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SECTION
4
THEORY OF OPERATION
4.1
INTRODUCTION
4.2.1
Signal Conditioning
This
section contains
circuit
descriptions
for
the
Model 177
DMM
and
the
optional
Model 1788
Battery Pack.
An
overall
block
diagram
of signal
flow
Is
provided
In
Figure
4-1
.
The
schematic diagrams
are
contained
In
Section
6
of
this
manual.
The Model
177 schematic
is
drawing
number
29658E
and
schematic drawing
number 29659C
is
for
the
Model
1788.
4.2
OVERALL OPERATION
The
Model
177 uses
a
2V
(20,000 count)
full
scale anaiog-to-
digital
(A/D)
converter with a 414
digit
multiplexed
display.
Signal
conditioning
permits
the
A/D
converter
to
ac-
comodate
full
scale
DC
voltage
and
current
measurements
over seven
ranges.
Signal conditioning
iciudes:
DC
attenuation (on the
20,
20C
and
1200V
ranges),
modulation
and
demodulation
of the
DC
signal
by
the
chopper
buffer/amplifier
with
X100
and X10
gain (on the
20 and
200mV
ranges,
respectively),
AC
atten-
uation (on the 20,
200 and 1000V
ranges),
AC
amplification
(on
the
200mV
and
20V
ranges),
AC-to-DC
conversion,
ohms
source
and
conversion,
and
current
shunts as
shown
in
Figure
4-1.
1
.
in
the
DCV
mode,
signal
conditioning to
the
A/D
converter
consists of
range
and
function switching,
range
resistors
(attenuator)
and
the
chopper
buffer/amplifier.
For
all
ranges, the
DC
input
signal
is
attenuated/amplified
to pro-
vide
a
±2V
full
scale
output
to
the
A/D
converter.
In
the
DCA
mode,
the voltage
developed across
the shunt
resistor
is
applied
directly
to the input of the
chopper
amplifier.
A
chopper
gain of
X10
is
used
for
all
ranges
ex-
cept
20A
which
uses X100.
LO>-
OPTfONAL
BATTERY
PACK
ATTENUATOR
\c
+ 2V
FS
?
TT:
,
Q
CURRENT OUTPUT
'
DC
INPUT
AC
OUTPUT
DC
ATTENUATOR/fl
SOURCE
RESISTORS
FUNCTION
& RANGE
SWITCHING
SOURCE
<7
FEEDBACK
±2VF8
OUT
-HIV
REFERENCE
390Hz
CHOPPER
DRIVE
100
kHz
CLOCK
u
1
AC MAINS
POWER
SUPPLY
OSCILLATOR
&
DIVIDER
«
-t-5
o
±15
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