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Model
3455A
THEORY
OF OPERATION
Section VIII
Figure
8-3.
Operational Attenuator Diagram.
analog
circuitry
of
the
Voltmeter. This
is
accomplished
by
measuring
the
offset
and
gain errors
and mathematically
correcting
for
them.
Each
error
measurement
is
stored
in
"memory"
by
the
main
controller as a constant.
These
constants
are sequentially
updated.
The
output
reading
of
the
Voltmeter
is
computed by
the
Main
Controller
and
is
equal
to the
ratio
of
the external input to the
internal
refer-
ence, times
a
range
factor.
Figure 8-4
shows
a
very
basic
diagram of
the
Voltmeter.
A
basic
equation
describing a
measurement of one of
the
three
inputs
is:
V(
)
=
(E(
)
+ Eq)G
;
where V(
^
is
the
par-
ticular
output, E(
)
is
one
of
the three inputs.
Eq
is
the
internal
offset
error,
and
G
is
the
circuit gain.
Closing
switch SI
applies
the
internal reference voltage.
The
circuit
output
would
be:
Vjef
=
(Eref
* ^O)
offset
error
is
measured by
closing
S2,
grounding
the
input.
The
resultant
output
would
be
Vg
= EqG. Measurement
of
the external
input
would
yield
Vjn =
(Ejn
+
EoX*-
The
equation
describ-
ing
the Auto-Calibration
is:
Output Reading®
_
fh
'
x
Kr
Yref'
Yq
Substituting
the basic
equations
into the
Auto-Cal
equation
would
yield:
Output
Reading
=
Eq)
G
•
EpC
(Etef
* Eg)
G
-
EgG
This equation reduces
to:
Output Reading ®
x
Eref
or:
the
output
reading
is
equal
to the ratio
of
the
8-3
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