DIY cable for picoammeter - Page 1
Forum section: electronics / general

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 11, 2019, 04:03:54 pm
content: Hi every one!I have an idea to make a cable for a picoammeter. Such cables have a conductive layer on the insulation to suppress the triboelectric effect. I think there are people here who know what it is, and I don't have to explain.I have a RF coaxial cable with Teflon insulation. And I have a conductive glue with a suspension of graphite. If I remove the outer copper shield from the cable and cover it with this glue, will I get a decent result?Will the glue stick to the Teflon?Thanks!

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 12, 2019, 02:21:18 pm
content: Another idea is to cover the cable with spray paint. Such paints have a metal powder and must conduct current.Don't you have any thoughts about that?

author: Gyro
author_rank: Super Contributor
timestamp: November 12, 2019, 07:09:01 pm
content: How much are you likely to be moving the cables during use? The triboelectric effect is transient during movement (as in a microphone lead for example). If you are using some kind of test fixture and can keep the cables to the picoammeter still during measurement, then you shouldn't have a problem.I think you would need to put the outer copper shield back regardless. Conductive glue or whatever would be too resistive for effective screening.The other thing you need to ensure is absolute cleanliness of the insulation at the terminations so you would need to avoid accidental contamination with whatever coating you use.Polyethylene insulation might be better that Teflon for lower triboelectric effect too.What sort of current levels are you measuring?P.S. The Tektronix (formerly Keithley) Low level Measurements handbook is the bible for this sort of thing...https://www.tek.com/document/handbook/low-level-measurements-handbookP.P.S. You might want to ask a Mod to move this thread to the Metrology section.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 12, 2019, 07:51:08 pm
content: Thanks!Yes, I understand what you're saying. I have this Keithley book. I'll read it all over again when I need to.All this will lie on the table during the measurements. Nevertheless, I have read horrors in the special literature that such sensitive equipment can detect the slightest vibration up to seismic:) And I thought about the proper cable...

author: guenthert
author_rank: Frequent Contributor
timestamp: November 15, 2019, 03:33:49 am
content: Not sure about shipping to Russia, but on eBay one finds low noise triaxial cables for a modest price (much unlike brand-name ready-to-use triaxial cable assemblies). The thought of making the actual cable didn't cross my mind.

author: David Hess
author_rank: Super Contributor
timestamp: November 15, 2019, 04:18:37 pm
content: I think the triboelectric suppression (graphite?) is between the inner and outer conductors and the dielectric so adding something to the outside of the cable or outside of the shield will not help.I would first try using solid and foamed polyethylene cables to see if they are good enough for your application.

author: MiDi
author_rank: Frequent Contributor
timestamp: November 15, 2019, 04:45:36 pm
content: There is a conductive black "foil" between inner insulation and inner shield e.g. Keithley SC-22, Lemo 001 101.I think Kirill did not mean on outside:Quote from: Kirill V. on November 11, 2019, 04:03:54 pmI do not think you get decent results with diy conductive layer - not mentioning the time and efford.Simply buy proper cables, there are some low noise coax cables awailable.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 15, 2019, 04:47:03 pm
content: It sounds like you're describing a typical low noise cable for charge mode accelerometers - while they usually use a different connector, they are readily available on ebay and elsewhere - not super cheap, but not prohibitively expensive.The problem with using these for a picoammeter is shielding - they are coax, not triax and they generally don't have full shielding coverage (it seems like 90% is common). The triboelectric effect is specifically an issue for charge mode operation (so more nanovoltmeter than picoammeter), and should not generate much while measuring a DUT provided it's not physically moving, so I'm not sure it's necessary for such an application. Triax cabling (like Belden 9222) offers better shielding plus the guard inner shield to eliminate EMI and leakage currents, which is what the picoammeter would be measuring.

