Fluke 8024B - Page 60

8024B
3-23.
Conductance
Measurement
Function
3-24.
Conductance
measurements
are
made
using
a
ratio
technique
similar
to
that
used
in
making
resistance
measurements
(Figure
3-7).
The
main
differences
are:
only
one
range
is
provided
(200
nS),
and
the
function
of
the
range
resistor
and
the
unknown
resistor
in
the
measurement
cycle
is
reversed.
That
is,
the
voltage
drop
across
the
range
resistor
is
used
as
the
unknown
input
during
the
integrate
period,
and
the
voltage
across
the
unknown
resistor
is
used
for
the
reference
input
during
the
read
period.
As
a
result,
the
display
provides
a
reading
that
is
the
reciprocal
(1/1)
of
the
unknown
input
resistance;
i.e.,
the
higher
the
input
resistance,
the
lower
the
display
reading.
The
PEAK
HOLD
switch
should
always
be
in
the
OFF
position
when
making
conductance
measurements.
3-25.
Peak
Hold
Circuit
Level
3-26.
As
Figure
3-8
shows,
the
peak
hold
circuit
consists
of
an
operational
amplifier
(U19)
and
a
capacitor
(C19)
which
is
across
the
a/d
converter
(U8)
input.
When
the
PEAK
HOLD
switch
is
set
to
the
ON
position,
switch
action
removes
the
normal
input
to
the
a/d
converter
and
routes
the
output
of
the
signal
conditioners
to
U19.
The
operational
amplifier
charges
C19
to
the
peak
positive
input
signal
to
the
DMM.
The
charge
on
C19
is
the
unknown
value
that
the
a/d
converter
reads
to
determine
the
displayed
value.
As
the
charge
on
C19
bleeds
off
through
U19,
U14,
and
Q10,
the
display
value
will
decay.
Peak-
Hold
AC
will
give
the
peak
rms
value
of
a
sine
wave
since
the
signal
is
routed
through
the
AC
Converter.
Peak-Hold
DC
should
give
the
positive
peak
of
any
input
waveform.
3-27.
Level
Detector
Circuit
3-28.
As
Figure
3-9
shows,
when
the
9
or
S
functions
are
selected,
the
DMM
input
is
routed
both
to
the
resistance
/
conductance
signal
conditioner
and
to
the
comparator
of
the
level
detector
circuit.
The
other
input
to
the
comparator
isa
+0.8V
reference
level
(200
kQ.
range).
When
the
DMM
input
is
open
circuited
or
greater
than
the
reference,
the
level
detector
circuit
causes
the
Aup
arrow
to
appear
in
the
display.
If
the
DMM
input
is
less
than
the
reference
level,
the
level
detector
circuit
causes
the
¥
down
arrow
to
appear
in
the
display
and
the
audible
tone
to
sound
if
the
AC/DC
switch
is
at
the
AC
position.
NOTE
The
Peak-Hold
switch
should
be
in
the
OFF
position
when
using
the
Level
Detector
Circuit.
-