Zero Ohm DIY 4-wire standard

[Conrad Hoffman]

As a practical matter, it isn't needed. As proof, look how many are offered on the commercial market and for how long. For setting zero on a meter there are certainly simple methods like shorting bars that work fine. That said, the use in the resistance transfer standards is interesting, as is the entire subject of equi-potential lines on a conductor. I read about it and ran to the basement to make an measure one. Not everybody is fascinated by the same things and I suspect those with an interest in sub-ppm measurements would be the target audience.

[Kleinstein]

Just to have a 4 wire zero ohms reference for a meter, it is not needed to have the contacts interchangeable. So no need for a fully symmetric zero. 

A zero ohms with fixed current / voltage path is much easier (and can be lower value) - here the PCB version (like the ones sold by Fluke) are a good solution:  The current contacts (and voltage contacts) connected by a relatively thick line and a thin line from the center connecting the voltage and current pairs. Even with low precision and without adjustment the 4 wire resistance for the right current path will be very low, like not measurable by normal means (e.g.  < pOhms range).

Only if you have a wrong current path the resistance will be quite high.

The fully symmetric one is needed for something like Hamond dividers or similar circuits. Here requirements are not that bad - so the simple 4 wires crimped together can be good enough.

[VintageNut]

DMMs have a limit for the potential between FORCE HI and SENSE HI. This precludes anything with appreciable resistance difference between the two HI terminals. The same goes for the LO terminals. You cannot have an appreciable resistance between these terminals.

Certainly true, and the question is how much? This is something I've never seen in a meter data sheet unless I don't know what I'm looking for. My WAG is that it might be quite different for different meters, unless everybody happened to settle on the same basic circuit.

Example:
http://literature.cdn.keysight.com/litweb/pdf/03458-90014.pdf
Appendix A, page 286.

I spare you the looking around. It is +/- 200V.
Seriously, what is the deal with this "tetrajunction"? Is it really hard to make 0 ohm on a multimeter? You need 2 shorting bars, three if you must be proper, and a Pomona shorting bar is 5 EUR on Farnell. Can someone tell me what is the deal with it? Is this being used in turboencabulators?

For me, it has nothing to do with making zero on a DMM. I wanted to make zero on a nanovoltmeter. That is the challenge.

The DMM shorting plug is easy to replicate with DIY copper but is useless for finding the floor of a nanovoltmeter.

[Vgkid]

Bonus points if you can find this for me, I can't.
Hess, A. E. , Nano·ohm junctions for accurate ratio instruments,
NBS Report No. 9158 (Jan. 1966).

[Assafl]

DMMs have a limit for the potential between FORCE HI and SENSE HI. This precludes anything with appreciable resistance difference between the two HI terminals. The same goes for the LO terminals. You cannot have an appreciable resistance between these terminals.

Certainly true, and the question is how much? This is something I've never seen in a meter data sheet unless I don't know what I'm looking for. My WAG is that it might be quite different for different meters, unless everybody happened to settle on the same basic circuit.

Example:
http://literature.cdn.keysight.com/litweb/pdf/03458-90014.pdf
Appendix A, page 286.

I spare you the looking around. It is +/- 200V.
Seriously, what is the deal with this "tetrajunction"? Is it really hard to make 0 ohm on a multimeter? You need 2 shorting bars, three if you must be proper, and a Pomona shorting bar is 5 EUR on Farnell. Can someone tell me what is the deal with it? Is this being used in turboencabulators?

For me, it has nothing to do with making zero on a DMM. I wanted to make zero on a nanovoltmeter. That is the challenge.

The DMM shorting plug is easy to replicate with DIY copper but is useless for finding the floor of a nanovoltmeter.

Hmm. Does it really go lower (with the zero ohm) than a thick copper wire shorting the input? If so - I'd really love to figure out why....

I ask because to me the concept of looking at the noise floor a high gain amplifier is about getting rid of picked up noise (lots of shielding, shortest wires, no pickup loops, cleanliness) and eliminating spurious voltage sources (thermal equilibrium by waiting a long time, compatible materials for low thermal EMF, etc.)... In RF that may be finding the right Zo short (or low inductance resistor) but in DC it is about "real 0 ohm".

