Author Topic: Recycling of precision current source & noise reduction for low burden shunts  (Read 2101 times)

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Offline Dr. Frank

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I designed this precision current source over 25 ago, serving well for low temperature thermometers like PT100, diodes, carbon resistors. It provides 100nA to 100mA, in six decades, and a decade vernier.
It replaces much more expensive current sources like the ones from Time Electronics, Keithley or Lake Shore, and is even more precise.



Later, it also served well for checking current ranges of DMMs, and even calibrating my 34401A, by a comparison to the 3458A.



I also wanted to check the ranges of my newly bought 34465A, as its 1µA to 100µA ranges are not calibrated. The specification denotes 'for typical performance' only.
The 100nA to 10µA ranges of the 3458A are also specified 'typical' only, and these were not verified in calibration reports.
Reason for that may be, that calibrators like the Fluke 5720A have 220µA as lowest range.
« Last Edit: February 22, 2018, 01:06:38 am by Dr. Frank »
 

Offline Dr. Frank

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Recycling of precision current source & noise reduction for low burden shunts
« Reply #1 on: February 21, 2018, 08:27:06 pm »
The 34465A always showed an overflow in its 1µA range.
By using its unique current digitizing function, a 50Hz hum and spikes of 3µA(pp) became apparent.



The 3458A accepts this noisy signal w/o any problem.

The different behavior arises from the low burden circuit, which is designed into the 34465A, similar to Daves µCurrent design.

The voltage drop over a 1kOhm shunt for the 1µA .. 100µA ranges is amplified by x1, x 10 and x 100 by two successive x10 stages of AD8638 zero offset amplifiers.
That also gives a high bandwidth for digitizing the current measurement at 50kHz sampling rate.
Therefore, the 1µA range might easily be clipped, if noise is present. The 3458A has unique shunt resistors for each range, and there's enough headroom to average out the noise, instead of clipping.

The hum in the current output comes over the transformer, and can't be easily blocked by the usual shielding measures.
A transformer with a shield between primary and secondary would be required, but these transformers aren't available from stock.
The spikes may be generated by the AZ circuit of the DMM itself.

The whole assembly didn't look so nice any more, due to many changes and extensions, like the vernier, built from 10 EA of old 2kOhm, +/-50ppm, +3ppm/K PWW resistors.
So I redesigned the whole PCB, and also the mechanical assembly.
The drifting REF01 was replaced by an LM399 circuit, the vernier now can be trimmed to 1ppm linearity, and the power supply was separated from the PCB, introducing a shield.




« Last Edit: February 22, 2018, 01:09:14 am by Dr. Frank »
 

Offline Dr. Frank

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Recycling of precision current source & noise reduction for low burden shunts
« Reply #2 on: February 21, 2018, 08:28:25 pm »



These measures further increased the stability of the circuit, but the hum and spikes were not reduced, even by an additional magnetic shield.

After many experiments, a 470nF/250V capacitor from circuit ground to case and mains earth reduced the noise greatly (see principle schematic).




The 1µA range and even 100nA were now much less noisy and more stable, so the standard deviation at NPLC100 in both ranges is about 10pA only. All other ranges have a standard deviation of about 0.5ppm of F.S.




The 2h stability can be as low as 0.3ppm, at 100µA as an example.



The differential output resistance is typically about < 1ppm/10V, or 167MOhm for the 100mA range, going up to 100GOhm for 100µA.
The 10µA range also has about 100GOhm, which is about 8ppm/10V. On these low current ranges, leakage currents come into play.

The current ranges may be trimmed to 1ppm, the absolute accuracy is as good as the reference, e.g. 10ppm by using the 3458A.

The T.C. is not determined or compensated yet, so might be as high as about 5ppm/°C. 

In the end, these parameters are way better than any commercially available current sources, and may even compete with calibrators like the 5720A.
« Last Edit: February 21, 2018, 10:07:44 pm by Dr. Frank »
 
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Offline TiN

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Yay. Remember seeing this years ago. Surprised noone repeated it already in all this time :)

R76 is for calibration?
« Last Edit: February 21, 2018, 10:24:15 pm by TiN »
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Offline chuckb

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Dr Frank, that’s a very nice, very high performance design! I like the star connection between the 10 ohm and the LM399. Thanks for sharing.

I wonder if the 100k and 899k HP resistor are picking up some of the magnetic field from the 50Hz transformer. Would a cap directly across the resistors help with the low frequency noise? A 0.1ufd cap has a 31k ohms impedance at 50 Hz. That may help attenuate any 50Hz picked up by the large wire wound resistor.  It would add a delay to the control loop response.

I did not realize the 34465A measured the low currents that way. Are there other meters that use an amplified low resistance (1k ohm) shunt?

