Fluke 8024B - Section 3 Theory of Operation

8024B
Section
3
Theory
of
Operation
3-1.
INTRODUCTION
3-2.
This
section
of
the
manual
describes
the
theory
of
operation
of
your
8024B.
The
overall
function
of
your
8024B
is
presented
first
at
an
overall
functional
level.
Then
the
operation
of
the
a/d
converter
and
each
function
of
your
8024B
is
described
in
more
detail.
A
detailed
schematic
of
your
8024B
can
be
found
in
Section
7.
3-3.
OVERALL
FUNCTIONAL
DESCRIPTION
3-4.
Figure
3-1
shows
the
major
circuits
of
your
8024B
arranged
in
a
simplified
functional
block
diagram.
Input
signals
are
routed
by
the
range
and
function
switches
through
the
appropriate
signal
conditioners
so
that
a
dc
analog
signal
that
is
proportional
to
the
input
signal
is
applied
to
the
input
of
the
a/
d
converter
if
the
PEAK
HOLD
switch
is
at
the
OFF
position.
If
the
PEAK
HOLD
switchis
at
the
ON
position,
the
dc
analog
signal
will
be
stored
ona
capacitor
in
the
peak
hold
circuit
which
will
drive
the
a/d
converter
with
a
dc
voltage
that
is
the
same
as
the
stored
charge
on
the
peak
hold
capacitor
until
the
PEAK
HOLD
switch
is
set
to
the
OFF
position.
The
a/d
converter
will
drive
the
display
to
a
digital
display
that
is
numerically
the
same
as
the
proportional
input
signal.
Decimal
point
position
is
determined
by
the
range
switch
selected.
When
the
©
function
is
selected,
the
input
signal
is
also
routed
to
the
level
detector
circuit.
The
level
detector
circuit
compares
the
input
signal
level
to
a
+0.8V
reference
(200
kf
range).
If
the
signal
is
more
positive
than
the
reference,
the
level
detector
circuit
will
cause
the
Aup
arrow
to
be
displayed
(over
the
minus
sign
position).
If
the
input
signal
is
less
positive
than
the
reference,
the
level
detector
will
cause
the
down
arrow
to
be
displayed
(under
the
minus
sign
position).
If
the
audible
tone
is
enabled
(AC/DC
switch
at
the
AC
position)
the
level
detector
circuit
will
cause
the
audible
tone
to
sound
when
the
input
signal
is
less
positive
than
the
reference.
3-5.
A/D
Converter
3-6.
The
entire
analog-to-digital
conversion
process
is
accomplished
by
a
single
custom
a/d
converter
and
display
driver
IC,
U8.
The
a/d
converter
employs
the
dual
slope
method
of
a/d
conversion
and
requires
a
series
of
external
components
to
establish
the
basic
timing
and
reference
levels
required
for
operation.
These
include
an
integrating
capacitor,
an
autozero
capacitor,
and
a
flying
capacitor
(for
applying
a
reference
level
of
either
polarity).
Since
the
power
consumed
for
display
operation
is
very
low,
the
a/d
converter
IC
also
contains
the
display
latches,
decoders,
and
drivers.
3-1