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Offline ManateeMafia

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MX Reference
« on: July 29, 2017, 11:06:31 am »
Once in a far away galaxy a volt-nut was born. As typical, collection of useful and not, expensive and cheap, rare and common test equipment gear and accessory grew, as the volt-nut explored the world of fleabay. But no volt-nut journey will be complete before the final unique artifact is built. Being such, allmighty Repus-Renez reference, which is beginning and end of all the references!

..oh, it's EEVBlog here, people here don't like watery intros.. straight to the meat now.

Project goal and specifications

* Maximum stability for DC voltage reference.
* Low tempco precision wire-wound resistors, PCB to accept for both PWW Ultrohm Plus and Vishay VPG resistors.
* Linear LTZ1000 ultrazener URL.
* Thermal sensor NTC for external calibration purpose.
* Compact 80 x 120 mm form factor to fix Hammond metal enclosure.
* Thermostat setpoint +55C.
* Kelvin sense output.
* Single-rail voltage input.
* PCB space for add-on board to use direct output from LTZ1000 for further down/up-conversion.
* Basic reverse supply voltage protection.
* Room for 3D printed cap on top and bottom of board for LTZ
* Room for 3D printed cover for rest of board.

Typical schematics from datasheet, which is a proven baseline with already great performance. This datasheet schematic well capable to surpass any precision DMM in long-term stability. Hence it's used by many proven instruments, such as HP 3458A, 34470A, Keithley 2002 with just minor tweaks.



Now to stay on the safe side on this project, base reference circuit will repeat datasheet schematics almost exactly. No controversial chopper amplifiers, no voodoo magical stress-relief slots, no exotic PCB materials. This will still have good learning curve, for one who only took apart instruments with these references, not built from scratch.


   
The schematic is straight-forward, and was explained and covered on EEVBlog many many times. Resistors of choice this time are custom PWW's or more expensive VPG BMF's. Opamp is recommended LT1013, which performance is well matched to design goals of LTZ module. Most of noise and drift on the output voltage will be due to own LTZ1000 instability, than used opamp.

Diode D3 provides reverse polarity protection, as no matter how careful you are, one day you will connected positive PSU output to the ground reference of the module. "Woops" would be rather sad portmortem note for the 55$ USD zener.

Layout design

Layout is done on two layer FR4 PCB with "Dangerous Red" solder mask color, as should be for the prototype. To give it a nice touch, exposed metal pads have electropated ENIG coating. This will let one to assemble board without soldering issues even after a year of storage.



Almost all components are thru-hole, which makes assembly a breeze, compared to much smaller and difficult to assemble xDevs.com KX module. This was fourth or fifth iteration of the layout, since this was my near entry to electronics design and manual layout. First iterations were not worthy of even the "Drunken spider" nickname, so it's best we don't see them here...



This PCB designed specifically for Hammond 1457K1202 metal enclosure, which is large enough to feature future upgrades with 10V scaling circuitry and other possible additions to design. It's important to select good form factor with some room to scale up, than scratching head how to jam in that extra capacitor needed.

Assembly notes

Other than traditionally long lead times for precision custom resistors, assembly and rest of components are straight forward. Total BOM list of components used with Digikey P/N is listed in table below.

Digikey Part Number Manufacturer Manufacturer Part Number Customer Ref Number Customer Description Quantity for module
N/A Ultrohm Plus N/A R1-R5 Custom made wirewound set 1
495-2479-1-ND EPCOS (TDK) B32529C1104J289 C1,C2,C4 0.1uF Cap Film 5% 100VDC 3
495-2489-1-ND EPCOS (TDK) B32529C1223K289 C3,C6,C7 22nF Cap Film 5% 100VDC 3
399-3905-1-ND KEMET T495X476K035ATE300 C5,C8 47uF Cap Tant. 10% 35V 2
1N4148FS-ND Fairchild/ON Semiconductor 1N4148 D1-D3 1N4148 Gen Purp DO35 3
PZT3904CT-ND Fairchild/ON Semiconductor PZT3904 Q1 Transistor NPN 1
1.00KXBK-ND Yageo MFR-25FBF52-1K R6 1k ohm 1/4w 1% 1
10.0KXBK-ND Yageo MFR-25FBF52-10K R7 10k 1/4w 1% 1
1.00MXBK-ND Yageo MFR-25FBF52-1M R8 1M 1
402KXBK-ND Yageo MFR-25FBF52-402K R9 402k ohm 1/4w 1% 1
10.0XBK-ND Yageo MFR-25FBF52-10R R10 10 ohm 1/4w 1% 1
BC2299-ND Vishay BC Components NTCLE203E3103FB0 R11 Thermistor 10k 1
LTZ1000CH#PBF-ND Linear Technology LTZ1000CH#PBF U1 Shunt Voltage Reference 1
LT1013ACN8#PBF-ND Linear Technology LT1013ACN8#PBF U2 IC OpAmp 1
HM1023-ND Hammond Manufacturing 1457K1202BK N/A Enclosure, Black 1
HM1022-ND Hammond Manufacturing 1457K1202 N/A Enclosure, Natural 1
1457K1202EBK-ND Hammond Manufacturing 1457K1202EBK N/A Enclosure, Black EMI Shielded 1
1457K1202E-ND Hammond Manufacturing 1457K1202E N/A Enclosure, Natural EMI Shielded 1

Linear parts were purchased directly from their website shop, custom resistors ordered directly, and rest of parts come from Digikey USA. The initial build was made using Riedon USR series resistors that were readily available. Several sets are on order from Edwin and a board will be assembled when they arrive.

Assembly passive components, such as capacitors, diodes, resistors first. LTZ, opamp and precision resistors install last after board is cleaned and checked, keeping soldering times minimal and keeping parts cool to reduce thermal stress.











MX Voltage reference module designed for education and hobby experimentation purposes only, which resulted in used components choices. If one need ultra-stable voltage reference for production device or practical equipment, very different approach would be required. This includes adding necessary polarity/overload/ESD protection circuits to the input and outputs of the reference, etc.

To reduce thermal stress to foil resistors, heatsink legs with small pliers or braid during soldering, so resistor body would not heat up as much. Excessive heat on resistive element will cause stress and possible hysteresis, which will require long-term recovery to original state afterwards. Make your soldering brief and short. If solder joint not good, wait till board and resistor cool down back to ambient and then try brief reflow for bad connection again. Do not keep iron tip heating up resistor lead longer than few seconds.

Initial data results

Connect power to the modules from quiet (electric noise wise) linear power supply, leave to settle for few days/weeks/months and then measure with best stability multimeter available.

Here are some measurements and test data completed on LTZ1000 module, with alternative resistors, as lead time exceeded amount of patience I had:

Test result description Test setup                   Datalog
 
  Data MX1, initial       3458A NPLC100, PD PSU +15V  MX1 Test 1 realtime log 


Or in graph form:



If you interested in this project build or have any questions, feel free to jump in comments. We also have more detailed and less traditional "KX LTZ1000-reference build":https://xdevs.com/article/kx-ref/ so be sure to check that one out as well.
 
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Offline cellularmitosis

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Re: MX Reference
« Reply #1 on: July 29, 2017, 11:24:01 am »
Fantastic write-up!  Always great to see a published result, and more participation in this pursuit.   :clap:
LTZs: KX FX MX CX PX Frank A9 QX
 

Offline ManateeMafia

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Re: MX Reference
« Reply #2 on: July 29, 2017, 12:31:54 pm »
Thanks, there was a lot of "influence" in this article from a fellow member.   :-+

The first rev has brought some ideas and changes to this layout and it is waiting based on test results. The free space on the next board will almost double.
I will be building a thermal chamber to house the board and run it through some testing similar to how others have tested their designs.
 

Offline Andreas

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Re: MX Reference
« Reply #3 on: July 29, 2017, 03:14:28 pm »
Hello,

just some annotations:

C7 would work much better if you place a series resistor (10K) between R1 and C7.

Do you plan to use the 805 style (6*13mm) of Edwins resistors or the smaller type that I have seen in other (KX-based) designs?

For the 805 style (as well as UPW50 or 8G16)  the footprint of the resistors should be minimum 700 mils for the hole distance.
For me it looks that you have only 600 mils.

With best regards

Andreas

 

Offline ManateeMafia

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Re: MX Reference
« Reply #4 on: July 29, 2017, 03:53:36 pm »
Good catch on the measurements. I realized the size error when the boards arrived. I am certain I used the measurements of the smaller resistors in the TC Measurements thread when creating the footprints. My library has been updated and the Rev A01 board layout has been shifted to accomodate the spacing.

For now, I have a second set of BMF resistors for another board. I would hate to bend any PWW resistors into an unforgiving shape.
 

Offline MK

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Re: MX Reference
« Reply #5 on: July 29, 2017, 03:56:06 pm »
If you place a BF245C at the output of the LT1013 then the diode d1 is not needed, as it becomes self starting, and as a bonus any extra current drawn does not heat up the lt1013, so that stays cooler so the offset does not change as much, and a current limit is built in.
just a suggestion for future versions
« Last Edit: July 29, 2017, 08:42:32 pm by MK »
 

Offline quarks

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Re: MX Reference
« Reply #6 on: July 29, 2017, 04:49:27 pm »
Thanks a lot for sharing
 

Offline TiN

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Re: MX Reference
« Reply #7 on: July 29, 2017, 08:47:02 pm »
Since I was trusted with hoarding full data, here's plot from the beginning:



It's too early to tell if there is problem with the build or not, need to log for another few weeks to be sure, if we reach the stability.
Just a reminder for newcomers that ppm-level measurement do NEED weeks of undisturbed test time, accuracy in setup, and above all - patience.

There is ACAL +/-0.3C for 3458A, to ensure measurement stability to own meter's LTZ level.
Orange line is BME280 temperature.

RAW DSV-file for the analytic minds.

Green curve is bonus, stability of Power Designs 2005A ovenized power supply. It set to output +15V to power MX module, and drifted up only +42 ppm since the beginning. Output noise is only few times worse than LTZ output itself, so I'm quite impressed with this vintage gem (expecting paybay prices to rise now!).

Another idea for the cal-clubs - have two level loops of rovers. Basic level loop with usual LM399/non-ovenized zener references etc for quick turnaround and comparisons for <6.5d meters and slow "Volt-nut grade" loop with LTZ1000/LTFLU references with 1 month/1 member turnaround time for 7.5+d meters :).
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Offline Kleinstein

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Re: MX Reference
« Reply #8 on: July 29, 2017, 09:45:05 pm »
The 120 Ohms resistor has quite a lot of long thin line in series. My feeling it the copper resistance might get relevant. There is also quite a large loop area - so unless in a good case, it might tend to pick up radio signals.

The JFET at the output of the OP, does not really offer an extra current limit. With a positive gate voltage the current can go higher then IDSS and the gate current (from the OP) can also add to this. So for a current limit it would need a resistor in the drain line and maybe at the gate.
 
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Offline Andreas

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Re: MX Reference
« Reply #9 on: July 30, 2017, 01:47:45 am »
or perhaps better only one resistor at the source like here?
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/?action=dlattach;attach=52101

With best regards

Andreas
 

Offline ManateeMafia

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Re: MX Reference
« Reply #10 on: July 30, 2017, 01:57:38 am »
@Andreas

Thanks. Are you referring to R13?
 

Offline Andreas

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Re: MX Reference
« Reply #11 on: July 30, 2017, 02:14:30 am »
no I refer to R18 (the 220 R)
R13 (47K) is only a "startup" resistor. (but of course optional when the BF245 is populated).

By the way the BF245 is no longer manufactured.
So you may want to use the SMD-version (SOT-23) BF545C instead.

With best regards

Andreas
 

Offline ManateeMafia

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Re: MX Reference
« Reply #12 on: July 30, 2017, 02:32:40 am »
I discovered that the BF245C was discontinued last night after MK recommended it. I will add the modifications to rev A01.
 

Online dr.diesel

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Re: MX Reference
« Reply #13 on: August 23, 2017, 06:43:01 am »
Bump, any updates?   ;D

I've got some spare Edwin resistors itchin for a purpose.

Offline TiN

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Re: MX Reference
« Reply #14 on: August 23, 2017, 01:43:32 pm »
Bump-bump, can't wait for tempco results
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Offline ManateeMafia

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Re: MX Reference
« Reply #15 on: August 23, 2017, 02:06:14 pm »
It has been burning in since it was built. Data collection has been problematic with a bad SD card on the RPi but the board has shown signs of settling.

I have some sets of resistors coming in from Edwin eventually so I hope to get the next rev made and tested before they arrive. I still have to make a few more changes before I send out the gerbers.
The next build will be with the LTZ1000A.

I have all the parts for the test chamber so I need to go ahead and assemble it.
 
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Offline RandallMcRee

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Re: MX Reference
« Reply #16 on: August 24, 2017, 02:11:06 am »
Very nice!
If this is a kicad project would it be possible to post the kicad project folder? Would help me (and presumably others) to jump-start our efforts.

Thanks,
Randy
 

Offline ManateeMafia

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Re: MX Reference
« Reply #17 on: August 24, 2017, 11:23:49 am »
Thanks.
This one is in Eagle CAD. One day I will make the jump but I don't have a need for it yet.

Once I get the next rev tested, I won't mind sharing what I have. I am certain there will changes based on what others have recommended and already done.
 

Offline TiN

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Re: MX Reference
« Reply #18 on: September 06, 2017, 02:40:24 pm »
I think she's ready for the tempco testing.

Attached plot of MX1 prototype stability over 1015 hours since power on.

We can see rather harsh voltage drop first few hundred hours. Last 100 hours it's stable ~0.4ppm.
Overall delta from fresh built state to stabilized is -7 ppm, give or take one.

"Jump" between 06/08 and 01/09 is due to data was sampled in other files. Reference was continuously powered, without interruptions.

15MB DSV-file with RAW data, if you want to fiddle with it.
« Last Edit: September 06, 2017, 02:48:53 pm by TiN »
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Offline Andreas

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Re: MX Reference
« Reply #19 on: September 07, 2017, 01:00:47 am »
Hello,

(I know I am repeating myself but)
those negative peaks of around -1.5 ppm at the end of the plot are most likely due to EMI-noise.
So I would place 2 * 100nF capacitors directly between pin 6+7 and 4+7 of the LTZ to reduce those peaks by around factor 10 before any other adjustments.

With best regards

Andreas
 

Offline Kleinstein

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Re: MX Reference
« Reply #20 on: September 07, 2017, 01:39:03 am »
One has to be careful with an additional capacitance between pins 4 and 7  - this can upset the voltage loop. It depends on the rest of the circuit if it can tolerate it.

