behind the scenes: Working on the B revision voltage standard - Page 1
Forum section: electronics / projects

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 25, 2015, 02:50:24 pm
content: I expect to finish the design of my revision B voltage standard in about 2 weeks. I want to share some toughs of things I am considering, revoking or other interesting stuff.Currently I have tested a voltage doubler for the 10.00000 V version. I need a voltage doubler, because I want to use one 9V battery. One of the IC's I'll be able to use is the LT1021. The board will have a layout, so that other, pin compatible, IC's will fit.I already tested a nifty layout with an 8 pin dip, and a so8 package in the same place.Anyhow, I was reading in the datasheet of the LT1021 and found this graph:I marked a spot yellow. At this point a 14 bit converter can have a 3 ppm/°C TC, when the delta operating temperature is 10°CIn normal human language:When the operating temperature of a 16384 count (= 14 bit) ADC is between 18°C and 28°C (delta=10°C), at least a 3ppm/°C reference is needed.Interesting ..

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 25, 2015, 06:38:08 pm
content: Here is that nifty construction with a choice between DIP8 and SO8:The 3D view only shows the DIP8, but there is a choiceI am in doubt over the mechanical stress relief. Should it be as shown, or are 2 slides on the edge of the pcb sufficient?Hmm .. I will have a good night sleep over that.From the datasheet:Effects of Air Movement on Low Frequency NoiseThe LT1021 has very low noise because of the buried zener used in its design. In the 0.1Hz to 10Hz band, peak-to-peaknoise is about 0.5ppm of the DC output. To achieve this low noise, however, care must be taken to shield thereference from ambient air turbulence. Air movement can create noise because of thermoelectric differencesbetween IC package leads (especially kovar lead TO-5) and printed circuit board materials and/or sockets. Powerdissipation in the reference, even though it rarely exceeds 20mW, is enough to cause small temperature gradients inthe package leads. Variations in thermal resistance, caused by uneven air flow, create differential lead temperatures,thereby causing thermoelectric voltage noise at the output of the reference.So, is it necessary to get some kind of shield over the reference IC? And where do I get something like that? Has to be custom made, I presume.

author: Vgkid
author_rank: Super Contributor
timestamp: February 25, 2015, 06:52:03 pm
content: I do not believe that the mechanical reliefs are necessary. And for the shield, is it (the board) going to be mounted in a case?

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 25, 2015, 06:59:19 pm
content: Quote from: Vgkid on February 25, 2015, 06:52:03 pmThere is a recommendation about that .. I thinks it was from Lineair. They did a video about that.QuoteNope .. on a battery holder ..

author: nuno
author_rank: Frequent Contributor
timestamp: February 25, 2015, 07:44:36 pm
content: Sometimes the reference chip is on a PCB corner, one I did once is in the middle but having 2 cuts (U1 is the reference chip, the black line is the board outline so there's a cut on both left&right):Still the chip is on the corner on purpose.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 25, 2015, 08:46:08 pm
content: Hi JohnnyBerg,Why do you not make a circuit board, that fits into one of these nice aluminum boxes.http://www.eoo-bv.nl/index.php?_a=viewProd&productId=15907"Dutch Mode on"Weet je nog, een referentie hoort in een metalen behuizing!"Dutch Mode off"Kind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 25, 2015, 08:57:09 pm
content: Quote from: blackdog on February 25, 2015, 08:46:08 pmIf I make a very stable one (< 2ppm), I consider that.Problem with that enclosure is that it needs a lot of work (machining and making it attractive), before it is a product that can be sold.The never ending battle between technics and salesQuoteJa .. ik weet het. Maar zo'n metalen bakje smoelt gewoon niet lekker op een foto