author: MiDi
author_rank: Frequent Contributor
timestamp: November 15, 2019, 04:51:09 pm
content: As far as I remember the Belden 9222 is not low noise type.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 15, 2019, 05:18:38 pm
content: Wow, thank you guys so much!Yes, finding the right cable is a very difficult task, if we do not look at cables specifically for picoammeters. There are just "low noise" cables, there are for vibration sensors, there are triaxial for video applications. And the specifications do not say about ultra-low currents and I will have to think on their own about the suitability of a cable for my application.You've told me so much I need to think about it:)The basic idea is that the outer shield (which is the very first after the Central conductor in triaxial cables) is integral with the insulation and cannot slide over it, generating triboelectricity. This role is performed by a layer of graphite and it does not carry a significant current by itself. But maybe I'm wrong?

author: MiDi
author_rank: Frequent Contributor
timestamp: November 15, 2019, 05:36:24 pm
content: Quote from: Kirill V. on November 15, 2019, 05:18:38 pmWhen the shield sticks strong to the insulation the cable will become rigid.To overcome that a really thin sticky layer is needed to allow flexibility.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 15, 2019, 08:13:37 pm
content: Quote from: MiDi on November 15, 2019, 04:51:09 pmAt least in terms of triboelectric effect and current measurement, does it matter? I don't see a noise specification for it, and it doesn't specifically mention low noise operation, but at least some triax advertised for low noise seem to have similar construction - Huber & Schuner G_01330_HT-23 seems to be the same materials (and no mention of graphite for specialty noise reducing material), just PE, PVC, and copper braided shields.And maybe my understanding is wrong, but provided you're not flexing the cable while measuring, tribolelectric effect should have minimal effect on current measurement, correct? Charge is built up because of friction, and it stays there because of the insulating layers, but then any buildup of charge from handling would only effect current measurement through dielectric leakage, which is going to be extremely low with a guard shield in the case of triax. Accelerometer cables need it because the whole point of the device is to subject it to vibration while measuring, but I would assume that the vast majority of benchtop uses wouldn't run afowl of the triboelectric effect, unless you're trying to measure built up charge on an apparatus or something where that residual charge built up from handling would effect the reading.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 15, 2019, 08:57:04 pm
content: Guys, I found an article that mentions an interesting cableSignal conditioning for high-impedance sensors Glen BriseboisThis is Belden 9239, and it has Conductive PVC - Polyvinyl Chloride layer. In my opinion, there are other similar cables in the range of the company. It looks like an alternative to graphite.And then a reasonable question: is the picoammeter needed only a triaxial cable? It seems to me that a simple coax will suffice

author: DaJMasta
author_rank: Super Contributor
timestamp: November 15, 2019, 10:21:37 pm
content: What picoammeter do you have? Triax connectors look much like coax, but they have 3 locking points on the outside, and two shield ring contacts around the center conductor, the picoammeters I've dealt with use triax because the guard trace is essential to keeping noise out of your readings (also measurements are best taken in a shielded box).The H&S cable I referenced in my last post doesn't mark any of its PVC as conductive, but the big difference in construction between it and the Belden 9222 is that it uses a PVC insulator between the guard and outer shield (the H&S) instead of PE for both dielectrics (the Belden 9222) - it could be that the inner PVC layer is conductive to make the cable low noise, it's just not specified.

author: guenthert
author_rank: Frequent Contributor
timestamp: November 16, 2019, 02:23:58 am
content: Quote from: DaJMasta on November 15, 2019, 10:21:37 pmModern ones. Old (Keithley) ones might come with 2 lug connectors.

