Best way to protect my voltage reference - Page 1
Forum section: electronics / general

author: eurofox
author_rank: Unknown
timestamp: July 12, 2018, 05:36:21 pm
content: Hi voltnuts friends,I would like to get some ideas how to protect the output of an LTC1052 against over voltage from static electricity.2 weeks ago I killed my voltage reference by connecting it to a DMM.The pamona cable was connected to the guard of the DMM and of course the DMM is connected to my home ground by the power connector.There is a red led connected to the power supply and when I connected the cable to my voltage reference the led was shining a fraction of a second brighter .... I suspected something was wrongMy LTZ1000 based reference voltage aged for one month and was very stable, it was mounted in a Teco box powered with a external very stable and ultra low noise power supply. The power supply was as well checked free of over voltage when powered up.All this is mounted in an aluminium enclosure well grounded.Thanks in advance

author: TiN
author_rank: Super Contributor
timestamp: July 12, 2018, 06:28:27 pm
content: RIP LTZ?What kind of crappy DMM is it that does not have isolated front-end?Best way to protect devices is to limit/avoid the chance or user errors. By using different keyed connectors for power, consistent cable colors/labels, fixed connections, etc.

author: rhb
author_rank: Super Contributor
timestamp: July 12, 2018, 06:48:41 pm
content: I lost several hard drives while handling them during an unusually dry period a few months ago. I mounted a piece of 1/16" x 1/2" aluminum angle to the edge of the desk and grounded it through a 1 meg resistor.I've got an ESD mat, but those don't help if you're working on something which is not on the mat.

author: eurofox
author_rank: Unknown
timestamp: July 12, 2018, 07:23:20 pm
content: Any experience with TVS diodes?

author: The Soulman
author_rank: Super Contributor
timestamp: July 12, 2018, 07:40:20 pm
content: Quote from: eurofox on July 12, 2018, 05:36:21 pmI can't imagine that a static discharge would discharge slow enough to notice the increase in brightness of the led,you sure the voltage regulator hasn't failed?And how is your grounding scheme, the v-ref negative (nor positive..) should not be connected to any guard or earth connection.

author: Brak
author_rank: Contributor
timestamp: July 12, 2018, 07:53:15 pm
content: I've done a lot of googling and found almost nothing about output protection for a voltage reference. It seems almost anything you do affects the output voltage.I did find an article, "Statistical Voltage Reference" by Mark Hennessy that discusses this a bit. He used a 100 ohm resistor in series with a JFET current source. And a zener diode across the output.See:http://www.markhennessy.co.uk/dvm_reference/#output_protectionBrak

author: eurofox
author_rank: Unknown
timestamp: July 12, 2018, 07:53:24 pm
content: Quote from: The Soulman on July 12, 2018, 07:40:20 pmPower supply is ok and of course none of pos & neg are connected to GNG.

author: Andreas
author_rank: Super Contributor
timestamp: July 12, 2018, 08:10:39 pm
content: Hello,a picture says more than 1000 words:- cirquit diagram of the reference + the set up?what do you mean with static electricity?- static DC-voltage source- electrostatic discharge?with best regardsAndreas

author: cellularmitosis
author_rank: Unknown
timestamp: July 12, 2018, 09:17:33 pm
content: Quote from: eurofox on July 12, 2018, 05:36:21 pmThat is terrible! If this thread comes up with any good suggestions I'm happy to add them to the next round of px-ref board tweaks.

author: eurofox
author_rank: Unknown
timestamp: July 12, 2018, 10:30:51 pm
content: Quote from: TiN on July 12, 2018, 06:28:27 pmI think it is not a DMM issue, for the moment when I touch my car outside I get a painful arc between my car and my fingers, this mean that there is lack of humidity and with isolating shoes we are loaded with high voltage, the same with the car that is isolated on tires and with air movement loaded with high voltage.It is very important in this season to have a wrist strap connected to ground when working with sensitive electronics.

author: orin
author_rank: Frequent Contributor
timestamp: July 12, 2018, 10:48:32 pm
content: Looking at the Fluke 732B manual, they have an 11V 5% TVS in parallel with a reverse biased 100 PIV 5A diode. Also a 230V gas discharge tube from their 0V/common terminal to chassis ground. There is also a 10uF and 0.22uF capacitor in parallel across the output, but I haven't found these capacitors (nor any components in this part of the circuit) in the parts lists. I'd guess 10uF 20V tantalum and 0.22uF film.I'm going to add something similar to my references.