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 26, 2015, 03:43:00 pm
content: I finished placement of all the components on the board. Sort offFor mechanical stress relief I looked again in some datasheets, and I decided to do it like this.I added 3 slot holes around the reference IC. In there I can put some kind of metal shielding. It will protect against noise radiated in, and drought over the board. As the backside of the board will be a ground plane, the whole reference IC is well shielded.As I plan to deliver a calibration report with each device, there has to be a serial number on the board. I do not write values any more on the board, as I did in the A rev.The potmeter is drawn straight, but it will bend on the pcb. I replaced the smd potmeter with a 10 turn type. The bending is necessary for transport. After calibration I seal the potmeter position.I have some nice gold plated testpoints. They will replace the hand made testpoint from rev. A. I won't be using this switch anymore. It is likely to damage during shipment as it sticks out on the top. Actually the same reason as with the potmeter.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 26, 2015, 03:49:18 pm
content: Hi JohnnyBerg,If you want, i can buy/test one of your first sample's, to see how stable it is and test the noise level?Maybe its a good point, to remove the pins on the DIP that are not in use, cut them! (pin 1, 3, 7 andKind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 26, 2015, 03:57:21 pm
content: Quote from: blackdog on February 26, 2015, 03:49:18 pmYes, please! I send you one for free!Edit: speaking with Louis van Gaal: "I hope for your unsalted opinion"QuoteI think that sales has a problem with thatFor the 10V version I've got a LT1021DCN8-10 in order. I'll think I will order a REF5010AIDR for the smd version, as the board provides for both packages.Nothing to fancy yet, all above 2.5PPM/°K (typ.)I'll have a good night sleep over the shielding ..Yeah, deadline is still 2 days away .. 1 ready, 2 to go

author: metacollin
author_rank: Contributor
timestamp: February 26, 2015, 04:43:36 pm
content: Quote from: JohnnyBerg on February 25, 2015, 02:50:24 pmThis is a terrific project, the board looks very well done, and I don't mean to disparage it at all. But, assuming an LT1021 as the reference...it seems like there is 1 too many significant digits there.Well, or maybe not, I'm just asking if that number is correct or a typo.10.00000 translates into 1,000,000 counts - meaning you need stability as good as 1ppm. I've seen LT1021s with temp co's better than 0.2ppm at room temperature in the wild, and indeed, they're not even that uncommon, and that would give you a small but not impractical temperature window for 1ppm stability. But you'd still have to do some pretty serious cherry picking of a not particularly inexpensive part to achieve that. The bigger issue though is the long term stability. The epoxy DIP packaging does absorb moisture over time and is definitely not hermetically sealed, so I doubt any parts will do much better than the 10ppm/kHr drift stated in the data sheet. Indeed, I found a post by Andreas on the volt-nut mailing list stating that these parts see 0.5ppm drift per % change in relative humidity of the environment. As a desert-dweller with an evaporative cooler... the relative humidity inside can go from 10% to 70%+ no problem, which is more or less a hell tailor-made for the LT1021. Or really any reference in a non-hermetically sealed package.So, you would need to hermetically seal those puppies somehow... I am not really sure what is involved in that, as being completely encased in epoxy seems pretty sealed to me, but obviously it is not. And I can't find any data on if that helps with the 10ppm /kHr drift. Maybe rock out with your caulk out and go nuts on the DIP part? I dunno heh.Basically, 10.0000, or 100,000 count (10ppm drift budget) is very reasonable. 10.00000 (1ppm drift budget) seems doable, but you'd need some real special sauce going on that you haven't mentioned in this thread. But that'd be pretty sweet.Just wanted some clarification. Regardless, its a great little standard regardless of ±1 one significant digit. Again, I don't mean to disparage the project at all, but just wanted to double check what was going on. Anyway, looking forward to seeing how this project evolves!

