Scanner/Multiplexers for voltage references

[branadic]

Hi folks,

multiplexing between different voltage references is neat for automated test setups.
So while some of you have already build the DIY Low Thermal EMF Switches, others owned an expensive Dataproof scanner such as Model 160, 164 or 320, I tested my Prema 2080, which can be configured for 80x common ground, 40x 2W measurement or 20x 4W measurement, for thermal emf.

Therefore, I shorted two channels on one of the D-sub50 connectors on the back, connected K2182A to its output and switched between both channels. The connector on the back was -  metrology-like - covered with a towel and a Lemo - PTFE cable - Stäubli SLS4-B connector 22.2642-* connected scanner and nanovoltmeter.

Turns out the emf measured is <<100 nV and thus good enough for measurements on voltage references in the typical 10V range, even with a 8.5-digit meter. The scanner uses G6AK-234P-ST-US 12V relay. I received the full schematics of the scanner on request at Prema, which is basically used the same way in Prema 5017 SC.

So here you have it, even though it is specified to have 1 µV typ. and 2 µV max after 1.5h warmup only, it performs much better than expected.

What is your solution?

-branadic-

[TiN]

Not bad, thanks for data.

How was TEMF measured? Was is just 2182A measure scanner output with direct cable, and select CH1 or CH2 shorted inputs? How short was made?

Perhaps more details on method, to allow reproduction by other scanners?

[Mickle T.]

Prema Type 2000 10x4W scanner have about the same 100-200 nV TEMF offset.

[branadic]

How was TEMF measured? Was is just 2182A measure scanner output with direct cable, and select CH1 or CH2 shorted inputs? How short was made?

As I said, I made a D-sub50 connecter with a short on channel 1 and 2 using copper wire and standard leaded solder, so nothing special or low thermal. The setup was warmin up overnight and the connector covered with a towel.
K2182A was read via serial interface and GNU Ocatve, while P2080 was switched manually. Prior to measurement ACAL on K2182A was performed and measurement done using Rel button, while Ch2 was active on P2080. Hope that answers your question?

Prema Type 2000 10x4W scanner have about the same 100-200 nV TEMF offset.

Ah, thanks MickleT., is that the scanner with the fancy unobtanium relay you showed some years ago in another thread?

-branadic-

[TiN]

Abovementioned Data Proof 160A results. It is version without silly connectors on the back, just pure copper single-strand cables come out to the DUT.



Data points collected with Keysight 34420A nVM on 1mV range CH1, NPLC100. nVM measure A-B HI difference, while LO is shorted with copper bar (just like photo shows)



Pretty happy with results. I might replace relay board that house CH5,CH6,CH7,CH8 relays with spare board, to rectify higher TEMF for these channels.

But even bad CH8 is still within maximum vendor spec, just a tiny bit under 35nV. 35nV error in case of 10V DC voltage measurement error would translate into 0.0035 ppm, which can be ignored for all practical reasons.

Looks inside, with massive metal lid removed from isothermal box for clarity:



Latching relay card:



Rest of teardown photos are in ancient ManateeMafia's article.

[dietert1]

Your scanner has six cables in one photo and eight in the other one. What happened?
I was wondering how the cables enter into the temperature protected inner cabinet. There seem to be some centimeters of cable between outside and inside plus some kind of thermal clamp.
Could you indicate which one was the "bad" relay you want to improve, i mean the position in the inner cabinet?
As far as i understand there is something like an analog bus to collect the output signal, with card edge connectors in between. If that is correct, your < 10 nV results are wonderful.

Regards, Dieter

[Echo88]

The card edge connector is only used for the control/protection signals.
The cables used to connect the references are going directly from the outside through the insulation foam into the internal housing, soldered to the visible big vias on top of the cards.

https://doc.xdevs.com/doc/Data_proof/img/rly_top.jpg
https://www.eevblog.com/forum/metrology/teardown-data-proof-160a/

[niner_007]

I looked through and bought some research papers on the subject, and seems a lot of people had great success with motor controlled rotary switches, resulting in ultra low EMF, I don't have the papers in front of me, but it was 40nV or much less in their measurements, IIRC.

The setup was basically an Electroswitch series C4 rotary switch in a dual pole, multiple throw configuration, being rotated by a stepper motor. The setup is fairly simple, and the switch is fairly inexpensive. Something like C4D0206N-A, gives you a straight two 1:6 muxes, just perfect for reference measurements.

[branadic]

Even though the Prema2080 contributes <600 nV absolute offset from input to output it is still good enough to be used for voltage references, as the difference between two channels seems to be <100 nV. So the 40x 2W channels available on the unit configured as two channels for one reference with polarity reversal still results in 20 channels available in total and offset of the setup of <50 nV. I call that good enough, even though I have only measured channel 1 and 2 by now.

-branadic-

[e61_phil]

Here is my DIY multiplexer. It uses Panasonic relays. The performance seems to be in between a real Dataproof and the Prema.

Keithley 182 was shorted in front of the multiplexer and after that connected to the multiplexer.

[TiN]

Which relays? Maybe post some more details

Replaced Data Proof relays in channel 5,6,7,8 and restarted zero test. This time we catch warm-up time.
It took about 8 hours to fully settle under typical +/-20nV limit.

[dietert1]

A more contemporary method would be using a little ADS1256A ADC with integrated multiplexer. Board with reference is € 20 at amazon.
If you look up the datasheet page 8, offset drift with PGA 64 (measuring small voltage differences) is within +/- 15 nV/K. Input referred noise of the PGA is 27 nV (chopper).
Today i got two such ADC modules to monitor a 5x LM399 reference setup at 7 V and to monitor some other references at 10 V. Will report later.

Regards, Dieter

[branadic]

A more contemporary method would be using a little ADS1256A ADC with integrated multiplexer. Board with reference is € 20 at amazon.

Not sure how that is a contemporary method, unless you mean buying stuff off Amazon, instead of using what you already have at hand. 

I've spent some cotton-wool underneath and above the relay card in Prema 2080 and repeated the measurement.

First 500s: both Stäubli SLS4-B connectors shorted at one of the P2080 posts
500s - 1000s: both connectors connected to the output of P2080 and switched to shorted channel 1, which is an imbalance in thermal mass
1000s - 1500s: channel 2
1500s - 2000s: channel 1
2000s - 2500s: channel 2
2500s - 3500s: MULTI-CONTACT 4mm short at the ouput of P2080, which added a large thermal mass

Lower diagram is zoomed in, the difference between the channels decreased, I'm happy with that.

-branadic-

[dietert1]

If you have a museum, that's fine with me. And if your pieces are working (more or less), even better.
Yet somebody should mention how those things would be done nowadays. For me a solution that is available as a regular product is more interesting than using 30 year old surplus.

Regards, Dieter

[e61_phil]

Here is some more 30+ years old equipment

I played around with my HP 3488A switch unit. No low thermal cables, just Hirschmann stuff not even gold plated..

The Keithley 182 was shorted before connecting it to the multiplexer. Therefore, the measurements show the absolute voltages not just channel differences.

Channel was switched to the next every 120s. One measurement point is the average of 10 measurements (100ms integration time)

[branadic]

If you have a museum, that's fine with me. And if your pieces are working (more or less), even better.
Yet somebody should mention how those things would be done nowadays. For me a solution that is available as a regular product is more interesting than using 30 year old surplus.

Regards, Dieter

I wouldn't call a Prema 2080 a museum device and on the other hand you could have just kindly asked for the schematics, if you want to rebuild it. It uses components that are easy to buy at any distributor and a lower voltage relay version with some "contemporary" low volt controller like Arduino (in the past people simply programmed their microcontrollers themself instead of clicking stuff together, not sure if I would call that an improvement but obviously that is contemporary), Nucleo board or whatnot can be used.

-branadic-

[Kleinstein]

A more contemporary method would be using a little ADS1256A ADC with integrated multiplexer. Board with reference is € 20 at amazon.
If you look up the datasheet page 8, offset drift with PGA 64 (measuring small voltage differences) is within +/- 15 nV/K. Input referred noise of the PGA is 27 nV (chopper).
Today i got two such ADC modules to monitor a 5x LM399 reference setup at 7 V and to monitor some other references at 10 V. Will report later.

Regards, Dieter

The ADS1256 is limited to voltage inside the supply (5 V max). So the multiple inputs on a ADC chip are not a full replecement for a multiplexer for 7 or 10 V sources. It can work for things like the differenes, but keep in mind the limited voltage range.

[dietert1]

Built several some multiplexers with bistable relays, that's boring. The results are in my contributions to the precision resistor TC thread. In my opinion precision measurements of voltage references are always measuring small difference voltages between voltage references. Measuring a reference with a high resolution DMM fully includes the ADC errors (bad!). If you think measuring voltage differences is difficult, i can tell you it is easy, once you understand the isolation and guard status of your equipment. Even Mr. Frank recently got nice results using the difference method.

