Fluke 8021B - Page 44

8021
B
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