Electronics > Metrology

Scanner/Multiplexers for voltage references

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The results are now ready. After doing the mentioned modifications: copper wrapped relays, some insulation, add one more scan card to fill out the machine, reduction of TEMF turned out to be 35 nV on average. What is more important that the improvement was bigger on the relays that previously showed more TEMF bringing down the min to max from 217 nV to 182 nV.

Results table below with per relay TEMF in nV, after mod, before mod and difference.

The setup was already good enough for my purposes before, but better is of course better. It is not certain which of the 3 modifications made a difference, but I could think that all helped.

I already raised my question in DIY Low Thermal EMF Switch/Scanner for Comparisons of Voltage and Res. Standard, but did someone with such a low thermal emf scanner already implemented a remote control for a fully automated setup? I was told the following already:

--- Quote ---nV scanner: it is powered via the SMA connector (center positive). When you apply 5VDC, you can use the Reset and Next buttons to select the channel. If it is connected to the DVM's 'voltmeter ready' connector, it advances automatically, but be sure to set some slow measurement time - 1s or more.  Otherwise it won't have time to charge it's internal capacitors and won't have enough energy to switch the relays. Best performance is obtained when You delay the moment of taking measurement by 5...10s from the activation of the relay coil.
--- End quote ---

My meter doesn't have such a 'voltmeter ready' connector, hence why I'm asking for remote control.


The low thermal EMF scanners i built have a nucleo32 STM32 controller (USB connection to host). The STM32 firmware also implements ambient sensors and a transparent RS232 link for the Keithley 2182A nanovoltmeter. In some sense the controller represents a meter with a scanner. These things were easy to do.
Meanwhile i learned how to implement GPIB using STM32 controllers. This improves measurement time (sampling rate) in relation to data transfer time. What is "missing" is the precision temperature readout of the scanner internal temperature sensor (glas ntc) that was previously implemented using a HP 3456A meter and got voltage deviations to below 1 nV. I made some prototypes including enclosures, but then i burnt one of the ADS1256A ADC modules and i continued with other stuff.

Regards, Dieter


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