There are other possible reasons for such peaks.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #21 on: September 07, 2017, 01:56:54 am »
You are on the right track - That data set is very typical of LTZ operation as the substrate crystal lattice relaxes post-soldering over the first few thousand hours.  Let it run another year or so (we go 18 months minimum  burn in for customers) to see the real story - most units will see some more improvement. 

As noted by Kleinstein - Adding capacitors:  Definitely not recommended by LT apps engineering: that can cause instability in certain situations depending on board thermal flow.. If you need to do that find out where external noise is coming from. HP3458a Vref board / datasheet is an example of a very good, proven implementation over hundreds of thousands of boards. We have never seen any need to add caps, but your application might be different.

You probably have an external noise emitter somewhere near your lab - for instance when the AC compressor kicks in, or somebody turns on the microwave down the hall...or your neighbor across the street turns on their microwave or a water well pump kicks on somewhere in the vicinity of your lab.  Something like that.  We have tracked down those types of noise sources that show up on an LTZ especially if the traces are too long and current loop antenna area too large. Generally that's not the LTZ circuit per se.
« Last Edit: September 07, 2017, 02:15:24 am by MisterDiodes »
 
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Offline MisterDiodes

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Re: MX Reference
« Reply #22 on: September 07, 2017, 02:05:25 am »
Something else to look around for:  We tracked down one noise source similar to what you see as coming from a Weller soldering iron station that was left on in the next room.  It was on a different mains circuit but still was causing enough EMI pollution every time it cycled off to disrupt a ppm level measure - the glitch would appear only every once in a while depending on where the turn-off event happened during the power mains cycle.  Very random.  This was the type of iron that has the thermo-mag switch in the hand piece.

You have to be on the lookout for anything like that.
 

Offline NANDBlog

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Re: MX Reference
« Reply #23 on: September 07, 2017, 02:36:11 am »
When I was doing PPM measurements, with a 3458A, I could tell, when the last person left the office down to the minute. And air conditioning added really a lot of noise on top of that. Also, I've tried to do the Tempco measurements when the device was cooling down. Because the heater is so random. Just insulate it, make part of the insulation adjustable, so it warms up or cools down in 8 hours, setup the measurement and leave.
And you can preatty much forget about measuring the tempco of a well built LTZ1000 circuit. You can maybe heat it up to 50 degrees,  based on the temp setpoint, and cool it down to 25. So I give them (management) a number, less than 0.3ppm/K. Or something. Buy me a better multimeter, then I will measure it for you.
 

Offline TiN

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Re: MX Reference
« Reply #24 on: September 07, 2017, 03:41:07 am »
I tend to agree on caps with MisterDiodes and Kleinstein. I tried them on one of my KX boards before, that didn't show any improvement. Best way to fix EMI/RFI issues is not to generate them, instead of battling with the consequences. However it's up to ManateeMafia, if he wishes to enter the murky waters...

Also one can see the temperature stability was not that great, due to use of portable AC unit in same room, as far I know. :)

Quote
So I give them (management) a number, less than 0.3ppm/K. Or something. Buy me a better multimeter, then I will measure it for you.
.
That's the good one, as there are not an awful lot of the voltmeters with tempco better than 0.3ppm/K  :-DD.

Tempco measurements on unverified/new LTZ1000-REF builds and designs however are useful, as a proof that design does not have some unexpected horrible terrible error or problem that give unexpectedly large TC. If tempco is buried in noise of own 3458A data on 20-30C span, that's good. If it's over 0.2ppm/K, then there is a problem that need fixing. This test data can be a good example of exactly that case.

 
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Offline ManateeMafia

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Re: MX Reference
« Reply #25 on: September 07, 2017, 03:59:52 am »
It looks like most of the peaks are during the waking hours though a few are not. It could be the microwave or other issues outside of A/C since mine runs 24/7 nearly year round. My noise generating activities usually include cups of coffee from the Keurig, reheating the coffee that got cold from doing emails, etc... It could also be the refrigerator. The pool pump is just outside the  wall and runs several hours a day. It is not speed controlled so I would expect something around 1200 and 2000. Also, the pump died and was not working during some of the noise.

The Power Designs ps has an internal heater that cycles on and off. It could also be a source but it cycles on/off a 100 or more times a day. I would think there would be more spikes but I wouldn't bet on it. It could be swapped with an HP supply for the next round of testing the tempco.

The board is located inside an aluminum enclosure and the end is sealed with tape where the wires come out. It is temporary and I think the cable needs to be swapped from Cat5 to STP with the shield tied to ground. The end cap also needs to be installed. There are definitely lots of things to tweak with this setup and I want to try modifying the setup before adding parts.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #26 on: September 07, 2017, 04:20:53 am »
You can also try some MuMetal or steel foil (or both) around the circuit, depending on the noise source.  Sometimes powering the 3458a thru a 1:1 toroidal transformer or adding additional chokes on the mains wiring helps attenuate power spikes running thru the meter.  Run the LTZ on battery power during tests if possible.  Keep all measure leads twisted, short as practical and shielded - that'll help.   Think "Minimal current loop antenna area", always at low ppm measures of anything.
 

Offline ManateeMafia

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Re: MX Reference
« Reply #27 on: September 07, 2017, 04:41:59 am »
I have some batteries charging for the imminent hurricane threat this weekend. I can certainly power the board with a 12V SLA and I will look at getting something on the 3458A mains side.

Could these spikes also be a sign of an issue with the 3458A internal reference? The meter is sealed and under service agreement. I could always test it with a 732B over a period of several hours on battery power.

I am planning on adding a 3D printed cover for the board. Is there any benefit with using a conductive coating on it similar to how Keithley did their 2001/2002 design? MG Super Shield comes to mind but I wouldn't use it unless it is worth the investment.
 

Offline cellularmitosis

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Re: MX Reference
« Reply #28 on: September 07, 2017, 05:19:36 am »
Is there any benefit with using a conductive coating on it

I'd say its worth a shot.  Here's an interesting video about using static-shielding bags to block EMI.



Searching for "faraday cage", etc on youtube is a bit of a frustrating experience.  90% of the material is aimed at doomsday preppers trying to save their walkie-talkies in the event of an EMP, and maybe one out of a thousand videos actually has any sort of empirical data (in the above video, he puts a spectrum analyzer inside of the bags!).
LTZs: KX FX MX CX PX Frank A9 QX
 

Offline texaspyro

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Re: MX Reference
« Reply #29 on: September 07, 2017, 05:28:50 am »
When I was doing PPM measurements, with a 3458A, I could tell, when the last person left the office down to the minute.

My sub-millidegree res temp logger could easily determine the number of bodies in a somewhat large-ish lab (well, actually body-equivalents... one particularly obese person counted as two people)  from the temperature gradient (BTW,  one body equals around a 100 watt heater).

I also have a double oven OCXO that can tell whenever I go to the kitchen to fetch a cool, refreshing, adult beverage... I can measure that thing to sub parts per trillion.  The temp logger clearly shows person entering room,  opening fridge,  closing fridge, leaving room.
 

Offline Kleinstein

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Re: MX Reference
« Reply #30 on: September 07, 2017, 06:06:07 am »
The spikes could be from EMI acting on the meter or the cables as well. If it is just a few rare events, I would not worry that much. Some EMI can be really strong - hard to protect against.

I remember an arc lamp at the university that stopped most computers (different brands) within about 6 meters around, when starting. It was really pain to make is EMC acceptable - they finally used cables inside copper tubing at some places.

The reference might profit from a steel enclosure to keep the magnetic field approximately constant. I Don't think it needs to be Mu metal. Just Mu metal might go into saturation to fast anyway. So it would be more like a second layer only. So far I have not seen how sensitive the LTZ1000 is to magnets. For EMI problems it is likely more the supply leads that bring in noise. Due to eddy currents even a normal metal case can shied AC magnetic fields at higher frequency.
 

Offline NANDBlog

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Re: MX Reference
« Reply #31 on: September 07, 2017, 06:12:56 am »
Quote
So I give them (management) a number, less than 0.3ppm/K. Or something. Buy me a better multimeter, then I will measure it for you.
.
That's the good one, as there are not an awful lot of the voltmeters with tempco better than 0.3ppm/K  :-DD.
I think eventually I measured a few. The methodology was:
Select two with <100mV difference in the output. Connect ground, place one in the temperature chamber, measure difference. I know, it is not perfect. Even then I wasnt sure if the 3458A wasnt drifting more than the reference.
My sub-millidegree res temp logger could easily determine the number of bodies in a somewhat large-ish lab (well, actually body-equivalents... one particularly obese person counted as two people)  from the temperature gradient (BTW,  one body equals around a 100 watt heater).
Interesting. Is it a PT1000 with some 24 bit ADC or is it much more sophisticated than that?
 

Offline MisterDiodes

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Re: MX Reference
« Reply #32 on: September 07, 2017, 07:01:07 am »
Some other tid-bits to keep an eye on if you don't monitor already:

1.) http://blitzortung.org/
Use the above map for your location and see if there is a lot of lightning activity within 1000 miles (sometimes even more).  A large bolt will generate a lot of lower freq (some kHz to low MHz) mag field energy that's going to make any susceptible current loop circuit "ring" a little depending on it's orientation - and that ripple in the mag field travels thousand of miles around (and through) the globe.  A steel and/or MuMetal shield (or combo) will help attenuate that stuff.  If you're getting lightning within a few hundred miles might as well give it up for the day if you're trying for quiet measures at low ppm - with lightning that close now you're getting jolts to the local electrostatic E-field which is going to show up everywhere.

You can get MuMetal foil even at Amazon these days, a little thicker like .005" is good.  .01" works well also.  A good -steel- box over your circuit will help and the combination with MuMetal will make a good broadband mag H-field shield.  Aluminum isn't doing too much for mag field shielding but does help with air drafts.

2.) http://www.spaceweather.com/

You're always going to have more or maybe less problems during solar CME's and such (sometimes the CME event deflects cosmic rays, sometimes not) .  Even a random cosmic ray flying through the LTZ crystal substrate will cause a momentary glitch every now and then (usually with a longer recovery tail as the crystal de-stresses again over time) - and not a lot can be done about that.   This is exactly what happens to higher capacity computer memory chips at random times - you'll suddenly see a flipped bit for no reason...but that's the evidence of a cosmic ray on one memory cell.  It will happen more the smaller and more dense the memory cells are, and on analog chips you'll see this effect on die with more dense-packed transistor counts.


« Last Edit: September 07, 2017, 07:22:00 am by MisterDiodes »
 
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Offline ManateeMafia

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Re: MX Reference
« Reply #33 on: September 07, 2017, 07:15:04 am »
Lightning? https://weather.weatherbug.com/news/Top-30-Cities-in-the-U-S-with-the-Most-Lightning  :scared:

I live ~50 miles south of Tampa. Not much to do except move. I have seen the most incredible lightning storms while driving through Cape Coral (100 mi south). There are a few of us forum members living in these areas.

If I wanted to experiment with Mu Metal, where is a good place to buy it? I see several sellers online, but I would guess some sellers are more trustworthy.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #34 on: September 07, 2017, 07:26:24 am »
Amazon is easy, but magnetic-shield.com works and a bunch of others.

Yes, summer afternoon are going to tend to show a bit more random noise on your data when there is a storm around - it just depends on how your current loops are going to ring when they get "tickled" with a bit of mag field ripple
 

Offline lukier

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Re: MX Reference
« Reply #35 on: September 07, 2017, 07:43:57 am »
You can get MuMetal foil even at Amazon these days, a little thicker like .005" is good.  .01" works well also. 

Is that good enough? I thought that after any mechanical processing (bending etc) mu-metal needs annealing which is done in hydrogen atmosphere. Heating pure hydrogen to 1000 deg C for hours is not something I would like to do at home.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #36 on: September 07, 2017, 07:47:39 am »
More noise sources that show up on LTZ measures, especially if you're in a rural area - just as some fun examples literally from "left field":

1) Electric fencers from the ranch down the road. 

2) When the rancher down the road starts arc-welding on his hay trailer... Ask me how I know.  This can even show up when the 3458a is running on a battery-back up inverter.

3) Somebody at the other end of the building operating a spot welder building some test leads.

I can go on all day....but you get the idea.  At low ppm and uV levels you look all around you for the noise sources.  Keep your eyes and ears open.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #37 on: September 07, 2017, 07:50:12 am »
You can get MuMetal foil even at Amazon these days, a little thicker like .005" is good.  .01" works well also. 

Is that good enough? I thought that after any mechanical processing (bending etc) mu-metal needs annealing which is done in hydrogen atmosphere. Heating pure hydrogen to 1000 deg C for hours is not something I would like to do at home.
Don't make -sharp- bends in it...You can still see if it has a reduction effect for your situation with somewhat gentle bends.  You might find that a good steel box alone works OK also - you just test and find out.
« Last Edit: September 07, 2017, 07:56:28 am by MisterDiodes »
 

Offline ManateeMafia

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Re: MX Reference
« Reply #38 on: September 07, 2017, 08:17:10 am »
custommagneticshielding.com has this product that doesn't require annealing after bending. http://custommagneticshielding.magneticshield.com/viewitems/mumetal-sheet-and-foil/mumetal-foil
They mention that severe forming does not apply. They failed to mention what they considered severe forming. Is it making sharp bends to form a cover? Looks like a CYA catch-all.

I would consider using the adhesive backed foil and apply it inside a printed cover.
 
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Offline branadic

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Re: MX Reference
« Reply #39 on: September 07, 2017, 08:22:05 am »
Quote
My sub-millidegree res temp logger could easily determine the number of bodies in a somewhat large-ish lab (well, actually body-equivalents... one particularly obese person counted as two people)  from the temperature gradient (BTW,  one body equals around a 100 watt heater).
Interesting. Is it a PT1000 with some 24 bit ADC or is it much more sophisticated than that?

This is also of interest for me. I'm sure it's not a PT1000 + 24bit ADC, there must be something more going on. I have a calibrated temperature measurement system based on PT100 + LTC2440 in ratiometric measurement mode (fast digital approach) with accuracy of 0.02°C over a range of -55 ... +150°. The resolution is sadly limited to 0.01°C by the manufacturer.

-branadic-
« Last Edit: September 07, 2017, 08:24:14 am by branadic »
Fluke 8050A | Prema 5000 | Prema 5017 SC | Advantest R6581D | GenRad 1434-G | Datron 4000A | Tek 2465A | VNWA2.x with TCXO upgrade and access to: Keysight 3458A, Keithley 2002, Prema 5017 SC, 34401A, 34410A, Keithley 2182A, HDO6054, Keysight 53230A and other goodies at work
 

Offline texaspyro

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Re: MX Reference
« Reply #40 on: September 07, 2017, 08:42:51 am »
Interesting. Is it a PT1000 with some 24 bit ADC or is it much more sophisticated than that?

