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INSTRUCTION
MANUAL
Dlgilal
MultImeter
Models 172A. t73pI
THEORY
OF OPERATION
2)
As shown
on Sheet
I of
Schematic
29145E,
the
-5volts
is
supplied
t&Q703
by zener
diode
VR70l
and resistor
R710,
which
is connected
to
-15
volts.
h.
Active
Filter
(Figure
5-10).
1)
The DC filter
is
used
in front
of
the
A/D converter
on all
functions.
The comp-
onent
values
used
in
the
circuit
provide
25O:l
attenuation
to 50 Hz on dc volts,
and
i7OO:t
filtering
on ac volts
output
at
50 Hz.
Minimum
rise
time
is
achieved
without
overshoot
by having
the
following
component
relationships:
~70'2 = ~701,
~604 = ~603.
It* + R707 = 1.5
(~603)
and c702 = 1.5
(~601).
The filter
requires
an input
resistance
of
200K0,
49.9Ka
of which
is
R707.
The other
150KR comes
from
various
sources,
depend-
ent
upon
range
and function,
as given
in Table
5-5.
The 200KO is also
maintained
so
that
the
effect
of
A/D converter
input
current
is autozeroed
out
(see
A/D converter
discussion,
Paragraph
5-4).
The filter
does
not
contribute
any dc offsets
to
the
signal
path.
They
are
blocked
by C702 and C701.
C903
(not
shown)
provides
frequency
compensa-
tion
for
lJ602.
TABLE 5-5.
Active
Filter
Input
Components.
12oov
ACV
ALL
Cl02
j-10
R':
R217
R213,
R209,
RZl5B,
R208
~213.
~209.
214.
R215B.
R2l5C,
R208,
Q206
1x106
0.
I
ALL
I
R704
Ran e Selection.
Ranging
is accomplished
by RSA F. RSB lines
going
to LSI
Fircuit
U9hi.
Th&
are
contro\led
by the
front
panel
switches
AUTO/MAN.
DN 6 UP, according
to
Table
5-6.
RSA & RSB are
edge
sensitive,
causing
range
changes
when
their
levels
are
changed.
This
causes
one uprange
or one downrange
per
button
push.
TABLE 5-6.
Range
Selection.
Range Switches
Range
Lines
AUTO/MAN
DN
UP
RSA
RSB
ACTION
OUT
X
X
ov
+av
AUTORANGE
IN
OUT
OUT
OV
ov
HOLDRANGE
IN
IN
OUT
t8V
ov
DOWNRANGE
IN
OUT
IN
+8v
+8v
UPRANGE
X = DON'T CARE
5-1:
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