Optimizing Low-Voltage Measurements
As discussed in Thermal Voltages, pairs of dissimilar metal junctions—rather than a single junction—potentially can create problems. The NI 4070/4071/4072 uses proprietary gold-plated copper input leads (HI and LO). If you just connect these two jacks directly with some wire that is not copper, then you have two junctions of dissimilar metals. This basic concept can be extended to just about any other setup.
If the two jacks are at different temperatures, then a voltage is generated. The amplitude of the voltage depends on the other metal that is being used. If the other metal is copper, then the Seebeck coefficient for copper to copper is very low-well under 100 nV/°C, as long as the connections are clean and tight.
A temperature difference between the two jacks greater than 1–2 °C would be rare, but a temperature difference does exist. The interior of the PXI chassis is warmer than the outside air creating heat flow that ultimately results in a temperature difference.
For copper-to-copper connections, the generated voltage is relatively small. If you used brass instead of copper where a 1–2 °C temperature difference exists, there could be a 6 µV difference in the measurement value detectable by the DMM.
One rule for low-voltage measurements is to always use metals with Seebeck coefficients close to that of copper. Copper, gold, and silver are all good candidates. Typical Seebeck coefficients for common conductors are listed in the table in Thermal Voltage.
If the connections do not change temperatures, then the thermal voltage is stable and can be corrected; it is temperature changes between these junctions that create problems (such as offset drift, instability, and very low frequency noise). The key to preventing changes in temperature is to prevent circulating air currents that can disturb the thermal equilibrium of the junctions. This leads to another rule for low-voltage measurements-keep junctions and connections at a stable temperature and away from circulating air currents caused by movement, fans, and so on. You can prevent these temperature differences by creating a "thermal baffle," such as wrapping the junctions with common foam padding (even Styrofoam sheet can work) and keeping them away from sources of heat such as equipment heat sinks and sunlight.
Even common lead-tin solder connections can create thermal offsets of (1–3 µV/°C). While not insignificant, they can be managed using the techniques described above. For increased performance, cold weld connections by tightly twisting clean copper-to-copper connections together and then using either crimped or twist-on plastic connectors.
When copper oxidizes, the Seebeck coefficient can easily increase by several hundred µV/°C. This leads to another rule for low-voltage measurements-keep the connections clean. One option is to simply use a pencil eraser to clean the bare wire until the wire is shiny, then clean off any rubber fragments with a paper towel. Another way to clean connections is to use Scotchbrite pads to clean the wire. After cleaning the connections, do not handle the connections with your fingers. Skin oil contains a very effective corrosive that accelerates oxidation of many metals.
Refer to Switching Voltages for recommendations on reading low voltage signals through switches.