It used a precision V/F converter and a synchronized gate frequency counter circuit.  The temp sensor could be anything that can be coaxed to generate a voltage.   In this case a precision micro thermistor (needed for fast temp response) calibrated (data by NIST) with a 24 degree polynomial curve fit.  Absolute accuracy was in the low milli degree range.   Resolution was in the micro degree range.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #41 on: September 07, 2017, 09:32:10 am »
custommagneticshielding.com has this product that doesn't require annealing after bending. http://custommagneticshielding.magneticshield.com/viewitems/mumetal-sheet-and-foil/mumetal-foil
They mention that severe forming does not apply. They failed to mention what they considered severe forming. Is it making sharp bends to form a cover? Looks like a CYA catch-all.

I would consider using the adhesive backed foil and apply it inside a printed cover.

..That might work, but really, bending a MuMetal sheet over a dowel isn't going to have a huge detrimental effect.  Usually a "severe" bend is where the inside bend radius is < 2X thickness, or where you take the metal well past its yield point on the outside corner (cracks appear).  Yes then you anneal.  For just playing with a test you get a sheet, put some gentle bends in it or make some corners over a 3/8" or  ~ 1/2" dowel, and see what happens. Keep it simple for first test...You'll know if it will help or not.  A lot depends on your PC board and how long are the traces, how big the current loops are, etc.

Or just try a steel box by itself and see if that helps enough.

Or just ignore the glitches if they don't really interfere with it's final application.  You're in a -very- lightning prone area, you might see the glitches fall off as storm season slows down.
 

Offline ManateeMafia

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Re: MX Reference
« Reply #42 on: September 07, 2017, 12:51:30 pm »
I will keep you recommendations in mind.

Also, thanks to everyone for their suggestions.

Speaking of CME, I just seen this article. http://www.foxnews.com/travel/2017/09/06/northern-lights-will-be-visible-across-much-continental-u-s.html
Anyone logging data in this part of the country?
 

Offline Dr. Frank

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Re: MX Reference
« Reply #43 on: September 07, 2017, 04:56:40 pm »
Hello,

to my experience, the initial drift is far too high, should be on the order of < 3 ppm over the whole year, at 50°C oven temperature.
Your measurements are an indication of hysteresis after overheating the LTZ1000.
No wonder, if you did not cool its legs during soldering, and maybe these exterior spikes once made the oven regulator to unlatch, heating it to > 100°C once.

I explicitly recommend Andreas design, especially the additional capacitors in parallel to each base-emitter diodes of the LTZ1000s transistors, that's C11 and C12 in his schematic.
It's definitely not true, that these capacitors affect the stability of the circuit, the opposite is correct.

I have added these two capacitors to my original prototype design from 2009 (it's in principle the datasheet circuit).
That greatly improved the EMI suppression, and the circuit shows no longer such spikes, even when a switch mode P.S.U. is present directly near the box.
Before that, the RF shifted the ovens set point and in turn the reference voltage (reversible change)

Also, I have built Andreas complete circuit once, initial drift was very low, and the EMI susceptibility was much lower than the original circuit... I also tested with switch mode P.S.U. and inductive loads (coffee machine test).


In the 3458A, the LTZ1000A is enclosed in the big analogue guard box, and neither its reference output, nor its supply rail is prone to the exterior, therefore it's not possible, that the 3458A may be affected by external disturbances. Whenever I measure 10V stability of 3458A versus my 5442B, there are absolutely no spikes visible, nor any voltage shifts, even when strong RF disturbance is present.


In stand-alone applications of the LTZ1000 or LTFLU, the disturbance mostly enters via the output connectors, or the P.S.U.
So a shielding / guard box is appropriate, also ferrite rings around the output jacks, like in the 732B. 


I'm just building another 4 new references, that's the successor of my prototype build, including supplements from Andreas design.
It's a single sided PCB, all leaded components, and it's compact, half size of the prototype, i.e. 50 x 80 mm, so four PCBs from one Euro-PCB.
That fits nicely in a small tuner box, which will be assembled inside another case.

T.C. compensation and 7=> 10V step-up amplifier will be trimmed next.

Anyhow, if you want to further improve your circuit, maybe you want to have a look on my solution.

Frank

« Last Edit: September 07, 2017, 05:34:42 pm by Dr. Frank »
 
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Offline Andreas

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Re: MX Reference
« Reply #44 on: September 07, 2017, 06:42:20 pm »

I'm just building another 4 new references, that's the successor of my prototype build, including supplements from Andreas design.
It's a single sided PCB, all leaded components, and it's compact, half size of the prototype, i.e. 50 x 80 mm, so four PCBs from one Euro-PCB.
That fits nicely in a small tuner box, which will be assembled inside another case.

T.C. compensation and 7=> 10V step-up amplifier will be trimmed next.

Anyhow, if you want to further improve your circuit, maybe you want to have a look on my solution.


looking forward for schematics and photos...

with best regards

Andreas
 

Offline hwj-d

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Re: MX Reference
« Reply #45 on: September 08, 2017, 12:40:51 am »
Quote
I'm just building another 4 new references, that's the successor of my prototype build, including supplements from Andreas design.
This awakes a great tension on what is announced now.  :)

Apologize that i post this here. I'm building a KX-reference too, powered it with simple 4x 18650 in series now about 2 weeks with short interruptions for changing akkus. In my opinion, now it is relatively konstant, but with TC ~1ppm per 1°C wich reflects the overall system with my 1 yr old 34461a. Sometimes, but mostly at the beginning of the overall test period, i have had relatively abrupt voltage rises or falls. But last time getting rare and <0.5 ppm, but noticable, if i touch my dmm for example. Screenshoot shows one caused putting in USB-stick just before. At irregular intervals i have this spikes too. In the near is wlan, router, i can't prevent. 

My  LTZ1000CH sits down on the board, soldering the legs shortly as possible with weller-iron and very little amount of solderpaste taking note of melting point. For measurement im using cat6-twisted with one side shielding to ground at the pcb and alu-housing. At the other side i have this hollow golden spring-suspended bananas, don't know their name.

Let it sit a couple of some more days or weeks, and then i'm lookin for someone in germany or maybe Tin, to kalibrate it. But i would build some more, with A-type LTZ too.

Quote
T.C. compensation and 7=> 10V step-up amplifier will be trimmed next.
One question to that, because it is the next logical step to build a useful reference. Most referencepoints of DAC's ADC's need 5V, 4096V or something like this but <=5V. Maybe it's useful to have one of these voltages directly driven from LTZ (second) buffer too, or must it be 10V first? As you can see, this is my first attempt to build such things...

Thanks and best regards
 

Offline TiN

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Re: MX Reference
« Reply #46 on: September 08, 2017, 12:46:03 am »
Quote
Quote
I'm just building another 4 new references, that's the successor of my prototype build, including supplements from Andreas design.
This awakes a great tension on what is announced now.  :)

I love it. Getting tired to see all these TiN's KXes with noisy choppers.  ::)
Way to go, Dr.Frank. My bank of LTZ's (7 various units) currently cold for 2nd month, but I'll be firing it up when you ready.
Is it some specific project you going to do (number of 4 refs sorta imply that, 3 working standards and one for sanity checks) or you just miss old days building LTZ1000 circuit?
 :clap:

I'm going to build few more LTZ soon enough too, need ultrastable 1mA reference..

hwj-d,

One of the possible options to get stable <5V voltage for ADC/DAC is to use LTC1043 to divide 7.2V in half, but that likely to need additional buffer to get driving strength into DAC/ADC.
There is no need to get 10V if you have own DAC/ADC system which can autocalibrate for random voltage.

Hopefully you have cap on top and bottom of LTZ.
« Last Edit: September 08, 2017, 12:54:28 am by TiN »
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Offline hwj-d

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Re: MX Reference
« Reply #47 on: September 08, 2017, 01:04:15 am »
Quote
Hopefully you have cap on top and bottom of LTZ.

Inside the box? Simple i have two of these cotton make-up pads, one under, one over the LTZ.
TiN, you need more than one thanks from me to you ...  :-+
« Last Edit: September 08, 2017, 01:06:08 am by hwj-d »
 

Online dr.diesel

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Re: MX Reference
« Reply #48 on: September 08, 2017, 01:08:24 am »
I'm just building another 4 new references, that's the successor of my prototype build, including supplements from Andreas design.
It's a single sided PCB, all leaded components, and it's compact, half size of the prototype, i.e. 50 x 80 mm, so four PCBs from one Euro-PCB.
That fits nicely in a small tuner box, which will be assembled inside another case.

Please share with us!  Possible group PCB purchase?

Offline texaspyro

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Re: MX Reference
« Reply #49 on: September 08, 2017, 01:26:34 am »
Inside the box? Simple i have two of these cotton make-up pads, one under, one over the LTZ.

I would be concerned with cotton absorbing and holding on to moisture from the air.  Would not ge a good thing if the cotton can contact components / circuitry.  Also make-up pads may be treated with who knows what.
 
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Offline hwj-d

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Re: MX Reference
« Reply #50 on: September 08, 2017, 01:44:37 am »
Quote
Also make-up pads may be treated with who knows what.
They are used, but i washed them.  :-DD

No, joking aside, thanks for the hint. You are right. Something others must go there. A little cap of foam not touching other parts will help much better.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #51 on: September 08, 2017, 01:46:41 am »

I explicitly recommend Andreas design, especially the additional capacitors in parallel to each base-emitter diodes of the LTZ1000s transistors, that's C11 and C12 in his schematic.
It's definitely not true, that these capacitors affect the stability of the circuit, the opposite is correct.

I have added these two capacitors to my original prototype design from 2009 (it's in principle the datasheet circuit).
That greatly improved the EMI suppression, and the circuit shows no longer such spikes, even when a switch mode P.S.U. is present directly near the box.
Before that, the RF shifted the ovens set point and in turn the reference voltage (reversible change)

I think I might have been misunderstood and didn't explain very well - apologies:  When I mentioned that caps can disturb the stability of the circuit, I should have said "degrades the stability of the LTZ die".

So:  If you're going to run the LTZ circuit exposed to noise: on a bare exposed PCB without of a proper shielded enclosure - in a way that it was never intended  - I can see that the extra caps on the LTZ pins would be a band-aid type of fix to hide some of those noise glitches.  The problem with that is that it can increase the magnitude of current noise -across the LTZ die.  The concern is the extra caps have provided a low-impedance reservoir of charge to allow higher LTZ peak noise current flow in response to external noise energy. Excess LTZ noise current is a direct contributor to degrading long term (> 5yr) stability of the LTZ die substrate, or cause the die to never quite achieve it's most relaxed, lowest energy crystalline state.  The LTZ zener makes it's own noise for sure, but you -really- don't want to do anything to add to that.

If you're running a switcher power supply next to the LTZ circuit and those caps made the glitches disappear - you've certainly hidden the voltage glitches, but the LTZ die still is exposed to the current noise, which is not recommended. This is the same reason LT explicitly does not recommend a '2057 current driver with it's very high input current spikes injected onto the LTZ die if you're not careful.

Before Andreas complains, I know: The circuit will still work with AZ amp and extra caps of course - but if you want best long term stability for decades adding caps and/or AZ amps is not the recommended best practice.  If you've tried every other way and the caps are the only solution for your application, then that's what you have to do...But there is probably something else wrong, like inadequate shielded enclosure or too much noise nearby in the lab.  We would never, ever run a switcher power supply next a sensitive circuit during a measure, we always go to battery power for serious measures - Period.  But that's how we have to do it for many more reasons than operating an LTZ.

Again - I've built several hundred compact boards, no slots or crop circles, and they go into well shielded enclosures, LTZ-A version soldered directly down onto the board, air-draft covered,  and no real stability issues for decades.  A working LTZ circuit will not generate random large output spikes on it's own.  The circuit -itself- should NOT need those extra caps if you build it correctly, as per LT recommendations.  They are the manufacturer and have a good source of long-term feedback data working with every major test equipment manufacturer - so I rely on their experience along with what I've witnessed for decades.  That's all I can offer.

If you find the extra caps are required for your application, then you do what is required.  For me I'd find the root cause of the noise first and deal with it at that level - remove the noise source or increase LTZ circuit shielding as required, quit making the board too big with long antenna traces, etc, etc, etc.  That way you know you're protecting the LTZ die as much as possible from injected noise energy.  You -never- want to throw caps at something like an LTZ to try to hide a voltage glitch that shouldn't be there if a better method is available.  The "let's throw capacitors at the circuit until the output noise goes way"  approach should be last line of defense for LTZ circuits, and usually indicates a bigger problem with the board design or enclosure - or lack of enclosure.

For sure, do what you need to that best works for your application - everyone has different needs.



 
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Offline Kleinstein

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Re: MX Reference
« Reply #52 on: September 08, 2017, 04:41:52 am »
At least in the simulation an extra capacitor at the transistor base to emitter does reduce the phase margin quite it bit. The circuit seems to be still stable, but less tolerant to capacitive loading at the output.

The more appropriate way is to keep RF noise away from the circuit, with something like shielding and filtering (e.g. ferrites) of the supply and output. For magnetic shielding the material for transformer cores is also rather effective - not as soft as MuMetall, but much cheaper and higher saturation. There should be no need for a super low field, mainly a voiding large variations. For annealing MuMetal one does not need pure hydrogen - often an argon hydrogen mixture is used, that is hardly flammable. As an other option there is amorphous / noncrystalline material with similar magnetic properties, but less sensitive to bending (and much more spring like).

I don't think the rather small current noise and even the current peaks at the input of an AZ OP would be a problem for the LTZ1000 chip. These are just minute changes and thus should not be enough to really disturb the silicon. I won't be too much concerned about the silicon itself. A point much more susceptible to aging is the glue to hold the chip. Also the bond-wires and thin metal layers are more susceptible.

The more problematic cases are large transients, when turning on a circuit that is only marginally stable. Also connecting a capacitive load can be problem. Ringing of the output voltage could easily stress the transistor from too much base current. Also excursions of the temperature could be a problem - the quadratic heater curve could cause rather high power spikes.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #53 on: September 08, 2017, 05:22:36 am »

I don't think the rather small current noise and even the current peaks at the input of an AZ OP would be a problem for the LTZ1000 chip. These are just minute changes and thus should not be enough to really disturb the silicon. I won't be too much concerned about the silicon itself.


You'd be surprised at how little current noise it takes over long term to alter the lattice strain level in the substrate to affect long term ppm-level aging effects on analog circuits.  That's what we're concerned about here. Even some added pA pulsing on a die the size of a LTZ will have an effect long term and you can even measure the strain changes even after a year acoustically and with atomic force 'scopes - that's why proper circuit shielding is -very- important for the best chance at max performance long term.  Of course temperature cycling plays a big role.  The wire bonds and epoxy have some ageing effect but the crystal lattice strain changes completely overwhelm those.

