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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