4.3.8 Guarding High Value Resistance Measurements (SMX2044)
Measuring high value resistors using the 2-Wire function require special attention. Due to the high impedances
involved during such measurements, noise pickup and leakage could be very significant. To improve this type of
measurement it is important to use good quality shielded cables with a low leakage dielectric. Even with a good
dielectric, if a significant length is involved, an error would result due to leakage. Figure 4.7 exemplifies this error
source. It is important to emphasize that in addition to the finite leakage associated with the distributed resistance,
R
L
, there must also be a voltage present between the two conductors, the shield and the center lead, for leakage
current to develop. Provided there was a way to eliminate this voltage, leakage would have been eliminated.
Figure 4-7. Error due to cable leakage.
The SM2044 provides an active guard signal that can be connected to the shield and prevent the leakage caused by
the dielectric’s finite resistance. With the shield voltage guarded with Vx, as indicated in Figure 4-8, there is 0V
between the shield and the high sense wire, and therefore no current flows through R
L
.
Figure 4-8. Guarding eliminates errors due to leakage associated with high resistance measurements.
4.4 RTD Temperature Measurement (SMX2044)
For temperature measurements, the SMX2044 measure and linearize RTDs. 4-wire RTD’s can be used by
out. The connection configuration for RTDs is identical to 4-wire Ohms.
selecting the appropriate RTD type. Any ice temperature resistance between 25
Ω
and 10 k
Ω
can be set
for the platinum type RTDs. Copper RTDs can have ice temperature resistance values of 5
Ω
to 200
Ω
.
The highest accuracy is obtained from 4-wire devices, because the resistance of the test leads is nulled
4.5 Internal Temperature (SMX2044)
Signametrics
32
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