Signametrics SMX4032, SM4022 and SM4020 switching cards have a hundred times lower Thermal EMF than
most other switches. Even the lower grade Signametrics switches will be 10 times better.
Ohms law is used to
provide the conversion of the thermal voltage to resistance error. If you can’t tolerate 100
m
error, you
should consider using the Signametrics SMX4030, SMX4032, SM4022 or SM4042 switches, as well as use the
SMU2064 or SMU2064 DMM.
SMU2064
Range
Ohms
Current
DMM
Resolution
Error due to 10
V EMF
Error due to
100
V EMF
Error due to
1mV EMF
24
10 mA
10
1
m
10
m
100
m
240
1 mA
100
10
m
100
m
1
2.4 k
1 mA
1 m
10
m
100
m
1
24 k
100 uA
10 m
100
m
1
10
240 k
10 uA
100 m
1
10
100
2.4 M
1 uA
1
10
100
10
24 M
100 nA
100
100
1
k
100
240 M
10 nA
10 k
1
k
10
k
100
k
Figure 4-6. Resistance measurement errors contributed by Thermo-Voltaic offset.
4.3.7 Guarding High Value Resistance Measurements (SMU2064)
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 4-6 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. Depiction of the error caused the cable leakage, R
L
.
The SMU2064 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 4-7, there is 0V between the shield and the high sense wire, and therefore no current flows through
R
L
.
41
Signametrics
Содержание SMU2060
Страница 26: ...Signametrics 26 PXI Instrumentation Switching modules SMX4030 SMX4032 IVI COM driver...
Страница 81: ...81 Signametrics Postive Value 100 Warning code Example char cBuf 64 int status status DMMGetCalDate 0 cBuf...
Страница 106: ...Signametrics 106 Postive Value 100 Warning code Example char cBuf 64 int status status DMMPeriodStr 0 cBuf...
