Service Manuals, User Guides, Schematic Diagrams or docs for : Keithley Appnotes offsetCompensation
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What is Offset Compensation?
Offset compensation is a measuring technique that reduces or eliminates thermal
EMFs in a low level resistance measurements. The presence of thermal EMFs
can adversely affect low resistance measurement accuracy.
Thermal EMF in Connections
Equivalent
DMM
signal source Connections
RS
EMF C 2
VS
V IN
EMF C 1
Example:
Every connection made with dissimilar metal creates
an unwanted thermocouple.
Every connection made with dissimilar metals creates an unwanted
thermocouple in the measurement circuit. Each of these unwanted
thermocouples generates an error voltage that varies with temperature gradients
in the system. These can be on the order of many microvolts that can cause
significant errors in thermistor, RTD, strain gauge, and low-ohms measurements.
One method of minimizing thermal EMF errors is by using the offset-
compensated ohms measurement technique. Using this technique, test current
is applied to the circuit and the resulting voltage measurement will include the
desired DUT measurement as well as the effect of thermal EMFs.
During the second half of the cycle, however, test current is turned off. The
resulting voltage measurement is therefore attributable only to the thermal EMFs
present in the circuit. The desired measurement is the difference between the
two values. The microprocessor in the DMM compensates for the offset voltage.
Thermal EMF Offset
Compensated Ohms
a. Offset compensation
measurement cycle
One
measurement
cycle
On
Source
Current
Thermal offset
measurement
b. Voltage measurement with c. Voltage measurement with
source current on source current off
VEMF V EMF
V M1 IS VM2
RS RS
VM1 = V EMF + IS RS V M2 = V EMF
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