Operator`s manual

4.1.1 Two Wire Multiplexing
In the TwoWire multiplexed configuration, the SMX4000 family first opens all channels, than it connects the Hi and
Lo terminals of the selected channel, to the Hi and Lo terminals of the A-bus. Ch1 to Ch35 are routed 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 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 switching
card for the job. Make sure the relay scanner selected has sufficiently low thermal EMF (offset) specification. At
1µV thermal EMF, the SMX4032 is the best choice for low level voltage and resistance measurements. The Digital
Multimeter you select is also important. High end DMMs such as the SMX2042, SMX2044, SMX2064 and Agilent
34420A have the advantage of having an order of magnitude more current, reducing low resistance measurement
errors by a factor of 10. The following table quantifies the effect of TEMF on Ohms measurement, using a high
quality DMM such as the SMX2042, SMX2044 and SMX2064 in conjunction with a low thermal EMF switches
such as the SMX4030 and SMX4032 will result in the lowest possible error. Be aware that this effect is the same for
either, 2-Wire or 4-Wire Ohms measurement. The resistance measurement error is equal to the thermal voltage
divided by the DMM Ohms test current. In other words, no use in getting a high end DMM while using low end
switching card.
SMX2064
Range
Ohms
Current
DMM
Resolution
Error due to
1µV EMF [1]
Error due to
10µV EMF [2]
Error due to
300µV EMF
24
10 mA
1  100  1 m 30 m
240
1 mA
10  1 m 10 m 300 m
2.4 k
1 mA
100  1 m 10 m 300 m
24 k
100 uA
1 m 10 m 100 m 3
240 k
10 uA
10 m 100 m 1 30
2.4 M
1 uA
100 m 1 10 300
24 M
100 nA
10 10 100 3 k
[1] SMX4032 performance [2] SMX4030 performance
Table 4-1. The Scanner thermal EMF affect on Ohms measurement accuracy.
Figure 4-1. 32 Channel 2-Wire measurement application.
4.1.2 Four Wire Multiplexing
In the FourWire configuration, the SMX4000 family connects simultaneously two channels. They route Ch1
through Ch16 to the A-bus, and the corresponding channels, Ch17 through Ch32 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 18