Precision binary (R2R) divider

[VintageNut]

Reading the datasheet for the Measurements International model 8000B binary divider has me wondering if a DIY implementation could be devised.
The MI8000 datasheet states 0.01 ppm resolution and 0.05 ppm accuracy.

The attraction is that the system is self-calibrating and has better resolution than a Fluke 720A KVD.

My assumption is that MI characterizes every stage of the R2R ladder individually and saves each stage's error to use in the calculation of total deviation.

A 30-stage R2R ladder would give a resolution of 10^-9.

The DIY implementation that I am thinking of is to use a DMM with switch cards to perform the programming of each stage and to perform bypassing for measuring each stage.

The downside of this architecture is that the entire input voltage will be across one stage limiting the usefulness of the architecture to low voltages. It would seem impractical to make this exact architecture able to accept 1000V input. The MI system has a high voltage extender that changes the sub-ppm goodness of the base divider to 2ppm for the high voltage enabled two-box system.

Has anyone found a discussion of a practical implementation of a DIY version of a sub-ppm accurate R2R divider ?

[zlymex]

There was an IEEE article by TSAO giving more detailed and more accurate block diagram than 8000B DS.

[VintageNut]

Thank you Zlymex. That answers some questions.

Do you have the article?

[zlymex]

Thank you Zlymex. That answers some questions.

Do you have the article?

Here it is.

Add circuit: In case anyone is interested in the Cutkosky divider on which 8000B is based.
All resistors are equal in value, R1b and R1c can be omitted.

[Kleinstein]

The  Cutkosky divider is more similar to a binary version of the KV divider than to an R2R DAC. A slight advantage might be the use of polarity changing switches and the possibility to use resistors with R and 2R value instead of separate resistors like shown. Though at the MSB one might still use 3 identical resistors instead.

The advantage is that the total resistance is constant. The advantage of a R2R chain would be having half the number of switches (contacts), but at the price of a non constant and possibly low load of the total divider.

The interesting part is still how to do automatic adjustments / measurement of the correction values. 

[VintageNut]

Hello Zlymex

The article is very helpful. Do you have the other articles referenced? Some more pieces of the puzzle will help.

6. Self-checking resistive ratios, A.M. Thompson

5. On the output resistance of self-checking voltage dividers S. H. Tsao

2. A sub-ppm automated 1-10 volt dc measuring system B.F. Field

1. A reference 25-bit resistive voltage divider S.H. Tsao

[JS]

That unit is huge, must be a lot of goodness inside!

Dealing with the few MSBs is quite a thing to archive sub ppm performance, I don't know how the trimming is done there, you do need some serious way of doing so. It states to be self checking, as long as you can measure the deviation you have enough precision to correct the INL. For checking it I don't know, swapping the polarity and comparing with it self (some sample and hold or ratio reference) would be a first degree of self checking.

JS

[zlymex]

Hello Zlymex

The article is very helpful. Do you have the other articles referenced? Some more pieces of the puzzle will help.

6. Self-checking resistive ratios, A.M. Thompson

5. On the output resistance of self-checking voltage dividers S. H. Tsao

2. A sub-ppm automated 1-10 volt dc measuring system B.F. Field

1. A reference 25-bit resistive voltage divider S.H. Tsao

I have them all, and I'll attach them here.
It may takes a little while to upload because some file is large.

[zlymex]

Have to start new posts to get around that 2M per post limit

[zlymex]

More.....

[zlymex]

And the last.
You may have to rename these five .zip files to .rar before decompress. I use WinRar but the forum does not accept .rar extension so I have to rename them before upload. I can decompress those .zip files direct by WinRar though. I tried to compress to zip file, but have to split as well due to the forum limit, but .zip file split to z01, z02,,, which not allowed.

[VintageNut]

Thank you very much, Zlymex. This will keep me busy trying to figure out what the original inventors did.

[zlymex]

.......... and the possibility to use resistors with R and 2R value instead of separate resistors like shown. Though at the MSB one might still use 3 identical resistors instead.
.........

Another meaning to use identical resistor is the self-calibration where voltages is referred at the middle point of 2R.

I guess 8000B must use mechanical relays for the switches to minimize  the contact resistance at least for MSBs.

[VintageNut]

.......... and the possibility to use resistors with R and 2R value instead of separate resistors like shown. Though at the MSB one might still use 3 identical resistors instead.
.........

Another meaning to use identical resistor is the self-calibration where voltages is referred at the middle point of 2R.

I guess 8000B must use mechanical relays for the switches to minimize  the contact resistance at least for MSBs.

Is there an article for using the middle of the 2R element as the reference point?