The ADS1256 was made for measuring thermocouples. It's something like a nanovoltmeter with a low thermal EMF input multiplexer. Depending on ambient setup it reaches about 10 ppm accuracy and is good enough for monitoring the five LM399s with an accuracy of about 100 to 200 nV. Resolution will be ten times better. I ordered some MAXIM SPI isolators for it, so with a guarded isolated supply it can be used like a DMM. There is tested firmware since i used the part before to implement a linear TEC controller.

By the way:
The NASA Perseverance robot that recently arrived on Mars has an electric energy generator that produces up to 110 W by thermal EMF (Seebeck). The temperature difference stems from 4.8 kg of Plutonium 238, half life about 8 years.

Regards, Dieter

PS: In order to measure larger voltages one can use a differential voltmeter setup. I recently made a PWM divider for our 5x LM399 reference. That is already good for a fraction of a ppm and still under development.

[branadic]

In the last days I've set up an automated measurement measuring Prema2080 with K2182A, switching through all channels (1 to 40) and acquired quite some data.

Figure P2080-TEMF3: The relay card was fully covered in cotton wool, initial zero was done with a short at the scanner output, afterwards channel 1 (very left) to 20 were monitored (1000 samples per channel / 500s), at around minute 180 I connected the short again at the scanner output, then measured channel 21 to 40 (very right) and finally connected short at the scanner output again.

I then removed all of the cotton wool at the component side of the board.

Figure P2080-TEMF4: The initial zeroing was done with scanner switched to channel 1, each channel was monitored over 1000 samples (500s) before switching to the next channel and this for ten complete runs.
As can be seen there is quite some overall distribution and difference between the first 20 channels (connector X2) and the second 20 channel (connector X3). It can also be extracted that some channels are close to each other, so that they could potentially be used for reversal measurements of voltage references without contribution of large errors.
We can further extract that there is obviously some thermal gradient, so that improving thermals could improve overall behaviour. At least for channel 21 - 40 this can be directly correlated with the position of the relays inside the scanner, while on channel 1 to 20 it's less obvious.

Is someone experienced? Would a copper plate covering all relays improve thermal gradients significantly and thus TEMF?
Just for clarification, the scanner is fully within spec (typ. 1 µV, max 2 µV after 1.5 h warmup time), but I just wonder if it can be made behaving a bit better.

-branadic-

[Echo88]

You could design a pcb(70µ copperplane would be good) with relay cutouts, that is laid on the bottomside of the existing relay-pcb and then thermally couple it to the relays with Vishay THJP for example...but i doubt it will be yielding the same results as a design where theres no heat producing stuff next to the relays.

[niner_007]

I wonder what if you heat the relays with a heater substrate, or keep them relays in an oil bath.

[Andreas]

Is someone experienced? Would a copper plate covering all relays improve thermal gradients significantly and thus TEMF?

I fear no. In my experiments with 1.5 mm thick Aluminium sheets I have seen quite large temperature differences within few cm.

The first thing that I would do is looking with a thermal camera where the gradients are. (If I had one).
I fear that the processor, the voltage regulator and the bridge rectifier produce a lot of heat.
There also seems to be an old 74LS device on the PCB which probably could be replaced by a CMOS device.
In my scanner the average current consumption of the whole cirquit is well below 1 mA.
So I would try to replace the heat generating parts. (or at least remove the voltage regulator from the PCB).

with best regards

Andreas

[branadic]

There is at least one possible solution, that I will give a try. Between board and metal sheet is a distance of 8 mm. Filling this gap with cotton wool didn't much. So I will install a 6mm aluminum plate with the shape of the board and put some 2mm thermally conductive, but electrically insulating silicone rubber pad (4 kV/mm) in between. I hope to decrease thermal gradients this way a bit better.

-branadic-

[branadic]

Meanwhile, I made an aluminum plate, installed it to the scanner and the silicone pads arrived, which are sandwiched between aluminum plate and relay card. I also repeated the measurement relative to channel 1. The picture slightly changed, which proofs that the scanner has a thermal issue, but the result is still not satisfying.

-branadic-

[dietert1]

The datasheet of those bistable OMRON relays https://www.mouser.de/ProductDetail/Omron-Electronics/G6AK-234P-ST-US-DC12/?qs=CX134%252BdLMDH9uKRlS2Zpog%3D%3D does not contain the terms "thermal" nor "EMF" nor "Seebeck".
It does contain though a specification for "Min. permissible load". Somewhat surprising for a gold plated contact, but it may be unreliable for dry switching.
Can you check that?

Regards, Dieter

[Echo88]

https://www3.panasonic.biz/ac/e_download/control/relay/signal/catalog/mech_eng_txs.pdf TXS2SA-L2-4.5V
https://www.buerklin.com/medias/sys_master/root/h5c/hea/9313902428190/8873768026142.pdf AGQ210A4H
https://content.kemet.com/datasheets/KEM_R7002_EC2_EE2.pdf  EC2-3NU
https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F108-98001%7FZ.1%7Fpdf%7FEnglish%7FENG_DS_108-98001_Z.1.pdf%7F5-1462037-5 5-1462037-5
All spec a minimum load (10µA 10 mV DC / 100μV/ 10μA), probably just to be on the save side legally and otherwise DMMs couldnt measure small voltages reliable due to negligible current over the switching relays.

[3roomlab]

i butchered the omron pdf chart ...
very messy but the lines seem to tell a story
in general, it looks like there are 2 groups of performers, the better ones have a deliberate steeper upward curve

the lines ending in blobs indicate pdf does mention about low thermal emf ability. the 274P pdf mention that it has a special low thermal model. but which model? is it a typo?
the 2 MEDER reeds are 1billion ops @ <5v switching.
has anyone seen a S4EB DC performance curve published by panasonic?

edit SEE BELOW POST for updated butchered pic

[dietert1]

Assuming the gold-plated contacts are good for dry switching, one could invent another problem. What i have seen in the past are relays using composite contacts where the signal needs to pass another contact/oxide barrier. For example the solder pins are made from thicker sheet than the contacts themselves. Oxide is known to cause EMF problems. Maybe one needs to switch some current every now and then to clear the relays. Easy to check whether a certain type of relay has this "feature", if you have one. Anyway, before using a relay outside specs, one could take one apart to understand whether it may be good enough or not.

Regards, Dieter

[kleiner Rainer]

Dieter, there is a good resource about relays and their contacts:

https://www.panasonic-electric-works.com/pew/de/downloads/technical_information_relay_de.pdf (sorry, only in german)

There was an excellent relay handbook published by SDS Relais , one of the predecessor companies, but i could not find a pdf-version yet.

My experience with their relays is twofold: my company used about a million of them, and as a ham radio operator i can testify, that they are able to switch microvolt signals well - the RX-TX-transfer switch has to, otherwise the very weak received signals would not be able to pass the relay contact.

BTW it is a bad idea to try to clean a gold plated contact with high current - in most relays the gold film is very thin for obvious reasons. Relays in telecom apps used a concept named "wetting current" to avoid dry switching, but adding a small voltage drop across the contact while being unimportant for AC applications is not desirable for metrology.

For improved reliability i recommend to look for bifurcated contacts, this gives very high reliability due to the fact that you are switching two contacts in parallel.

Panasonic relays that seem usable for metrology application are TQ (thermal EMF specified max. 2uV for the SMD version), AGN (bifurcated contacts with AgPd, Au clad) and DS (Au clad Ag contacts).

Greetings,

Rainer

[Echo88]

Afair the STLT-suffix in the G6AK-274P-STLT-US-DC5 marks the "Low Thermal"-version.
Seems Omron discarded all datasheet-versions where the LTST-suffix was described.
Funny enough they are still actively sold by e.g. Mouser/Digikey.

Anyway: even the Low Thermal versions of non-latching relays will have more TEMF than latching relays due to their construction.
Latching relays switch with one short pulse and the heat in the relay decays, while non-latching relays will always have a thermal gradient during switching and therefore TEMF in the relay contacts all the time, despite good thermal symmetrical construction.

[kleiner Rainer]

For non-latching relays there is a trick to reduce heating: the pick-up voltage is much higher than the drop-out voltage. Often the drop-out voltage is 10% of the nominal operating voltage. So option one is to use a parallel RC circuit is series to the coil and select the cap so that the charging current pulse is sufficient to close the relay while the resistor just barely holds the relay closed. Option two is described here:

http://www.ko4bb.com/getsimple/index.php?id=how-to-operate-24v-relays-from-12v

Hope that helps.

Greetings,

Rainer

[dietert1]

Of course, it would be extremely stupid to derive from the datasheet requirement of some micro-amperes the destruction of relay contacts by overcurrent.
As stupid would be the assumption that the average user of a DVM would notice a thermal EMF of 1 or 2 uV, as he/she would use banana plugs instead of copper lugs.
Before commenting here, people should try to understand what branadic is trying to achieve.

