[volt-nuts] Traveling Standards

Sat Oct 20 08:47:12 UTC 2012

I have made a LM399 standard, dead bug build, at 10V. A uA723 for powersupply at 15V. In a metalhousing and shielded internal extra using double sided pcb. 
It is now 24/7 on for about 5 weeks.
I modified my solartron 7601 so it stays at 38C plus/minus 1 degree internal. ( mounted two heatsincs on top, one above the transformer, a ntc inside, and a speedregulated fan on the main heatsinc. Thanks to your post i started monitoring it. So since a few days that meter is also on 24/7. A few times a day Zi take readings ( i am digital disabled so logging probably always will stay a dream)  Room temp changes from 19 to 24 degrees.

I made a 10 turn potentiometer on the front that gives me some adjusting. Still more as needed but until it is aged enough I keep it like this. Resistors are all 0.02% or 0.01% 
One whole turn of the multiturn is around 100 uV.  
I adjusted it to 10.000,000 Volt at 19 degrees Celcius. This was a cold mornig and a lot of rain. According to my bones humidity was high. Later that day it became 21 degrees ( heath from instruments and light spots)  It was the 10.000,030V, yesterday a dry and warm day it ( 21 to 24 degrees) raised to 10.000,052) this moring 20 degrees, rain outside, it was 10.000,058
The ten turn pot has a reading ( a dial with two arms like a clock) and  i made the standard so that the pot at 10.000,000 V was at the 5.00 position. Until that, I had to turn it between 4.45 and 5.15 to get it back at 10V. So it still has a total drift around 50 to 100 uV ( monitored it als a few days three weeks back using two meters and they showed the same trend.

But i think I have a problem regarding magnetic fields or other sources because I have strange issues while meauring using the 332 and KV deviders or my LM399. 
I looked to it with a friend and he thinks it is some magnetic field or common mode thing.
I had powered everything off, even the lights. Then both 7,5 digit meters showed the same values, turning the polarity did not matter. Both KVs were lineair upto 1 uV. Using my 399 or Fluke 332, also tested the 332 divider dircect. 
Then with the lights on severa instruments ect. A big difference if i change polarity, both meters gave different readings, both KV dividers gave excact the same non lineairity. I did this several days and up to about 1 mV !! between 100 mV and 10V but an verage of a few hundered mV) if I power all stuff down exept the 332 and 7106 all is perfect again. All intruments are grounded. I use shielded cables to the meters and from 332 to KVs from Beats me :-(
So I think your standard performs well if it is this stable while tossing around, i wish my 332 and LM399 was that stable ( or more, the environment was more standard friendly becaus all powered  down things are much more stable)

Fred PA4TIM

Op 20 okt. 2012 om 04:27 heeft Bob Smither <smither at c-c-i.com> het volgende geschreven:

> On 10/18/2012 02:05 PM, Andreas Jahn wrote:
>> Hello Bob,
>> 
>> whats the matter with you. Infected by precision virus like me?
>> You wanted to have a standard with about 10ppm and now you blame a 3-4ppm drift.
> 
> :-) - afraid so Andreas!
> 
>>> 
>>> The LM199A is hermetically sealed.
>>> 
>> 
>> The PCB, the 8K Resistor and the voltage Regulator are not.
>> 
>>> Although a small sample, the two references appear to be similarly affected by
>>> whatever caused the drift - similar range of drift, similar time constant.
>>> 
>> On the first view I would blame it on the meter.
>> It is very unusual that the drift of 2 different references has nearly exact the
>> same amount of ppm and direction.
>> But on the other side you state that there are several HP3458A which recorded
>> the drift.
>> It is not probable that all came freshly from calibration of a other location.
>> 
>>> Any ideas about what could cause the drift we are seeing?
>> 
>>> From time constant it could be the humidity change.
> 
> This is my best guess.
> 
>> My 2 LT1027CCN8-5 references which ara mounted only with 1 Pin
>> to the PCB have time constants in the range of 4-7 days.
>> The epoxy material of a PCB should lie in the same ball park area.
>> The LT1027 are influenced by around 0.5 ppm per percent humidity change.
>> 
>> For the hermetically sealed brand new references LT1236AILS they state in their
>> new product catalog
>> a humidity change of less than 10ppm for 25% humidity change. (page 36)
>> http://cds.linear.com/docs/Product%20Info/NPC.pdf
> 
> This is most interesting - so even "hermetically sealed" units are influenced by
> humidity!
> 
>> I asked them whether this is from mechanical stress from the PCB and they
>> confirmed to me
>> that with a dead bug mounting the influence of humidity will be virtually
>> unmeasurable.
>> So they will delete the parameter from the data sheet.
> 
> Mine are not "dead bug" mounted.  The 'PCB' is in fact a Radio Shack perf board
> - certainly not the best substrate to mount them on - I don't think it is FR-4
> material.
> 
>> So for the LM399 it might be mechanical stress introduced by the PCB.
>> 
>> 
>> When looking at your cirquit there are several points to mention:
>> One common failure source will be the LM78L15. A output voltage change will
>> influence the supply of MAX6350 and the reference current of LM399.
>> PSRR of MAX6350 is about 2-5 ppm/V above 10V supply. (without self heating
>> effects).
> 
> LM78L15 spec is 1mV/C.  This would result in .02 ppm/C on the LM199A (operated
> at 1 mA with 8K resistor providing the current).
> 
> For the MAX6350 1mV/C and 5 ppm/V => .005 ppm/C
> 
>> The LM399 resistor will give a current change of about 10% per Volt (100uA)
>> resulting with 0.5 Ohm impedance in about 50uV/V or 7ppm/V
> 
> see note above.
> 
>> Other weak points of the cirquit are:
>> The LM399 heater voltage is not stabilized. this will give about 0.5ppm/V
> 
> It is stabilized by the power supply - a 24 volt, .02%/C unit => 5mV/C => .0025
> ppm/C.
> 
>> And finally: was the LM399 always in the same orientation during measurements?
>> (will be difficult with a cylindrical housing).
>> My LM399 drift 3-4 ppm by tilting orientation.
> 
> Not sure - but from the consistent results (consistent drift and apparent
> settling) it likely was.
> 
> The above still leaves humidity induced PCB changes causing mechanical stress as
> likely.  I am going to look for some better board material for when I construct TS2.
> 
> Thanks Andreas!
> 
> --
> "As we act, let us not become the evil that we deplore."
> <smither.vcf>
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