Current measuring resistor calibration

Actually, you could do it with just one resistor assuming that the current is DC. With a chopper op-amp and careful design, you can get down to a micro volt. You have to beware of the metals involved.

Look very carefully at how you have things wired. It could be that the wiring resistance is getting into your measurement.

I assume you didn't let the drop in the switches get into it.

Check the tempco first. Look at the metals you have in the design to see how many little thermocouples you have made. Look at you op-amps last but don't forget to check.

You can use "electronic pots". These are sort of like DACs with EEPROMs built in.

The first resistor dissipates 10 watts max, and the next one dissipates 1 watt. They'll get hot, and the heat will change their values and generate thermoelectric potentials. Both will need 4-wire connections to avoid copper errors, especially the first one.

Use lower-value 4-wire shunts to keep the errors down, and low-drift diffamps with lots of gain. 10 to 50 mV max drop is reasonable. Do the zero and span calibrations in the computer.

One good trick is to make provisions for shorting the diffamp input occasionally and auto-zeroing in software. Then you can tolerate a cheaper, driftier diffamp. Software is free in production.

As noted elsewhere, when you switch resistors, make sure the resistance of the switch contacts isn't part of the measurement resistance. In other words, switch all 4 wires to each shunt.

John

p.

The first problem might be the opamp DC offset. The second is Joule heating (V*I) and TC of resistor. Drop the value of the resistors, or heatsink them, or both.

"If I want to mass produce this,..."

Good low TC, low value resistors are expensive.

George H.

lp.

Yeah, that about nails it! Certainly four leads, And I love the shorting trick. Thanks,

George H.

----------------------------------------------------------------------- I need 0.1uV resolution if I want to measure 1mA with 0.1 Ohm Resistor with 0.1% accuracy. I never could measure lower than 10uV in practice and that only happens if I use every trick that I know. for me 100uV sounds more practical. So if you excuse me I can't do this unless you teach me practically how.

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--------------------------------------------------------------------------- I have used very wide tracks to ensure there is no IR drops in tracks and also in switches.

but even in lower currents this problem exists. I mean 100mA is not much current for a short(a few Cm) 200 mil track. It should not cause this problem.

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---------------------------------------------------------------------------- But how to calibrate these digipots? calibration takes a lot of time and I hoped to somehow do it automatically.

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------------------------------------------------------------------------------ Do you think it is possible to produce a board that can produce very accurate currents for this purpose ? I mean how can I make sure that my reference current is let's say exactly 100mA. I mean (100.0mA). Is there an alternative to check it with a very accurate galvanometer? I can produce very accurate voltages using high precision reference ICs but what about current?

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Surface-mount 4-wire low-tc shunts are affordable. IRC, Vishay, KOA, Isotek, Digikey. They are usually stamped out of manganin... production cost must be pennies each.

We punch or photoetch our own current shunts out of sheet manganin, using top-secret shapes to minimize thermals, mag field pickup, and eddy current errors. Typically in the 15 milliohm ballpark.

You can make a pretty good shunt out of a short piece of manganin wire soldered between two pcb thru-holes, with careful 4-wire connections to the pads. Manganin is just a little tricky to solder.

John

Sure, depending on what exactly is your purpose. That's not clearly defined here.

Fluke makes bench DVMs that are very accurate on current, like 0.02% to 10 amps. Use one of them as a tracable cal thing.

John

-------------------------------------------------------------------------- John, My purpose is to automatically calibrate those current measurement resistors but for that I need to either know the very exact value of each resistor or pass a known(reference) current through them and measure their voltages. Yes there are accurate measuring instruments but problem is they also have to be calibrated. If I remember correctly Fluke guarantees its calibration for just 2 years. Is it possible to produce a reference current without such instruments?

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Hi Petkovic

Take a look at this chip:

It looks like this can measure to a high dynamic range due to the low input offset voltages of the (chopper stabilised) op amps. DO NOT buy one without the current sense resistor soldered to it. Soldering the resistor to the board will present problems with the Siebeck effect (thermocouple junctions). If you can buy a module (resistor and ASIC) it will save you lots of production problems.

help.