author: Gyro
author_rank: Super Contributor
timestamp: November 16, 2019, 04:49:59 pm
content: Quote from: Kirill V. on November 15, 2019, 08:57:04 pmIf the current is referred to ground / chassis then I think you're right, however if the circuit is floating then you need an overall screen too.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 16, 2019, 05:37:26 pm
content: It will be a homemade picoammeter. Now I'm looking at different implementation options. I have some books and articles on this subject, but they describe typical solutions, and these articles do not have feedback from people. And to discuss my strange ideas, I turned here. You deal with this technique in practice, right?There are several reasons why I don't want to do a triaxial interface. First, these are scarce and expensive parts. Second, I don't know yet how to technically provide middle screen control in triaxial cable when measuring current. If I were designing an electrometer, I wouldn't ask such questions. Indeed, I would use a high impedance buffer at the input and know exactly the potential of the central conductor. Then with the help of some reception would ensure equality of potentials of the central conductor and the first screen.But I will use a transimpedance amplifier, in which case it is unclear where to take the potential for the screen. I have several service manuals of industrial picoampermeters/electrometers but I have no business to study them carefully. If someone knows how industrial picoammeters control the middle screen in the cable and at least in general describe the principle, I will be very grateful.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 16, 2019, 06:38:48 pm
content: You can think of it as driving an opamp in a buffer amplifier configuration or similar. Your signal comes into the noninverting input, your ground is attached to the outer cable shield, and your guard internal shield is attached to the inverting input for the amplifier. Since the opamp is trying to drive the output to make the two inputs match, the inverting input is driven to match the input signal, so then your guard trace on the PCB and guard shield (which are both connected to the inverting input) in the cable is driven with the same signal you're measuring. Because the input and the guard are at the same potential, there is no leakage between them, and since the guard trace is driven, any leakage between the guard and the ground is compensated for by the output of the buffer amp.https://en.wikipedia.org/wiki/Driven_guardIf you're designing an electrometer, regardless of your cable, this technique will likely be required on the PCB. Leakage currents over clean FR4 can easily be high enough to make problems with this, though if you're using teflon standoffs and point to point wiring or similar, you may be able to get away with out this technique.... though then if your cable isn't two individual wires, insulation like PVC can have high enough leakage to be a problem if the wires' insulation is in physical contact. Also worth mentioning that you want to establish a cleaning method for your frontend after assembly and make sure to not touch it with bare hands (and keep it covered from dust) as much as possible - even the oil from your hands can dramatically reduce the insulation resistance of the separation between sensitive parts and can give you a leakage problem in an otherwise good design.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 16, 2019, 07:42:41 pm
content: Great Thanks!But you described it as an electrometer, and now look at my ideaI plan to connect the outer shield of the coaxial cable to the signal ground, and the voltage between the conductors of the cable will be close to the offset voltage of the amplifier. I very much hope that leakage currents will be negligible therefore.How to organize a triaxial interface in this case?So far I have made the first important conclusion: I should not worry about the triboelectric effect. And it's great!

author: Gyro
author_rank: Super Contributor
timestamp: November 16, 2019, 07:47:46 pm
content: Quote from: Kirill V. on November 16, 2019, 05:37:26 pmThis thread will hopefully help you...https://www.eevblog.com/forum/projects/picoammeter-design/P.S. The protection resistor is best put on the opamp input, inside the feedback loop.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 16, 2019, 08:04:09 pm
content: Yes, thank you, I have seen this thread before, I will definitely study it.I studied the specifications of the cables that I was advised here. There are a few comments:SC-22 cable spec says about 1.2*10^11 Ohm resistance between center and inner conductors. Isn't that too bold for plain polyethylene?QuoteIt seems that such an alternative to the graphite layer is now used:The low noise technology with semi-conductive layer. This is HUBER+SUHNER.In Belden datasheet says about semi-conductive layer, PVC

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2019, 08:29:45 pm
content: Quote from: Gyro on November 16, 2019, 07:47:46 pmSpecifically, directly clamp the inverting input with low leakage diodes and place a current limiting series resistor between the inverting input and the summing node. For stability, feedback capacitance will need to be added to cancel the RC time constant between the series resistance and capacitance from the clamp diodes.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 16, 2019, 09:11:04 pm
content: Quote from: David Hess on November 16, 2019, 08:29:45 pmI'm not going to use external protection diodes. I will limit the input current to 2 milliamps in case of an input overload with a protection resistor.So, I can make the second important conclusion: I can quite use simple or" low-noise " cables with polyethylene which do not cost mad money. Am I right?

