[volt-nuts] Sub PPM 10V transfer standard

[John Devereux]

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