Unreliable dry switching was new to me until i saw it in active speakers for home use. Output relays need to be strong for several amperes to survive drive currents for the bass channels and they would fail for the tweeter, since during normal home use tweeter currents are to small to clear the relay contacts. So after some years those speakers would sound dumb and one could solve the problem by turning up sound level for some seconds. Nowadays one can use MOSFET switches instead.

Regards, Dieter

[Echo88]

Okay, you were the first to mention minimum load for the relays branadic used, as if that would be in any way relevant to the problem.
Then talking about missing TEMF-terms in the relay datasheet, as if that would change the fact that the TEMF in branadics scanner comes from the external temp gradient due to the surrounding electronic heat sources.

And then comparing some several ampere/inductance load switching relays with DMM-switching and claiming dry switching problems.

Regards to you too. 

[3roomlab]

For improved reliability i recommend to look for bifurcated contacts, this gives very high reliability due to the fact that you are switching two contacts in parallel.

Panasonic relays that seem usable for metrology application are TQ (thermal EMF specified max. 2uV for the SMD version), AGN (bifurcated contacts with AgPd, Au clad) and DS (Au clad Ag contacts).

Greetings,

Rainer

i stumbled on something interesting

there are 2 version of TQ pdf
TQ2 states 1A life is 0.2million with therm emf ability
TQ2S states 1A life is 0.5million no therm emf ability stated
both look like current pdf can be downloaded from the usual octopart etc
but in the TQ2 pdf, it include TQ2S as part of the "total" lineup. it is a confusing story 

this confusion almost infects TXS2 / TX2S. the TXS2 emf spec is 0.3uV. like the S and no S naming problem, the TX2S has no thermal ability stated. try read this line a few times, it helps in the confusion. now which is the model with low emf? 2S ? X2? S2? 2X ?
then there is TE, sometimes it is listed as part number, sometimes it is listed as model number.

(edit maybe all good DMM should have firmware to count relay ops to advise user of reliability issues)
updated butchered pic

bonus article : relay contact adhesion effect
maybe the contacts collided too hard !

[NWerner]

By chance I opened a TXS2-L2-4.5 just yesterday.

in the attached photo one may identify
bifurcated contacts and some contact blocks.

Aside from those blocks all other conductors
have a reddish colour (maybe phosphor bronze or CuBe ?)

[dietert1]

Here i have some results from our HP 3457A with 44492A reed relay scanner (10 channels). Since the relays are in two rows of five relays, the result looks very similar to branadics Prema 2080. Channel 10 is an exception - it sits next to the power supply. I think i could use some tape to close the 44492A on that side to improve that channel. Of course - if one has more channels than needed, one can use a subset. Resolution of the HP 3457A is 10 nV, stdev was 45 nV during a 4 hour run.

When you see a spec of "maximum thermal EMF" in a relay data sheet, you already know that is strange. Thermal EMF is a product of a physical characteristics and temperature gradient. EMF = K * dT. When switching Hi and Lo in a DVM scanner, it gets something like EMF = K * dT(Hi) - K * dT(Lo). Assuming the construction of both relay contacts is similar. There won't be a maximum, unless you specify the maximum gradient. Maybe what they mean is the gradient from self heating, but then again: What is the self heating of a bistable relay?

One of the main problems of those scanners has been discussed and the solution has been shown before by Tin: Don't use connectors at the border to the cold outside, but solder thin shielded cables to the relay board and make a loop of the cables inside the scanner.

Regards, Dieter

[MiDi]

When you see a spec of "maximum thermal EMF" in a relay data sheet, you already know that is strange. Thermal EMF is a product of a physical characteristics and temperature gradient. EMF = K * dT. When switching Hi and Lo in a DVM scanner, it gets something like EMF = K * dT(Hi) - K * dT(Lo). Assuming the construction of both relay contacts is similar. There won't be a maximum, unless you specify the maximum gradient. Maybe what they mean is the gradient from self heating, but then again: What is the self heating of a bistable relay?

I think T-EMF given in uV is meant to be due to self-heating between contact pins.
In contrast if it is given in uV/K I would assume it is due to external temperature difference between contact pins.
For a bistable version self-heating is negligible or at least decays after switching, so not sure what T-EMF in uV implies for those.
With 2 contacts involved the max. T-EMF could be worst case twice as high or best case cancel each other.

[dietert1]

After applying some mods to the HP3457A with built in relay scanner i reduced spread of thermal EMF errors to +/- 400 nV after heat-up of some hours. The mods i tried are:
- Move the floating power supply regulators to the aluminum sheet below the board.
- Cover onto the reference module
- Remove inner signal connector and solder teflon wires to scanner board.
There are more mods on my list.

One can subtract the remaining offsets per channel. Changes in ambient temperature can be corrected by declaring one of the relays a thermocouple and using its temperature dependent residual voltage for compensation of the other channels. Then the residual thermal EMF errors remain below +/-50 nV - a reasonable result. By the way the first two rounds after turning on the DVM with scanner give a channel spread of +/- 56 nV (stdev) with 155 nV p2p. Whatever you do, that scanner won't get better than this.

Regards, Dieter

[dietert1]

Here i have the results of a calibration as outlined above. Some hours at the beginning of a continuous run were used to calculate the calibration, with channel 6 of 1 .. 10 as reference ("thermocouple"). This channel was found to be representative for the others.

Then a 24h test was evaluated using that calibration. HP 3457A noise was reduced by application of 21x boxcar lowpass filters. Later i will try to reduce the protection resistors at the meter input. And run the DVM without Autozero to collect twice as many readings. The device should go into a cabinet with a bit of protection against air drafts.

Residuals with low thermal shorts exhibit standard deviations of 13 to 22 nV for the other nine channels. p2p statistics appears normal. Channel 10 is still worse than the others. Don't know yet what will happen when the lab temperature rises to 30°C in summer, yet for the time being the proposed calibration method works. The HP3457A can be used as a nullmeter for monitoring a set of voltage references to 0.01 ppm.

In the second diagram i stacked the curves by 50 nV per channel to show individual logs better. Y scale is correct only for first channel.

Regards, Dieter

[Kleinstein]

The 3457 is not very suitable as a null-meter. The resistors in the input protection contribute quite a bit too the noise, but I would expect the actual input amplifier to contribute about as much.
The noise shown shows quite a bit if low frequency noise - e.g. thermal fluctuations. The efffective bandwidth for the average over 21 readings at 100 PLC is quite low, at some 1/84 Hz. So some 40 nV/sqrt(Hz) from the resistors (102 K) would only be 4.5 nV of noise over this bandwidth. This is quite a bit less than the obersvend 15 nV RMS.

The fluctuations could be at the scanner or possibly at the voltmeter inputl - there are already relays to switch from front to back/scanner and to isolate the  low voltage path when a higher votlage is measurend.

Using the non AZ mode would only make the noise worse for a slow reading (like 10 PLC).  Swtiching to the reference channel is slow and thus a rather low effective frequency for the 1/f noise. So the higher reading rate would not compensate for more 1/f noise. The non AZ mode is usually only good with very fast readings << 1 PLC.

[z01z]

Dieter, there is a good resource about relays and their contacts:

https://www.panasonic-electric-works.com/pew/de/downloads/technical_information_relay_de.pdf (sorry, only in german)

There's also an english version: https://www.panasonic-electric-works.com/pew/cz/downloads/technical_information_relay_en.pdf

[dietert1]

Many years ago i bypassed the HP 3457A front end for a certain project. Our HP 3457A has U111 in a socket and there is a pigtail from one of the 51K protection resistors to plug there and feed the input terminal to the ADC directly. For a null meter application one could replace the front end by an integrated chopper amplifier with the correct gain. Monitoring voltage references does not require extremely low leakage.

I used the scanner in two wire ohm mode to put a small current into the relay contacts before. Anyway, for me nine channels with 80 .. 100 nV p2p are useful and i did not expect it from that old meter.

Regards, Dieter

[dietert1]

Meanwhile i repeated the low thermal short test for a DIY scanner i made last year. It has a FTDI UM245R module and a XC9572XL CPLD and drives an 8x4 matrix of 32 coils in 16 bistable Axicom relays. There are four pnp transistors as pull-ups. The eight pull-downs are CPLD pins (5V tolerant). Although the relays are general purpose in contrast to the Coto relays of the HP 44492A, the DIY scanner is superior, as it remains cold. The relay coils are 180R and operate with 20 msec pulses. Average total power consumption in all 16 relays is about 1 mW during this test. Two coils get operated every five seconds to advance the scanner. While the span in the 44492A was +/- 400 nV after some mods, this time it is more like +/- 150 nV. Relays 1 and 16 are significantly high.
When calibrated as discussed above, this DIY scanner operates at p2p = +/- 20 nV over 24 hours (15 channels).