[zlymex]

.......... and the possibility to use resistors with R and 2R value instead of separate resistors like shown. Though at the MSB one might still use 3 identical resistors instead.
.........

Another meaning to use identical resistor is the self-calibration where voltages is referred at the middle point of 2R.

I guess 8000B must use mechanical relays for the switches to minimize  the contact resistance at least for MSBs.

Is there an article for using the middle of the 2R element as the reference point?

See attachment.

[VintageNut]

Once again thank you very much. That is quite a lot of info and will keep me busy studying and deciding what to try first.

It is interesting that the 1984 article by Bruce Field uses a common 10:1 divider. The method of calibrating each step of the divider is very clever.

 I think that using an ordinary DMM with a switch card could replicate this calibration of the divider.

You could automatically calibrate the 10:1 string, then take the measurement(s) that you want to compare to the divider and then re-run the calibration to see if the divider drifted.

[TiN]

Thanks zLymex!

Rehosted PDFs without stupid size limits.

A_25-Bit_Reference_Resistive_Voltage_Divider.pdf

Microcomputer-Based_Calibration_Systems_for_Variable_Voltage_Dividers.pdf

[Cerebus]

And the last.
You may have to rename these five .zip files to .rar before decompress. I use WinRar but the forum does not accept .rar extension so I have to rename them before upload. I can decompress those .zip files direct by WinRar though. I tried to compress to zip file, but have to split as well due to the forum limit, but .zip file split to z01, z02,,, which not allowed.

Thanks mate. That little pile has sorted out my bedtime reading for tonight.

[Echo88]

Has anyone ever actually built a KVD based on Conrad Hoffmans suggestion to use selected resistors out of an normal batch? Im interested in the achievable performance: linearity/short term stability and so on, of course no one can predict what the long term drift of those resistors will be.

Also: The "A 25-Bit Reference Resistive Voltage Divider" document is very interesting, since they use selected +-20ppm and normal relays, while achieving excellent results.
Are there more documents available on cutkosky dividers? Especially error comparisons between a normal automated KVD and cutkosky-divider considering switch resistance would be nice.

[VintageNut]

Has anyone ever actually built a KVD based on Conrad Hoffmans suggestion to use selected resistors out of an normal batch? Im interested in the achievable performance: linearity/short term stability and so on, of course no one can predict what the long term drift of those resistors will be.

Also: The "A 25-Bit Reference Resistive Voltage Divider" document is very interesting, since they use selected +-20ppm and normal relays, while achieving excellent results.
Are there more documents available on cutkosky dividers? Especially error comparisons between a normal automated KVD and cutkosky-divider considering switch resistance would be nice.

...normal automated KVD...   

Do you have an example of such an animal?

[Echo88]

@VintageNut: Instead of a rotating switch a lot of bistable relays are used, while maintaing the original KVD-concept with 11 resistors in each decade.

[VintageNut]

@VintageNut: Instead of a rotating switch a lot of bistable relays are used, while maintaing the original KVD-concept with 11 resistors in each decade.

What I am asking for is a link or documentation of a working example.

[Echo88]

Ive never seen an automated KVD, it was just an idea of mine and it certainly is possible to build it this way.

[VintageNut]

Ive never seen an automated KVD, it was just an idea of mine and it certainly is possible to build it this way.

Your use of the word "normal" seemed to imply that it was something that existed.

The problems with contact resistance of relays will make automation of a KVD a very difficult engineering task.

I suspect that Measurements International goes to heroic lengths to make characterize the contact resistance of the relays.

There is a picture of a method that Fluke uses to rotate the knobs of a KVD using stepper motors. This tells me that it is more cost effective to build a robot to turn the knobs of a KVD than it is to invent a way to switch relays inside a KVD.

[Echo88]

Yeah, but were talking about self built dividers, not modifying existing KVDs. Of course it is easier and more suitable to use steppers to adjust the rotating switches on existing KVDs to automate them. I would love to have an Fluke 720. 

So either one gets those super low emf/resistance rotating switches (where and for how much money?) or designs something new based on COTO-Relays (guaranteed low emf) or those proprietary Guildline-relays: latching relays which press a goldplated copperfoil on gold plated copper traces on the PCB, which results in <50nV emf error. Either way, one can parallel many less costly relays to achieve lower switching resistance, like it is done in the pdf of the cutkosky-divider.

I suspect MI uses the same relay-concept like Guildline uses. Either way, im still very sceptical about selecting standard 1% 50ppm resistors to use for a divider, since no one can guarantee what the long time drift will be.   

Edit: Source for the Guildline-relay-description http://www.dataproof.com/Guarded%20Scanner.PDF Page 2

Edit2: https://xdevs.com/review/dp160_scanner/ Pictures of the mentioned relays.