author: David Hess
author_rank: Super Contributor
timestamp: July 12, 2018, 11:51:43 pm
content: You protect an output the same way that you protect an input. Add series impedance to limit the current and shunt protection. In the case of an operational amplifier output, that means protecting both the output and feedback network or connection to the inverting input.An output capacitor is not a bad idea but consider how the pole formed by the series impedance and output capacitor affects the frequency compensation; that usually means adding another AC feedback network directly from the output to inverting input.Leakage will not be a problem so normal diodes can be used for shunt protection. Series current limiting can be implemented with a resistor, current sources like depletion mode FETs, or if you want to madly cackle, with some current sources and a diode bridge as shown below.For protection against ESD, a shunt capacitor and shunt diodes may be sufficient.

author: iMo
author_rank: Super Contributor
timestamp: July 12, 2018, 11:54:40 pm
content: A 15V transil?https://www.st.com/content/ccc/resource/technical/document/application_note/30/f6/d4/72/7d/74/45/bc/CD00003857.pdf/files/CD00003857.pdf/jcr:content/translations/en.CD00003857.pdf

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: July 13, 2018, 01:38:56 am
content: Quote from: eurofox on July 12, 2018, 05:36:21 pmDamn! The dingo done 'et yo' baby!Try a P6KE12A across the output terminals.

author: Nusa
author_rank: Super Contributor
timestamp: July 13, 2018, 02:09:08 am
content: Climate controlled room, anti-static flooring or floor mat, anti-static work surface, anti-static clothing, anti-static wired wrist strap, and anti-static ankle/heel strap for whenever the wrist strap isn't connected.And that's probably not even close to the BEST way.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 13, 2018, 09:31:16 am
content: Most important thing for me is to wear anti static shoes in the lab.Since I do that, I have not killed any electronics from static discharge.People just look at me strangely, when I take an Ohmmeter along when I go shoe shopping.

author: Andreas
author_rank: Super Contributor
timestamp: July 13, 2018, 02:33:58 pm
content: Hello,to the protection diodes:there are 2 different voltage specifications.the 11 V TRANS SUPRESS in the Fluke might most probably be aSA11A - the 11V specify the "stand off" voltage. The breakdown voltage is more on the 13V side.A P6KE12A has a stand off voltage of 10V and a breakdown voltage of 12V +/- 5%.So a SA11A has lower (temperature dependant) leakage current as a P6KE12A at 10V.In the Fluke 734 handbook the output capacitor is specified as mylar capacitor.Against ESD it builds a capacitive voltage divider of around 330pF/220nF.But most probably there is no resistor from output to the negative input of the output buffer.So together with the input protection diodes between the positive and negative input there is no current limitation for the ESD.With best regardsAndreas

author: eurofox
author_rank: Unknown
timestamp: July 13, 2018, 02:50:26 pm
content: Quote from: Andreas on July 13, 2018, 02:33:58 pmI order this morning 4 5KP11CA transil 5kW 11V bidirectional.I have a 1.5 KV power supply and will make some test but not on my LTZ1000 reference

author: SvanGool
author_rank: Regular Contributor
timestamp: July 13, 2018, 03:11:53 pm
content: With the 5KP11CA clamp voltage of 18.2V and assuming a tolerance of 5%, I don't think your LTC1052 will always survive a test with your 1.5 KV power supply and a random 5KP11CA.Like Andreas, I would also recommend a lower clamp voltage.

author: Andreas
author_rank: Super Contributor
timestamp: July 13, 2018, 03:13:33 pm
content: Hello,I hope you know that the 5KW are only for a single 1 ms pulse. (5 Joule).And a unidirectional device would be better since then you have only 1V in forward direction instead of 11V in both directions.The bi-directional devices are meant for AC.And be aware of the parasitic capacity of the Zeners.Some OPs have to be isolated against the capacity in nF range.with best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: July 13, 2018, 03:17:20 pm
content: Quote from: SvanGool on July 13, 2018, 03:11:53 pmThe clamp voltage is for 280A current through the device.ESD is usually below 15A. So the clamping voltage for ESD will be lower.I have more concern with the leakage current on a sub ppm device.With best regardsAndreas

author: eurofox
author_rank: Unknown
timestamp: July 13, 2018, 04:11:04 pm
content: Quote from: Andreas on July 13, 2018, 03:13:33 pmYes of course I know this.Is the first time I use this kind of component, they are not expensive and will order other type if needed.

