[volt-nuts] Traveling Standards

[Bob Smither]

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."
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