Recycling of precision current source & noise reduction for low burden shunts - Page 1
Forum section: electronics / general

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 21, 2018, 09:26:20 am
content: 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.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 21, 2018, 09:27:06 am
content: 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.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 21, 2018, 09:28:25 am
content: 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.

author: TiN
author_rank: Super Contributor
timestamp: February 21, 2018, 10:33:21 am
content: Yay. Remember seeing this years ago. Surprised noone repeated it already in all this timeR76 is for calibration?

author: chuckb
author_rank: Frequent Contributor
timestamp: February 21, 2018, 04:03:41 pm
content: 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 outputvoltage stabilityrequirements for the calibrator would become 100 times easier and more practical.

author: Andreas
author_rank: Super Contributor
timestamp: February 21, 2018, 10:38:26 pm
content: Quote from: Dr. Frank on February 21, 2018, 09:27:06 amHello Frank,perhaps a wall wart would helpif 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%2012vacwith best regardsAndreas

author: BU508A
author_rank: Super Contributor
timestamp: February 22, 2018, 07:33:45 am
content: Quote from: Andreas on February 21, 2018, 10:38:26 pmYep.My second choice would be a toroidal transformer, something like this:https://www.ebay.com/itm/312029977537

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 22, 2018, 09:56:12 am
content: Quote from: Andreas on February 21, 2018, 10:38:26 pmHello 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

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 22, 2018, 10:51:00 am
content: Quote from: chuckb on February 21, 2018, 04:03:41 pmYes, 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.Quote from: chuckb on February 21, 2018, 04:03:41 pmNo, 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.Quote from: chuckb on February 21, 2018, 04:03:41 pmWhy? 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.Quote from: chuckb on February 21, 2018, 04:03:41 pmThere 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.Quote from: chuckb on February 21, 2018, 04:03:41 pmWell, 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.Quote from: chuckb on February 21, 2018, 04:03:41 pmNope, 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 relevantvoltagelevel 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

author: BU508A
author_rank: Super Contributor
timestamp: February 22, 2018, 12:15:46 pm
content: Quote from: Dr. Frank on February 22, 2018, 09:56:12 amHow about this one?http://induktor.de/transformatoren/<--- konfektionierte RingkerntransformatorenOr Sedlbauer:http://www.sedlbauer.de/de/produkte/induktive-komponenten#rkt-kundenspezifischThe 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.

author: zhtoor
author_rank: Frequent Contributor
timestamp: February 22, 2018, 12:51:02 pm
content: hello,how about just using a couple of rechargeable 18650 style LiPo batteries and ensure a very clean and portable power?regards.-zia

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: February 22, 2018, 01:47:49 pm
content: Quote from: BU508A on February 22, 2018, 12:15:46 pmHallo 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.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 22, 2018, 01:55:32 pm
content: Quote from: zhtoor on February 22, 2018, 12:51:02 pmYou'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

author: Andreas
author_rank: Super Contributor
timestamp: February 22, 2018, 07:11:55 pm
content: Hello,This is one supplier which makes toroid transformers to spec.https://www.multi-circuit-boards.eu/preise/ringkerntrafos.htmlThe question is: is one shield enough or do I need 2 shields: one for the Mains GND and one for the guard.with best regardsAndreas

author: TiN
author_rank: Super Contributor
timestamp: February 22, 2018, 07:50:40 pm
content: QuoteWell, i don't own 5700A, just the bits and pieces of one. But I'll see what I can do about that.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 22, 2018, 10:32:19 pm
content: 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

author: TiN
author_rank: Super Contributor
timestamp: February 23, 2018, 04:36:59 am
content: 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.

author: BU508A
author_rank: Super Contributor
timestamp: February 23, 2018, 06:47:10 am
content: Quote from: Andreas on February 22, 2018, 07:11:55 pmThanks, that was the third one which I was looking for but didn't find him with my first try.QuoteHP 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

author: F64098
author_rank: Regular Contributor
timestamp: February 25, 2018, 08:11:52 pm
content: 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 regardsFrank