Regards, Dieter

[MegaVolt]

When calibrated as discussed above, this DIY scanner operates at p2p = +/- 20 nV over 24 hours (15 channels).

Is there a link to the topic with the project?

[dietert1]

No there hasn't been a thread, but it was mentioned before: https://www.eevblog.com/forum/metrology/hp-mux-e1476ae1442a-cards-for-ref-scans/msg3080729/#msg3080729.
You may notice that i increased pulse timing to 20 msec as recommended in the Axicom datasheet. And channel enumeration was rotated by 180° after some tests with a resistor ladder. Design of a board for that MUX is not yet finished. The board will definitely be bigger than that hand wired prototype as i want to increase distance between drive circuitry and the relays. Also i want to reserve a region of the board for a thermal clamp, with the relays on one side and all external connections on the other side of the clamp. The board will have a connector for the controller, as others may prefer an Arduino or the like instead of the CPLD

Regards, Dieter

[dietert1]

Here i have a test of a spare Axicom bistable dual coil relay. One coil is driven using a 100 uF capacitor - yellow trace. Blue trace shows the inactive coil. One can see the EMF with indication that the contact movement finishes within about 3 or 4 msec. That EMF should be taken into account when designing drive circuitry for the scanner.

Regards, Dieter

[dietert1]

Meanwhile i finished the next version of a USB scanner with Axicom bistable relays as mentioned above. After spending two days on the design of that 4-layer board, i made three of those for our lab. Later i added a small mezzanine with 2x 74AC138 to make the control interface fit into a DSUB-9. The image shows the thermal clamp made of two 8x12 mm aluminum bars, one above the board, the other one below.
In a first test today the channel to channel variations remained within +/- 6 nV p2p without any compensation or calibration. And the 4x4 geometry is still visible as a pattern - so maybe there is still room for improvement.

Regards, Dieter

PS: In fact the decoders are 2x 74ACT138 to implement translation from 3.3 to 5 V at the same time. For the tests i made a STM Nucleo-L432KC as a controller, external to the box. Communication with host is over USB CDC (virtual com port that is part of the ST-LINK debugger).

[dietert1]

Testing a scanner in the low nanovolts with a HP 3457A requires a lot of data. The instrument has a resolution of 10 nV but a noise level of about 43 nV (standard deviation at 100 PLC with AutoZero).
Meanwhile i collected 3000 rounds with the new revision of the scanner. Standard deviation of the 16 channel averages came down to 1.1 nV, with +/- 2 nV p2p. As a cross check i disconnected the scanner from the 3457A input terminals and wired a low thermal short instead. After collecting another 3000 rounds i got 0.6 nV StDev of the channel averages with +/- 1.2 nV p2p. These numbers are as expected from the basic 3457A noise. The diagram shows both sets of data. The visible 20 nV shift between them is very low frequency "noise" of the 3457A (e.g. ambient temperature drift). Average noise level per channel was 42.8 nV rms when measuring through the scanner and 43.2 nV rms with the one low thermal short at the input terminals.
Conclusion: With a 95 % confidence level detection limit of +/- 3.3 nV there are no imperfections of this scanner, neither channel offset nor enhanced noise. Good enough for monitoring voltage references.

Regards, Dieter

PS: You may have guessed it, channels 9 .. 11 (the most suspect ones in the blue plot if at all) are the four relays where the screw in the thermal clamp is missing. Need to fix that asap.

[ch_scr]

Just arrived Fluke 2300A Thermocouple Scanner. Instrument is copyright 1979 and has transformer isolation and SMPS for the 5V rail. Propably to reduce thermal gradient. Scanner card has draft shield for input connections, draft shield on bottom side and what looks like draft shield between relais board and main scanner pcb? Also copper sheet below relais sub-pcb. Input screw connectors are on big block of what looks like brass, whole scanner cards weighs 713g (25oz). Unit has shielded cage around scanner cards, should block interference from transformer as well and even out thermals. Interesting enough, seems like muxed signal is routed through backplane/front panel pcb, main pcb, communication pcb and out the ribbon cable / db connector on the back  where it would connect to thermometer unit.

[ch_scr]

Took apart the scanner board. Screws holding the bottom draft shield hold a coil board on top. Between coil board and reed relais is a 0.6mm thick copper sheet. Below is an isolating plastic and reed relais in little plastic tubs, soldered to ceramic interposer boards. 

[Kleinstein]

This is an interesting construction to get low thermal EMF reed relais.

The contact block does not look that impressive: I kind of miss more area to thermally couple the incoming wires to the metal block.

[Andreas]

Hello,

the effort is absolutely necessary.

The wires through glass are most probably out of Covar with a high thermal EMF against copper.
And the wires are not closely neighboured.

with best regards

Andreas

[dietert1]

Meanwhile i started using the low thermal scanner i described above, with two 3457A meters connected to the output. I am monitoring one of the 10 V references in 30 V range and voltage differences in 10 mV range. Today i found that avoiding transient overvoltage to the meter can make a difference. This means:
- not using autorange
- setting the meter for a higher voltage range before switching the MUX
- switching the MUX before setting the meter for a lower voltage range

When measuring 10 V the average TC of the two meters is about 0.6 ppm/K. One thing i still need to study is the guard of the HP 3457A. It does have a cage around the floating part but no separate guard terminal. Maybe i can disconnect the cage from the low terminal and add a guard terminal.

Regards, Dieter

[dietert1]

Here i have some results. The diagrams show daily averages since June, 24. As one scanner round takes about 40 seconds, each average contains about 2270 * 2 = 4540 measurements of each voltage.

The setup includes three experimental voltage references: Two with LTFLUs, one with 2x JFET. The best agreement is between the two LTFLUs, the diagram is +/- 0.1 ppm now! Biggest day to day step observed was 0.06 ppm. The drift  of 17.2 nV/day between the LTFLUs continues as expected from an exponential relaxation fit i showed some time ago in the LTFLU thread. It predicts 18.1 nV/day.

The 2x JFET JVR i made in February still isn't tired of drifting. 120 nV/day amounts to 4.4 ppm/year. Noise appears to be low, though.

Regards, Dieter

Edit: 120 nV/day * 365 days/a gives 44 uV/a, i.e. 4.4 ppm of 10 V. Not as bad.

[branadic]

I was struggeling for quite some time, but I have now ordered G6AK-274P-STLT-US-DC12 to replace the original G6AK-234P-ST-US-DC12 in my Prema2080. Reason for the decision was that the unit already has everything I want: case, display, GPIB interface, 40-channels, ...
So we will see if that is improving things further and fingers crossed it does. I'm not expecting too much, but you never know.

-branadic-

[branadic]

Yesterday the new relays (G6AK-274P-STLT-US-12) as well as a desoldering station arrived, so I spent the whole evening to desolder all of the original relays (G6AK-234P-ST-12) and cleaned the board (first IPA, afterwards Ethanol).
This morning, after some coffee break, I've soldered all the new relays in place and cleaned the back of the board. The unit is now back together, working and warming up. Hopefully I can manage to start a measurement on it and see what has changed. I'm not expecting any miracles though.

-branadic-

[Anders Petersson]

... cleaned the board (first IPA, afterwards Ethanol).

If you forgive an off-topic question, what's the advantage of combining the two cleaning agents? A basic Google search didn't help.
IPA is readily available but access to pure ethanol is regulated so perhaps some denatured version is acceptable?

[branadic]

IPA disolves most of the dirt, but sometimes leaves a white film on the top when evaporating. Ethanol cleans the board much better, at least that is my experience. Pure ethanol can be bought at the drug store, at least here in Germany.

-branadic-

[e61_phil]

I also experienced this white film after cleaning with IPA.

To investigate if it really came from the IPA, I filled a lot of IPA in a test tube and let it evaporate completely. There was nothing left in the end (no white film).

It seems that this film isn't directly from the IPA.

[CDN_Torsten]

I've always assumed that the white film remaining after an IPA wash was caused by small amounts of water in the IPA (which reacts with the flux)....but I'm not 100% certain.

[branadic]

Why is there white residue when I clean my PCB?

White residue is generally a symptom of ineffective PCB cleaning. Common conductive flux residues from the soldering process can include various unreacted activators, binders, rheology components, and saponifiers. Among these are numerous iterations of acids (abietic, adipic, succinic among others), highly basic ingredients (amino compounds), and even constituents found in “soaps” such as phosphate and sulfate ions. When a cleaner does not fully dissolve all the constituents, or the cleaner is not allowed to flow off the PCB, the remaining solvent can evaporate off and leave behind residue that is either white or like water spots.

Source: https://www.chemtronics.com/why-is-there-white-residue-when-i-clean-my-pcb

However, I only have that issue from time to time with IPA, not with Actenone or Ethanol. As Acetone can melt some thermal plastics I try to avoid it on such boards, but go IPA and Ethanol instead. Works best for me.