Some of my clients are building devices with 25yr / 35yr and 50yr guaranteed specs, so yes there's a LOT of very expensive research going into this problem.

What I can promise you:  Best practice==> Don't just hide the injected current noise, prevent noise from getting to LTZ in the first place.  As much as practical at least.





 
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Offline Dr. Frank

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Re: MX Reference
« Reply #54 on: September 08, 2017, 07:49:15 am »
I'd like to capture this thread, to publish in a quick 'n dirty manner my latest design.. it's a compressed version of my ~ 10 years old prototype, which works very fine.

The schematic is nothing spectacular, it's the original schematic @ 45°C, plus Andreas blocking capacitors. The buffer amplifier is an ICL 7650, or a LT1052, as displayed in the board drawing (these have a different connection for the chopper caps). It provides a buffered 10V output, and the direct LTZ voltage output, which can accessed better at +Ref_buf, to protect the reference circuit from unlatching the oven control.


The smart things are:
- the size, i.e. 50x80mm, so that the whole PCB fits into a tuner box
- single sided, so that all thermal junctions are on one side, and virtually at the same temperature.
- The layout is optimized for thermal symmetry, and all reference resistors and supplies are strung rigorously towards the two star points, -Ref and  +Ref.
The whole PCB will be thermally isolated by styrofoam inside the tuner box, the LTZ itself gets an additional foam cap
- all components are  through hole, only film capacitors, and one wire  bridge only required
- PCB design accepts both PWW and bulk metal film resistors (from AE, 0.2" / 5.08mm). VHP resistors will fit with a bit of bending of their leads.
- Pomona jacks will be assembled directly into the tuner box, which serves as Guard. Later, an outer case, connected to power ground, encloses the tuner box, and the jacks will be mounted directly through both case plates.
- All PWW resistors are T.C. characterized, and my 5 sets are matched to have a calculational over all T.C. of about -0.045ppm/K. That allows to trim the overall T.C. to zero by R10 ~ 820k. (200k ..1M)
- The first reference made with Andreas design has been trimmed to estimated ~ 0.02ppm/K and will serve as a bootstrap reference for further T.C. trimming, as the 3458A is not stable enough.
- as the LTZ reference voltage is fixed, 10V can be calibrated to a few ppm uncertainty by using any 6 1/2 digits  bench  DMM with ratio function, as the linearity should be sufficient.
- The T.C of the 10V output will be minimized by T.C. pairing of the 10K / 4k or 15k / 5k6 resistors.
Then, the residual T.C. will be characterized by means of the precision NTC, so that a correction can later be determined, for any internal temperature.
- 4 boards fit tightly on a standard Europe Card, so I etched and drilled the boards myself.

Frank

Edit 21.4.18: Updated schematic for LTC1052, C7, C8 tied to GND
« Last Edit: April 21, 2018, 04:37:22 pm by Dr. Frank »
 

Offline MisterDiodes

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Re: MX Reference
« Reply #55 on: September 08, 2017, 08:56:08 am »
I know it's not quite done - but don't forget a little access hole for your 10V pot!
 

Offline ManateeMafia

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Re: MX Reference
« Reply #56 on: September 08, 2017, 01:16:25 pm »
Dr. Frank,

Thanks for sharing. I originally tried for that form factor but I wanted it to fit in an extruded enclosure which resulted in much less space along the two edges that fit in the slots. I agree with the dual resistor option for whatever parts are available.
I look forward to your build and testing of the multiple references.

You are probably correct on the high initial drift of my reference. There could have been excessive heat applied at soldering. I hope this one isn't a wash but it certainly won't stop me from building more. I sense my Ultrohm Plus resistors are not far from being finished and I would like to be ready with the next version. All options will be considered and I will add places for the extra film caps but I won't initially install them. They will at least be optional and won't need to be bodged into the board.
 

Offline Dr. Frank

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Re: MX Reference
« Reply #57 on: September 08, 2017, 06:00:03 pm »

I think I might have been misunderstood and didn't explain very well - apologies:  When I mentioned that caps can disturb the stability of the circuit, I should have said "degrades the stability of the LTZ die".

So:  If you're going to run the LTZ circuit exposed to noise: on a bare exposed PCB without of a proper shielded enclosure - in a way that it was never intended  - I can see that the extra caps on the LTZ pins would be a band-aid type of fix to hide some of those noise glitches.  The problem with that is that it can increase the magnitude of current noise -across the LTZ die.  The concern is the extra caps have provided a low-impedance reservoir of charge to allow higher LTZ peak noise current flow in response to external noise energy. Excess LTZ noise current is a direct contributor to degrading long term (> 5yr) stability of the LTZ die substrate, or cause the die to never quite achieve it's most relaxed, lowest energy crystalline state.  The LTZ zener makes it's own noise for sure, but you -really- don't want to do anything to add to that.

If you're running a switcher power supply next to the LTZ circuit and those caps made the glitches disappear - you've certainly hidden the voltage glitches, but the LTZ die still is exposed to the current noise, which is not recommended. This is the same reason LT explicitly does not recommend a '2057 current driver with it's very high input current spikes injected onto the LTZ die if you're not careful.

Before Andreas complains, I know: The circuit will still work with AZ amp and extra caps of course - but if you want best long term stability for decades adding caps and/or AZ amps is not the recommended best practice.  If you've tried every other way and the caps are the only solution for your application, then that's what you have to do...But there is probably something else wrong, like inadequate shielded enclosure or too much noise nearby in the lab.  We would never, ever run a switcher power supply next a sensitive circuit during a measure, we always go to battery power for serious measures - Period.  But that's how we have to do it for many more reasons than operating an LTZ.

Again - I've built several hundred compact boards, no slots or crop circles, and they go into well shielded enclosures, LTZ-A version soldered directly down onto the board, air-draft covered,  and no real stability issues for decades.  A working LTZ circuit will not generate random large output spikes on it's own.  The circuit -itself- should NOT need those extra caps if you build it correctly, as per LT recommendations.  They are the manufacturer and have a good source of long-term feedback data working with every major test equipment manufacturer - so I rely on their experience along with what I've witnessed for decades.  That's all I can offer.

If you find the extra caps are required for your application, then you do what is required.  For me I'd find the root cause of the noise first and deal with it at that level - remove the noise source or increase LTZ circuit shielding as required, quit making the board too big with long antenna traces, etc, etc, etc.  That way you know you're protecting the LTZ die as much as possible from injected noise energy.  You -never- want to throw caps at something like an LTZ to try to hide a voltage glitch that shouldn't be there if a better method is available.  The "let's throw capacitors at the circuit until the output noise goes way"  approach should be last line of defense for LTZ circuits, and usually indicates a bigger problem with the board design or enclosure - or lack of enclosure.

For sure, do what you need to that best works for your application - everyone has different needs.

MisterDiodes,
that noise / die stress problem you have explained several times, I find it being a quite an interesting solid state physics aspect.

You're fully right, about avoiding or shielding external noise as much as possible.. I already tried that by moving the whole metrology stuff in our genuine German basement .. solid concrete walls and ceiling, with a lot of steel mesh inside, banned any switch mode P.S.U. in the lab, and the temperature change is very small, w/o any air con, which might produce mains glitches.
The prototype double reference has a single shielding box only, and it's running on mains by an external DC supply.

But anyhow, I still observed some glitches in the LTZ output, during 24h monitoring, which will very probably be caused by spikes in the mains supply, which can't be avoided in an ordinary building.
The LTZ circuit is also used in other mains powered applications, like in the 3458A, the Kei 2002, or in the Datron instruments 1281, 4910, and these are also exposed to these kinds of disturbances, w/o obvious penalty on their performance.

So it's too easy that you call for pure battery operation, or for avoidance of any external disturbance, all that is not practical nor realistic.

I assume that the double shielding / guarding, like in Flukes calibrators, or in higher grade bench DMMs, will improve the E.M.I. behavior. (But I'm still missing these shielded / guarded transformers.)


Anyhow, I still can not fully comprehend the magnitude of this degradation effect on the LTZ die by external spikes, in relation to the expected / typical -0.8ppm/year drift @ 45°C oven temperature. I have the gut feeling, that this might be a minor effect only, especially over long observation times of 5 years or more.

Please, would you mind to give some more details about the magnitude of this effect, and how you have experimentally determined / extracted this effect and its magnitude apart from the natural ageing by temperature.
Maybe you also have more practical recipes, how to avoid / shield external spikes.


Thanks a lot - Frank
 

Offline Dr. Frank

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Re: MX Reference
« Reply #58 on: September 08, 2017, 06:06:10 pm »
Dr. Frank,

Thanks for sharing. I originally tried for that form factor but I wanted it to fit in an extruded enclosure which resulted in much less space along the two edges that fit in the slots. I agree with the dual resistor option for whatever parts are available.
I look forward to your build and testing of the multiple references.

You are probably correct on the high initial drift of my reference. There could have been excessive heat applied at soldering. I hope this one isn't a wash but it certainly won't stop me from building more. I sense my Ultrohm Plus resistors are not far from being finished and I would like to be ready with the next version. All options will be considered and I will add places for the extra film caps but I won't initially install them. They will at least be optional and won't need to be bodged into the board.

At last, I recommend most urgently, that you also implement a buffer amplifier behind the LTZ output.
I encountered oven unlatching (to >100°C) on my prototype, during some experiments and in regular use, which caused severe sifts of the  output voltage.
I could bring it back within a few ppm of the original value by temperature cycling, but afterwards it took over a year to reach the same low annual drift again.
Therefore, this is the most important protection measure for this circuit.

Frank
 

Offline SvanGool

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Re: MX Reference
« Reply #59 on: September 08, 2017, 08:15:12 pm »
@Dr. Frank:

Finally a "volt-nut" application for my Altoids boxes (Maxim article)
Your PCB would fit it exactly :)


# Don't hurry, the past will wait. #
 

Offline Theboel

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Re: MX Reference
« Reply #60 on: September 08, 2017, 08:56:19 pm »
@ dr Frank,
I really interested about double shielding / guarding, like in Flukes calibrators but I have doubt about what exactly You talking about, can you tell / show a little bit about it.
 

Offline lukier

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Re: MX Reference
« Reply #61 on: September 08, 2017, 10:31:30 pm »
@ dr Frank,
I really interested about double shielding / guarding, like in Flukes calibrators but I have doubt about what exactly You talking about, can you tell / show a little bit about it.

Check Dr Frank's article on Fluke 5442A:
http://www.amplifier.cd/Test_Equipment/other/Fluke5442A.html

The whole case is mains earth shielded, but the inner cage (with the analog bits), mounted on a plastic insulated spacers, is grounded to the analog ground of the inguard floating power supply AFAIR.


The guarded transformed also has shields in between the primary and secondary.
 

Offline d-smes

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Re: MX Reference
« Reply #62 on: September 08, 2017, 11:03:38 pm »
@Dr. Frank-
Does your current regulator have any transients or ringing at turn-on?  I ask because of the C9 100nF loading on the output of the current regulator op-amp.  If the LT1013 open-loop output impedance is ~10 Ohms (unspecified, but not unreasonable), this plus the dynamic impedance of the 1N4148 form a pole with C9 that gives rise to considerable phase shift near the loop crossover point.
Edit-
Opps... I overlooked C3 being at 22nF (some LT schematics show 2nF) which brings your unity-gain crossover down to 20 kHz or so; well below where excess phase delay could lead to instability.
« Last Edit: September 09, 2017, 02:58:57 am by d-smes »
 

Offline 3roomlab

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Re: MX Reference
« Reply #63 on: September 09, 2017, 04:58:37 am »

The smart things are:
- single sided, so that all thermal junctions are on one side, and virtually at the same temperature.
- The layout is optimized for thermal symmetry, and all reference resistors and supplies are strung rigorously towards the two star points, -Ref and  +Ref.

wouldnt guard traces be important to the uA level traces ? and since traces have no mask? or it doesnt matter as long it is cleaned and sealed?
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Offline MisterDiodes

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Re: MX Reference
« Reply #64 on: September 09, 2017, 05:32:40 am »
Dr Frank:

To avoid getting in trouble with a customer -  and how bypass caps can affect crystal lattice aging I will have to leave it at:  The solid state physics theory that academia teaches is the first layer, and the reality layer of actually what happens in a production situation is something on a completely different plane.  This is an area where theory ends and and reality begins.  I am not trying to avoid your question but I am not at liberty to go into extreme detail in public.

I want to say overall you did a good job on your board - you have the LT1013 in there on a compact board, star + - Ref and avoided SMT caps !!  Those are the very best techniques for low ppm!!

Lets just say:  Do everything you can for shielding that LTZ FIRST and only resort to adding extra bypass caps if there is no other way for your lab.  Maybe you have hit the "No Other Way" point but here are some thoughts I -can- freely share with you.

1)  A working LTZ circuit (on a compact board) on it's own does not generate those spikes spontaneously, I promise.  I think you have figured out this is from an external event, and that probably means your circuit is not adequately shielded.

2) RE: Double shield boxes - Yes you are learning why all that "old boat anchor" equipment is designed with an inner steel box + outer steel box.  Your basement and your RF tuner box, Altoids boxes etc. aren't really the best for shielding against low to mid freq noise, but multiple steel layers and perhaps MuMetal is what we use as required.  You are still prone to H-field problems and junk on the power lines in your basement.  The big problem (for low ppm)  with any switcher bench power supply is somewhere in there I guarantee there is a "Y" capacitor that does a great job of delivering power line spikes directly to it's output - and I'm highly suspicious that you might be seeing that.

Not all sheet steel is the same, you can experiment with different alloys and thickness for best effect.  You need something of much more substantial thickness than a candy tin.

3) Outer steel box is ground, then teflon spacers, then inner steel box as Guard.   If required you add also MuMetal depending on what freq you're shielding against.  We have steel tables with a MuMetal sheet (to help prevent UN-intentional coupling transformers with wires laying on the steel) and and anti-stat on top of that just as a direct metal insulator, but use what works for your situation.

4) We use an voltage internal regulator just ahead of the LTZ circuit.  Learn from '732 and buffer the output.  Keep your input power leads quiet and choked / bypassed where they enter the in-guard box.

5) Keep all your test leads short, twisted and shielded.  Even in your initial test box you want to keep your output leads twisted.  You've soldered your leads onto the binding posts, but I know that's your fist test unit.  Normally you use a crimped copper + gold spade or ring lug on the inside connection of the binding post for better thermal performance and to keep the system balance with the wire or spades connected on the front side.

In short:  Look at any piece of old but good precision gear and learn from that.  You'll do the same.  No ventilation slots, keep the inner and outer boxes well isolated except maybe at one point, star grounds, keep any and all leads going into and out of the box choked, protected  and / or buffered. All of that -really- helps when you're at low ppm.

