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 <[email protected]> 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> > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
