[volt-nuts] Traveling Standards

Andreas Jahn Andreas_-_Jahn at t-online.de
Sat Oct 20 14:58:08 UTC 2012


Hello Fred,

I also had large common mode effects on the LM399 until I spent 2 100nF 
ceramic multilayer capacitors to the heater and the zener
See C6 and C8 on Bobs cirquit.
http://c-c-i.com/sites/default/files/reference_voltage.pdf
Further you could try using batteries during measurement for the standard. 
And charging only when no measurement is made.

Without these capacitors there was a difference up to 100uV depending where 
I put my hand on the wiring.
(either near the Reference or directly to the NiMH-Batteries of one of the 
references).
The voltage was measured as difference of 2 references to get improved 
resolution.

The common mode effect came from the switch mode supply  of my personal 
computer.
With these capacitors the common mode effect was reduced to
My first ADCs did not have photocouplers and where galvanically coupled to 
the PC.

With the decoupling capacitors the common mode effect was below 2uV.
Maybe also this were resulting thermocouple effects.
Now I have galvanic decoupling between PC and ADCs further reducing effects.
Common mode problems are only visible when I forget to disconnect the 
switchmode chargers for my batteries.

With best regards

Andreas




----- Original Message ----- 
From: "Fred Schneider" <pa4tim at gmail.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Saturday, October 20, 2012 10:47 AM
Subject: Re: [volt-nuts] Traveling Standards


>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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