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 25, 2018, 08:24:24 pm
content: 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

author: TiN
author_rank: Super Contributor
timestamp: March 07, 2018, 07:00:53 pm
content: QuoteHe's not the only one.Dr.FrankSome 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 3458ADC Current measurement logs, Keithley 6221 src vs 3458A

author: F64098
author_rank: Regular Contributor
timestamp: March 24, 2018, 08:15:21 pm
content: 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 regardsFrank

author: KNE
author_rank: Newbie
timestamp: February 23, 2022, 05:58:09 pm
content: Hi F64098,Is the PCB avaiable on PCBway?I'd like to resuse your layout.Best regards,Kris

author: trobbins
author_rank: Frequent Contributor
timestamp: January 04, 2025, 10:57:53 pm
content: Frank, as briefly discussed onhttps://www.eevblog.com/forum/metrology/precision-current-source-single-mosfet-or-twin-transistor-setup/, it's quite likely that the root cause of the transient spikes seen in the scope plot of reply post #1 in this thread arise from the diode bridge commutation of the +15 and +5V power supplies. I appreciate that you then identified a noise suppression method to mitigate the influence of those spikes.It's uncommon to find and then characterise such an example of that transient noise, as it typically has no tangible consequences for mains powered equipment, but in this application it actually did - so thanks for elaborating on it.I appreciate you were just using what was available at the time, but thought it worthwhile to add some insight on the cause of the spikes, as it rarely gets discussed. Capacitor input filter dc supplies such as you used cause a quite high crest factor current pulse through the transformer secondary winding and steered into the first filter capacitor by the rectifying diodes. At the instant of diode commutation, the leakage inductance in the secondary winding is the cause of residual energy that transfers out as a transient. Apart from using a transformer with lower leakage inductance secondaries, a tuned C-RC snubber across each secondary is the first opportunity to critically dampen the transient and constrain transient current flow to the smallest local area. Using fast, soft-recovery diodes or non-pn diodes also supresses the transient current behaviour at the time of commutation. And as you sought out, local region filtering can further constrain transient current path.Ciao, Tim

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 05, 2025, 03:10:53 pm
content: Quote from: F64098 on March 24, 2018, 08:15:21 pmHello100x100 PCB are cheaper as Chinese manage their production in panels iyou over 100x100 they need bespoke nesting , that the same or all uncommon request as copper thickness , silkscreen funny color or silver plated coating , they run mass production.RegardsOS

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 05, 2025, 03:25:01 pm
content: Quote from: trobbins on January 04, 2025, 10:57:53 pmHelloAs the current needed is not high do you think that possible to use a bank of batteries and use TI ultra low noise as TPS7A and the negative counter part , they are designed for 1A and can be assembled in // to increase the output current or limit the current load per IC and thermal problem as that quite small IC .RegardsOS

author: trobbins
author_rank: Frequent Contributor
timestamp: January 05, 2025, 11:08:53 pm
content: OS, I'd suggest that Frank doesn't need to modify his 'spare parts box' creation, as there would be no tangible benefit to him - he has a working item that meets his performance requirements.

author: Overspeed
author_rank: Frequent Contributor
timestamp: January 07, 2025, 10:42:12 am
content: Quote from: trobbins on January 05, 2025, 11:08:53 pmHelloMy point was to propose another PSU solution as 7805 voltage source are quite old and noisy , last generation LDO provide impressive performances or the costI agree that the original built source works but for a new one that can be an improvement that the same or old amp op which are now low in performances compared to last generation .RegardsOS

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 07, 2025, 11:21:37 am
content: Quote from: trobbins on January 04, 2025, 10:57:53 pmHello Tim and OS,thank you for your valuable hints.. a silent, de-coupled PSU is the most crucial part in any current or voltage references.I will for sure put more energy on the design of these, when I will work on a successor of my Precision Current Source.Frank

author: tiki
author_rank: Newbie
timestamp: February 18, 2025, 10:41:19 am
content: Hello,if it's going to lower currents, shielding becomes more critical even for the switches/relays.In my case (fA/pA measurements) I use Meder for some years, CRF05-1A or CRF05-1AS.Is there any other commercial reed relay out there, containing a shieldbetweenthe contacts and the coil?It's easy to be seen at the X-Ray pictures. To check this, I made an angled cut.The Cu foil is a bit longer than the coil and sits between the coil and the glass tube.The foil is contacted at the corners and may be connected to a guard, which works great in my application.If used as a guard, it isolates the contacts from each other effectively, minimizing leakage.As Meder informed me, the open contact capacitance is already low:"Der typische Wert über den offenen Reedschalter beträgt 0,2pF."This will be minimized too by using the shield as a guard.I do not know, if this information is already well known. In this case, I would like to apologize for the duplication.Cheers,tiki