Block Diagram - Fluke 8024B Instruction Manual

8024B
Figure 2-19.
Thermocouple Termination Unit
2-32
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
presenlcd
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 he 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
switch
is
at
the
ON
position, the
dc analog
signal
will
be stored
on a 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 fl
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
kfi range). If the signal
is
more
positive
than the reference, the level
detector
circuit will
cause the
A
up 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-lo-digital
conversion process
is
accomplished by
a single
custom
a/d
converter and display driver 1C, 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.
1
hese 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