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 26, 2015, 05:16:01 pm
content: Quote from: metacollin on February 26, 2015, 04:43:36 pmYou are absolutely right!I was a bit over enthusiastic, and I wrote 10.00000V. That is what my 34401A shows. I know that 1 of my 2 34401A's is only 1 or 2 count out, but I would have to say that I'd be able to trim to 10.00000V within 35ppm accuracy. (being the guaranteed accuracy of the 34401A). And I would have to specify a guaranteed time period.Quote from: JohnnyBerg on February 26, 2015, 03:57:21 pmThere you have it. That is what I expect to get.This project will have 2 siblings, a low cost version with a LT1004 and another REF5050-isch. These siblings will have a precision current source and 2 precision resitors on them. They are intended for multimeter testing and calibration.Behind the scenes I am playing with some other stuff, but i'll save that for later

author: blackdog
author_rank: Frequent Contributor
timestamp: February 26, 2015, 05:16:26 pm
content: Hi JohnnyBerg,This is the only language i speak =>"I hope for your unsalted opinion"I am a Dutch and you too, you will few problems, concerns saying it like it is.Yo can find my e-mail adress on C.O.Kind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 26, 2015, 07:08:15 pm
content: After some iterations with the autorouter, and some manual work I'm done. And satisfiedBackside almost complete copper. Good shieldingUnless I get some brainwave the next few days, this is how its send to manufacture.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 26, 2015, 07:45:22 pm
content: Hi JohnnyBerg,If you let me see the schematic, i can give you a better review.Think about the mounting hole next to: "REV. B"If you use shielding, there is no place for the screw.Kind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 26, 2015, 08:03:11 pm
content: Quote from: blackdog on February 26, 2015, 07:45:22 pmGood point! Thanks. I was getting tired and wanted to finish.I can't move the mounting hole, as it has to match the battery clip holder. Hmm .. have to move the reference IC, and the slots .. and the rest.Tomorrow is a new day

author: blackdog
author_rank: Frequent Contributor
timestamp: February 26, 2015, 08:11:20 pm
content: Hi JohnnyBerg,Just a lesson of a old man ;-)Do not push yourself, your creativity disappears with fatigue and you start experiencing tunnel vision.The efficiency can become nagative!!!Kind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 26, 2015, 08:21:04 pm
content: Quote from: blackdog on February 26, 2015, 08:11:20 pmHaha .. you're not the only old manQuoteYeahh .. you're right.On the other hand, I try to send out a order for some panelized, multi design pcb at the end of each month.I don't wan't to design forever

author: Andreas
author_rank: Super Contributor
timestamp: February 27, 2015, 07:36:07 am
content: Quote from: JohnnyBerg on February 25, 2015, 06:38:08 pmHello,something like this?(a coin cell holder and a metal cage for a optic fiber sealing ring)can be etched like a pcb or made by every PCB manufacturer (similar to SMD-Stencil)With best regardsAndreas

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 27, 2015, 09:59:19 am
content: Pff .. another iteration. Unless someone comes up with something realy good, I'm doneBackside, almost complete copperI will try to finish the low cost version today, hopefully

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 27, 2015, 10:01:23 am
content: Quote from: Andreas on February 27, 2015, 07:36:07 amthat is exactly what I meant.

author: nuno
author_rank: Frequent Contributor
timestamp: February 27, 2015, 12:01:06 pm
content: Looks excellent!

author: blackdog
author_rank: Frequent Contributor
timestamp: February 27, 2015, 12:49:48 pm
content: Hi JohnnyBerg :-)Is there a reason why u use a vertical model for the trimpot?Kind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 27, 2015, 01:00:09 pm
content: Quote from: blackdog on February 27, 2015, 12:49:48 pmIt's a 3296W model, from Bourns. I'd rather had the 3296P model, but they are not as widely available as the 3296W model.Back in the 80's my mentor on my first job after I graduated brainwashed me: "is there a second source available for the part you are using?" he kept asking ..pffSo, I had to make a choise between bending the legs or use a not so common potmeter. I choose bending the legs. The 3296W has support for horizontal mounting, by the way.Edit: horzontal mounting is only needed when sending 1 piece out, by normal mail. Bigger orders go in a karton box, and then the horizontal mounting is not nessasary.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 27, 2015, 01:24:30 pm
content: Hi JohnnyBerg,OK that's clear :-)But....Mmmm, LTZ1000A, LTC2057, ADA4077, ect. i wil remove them from my stock parts.And my Agilent 3458A, i will throw him out of the window.Kind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 27, 2015, 01:33:46 pm
content: Quote from: blackdog on February 27, 2015, 01:24:30 pmYeah .. I know. My first job was at a compagny that was in the Dutch AEX. So no LTZ1000A etc, because of the risk that production would stop, because there where no parts.Not as important for a small compagny, but hey .. I'm brainwashedQuoteWhere do you live, and when are you throwing it out?

