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Model
34SSA
THEORY
OF OPERATION
Section
VIII
8-32.
A
separate gain error
measurement
is
not
made
for
the
1000
V
dc
range.
Since the only difference
between
the
100
V
dc and
1000
V
dc
circuit
configuration
is
a
precise
attenuation difference
of
10,
the
1000
V
dc
gain
error
constant
is
computed by
the
main
controller.
$•33.
100
V
dc
Reference
Offset
Error
Measurement.
Since the reference voltage
for
the
100
V
dc
range
is
divided
by
the operational attenuator;
a
separate
offset
error
measurement
is
made
to include
any
offsets
which
might be
associated
with the attenuator
and
FbT
switches
used. Figure
8-14
illustrates
the
circuit
configuration
for
this
measurement.
8-34.
AUTO-CALIBRATION -OHMS.
8-35. General.
8-36.
During
the
ohms
function the
ohms
converter
sup-
plies
a
current
through both
the
unknown
resistance
and
the reference
resistance (see
Figure
8-15).
Since the
same
current flows
through both
resistors,
their respective
volt-
age
drops
are
proportional.
As
with the
CC
Auto-Cal
sequence,
the offset errors are
measured and
subtracted
from
the
unknown
and
reference
resistance
measurements.
The
voltage
developed
across the
unknown
resistor
is
mea-
sured
by
closing
SI while
the reference voltage,
developed
across the reference
resistor,
is
measured by
closing S2.
The
value
of
the
unknown
resistance
is
computed by
the
main
controller.
An
equation
describing
this
computation
is:
-HVRx
+
6o>Gi
-EoGil
R„
Kr
IVrEF
* Eol
G2
-
Eo
Ga
Figure 8-15. Basic
Ohms
Measurement
Diagram.
where
Rx
is
the
unknown
resistance value,
V(^x
is
the
volt-
age
drop
across the
unknown
resistance,
V|^
ref.
is
the
volt-
age
drop
across the reference
resistance,
is
(he
input
offset error.
G|
and
G
2
are
the
circuit
gains
of
the
particu-
lar
measurements, and
fCr
factor.
This equation
simplifies
to:
,
VRx
Cl
VreF
G2
Kr
8-37. Circuit Description.
8-38.
.1
kn,
1
kf2,
1
Mf2
Offset Error
Measurements.
The
offset error
constants derived
for
the
.1
V
dc and
I
V
dc
ranges
are
also
used
for
the
.1
kf7,
1
kfl,
and
1
Mf2
offset
error constants, since the
circuit
configurations
are the
same. Refer
to
Paragraph 8-25
for a description
of
these
offset
error
measurements.
8-39.
10
kn, 100 kn,
10
Mn
Offset Error
Measurements.
Two
additional offset
measurements
are
made
to
compen-
sate
for errors
which
might be
present
when
making
mea-
surements
on
the 10
kn,
100
kn,
or
10
Mn
ranges.
Figure
8-9
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