author: CopperCone
author_rank: Super Contributor
timestamp: July 14, 2018, 12:59:13 am
content: Well, since its a LTZ1000 and its the cream of the measurement world, an good idea would be to use a over the top protection system that destroys accuracy, like the full heavy RLC filter, Fast Diode, TVS, GDT, Fuse, with a relay or switch across it, so you can connect it then shunt the protection network after you are sure its ok, then reapply the protection before disconnecting.actually since it has stuff connected to ground it would be better to just disconnect that bit from the circuit.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 14, 2018, 11:10:47 am
content: I don't have a great understanding of the noise levels involved, but I would keep in mind noise when you start adding anything that presents itself as a high impedance.

author: iMo
author_rank: Super Contributor
timestamp: July 14, 2018, 12:59:34 pm
content: Quote from: Andreas on July 13, 2018, 02:33:58 pmThe transils are not reference-voltage-like devices. The "knee" of the transils is pretty similar to a standard zener diode one (rather "wide", not brick sharp). You have to buy several types (ie. from 11V-15V) and measure them individually..

author: David Hess
author_rank: Super Contributor
timestamp: July 14, 2018, 01:43:24 pm
content: Quote from: TheUnnamedNewbie on July 14, 2018, 11:10:47 amThe high impedance series element has to be enclosed inside the feedback loop of the operational amplifier.Some designs like the diode bridge example I gave are only high impedance under overload although in that case, it still needs to be inside of the feedback loop because of offset. Series protection using depletion mode FETs might be able to do without that.The feedback loop removes any added noise within its bandwidth but since it needs to be protected also, care is needed to make sure it does not add its own noise. This is much easier however since the feedback may feed a high impedance load like the inverting input of an operational amplifier.

author: IconicPCB
author_rank: Super Contributor
timestamp: July 15, 2018, 07:21:52 am
content: A PNP voltage clamp referenced to Vref output plus a reverse biased diode in parallel followed by a fuse provides a bidirectional "zener brier" with potentially adjustable upper voltage limit.Transistor collector to ground.transistor emitter to Vref.The base via a resistor to adjustable clamping voltage slightly lower ( a few hundred milivolts ) than Vref and independent of Vref.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: July 15, 2018, 04:43:07 pm
content: TheP6KE12A TVS diodehas an absolute minimum stand-off voltage of 10.2V, andat that voltagewill "leak" no more than 5uA. It's the right TVS diode to use for clamping a 10V reference output that will (very likely) be set to very near 10V. That said, every zener will create noise when you reverse bias them-- the question is how much LF noise will this diode create when you reverse bias it at 10V. The data sheet on these devices is silent on this subject-- so it would have to be measured. Pretty easy to do for someone with a very quiet reference (LTZ1K based) and an LF noise amplifier. Measure the output of the 10V reference with the noise amp before and after the attachment of the P6KE12A-- and note the difference. Worrying, pontificating, speculating and theorizing will get you nowhere-- theReal TruthTMwill be known after actual measurements.

author: eurofox
author_rank: Unknown
timestamp: July 15, 2018, 06:28:36 pm
content: Quote from: Magnificent Bastard on July 15, 2018, 04:43:07 pmI order different voltages including the P6KE12A.

author: Brak
author_rank: Contributor
timestamp: August 03, 2018, 05:54:12 am
content: I knew something about electronics thirty years ago, but I've forgotten everything I once knew. Re-learning at my age is going slow. I think I'm vaguely understanding this but I'm not sure.Quote from: David Hess on July 12, 2018, 11:51:43 pmAs I understand it, for proper, official ESD protection a series impedance is necessary.With a voltage reference the series impedance, usually a resistor, will introduce error. For example, a 470 Ohm resistive series impedance and a voltmeter with a 1 megohm input impedance would introduce an error of about 470 parts per million. With a 10 megohm voltmeter impedance the error would be about 47 parts per million. Which is not good for a precision voltage reference.I figured, and Mr. Hess seems to be saying, that shunt protection without a series impedance is better than nothing and might protect a device from a low energy ESD event. Any series impedance you can fit in within your error budget would would be even better.Am I understanding this correctly?Would a series inductance help with ESD protection? A ferrite bead or two, or a larger inductance?I'm going to build some sort of semi-precision voltage reference that I'll have to carry around and find someone or someplace to measure the voltage precisely. I'd like as robust ESD protection as possible but am not sure how to go about it.Brak