[volt-nuts] LM399 newbie - questions

[Ian Johnston]

Hi all,

Am playing around with designs for a precision voltage source and have a 
prototype up & running (may have seen on EEVBlog mailbag video recently). 
It's based on an 18-bit DAC and at the moment I'm using a MAX6350 vref IC 
(plastic package). I chose the 6350 originally for it's low tempco.

The 6350 is only really good for 16-bit, it's kinda pushing it to be used 
for 18-bit and this becomes apparent by way of thermal drift & thermal 
hysterisis.

So, I am looking at using the LM399. I have breadboarded it just to get my 
feet wet in regard to it's usage and after a bit of playing around have it 
working pretty good. I'm self-bias'ing it using a chopper op-amp and it's 
pretty stable, totally independant of supply variations, not really effected 
by temperature that much (extremely low tempco resistors used) etc etc.

I've logged a good bit of data (6.5 digit DMM & BenchVue) and noticed that 
when I first got her up and running there was quite a steep rise on voltage 
on it for a few hours and then it started to level off.

After a day or two it's much flatter but still on the rise i.e. 40uV 
overnight. Sometimes it will level off a bit then it may start climbing 
again.
I've ruled out a tempco issue and am starting to think it's an issue with 
having the LM399 on the breadboard......i.e. the mechanical stress is 
continually varying.

Of course it's probably aging as well, and thats where I have a couple of 
questions:-

If I go into production with my product (low, low volumes) then can I 
advance the aging process to at least get it down to a reasonable level i.e. 
single digit uV over a few days/weeks.

Does the aging process have to be done with power applied, or can it be done 
by heat cycling the LM399's.

If I build a special PCB where I can pre-age say 10off LM399's at the same 
time, what effect will removing from that PCB and then soldering into my 
target PCB's have. The special PCB could have sockets or possibly they would 
be soldered in.

So far the LM399 is looking good, even on the breadboard, compared to the 
MAX6350 which is in place on the target PCB.

Thanks in advance,

Ian.




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