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 17, 2019, 09:07:00 pm
content: My concern is that Keithley indicates a very high insulation resistance of its cables, while other firms give more modest values. And we are talking about polyethylene.And here they talked about polyethylene foam, but as far as I know, such a dielectric is used in simple coaxial cables for TV. Do any of you have experience using such cheap cables?And if you haven't done it in practice, you can make a theoretical assessment of it. I give you the input: let it be a resolution of 1 picoamp and a cable length of no more than 1 meter. So, what do you think about it?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 18, 2019, 09:19:41 am
content: Decent quality solid polyethylene is fine. I use the H&S low noise coax (G_01130_HT) with a polyethylene isolation and a PVC conductive layer and it is good for femtoamp level leakages even at considerable length (a few meters), for low voltages. I've measured the leakage current below 10fA at 100V DC for 2m of that cable at room temperature and about 40% RH. There is some DA so you have to wait some minutes for the leakage to settle.CheersAlex

author: serg-el
author_rank: Regular Contributor
timestamp: November 18, 2019, 09:40:03 am
content: Advantest triax cables have a coating on the inner insulation. Conducts current, or not - did not check.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 18, 2019, 10:46:25 am
content: Quote from: Alex Nikitin on November 18, 2019, 09:19:41 amThanks Alex. I found this cable in Mouser and the price suits me perfectly. It is clear that this is ideal for measuring picoamps. Now I'm wondering how cheap polyethylene cables behave.In any case, when I start building my picoammeter I will be forced to buy parts abroad, because such things are not sold in Russia(teflon standoffs and TO-99 socket for AD549). At the same time buy 1 meter HUBER + SUHNER cable.But first I'll make a simple prototype with a cheaper and less sophisticated op amp to test my ideas. And nothing but TV cables made in China in our stores do not sell.

author: serg-el
author_rank: Regular Contributor
timestamp: November 19, 2019, 03:18:24 pm
content: My google-fu tells me - low noise triax cable!

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 19, 2019, 10:24:23 pm
content: Thanks, but unfortunately this cable is out of stock in Mouser

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2019, 05:00:07 pm
content: Quote from: Kirill V. on November 16, 2019, 09:11:04 pmYes, that will work if you know that you can rely on the input protection diodes. Some low input bias current operational amplifiers completely lack input protection diodes and would have to rely on external ones for protection.There are more complicated circuits which bootstrap external protection diodes to reduce their leakage but I doubt you will require that.

author: Gyro
author_rank: Super Contributor
timestamp: November 23, 2019, 07:05:09 pm
content: Using the internal protection diodes is a valid thing to do with certain low input current opamps which already contain bootstrapped protection diodes such as the LMC6001 or, more usefully the low cost LMC662.The maximum rating for the LMC6001 input diodes is 10mA and the LMC662, 5mA. As long as you resistively limit the maximum overload input current to well below these levels (and ensure that the series resistor has sufficient voltage rating and minimal parasitic capacitance) then you are fine.Opamps with internal bootstrapping of the protection diodes makes things a whole lot easier these days.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 23, 2019, 08:10:43 pm
content: AD549 hasn't internal protective diodes. However, the same rules apply here as when using amplifiers with a JFET input, the input current must not exceed 5 mA. I have already received advice from ADI experts about this.Of course, it is possible to add external protection, I thought of JFET transistors as diodesAnd you can use bipolar transistors, but I do not understand by what criteria to select transistors for such protection.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 23, 2019, 11:11:16 pm
content: Quote from: Kirill V. on November 23, 2019, 08:10:43 pm1) The AD549 is a poor (and expensive) choice unless you really need a metal case or +/-15V capability. The ADA4530 will be much better or the LMC622 on the cheap will be better in most respects (input current, drift).2) The BAV199 diode from a decent manufacturer (TI, NXP) will give you a much better performance than either BJTs or JFETs as diodes.CheersAlex