6) Battery power not practical?  We have whole rooms that are battery powered, or have a 24VDC / 48VDC and battery powered 24VAC distribution bus.  If you HAVE to have mains power, we use a 1:1 toroid with high-isolation windings.  Transformer winding shield is part of the guard.  This will help if you place it ahead of your 3458a.  Build a smaller version for powering your smaller circuits.

7) Everywhere keep an eye open for current loops that encompass any surface area.  Every one of those loops becomes an H-field antenna and EVERY P-N junction in your circuit becomes a demodulator.  That includes the leads on your desk - keep them short and twisted.

Once you try all of those things, then you might add the extra LTZ caps - but for maximum performance and the best odds at a successful aging die you want to try to leave those out. 









« Last Edit: September 09, 2017, 05:43:31 am by MisterDiodes »
 
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Offline Kleinstein

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Re: MX Reference
« Reply #65 on: September 09, 2017, 06:13:03 am »
....

wouldn't guard traces be important to the uA level traces ? and since traces have no mask? or it doesnt matter as long it is cleaned and sealed?
The currents in the LTZ1000 circuit are not that small. Even the collector current of the transistors is at around 100 µA, and these nodes are not that sensitive to small voltage variations. So there should be no need for guard traces. The heating and thus usually slightly elevated temperature keeps humidity and thus leakage low too.
 
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Offline Dr. Frank

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Re: MX Reference
« Reply #66 on: September 09, 2017, 07:49:01 am »

The smart things are:
- single sided, so that all thermal junctions are on one side, and virtually at the same temperature.
- The layout is optimized for thermal symmetry, and all reference resistors and supplies are strung rigorously towards the two star points, -Ref and  +Ref.

wouldnt guard traces be important to the uA level traces ? and since traces have no mask? or it doesnt matter as long it is cleaned and sealed?

guard traces are required at about pA of signal currents, the LTZ circuit has 100 uA to 4mA, so that's not necessary at all.

In complement, you'll need proper star points for the reference voltages, to mitigate or to avoid any voltage drops caused by unwanted consumers, like the oven current (20..40mA), and the OpAmp supply.
Therefore the negative supply for the LT1013 is so strangely bend around the neg reference star point.

Equivalently, the temperature distribution should be symmetric. As the LTZ1000 also generates some heat on the board, the two reference star points are equally designed, regarding shape and distance from the LTZ solder junctions. Due to equal heat transport, both star points should be on the same temperature, and therefore their solder joints generate the same, but opposite thermo-couple voltage - hopefully.

Also worth to mention, that the signal paths with highest currents are closest to the LTZ, and are the shortest, so first comes the 120 Ohm resistor with 4mA, then the 12k/1k divider with 500uA is next, then the 70k / 100uA.

Hope, that explains the details of my layout.

Frank
 
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Offline Andreas

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Re: MX Reference
« Reply #67 on: September 10, 2017, 02:30:48 am »
For sure, do what you need to that best works for your application - everyone has different needs.

Hello,

these are true words.

But it is an illusion that you can keep out all EMI-sources even with the best metal housings
if you have a wide open door i.e. the reference negative line.
(it makes no difference if the positive reference line is buffered or not).

The best chokes have around 10 dB EMI reduction. (perhaps up to 20 dB if you have luck and the right frequency).
A capacitor easyly dampens > 20 dB if the wires are kept short.

Of course it is good to do some measures to keep out the EMI-noise (thats why I have a double shielded design + battery supply).
But it gets perverse if I have to switch off all other devices in my house and the neighbour is sitting in 2 m distance with his mobile phone separated only by a normal stone wall. Or he is using his "green" switchmode power supplies because linear wall warts are de facto prohibited.

So If you can switch off all EMI-sources during a measurement, can use shielded transformers which are hard to get in single quantities then its the best solution for you. But I fear most hobby volt-nuts do not have a shielded EMI-cabinet with multiple stage filters for the power supply.

So my needs are clear:
- I do not want to have different output values in my "lab" independent of the gear that I or my neighbour is using.
- I also do not want to have different readings when I take my references into a unknown environment e.g. for calibration.

This is an area where theory ends and and reality begins. 

So what do we have:
- no official documents from LT (no application note, no design note)
- no official published paper
- no measurement values nor a test setup to verify the story

On the other side:
I have recorded the ageing from my first 2 references over more than 6 years
there is only unusual ageing against other published papers of LTZ1000
if I short the output of the unbuffered reference.
-> Shi(f)t happens if you are working.
So what you describe (if it really exists) can only be a 2nd order effect.

with best regards

Andreas


 
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Offline Kleinstein

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Re: MX Reference
« Reply #68 on: September 10, 2017, 05:12:46 am »
AFAIK chokes (ferrite beads) can have quite a good effect on EMI. Though it usually take both a choke and a capacitor of some kind together. EMI filtering can be tricky and sometimes works not as good as one hopes for - especially at a few bad frequencies.

At least the simulations show that adding capacitance at the transistors (base to emitter) reduces the phase margin. Together with capacitive loading this could go all the way to oscillation. So at least for frequencies in the 100 kHz-1MHz range the extra caps would do more harm than good. There already is a capacitance from collector to emitter that does slow down the transistor. A way to dampen RF signals would be more like adding a RC series combination in parallel to the existing 22 nF (or 2 nF) capacitor - within reasonable values this is more like an improvement on loop stability. If the circuit is not to sensitive to capacitive loading this also allows for better filtering at the output.

It should be possible to limit excursions of the heater for the case of a short at the output. It would not fully prevent anything bad, but could limit the temperature excursion to maybe 10 or 20 K. So the possible damage from failure in the heater loop would be limited.
 
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Offline Andreas

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Re: MX Reference
« Reply #69 on: September 10, 2017, 07:47:14 am »

At least the simulations show that adding capacitance at the transistors (base to emitter) reduces the phase margin. Together with capacitive loading this could go all the way to oscillation.


no.

you have overlooked that loop stability is maintained by a additional 10K (in series to negative OP-Amp input)  + 100nF across the Op-Amp
in Franks and my cirquit.

Opposite to your assumption the simulation shows more overshoot with the original AN86 cirquit than with Franks design.
Of course the original AN86 cirquit will oscillate with a load capacitance of more than some nF.

with best regards

Andreas




 

Offline 3roomlab

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Re: MX Reference
« Reply #70 on: September 10, 2017, 07:49:48 am »
You need something of much more substantial thickness than a candy tin.



would 3mm be considered thick in your lab?
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Offline MisterDiodes

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Re: MX Reference
« Reply #71 on: September 10, 2017, 08:27:04 am »
Yes, 1mm ~ 3mm would be a good place to start.  We use .05 or .0625" as a starting place if you're on imperial measurements.  The inner box can be slightly thinner, and maybe a single box is all you need.  We normally go for a 1/4" spacing between inner and outer but 1/8" would probably be OK.  It depends on what you're shielding from.  Aluminum will help some against E-fields but not much for mag field issues.

Also note:  If you're using PWW resistors it's important to at least have some minimal shielding around your circuit if you're near stronger H-fields - say an industrial area where you've got power mains in conduits, motors, ovens, etc.  Those resistors are wound with balanced windings but they will see some magnetic interference effects in a strong field unless you give them at least some protection - otherwise they will give you low noise performance.


 
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Offline Andreas

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Re: MX Reference
« Reply #72 on: September 10, 2017, 08:28:59 am »
would 3mm be considered thick in your lab?

Hello,

it all depends on frequency.

Shielding seems to be another dark chapter in knowledge.
You have to distinguish between electrical field and magnetic field.
For a low frequency (in near field) electrical field a thin foil (capacitive shield) is all you need for shielding.

For a magnetic field the skin effect is the mayor mechanism for shielding.
(the induced current gives a counter field which eliminates the original field).
So a magnetic shield is much more demanding than a electrical (low frequency field).

the formulas are given here for example for some materials:
http://www.chemandy.com/calculators/skin-effect-calculator.htm

Additional you have to know that all cirquits which are low ohmic
(below 377 Ohms which is the impedance of "free air" in far field)
are more sensitive to magnetic fields. (Our references are usually below 10 Ohms.)
At higher frequencies also a electrical field generates a magnetic component -> again the magnetic part is the more critical.


now you can calculate:
eg at 1 MHz and copper the skin effect depth is 65 um (that is where the field has gone down by 1/e or to around 36%)
So for a effective shielding you need a multiple (3-5) of this value to get a shielding by around 20-40 dB.
More than that is usually not doable with a practical housing because you cannot completely live without any openings for connectors etc.

If the frequency goes up by a factor of 100 the shielding can be a factor 10 thinner.
At 10kHz you will need 3-5 * 650 um or 2-3 mm with copper.

If you take a magnetic material a my of 50 (around that of tinned can) will give you together with a worse conductivity a useful thicknes of around 0.5 mm at 10kHz.
(you can get sheets of tinned steel e.g. at RS-Components but shurely there are cheaper sources).
http://de.rs-online.com/web/p/stahlblech/0682472/

If you have to go lower in frequency (eg 50-60 Hz) the thickness of the shield has to be increased if you do not want to use Mu-Metal.

The mayority of EMI emissions is in the 50-200 MHz range.
Simply because the connected antennas (power line cords) are in the 1-2 m length. (and have a resonance there as lambda/4 antenna).
Lightning has a typical current pulse of 8/20 us which corresponds to a main frequency at about 25kHz.

I am planning on adding a 3D printed cover for the board. Is there any benefit with using a conductive coating on it similar to how Keithley did their 2001/2002 design? MG Super Shield comes to mind but I wouldn't use it unless it is worth the investment.

you can calculate by yourself considering that any coating has a very bad conductivity against pure metal and the effective metal thickness is very thin.
So for a electrical / capacitive shield you can get good results if you have no other metal shield.

But for frequencies which are annoying its a waste of money.

with best regards

Andreas






« Last Edit: September 10, 2017, 08:46:59 am by Andreas »
 
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Offline MisterDiodes

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Re: MX Reference
« Reply #73 on: September 10, 2017, 10:03:21 am »
For sure, do what you need to that best works for your application - everyone has different needs.

Hello,

these are true words.

But it is an illusion that you can keep out all EMI-sources even with the best metal housings
if you have a wide open door i.e. the reference negative line.
(it makes no difference if the positive reference line is buffered or not).

The best chokes have around 10 dB EMI reduction. (perhaps up to 20 dB if you have luck and the right frequency).
A capacitor easyly dampens > 20 dB if the wires are kept short.

So If you can switch off all EMI-sources during a measurement, can use shielded transformers which are hard to get in single quantities then its the best solution for you. But I fear most hobby volt-nuts do not have a shielded EMI-cabinet with multiple stage filters for the power supply.

So my needs are clear:
- I do not want to have different output values in my "lab" independent of the gear that I or my neighbour is using.
- I also do not want to have different readings when I take my references into a unknown environment e.g. for calibration.

This is an area where theory ends and and reality begins. 

So what do we have:
- no official documents from LT (no application note, no design note)
- no official published paper
- no measurement values nor a test setup to verify the story

On the other side:
I have recorded the ageing from my first 2 references over more than 6 years
there is only unusual ageing against other published papers of LTZ1000
if I short the output of the unbuffered reference.
-> Shi(f)t happens if you are working.
So what you describe (if it really exists) can only be a 2nd order effect.

with best regards

Andreas

Andreas:

Have you ever heard of an NDA?  Non Disclosure Agreement.  You have to sign those when you work in the industry.  That's why I can't go into extreme detail here.  I am not a hobbyist, I make my living making sure high performance semiconductor devices perform to spec long after we're all long gone and forgotten.  From manufacturing to packaging to final application.

At this end we've been helping our customers build longevity into devices for over 30 (35?) years, across hundreds of LTZ's for our own custom test gear - and that know how also goes into billions of devices you use every day  - and if you build the LTZ circuit correctly and shield it correctly it will work without glitches, because that is how they have always worked for us - at least that's how they should work in spirit and in application.

Extra caps or AZ amps are not required nor desired, but you do what you need to do. I apologize that I can't share detailed data publicly regarding advanced die aging processes because that R&D is at the -great- expense to our customers, and I've already taken some heat trying to offer aid here on a public forum.  Obviously I should not have done that in your eyes.  Obviously the manufacturer doesn't know what they are doing either after millions of devices and centuries of man-hours testing - so why should anyone take LT's advice over yours?

Most of what I have learned comes from working with LT and National Semiconductor and HP/Agilent/Keysight and Avago and Broadcom and TI and General Instrument and Siliconix and Fairchild and xxx, xxx, xx directly over the years, person to person...not a datasheet, not a college textbook written by some inexperienced PhD who never built a PN junction from sand in the industry.  It comes from working hand's on directly with the people in the lab: learning from the current device processes + customer test feedback to make the next process even better.  If that sounds like some story...OK, I understand.

Even if you don't believe me, have some faith there is a great deal going on inside the LTZ die (and similar structures) that you're not aware of .  There are several interesting proprietary methods to measure internal current-induced strain / stress waves traveling across the die... and how that affects the die crystal physics down the road.    All I can offer you in a very condensed, non-datasheet rule of thumb for buried zeners:  If you want the die to be as stable as possible 5 / 10 / 20 yrs from now, then keep the current flow as quiet as possible.  To that end good engineering practice doesn't call for adding low-impedance caps or AZ amps across a buried zener structure IF there are other methods available.  As I pointed out before:  If there is no other way to add appropriate sheilding then add the caps! Add the AZ amps!! Whatever you like!!

Of course you can't control your neighbors, so that might limit what you can do for low ppm measures.

My intent is only to pass along some tid-bit information on how it's done in the real world.  EEVblog is a great forum for that.  I try to give you what little I can share here, and believe me or not - there is a ton of information here but that is about 1/10 of 1% of the iceberg tip of some of the strange effects we see when you start working directly with the die.

I understand a hobbyist has budget restraints, but that means you get creative and learn.  Take an old box with non-working gear, strip out the guts and make that a Vref housing for testing.  Learn how to track down EMI and defend against it.   Wind your own transformer for instance if you can't buy one - or remove one from some other piece of gear.  Learn how balanced chokes work and you can build those also - if you learn what/where your EMI is coming from you can build a very good defense for it, and usually at not a lot of cost.  You can wind wire around a form, right?   OF COURSE you can filter/ choke the positive AND negative power leads!!  OF COURSE buffering the output helps keep out noise.  Even better: for noisy environments deliver a differential output by driving both the + and - output lines, , use FORCE and SENSE techniques, etc.   At least that's been our long experience and all of those things work for us when required. 

In other words, if you're going to build a Vref like a 732a/ b, then build it like you mean business for low ppm - within the means of your budget of course.  Do what you have to, but I wouldn't give advice for circuit changes that might not be best approach until more robust shielding methods are tried.

