[volt-nuts] Sub PPM 10V transfer standard

John Devereux john at devereux.me.uk
Sun Sep 4 18:39:45 UTC 2011


Hi, could you be more specific?

m k <m1k3k1 at hotmail.com> writes:

> Hi, The schematic has the opamp+ connected to the wrong point and
> there are some other details wrong. 

Do you mean the +input or the +supply? They both look OK to me but noone
is saying what the problem is!

> In another job there was an instrument that used an 1n829a run at
> about 7.5mA for a 10 V reference that used to hold to better than 1uV
> over lab temperatures. It was done using the Op amp to generate the
> 10V that directly drove the 7.5mA into the zener, select on test
> resistors. but for 0.1ppm you need to address opamp heating and
> unintended thermocouples everywhere.

This is essentially what he is doing as far as I can see. But the opamp
heating is addressed by using the 78L05 as a "power driver", so the
opamp generates no heat! The heat-generating part is taken off-chip (or
off-board perhaps) so it does not affect the input offset voltage.

I agree 0.1ppm seems optimistic, but then I haven't built it. In my
admittedly limited experience thermocouples are not such a problem if
you keep everything isothermal, i.e. avoid temperature
gradients. Stating the obvious perhaps.

Even if it were 1ppm instead 0.1ppm I think this is still an order of
magnitude better than the homebrew ideas discussed so far (if
achievable).

John

>
>> From: john at devereux.me.uk
>> To: volt-nuts at febo.com
>> Date: Sun, 4 Sep 2011 15:56:00 +0100
>> Subject: Re: [volt-nuts] Sub PPM 10V transfer standard
>> 
>> 
>> Hi Bill, Warren,
>> 
>> Whats the matter with it? The schematic is clear enough to me,
>> interesting and seems quite clever to this wannabe nut.
>> 
>> If I understand correctly, the opamp adjusts the divided-down output via
>> the regulator so that it matches the reference. Everything is powered
>> from the 10V output, eliminating problems with variable input voltages
>> and opamp PSRR.
>> 
>> I would be interested to know:
>> 
>> - Any suggestions for the wirewound resistors? Vishay bulk metal foil
>>   resistors are not cheap but say 5ppm wirewounds are not pennies
>>   either, I don't know how many you would need before a sufficient
>>   number could be matched.
>> 
>> - I am unclear as to the precise function of the 160k and 40M
>>   resistors. I suppose one of them shifts the output, and the other
>>   compensates the zener so it still runs at the same minimum-tempco
>>   current?
>> 
>> The zeners do look interesting as references, I admit I have been
>> fixated on LTZ1000s.
>> 
>> Farnell/Newark don't stock the 1N825 but they do stock the
>> 1N825A.
>> 
>> <http://uk.farnell.com/american-power-devices/1n825a/diode-zener-6-5v-0-25w-do35/dp/1651077?Ntt=1N825>
>> 
>> According to its "datasheet"
>> <http://www.farnell.com/datasheets/332215.pdf> there are also some
>> similar looking parts that may have have improved tempco, for example
>> 1N829A at 5ppm/K vs 1N825 at 20ppm/K. These are also available. There
>> are even some 2ppm/K parts but these seem unobtainium.
>> 
>> Are there any better datasheets? For example how does the tempco itself
>> vary with temperature? The company does not seem to have a web presence.
>> 
>> There
>> 
>> Regards,
>> 
>> John
>> 
>> 
>> WB6BNQ <wb6bnq at cox.net> writes:
>> 
>> > Warren,
>> >
>> > You need to do your schematics a little bit better.  Besides, I
>> > think you attached the wrong one.  There are a number of errors on
>> > the attached schematic.
>> >
>> > Bill....WB6BNQ
>> >
>> > WarrenS wrote:
>> >
>> >> Want a stable 10V ref?  Best thing to do is get a fluke 732A/B and
>> >> have it calibrated regularly for a few years.  BUT For the Extreme
>> >> (or cheap) Volt Nut, There is another choice.  Attached is the
>> >> circuit of a 0.1 PPM, 10 volt transfer standard.
>> >>
>> >> It has ONLY ONE critical part that limits its accuracy and
>> >> stability.  Everything else can be made to have so little effect
>> >> as to be insignificant or else can be trimmed and calibrated out
>> >> without using a precision voltage source using a method similar to
>> >> what the daily auto cal in the HP3458 does. It measures it's own
>> >> internal 6+V reference.
>> >>
>> >> Every year or more, best to record the value of the nominal 6.2
>> >> volts, to measurement the ageing rate, by using a known external
>> >> 10 volt standard.  To do that, an accurate external way is needed
>> >> to find the exact ratio of the 6.2 volts to the external 10 volt
>> >> standard.  This can be done by using a high resolution Linear DMM
>> >> (aka HP3458A) OR a Kelvin-Varley divider (aka fluke 720 ) and null
>> >> meter (aka Fluke 845).  Lets assume if one wants to do 0.1 PPM
>> >> things, then they have the equipment and skill needed to measure a
>> >> 10v to 6v ratio to 0.1 PPM.  After that ratio is known and
>> >> recorded the units internal 10 volts can then be set to that same
>> >> ratio without further need of the external 10V standard.
>> >>
>> >> The main advantage of all this extra trouble is low cost accuracy
>> >> and stability.  There are no critical or expensive resistor needed
>> >> or anything else critical, because all the TC and long term drift
>> >> are canceled out whenever a manual trim is done using an external
>> >> ratio device to check its own 10 V against it's own 6.2 volts.
>> >>
>> >> Nothing very special is needed to build this beside care and time.
>> >> To get the best performance, It does take a lot of Nut-time to
>> >> match and select things.  Total parts cost with a well supplied
>> >> analog junk box (or EBay) can be under $10.
>> >>
>> >> Any good 6V type reference device could be used.  I'm using a well
>> >> aged and selected 1n825 because I have a lot of history with them
>> >> and they work good without an oven.  I have not done any mailing
>> >> test on these parts but they can be Zero TCed so that they do not
>> >> change 0.1PPM over normal room temperature and they show no change
>> >> when powered down for short periods of time and tend to return to
>> >> their original value when hit with a heat gun and cold stray, so
>> >> it makes a good part to start experimenting with.

-- 

John Devereux



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