author: serg-el
author_rank: Regular Contributor
timestamp: November 23, 2019, 11:37:42 pm
content: Took measurements on his cable.Advantest A01009-200.Autopsy results in photos.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 24, 2019, 12:11:23 am
content: Great-great thanks Alex for diodes information and serg-el for cable disassembly!I know the latest op-amps are much better, but it's not critical for me because I'm not aiming for the femtoampere range yet.

author: David Hess
author_rank: Super Contributor
timestamp: November 24, 2019, 02:17:30 am
content: Quote from: Kirill V. on November 23, 2019, 08:10:43 pmJFETs only protect themselves when the junction is forward biased so an additional diode is required.Quote from: Alex Nikitin on November 23, 2019, 11:11:16 pmAs with 2N3904s used as low leakage diodes, the BAV199 is only suitable if you grade them yourself; they are only tested down to like 2 nanoamps or something.If you want a guarantied low leakage diode, 2N4117 or 2N4117A JFETs are the least expensive option. But personally I just grade 2N3904s which only takes a couple minutes and can be done with a power supply and any 10 megohm input voltmeter. Use the 0.2 or 2.0 volt scale as a picoammeter unless you have an electrometer handy.

author: MiDi
author_rank: Frequent Contributor
timestamp: November 24, 2019, 07:09:49 am
content: Quote from: serg-el on November 23, 2019, 11:37:42 pmInteresting,Keysight N1415Aseems to use same triax connectors

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 24, 2019, 11:49:21 am
content: BAV199 specifications says about 3 pA minimum at 75 Volts reverse, in my case they will work at 1 mV maximum.In older electrometers, a pair of bipolar transistors can be found as diodes, and often only one pn junction is used.

author: Echo88
author_rank: Frequent Contributor
timestamp: November 24, 2019, 12:52:14 pm
content: https://www.google.com/url?q=https%3A%2F%2Fwww.dropbox.com%2Fs%2F11wp4nlumil1koq%2F4x.3_TIA_final-DRAFT.pdf%3Fdl%3D1&sa=D&sntz=1&usg=AFQjCNHlXTnDRvTaCN_XTzJ73ML6ytRQNgOn page 9 they mention a trick on how to short the integrating cap of an integrator by manipulating the supply rails of the OP-Amp to get the ESD-protection-diodes to conduct.Can anyone share a suitable schematic (couldnt find any info on this by Mr. Gottschalk) for this and is it a better solution than just using a relay to short the cap for example?I assume the ESD-diode-trick doesnt produce charge injection on the cap, unlike the shorting-switch?

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 24, 2019, 03:33:01 pm
content: I want to ask forum members to measure the actual leakage current inRGtype coaxial cables. Then I can make prototypes without having to buy cables on the other side of the world.1 meter cable with decent pure copper screen.

author: David Hess
author_rank: Super Contributor
timestamp: November 24, 2019, 11:00:29 pm
content: Quote from: Kirill V. on November 24, 2019, 11:49:21 amThe maximum specification is what the part is actually tested for. The typical or minimum leakage is not guaranteed. From the datasheets:BAV199Typical 3pAMaximum 5nA2N4117Maximum 10pA2N4117AMaximum 1pANow like most 2N3904s, most BAV199s will leak at around 3pA but the only way to know is to grade them yourself because unlike with 2N4117 JFETs, the manufacturer is not testing to that level.Quote from: Kirill V. on November 24, 2019, 11:49:21 amNewer instruments may still do that. The manufacturer graded the transistors for low leakage. Or if they got tired of doing that, bought 2N4117s or 2N4117As which they did not have to test individually.True low leakage diodes can be had but it comes down to paying the manufacturer to do the testing for you and without the economy of scale of the 2N4117, they end up costing an awful lot. Contrast the price between the LMC6081 and LMC6001 operational amplifiers; they are the same part tested to different leakage specifications.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 25, 2019, 09:01:32 am
content: https://www.eevblog.com/forum/metrology/measuring-nanoamps-and-below-like-a-ninja/msg1119731/#msg1119731The spread from good makes is surprisingly small. You need to keep things very clean though.CheersAlex