The tetrajunction requires longer wires, creates loops (even within a disk of copper, albeit more eddy current like...), is very hard to generate thermal equilibrium (I'd consider putting it in a big box)... i.e. everything that isn't good for noise floor measurements... Also, the extra wires (unless you can find a tiny 0 ohm shape that perfectly fits the layout & configuration of the input jacks of the Keithley) means that the tetrajunction 0 Ohms is virtual rather than real (so the 2 wire resistance measurements - done at the wire ends would be somewhat higher than just a short instead of wires).

Lastly, there is something oddly suspicious in pushing 5A across a device that you want to measure 0 ohm. Not something that we haven't done before (think of a Peltier device cooling a input device pair to lower noise - high current that helps drop noise like a brick)... just that the high current/voltage were always separated galvanically from the input circuit....   

[VintageNut]

Shorting the input of an amplifier is a completely different problem than providing a zero for a 4-wire measurement system.

The tetrajunction was developed for connecting 4-wire junctions of transfer standard resistor arrays. The tetrajunction is not just a trick for fooling an instrument. It is a useful device where nearly zero resistance is required for an array of metrology resistors.

Keithley had a common plug for nanovoltmeters and microvoltmeters from the 1960s through the 1990s and there is a  short, the model 1488. It is a simple copper wire in a can. It is used to zero the nanovoltmeter before connecting a measurement cable.

[Assafl]

Shorting the input of an amplifier is a completely different problem than providing a zero for a 4-wire measurement system.

The tetrajunction was developed for connecting 4-wire junctions of transfer standard resistor arrays. The tetrajunction is not just a trick for fooling an instrument. It is a useful device where nearly zero resistance is required for an array of metrology resistors.

Keithley had a common plug for nanovoltmeters and microvoltmeters from the 1960s through the 1990s and there is a  short, the model 1488. It is a simple copper wire in a can. It is used to zero the nanovoltmeter before connecting a measurement cable.

Your original statement: "For me, it has nothing to do with making zero on a DMM. I wanted to make zero on a nanovoltmeter. That is the challenge. The DMM shorting plug is easy to replicate with DIY copper but is useless for finding the floor of a nanovoltmeter."

My assumption was the "finding the floor" meant figuring out the noise or "measurement floor" of the Nanovoltmeter. In reading this again I think you meant to "zero the 4w zero using a nanovoltmeter" (floor referring to "Flooring" the 4w-zero, not the nanovoltmeter).

BTW -  since you are measuring such low signals - why not up the current? Say at 10A or 20A it should be easier to measure the offsets since the offset will be 2-4 times as high. If you do it quickly so as not to heat the copper...

[Conrad Hoffman]

Now I'm a bit baffled, not being familiar with nanovoltmeters. I'm not aware of any potential measuring device that uses a 4-wire connection, or any reason why it would or should. The tetrajunction does not have a lower resistance than any similar lump of copper. It doesn't have any lower noise than any similar lump of copper. A tetrajunction in a box labeled "zero ohm standard" isn't really, in fact one could say it isn't even a resistance standard, as the fundamental source-input hook-up for making a 4-wire resistance measurement isn't being observed. It's just one of several ways of setting zero on that type of meter. (Are we talking a nanoampmeter here, not nanovolt, as there is that other thread on measureing nanoamps?)

[3roomlab]

a strange idea came to me, about extending the "accuracy" of a PCB or flatten tetra thingy. but i am sleepy, so i might be wrong.
see pic
top 2 pin are force.
centre pin = sense +ve or -ve
the bottom thingy, has a wave trace sitting within some zone of the neutral "zero", but we dont know where is this zero
between A-B, is about 9mm. however the height of the trace is 22mm approx (i just randomly made it so), and it is (6x 22mm++) worth of traces connected to get from A to B. (practically expanding 9mm into 130mm+++)

would it be true to say if 1 is to tap the -other sense probe on the exposed wave trace, at some point, you should find a zero? i suppose wire can be soldered there to make this accuracy extension even longer to 1:50? 1:100?
in conrad's case, the neutral zone is somewhere "there", by soldering 2 long wires to 2 spots just outside this zone, then "dowse" out the zero? hopefully the trace doesnt become an antenna ?
i hope i didnt bend the laws of physics too much?

(or after connecting bottom pin, you know you are off zero by X, by connecting a parallel "dowsing" pin into the "wave" trace, to find the counter-potential and offset the error? so instead of shaving material off the copper block, find an additional offset spot?)

