Keithley 2182, User Manual, Page: 96 / 339

5-6 Ratio and Delta
Delta
Delta provides the measurements and calculation for the DC current-reversal technique to
cancel the effects of thermal EMFs in the test leads. Each Delta reading is calculated from two
voltage measurements on Channel 1; one on the positive phase of an alternating current source,
and one on the negative phase.
Basic Delta Calculation:
where:V1t1 is the voltage measurement on the positive phase of the current source.
V1t2 is the voltage measurement on the negative phase of the current source.
Delta Calculation using Filter and Rel:
where:
• Filt V1t1 and Filt V1t2 are filtered voltage measurements on the positive and negative
phases of the current source. The “FILT” annunciator will be on when Filter is enabled.
• Rel V1 is the Rel value established for DCV1. The “REL” annunciator will be on when
Rel is enabled.
The Model 2182 is optimized to provide low-noise readings when measurement speed is set
from 1 to 5 PLC. At 1 PLC, current can be reversed after 100msec. At 5 PLC, current can be
reversed after 333msec. At these reading rates, noise induced by the power line should be
insignificant. Filtering can be used to reduce peak-to-peak reading variations. For more
information on Filter in regard to Delta measurements, see “
Filter considerations ” in this
section.
The following example shows how a bipolar current source and Delta can be used to cancel
the effects of thermal EMFs:
In Figure 5-1 A, a constant 1mA is being sourced to a 0.1 Ω DUT. Under ideal conditions, the
Model 2182 would measure 100µV across the DUT (1mA × 0.1 Ω = 100µV). However,
connection points and temperature fluctuations may generate thermal EMFs in the test leads.
Note that the thermal EMFs drift with temperature.
Figure 5-1 shows 10µV of thermal EMF
(V
THERM
). Therefore, the Model 2182 will measure 110µV instead of 100µV:
V
2182
= V
THERM
+ V
DUT
= 10µV + 100µV
= 110µV
Delta
V1t1 V1t2
–
2
-------------------------------- =
Delta
FiltV1t1 FiltV1t2 –
2
--------------------------------------------------- RelV1 – =