Keithley offsetCompensation free download

File name offsetCompensation.pdf

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