Author Topic: interested in hobbyist voltage reference standards?  (Read 39146 times)

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Offline rf-loop

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Re: interested in hobbyist voltage reference standards?
« Reply #50 on: August 24, 2010, 12:21:20 pm »
Example LM399A. Tempco: Not very well ovenized.  After this heating and temperature stabilization still component tempco is 1ppm/celsius (or 2ppm without A) . Without oven example REF102C have 2,5ppm/C max. (with box method) and example around 40C there is nearly zero tempco.

LM399 average long time drift (ageing) is around 4ppm/a but 3 sigma is 10ppm/a (and 8 ppm/?kH as also in dtasheet) Noise 10Hz - 10kHz ... 50uV and it is RMS value... real p-p is maybe too bad numbers to tell... (yes 199A is less noise (20uVrms) but it is obsolete)

What means this RMS noise specially in very low freq. (what is real worst case noise p-p value (this is important in reference use) example in most difficult (0.01) 0.1 - 10Hz.  Some reason they do not tell it. There is only one picture about noise but not data how it have get and there can see more than 1.5ppm p-p values. No information how they get this. (maybe best available single chip?)

What is advantage for this "heating". maybe something for cheap mass production but not anything for make good voltage reference example for simple DIY transfer standard. Just I understand well why someone was thinking (and make some drawing) for connect many 399 parallel for sqr reduce short time flicker noise and maybe also some advantage for more predictable ageing drift.


Ovenized one handfull  of REF102C's  is maybe quite good also for DIY reference. (I have make some exercises and test with it but I have not enough good instruments in home for get good reliable data but my HP3457A is poor DVM what can not give nearly anything in this accuracy class. This kind of measurements need  something like HP3458A + Fluke 7001 or 732 references. HP3457A have 399 reference.. and it is its weakness)   For average units maybe 40-45Celsius is best (tempco turn point so temp control is not so critical in this temp area)... only if keep oven all time on so that can delete thermal hysteresis possible problem.

What is wrong with 399. I can not understand tempco 1 or 2ppm/C for "ovenized" reference. Not so much advantage if compare best not ovenized references and some times they have also better long time stability and less noise. How can make so poor oven that not ovenized reference have 2.5ppm/C and ovenized have 1 or 2ppm/C.

What is problem with REF102C? Without any temp control/heating maximum is 2.5ppm/C (keep this component example +40C +-0.1C and you have +-0.25ppm...  Exept ... it is much more better becouse tempco between 40-45 is much less than 2.5ppm/C ... it is nearly zero. Curve is littlebit S and one turn point is somewhere around 40C or littlebit more typically.

If can nearly zero tempco then there is one variable less for good reference.
 
Of course better is more low temp but more and more low then need maybe (peltier)cooling and warming together... (it is not difficult today but physical design come more hard also becouse humidity dev point. )

Of course this kind of things are nice for DIY use but sometimes impossible in mass production becouse some (still simple) things may be expensive in production. So "rules" in DIY and mass production are nearly totally different case. I can use hundreds of hours for some adjusting and follow up work... becouse it do not need money.... (wit this thinking I well understand HP and Fluke selections...) But also... what they do next years if they now do something what can not do better new model... (this is included also to inside design...even if we want or not)
« Last Edit: August 25, 2010, 05:34:14 am by rf-loop »
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