Re: [volt-nuts] The "averaging reference"
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!
On 19 Dec 2014 19:30, "Jan Fredriksson" jan@41hz.com wrote:
It's no coincidence that virtually all 8.5 digit DMMs use the LTZ1000.
It's in a class of it's own.
What do the 8.5 digit meters use if they don't use the LTZ1000?
Dave.
On 19/12/2014 22:13, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
On 19 Dec 2014 19:30, "Jan Fredriksson" jan@41hz.com wrote:
It's no coincidence that virtually all 8.5 digit DMMs use the LTZ1000.
It's in a class of it's own.
What do the 8.5 digit meters use if they don't use the LTZ1000?
Dave.
Well the 7081 uses well conditioned 1n829 zeners with a parabolic
temperature compensation scheme, but Mickle does not have a good opinion
on how well it works. but it could possibly be that the drift over the
decades since manufacture means it is running at the wrong current now.
The Datron 1081 used 4 zener diodes
Regards
Frans
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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On 20 Dec 2014 21:18, "Joel Setton" setton@free.fr wrote:
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.
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.
I suspect if you built something very stable using an LTZ1000, it would be
possible to get one or more volt-nut with a 3458A or similar to measure it
for you.
You could even average the result from several volt nuts.
Dave.
On Sat, 20 Dec 2014 22:17:57 +0100
Joel Setton setton@free.fr wrote:
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!
Well, that's where physics strikes back... and economics.
None of the materials known to men are completely gas or water tight.
And an exchange of gas/water leads to change of physical properties or
even chemical reactions. AFIK the most gas-tight enclosure is a
metal can (afaik only hydrogen and helium diffuse trough a steel can,
surface does not readily react with most substances found in air, but
needs non-metalic isolation for the wires going in/out),followed by glas
packages (AFAIK water tight, but not completely gas tight. also can act as a
getter material if outside surface is clean and in vacuum), followed
by ceramics (little gas exchange with the inside, but porous, ie can
store gases/water on the surface).
Plastic packages are mainly one thing: cheap. Neither gas nor air-tight,
they even store a lot of chemical compounds from production within the
material, that slowly leaks out. Also they are quite hygroscopic, to
the extend that chips are backed out before soldering, in order to
prevent the vaporizing water from breaking the chip. Even small changes
in the composition of the plastic can change the pin-to-pin resistance
from 10M to 2M. For normal electronics this doesn't matter, but here...
So, yes, it is possible to have better packages than just plastic.
But it is not economical to keep these around for the one or two people
a year who actually need them. (well, they do it with space grade components,
but they charge you 1000 times the price of the commercial equivalent).
Attila Kinali
--
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the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
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