5-20
Delta, Pulse Delta, and Differential Conductance
Model 6220/6221 Reference Manual
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Part 2
Delta
Basic measurement process
Delta voltage measurements use a current-reversal technique to cancel the
effects of thermal EMFs in the voltmeter test lead connections. The simplest
algorithm for a delta measurement uses the following 2-point source/measure
process:
1.
Source a positive current through the DUT and measure the voltage (V1).
2.
Source a negative current through the DUT and measure the voltage (V2).
3.
Calculate the delta reading as follows:
delta = (V1 – V2) / 2
Each voltage measurement will include the thermal EMF. When the two
measurements (V1 and V2) are averaged, the thermal EMF will effectively be
cancelled out of the delta reading.
For example, assume the DUT is exactly 0.1
Ω
and the test current is 1mA. Ideally,
the voltage across the DUT would measure exactly 100µV.
However, assume that the voltage measurement test circuit has +10µV of EMF.
For a positive test current, the voltage measurement will be +110µV (100µV +
10µV). For a negative test current, the voltage measurement will be -90µV
(-100µV + 10µV).
When using the fundamental delta process, the two voltage readings (V1 and V2)
are averaged to cancel the 10µV of EMF:
delta = (V1 – V2) / 2
= [+110µV – (-90µV)] / 2
= (110µV + 90µV) / 2
= 200µV / 2
= 100µV
The Keithley Models 2182 and 2182/2182A can perform fundamental Delta mea-
surements using a conventional bipolar current source. For details on the funda-
mental Delta process, see “Delta” in Section 5 of the Model 2182/2182A User’s
Manual.
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