-branadic-

[branadic]

So here is the measurement of all the 40 channels, one after the other. Some improvement, but not as good as a Dataproof. Some of the channels are quite close to each other though, thus by selecting the proper channels the influence of the scanner can be negligible .

-branadic-

[maat]

I figured the white residue are salts left from whatever acid there is in the flux, therefore IPA does not dissolve it. When I wash the boards by hand I typically use destilled water first to get those water soluble salts and then I throw the boards in IPA and let them dry (in the liquid). Then I give them a good brush. Works every time. What works even better is a good ultrasonic cleaner at 130 kHz and 60 °C. Don't go for the cheap chinese rubbish. Been there, done that.

But you have to be careful when using an ultrasonic cleaner, because some parts just don't like that. ALPS rotary encoder for example. I don't know what kills them, but they die. Every time so far...

[ramon]

I remove the white residue just by rubbing it with a cotton rag  (for small areas a few cotton buds also works, but is not as good). The trick is to do it 10 or 15 seconds after applying IPA (maybe because the PCB still has some water content and helps it to adhere into the cotton rag?. )

I guess that this trick just removes the visible white residue but doesn't remove it 100%. Probably not good for ultra high impedance or femtoampere currents. In that case, what branadic proposed would work best.

[dietert1]

So here is the measurement of all the 40 channels, one after the other. Some improvement, but not as good as a Dataproof. ...

For scanning voltage references low leakage is a pointless exercise (and it doesn't improve on low thermal EMF). Why don't you try to understand the solution i proposed and demonstrated. Some people successfully copy things even without understanding all the details. Better than buying old stuff of unknown origin. I mean exactly with relays you don't want the old ones that already executed millions of cycles.

Regards, Dieter

[branadic]

Why don't you try to understand the solution i proposed and demonstrated. Some people successfully copy things even without understanding all the details.

Don't get me wrong dieter1, but the way you communicate is really weird and strange. You act like you know it all and I'm pretty sure that is not the case.

Better than buying old stuff of unknown origin. I mean exactly with relays you don't want the old ones that already executed millions of cycles.

Carefully reading what I wrote before reveals, that the relays have all been replaced, so they are in new condition directly from a valid distributor. And you don't seem to understand, that I like all my gear with the very same interface and as all of my equipment has GPIB interface, I like the scanner to have that too, so that all of it can be connected to the same bus, without multiple different cables for different interfaces running all over the place. If you for yourself have a different view on that, that is your very own opinion and I leave it like that. Please accept that other people have different preferences. Thanks.

e61_phil suggested to run the scanner in 4 wire mode and to use the sense lines only, as routing is more even, so I will give that a try next. Thanks for that suggestion.

-branadic-

[Kleinstein]

The problem with the prema board is that there relatively hot driver part is relatively close to the relays.
Even if old, the scanners are often not used for that many cycles and if so, often just a few channels.

I think it is mainly the software / interface part that makes it attractive to reuse the old PCB and not start from scratch, like Dieters design, which looks better and also has better measured results.

If the scanner is 4 wire (e.g. 2 sets with 2 poles), one could chose a better suited set of contacts, and even mix the 2 sets.
Chances are much of the thermal EMF effect is relatively stable. So it would not hurt measurements of drift and noise vey much.

[branadic]

I think it is mainly the software / interface part that makes it attractive to reuse the old PCB and not start from scratch, like Dieters design, which looks better and also has better measured results.

If the scanner is 4 wire (e.g. 2 sets with 2 poles), one could chose a better suited set of contacts, and even mix the 2 sets.
Chances are much of the thermal EMF effect is relatively stable. So it would not hurt measurements of drift and noise vey much.

It works better, agree, but only after some iterations and its form follows function (ugly Hammond case). If that works for him, fine.

But experience is not about simply copy&paste, but about making experiements yourself, taking some traps and improve where you spotted an issue and it's hobby to see, were the limits of improvements are, even on old test gear. That way and only that way you can develope a gut feeling, not by reading and simply copying. And noone should explain himself, why not to simply copy some others design, but share results and also drawbacks during the way.

Otherwise we could all wait until someone else made a solution or buy what is already out there, stop hobby and buy what others came up with. Nothing I like though.

-branadic-

[dietert1]

My remark about old relays was a response to your mention of dataproof scanners. Somewhere in this forum you can find a report of TiNs attempt to fix an old dataproof scanner.

Regards, Dieter

[branadic]

As suggested I've measured TEMF of the Prema 2080 in 4W mode, first the sense channels and as a consequence also the force channels. Results attached.

-branadic-

[dietert1]

branadic was also wrong with his comment on the aluminum enclosure i used, it is no Hammond.

That is one of the details i did not write down until now. The case is a RND 455-00396 IP65 enclosure. It was selected as it is moisture proof (rubber seal). And its wall thickness is such that it serves as a "temperature equalizer". I am using it inside an outer case made from MDF boards plus 20 mm of styrofoam isolation. The MDF box is almost air tight, too. There are no connectors but pigtails long enough to pick up input signals at their origin. The output cable directly feeds the DVMs.
That RND aluminum case is available for € 12 at Distrelec, order code 300-64-617. For € 20 they also have a painted version.

Regards, Dieter

[guenthert]

    There are different approaches.  I was a bit surprised seeing a national lab using rotary switches actuated by stepper motors in what superficially looks like a home-made robot kit 

    They do put quite some effort in assuring low thermal EMF across the switches, a thick copper base and the bane of home construction, Cd based solder: https://www.bipm.org/documents/20126/27085544/bipm+publication-ID-2250.pdf/0b48f272-169c-c175-72df-6aa670c4b763?version=1.7&download=true

[dietert1]

Above i recommended trying a TI ADS1256 with its built-in muliplexer.
Meanwhile i learned that there is an improved part ADS1263 and today there are first results with a raster board test circuit including a MAX14483 SPI isolator and +/- 2.5 V voltage regulators for the analog front end. The setup includes another board with a linear +/- 3.3 V isolated power supply and a STM32F429I-DISCO.
I put the ADS1263 board into a plastic bag to reduce air draft. The on-chip scanner exhibits thermal offsets with a standard deviation of 45 nV over several channels with low thermal shorts. The offsets are very stable with a standard deviation of 9 nV, so the scanner is already useful to this level. May improve yet when i put the board into a constant temperature metal enclosure similar to the HPM7717.

Regards, Dieter

[Kleinstein]

The standard CMOS MUX chips also behave reasonable as a low voltage MUX. They have low power which makes it realtively easy to get small temperature gradients.  A problem is however that they have little protection, so ESD or to high voltage (outside the supplies) may damage them. Especially old types are supposed to be relatively susceptible to latch-up.

[dietert1]

The ADS1263 is much more than a MUX. Next step is to read the on-chip temperature sensor and see how it correlates with the residual offsets. Maybe it's possible to calibrate away large parts of the offsets

The numbers above are with the PGA at G=2. Then, using the on-chip 2.5 V reference i have an input range of +/- 1.25 V. A 1 V test voltage divided from the +2.5 V supply gets measured with a standard deviation of about 2 or 3 ppm. The on-chip reference is supposed to be good for 1 ppm.

Regards, DIeter

[Andreas]

Hello,

more interesting would be wether the gain and reference drift could be calibrated out over temperature.

is there any possibility to measure the whole 0..5V range by using Gain = 1 and put the negative input to the 2.5V Vref?
So with a LTC1043 2:1 divider one could measure the whole 0..10V range.
Best would be a whole system calibration over temperature.

Edit: but I fear that the internal voltage reference is sensitive to humidity.

with best regards

Andreas

[dietert1]

My plan is to try and use this as a nanovoltmeter for scanning the voltage differences of a group of references in a round-robin manner. Want to compare results with the low thermal EMF relay scanner i made recently (see above). Still many steps to get there, like a good input protection for the ADS1263.

For measuring voltages up to +/- 14 V i will probably try and combine the second ADS1263 board i made with a programmable PWM reference, implementing a differential voltmeter. I want a precision DVM without resistor arrays and this may be a path.

Regards, Dieter

[Kleinstein]

I way to get low noise protection is using 2 back to back MOSFETs with a photovoltaic coupler to drive the gates and with a AC optocoupler for clamping and to turn off the input.
This is a bit like the input protection for the Keithley 2000 (ohms part) and 2182. It can be reltively low resistance.
For me 2x STU2N95 + TLP3906 + TLP290 work OK and limit the input current in case of a overvoltage to a reasonable level (< 100 µA).
There can be some thermal EMF (a few µV when heating a fet with the finger), but the FETs can be separated from the heat source (TLP3906).

To a limited extend (a bit high leakage) it can also be used for input switching. Compared to ready made photomos there may be lesser offset from thermal EMF.

