Overall Functional Description; Block Diagram Analysis; A/D Converter - Fluke 8021B Instruction Manual

3-2.
This
section of the
manual contains an overall
functional description followed by a
block
diagram
analysis
of the 8021 B.
A
detailed
schematic of the 8021
B
appears
in
Section
7.
3-3.
OVERALL FUNCTIONAL
DESCRIPTION
3-4,
The
Model 8021
B
as
shown
in
Figure
3-1,
is
a
hand-held
six
function
digital
multimeter.
It
features a
total
of
24 measurement ranges (dcvolts-five,ac
volts-five,
ohms-
six,
dc
mA-lbur, ac mA-four),
audible continuity a high contrast, easy-to-read, 3-1/2-
digit,
liquid crystal display;
long battery
life
(up
to
200 hours);
and overload protection for
all
ranges.
3-5.
Operation centers
around a
custom LSI
chip,
U
1,
which contains
a
dual
slope
a/d
converter
and a display
driver.
Peripherals to
U1 include range and function switches,
input signal
conditioners,
and
the display.
When
an input signal
is
applied to the 8021
B,
it
is
routed
through
the
range
switches
to
one
of four
input signal conditioners as determined
by the
function switch setting.
Each conditioner scales
and,
if
necessary,
rectifies
the input
so that
an acceptable dc input level
(-0.2 to
+0.2V
dc)
is
presented to the
a/d
converter.
3-6.
Timing
for the overall
operation of the
a/d converter
is
derived
from an
external
quartz crystal
whose frequency
is
a multiple of
the local line
frequency.
This allows the
conditioned dc input data to be
integrated
over a single line cycle,
thus
optimizing both
common mode
and normal
mode
rejection.
3-7,
Digitized
measurement data
is
presented to
the display as four
decoded
digits
(seven
segments) plus polarity.
Decimal point position
on
the
display
is
determined by the
range
switch settings.
3-8.
BLOCK DIAGRAM
ANALYSIS
3-9.
A/D Converter
3-10.
The
entire analog-to-digital
conversion process
is
accomplished
by a single
custom
A/
D
Converter and Display Driver
1C,
Ul.
The IC employs the dual slope
method
ofa/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 latches,
decoders,
and drivers
required for the
display.