[Conrad Hoffman]

Not too much bend. Me thinks you've been looking at the old lead compensation boxes for old bridges. They did similar things long ago, just without the tetra part. This isn't that different from the way I used to balance the grounds of two KVDs I wanted to compare. I connected the grounds with a length of copper wire, then a clip lead to the center point. By moving the clip a bit, I can insure a null at zero, which is harder to do with separate leads. It's been a while, but it was something like that. No reason you can't do it in 2D.

[VintageNut]

Shorting the input of an amplifier is a completely different problem than providing a zero for a 4-wire measurement system.

The tetrajunction was developed for connecting 4-wire junctions of transfer standard resistor arrays. The tetrajunction is not just a trick for fooling an instrument. It is a useful device where nearly zero resistance is required for an array of metrology resistors.

Keithley had a common plug for nanovoltmeters and microvoltmeters from the 1960s through the 1990s and there is a  short, the model 1488. It is a simple copper wire in a can. It is used to zero the nanovoltmeter before connecting a measurement cable.

Your original statement: "For me, it has nothing to do with making zero on a DMM. I wanted to make zero on a nanovoltmeter. That is the challenge. The DMM shorting plug is easy to replicate with DIY copper but is useless for finding the floor of a nanovoltmeter."

My assumption was the "finding the floor" meant figuring out the noise or "measurement floor" of the Nanovoltmeter. In reading this again I think you meant to "zero the 4w zero using a nanovoltmeter" (floor referring to "Flooring" the 4w-zero, not the nanovoltmeter).

BTW -  since you are measuring such low signals - why not up the current? Say at 10A or 20A it should be easier to measure the offsets since the offset will be 2-4 times as high. If you do it quickly so as not to heat the copper...

I was unclear. Not just the noise floor of the nanovoltmeter. The floor of the nanovoltmeter while measuring a device under power.

The net result is that the device really has to be as close to true zero as possible. Not just an apparent zero.

The Ohm Labs device has a specified limit of 5A. Beyond that damage may occur. The manufacturer tests the device at 5A for the calibration report.

[VintageNut]

I do not understand you assertion. What fundamental hookup is not being observed?

A tetrajunction in a box labeled "zero ohm standard" isn't really, in fact one could say it isn't even a resistance standard, as the fundamental source-input hook-up for making a 4-wire resistance measurement isn't being observed. It's just one of several ways of setting zero on that type of meter. (Are we talking a nanoampmeter here, not nanovolt, as there is that other thread on measureing nanoamps?)

[Assafl]

I can't answer for Conrad but for me the fundamental difference is that in a "true" Kelvin Connection the Force and Sense are connected at the DUT. In a tetrajunction, symmetry means that there is no such connection point.

Dealing with nomenclature, one could say that for devices that measure trans-resistance (like 4w DMM), the tetrajunction could, in theory, demonstrate the lowest trans-resistance. But switching to a resistance measuring mode (e.g. 2w DMM) - I can't see a tetrajunction beating a thicker slab of copper.

Given short and thick leads, a real standard should probably measure close in both resistance and trans-resistance measurement modes...

Obviously, a physical device with 0-ohm doesn't exist in a tetrajunction. So, as an example, you can't use it for levitation (instead of superconductors in cryogenic experiments). 

[Assafl]

The net result is that the device really has to be as close to true zero as possible. Not just an apparent zero.

If you measure a real (not apparent - e.g. a superconductor) "zero" in both 2w (resistance) and 4w (trans-resistance) measuring instruments, they should come out as "0" (give and take cabling - so hook them directly to the input jacks).

A tetrajunction will come out with a substantial difference between 2w and 4w measurement as a result of the onus to be symmetrical. It won't "fool" the 2w mode, but the trans-resistance 4w mode will show an apparent reading as close to "0" as construction symmetry allows. 

[Assafl]

Some here ask "what is the use for this" outside of a Hamon ladder. I ask that too.

I guess the question I have is: As a source for an "apparent" zero - what does that mean for the DMM? If one were to calibrate 4w zero using a tetrajunction - would the stored calibration figures be different (ignoring the hookup wires - bend the tetrajunction to fit the front plate - but gently so the symmetry doesn't change) than the Fluke 4w zero?

I tried drawing this. The main difference is an additional "bridge" overlaid between the 4 wires.


So let's assume the HP34401A. My guess is that if there is a difference - it will be due to the 30pA bias current which will unbalance the legs of the "bridge". But 30pA out of 5A??? For the HP34401A the Force current is 100mA. Even then, 30pA is sub-sub-sub 1ppm....