For high speed pulses one may want some input inductors.

[dietert1]

OK, in order to try that scheme i ordered some parts from Digikey. TLP290 is "not recommended for new design", but Digikey have LITEON LTV-814 and LTV-844 that appear to be the same. STU2N95 is rare, so i ordered Infineon IPU80R4K5P7AKMA1.

Meanwhile i learned that noise and zero offsets of an ADS1263 depend a lot on PGA gain. At the highest gain G=32 i am getting 3 nV as stdev of the zero offsets. Noise when combining 2x 10 PLC measurements with polarity reversal is about 11 nVrms. For comparison a 3457A does a 100 PLC measurement including Autozero in 5 seconds and it gives 45 nVrms noise. At the same time the ADS1263 repeats the 2x 10 PLC measurement 12 times, so noise becomes 12 nVrms/sqrt(12) = 3.5 nVrms. Including the 10 channel scanner with calibrated offsets, there will be about 5 nV rms. Let's see how much input protection adds into this.

Regards, Dieter

[Kleinstein]

The FETs and optocouplers I noted are the parts that I use, they type are not special and alternatives are OK. The AC input optocoupler effects the current one sees in the fault case. For some reason the leakage specs for the STM FETs are relatively low - could be still just a point of the measurement. Just for protection and if not used as a switch the leakage is not critical anyway.

The noise level with the gain looks really good.  A relevant point may still be the input current.
Another possible source for trouble may be if the capacitance at the input effects the reading. This does happen with some AZ OPs and may also efect the ADC input. So one may want some filtering there (e.g. capacitance to ground and series inductance / resistance). Some part may be before the protection too. 

[dietert1]

LT MUX check with K2182A

The setup has one of the LT MUXes i made last year, with 16 low thermal shorts - soldered copper wire bridges inside scanner. Output is connected to a Keithley 2182A nanovoltmeter. A Nucleo32-L432 module with an ICL3232 is used to control the scanner and the nanovoltmeter by a USB connection to a W7 laptop. Besides the Nucleo32 has ambient sensors SHT35 and BMP388.
The nanovoltmeter collects 40 samples of 2 PLC with FAZ polarity reversal per sample in its trace buffer and sends the data to the host. This takes about 5 seconds per channel and about 80 seconds for one scanner round. Total number of samples is about 640 samples per scanner round plus some more data from the ambient sensors and the K2182A temperature sensor.
First diagram shows all samples from four rounds. Transients from relay operation are not visible.
Second plot shows 16 scanner rounds, this time with averages of each 40 consecutive samples. And there is an average curve from all 16 rounds. Once more the DIY scanner shows thermal EMF at a 1 or 2 nV level or less. This time there may be significant deviations.

What took weeks with a HP 3457A was finished after 30 minutes with the Keithley 2182A. HP 3457A five second measurements show about 45 nVrms noise, while a 2182A provides five second measurements with about 1 nVrms noise.

Regards, Dieter

[Kleinstein]

That is impressive low noise and also low variations in the thermal EMF for mechanical relays.

The overall offset of some 140 nV I assume is from the meter and maybe the cable there.

Not directly related to the scanner:
How fast is the FAZ in the K2182A running: is it 2 PLC with one polarity and then 2 PLC with the revese and than 2 PLC for the ADC zero ?

[dietert1]

Our second K2182A runs in a thermal chamber and with synthetic mains power. It exhibits noise of hourly averages of 0.18 nVrms, yet walks around like +/- 5 nV, so it needs continuous nulling. Maybe the LT Muxes i made are good enough to help improve below 5 nV. The first measurements looked promising.

When set for nominal 2 PLC, the K2182A executes two 2 PLC integrations for each sample if FAZ (polarity change) is on. So 40 x 2 x 40 msec = 3.2 sec total integration time. I don't think there is an ADC zero integration, as the conversion time totals 4.51 seconds including setup delays and delays from line sync. With three integrations per sample one would expect at least 4.8 seconds. I used the 2 PLC setting for lowest noise after looking at a diagram in one of the Keithley manuals.

Regards, Dieter

[Kleinstein]

Not using ADC AZ is a bit strange, as the non AZ mode has some low frequency noise, that can be visible even with a gain of 100 in front.
Not using AZ for the ADC may explain the extra slow walk around. A different range may give more hints on this.

With not AZ for the ADC part more gain for the front end could help.

In some cases the Keithley meters may use a bit strage type of AZ mode, like averaging over multiple readings. So the AZ part for the ADC may be faster than expected, though also not as efective. Still with just repeated 2 PLC readings (atually 2x2 PLC) there are not that many options.

[dietert1]

As far as i understand with polarity reversal there are two ADC readings ADC_P and ADC_M and the sample value 0.5 * (ADC_P-ADC_M) is independent of ADC offset. In the manual they mention about turning off autozero, but more warnings than explanations. The measurements above are with AZERO on. Maybe AZERO uses ADC_P+ADC_M to null the ADC.

Regards, Dieter

[Kleinstein]

If the description / crude plan is correct, the front end reversal only changes the sign of the offset. So one reading is actual signal + front end offset and the other is signal - front end offset. This should not correct the ADC offset. It is only the gain of the amplifier that reduces the effect of the ADC. There is still the amplifier before the ADC and the ADC itself with no zero correction. For the amplifer at the ADC input there may be a hidden AZ OP, I have not yet seen and than one may get away without an ADC AZ part. With high total gain the ADC drift would be less important - a really good amplifier may be sufficient.
There is still some residual LF noise from the 2nd amplifier and the ADC.

[dietert1]

Yes, that's correct. There is no polarity reversal, but an "exchange of parts" to simulate a symmetric input stage without offset.
Don't know why they didn't implement polarity reversal. Maybe the floating section coupling capacitance is large and takes current on polarity reversal. Then one would need a floating section that has its Gnd potential unaffected by input polarity reversal. Looking at the noise of 2SK170 below 1 KHz one would really like to have polarity reversal at a frequency of 50 or 100 Hz to supplement recalibration by the relay MUX.

Regards, Dieter

[Kleinstein]

The part exchange is just a slightly modified chopper amplifier. There is no significan analog low pass and the filtering is done digitally with the ADC. The method is kind of limited by the ADC speed and settling. The special switches (kind of Photomos) used for the input are also a bit slow.
Classical polarity reversal would need the switches to operate with the full input voltage and this add to the input current spikes.
There is a way to do a kind of polarity reversal for the ADC by moving around the low side (I use it with my DVM), but it really does not look like it is used here. This would allow a kind of AZ for the ADC withput loosing time, but it may not be compatible with the ripple from the front side AZ or at least add to the settling time.

Faster switching could help for the amplifer, but the ADC speed sets a practical limit to a 25 Hz cycle (2x1 PLC).

The strange part to me is more that the 2nd amplfier stage seems to not use an AZ op, but switching to zero with the MS ADC.
There are some AZ OPs in the circuit - maybe one is also correcting the 2nd amplifer, though it does not look like that.

[dietert1]

Meanwhile the LT Mux with K2182A revealed significant thermal voltages of up to +/- 6 nV on some channels. The offsets depend on the daytime and correlate with ambient temperature change. Our lab has a daily temperature cycle of about 2.5 °C, with rapid temperature rise after heaters turn on in the morning.
Due to construction of the LT MUX (see above) basic temperature gradients inside are near zero and cause thermal EMF of less than 1 nV on all channels.
As ambient temperature changes creep into the LT MUX and its relays, they cause additional temperature gradients proportional to speed of temperature change. I could measure the speed of temperature change (up to +/- 0.2 °C per hour) after mounting an additional temperature sensor inside the MUX and thus predict null offsets. Looks like the LT MUX+K2182A measurement can be calibrated to nV level. The diagram shows this for one channel of the MUX. All other channels exhibit the same behaviour with lower null offsets. In previous tests with a HP 3457A multimeter these effects did not show up, as that meter requires much longer integration times to reveal nV effects.
The internal temperature log demonstrates the quality of thermal isolation. The scanner alu box is inside an outer box with 3 cm of PUR foam.

Regards, Dieter

[dietert1]

Meanwhile there is a one week log with the setup. The image shows the LT Mux in its box, the Keithley 2182A nanovoltmeter and the Nucleo32 controller. Not shown is an HP 3456A used to log the MUX internal temperature, a USB to GPIB controller and a laptop computer.

There is a diagram to show all 15 calibrated channels over one week. The logs are stacked by 10 nV per channel, so channel 5 zero is at 50. The red lines are running averages of about one hour.