Over n' Out.  Have fun folks!  Sorry if I've offended anyone here with advice &  tips that has kept our equipment running to spec for some 30+ years, including LTZ's.
« Last Edit: September 10, 2017, 10:39:36 am by MisterDiodes »
 
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Online dr.diesel

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Re: MX Reference
« Reply #74 on: September 10, 2017, 10:29:05 am »
Over n' Out.  Have fun folks!  Sorry if I've offended anyone here with advice &  tips that has kept our equipment running to spec for some 30+ years, including LTZ's.

Keep bringing it, we value your input, many thanks for all the time spent!   :-+

Offline lukier

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Re: MX Reference
« Reply #75 on: September 10, 2017, 10:54:06 am »
Thanks for your valuable insights and don't get offended, it is actually a good thing that fellow forum members would like to see a scientific evidence to back up the claims, but well bloody NDAs.

I am not a hobbyist, I make my living making sure high performance semiconductor devices perform to spec long after we're all long gone and forgotten.  From manufacturing to packaging to final application.

At this end we've been helping our customers build longevity into devices for over 30 (35?) years, across hundreds of LTZ's for our own custom test gear - and that know how also goes into billions of devices you use every day

In which industry you are working, if you don't mind me asking. I got curious because of these timescales, 30-35 years or exceeding human lifetime - we only do commercial ICs since the 60's, that is around 50 years and while some ICs are still around (741, 555) I doubt they generate much revenue nowadays and have significant market share.

Military tends to be on the conservative side, but even here 30-35 years I'd say it's much. But then military is not high volume.

What is high volume, is for example smartphones, now I think just above billion a year. But these things have very short lifespan, usually the of royalties for some chip/IP in a phone are mostly gone in 3 years.

I cannot think of any IC that is produced in billions and has many decades of lifetime - except jellybean parts probably.
 
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Offline MisterDiodes

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Re: MX Reference
« Reply #76 on: September 10, 2017, 11:19:53 am »
Thanks Diesel ,Lukier, etc.!

Regarding noise shielding:  You can measure some circuit noise at some frequency X, some duration Y and then convince yourself that some sheet metal of some thickness Z from some affected part and it's all coming from a line cord of some length L (like the length of the shielded / unsheilded mains cable doesn't have an effect as well as how many other loads on that power pole pig). 

About then you realize that the current loop in your circuit that's resonating at whatever damn freq it wants to, regardless of whatever external field ripple gave it the energy to start ringing. :-DD

It's been our experience that you -test- an enclosure and find what works best, but in general we start with .05" or .063" (or as I said 1~3mm but 2mm would be a good starting point). steel box, cold rolled steel.  That is the minimum, see if that is enough.  Second box as required.  MuMetal additional as required and yes we have had to do that on more than one occasion.  A -combination- of shielding works well in challenging environments.

Again: All wiring going in and out is protected as required - and if that means balanced chokes and output buffers or whatever extremes you need, that's what you need.  Not everyone needs it all, sometimes one box is fine with some simple ferrites.  Other times you don't get so lucky.

Find out by testing in your environment and what your application -actually- needs.  It could be that an occasional glitch has no effect on the end result, or it could be that the application demands perfection 24/7/365.

A very fun project is to build yourself a mag-loop antenna & amp for detecting lightning, and then use that to sniff around your lab and power mains and see if that matches up with your Vref glitches.  The only thing you have to build is a loop of wire and an amp, and it'll resonate at it's own personal freq. when you find a noise source.  Just remeber that's the res. freq of the antenna, not necessary the noise source.   The blitzortung.org site, etc. has some great ideas and circuit ideas -  Something like that basic amp design & filter is a good starting point to build yourself a simple loop antenna EMI sniffer.

Lukier:  Our company builds and maintains semiconductor process equipment <exact description redacted for privacy>.  The owner started in 1965, Silicon Valley, I've been on board since the late 70's. Started as a young smart ass with a degree and an equation for everything - and then finally figured out what I really didn't know by listening to the older staff scientists who knew what they were doing.
« Last Edit: September 10, 2017, 11:49:16 am by MisterDiodes »
 
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Offline hwj-d

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Re: MX Reference
« Reply #77 on: September 10, 2017, 11:26:00 am »
Over n' Out.  Have fun folks!  Sorry if I've offended anyone here with advice &  tips that has kept our equipment running to spec for some 30+ years, including LTZ's.
I'm at the other end, as hobbyist, that's not so vital to me, BUT, that's extremely grounded to me too, what you are talking about, and i understand every word that you mention here (ok, after some learning effect). I think, your input here is one of the best, and you're right! Others too, for sure. I understand, that must be very frustrating, if someone cannot go too deep in details to prove oneself, if others do because, maybe, they're not so legally bounded. But your sometimes opposite meaning is really an eyeopener to me and others, so, please keep doing that.  :-+ :-+

Thanks, thanks to all
 
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Offline MisterDiodes

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Re: MX Reference
« Reply #78 on: September 10, 2017, 12:02:12 pm »
Thanks. I try to give as much detail as I can but I really have to stay out of trouble with customers, and I have to not mention the company name.  The end result here is sometimes it looks like a wishy-washy answer pulled out of my butt I know - and sometimes it's better if I just stay quiet.  I have to respect our customer's respect for keeping their R & D private; so sometimes I just have to observe here, and toss out a tidbit only if it's common helpful knowledge.



 
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Offline MisterDiodes

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Re: MX Reference
« Reply #79 on: September 10, 2017, 02:57:58 pm »
Dr. Frank:

I was looking at the photo of your setup, and I wasn't sure where your analog signal ground is referenced to Earth.  You probably already did this but just in case -   In your basement lab, do you have good access to a buried metal water pipe leading outside that you can use as a ground point? - or some way to setup a separate dedicated lab signal ground?  If you're using the ground from the power mains, that can be a source of noise issues.

We have a separate dedicated lab signal ground (we like the lightning rod downlead for low impedance / good bandwidth) that travels outside (and shares no close routing with any power mains wiring) to its very own dedicated ground rod.  The idea is that your lab test area has it's own dedicated clean analog lab signal earth ground point with no power mains wiring bundled in close proximity. 

At least that's what we do here to help prevent some of those ground-coupled power line noise spikes sneaking into a test.



 
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Offline Kleinstein

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Re: MX Reference
« Reply #80 on: September 10, 2017, 06:48:14 pm »

At least the simulations show that adding capacitance at the transistors (base to emitter) reduces the phase margin. Together with capacitive loading this could go all the way to oscillation.


no.

you have overlooked that loop stability is maintained by a additional 10K (in series to negative OP-Amp input)  + 100nF across the Op-Amp
in Franks and my cirquit.

Opposite to your assumption the simulation shows more overshoot with the original AN86 cirquit than with Franks design.
Of course the original AN86 cirquit will oscillate with a load capacitance of more than some nF.

with best regards

Andreas

The modified circuit with the additional 10 K and local feedback around the OP is a different thing. Here these modifications are effective in improving stability even with capacitive load. But even in this circuit the capacitor at the transistor is reducing the phase margin, though only slightly. Without the extra 10 K and capacitor in direct feedback, adding a capacitor to the transistor brings it close to the edge - thus not a good idea.

In my opinion the modified circuit looks like a good idea - still not convinced of the capacitor at the transistor.
 

Offline TiN

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Re: MX Reference
« Reply #81 on: September 10, 2017, 08:07:53 pm »
Thanks. I try to give as much detail as I can but I really have to stay out of trouble with customers, and I have to not mention the company name.  The end result here is sometimes it looks like a wishy-washy answer pulled out of my butt I know - and sometimes it's better if I just stay quiet.  I have to respect our customer's respect for keeping their R & D private; so sometimes I just have to observe here, and toss out a tidbit only if it's common helpful knowledge.

We indeed value all the feedback. I honestly didn't even expect to see any feedback from people who actually build and use ultrastable references like LTZ circuit for real applications, due to NDA's, R&D NRE agreements, legals, etc. So you have my respect on trying to share what's possible.

Hobbyist which built 1-5-10 references for fun rarely even have ability or dedication to measure the results over decade, maintaining constant environment (temperature, RF, EMI, noise, etc) over all that time. It's just amount of man-hours required to get a proof. Same reason why HPAK/Keithley/Fluke can provide those tight specs on their instruments and charge $-$-$-$-$ for it. Surely, some big customers can get even special versions of 732, with even tighter spec if they need so, for attached $-$-$-$-$-$ :).

I'm bit lucky as my daily professional field is much far from precision analog design, that I'm not bound by NDA's on these topics so far. (yet?).  :phew:

Quote
I understand a hobbyist has budget restraints, but that means you get creative and learn.
Or move the restraints, but that does little good actually, leading to equipment hoarding issues with zero knowledge learned.  :-DD
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Offline ManateeMafia

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Re: MX Reference
« Reply #82 on: September 10, 2017, 10:50:36 pm »
Or move the restraints, but that does little good actually, leading to equipment hoarding issues with zero knowledge learned.  :-DD

I am making a storm shelter out of my gear. I hope I don't have to find out if any of it floats.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #83 on: September 10, 2017, 11:43:33 pm »
Or move the restraints, but that does little good actually, leading to equipment hoarding issues with zero knowledge learned.  :-DD

I am making a storm shelter out of my gear. I hope I don't have to find out if any of it floats.

Good wishes to you Manatee...That's a bit of a breeze blowing your way.  We'll send you good thoughts and hope you're on a little higher ground.
 

Offline Andreas

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Re: MX Reference
« Reply #84 on: September 10, 2017, 11:57:42 pm »
Hello MisterDiodes,

don´t take it personally.

Have you ever heard of an NDA?

Of course, but that´s not the way a public forum is working.
And usually on important things the NDA is running out as soon as the patend is claimed.
If you say throwing capacitors is a "no no" you should be able to explain that.

I get the impression that with your statements some of the members are insecured somewhat.
And I can not estimate if it is really worth that.

with best regards

Andreas
 

Offline MisterDiodes

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Re: MX Reference
« Reply #85 on: September 11, 2017, 12:01:25 am »

Or move the restraints, but that does little good actually, leading to equipment hoarding issues with zero knowledge learned.  :-DD


Ha!  Good one...  At least you have some really good boxes to study, and then you try to build you're own version you come to that moment: "Oh - That's why they did it that way!"  THEN you appreciate the details that went into the design of very high quality equipment.

So you're not hoarding, you just bought some very good educational class time - and a much better value than a lot of university courses.  Sometimes you don't realize right away when you first pop the lid for a peek inside.

It's true:  If you're into low ppm / high stability / high reliability, it's not for the faint of heart when it comes to spending time & money on equipment for what amounts to something less exciting than watching paint dry - if it works correctly.

"Nothing happened this decade...and I'm thrilled!! Yay!!"   Volt-"nuts" indeed.  :-DD

It's all fun.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #86 on: September 11, 2017, 12:15:29 am »
Hello MisterDiodes,

don´t take it personally.

Have you ever heard of an NDA?

Of course, but that´s not the way a public forum is working.
And usually on important things the NDA is running out as soon as the patend is claimed.
If you say throwing capacitors is a "no no" you should be able to explain that.

I get the impression that with your statements some of the members are insecured somewhat.
And I can not estimate if it is really worth that.

with best regards

Andreas

If I gave you the complete explanation that would cross over into current R & D onto die aging / stability / reliability.  So take that little gem of info or not, it's entirely up to you how you want your LTZ to age or how you want it to drift long term.

What I do invite you to do is to think about the effect of converting what you see as a small voltage noise spike into a surprisingly large current noise spike flowing across a zener...  That noise spike is the symptom of a bigger problem.

You're treating the LTZ zener like it's a digital IC or an op-amp power rail.  There are times you don't ever want to heavily bypass a PN analog junction, and this is one of them.

I'll leave it at that.
 

Offline TiN

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Re: MX Reference
« Reply #87 on: September 11, 2017, 12:51:36 am »
The way NDA works is that you get resources and honor to acquired knowledge to be kept private between the involved parties.
Also the way public forum works is anything published is up to poster discretion. Be it accurate or not, complete or not, up to the poster, and nobody else. It's also better be safe than sorry, and stay out of sensitive subjects, no matter how much we would love to see the data.

Surely MD will be happy to offer you full details and certificates and test data results, if you become his customer and pay all that research and time, and likely you would wish that data not to be freely distributed for anyone else, undermining your product advantage. :)

I'd suggest we leave this OT at this, and move on  :-DMM.
After all any of our data or results posted (including yours, with all due respect) are yet to be proven to be the only "hard rock solid truth". I don't see issues with my AZ modules without extra caps, nor I see any improvement with the caps. But that only means it works for my specific case, in my specific environment, and with my equipment, and those few references for hobby voltnutting purposes only.

« Last Edit: September 11, 2017, 12:53:13 am by TiN »
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Offline Echo88

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Re: MX Reference
« Reply #88 on: September 11, 2017, 01:21:13 am »
Back to the topic: Does anyone know if our diligent chinese pals on the http://bbs.38hot.net-forum have already built working power-supply/battery-pcbs which resemble those working in the 732B? This way i wouldnt have to layout one myself.
 

Offline MisterDiodes

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Re: MX Reference
« Reply #89 on: September 11, 2017, 03:47:45 am »
The way NDA works is that you get resources and honor to acquired knowledge to be kept private between the involved parties.
Also the way public forum works is anything published is up to poster discretion. Be it accurate or not, complete or not, up to the poster, and nobody else. It's also better be safe than sorry, and stay out of sensitive subjects, no matter how much we would love to see the data.

Surely MD will be happy to offer you full details and certificates and test data results, if you become his customer and pay all that research and time, and likely you would wish that data not to be freely distributed for anyone else, undermining your product advantage. :)

I'd suggest we leave this OT at this, and move on  :-DMM.
After all any of our data or results posted (including yours, with all due respect) are yet to be proven to be the only "hard rock solid truth". I don't see issues with my AZ modules without extra caps, nor I see any improvement with the caps. But that only means it works for my specific case, in my specific environment, and with my equipment, and those few references for hobby voltnutting purposes only.

Exactly.

Back to LTZs, hard rock data style:  When you build your own circuit, take a look at at the datasheet circuit and HP3458a implementation.  Tell me what you -don't- see added, and tell me why.  Sometimes its what you -don't- see is the lesson.  Pay attention to that also.

When you understand that an AZ amp does absolutely -nothing- for an LTZ circuit except increase current noise in the zener (Vofs in the LT1013 has no effect on output), then it'll dawn on you what's happening with added direct-connected bypass caps energy storage (especially see C5 and C9 on Dr. Franks circuit, which are covering up the much bigger problem).  You've taken the same problem with the AZ amp current noise and magnified it to an even more glorious scale.  The voltage glitch is the LTZ crying out for you to help, but strapping a pillow over its face is not the correct answer.