author: iMo
author_rank: Super Contributor
timestamp: November 25, 2019, 10:58:43 am
content: Metal 2N4117A costs 14Euro, TO92 epoxy PN4117A 1.60E..Are those comparable in the max leakage??

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 25, 2019, 11:00:00 am
content: Quote from: Kirill V. on November 24, 2019, 03:33:01 pmI've measured what I had available - a~180mcoil (18nF total capacitance) of the RG58C (LSZH outer isolation, tinned copper screen and solid PE internal isolation). At100V DCafter settling the leakage of the coil is about0.1pA (100fA). This is a high quality mil-spec coax. I've also measured ~15m of RG-62 but it was terminated with a nylon isolated BNC, which leaks badly, so the result was the connector leakage (~0.5pA at 100V). In my experience at a room temperature and reasonable humidity (<50% RH) quality standard coaxial cables with solid PE isolation are rather good for low leakage.CheersAlexP.S. my measuring setup (K617 + 1m triaxial cable to an external box with PTFE isolated BNC connectors and 1G test resistor) leakage with nothing connected to it is below 5fA at 100V.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 25, 2019, 12:44:12 pm
content: Great thanks for measuringsMe available for purchase RG-58C/U. I will proceed from the fact that my picoammeter must have a resolution of 1 pA so that the leakage in the cable is an order of magnitude smaller and does not affect the measurement results.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 25, 2019, 12:49:24 pm
content: Quote from: Kirill V. on November 25, 2019, 12:44:12 pmAbout 3 orders of magnitude, as for 2m cable length the leakage at 100V DC would be below 1fA and obviously less for lower voltages.CheersAlex

author: David Hess
author_rank: Super Contributor
timestamp: November 25, 2019, 11:53:30 pm
content: Quote from: imo on November 25, 2019, 10:58:43 amYes, I have never noticed any difference. Plastic packages have been outstanding for decades now.Metal can packages in theory could be better however Bob Pease recounted an incident where light leaking through the bottom of a TO-72 can caused high leakage. I have had the same problem with glass packaged low leakage diodes.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 26, 2019, 12:33:28 am
content: No one else here has mentioned InterFet products and their low leakage JFET pairs. Have you tried this?

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2019, 12:38:03 am
content: Quote from: Kirill V. on November 26, 2019, 12:33:28 amI have not but Linear Systems also produces low leakage JFETs and JFET pairs as well as low leakage diodes.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 26, 2019, 12:46:54 am
content: Quote from: David Hess on November 26, 2019, 12:38:03 amThese diodes are actually JFET connected as diodes to two external pins, right? Unfortunately, this is a rare and not cheap product. At least for prototypes or the first attempt, as in my case:)

author: Gyro
author_rank: Super Contributor
timestamp: November 26, 2019, 10:08:30 am
content: How difficult is it for you to buy European / US commodity parts in the Russian Federation? An LMC662 costs $1 - $2 in single quantity. Probably much cheaper that trying to source low leakage FETs etc.

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2019, 10:33:11 am
content: Quote from: Kirill V. on November 26, 2019, 12:46:54 amThey could be JFET dies connected externally as diodes but I do not know. Planar diodes constructed like planar transistors do exist.2N4117s and 2N4117As are inexpensive and readily available. It only takes a couple minutes to test some very inexpensive 2N3904s or similar small signal transistors for low leakage. So there are several inexpensive and readily available low leakage diodes available for those who need them.Quote from: Gyro on November 26, 2019, 10:08:30 amThe LMC6081/LMC6082 is also suitable or the LMC6061/LMC6062 if lower supply current is required.Testing an operational amplifier for input bias current is very easy; configure it as an integrator so that it integrates its own input bias current.