The 34465a uses a 1k ohm res shunt. The HP3458A uses about 45k. With a 1ua current the 34465 develops a 1mV dc drop across its shunt. For 1 ppm display resolution the shunt voltage needs to be measured with 1nV resolution. Ok, that should be a Keysight problem right? But that also adds a very difficult requirement for the calibrators. It means your output voltage needs to be stable to 1nV. That could be difficult. Am I thinking about this correctly?

Everything is much easier with a normal 45k ohm shunt in the meter (like the HP3458A). The voltage developed is 0.045V and 1ppm of noise is 45nV. That’s not easy but it is 45 times easier.

Maybe a 100k ohm resistor in series between the calibrator and the 34465A would eliminate the noise. Now the output voltage stability requirements for the calibrator would become 100 times easier and more practical.
 

Online Andreas

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The hum in the current output comes over the transformer, and can't be easily blocked by the usual shielding measures.
A transformer with a shield between primary and secondary would be required, but these transformers aren't available from stock.


Hello Frank,

perhaps a wall wart would help
if it is magnetic coupling and not capacitive coupling over the transformer distance is the best shielding.

RS-components still has some linear (AC) wall warts.

https://de.rs-online.com/web/c/stromversorgungen-transformatoren/netzteile/steckernetzteile/?searchTerm=steckernetzteil%2015vac%2018vac%2012vac

with best regards

Andreas

 

Offline BU508A

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if it is magnetic coupling and not capacitive coupling over the transformer distance is the best shielding.

Yep.
My second choice would be a toroidal transformer, something like this:

https://www.ebay.com/itm/312029977537

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Offline Dr. Frank

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Hello Frank,

perhaps a wall wart would help
if it is magnetic coupling and not capacitive coupling over the transformer distance is the best shielding.

RS-components still has some linear (AC) wall warts.

https://de.rs-online.com/web/c/stromversorgungen-transformatoren/netzteile/steckernetzteile/?searchTerm=steckernetzteil%2015vac%2018vac%2012vac

with best regards

Andreas


Hello Andreas and 'BU508A',

it's definitely not been magnetic coupling, as I also experimented with µ-metal and steel sheets which did not make any difference.

I also removed the whole PSU assembly from the box, and the noise did not change at greater distance, or different orientation from the current source circuit.

This toroid transformer will reduce the magnetic stray field, that's correct, and I will very  probably use such a type in my next design of an upcoming  'Super Precision Current Source'.. but this project was just a recycling project, as mentioned in the title.

Anyhow, there's a reason, why especially Keithley and FLUKE always use transformers with capacitive shielding for their nV and fA instruments..

I still want to implement the mains transformer directly inside my LTZ1000 boxes, as soon as I find a source for these shielded transformers.. preferably a toroid type also.

Frank
« Last Edit: February 22, 2018, 09:00:25 pm by Dr. Frank »
 

Offline Dr. Frank

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Dr Frank, that’s a very nice, very high performance design! I like the star connection between the 10 ohm and the LM399. Thanks for sharing.

Yes, that's right. All six ranges are strictly laid out for Kelvin connections. On top of the shunt resistors, the double pole relays form drive and sense connections, on their bottom, the supply current is fed from one side, and the sense side, including the LM399 ground point, arrives from the opposite side.

I wonder if the 100k and 899k HP resistor are picking up some of the magnetic field from the 50Hz transformer. Would a cap directly across the resistors help with the low frequency noise? A 0.1ufd cap has a 31k ohms impedance at 50 Hz. That may help attenuate any 50Hz picked up by the large wire wound resistor.

No, it's not magnetic pick-up, as an additional magnetic shield, or a different orientation of the transformer did not make a difference.
I may give the 0.1µF capacitor a try, but the disturbances might enter or influence the circuit everywhere else.. obviously the whole ground is riding on this 50Hz hum, and the 470nF capacitor short-cuts that.
This noise is also present in the other ranges, on the same order of magnitude, but due to the higher absolute currents, are of no consequence there.

It would add a delay to the control loop response.

Why? The circuit is stable under all conditions, I see no oscillations, only this noise with 50Hz repetition rate.
If I put the 3458A in series, it will add spikes, which might drive the 34465A into overload, again.
Therefore, much of the noise seems to be created outside my circuit.

I did not realize the 34465A measured the low currents that way. Are there other meters that use an amplified low resistance (1k ohm) shunt?

There are not so many other DMMs on the market, which resolve 1pA, or have a 1µA range.
Usually, that's the reign of electrometers, which operate a bit different, and feature double shielding by means of a triax cable.
Anyway, I also wondered why KS designed it that way. Other ranges are not consequently designed for low burden, and a chopper OpAmp might be more expensive than an additional quality shunt resistor.

In the past, HP justified the lack of more current ranges on the 34401A by the additional price of such shunt resistors.


