Keithley 2750 User Manual page 397

D-4 Signal Processing Sequence and Data Flow
OComp (offset-compensated ohms)
The Model 2750 performs a normal ohms measurement by sourcing a known current (I),
measuring the voltage (V), and then calculating the resistance (R = V/I). Offset-
compensated ohms cancels the effects of thermal EMFs which can adversely affect low-
resistance measurements.
With OCOMP enabled, the Model 2750 performs one normal resistance measurement,
and then loops back to perform a second resistance measurement with the internal current
source set to its lowest level. The offset-compensated ohms reading is then calculated as
shown in
NOTE
Filter
The filter is used to stabilize noisy readings. With the filter enabled, the specified number
of readings are averaged to yield a single filtered reading. There are two types of filters:
moving and repeating.
A filter stack is used to temporarily store the specified number of readings to be averaged.
In general, for the moving filter, each measurement process adds a reading to the stack
(oldest reading discarded), and then averages the stack to yield a filtered reading. For the
repeating filter, each measurement process fills the stack with new readings (all previous
readings discarded), and then averages the stack to yield a filtered reading.
NOTE
Output trigger pulse (VMC)
An output trigger pulse from the Model 2750 can be used to trigger an external instrument
to perform an operation. In general, a trigger pulse is output at this point in flow chart for
each processed reading.
An exception is the SCAN function for scanning. For the SCAN function, an output trig-
ger is not output until after the specified number of channels (as set by the sample counter)
are scanned.
NOTE
Rel
Next in the signal processing sequence is the Rel operation. Rel is used to null offsets, or
subtract a baseline rel value from the reading. With Rel enabled, the Rel'ed reading is cal-
culated as shown in Figure D-2.
NOTE
Figure D-2.
For details on OCOMP measurements, see
page 3-20.
For details on filter operation, see
For details on scan operation, see
For details on Rel operation, see
Model 2750 Multimeter/Switch System User's Manual
"Offset-compensated ohms,"
"Filter," page 4-14.
"Trigger models," page
"Relative," page
7-4.
5-2.