[volt-nuts] The "averaging reference"

[Joel Setton]

Jan,

Thanks for a good summary f the pros/cons. Of course the LTZ1000 is much 
closer to the current state of the art, but the REF102 is far easier to 
use and to calibrate. I'm definitely not shooting for sub-ppm 
performance, if I can build anything that stays within (say) 20 ppm 
long-term, that would be more than adequate as a home standard.

I wasn't aware of the degraded long-term drift performance in the 
plastic packages, as compared to the metal can. I'm surprised they can't 
protect the chip from package-induced effects!

One thing I don't like about the LM199 and LTZ1000 is that although they 
are stable, they are sold uncalibrated. As a result, building a 10-V 
reference with either of them would require at least two very stable 
resistors, one of which must be selected within a range of several 
percent to get an accurate 10V output. Most of the DVMs I have seen with 
the LM199 / LTZ1000 use "soft calibration" in which the calibration 
coefficient is stored in memory, and the voltage measurement is 
performed in ratiometric mode. Building a 10V reference is a rather 
different problem.

As before, comments and suggestions will be welcomed!

Joel Setton



On 19/12/2014 19:28, Jan Fredriksson wrote:
> It's no coincidence that virtually all 8.5 digit DMMs use the LTZ1000.
> It's in a class of it's own. REF102 is not in the same class, even if
> you average a handful.
>
> But there are a couple of nice things about the REF102, though for
> more moderate requirements
> - You get a reference at 10V, +/-0.0025V, trimmable (not a 5% 7V of the LTZ1000)
> - Moderate power / current
> - Low sensitive to supply voltage
> - Very simple to implement
> There was a metal can version but it's obsolete. But be aware that
> the TI site still shows the metal can spec 5ppm/1000h while the
> available packages are actually 20ppm/1000h!
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