4.1.1 Two Wire Multiplexing
In the
TwoWire
multiplexed configuration, the SM4000 family first opens all channels, then it connects the Hi and
Lo terminals of the selected channel, to the Hi and Lo terminals of the A-bus. The SM4020 and SM4022 route Ch1
through Ch20, and the SM2040 and SM2042 can route Ch1 to Ch40 to the A-bus. Selecting Ch1 will result in
Ch1Hi connected to A-bus Hi and Ch1Lo connected to A-bus Lo.
When making very low level DCV measurements (<1 mV), you should use copper wires. A common source of
error can come from your test leads, which can introduce tens or hundreds of
µ
Volts of error due to thermal
voltages (Thermal EMF or TEMF). To minimize thermo-voltaic effects, after handling the test leads, wait a while
for lead and terminal temperatures to equalize before attempting to make measurements. Signametrics offers
several high quality probes that are optimal for low level measurements. It is essential to select the right scanner for
the job. Make sure the relay scanner selected has sufficiently low thermal EMF (offset) specification. The SM4022
and SM4042 are the best choices for low level voltage measurements, as well as for Ohms and temperature. It is not
unusual to find scanners with very high thermal voltages. The following table quantifies the effect of Thermal
TEMF on Ohms measurement in a typical DMM such as the SM2042. This holds for both, 2-Wire and 4-Wire
Ohms measurement:
SM2042
Range
Ohms
Current
DMM
Resolution
Error due to
1uV TEMF
Error due to
10uV TEMF
Error due to
100uV TEMF
33
Ω
10 mA
10
µΩ
100
µΩ
1
m
Ω
10
m
Ω
330
Ω
1 mA
100
µΩ
1
m
Ω
10
m
Ω
100
m
Ω
3.3 k
Ω
1 mA
1 m
Ω
1
m
Ω
10
m
Ω
100
m
Ω
33 k
Ω
100 uA
10 m
Ω
10
m
Ω
100
m
Ω
1
Ω
330 k
Ω
10 uA
100 m
Ω
100
m
Ω
1
Ω
10
Ω
3.3 M
Ω
1 uA
1
Ω
1
Ω
10
Ω
100
Ω
33 M
Ω
100 nA
100
Ω
10
Ω
100
Ω
1
k
Ω
Table 4-1. The Scanner thermal EMF affect on Ohms measurement accuracy.
Figure 4-1. 40 Channel 2-Wire measurement application.
4.1.2 Four Wire Multiplexing
In the
FourWire
configuration, the SM4000 family connects simultaneously two channels. The SM4020 and
SM4022 route Ch1 through Ch10 to the A-bus, and the corresponding channels, Ch11 through Ch20, to the B-bus.
The SM4040 and SM4042 route Ch1 through Ch20 to the A-bus, and the corresponding channels, Ch21 through
Ch40 to the C-bus. To measure a resistor using Kelvin connection, the A-bus can be connected to the DMM source
leads and the B-bus to the sense leads. Make sure the polarity of the lines is consistent.
Signametrics
16
Summary of Contents for SM4020
Page 61: ...Signametrics 60...