Calibration works in two steps:
1) Subtract measured null offset of channel 1, to continuously null the nanovoltmeter.
2) Subtract a linear term dependent on the speed of temperature change observed by the internal temperature sensor. There is a correlation diagram showing this relationship for channel 10 (same as in previous post).
First table shows the results of similar line fits for all channels. The fits yield positive and negative coefficients for different relays. Remaining offsets are all below 1 nV.
The residuals of calibrated measurements exhibit a standard deviation of about 2 nV (1.3 nV * sqrt(2) due to subtraction of channel 1)
Second table shows daily averages for all 15 calibrated channels. All 15 channels over all 8 days exhibit a standard deviation of 0.08 nV. As one scanner round takes about 80 seconds, there are 1000 rounds per day, so 2 nV / sqrt(1000) = 0.06 nV, so 0.08 nV is near perfect.

Next step is routing cables out of the LT MUX for all 15 channels and repeating test with external low thermal shorts.

Regards, Dieter

[dietert1]

As i did not have any free time to work on the setup i left it running. Now i have a one month log, with an overall standard deviation of each calibrated MUX channel = 0.3 nV. P2p is about 2 to 3 nV, roughly as expected.
I need to work on the determination of temperature drift after interrupts of operation. The math is incomplete during restart.

Second diagram shows a calibration check. There are two calibrations now, one from January (as above), one from February. They agree well. The per channel offsets are reproduced with a standard deviation of 0.09 nV.

Third diagram shows a Channel 1 log of the nanovoltmeter. That would be the result of the Keithley 2182A without the LT MUX. There is an uncertainty of 50 nV p2p and "events" that vanish after calibration by the LT Mux.

Regards, Dieter

[martinr33]

I did not see these papers posted before.

This one is on the Xdevs site:
https://xdevs.com/doc/_Metrology/qian2016.pdf
Interesting becasue:
 - it uses $4 relays (panasonic latching relays) and achieves very small offset voltages.
 - Noise appears comparable to the dataproof scanner system, below. +/-15nV
 - Uses terminal posts selected for similar thermal offset voltages
 - Pure mechanical connection to terminal posts.


NIST on a Dataproof scanner, showing about 30nV of noise per channel.
https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=32563

[dietert1]

The work i documented above includes a new method of calibration/compensation and it got explained in some detail. Another detail is a thermal clamp between nanovolt signal I/O and relays. In fact there are two thermal clamps, as shown above. Also i am using both relay contacts to implement symmetrical switching. My relay board isn't vertical but horizontal. My drive circuitry is USB without additional power supply. There have been some revisions to get there.
In my LT MUX design a delay of 5 seconds after switching a relay is not necessary.
In the next revision i want to implement a Kelvin connection for the internal temperature sensor.

Regards, Dieter

[Kleinstein]

The performance is really impressive. I had expected more trouble from using relatively normal relays for low level DC signals.
The thermal setup looks really logical: quite some separation the extra blocks.

With a good temperature sensor inside, it may be possible to also use some channels for thermocouples. So TC plugs connected with TC wire up to the contact block - these could be in parallel to the normal contacts.

[martinr33]

The general design approach for scanner control appears to be parallel controllers. However, this approach is wiring and power intensive.

I am thinking a serial shift register approach woud be better:
 - Fuve control lines, reset - power - ground - clock - bits
 - no operating current, just a quick pulse.
 - Very scalable, no meaningful limit to he number of relays
 - Board could be cuttable - design for 12 or 16 channels, and place cut lines so the board can be easily shortened.
 - there is a risk of multiple inputs being on simultaneously.

Two output channels might be good, ut they add another set of relay contacts.



Then, it needs a small controller. Arduino would be fine. The simplicity of the cable makes the separation very easy. Could also run off of a parallel port.

Another point, aluminum PCBs are now inexpensive. This approach would help with thermals. Surface mount relays would also be better.

[dietert1]

Yes, it's working well, yet for continuous logging i'd prefer a mosfet scanner.

With 5 seconds per channel and 5 msec relay coil pulse, the average power consumption is 2 * 5 msec / 5 sec * 4.5 V ² / 170 Ohm = 0.24 mW. One needs to operate two coils to step the scanner. Observed temperature rise of the internal temperature sensor above ambient temperature is about 1 °C when the unit operates inside its thermal box.
With photovoltaic mosfet drivers consumption would be about 3.3 V times 10 mA continuous = 33 mW. Roughly a factor 100 more than before. This would drive two contacts per channel like the relays (4 mosfets per channel).
For low leakage the couplers need to be close to the mosfets, but a temperature rise of 100 °C won't work. So the construction will be more complex, maybe with a peltier cooler to take away the "heat". Or give up on isolation and use mosfet drive circuitry similar to integrated muxes.

Regards, Dieter

[Kleinstein]

The single coil latching relays often get away with less power, but the driving with bipolar pulses is a bit more complicated. With half the coils it is still not that bad and there is a solution with N+1 CMOS drivers for N relais.

The PV drivers for a MOSFET gate can get away with a little less current. I get away with some 2 mA (2.2 K series resistor with 5 V supply). If needed as a pair, 2 diodes may be wired in series. The voltage at the PV driver is not the full 3.3 V, more like 1.2 V for an IR diode, though a few types may use 2 diodes in series internally. The resistor can be more with the control part.

The balance between heat and leakage is indeed a bit tricky.  Usually nV accuracy is not needed in combination with very high resistance and the MOSFETs are usually not super high isolation anyway. So the MOSFET based scanner would naturally have a bit more leakage, especially with many channels. For the heat of the PV drivers much would be about spreading it out / linking the different channels, as usually 1 channel is active at any time, just the position changes. A constant heat source is a smaller problem than one that is moving around.

In many cases one would not need that many channels, at least not the high precision ones. With slightly leaky switches more channels also come with a downside.  At least from my experiance one often has a combination of a few precision channels and than sometimes quite a few additional lower grade signals (e.g. temperature, supplies,...) to monitor. It can there make sense to have 2 (or even more) separate meters and scanners. The sometimes tricky point is combining the data.

[guenthert]

The general design approach for scanner control appears to be parallel controllers. However, this approach is wiring and power intensive.

I am thinking a serial shift register approach woud be better:
 - Fuve control lines, reset - power - ground - clock - bits
 - no operating current, just a quick pulse.
 - Very scalable, no meaningful limit to he number of relays
 - Board could be cuttable - design for 12 or 16 channels, and place cut lines so the board can be easily shortened.
 - there is a risk of multiple inputs being on simultaneously.

Two output channels might be good, ut they add another set of relay contacts.



Then, it needs a small controller. Arduino would be fine. The simplicity of the cable makes the separation very easy. Could also run off of a parallel port.

Another point, aluminum PCBs are now inexpensive. This approach would help with thermals. Surface mount relays would also be better.

     Well, the old (Keythley brown era) 705 uses (plenty of) shift registers.  With some luck you might get a good deal on the secondary market.  The low EMF relay cards however are less common and considerably more expensive.

[dietert1]

Five days ago i started another log with a low thermal relay multiplexer as shown above, this time using a different LT MUX unit. Unit three was finished very similar to unit two, ie. with internal low thermal shorts, with a shielded output cable and the special LEMO connector for the Keithley 2182A. It also got a 10 KOhm glass thermistor to track internal temperature.
The calibration obtained from this log resembles previous results, both for temperature drift and basic offset calibration. The standard deviation of hourly averages is 0.33 nV once more. If the calibration comes out similar between two units, that's another indication it will be stable.

Regards, Dieter

[miro123]

I have few questions.
1. Why the most people choose relays?
2. What are the relay advantages for this application ?  Thread description is - 'Scanner/Multiplexers for voltage references'
I asking this questions because I start building mux for my 6 channel lm399/adr1399 board- I choose different approach -> low power=low thermal EMF, solid state=small size. If this does not work I can ovalized it- I already made very good digital-TC oven. I have experience in digital control and it was easy task. With dual control loops and MISO control I achieve under 1mK stability in 24h
I have doubt that I'm in the right way - I'm curious to know why experience guys choose relay technology for such application

Thanks in advance

[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

[bobAk]

What relays are there?  I have a board 7701 2 pcs.

[dietert1]

The relays used in the 7706 have a red cover and are labeled "NEC Japan EC2-4.5SNJ". There is a number 389930, maybe a date code. They are single coil latch type relays, mouser have datasheet of Kemet version:
https://br.mouser.com/datasheet/2/212/1/KEM_R7002_EC2_EE2-1104574.pdf.
I mounted the multiplexer in the lower bay with the upper bay empty and with its cover on.

Regards, Dieter

[bobAk]

So the relays are the same.  Thanks. I will need to take measurements with a 7701 connector and a homemade cable.  Connector contact material is phosphor bronze.

[dietert1]

Meanwhile i solved the USB crashes and there is a complete 24h log now. Results are very similar to what i got before.
The day-night temperature cycle of about 2 °C causes offset variation of about 125 nV.
The third diagram shows the 8 input channel logs with continuous recalibration and stacked by 100 nV steps in order to show how well this is working. Total vertical scale is 1 uV and applies to channel 1. These curves involve a 5x running average.
With 5 second measurement time one gets about 47 nV RMS. This is 0.005 ppm of 10 V - good enough for monitoring a setup of multiple zener based references.
In this test the K2700 performs very similar to a HP 3457A + 44492A scanner plugin that tested with 45 nV RMS previously.

