The primary consideration for low voltage measurements is the Thermal EMF or Offset Voltage associated with the relay.
When two, dissimilar metals are joined a voltage is created. This voltage is known as the thermal electromotive force (EMF) or the Seebeck voltage. The Seebeck voltage is dependent on the temperature of the junction and the composition of the metals joined. The specific metal-to-metal junctions result in specific temperature coefficients (µV/°C), also known as Seebeck coefficients. The following table lists the most common metals and their respective Seebeck coefficients.
Thermal EMF in switches
The leads of electromechanical relays are usually composed of metal alloys, most often nickel-iron alloy, while the PCB of a switch module is usually composed of copper or copper alloy. The junction between these two, dissimilar metals creates a thermocouple, as illustrated in the figure below.
||Note A thermocouple developed in a relay is dependent on the temperature of the junction. The temperature of a junction varies according to the ambient temperature, the number of relays activated, the air flow inside the switch module, and the types of switch modules located in the adjacent slots.
A signal path can transverse a single relay or multiple relays. The sum of all the thermocouples in a signal path is expressed as the thermal EMF. Thermal EMF can be specified as single path (single wire) or differential path thermal EMF. The figure below illustrates thermal EMF measured in a single path.
The figure below illustrates thermal EMF measured in a differential path.
See "Other Low Voltage Tips and Techniques" below for minimizing Thermal EMF.
The discussion above applies to electromechanical armature and reed relays. FET and SSR switches do not have a thermal EMF in the same sense as reed and armature relays. They do, however, have an offset voltage due to the heating of transistors in the relay circuitry. This offset voltage is comparable to the thermal EMF of armature and reed relays and is expressed in the same units, thus we can compare thermal offset between all four relay types.
Thermal EMF or offset voltage ratings are given for the entire switch module. For instance, the NI PXI-2503 has a thermal EMF of <2 µV. This is not the thermal EMF for each relay in the switch module, but rather for the module as a whole. Additionally, this value is also taken as a worse-case scenario estimate being that the signal path may go through a variable amount of relays depending on the topology of the switch and the channel chosen.
An Additional Note on Accuracy
When measuring voltage with a switch and a DMM, be sure to account for thermal EMF in the overall system accuracy calculation.
For example, if the DMM has an accuracy of 4 µV and the switch has a differential path thermal EMF of 3 µV, the overall system accuracy can be calculated as follows: √(4² + 3³) = 5 µV
Thus, when measuring a 50 mV signal, the overall system accuracy is 0.01%.
Which Relays to Use for Low Voltage?
Electromechanical armature relays have a thermal offset that is comparitively low and can range from <1 µV to around 10 µV. For instance, the NI PXI-2503 armature switch has a thermal offset of <2 µV. Additionally, latching relays are preferred to non-latching relays because the lack of coil heating minimizes EMF which can affect your measurements.
Reed relays traditionally have a higher thermal EMF: from around 5 µV to well over 50 µV. This is due to the ferromagnetic materials from which reed relays are constructed. For instance, the NI PXI-2530B reed switch has a thermal offset of 50 µV.
FET relays have a thermal offset closer to electromechanical armature. For instance, the NI PXI-2501 FET switch has a thermal offset of 2.5 µV.
SSR relays have a higher thermal offset than either electromechanical armature or FET switches. For instance, the SCXI-1128 SSR switch has an offset voltage of <25 µV from 0 °C to 25 °C.
Top Low Voltage Picks
PXI-2501 multiplexer/matrix, PXI-2503 multiplexer/matrix, PXI-2527 multiplexer
All switches listed above have a low thermal offset and are excellent choices for low voltage measurements. The PXI-2501 has the added benefit of extremely fast switching: the relay scan rate is 15,000 cycles/s as compared to 100 operations/s with the electromechanical armature relays (PXI-2503, PXI-2527). However, the thermal offset for the armature relays is slightly lower in general than that for the FET switches.
Other Low Voltage Tips and Techniques
A common technique used in metrology labs for very accurate measurements is to take a measurement, switch the leads, repeat, subtract and average the readings.
Thermal EMF will always exist in connections, but by remembering that it is temperature dependent, error can be minimized. 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. These temperature differences can be prevented 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.
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, connections should not be handled with the fingers. Skin oil contains a very effective corrosive that accelerates oxidation of many metals.