 

[VintageNut]

If you want an accurate resistance measurement, you use 4-wire below something like 20k ohms. Your discussion of being able to measure low ohms the same 2W or 4W is a non-sequitur. AND you have to use offset compensated ohms procedure to remove the always present microvolts that are created by wires+connection neodes required for the circuit to take the measurement.

You cannot measure low ohms with 2W. Lord Kelvin was using 4W before the invention of electronics.

I can't answer for Conrad but for me the fundamental difference is that in a "true" Kelvin Connection the Force and Sense are connected at the DUT. In a tetrajunction, symmetry means that there is no such connection point.

Dealing with nomenclature, one could say that for devices that measure trans-resistance (like 4w DMM), the tetrajunction could, in theory, demonstrate the lowest trans-resistance. But switching to a resistance measuring mode (e.g. 2w DMM) - I can't see a tetrajunction beating a thicker slab of copper.

Given short and thick leads, a real standard should probably measure close in both resistance and trans-resistance measurement modes...

Obviously, a physical device with 0-ohm doesn't exist in a tetrajunction. So, as an example, you can't use it for levitation (instead of superconductors in cryogenic experiments).

[Assafl]

Of course you measure low ohms with 2 wires. Every resistance measurement has an excitation (a Force current or voltage) and a reaction (a reaction voltage or current, respectively) that happen together on the same device (a resistor or other DUT). Always 2 wire! (Edit: this truism is just Ohm's law - in a lumped version or the Maxwell equation version)

Kelvin is about separating the sense lines from the excitation lines as far as possible so that the only thing measured is the DUT. Eventually they MUST connect.

They have a few options to connect: they can connect on the resistor leads. Or they connect at the kelvin probe. Or they connect using stacked bananas. (Edit: they have to connect outside - encompassing - the DUT). If you run the 4w mode without connecting the force and sense - you'll get odd readings.

In the tetrajunction they never connect. Because it is a unique device whose trans-resistance is 0 no matter what its resistance is (within bounds inflicted by the measurement instrument - of course Edit - irrelevant since the transresistance will be 0 even if the DMM can't measure it... ). You can build a tetrajunction using Manganin wire - its resistance would be in many-many Ohms. Its Trans-resistance (if balanced) will be 0.

[VintageNut]

In theory only. In practice 2-wire low ohms would require zero resistance cables between the measurement instrument and the device under test. Practical cables have a total of a very significant fraction of an ohm which is many orders of magnitude larger than what is being measured.

Of course you measure low ohms with 2 wires. Every resistance measurement has an excitation (a Force current or voltage) and a reaction (a reaction voltage or current, respectively) that happen together on the same device (a resistor or other DUT). Always 2 wire! (Edit: this truism is just Ohm's law - in a lumped version or the Maxwell equation version)

Kelvin is about separating the sense lines from the excitation lines as far as possible so that the only thing measured is the DUT. Eventually they MUST connect.

They have a few options to connect: they can connect on the resistor leads. Or they connect at the kelvin probe. Or they connect using stacked bananas. (Edit: they have to connect outside - encompassing - the DUT). If you run the 4w mode without connecting the force and sense - you'll get odd readings.

In the tetrajunction they never connect. Because it is a unique device whose trans-resistance is 0 no matter what its resistance is (within bounds inflicted by the measurement instrument - of course Edit - irrelevant since the transresistance will be 0 even if the DMM can't measure it... ). You can build a tetrajunction using Manganin wire - its resistance would be in many-many Ohms. Its Trans-resistance (if balanced) will be 0.

[Conrad Hoffman]

Actually, force and sense only have to connect well enough to stay within the common mode range of the sense device. When I do a precision resistance measurement, I connect my standard in series with the unknown and measure across each with a floating meter. "Floating" in the sense that it can't be more than 500V or so away from the force circuit. 4-wire DMMs may be more limited! They also generally use a constant current source, though that's not (I think) relevant to the matter.

The nanovoltmeter problem seems to be one of common mode. If you fasten the leads together on a copper wire stub, you should be able to touch the stub anywhere on the high current line with no change in zero. Obviously this is only going to work with all copper connections, ideally crimped and not soldered, and every other effort made to eliminate thermal effects. Tetrajunction shouldn't be necessary to do this. Just curious, what's the input impedance of the nanovoltmeter? Bias currents? The surplus world has not yet blessed me with a nanovoltmeter. 