Cool I've only used manganin as heater wire. Not sure the gauge but about 12 ohms per foot. You can get this in thin sheets? I don't remember any problems soldering to the wire. But I was only making heaters, not sensors.

George H.

--- I don't mean to be unkind, but you don't seem to have grasped the fundamental fact that if the resistance of your "reference" resistor(s) changes when current through them changes, then you are lost.

Consider: (View in Courier)

Where Vin is the voltage source feeding the circuit, Rs is its source resistance, R1 and R2 are the resistances of the wires connecting Vin to the load, RL is the load resistance, and VL is the voltage appearing across the load,

then if the resistance of RL changes (because of its temperature or voltage coefficient of resistance, or both, or whatever) when Vin, Rs, R1, and R2 are perfectly stable, VL will also change and won't truly reflect the current in RL.

---

--- Yes, of course:

But once you build it what are you going to use to make sure it's working properly?

JF

There are other voltage reference sources - buried zener voltage references can do better than band-gap parts

while Analog Devices offer low voltage reference sources based on FETs.

Standards laboratories use quantum standards to do rather better

-- Bill Sloman, Nijmegen

I went on to answer the other questions because it is hard to do. I was only pointing out it was not actually impossible. Switching resistors is going to be a lot easier.

You need to run the connections directly to the resistors. This matters the most at the high current end where the resistor is small. You also want to make sure you aren't making thermocouples.

Temperature gradients along the copper make voltage gradients. (This is the real way a thermocouple works). Even at 100mA you are making some heat in the sense resistor.

If you have something that needs adjusting, it takes time to adjust it. Even if you make a computer do the work, it will take some time. It is better to try to design the need out.

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A micro can assist resistor switching and calibrate linear output. It can also monitor temperature changes for compensations. In productions, you can build lookup tables while calibrating and programming the micro. The so called "ASIC" are probably pre- programmed micro.

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If you want to be legit, anything that you sell as electrically accurate has to be tracable to a recognized standard, like NIST (in the USA) or equivalent. A Fluke that is in cal is tracable; after the cal interval has expired, it's not.

You can also buy lab standard resistors - very expensive - but they need to be periodically certified, too.

No instrument can calibrate itself, at least unless it relies on some fundamental quantum measurement. A primary-standard current source would cost as much as a thousand Flukes.

Why not buy a Fluke and have it cal'd every two years? Use that to calibrate whatever it is that you're making.

Since I don't know what you're trying to do, that's all I can say.

John

You can buy sheet manganin, but not in small quantities. We use some special solder that works better than the standard stuff. We anneal the shunts before we solder them down, which tarnishes them a bit and makes them harder to solder; a brief dunk in dilute HCl shines them back up.

If you stress manganin, it degrades the tc and stability slightly. Annealing after punching/rolling/bending (bake for 24 hours at, say,

John

------------------------------------------------------------------------ John Fields, The most usual way of measuring currents is to use a resistor as a current to voltage converter. Yes all resistors have TC,... but this is another issue. For most precise measurements one should uses a resistor with lowest possible TC. what you mentioned is a general issue and not addresses my specific problem. Yes TC is important and you can suppose I make all my effort to lower that effect as mush as I can.

I think I was not clear enough in previous post so I say it again.

I want to calibrate current measuring resistors -> From calibrate I mean to know exactly what voltage a specific current produces across that resistor. If I know this coefficient I can convert the measured voltage to a valid current value in computer and hopefully then there is no gap when I change from one resistor to another. what I said in previous post was that I have two different ways. I said

EITHER:

OR:

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------------------------------------------------------------------------- I should produce a ***REFERENCE CURRENT*** not a reference voltage. I explicitly mentioned there are many voltage reference ICs. what I want is either to measure the very exact value of a resistor or produce a

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