Electronics > Metrology

Scanner/Multiplexers for voltage references

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Kleinstein:
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.

MegaVolt:
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.

dietert1:
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

dietert1:
Recently i used a Keithley 2700 with a 7706 multiplexer for resistor array TC measurements.
Maybe it can also be useful for scanning voltage references (difference mode). After a little experimentation i got this setup, for the time being using the 2700 front panel inputs: The meter setup is DCV range 0.1 V, "Slow" with autorange and filter off. Every 5 seconds it takes a trace of 38 samples. That takes about 3.5 seconds. The remaining time is used to transfer the data to the RS232 host and calculate the median value. So there is a result every 5 seconds. Standard deviation over all 5122 results in about 7 hours is 67.4 nV. Need to repeat this with ambient temperature logging and the 7706 as scanner.

Regards, Dieter

dietert1:
Meanwhile i wired the 7706 scanner with 9 low thermal shorts and ran a 24 h test. One scanner round takes 45 seconds. Channel 5 gets used for continuous offset calibration. In order to reduce noise from the calibration channel i am using a running average of 5 scanner rounds, i.e. 225 seconds.
The diagram includes some data taking failures caused by a bad setup of the USB-RS232 adapter (CH340/341).
The table shows that with continuous recalibration each of the other 8 channels yields a resolution of about 50 nV. P2P appears a little more than expected due to the data taking breaks. The scanner needs some minutes to settle.
The residual offset of each channel from the calibration channel is between -88 nV and +20 nV, with an average of -21 nV over all 8 channels. These offsets are stable to +/- 5 nV when looking at partial data.
I think the Keithley 2700 with a 7706 plugin is a resonable solution for scanning voltage references in difference mode.

Regards, Dieter

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