IMO, though you can look at the tetrajunction modeled as bridges and resistors, the potential lines of the FEA are far more revealing in how it functions.

[Assafl]

IMO, though you can look at the tetrajunction modeled as bridges and resistors, the potential lines of the FEA are far more revealing in how it functions.

I am okay with how it functions and am having a blast with your FEA program. It is a hoot to see voltage (~E - Electric Field) and current lines (J - current flux)... It is looking at Ohm's law (the more generalized Paul Drude version J=?E) for free!

Bridges and resistors - I wanted to change the POV from the tetrajunction to the other side of the binding posts - and to answer a different question:

1. How would it affect the calibration parameters if one were to use a tetrajunction to calibrate their DMM?
2. Would the calibration parameters provide improved accuracy over a regular zero (piece of copper stub, or a Fluke zero)?

I can't make any inroads on that. Feel stupid (a bit) since theoretically speaking it is simply Ohm's law... But then again it isn't...

(This is in theory only - I think thermal EMF will get you long before you realize any upside, cabling to the tetrajunction will also get you, and lastly, even if all the rest was taken care of - my DMM - an HP34401A - zeros all ranges at once - so one can't use the tetrajunction just for the 4-w resistance).

[Assafl]

In theory only. In practice 2-wire low ohms would require zero resistance cables between the measurement instrument and the device under test. Practical cables have a total of a very significant fraction of an ohm which is many orders of magnitude larger than what is being measured.

When you do 4w Kelvin, the force and sense are shorted at both sides of the DUT. At that point, be it 2w or 4w, electrons are forced to flow by creating an electric field in the DUT that results in the current required (by the regulated current source), the potential is then measured by the A/D, and the ratio between the two is calculated and labelled resistance.

The flow of electrons is because of and in the direction (or opposite) of the electric field lines (not the equipotential lines - the flux lines). 

But a tetrajunction measured 4w is not physically a resistor: In no place in the tetrajunction is there an electric field that results in an current flow in the direction of measurement (except maybe that due to Bias currents and imbalance of the tetrajunction).

Ohms law (as J=?E or as I=V/R) is not just not applicable here - it is wrong to use it! You are not measuring the electron flow in the direction of the electric field. You are inducing current that is perpendicular to the potential measurement (so keep magnets away or have to deal with hall effect).

It is a curious device nonetheless...

The fluke zero has no wires. There are internal wires (PCB tracks) from where the current is injected into the 2w terminals (in the HP34401A I think this point is near the big Coto reed relay). The 4w vs 2w compensates for that extra length of PCB.

The fact that the DMM measures trans-resistance is interesting since it would suggest that one could use a DMM to balance a bridge. The two force contacts to the top and bottom, and the two sense to L/R. minimize the Ohm reading and your bridge is balanced....

[VintageNut]

In theory only. In practice 2-wire low ohms would require zero resistance cables between the measurement instrument and the device under test. Practical cables have a total of a very significant fraction of an ohm which is many orders of magnitude larger than what is being measured.

The fact that the DMM measures trans-resistance is interesting since it would suggest that one could use a DMM to balance a bridge. The two force contacts to the top and bottom, and the two sense to L/R. minimize the Ohm reading and your bridge is balanced....

No. For most cases, the requirement for maximum resistance between SENSE HI and HI is violated. The returned measurement is crap.

[Assafl]

I had never gave the max lead resistance any thought. I always kind of thought of it as a "safety margin", for abnormally high resistance leads.

I completely missed the fact that the spec is actually about the ability to resolve the transimpedance of the 4w as "Ohms" (i.e. as if they were a physical property of a conductor - which happens to be true only for Kelvin connections).

Looking at the specs for the HP34401A, the maximum lead resistance is 10\$\Omega\$ for 100\$\Omega\$ range, 100 for the 1k range, and 1k for the rest. So in theory balancing a 10 bridge should be doable (at the highest sensitivity) - or a 1k bridge at the lowest.

[VintageNut]

There are more things to consider. The open circuit voltage is limited on DMMs. You will be limited to bridge arms that develop a voltage less than or equal to the DMM ohms current multiplied by the bridge arm resistance.

Also, you normally want the galvanometer to be able to accurately measure 1uV. If you use your proposed method, you may not be able to see that microvolt with enough accuracy.

Give it a try and post your results.