Keithley 2700 User Manual page 119

3-28 Basic DMM Operation
Since I = V/R, Eq. 1 is modified using the V/R equivalents in place of I
Therefore:
I
SOUR
Eq. 1 is then rearranged to solve for R
mind that V
Model 2700 calculates the resistance of the DUT and displays the result.
NOTE
As shown in
10MΩ and 100MΩ ranges. There are actually 3-wire ohm measurements. Sense Hi is not
used.
need to be. It can be left open.
The measurement method is similar to the ratiometric method for Ω2, but it performs an
extra voltage measurement (V
stated in the specifications (Appendix A) to achieve rated accuracy, the Input Hi and Input
Lo test leads must have 10% matching of resistance. To meet that criteria, simply use
similar type test leads that are the same length.
Keep in mind that V
Input Lo). Therefore, the actual voltage drop across the DUT is V
voltage drops in the test leads.
Since matched input leads are used, the voltage drop for the two test leads are 2 x V
Therefore; V
The Model 2700 still uses Eq. 2 to calculate resistance, but it uses V
V
MEAS
Effects of open test leads on ohms readings
The Model 2700 will display readings up to 120% of range. Readings above 120% of
range will cause the "OVRFLW" message to be displayed. For example, on the 100Ω
range readings up to 120Ω will be displayed. Above 120Ω , the "OVRFLW" message is dis-
played.
The Model 2700 will also display the "OVRFLW" message if a test lead is open during an
ohms measurement. A hardware (H/W) detection circuit or software (S/W) detection is
used to detect an open input lead. For an Ω4 measurement, a software (S/W) detection rou-
tine is used to detect an open sense lead.
= (V / R ) + (V
MEAS REF
is measured by the Model 2700. With V
MEAS
Eq. 2 in Figure 3-11
Model 2700 "measures" the resistance of R
equation. The Model 2700 routinely does a "self-calibration". During this
process, the precision 0.7µA current is sourced through R
input. This reference voltage (V
R = V / 0.µ7
REF REF
3-11B, the Ω4 function can also be used to measure ohms for the
Figure
Figure 3-11B shows the Sense Hi terminal connected to the DUT but it does not
includes the voltage drops of the input test leads (Input Hi and
MEAS
= V - 2(V
DUT MEAS
. This ratiometric method cancels the effects of input test lead resistance.
Model 2700 Multimeter/Switch System User's Manual
/ R )
MEAS DUT
and is shown in Eq. 2 of
DUT
assumes that R is exactly 10MΩ . In reality, the
REF
) is measured and R
REF
) to compensate for IR drop in the input test leads. As
LEAD
).
LEAD
and I
REF
Figure 3-11. Keep in
, I , R
MEAS SOUR REF
and uses that value in the
REF
with an open
REF
is then calculated:
REF
minus the two
MEAS
in place of
DUT
.
DUT
known, the
.
LEAD