The 34465a uses a 1k ohm res shunt. The HP3458A uses about 45k. With a 1ua current the 34465 develops a 1mV dc drop across its shunt. For 1 ppm display resolution the shunt voltage needs to be measured with 1nV resolution. Ok, that should be a Keysight problem right? But that also adds a very difficult requirement for the calibrators. It means your output voltage needs to be stable to 1nV. That could be difficult. Am I thinking about this correctly?
Everything is much easier with a normal 45k ohm shunt in the meter (like the HP3458A). The voltage developed is 0.045V and 1ppm of noise is 45nV. That’s not easy but it is 45 times easier.

Well, the problem is somewhat different. The OpAmps only need to amplify a 1nV level to 100nV, which the A/D then is able to resolve properly. So, there's also no output voltage which has to be stable to 1nV.

The problem, you pinpointed here, are un-controllable / fluctuating thermocouple voltages on the order of 1nV across the 1kOhm shunt, to achieve 1pA resolution. Using higher shunt values, implying higher burden voltages, reduces this problem.. You're right, I prefer the 3458A for these µA ranges, although the 34465A works surprisingly well, anyhow.

Maybe a 100k ohm resistor in series between the calibrator and the 34465A would eliminate the noise. Now the output voltage stability requirements for the calibrator would become 100 times easier and more practical.

Nope, I don't think, that this will improve the noise.
The circuit intentionally operates at a high 1V level across the shunts, and that's the relevant voltage level you have to take into account.
The chopper has absolutely no problem to stably regulate 1V to < 1ppm.

In the end, the stability / noise for time scales > 20msec is extremely good, just compare that to specifications of other current sources from Time Electronics (model 1024), Lake Shore, and Fluke.
 

In the next design anyhow, I will even use a 100mV reference voltage for the higher current ranges, as here, the 100mA range is very non-linear (~20ppm), due to self-heating effects.. an output of 100mA  requires at least one minute to stabilize.

Frank
« Last Edit: February 22, 2018, 10:34:59 pm by Dr. Frank »
 

Offline BU508A

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Anyhow, there's a reason, why especially Keithley and FLUKE always use transformers with capacitive shielding for their nV and fA instruments..

I still want to implement the mains transformer directly inside my LTZ1000 boxes, as soon as I find a source for these shielded transformers.. preferably a toroid type also.

How about this one?

http://induktor.de/transformatoren/   <--- konfektionierte Ringkerntransformatoren

Or Sedlbauer:

http://www.sedlbauer.de/de/produkte/induktive-komponenten#rkt-kundenspezifisch

The disadvantage is: they will probably cost a fortune.

Maybe some of the folks at "elektor" will know a source for such custom specified transformers. Will write them an email and ask.
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Offline zhtoor

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hello,

how about just using a couple of rechargeable 18650 style LiPo batteries and ensure a very clean and portable power?

regards.

-zia
 

Online chickenHeadKnob

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Anyhow, there's a reason, why especially Keithley and FLUKE always use transformers with capacitive shielding for their nV and fA instruments..

I still want to implement the mains transformer directly inside my LTZ1000 boxes, as soon as I find a source for these shielded transformers.. preferably a toroid type also.

How about this one?

http://induktor.de/transformatoren/   <--- konfektionierte Ringkerntransformatoren

Or Sedlbauer:

http://www.sedlbauer.de/de/produkte/induktive-komponenten#rkt-kundenspezifisch

The disadvantage is: they will probably cost a fortune.

Maybe some of the folks at "elektor" will know a source for such custom specified transformers. Will write them an email and ask.

Hallo Doktor, I was just in the process of sourcing good a electrostatic shielded toroidal for similar end use. Found Antekinc.com. example:

http://www.antekinc.com/as-0515-50va-15v-transformer/

Carefull! they offer lower power 10 and 25 VA transformers but these do not show electro-inter-winding shield. so sad. I haven't ordered yet so don't know what their international shipping is like. Quite reasonable quantity 1 prices.
 

Offline Dr. Frank

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hello,

how about just using a couple of rechargeable 18650 style LiPo batteries and ensure a very clean and portable power?

regards.

-zia

You're right, that's always a solution.
The Time Electronics 1024 also has this option:
https://www.timeelectronics.com/portable-voltage-current-instruments/1024-dc-current-calibrator/
In fact, we had these boxes at university 25 years ago, and they cost a fortune, as of today over 2000 $..
It was my original intention, to build sources for 1/20 of that price, at better specification.


In my actual design though, the two relays consume about 50mA, the heater of the LM399 about 20mA, plus  100mA for the highest current range. That's a lot, so my design goal was to favorably implement a 'silent' mains P.S.U., instead.

