[volt-nuts] Sub PPM 10V transfer standard

[m k]

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

> 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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