[volt-nuts] "averaging reference" / "Perfect Volt"

[Joel Setton]

Frank (and all!),

These are all very valid questions.

The context is a hobby activity, and the purpose of the reference 
voltage is to have something which is good enough to calibrate, for 
example, a 30-year old DVM after restoring or repairing it. With 
currently available reference chips, 3 1/2 digits is relatively easy, 4 
1/2 digits is much more difficult, and 5 1/2 digits is almost completely 
out of reach. And as with many physical measurements, you may have 
access to an external standard at a given point in time (for example 
with help from a fellow volt-nuts list member), but after this initial 
calibration the stability of your reference over time becomes important 
since you can't send it out for recalibration every year.

The nice thing about the REF102 is that it's relatively easy to use, and 
averaging several chips is easy. But as I understand from previous 
replies, maybe it wouldn't match the long-term stability of an LM399.

Obviously Josephson junction technology is out of the picture, since 
cost and practical feasibility are in the top of the requirements list. 
The only commodities which are freely available to the average hobbyist 
are time and creativity. And of course the ultimate goal of the Perfect 
Volt will never be reached, but improving your previous design is what a 
hobby is all about!

Joel


On 22/12/2014 00:17, Frank Stellmach wrote:
> Joel,
>
> your requirement about a "Perfect Volt" is very diffuse.
> What shall that be?
> "Perfect" in terms of tolerance, stability over temperature, stability 
> over time?
>
> REF102 is far from either of these parameters, i.e. 1000ppm tolerance, 
> 2.5ppm/K, and in best case 5ppm/1000hr, that may be 40ppm/year only.
>
> Later you mention 20ppm "longterm" (??) stability..
>
> 1000ppm tolerance are 50 times worse than these 20ppm/year (?) 
> stability you tend to achieve.. and that REF102 is NOT easy to 
> calibrate .. you need a precise DVM for that, anyhow .. or how many 
> hundreds of REF102 do you want to average to reach reasonable 
> statistical tolerance and stability?
>
> There may be also better references with about 0.01% = 100ppm 
> tolerance, but that's probably still not sufficient, compared to these 
> 20ppm "longterm"..
>
> Perhaps you first clearly define your requirements in those three 
> technical parameters given above..
>
> Then you will for sure recognize, that you will only achieve that goal 
> with buried zener references, like LM399 or LTZ1000, stable external 
> components and trimming the output according to a standard. On 
> EEVBLOG, you may find two good and long threads about building your 
> own ones.
>
> Everything else is far away from being "The Perfect Volt", or you may 
> buy a JVS.
>
> Frank
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