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

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.