Fluke 8024B - Page 55

8024B
3-14. Voltage Measurement Function
3-15.
Both
ac
and
dc
voltage
measurement
functions
use
an
overvoltage
protected
10
MQ
input
divider
to
scale
down
the
input
voltage.
Under
normal
conditions
(assuming
a
dc
input
signal
on
the
proper
range)
the
divider
output
is
a
de
voltage
that
is
directly
proportional
to
the
input
signal
level.
If
the
AC
function
is
selected,
the
output
of
the
divider
is
ac
coupled
to
an
active
full-wave
rectifier
whose
dc
output
is
calibrated
to
equal
the
rms
level
of
the
ac
input
(for
sine
wave
inputs).
If
the
PEAK
HOLD
switch
is
at
the
OFF
position,
the
dc
voltage
from
the
divider
or
the
ac
converter
is
passed
through
a
filter
and
applied
to
the
a/d
converter
as
the
unknown
input
(Figure
3-4).
Peak
Hold
operation
will
be
covered
later.
3-16. Current Measurement Function
3-17.
Current
measurements
are
made
using
a
fuse
protected,
switchable,
current
shunt
(0.19,
10,
100,
or
1000)
to
perform
the
current-to-voltage
conversion
required
by
the
a/d
converter
(Figure
3-5).
The
voltage
(IR)
drop
produced
across
the
selected
shunt
may
be
either
ac or
dc.
If
the
input
current
is
dc
and
the
DC
function
is
selected,
the
IR
drop
is
passed
through
a
low-pass
filter,
and
presented
as
the
unknown
input
to
the
a/d
converter.
However,
if
the
input
current
is
ac
and
the
AC
function
is
selected,
the
IR
drop
is
rectified
by
the
ac
converter.
If
the
PEAK
HOLD
switch
is
at
the
OFF
position,
the
de
signal
from
either
the
ac
converter
or
the
current
shunt
is
routed
through
a
filter
to
the
a/d
converter.
This
unknown
input
voltage
to
the
a/d
converter
is
proportional
to
the
current
passing
through
the
current
shunt.
3-18. Temperature (°C) Measurement Function
3-19.
As
Figure
3-6
shows,
the
input
from
the
thermocouple
accessory
is
applied
across
the
mA
and
COMMON
terminals.
If
the
PEAK
HOLD
switch
is
in
the
OFF
position
the
input
will
be
routed
through
a
filter
to
the
a/d
converter
unknown
input.
The
COMMON
terminal
is
thermally
tied
to
the
collector
and
base
leads
of
transistor
Q3.
This
provides
reference
junction
temperature
compensation.
Q3
changes
with
temperature
and
provides
an
offset
voltage
to
counter
the
thermocouple
at
the
input
jacks
to
ensure
the
integrity
of
the
reading
at
the
measurement
end.
3-20. Resistance Measurement Function
3-2].
Resistance
measurements
are
made
using
a
ratio
technique
as
shown
in
Figure
3-7.
When
the
kQ
function
is
selected,
a
simple
series
of
circuits
are
formed
by
the
internal
reference
voltage,
a
reference
resistor
from
the
voltage
divider
(selected
by
range
switches),
and
the
external
unknown
resistor.
The
ratio
of
the
two
resistors
is
equal
to
the
ratio
of
their
respective
voltage
drops.
Therefore,
since
the
value
of
one
resistor
is
known,
the
value
of
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.
3-22,
Overall
operation
of
the
a/d
converter
during
a
resistance
measurement
is
basically
as
described
earlier
in
this
section,
with
oneexception.
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
of
the
unknown.
The
PEAK
HOLD
switch
should
always
be
in
the
OFF
position
when
making
resistance
measurements.
3-6