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 26, 2019, 01:04:58 pm
content: LMC662 - it is available under the orderLMC6001 - around 15$2N4117/PN4117 - not available in our storesLMC6081 - not available in our storesQuote from: Gyro on November 26, 2019, 10:08:30 amI had in view the use of their as diodes protection.I can buy these things in Mouser, but they do not work with Russia directly, only through an intermediary in St. Petersburg. They charge a certain amount of money for their mediation. I've never done that before. I'll leave it to the future.When you advise to buy something ready-made, you do not understand the specifics of it. I can't go to a store and leave with a package of parts. I can't even order it in Moscow or anywhere else, because these parts are not there

author: Gyro
author_rank: Super Contributor
timestamp: November 26, 2019, 01:42:13 pm
content: Quote from: Kirill V. on November 26, 2019, 01:04:58 pmI feel your pain.I suspected that it might be difficult, I just wasn't surehowdifficult.Edit: Actually the LMC6001 cost is fairly similar to the UK, LMC662 is a good cheap bet if it is available to order, as you indicated -typicalinput current of 2fA (lower than the 6081, of course you have to be lucky to get the typical one!)

author: Kirill V.
author_rank: Regular Contributor
timestamp: November 26, 2019, 02:23:28 pm
content: ThanksBut don't try to talk me out of using the AD549 because it's already in my drawer waitingThe strangest thing is that this item is available for purchase absolutely easily.Products LIS or InterFet is absolutely not available from us even to order. The vast majority of people in our country have not even heard of such a thing.H&S or Belden are available only under the order and the cost needs to request and the delivery time is not very fast. In fact, a Russian company will order it in America or Europe and sell it to me. They don't have it in stock.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 29, 2020, 05:14:18 pm
content: Quote from: Kirill V. on November 26, 2019, 01:04:58 pmhttps://efind.ru/offer/LMC662+?r=https%3A%2F%2Fefind.ru%2F&stock=

author: DeltaSigmaD
author_rank: Contributor
timestamp: March 30, 2020, 10:09:07 am
content: I would like to make some comments on this subject according to my experience.Triboelectric currents and microphonic effects are a general problem for the cable connection between signal source and first amplifier, since the dielectric material of the cable cannot be freely selected. Teflon/PTFE has excellent DC isolation, but it has very high triboelectric generation and microphonics. This can be easily understood: PTFE has C-F-chemical bounds, where the difference of the electronegativity of C and F are very high. After mechanical treatment or soldering, PTFE can freeze electrical charges inside its bulk volume (which cannot be removed via surface) with relaxation times of hours to several weeks. The relaxation is so slow since the C-F-bound of PTFE is extremely rigid (bound energy far away from kT, only tunneling?). But a relaxing charge generates surface currents (displacement current, see Maxwell equations). Nearly perfect in this aspect are dielectrics like PE (polyethylene) or PP (polypropylene), where the difference of the electronegativity between C and H are weak. Even after machining only weak bulk charges are produced. The limiting factor with these materials seems to be chemical additives to the PE/PP polymer material. Afaik, no cable manufacturer mentions any concerning information in the data sheets. Therefore, it is also not clear which influence the degree of polymer cross-linking has. PVC (polyvinyl chloride) has much too bad isolation and microphonics, it is a very dirty material even due to the required softener(s). Glass has relatively high surface conductivity due to the Ca-, Na-, and K-ions on its surface (hygroscopic, good wetting with water!). With glass-insulated reed relay contacts, you can easily measure the wandering charges on the glass surface. Ceramics also have high difference of the electronegativity (e.g. Al2O3), and are not flexible.One additional problem with all cables is that the noise is increased by the cable if non-steady-state (--> ac) signals must be measured. In most practical cases, the cable capacitance generates additional noise due to the amplification of the input voltage noise of the preamplifier, dominating factor is Ccable / Cfeedback.Consequences:1. Don't use PTFE, excepted you are measuring only DC signals and there is no movement or vibration.2. When a PTFE-isolated cable (even triaxial cable) was moved, consider a relaxation time of hours to stabilise the displacement current. After that period, subtract the offset current to get reliable results.3. Make the cable length as short as possible.4. If the mechanical/practical constraints permit, use a transimpedance preamplifier directly at the source. The amplifier's box size will be only a few cm³. That's the preferable solution.

