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Section
VIII
THEORY
OF OPERATION
Model
3455A
Figure 8-17.
Ohmt
Reference Meaiurement.
Notei:
VOLTAGE
LIMITED
TO
5
V.
FET
Switch
A10Q13. The
reference
is
measured
prior
to
8-16
illustrates
the
circuit
configuration
for
these
measure-
each
measurement of
the
unknown
resistance,
ments.
The
DC
Preamp
input
is
grounded through
a
100
kf2
resistor
by
FET
switch
A10Q2. The
feedback path
for the
8-41,
100
1
10
Mf2
Reference Measurements.
On
X2
gain
is
through
FET
switches
A10Q22
and
A10Q21.
the
100
kJ2
range,
AI0K7
is
opened and
the
.7
mA
current
Feedback
for
X20
gain
is
through
A10Q22,
isolation
amp-
source
is
applied
to the
combination of
R1
and
R5
(1
M£2).
lifier
AI0U3,
switch
A10Q27,
the precision
10-to-l divider
The
reference voltage
developed
across
R1 and R5
is
ap-
and
switch
A10Q18.
A
separate
measurement
is
made
for
plied
to
the
DC
Preamp
input
through
FET
switch
both
gains
and
the
results
are stored
by
the
main
controller.
A10Q14.
On
the
1
MJ2
and
lOMD
ranges, relay
A10K8
is
8-40.
.1
kSl,
1
kn.
10
kf2
Reference Measurements.
The
'^e
.7
M
current source
is
applied
to the
1
Mfi
ohms
reference voltage
is
developed
across the reference
reference
resistance.
The
reference voltage
is
applied
to
resistance.
On
the
.1
kfi
through 10
kfl ranges the
refer-
^
Preamp
input
through
A10Q14.
ence
resistance
is
I
kJ2
(see
Figure
8-17).
The
.7
mA
current source
is
connected
to the
I
kJ2 reference
through
8-42.
AUTO
CALIBRATION—
A/0
CONVERTER,
relay
A10K7. The 999
kfi reference
resistor
is
shorted
by
the
combination of
relay
AI0K7
and
AI0K8. The
refer-
8-43.
Two
Auto-Cal
measurements
are
made
to correct
ence
voltage
is
applied
to the
DC
Preamp
input
through
errors
which might
be generated
in
the
A/D
Converter.
One
Figure 8-18. Simplified
A/D
Converter Diagram.
8-10
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