Regards, Dieter

[aronake]

I plan to use an Agilent 34970a with a 34904a matrix switch card to have 4 3458a continuously monitor 7 voltage references where all meters measure all voltage references. This will also be a way to monitor the 3458a.

The 34970a and the 34904a are not metrology grade equipment. The 34904a is specified to have a thermal offset below 3 uV. If actual offset is close to 3 uV with much variations on the relays this would not bee good enough for my intended purpose. But usually specifications are very relaxed compared to real performance. Obviously I wanted to test this.

Test setup:
- Well warmed up 3458a, 34970a, 34904a.
- Around 1.5 meter Canare L-2B2AT cable connected to the 4 rows and 7 columns (I did not test the last column). Shield of all cables soldered to ground on the matrix switch card. Canare L-2B2AT is a tradeoff between being thin, being shielded, being flexible, but unfortunately not PTFE but PE insulation. But PE is not too far from PTFE when it comes to electrical characteristics.
- Reason for the length of the cables is that this is the length i will need once i put all in "production" so get a test of the whole system. Not too much care on keeping the cables free from electrical interference. But this will be similar to "production" environment. I do not have enough space to make this great.
- All row wires connected to 1 3458a, and from there connected to one more 3458a. The second 3458a used to validate the first.
- All column cables shorted.

The test:
- Python script to:
- Switch through all switches of the 34904a.
- For each switching of 34904a relay do 7 measurements at NPLC 100 with both 3458a. First one 3458a measure, then the second, then the first again.
- ACAL on 3458a for every third round of switching of the 34904a. The first round after each ACAL discarded in final analysis, as the 3458a take some time to settle in after ACAL.
- Total 20 runs across all switches were made.

Results:
TEMF averaged 123 nV. Max 249 nV and min 32nV.

I took average nV measurement of all measurements on all switches and mapped out as how they are located on the card.

See nV switchmap attached at end of this post.


Generally the result came out much better than expected. 249 nV will not really be much seen on 3458a at 10V. 249nV is around 1/10th of 34904a thermal offset specification. Also cables and electronic interference included here. It is also very clear that different relays have different amount of temperature gradient. The transformer of the 34970a is located close to where the highest thermal EMF can be seen.

I also wanted to check settling time of TEMF after a relay has switched. To do this, I grouped all measurements in which order it was made since relay switched and took average of each group.

Measure   nV
1           117
2           118
3           119
4           115
5           119
6           117
7           116
8           117

Conclusion here is that no settling in time can be observed. TEMF is same for first measurement since relay switched to last.

All in conclusions:
Positively good result and the 34970a / 34904a are good enough for my needs when it comes to having many 3458a monitoring each other and many voltage references.

Improvements that could be done:
Best nV measurement device i have is 3458a. It showed to be good enough to measure what I wanted to measure, so the reasonable approach would be to be satisfied with this. The more fun approach would be to get a nanovoltmeter (2182A or 34420a). That will probably be where I will go next.

Ideas in improvement on 34970a/34904a:
It is clear that the relays close to the transformer show quite a bit more TEMF. I will do some test putting copper sheets on top of the relays to have heat spread out more evenly and see if that reduce TEMF.
 

[alm]

- For each switching of 34904a relay do 7 measurements at NPLC 100 with both 3458a. First one 3458a measure, then the second, then the first again.

Nice test and write-up, thanks for posting! If you're not aware, a trick to do long measurements more efficiently is to set the 3458A to single trigger mode, then trigger each 3458A (using TRIG SGL command) so they start the measurement, and then poll each meter until the ready for instruction status bit is set. See this code for an example of reading both non-blocking (max_time = -1), polling for a certain amount of time (max_time > 0) or a plain blocking read (max_time = 0). The advantage of polling is that it keeps the GPIB bus free for other thing. This is especially a big deal with resistance readings with OCOMP and DELAY which can take minutes.

- ACAL on 3458a for every third round of switching of the 34904a. The first round after each ACAL discarded in final analysis, as the 3458a take some time to settle in after ACAL.

Interesting, I have observed this for resistance but not for DCV. But then I normally use the 3458A on the 10V range, so maybe it's only the lowest DCV ranges?

[aronake]

Hi Alm,

Thanks for comments.

- For each switching of 34904a relay do 7 measurements at NPLC 100 with both 3458a. First one 3458a measure, then the second, then the first again.

Nice test and write-up, thanks for posting! If you're not aware, a trick to do long measurements more efficiently is to set the 3458A to single trigger mode, then trigger each 3458A (using TRIG SGL command) so they start the measurement, and then poll each meter until the ready for instruction status bit is set. See this code for an example of reading both non-blocking (max_time = -1), polling for a certain amount of time (max_time > 0) or a plain blocking read (max_time = 0). The advantage of polling is that it keeps the GPIB bus free for other thing. This is especially a big deal with resistance readings with OCOMP and DELAY which can take minutes.

Background to why I have one 3458a to wait while the other is measuring is that I first had them to measure at the same time. But then saw some mysterious result that I thought could have to do with the meters somehow causing some interference with each other. So then changed. I later figured out this likely was related to making measurements straight after an Autocal.

My usual method for nonblocking reads looks like this:

while not (MySTB & (1<<4)):
            MySTB = 3458a.read_stb()
            time.sleep(2)

What you point at looks better. Do you have some example code where this is used?

- ACAL on 3458a for every third round of switching of the 34904a. The first round after each ACAL discarded in final analysis, as the 3458a take some time to settle in after ACAL.

Interesting, I have observed this for resistance but not for DCV. But then I normally use the 3458A on the 10V range, so maybe it's only the lowest DCV ranges?

As mentioned in my writeup I did one ACAL for each 3d round of scan across all switches. In the graph below (not very nicely made) I have marked when ACAL happens (3 times for this part of the measurement). Each horizontal line is 100 uV and I added some offset to actual measurements to keep measurements from the two meters more apart. The faint" digital looking line" is what relay I was meaning on.

One meter seems to take a jump upward and the other a bit lower after ACAL compared to for the relay sweeps for where no ACAL was made. But it looks like it is very small, maybe 20 nV on the one that jumps upward and maybe 10 nV for the one that jumps downward. 20nV when measuring 10 V would be 2 counts on the last digit at 8.5 digits so hidden well below the noise floor. So for 10V measurements ACAL stabilization should not matter. And 3458a not really the right tool for nV level measurement. But what to do if not having anything better at hand (yet)?

This behavior looks very similar for all other times ACAL (maybe 7 more) was done over these measurements.

Purpose of this test was to see TEMF on the scanner, so do not have too good data on ACAL stabilization time, so not very conclusive.. But thanks for giving me an idea on something to dig into. I have 5 3458a, so not a huge sample from a statistical point of view, but still quite some 3458a to test. Its intresting that the two I used here seems to move in different direction after ACAL.

[alm]

What you point at looks better. Do you have some example code where this is used?

Here you have an example of starting a measurement on two 3458As and then polling both meters (in series) until they're ready.

Purpose of this test was to see TEMF on the scanner, so do not have too good data on ACAL stabilization time, so not very conclusive.. But thanks for giving me an idea on something to dig into. I have 5 3458a, so not a huge sample from a statistical point of view, but still quite some 3458a to test. Its intresting that the two I used here seems to move in different direction after ACAL.

I misspoke, I did not observe any deviation after ACAL, but I did observe a deviation after running the TEMP? command, which I run regularly to check if the internal temperature changed enough to require ACAL. When measuring 10 MOhm standard resistor, I observed about 3 ppm drops in the reading the sample after executing TEMP? (see attached graph, blue is the resistance reading and red is temperature reported by TEMP?). I did not observe any difference in the second sample, so I just discard the one sample after TEMP?. I haven't observed any effect on the sample after ACAL that was above the noise floor, but I indeed haven't used the 3458A down to these levels.

[aronake]

Interesting. Thanks for this.

I realized i misspoke too. 20 nV which was kind of my rough estimate of ACAL TEMF drift of the most impacted meter is 0.2 of last digit at 10V and 8.5 digits. So not only below noise floor but also below number of digits that can be read. So seems fair to say that DCV measurements can be done straight after an ACAL with no impact. Apparently not resistance though from your findings. I will investigate this further.

[aronake]

Time for some modifications to see if the 34904a can be improved. Wrapped the relays in two layers of copper foil, added some insulation and added grounds copper foil as shield around the card. Previously had the 34904a and a 34901a multiplexer in the 34970a. Have added one more card now to fill out the space.

A bit too many changes to know what change will make a difference, but running tests now.

[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:

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.

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-