
voltage a reference
resistor
from
the voltage divider (selected
by range switches),
and
the
external
unknown
resistor.
The
ratio
of the two resistor valves
is
equal to
the
ratio ot their
respective voltage drops.
Therefore, since the
value
of
one
resistor
is
known,
the
value
ol
the
second can be
determined by using
the voltage
drop
across the
known
resistor as a
reference.
This determination
is
made
directly
by the
a/d
converter.
3-26.
Overall operation
of
the
a/d
converter during a resistance
measurement
is
basically
as described earlier in
this section,
with one exception.
The
reference voltage present
during a
voltage
measurement
is
replaced by the
voltage
drop across the
reference resistor.
This allows the
voltage
across the
unknown
resistor to
be
read
during the integrate
period
and compared against
the
reference
resistor
during the
read period.
As before, the length
of the read period
is
a
direct
indication of the
value ot the
unknown.
3-27.
CONTINUITY
MEASUREMENTS
3-28.
Continuity
is
a
measurement feature
that
supplements the
resistance
and
conductance
measurement functions.
The
feature
is
enabled
when
the Vi
ft
and
the
3-6