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3-28 Basic DMM Operation Model 2700 Multimeter/Switch System User’s Manual
Since I = V/R, Eq. 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
)
Eq. 1 is then rearranged to solve for R
DUT
and is shown in Eq. 2 of Figure 3-11 . Keep in
mind that V
MEAS
is measured by the Model 2700. With V
MEAS
, I
SOUR
, R
REF
known, the
Model 2700 calculates the resistance of the DUT and displays the result.
NOTE
Eq. 2 in Figure 3-11 assumes that R
REF
is exactly 10M
Ω
. In reality, the
Model 2700 “measures” the resistance of R
REF
and uses that value in the
equation. The Model 2700 routinely does a “self-calibration”. During this
process, the precision 0.7µA current is sourced through R
REF
with an open
input. This reference voltage (V
REF
) is measured and R
REF
is then calculated:
R
REF
= V
REF
/ 0.µ7
As shown in Figure 3-11 B, the
Ω
4 function can also be used to measure ohms for the
10M
Ω
and 100M
Ω
ranges. There are actually 3-wire ohm measurements. Sense Hi is not
used. Figure 3-11 B shows the Sense Hi terminal connected to the DUT but it does not
need to be. It can be left open.
The measurement method is similar to the ratiometric method for
Ω
2, but it performs an
extra voltage measurement (V
LEAD
) to compensate for IR drop in the input test leads. As
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
MEAS
includes the voltage drops of the input test leads (Input Hi and
Input Lo). Therefore, the actual voltage drop across the DUT is V
MEAS
minus the two
voltage drops in the test leads.
Since matched input leads are used, the voltage drop for the two test leads are 2 x V
LEAD
.
Therefore; V
DUT
= V
MEAS
- 2(V
LEAD
).
The Model 2700 still uses Eq. 2 to calculate resistance, but it uses V
DUT
in place of
V
MEAS
. This ratiometric method cancels the effects of input test lead resistance.
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 100
Ω
range readings up to 120
Ω
will be displayed. Above 120
Ω
, 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
Ω
4 measurement, a software (S/W) detection rou-
tine is used to detect an open sense lead.