Electronics > Metrology

Scanner/Multiplexers for voltage references

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aronake:
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.

branadic:
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.

-branadic-

dietert1:
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

branadic:
Just in case someone missed the thread, here are very first results of a measurement on the DIY Low Thermal EMF Switch/Scanner

-branadic-

dietert1:
Using the same technology as i recently showed in the "Experiments with reference ovens" thread, i instrumented one of the low thermal EMF scanners from 2021. Here i have some results from a first 24 hour test.
Temperature is measured with a 10 KOhm thermistor and a 10 KOhm metal foil reference resistor both inside the MUX. I am using a ADS1256 module with an ADG736 CMOS multiplexer for bridge voltage reversal. The half bridge is wired
with separate force and sense (5 wires in total). In order to fit this into the existing DSub-9 connector the relay driver was supplemented into a SPI like serial interface using a 74HCT595. The STM32L433 device includes a GPIB controller for the Keithley 2182A nanovoltmeter and a RS232 interface in addition to the USB CDC link. It also monitors ambient using I2C sensors.
The controller calculates temperature drift every 5 seconds and outputs a running average with a time constant of 100 seconds. In the diagram i marked the extrema of the LTMUX temperature curve (red), in order to check that the calculated drift result crosses zero at the same time. Notably the ambient temperature crosses the LTMUX temperature curve at the same time, except at the beginning, when the MUX started with its own temperature after working on it. The MUX is inside a thermal insulation box that causes a temperature time lag of about 2 hours.
The temperature drift determination is needed to numerically compensate residual EMF of the relays (about +/- 5 nV).

Regards, Dieter

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