author: dadler
author_rank: Unknown
timestamp: March 19, 2015, 08:06:22 pm
content: Are these new ones available yet?

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 19, 2015, 08:33:19 pm
content: QuoteThey have been send to the pcb fab. a week ago. I'll expect them next week.They have to be assembled then, and 1000 hrs aging.Besides this reference, I also have a new low cost version coming, for testing/calibrating multimeters up to 4000 counts.It will have a 5.000V output, a 1.000mA current source and 2 precision resistorsAnd a version with 10.0000V output from 1 9V battery.Top of the range will be the "ovenized" version. It has a temperature controlled oven that keeps the reference chip at a constant temperature of 54°CEach reference will come with a datasheet and a calibration sheet.I will offer calibration service as well, and one time free calibration with this one.At this moment I am working on gear to characterize the references. I build a 10.000x (80dB) preamp to measure noise. That was a challenge on his ownResult gives very nice pictures:scale is1.5 µV/div, I am quite happy with 5.88µV pp noise

author: Andreas
author_rank: Super Contributor
timestamp: March 19, 2015, 08:40:29 pm
content: Hello,minimum measurement time for 1/f noise (0.1..10Hz) is 10 seconds. So your 200ms (per div?) scale limits lower bandwidth.And it seems that there is no upper bandwidth limit (10 Hz) in your amplifier. There are much higher frequencies with large amplitude within the signal.With best regardsAndreas

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 19, 2015, 08:52:49 pm
content: Quote from: Andreas on March 19, 2015, 08:40:29 pmI know ..I am still struggling with the filtering in the preamp. And I'm fighting with LTSpice to get the right values.Here's the first stage:

author: blackdog
author_rank: Frequent Contributor
timestamp: March 19, 2015, 09:03:30 pm
content: Hi JohnnyBerg,What about presenting your schematic of the filter...Let us helpKind regarts,Bram

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 19, 2015, 09:18:19 pm
content: I did a quick and dirty adjustment, now it's more to standardI trying to lay an egg about some kind of noise reduction, but I think 7,26µV pp is not that bad. And just within spec of the REF5025.I have carefully laid out the new board, and I'm hoping for better results. Grounding was a real pain in the ass!

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 23, 2015, 04:08:18 pm
content: Got notice that the boards are ready and shipped!As a service, I got a photo from the shipped boards. Looks exactly like the design

author: blackbird
author_rank: Regular Contributor
timestamp: March 23, 2015, 10:27:16 pm
content: Nice pcbGegroet,

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 07, 2015, 03:15:27 pm
content: I ordered 2 batches with PCB's. One express, and one with normal shipment.And guess what?I received the normal shipment, and the Express are still in transit.Anyhow, I can start assembling the first 60 boards .. whow ..

author: blackdog
author_rank: Frequent Contributor
timestamp: April 07, 2015, 03:22:04 pm
content: Hi JohnnyBerg,Tomorrow i will wait next to my mailboxKind regarts,Blackdog

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 07, 2015, 08:28:54 pm
content: Haha .. be patient.Need to assemble and 1000 hr burn in. This is a bit of work .. who ..

author: blackdog
author_rank: Frequent Contributor
timestamp: April 07, 2015, 08:35:25 pm
content: Hi JohnnyBerg,No problem to burn one in overhere...The choice is yours, I will be patient:-)Nice looking circuitboards!Kind regarts,Bram