I succeeded, at last, and I think it's not worthwhile trying to squeeze the last noise figures out of it..
Please take notice of the noise specifications of the Fluke 5700, 5720, and 5730, that's incredibly high.
These calibrators were very probably not capable of calibrating/verifying the 1µA and 10 µA ranges of the 34465A/470A, due to the enormous 10nA rms noise at 10kHz bandwidth. I achieve < 100pA(rms)  @ 50kHz B.W., if I remember correctly.

Maybe TiN would be so kind to check on his own 5700 calibrator, if that is really the case?
I can't hardly believe that.. current metrology seems to be the hardest ...

Frank
« Last Edit: February 23, 2018, 01:09:37 am by Dr. Frank »
 

Online Andreas

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Hello,

This is one supplier which makes toroid transformers to spec.

https://www.multi-circuit-boards.eu/preise/ringkerntrafos.html

The question is: is one shield enough or do I need 2 shields: one for the Mains GND and one for the guard.

with best regards

Andreas
 
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Offline TiN

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Quote
Maybe TiN would be so kind to check on his own 5700 calibrator, if that is really the case?
I can't hardly believe that.. current metrology seems to be the hardest ...
Well, i don't own 5700A, just the bits and pieces of one. But I'll see what I can do about that :).
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Offline Dr. Frank

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Apologies, I wrote utter nonsense.. had the wrong numbers in my old head...

The source has noise of < 100pA(rms)  @ NPLC 1 only.



At 50kHz sampling rate, 1µA .. 100µA show identical 47nA (rms), which can be mitigated to 24nA (rms) by a 1µF capacitor in parallel to the output.



So that's in the same ballpark as the FLUKE 57xx, but latter is much less noisy overall.
I would be interested, how it behaves if measured in a similar setup.

TiN, I thought that you have finished that calibrator already, as there were photographs of a 5700A on your site .. what's missing?

Frank
« Last Edit: February 23, 2018, 09:39:24 am by Dr. Frank »
 
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Offline TiN

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My calibrator is still all over the room (in form of boards and pieces. Only missing few boards and a chassis now :-D).
I'll be getting back to it soon (sorta). The 5700A i use in all recent tests is remote unit in USA, to which I have GPIB access over internet ;).
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Offline BU508A

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This is one supplier which makes toroid transformers to spec.

https://www.multi-circuit-boards.eu/preise/ringkerntrafos.html

Thanks, that was the third one which I was looking for but didn't find him with my first try.

Quote
The question is: is one shield enough or do I need 2 shields: one for the Mains GND and one for the guard.

HP uses a transformer with three shields in their design for the 3456A. See screenshot below.
Maybe it is a good idea to use at least 2 shieldings.

Cheers,

Andreas
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Offline F64098

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Hello Dr. Frank,

your project inspired me, to "quickly" create a nice filling of one of my several empty Hameg modules.
Most of the resistors, OPs, LM399H and all relays are available in my "stock", so the pcb manufacturing will produce the biggest costs.
I just have to buy 15 or 20 S102K 2k0000 0,1% to select the 10 best ones. hifi-szjxic has a nice offer for this resistors.

At the moment I'm not sure how the control should look like. Only 2 simple rotating switches or a small keypad and display.
Because i use relays everywhere, i have both possibilities and will decide it later.

To which potential should i connect the "groundplanes"?
They are floating at the moment and the fastening noses of the housing have isolated contact surfaces.

Thanks in advance for your opinion and hints!

Best regards

Frank
« Last Edit: February 26, 2018, 07:14:38 am by F64098 »
 
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Offline Dr. Frank

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I left my top layer floating, to prevent additional leakage currents.. next iteration will include driven guard rings around the low current range relays and resistors.
I also did not experiment with different potentials of this plane, maybe setting it to ground or to the reference voltage (0.. 1V) would improve the low current ranges.

Fine, that my article inspires you to build a similar source.. let us know, what your results will be.

Frank
 

Offline TiN

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Fine, that my article inspires you to build a similar source.. let us know, what your results will be.
He's not the only one.  ::)

Dr.Frank
Some random samples for you. Azero is off on 5700A tests to get better NPLC timing.

DC Current measurement logs, HP3245A(LTZ) src vs 3458A and 3458B(second unit)
DC Current measurement logs, Fluke 5700A Mark I src vs 3458A
DC Current measurement logs, Keithley 6221 src vs 3458A
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Offline F64098

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It has said "Daddy!"  ;)

The PCB is from pcbway and it is a really perfect work.
Just the pricing is a little bit strange. 100x100mm and you will get 10pcs for 5$.
101x100mm and there are only 5 for "only" 32$... But it is cheap anyway.

Now i'm waiting for the 10 S102K and for the 16 relays.
And i'm hunting for a lot of 0R22 in 0207. Not really easy to get them in metal film.


Best regards

Frank


 


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