Series 3700 System Switch/Multimeter User's Manual
Section 5: Basic Digital Multimeter (DMM) Operation
3700S-900-01 Rev. C / July 2008
5-41
Figure 5-24: Two-wire ratiometric method
Because I = V/R, Equation 1 is modified using the V/R equivalents in place of I
REF
and I
DUT
. Therefore:
I
SOUR
= (V
MEAS
/ R
REF
) + (V
MEAS
/ R
DUT
)
Note that V
MEAS
is measured by the Series 3700. With V
MEAS
, I
SOUR
, R
REF
known, the
Series 3700 calculates the resistance of the DUT and displays the result. R
REF
is
learned during calibration and V
SOUR
is routinely self-calibrated when the
dmm.autozero
attribute is enabled (
dmm.autozero = dmm.ON
).
As shown, the 4W function can also be used to measure ohms for the 10M
and 100M ranges. To minimize the effects of charge injection when dmm-
autozero is enabled, the 10M to 100M is actually a 3-wire ohm
measurement. SENSE HI is not used. SENSE HI is connected to the DUT but is
not required (it can be left open). The measurement method is similar to the
ratiometric method for 2W , but it performs an extra voltage measurement
(V
LEAD
) to compensate for voltage drop in the input test leads.
