Theory Of Operation; Functional Description; Circuit Descriptions; Digital Voltmeter - Fluke 8010A Instruction Manual

Section 3
Theory Of
Operation
3-1.
INTRODUCTION
3-2.
The
theory
of operation of your Multimeter
is
discussed
on two
levels. First,
the Functional Description
describes the operation of your instrument
in
terms of
the
functional relationships
of
the
major
circuits.
Second,
the
Circuit
Description presents a
more
detailed discussion of
the
major
circuits.
Both
levels are illustrated
by block
diagrams and
simplified
schematics
in this section
and
the
schematic diagrams
in
Section
7.
Section 6 contains
the
theory of operation of the Rechargeable
Battery
Option.
3-3.
FUNCTIONAL DESCRIPTION
3-4.
The
major
circuits
of
your Multimeter
are
illustrated in the functional
block diagram
in
Figure
3-1.
The
functional block
diagram shows
that the multimeter
can be
divided into
two
general
circuits,
the Input Signal
Conditioners and
the Digital
Voltmeter.
The
Signal
Conditioners are responsible for converting current,
.
resistance,
conductance,
and
ac or
dc
voltage to
an
analogue dc voltage (proportional
to the input)
from -200
mV
to
200
mV
or
-2V
to
2V
for the
digital voltmeter.
The
signal conditioner
circuits
also
provide
protection
from
transients
and
overrange
conditions.
The
Digital
Voltmeter
is
composed
of a
custom IC
(U3)
and
the
LCD
Display.
The custom IC
contains
an analog
to digital
converter
(a/d),
a
controller,
and
the
LCD
drivers.
3-5.
These
two
circuit
groups
work
together as follows:
An
input signal (of
unknown
value)
is
applied to
its
respective signal
conditioning block
by
the selection of
the
mA,
V, or kfl (S=l
/ fi.)
front
panel function
switches.
(For
this
discussion,
the
lOA
function switch for the
8010A and
the
LO
RANGE H
function switch
for the
8012A
will
be
presented as extensions
of
the current
and
resistance signal conditioners.)
The
output
voltage of the
signal conditioners
is
a proportional analogue of
the
input
signal.
This output voltage
will
be
from -200
mV
to
200
mV
or
-2V
to
2V, depending on
the
selected range.
The dc output
of the conditioners
is
sent
directly to
the
'
A/D
Converter.
The
ac output of the conditioners
is
converted
to dc
by
a true-rms converter before reaching
the
A/D
Converter.
Once
the
analog
voltages have been
conditioned, the
A/D
Converter converts the
unknown
voltage (which
represents the signal
measured
with the
Multimeter)
into representative,
digital
information. This
digital
information
is
then used by
the
LCD
drivers to
present the reading
on
the
LCD.
The
position of the
decimal point
is
determined by
the
Range
Switch
settings.
3-6.
CIRCUIT DESCRIPTIONS
3-7.
The
following paragraphs
describe
each
of the
major
circuits in
detail.
3-8.
Digital
Voltmeter
3-9
.
The
Digital
V
oltmeter of
your Multimeter
consists
of the
custom IC
(U3) and
the 3-1/2
digit
LCD.
The
custom IC
contains
an
a/
d
converter,
a
digital controller,
and
the display logic
and
drivers for the
LCD. The
proper
function of
U3
depends on
external
components
to
establish the basic
timing and analog
levels.
Basic timing
is
provided by a quartz
crystal (Yl). Resistors, capacitors,
and an
external voltage reference
provide the analog
levels
for the
A/D
Converter.
3-10.
The
Digital
Voltmeter
has
two
ranges
of
measurement.
If
you
select either
the 200
mV
or
20V
ranges (or
their
equivalent ranges
in other functions) the
Digital
Voltmeter can read
voltages
from
-200
mV
to
200
mV.
Selection
of any
other range
will
enable the
Digital
Voltmeter
to
read
-2V
to
2V.
3-11.
A/D
CONVERTER
3-12.
The
A/D
Converter
uses the dual slope
integration
method.
Dual
slope integration takes
advantage of the natural laws governing
the charge and
discharge of capacitors. In dual slope
integration,
the
unknown
voltage
is
used
to
charge a capacitor
for
a
specific
length of time.
Then
a
known
voltage of
the
3-1