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INSTRUCTION
MANUAL
Digital
MultImeter
Models 172A. 173A
THEORY OF OPERATION
and 3V (ZA)
ranges.
F501
is accessible
from
the
rear
panel
and R514 is
mounted
on a
shield
above
the
display
board.
The Circuit
is shown
at
the
bottom
of
Schematic
2747&,
which
also
contains
the
I/V
converter
for
the
yodel
173~.
f.
Ohms Converter.
1)
The ohms converter
circuit
basically
consists
of
a current
generator
and summing
amplifier.
Simplified
operation
for
the
two wire
ohms configuration
is
shown
ip
Figure
5-7.
2)
The current
is generated
by applying
either
3.34
or
,334
Volts
(Ohms Cal.
Voltage)
from
buffer
amplifier
UlOl
to the
Range
resistors.
The unknown
resistances
(Rx)
is
placed
in
the
feedback
loop
of
the
summing
amplifier
(U202)
to
force
the
current
through
the
unknown
resistor.
Thus,
the
output
signal
to the
A/D converter
for
an
on
range
measurement
can be determined
by the
equation
-vodc
=
R, is
determined
by the
equation
I = vOhms CAL.
I-RX.
The current
flowing
through
R
Range
Six
decade
currents
are
generated
by the
six
combinationsof
the
three
range
resistors
and
two
levels
of
ohms cal
voltage.
Examp I e :
.334v=
IliA.
This
current
is
used
on 300Kf
LOR range
and 3M HIR range
(3V max.)
334K
3)
See Table
5-4
for
ohms ranges.
High
ohms puts
the
A/D converter
on the
3 Volt
range,
allowing
6 resistance
ranges
which
can turn
on semiconductor
junctions
for
on
scale
readings.
LOW ohms puts
the A/D converter
on the
3OOmV range,
allowing
6 resis-
tance
ranges
which
do not
turn
on semiconductor
junctions
for
on scale
readings.
4)
There
at-e time
and temperature
drifts
associated
with
amplifiers
UIOI
and U202.
These
effects
are
compensated
for
by the
A/D converter.
The
reference
used
to calibrate
the
A/D converter
in 0 function
is
the
output
of
UIOI.
Therefore,
any drift
in UIOI
is
compensated
for
every
conversion
cycle.
The negative
input
of
U202 is what
"auto-
zero"
I is
calibrated
to every
conversion
cycle.
Therefore,
ohms current
stability
is
a function
only
of
the
stability
of
the
Range
resistor.
The
lead
drop
in
the
high
terminal
of
the
ohms source
amplifier
is compensated
for
by connecting
it
to "auto-
zero"
2 when
in
the
ohms mode.
"Auto-zero"
2 is
the
zero
for
the
input
signal.
In
Z-wire
ohms the
leads
connect
at
the
front
panel
binding
post.
Therefore,
a 4-terminal
ohms system
exists
up to the
front
panel
terminals.
See A/D
converter
discussion
for
a more
thorough
explanation
of
error
correction.
5)
The ohms converter
circuit
has been
further
simplified
and
redrawn
to
show the
4-Wire
ohms configuration
in Figure
5-8.
a.
As previously
described,
the
circuit
measures
a resistor
by putting
a constant
current
through
Rx and measuring
the
voltage
drop
across
Rx,
which
is accomplished
by
putting
Rx in
the
feedback
of
a summing
amplifier
(a AMP) whose
input
is a reference
(n Vref)
voltage
through
an input
resistor
(RR).
b.
AZ-l,
ACAL,
AZ-Z,
t A/D Signal
are
the
4 A/D
inputs.
This
A/D operation
comp-
ensates
for
time
E. temperature
variation
of
Q VRef and R Amp, as well
as compensating
for
lead
resistance.
The A/D looks
at each
of
these
inputs
in
the
time
Sequence
as
follows:
1)
AZ-1
looks
at
R AMP input
for
40 ms and stores
this
zero
level.
2)
ACAL looks
at n VREF for
40 ms and calibrates
itself
to
the
difference
be-
tween
AZ-I
and ACAL.
Thus,
it
is
calibrated
to the
voltage
~CTOSS
R9.
Since
RR
js a fixed
stable
resistor
the
value
of
I is
now
known.
Since
I flows
through
Rx
the
calibration
is
fixed.
3)
AZ-2
looks
at
the
voltage
at
the
top of
Rx for
40 ms.
This
is
defined
as the
zero
level
for
signal
measurement
and
is stored
in A/D.
5-V
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