author: Kirill V.
author_rank: Regular Contributor
timestamp: April 01, 2020, 12:27:19 pm
content: Thank youAlmost everything you said is described in Soviet literature. I am once again convinced that this is a source of non-outdated knowledge.

author: klimm
author_rank: Contributor
timestamp: March 17, 2021, 02:18:45 pm
content: Quote from: David Hess on November 24, 2019, 02:17:30 amAccording to the above described measurement procedure, can anyone estimate the value of reverse current one could expect from B-C junction of 2N3904? If measured , even better.My measurement on a lot of 20 unknown bipolars(B-C, E not connected) with about 5V, results in 410±10 pA. I get same result when measuring 2 pcs of 2N3904. Not much difference when measuring B-E.For comparison a 4148 diode resulted in 39 nA at 5V and 53nA at 10V. Temperature was about 25C

author: klimm
author_rank: Contributor
timestamp: March 18, 2021, 07:03:07 pm
content: Well... I must admit, I forgot to null the DMM after 1 hour on. Therefore the intriguing current of 410pA, I got.There was a certain offset that I was able to notice today in the same conditions: 4.1mV which equates for 410pA at my 10M input resistance.Nulling the offset I now read 000.00(4 1/2 digits) when testing the B-C junctions. It is fine to know that the junctions have a leakage<10pA but can't see any difference between the 20 transistors I tested. Was expecting to see some variation.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2021, 07:47:32 pm
content: < 10 pA sounds reasonable. So it may need a more sensitive circuit or meter to see the differences.

author: klimm
author_rank: Contributor
timestamp: March 18, 2021, 08:00:43 pm
content: What intrigues me is that mr Hess says "any 10 megohm input voltmeter" and also said that he grades the juctions which implies that is able to see differences among these.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 18, 2021, 08:48:25 pm
content: Transistors from the same batch can be relatively similar. Grading in the case is more like check if they are actually <= 10-20 pA and not 1 nA. Many simple DMMs only provide 0.1 mV resolution with 10 MOhms and thus 10 pA resolution. In addition the measurement with such a high impedance may pic up mains hum and with a diode in the ciruit and not enough voltage this may see some rectification.

author: bsw_m
author_rank: Frequent Contributor
timestamp: March 18, 2021, 10:51:52 pm
content: Hi!Please note russian low noise cable AVK-6 (АВК-6) and cables in this series have a conductive layer on the insulation to suppress the triboelectric effect. But these cables is not a triaxial. But I not see any problem to make outer shield.P.S. MNIPI used these cables for their electrometers.Also I'm test B-E junction of 2T363B transitors at 1V reverse voltage.On average, the leakage current was 50fA with extremes of 100fA and 10fA

author: Kirill V.
author_rank: Regular Contributor
timestamp: April 19, 2021, 11:04:27 am
content: Hi all again,Quote from: bsw_m on March 18, 2021, 10:51:52 pmI believe that the question of a triaxial cable can be closed in this particular case. This refers to homemade current meters for the home. As proof - FEMTO uses a coaxial cableThe results for Soviet transistors are interesting and valuable, thanks for the measurements