Again: the Dr. Frank / Andreas circuit will work fine - at least for a while, considering what you're doing to the zener.   If you did this for a real company:  If you shipped that circuit out in production you'd certainly see some customer returns after a while, and some would be fine.  And then you'd be wondering why.

I have given you every clue you need right here.  The rest of discovery I leave to you.  The datasheet circuit will be all you'll need if you're building LTZ -and- shielding correctly, and that is my 85+ pages of EEVBlog summary.  That's what has been proven for decades over some hundreds of thousands / millions of devices and centuries of man-hour development work if you are after the hard facts.

That's how we do it here anyway.  It is a very good circuit that works fine.  A much much more interesting project is the conversion from 7.2VDC down to another voltage in a ppm-stable way.  If you ned an ultra-stable 10V, stick with a 732...you'll be done much faster.

 
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Offline NANDBlog

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Re: MX Reference
« Reply #90 on: September 11, 2017, 03:56:00 am »
The way NDA works is that you get resources and honor to acquired knowledge to be kept private between the involved parties.
Also the way public forum works is anything published is up to poster discretion. Be it accurate or not, complete or not, up to the poster, and nobody else. It's also better be safe than sorry, and stay out of sensitive subjects, no matter how much we would love to see the data.

Surely MD will be happy to offer you full details and certificates and test data results, if you become his customer and pay all that research and time, and likely you would wish that data not to be freely distributed for anyone else, undermining your product advantage. :)

I'd suggest we leave this OT at this, and move on  :-DMM.
After all any of our data or results posted (including yours, with all due respect) are yet to be proven to be the only "hard rock solid truth". I don't see issues with my AZ modules without extra caps, nor I see any improvement with the caps. But that only means it works for my specific case, in my specific environment, and with my equipment, and those few references for hobby voltnutting purposes only.
Yes thank you. I also made a design for the LTZ1000, few hundred was built, tested, calibrated and shipped.  And I had a lot of data about it, a lot that I produced myself. Under NDA. And though my NDA is not applicable anymore, I dont feel like I should be going around and spreading the info that I have. I could from memory, re-draw the schematic, aproximate the PCB, and open source it, or even start selling it.

But it is bad karma. I was payed to do it, and I dont own the IP. It was well over a year ago, and my NDA is not even valid anymore. And the product wasnt even a voltage reference, it was a very expensive test equipment, with a lot of other parts, in fact this was a only a tiny tiny part of it.

So that is why you would only hear tales from us, who worked with the LTZ1000 at work. Bad karma.
 
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Offline MisterDiodes

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Re: MX Reference
« Reply #91 on: September 11, 2017, 04:07:37 am »
A lot of people don't realize that you sign an NDA just to step into the back room of someplace like Broadcom.  Or Keysight. Or wherever.  It has nothing to do with patents, and nothing to do with a timeframe.

Many NDA's have no expiration date - so you sign it, and take the knowledge to your grave. 

 

Online tggzzz

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Re: MX Reference
« Reply #92 on: September 11, 2017, 04:18:39 am »
While in HPLabs, we were specifically instructed never to sign NDAs. Why? Because, when push comes to shove, NDAs didn't have much value but they did open the doors for the other party to sue HP at a later date.

No doubt NDAs were occasionally signed, but to do so would have been a heavyweight business decision.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline MisterDiodes

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Re: MX Reference
« Reply #93 on: September 11, 2017, 04:22:55 am »
BY the way - since this is apparently the LTZ100 thread now - and somebody might have already posted this bit but to be formal:

The LTZ1000 datasheet Page 6 that is currently online right now - that cap shown on Non-inverting input of '1013 is .022uF...NOT .002 uF as shown.

You will see on every other datatsheet for LTZ that cap as .02uF, and even on the first page of current sheet it is correct.

That error crept in 11/2015 when they made some pinout corrections to the negative circuit reference. This datasheet will get corrected at some point, but everyone who knows the LTZ knows what cap value for that position.

They do not really want .002uF in there as that leads to instability, but the circuit will work with that value.  Just use the correct .022uF value.

Confirmed with LT apps engineering in a phone call 3/16 and a subsequent email, for hard-core-facts record-keeping.
 
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Offline lukier

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Re: MX Reference
« Reply #94 on: September 11, 2017, 09:23:08 pm »
The LTZ1000 datasheet Page 6 that is currently online right now - that cap shown on Non-inverting input of '1013 is .022uF...NOT .002 uF as shown.

Thanks! When I was ordering bits and bobs for my references from Digikey I, of course, ordered 2.2nF. Now I need to get 22nF. Luckily, I'm slow with my DIY projects so I didn't even design the boards yet.

Side question to fellow LTZ enthusiasts. How much value there is in ovenizing (i.e. not only insulating but putting a heater as well) the reference board? In Fluke references it is obvious as LTFLU doesn't have a heater, but LTZ1000 does and the surrounding resistors have something like 0.01 factor impact on the output value. I'm planning to have 13k/1k resistor ratio, therefore AFAIR 55 deg C so the oven temperature would need to be be below that to allow LTZ heater operation and that would get close to room temperatures in the summer.

I'm asking because originally I was planning to design LTZ + LT1013 + other page 6 bits mini-module, very similar to shape and form factor to the 3458A reference, but inside a case filled with Spacetherm insulator. No oven. Then on another mini-module I was planning to have 10V step up and VHP202Z 10k (I want this modules to be 10V and 10kOhm standards) - this mini module would be similarly insulated, but also ovenized to 55 deg C, this could age the resistors slightly faster but would provide better stability. Overnizing only the second mini-module would bring the power consumption down which is important as I want this to be powered by 4x18650 cells.

I've ordered VHD200 10k/25k dividers from eBay for the 10V step up and I'm thinking if I should provide any means of adjustment at all. This divider in the non-inverting configuration would bring LTZ voltage 6.9-7.5V to 9.66-10.5V, close enough for any equipment that would need ~10V and adding extra parallel/series resistors/trimmers could be detrimental, I will be happy with just this divider and whatever value it settles onto and let it age. But on the other hand adjustment capability would allow nulling to another 10V standard, instead of just measuring my standard with e.g. 3458A which is less accurate. Any opinions?
 

Offline Dr. Frank

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Re: MX Reference
« Reply #95 on: September 11, 2017, 10:13:55 pm »
Side question to fellow LTZ enthusiasts. How much value there is in ovenizing (i.e. not only insulating but putting a heater as well) the reference board? In Fluke references it is obvious as LTFLU doesn't have a heater, but LTZ1000 does and the surrounding resistors have something like 0.01 factor impact on the output value. I'm planning to have 13k/1k resistor ratio, therefore AFAIR 55 deg C so the oven temperature would need to be be below that to allow LTZ heater operation and that would get close to room temperatures in the summer.

I'm asking because originally I was planning to design LTZ + LT1013 + other page 6 bits mini-module, very similar to shape and form factor to the 3458A reference, but inside a case filled with Spacetherm insulator. No oven. Then on another mini-module I was planning to have 10V step up and VHP202Z 10k (I want this modules to be 10V and 10kOhm standards) - this mini module would be similarly insulated, but also ovenized to 55 deg C, this could age the resistors slightly faster but would provide better stability. Overnizing only the second mini-module would bring the power consumption down which is important as I want this to be powered by 4x18650 cells.

I've ordered VHD200 10k/25k dividers from eBay for the 10V step up and I'm thinking if I should provide any means of adjustment at all. This divider in the non-inverting configuration would bring LTZ voltage 6.9-7.5V to 9.66-10.5V, close enough for any equipment that would need ~10V and adding extra parallel/series resistors/trimmers could be detrimental, I will be happy with just this divider and whatever value it settles onto and let it age. But on the other hand adjustment capability would allow nulling to another 10V standard, instead of just measuring my standard with e.g. 3458A which is less accurate. Any opinions?

It makes no sense to ovenize the LTZ1000 board.
That would only lead to problems with the LTZ1000 own oven stability.
And as you already mentioned, the influence from the 5 precision resistors is only 1/75 , down to 1/1000.
Typically, you'll end up with about +/-0.05ppm/K overall.
And it's much easier, to use compensation methods to bring the overall T.C. to around 0.01ppm/K.

If you put the step up resistors in an isolated oven, that would make sense, as this is the main feature of the 732A/B circuits, to have a low T.C. for the 10V output also.
Obviously, you're planning to build the very same solution/form factor, which I have published here, a few days ago.
For different LTZ voltages, there are several 'magic' ratio values to trim to near 10.000V, which are easily obtainable in PWW or BMF, like 4k over 10k, and 5k6 over 15k.
I decided to provide a trimming to exactly 10.000V, for usage in a precision Bridge configuration, where it's more convenient to have a 'round' value.
The trimming does not affect the stability of this amplifier circuit, and you can also omit that and just live with odd values.

If you don't design in a trimming, you'll have no choice, afterwards.

Frank
« Last Edit: September 11, 2017, 10:25:22 pm by Dr. Frank »
 
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Offline NANDBlog

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Re: MX Reference
« Reply #96 on: September 11, 2017, 10:55:01 pm »
Well, you can still order a custom resistor from Vishay Foil. In fact, if I would be volt nutting at home, I would do that. The S series has some 0.2ppm tracking, and the two resistors in one package. It will be for sure expensive, about an LTZ1000 brand new. They trim it to 6 digit. Just build the reference, burn it in, measure the value, and custom order the precision divider to get the 1-2ppm accurate  10V reference. If you really really want, trim that with a million turns pot.
 

Offline lukier

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Re: MX Reference
« Reply #97 on: September 11, 2017, 11:00:41 pm »
It makes no sense to ovenize the LTZ1000 board.
That would only lead to problems with the LTZ1000 own oven stability.
And as you already mentioned, the influence from the 5 precision resistors is only 1/75 , down to 1/1000.
Typically, you'll end up with about +/-0.05ppm/K overall.
And it's much easier, to use compensation methods to bring the overall T.C. to around 0.01ppm/K.

Thanks! That's what I thought, but I've seen some of the builds here with the oven so it made me wonder.

If you put the step up resistors in an isolated oven, that would make sense, as this is the main feature of the 732A/B circuits, to have a low T.C. for the 10V output also.
Obviously, you're planning to build the very same solution/form factor, which I have published here, a few days ago.

Similar, but I wanted separate 3458A-like ref module, so I could use that elsewhere (I want to build a variable resistance standard, based on Valhalla 2724A, like Mickle T did) and separate step-up/10k standard in the oven.

I have parts to build 4 (and 1 extra ref-module), so the final version will be a DIY Fluke 734A. I need to find a suitable case that would fit a toroidal transformer for charging and PCB slots to slide the modules in.

Initially, I was thinking about something sophisticated, with an MCU in the chassis for control and monitoring over isolated I2C, rerouting the reference voltages through the backplane for the averaging output or nulling, step up done by an ovenized MCU per-module doing the PWM like in 5440B DAC and doing other things like reading temperature sensors and switching a latched relay to switch the voltage output between raw zener and 10V but this all gets too complicated, noisy and I think I'll take MisterDiodes low noise advice and do instead:
* chassis with just some toroidal transformer + linear regulators to provide voltage for the LiPo charger, this voltage will go through a 2 pole switch per module, isolating the module from the PSU and turning off the on-board charger (so I don't need to slide the module out),
* each module will be a base board sitting on the plastic PCB guides, like EuroCards, on the base board there will be:
* 4x18650 - wouldn't last for many days, but small + some 4S charger IC (TBD) + low noise LDO to make 12V (might not be enough, if 15V is really needed then I need to rethink this section),
* a 3458A-like ref module, insulated with SpaceTherm and maybe instead of 3D printing a cover I'll get some 2mm steel sheet and spend some time with my sheet metal bender,
* another insulated and ovenized module with 10k/25k step-up (similar to your schematic)  and 10k resistance standard, need to add some protection circuitry as well (TBD, short circuit etc).
* cat6 cable coming from the oven to the front Taobao copper binding posts (on order) on the front with 7V, 10V, 10k force + sense + probably some cheap binding posts for guard and earth and LEDs to indicate oven temp OK, charging ON and LiPo low voltage warning.

I decided to provide a trimming to exactly 10.000V, for usage in a precision Bridge configuration, where it's more convenient to have a 'round' value.
The trimming does not affect the stability of this amplifier circuit, and you can also omit that and just live with odd values.

Maybe I'll provide footprints for series/parallel resistors/trimmer but leave them unpopulated. BTW What about using digital potentiometers, like MAX5436? They seem to have better TC than typical trimmers. As my step-up will be in the oven then the access to the trimmer will be difficult.

Well, you can still order a custom resistor from Vishay Foil. In fact, if I would be volt nutting at home, I would do that. The S series has some 0.2ppm tracking, and the two resistors in one package. It will be for sure expensive, about an LTZ1000 brand new. They trim it to 6 digit. Just build the reference, burn it in, measure the value, and custom order the precision divider to get the 1-2ppm accurate  10V reference. If you really really want, trim that with a million turns pot.

I've already ordered the page 6 circuit resistors (S102K and similar), VHP202Z 10k standard and VHD200 10k/25k from ebay hifi-szjxic. Not the best source, probably not the best batches of Vishays but good enough.

The reason is that well over a year ago I've created myself an account on VPG website, filled the Get quote form for 5 sets of page 6 resistors + 4 pcs of 10k oil filled and send the quote and guess what - nothing, zero, no reply. Very nice.
 

Offline NANDBlog

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Re: MX Reference
« Reply #98 on: September 11, 2017, 11:09:01 pm »
You should definitely talk to sales manager of your region from Vishay foil. On phone. It is not a huge company, and works differently than Vishay. Because they might not have something on stock at all since years, or you might break their production schedule completely by ordering a few thousand of their resistor, and they would deliver it in tiny, 5x5 trays.

No that definitely did not happen to me at all.
 

Offline lukier

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Re: MX Reference
« Reply #99 on: September 11, 2017, 11:14:42 pm »
You should definitely talk to sales manager of your region from Vishay foil. On phone.

Hate talking to sales people and also I would be wasting their time with my order for a couple of resistors while just before they were negotiating $1m supply contract, so I thought their website would be a better option.

That is also what it says on their website "How to Order" -> "Get a Quote", it must be there for some reason.
 

Offline Dr. Frank

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Re: MX Reference
« Reply #100 on: September 11, 2017, 11:57:19 pm »
Similar, but I wanted separate 3458A-like ref module, so I could use that elsewhere (I want to build a variable resistance standard, based on Valhalla 2724A, like Mickle T did) and separate step-up/10k standard in the oven.

