Electronics > Metrology

Scanner/Multiplexers for voltage references

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Latching relays are a quite attractive solution:
They only need relatively little power for a short time (e.g. 100 mW for 10 ms and thus on average not much power).
The isolation is very good, especially the typical values.
Isolation between control and signal.
Low capacitance and thus little EMI coupling.

Semiconductor switching is desirable in same areas, but these have more leakage, a limited voltage rating, ESD sensitivity (may lead to leakage going up over time) and still need some power.
The main factor is how fast to switch and what votlage.

For the temperature it is not about the absolute temperature, but about avoiding temperature gradients inside the relays or semiconductor switches to cause thermal EMF.
So for a scanner circuit temperature regulation does not really help. It is more about a careful thermal design to reduce the gradients from heat flow through the cables and from the control part.

With a limited voltage and common supply / ground, like in a multi channel reference it absolutely makes sense to use semiconductor switching. the LM399 and ADC1399 are limited accuracy anyway and it is not about the 10s of nV.

Semiconductors have a problem: they don't work without electricity. You need to ensure that the sensitive outputs are not wired to ground or between each other if power is lost.

Also the idea is that a relay implements switching between metal parts, while mosfets involve semiconductors, where the Seebeck effect is known to be much bigger.
Anyway, my study (like others before) indicates that one can reduce temperature gradients to such low levels that the residual thermo voltages are in the nV and can be predicted and subtracted.
A mosfet solution would be preferable for durability, but for the time being i made the relay scanners. Unit 1 has been continuously scanning a setup with three voltage references since last June, with a total of about 310 000 scanner rounds, where the relay spec in datasheet is up to 50 000 000 operations (Axicom V23079-B1201-B301). So it should last about 78 years. Of course with 16 relays the first one wil fail earlier, maybe after 5 or 10 years.

Regards, Dieter


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