I have parts to build 4 (and 1 extra ref-module), so the final version will be a DIY Fluke 734A. I need to find a suitable case that would fit a toroidal transformer for charging and PCB slots to slide the modules in.

We obviously had the same idea, and I also have 4+1 sets.

So my layout allows to cut off the amplifier part, and get a 57 x 50mm PCB, similar size like in the 3458A (48x41, I think), but with space for PWW resistors.
Direct pinning for the 3458A is not available, of course.

But a pinning for the 34465A would be interesting, to improve its noise and stability to 34470A level.
Had already begun to reverse engineer the 34470A reference .. but did not figure out, how LM399 and LTZ1000 are merged on these 8 pins.

Anyhow, the design of an isolated, silent, 4 fold PSU would be great.. much more important than YaRM - yet another reference module..  :-DD

Frank
« Last Edit: September 12, 2017, 12:05:01 am by Dr. Frank »
 

Offline lukier

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Re: MX Reference
« Reply #101 on: September 12, 2017, 12:08:08 am »
So my layout allows to cut off the amplifier part, and get a 57 x 50mm PCB, similar size like in the 3458A (48x41, I think), but with space for PWW resistors.
I was thinking about something like that, I think at seeedstudio they call it self-panelization, but because of the many mechanical components involved in the mini-module covers (steel sheet, spacetherm, kapton, nylon standoffs) or oven (heater - TBD, some resistive foil to test on order) I think I'll have 2 separate designs.

Anyhow, the design of an isolated, silent, 4 fold PSU would be great.. much more important than YaRM - yet another reference module..
Yeah, I was thinking about that initially, some form of Royer followed by low noise LDO and a lot of ferrites everywhere. My analog foo is probably not good enough for that and it is a lot of work.

Now I'm not as ambitious as that, there will be one linear supply in the chassis for all the 4 modules, just for charging the 18650 cells in the modules and when I need to use the module I isolate its charging supply (i.e. turning off the charger IC and floating the module) with a rocker switch on the front of the module, thus running the module from the LiPo cells.
 

Offline 3roomlab

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Re: MX Reference
« Reply #102 on: September 15, 2017, 10:09:29 am »
Yes, 1mm ~ 3mm would be a good place to start.  We use .05 or .0625" as a starting place if you're on imperial measurements.  The inner box can be slightly thinner, and maybe a single box is all you need.  We normally go for a 1/4" spacing between inner and outer but 1/8" would probably be OK.  It depends on what you're shielding from.  Aluminum will help some against E-fields but not much for mag field issues.

Also note:  If you're using PWW resistors it's important to at least have some minimal shielding around your circuit if you're near stronger H-fields - say an industrial area where you've got power mains in conduits, motors, ovens, etc.  Those resistors are wound with balanced windings but they will see some magnetic interference effects in a strong field unless you give them at least some protection - otherwise they will give you low noise performance.

ive been looking up more info on the net about the why and what, not much info until i saw this
http://www.compliance-club.com/archive/keitharmstrong/design_techniques4.html

i assume the 1-3mm being indicated have a correlation with skin effect vs frequency? esp for steel?


*edit : there are many TAOBAO shops offering 1J85 (permalloy equiv?) have anyone used these?
« Last Edit: September 15, 2017, 09:41:44 pm by 3roomlab »
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Offline MisterDiodes

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Re: MX Reference
« Reply #103 on: September 17, 2017, 03:54:34 am »
That's related more to induced current effects in the shielding material, and is more of a concern at higher freqs.  For low freq and DC circuits we worry about lower freq mag field problems, say at 100kHz and under...although noise injection can come from anywhere.

You use steel and / or MuMetal or whatever material you've got that has high permeability -without saturation- to protect your circuit from external mag fields.  In other words you need to steer the magnetic field -around- your circuit, so you give it a very easy path to follow in its own contained path around your device.  If the material is too thin it is likely to saturate if the mag field is strong enough, and then it's not doing much.  Once that shield material saturates, it's not steering the mag field anywhere in particular.

Depending on the local field strength and frequency you use the combination of shielding materials that works the best for best profit and effect - For instance you might find that a small steel box (a few dollars at the hardware store for a junction box for instance is a simple test, much heavier gauge than a candy tin) might give you enough noise attenuation for your lab situation.  Or a double shield box design.  Or you might need an annealed deep drawn MuMetal can that will shield your circuit all the way down to DC.  Copper and Aluminum can shield at higher freqs. and more for RF issues.  Sometimes a simple steel box with an overwrap of MuMetal foil works well enough at low freqs.  If you don't kink the foil in a really sharp bend that MuMetal will still work surprisingly well without any heat treatment at all - and is miles better than nothing.

It just depends on what you're shielding against, for your situation and budget.  You also look at protecting and shielding all leads (Chokes - ferrites usually) and equipment around the sensitive circuit - mag shielding is only part of it - you have to look at the noise delivered into your system from mains power and noisy high impedance grounds, noisy equipment in the lab etc.

The main thing is to keep your current loops with as small an area as practical, and try to minimize mag field interference as much as possible that way.  Twisted short & shielded connection leads, for instance.   Normally we'd never be using longer banana-plug and clip-lead style connections at low ppm for serious measures, but we see that all the time with the informal hobbyist setups - and those standard, separate red/black test lead assemblies are usually much too long and wind up acting as a pretty good loop antenna (transmit and receive) more than anything.

Do whatever works best for your situation, budget and measuring uncertainty goal. 
 
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Offline ManateeMafia

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Re: MX Reference
« Reply #104 on: September 30, 2017, 11:19:19 pm »
Received several sets of resistors from Edwin on Thursday. Thank you Edwin!
Most of them will be for building newer versions of the MX board but a few sets are earmarked for another project by a forum member.

These are the smaller body model 802 which is the same size that TiN had for the KX board here https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/700/
Coincidentally, these are the same size I used to create the original part library for my first board.  I have since created the larger version (805?) and the next rev can use either one.

I have vacation this week so I will be finishing the test chamber and will cycle it using my Keithley 2510. The tempco results from casual observation are not good but I will need a starting point.

Thanks again to everyone with suggestions. So far I am sticking with the datasheet for the components but will leave one or two unpopulated locations. I will need to sort out basic layout problems first. The next rev has all the PWW resistors closer together on one side of the board and the traces are shorter where possible.

Once we get tempco data, it will be posted hopefully later next week.
 
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Offline TiN

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Re: MX Reference
« Reply #105 on: September 30, 2017, 11:31:27 pm »
My Raspberry snakes are ready to run  >:D. Let's see some tempcurves!
YouTube | Chat room | Live-cam | Have documentation to share? Upload here! No size limit, firmware dumps, photos.
 

Offline ManateeMafia

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Re: MX Reference
« Reply #106 on: September 30, 2017, 11:35:27 pm »
I will need to mount the TEC and add a power connector. I think I will drill a small hole for the test cable. I have some STP and will use that instead of the CAT5. The pcb mounts are already done.

I have to work this weekend so maybe time late tonight or tomorrow. If not, I have staycation all next week. I just need to clear the bench, there is this large beast taking up room  :-X
 

Offline branadic

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Re: MX Reference
« Reply #107 on: October 01, 2017, 01:13:28 am »
Quote
Received several sets of resistors from Edwin on Thursday. Thank you Edwin!

I always wondered why they are not marked like any other resistor manufacturer does.

-branadic-
Fluke 8050A | Prema 5000 | Prema 5017 SC | Advantest R6581D | GenRad 1434-G | Datron 4000A | Tek 2465A | VNWA2.x with TCXO upgrade and access to: Keysight 3458A, Keithley 2002, Prema 5017 SC, 34401A, 34410A, Keithley 2182A, HDO6054, Keysight 53230A and other goodies at work
 

Offline SeanB

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Re: MX Reference
« Reply #108 on: October 02, 2017, 04:27:43 am »
Pretty much every hand wound resistor is hand labelled, even the Vishay special order ones that start off as a regular glass slab and then are precision trimmed to an exact value or tempco.
 

Offline Echo88

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Re: MX Reference
« Reply #109 on: October 04, 2017, 08:27:28 am »
@ ManateeMafia: Would you mind sharing your Eagle-Project on the MX-board or the LTZ1000-footprint-file? I plan on building a LTZ1000-bank and therefore would like to design a suitable pcb, which might divert a little bit from yours.
 

Offline branadic

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Re: MX Reference
« Reply #110 on: October 04, 2017, 08:35:42 am »
I'm not ManateeMafia, but I can share my LinearTechnology library with you. You will find LTZ footprint in there.

-branadic-
Fluke 8050A | Prema 5000 | Prema 5017 SC | Advantest R6581D | GenRad 1434-G | Datron 4000A | Tek 2465A | VNWA2.x with TCXO upgrade and access to: Keysight 3458A, Keithley 2002, Prema 5017 SC, 34401A, 34410A, Keithley 2182A, HDO6054, Keysight 53230A and other goodies at work
 
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Offline ManateeMafia

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Re: MX Reference
« Reply #111 on: October 04, 2017, 10:24:41 am »
@branadic,

Thanks for sharing. My library had a small error on the last pcb run. I think it is fixed but I am concerned people will get boards made with problems. My issue involved a solder mask over one of the pads. Fortunately the pcb house caught it and manually removed it for me. You can see it in one of the photos.

I hope to have the thermal chamber going late tonight or early morning.
 

Offline 3roomlab

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Re: MX Reference
« Reply #112 on: October 09, 2017, 02:45:01 pm »
on the stuff everyone talked about regarding magnetic in this thread, a really funny coincidental discovery fell on me when i saw a stainless steel bowls and pans product page

https://img.alicdn.com/bao/uploaded/i4/2454910677/TB1I_c8kRcHL1JjSZFBXXaiGXXa_!!0-item_pic.jpg_430x430q90.jpg
https://img.alicdn.com/imgextra/i4/507489737/TB23bkcXNaK.eBjSZFBXXagMXXa_!!507489737.jpg
plate version appears to be under 0.5mm, bowl appears to be >0.5mm

since it would be terribly difficult hobby wise to obtain cheap and sphere formed mu-metal spherical shielding ...
by cupping 2 of these bowls together (20cents stainless steel), it is looks like a readily available magnetic shield


it seems my assumptions above  about stainless steel could be wrong. 304 is likely not enough of permeability to be effective.
again based on FEMM 4.2 simulations (finally understood some magnetic functions). thickness plays an important role (even for mu metal which is assume to work well at 0.1mm thickness, but it highly depends on the quality). in the below text, i am using (u) as relative permeability.

https://3.bp.blogspot.com/-fH6EU786D1A/Wdtgva3295I/AAAAAAAAC2Y/xUPfJ0_OIr0cCckbzB42khGHMRlx4KHAACEwYBhgL/s1600/B006.gif
a sphere with 0.1mm 83k(u) mu material. approx 0.001T fields still can leak into the sphere. (5mm hole is deliberate)

https://4.bp.blogspot.com/-sHgsVDaZuxE/WdtgwMiv3FI/AAAAAAAAC2c/S6gItopk-YQRiRL3UIKDevrml7iqIUxHgCEwYBhgL/s1600/B007.gif
same sphere with 3mm thick ferrite mix approx 100(u) material (at least 500x worse than than mu metal). this inferior material appears to block more. (5mm hole is deliberate)

https://1.bp.blogspot.com/-QltxZtqCENU/WdtguRKeHCI/AAAAAAAAC2U/2tJPa_a32psPO66ruqW2bpItPSFWEMgqwCEwYBhgL/s1600/B005.gif
ferrite mix, odd corners, thickness can be a detriment? i am unsure of this.

https://3.bp.blogspot.com/-jPJ1FgDRmIY/WdthFopeAII/AAAAAAAAC2g/MCf1iFBh1Bws_tqF_ps8aoDJGsRNASo4wCLcBGAs/s1600/B008.gif
symmetry seem to play a bigger role in diverting field completely than using the usual long boxes.

https://3.bp.blogspot.com/-eNBh0UvGEsw/WdtgsAYjBKI/AAAAAAAAC2E/Bu0uq8FPiGsI6d6Uz2EYbc4LHgEzrJkRwCEwYBhgL/s1600/B002.gif
the 83k(u) material needs at least 0.5mm thickness. all older equipments seem to it this way.

so it appears, for a ferrite mix (made into like a liquid PU dip mix?), it may likely have more advantage than mu metal sheeting (single layer vs single layer?)
(Mn Zn ferrite powder can be searched and found on ebay and alibaba. my last find shows about S$5 per kg, my guess is a liquid form will conform to all kinds of shapes and form a seemless layer esp around wire inlets)

https://2.bp.blogspot.com/-jtuhlepFSMg/Wdtgt60bpEI/AAAAAAAAC2Q/tzYSXa8Nug4LI_NTOhzpGZCyhBnbxW9pQCEwYBhgL/s1600/B004.gif
thin wall mu metal and long box, even worse than the sphere model? (2x mm size holes are deliberate)

additional simulation
using 0.1mm mu but in 3 layers.

1) seemless with 1mm hole (which is impossible to fabricate and re-anneal at DIY level)
https://3.bp.blogspot.com/-8R09rMSGDXc/Wdx9SQ5WhGI/AAAAAAAAC20/enB7nueVFKE4fYu-Zl5tmWXRMP4gTsgdgCLcBGAs/s1600/B009.gif

2) segmented, which results in heavy field lines at corners
https://4.bp.blogspot.com/-uwsGXNL6W10/Wdx9R14v0vI/AAAAAAAAC2w/BdJFh20dkjoTyVSbMqYAQ0KeCMpxwfP-wCLcBGAs/s1600/B010.gif

3) segmented but with overlaps, which is worse than seemless by 1 magnitude, but better than single layer by 1 magnitude
https://4.bp.blogspot.com/-X4G7X3vKvUk/Wdx9S-MwJQI/AAAAAAAAC24/ax7-SDW784o-Z3EfWUW05adLiJz-JbmwQCLcBGAs/s1600/B011.gif

4) 0.1mm mu sheet. 2 overlap seams 1mm hole. shielding down to as good as 1uT (original incoming field strength is around 1mT)
https://2.bp.blogspot.com/-E7VQTriQgeU/WdyCDEZmHrI/AAAAAAAAC3I/KjEz_2U9IVoDZvGExx_Lypuh6egQjYUMACLcBGAs/s1600/B012.gif

5) 3 overlap seams and 1mm hole. seams inside start to generate fields between each other
https://2.bp.blogspot.com/-QnEqqy1pUfg/WdyFy4eZ27I/AAAAAAAAC3U/LwYz3iPGFu0AvOYfdLL0whrNkMFes5PVgCLcBGAs/s1600/B013.gif
« Last Edit: October 10, 2017, 07:38:51 pm by 3roomlab »
true-volt true-rms true-ohm true-lumen true-ampere true-decibel ... what else was false last year?
 


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