Dr. Frank,
No. Kleinstein has a good point.
I can't give you my client data but I can point you to a couple clues that -are- in public domain.
Carefully measure the input bias current spikes on a '2057 - you're going to find something that looks like what's shown on page 13, or worse.
https://e2e.ti.com/cfs-file/__key/telligent-evolution-components-attachments/00-14-01-00-00-70-21-03/Chopper-Noise.pdf
Now pay attention to page 20 that cautions what happens with those current spikes if you're not careful: "A high impedance sensor becomes a transducer" Even though the LTZ zener isn't considered high impedance, you ARE turning the LTZ substrate into a transducer, guaranteed - even if you can't see the effect with your equipment directly. You don't really want that.
Make a bunch of LTZ's with chopper spikes applied directly to the die, and without...and let me know what happens over time when you look at average drift rates on both groups 3 or 5 yrs from now.
If you believe that running the LTZ at no warmer than necessary will help decrease long term drift, then you will also learn why current spikes (even in uA range) will have a similar effect on the die over time.
The effects are small - but when you're chasing ppm it all matters.
Your circuit will certainly work, but you don't have the best topology for slowest aging. That might not be too important to you if you're building only a few LTZ's, and you might not even notice. If you're doing production an best long term stability, yes it really matters: Pay Attention to Chopper Noise! On Inputs, Output and power rails. These amps are very useful when used correctly - but they can cause trouble if you're not ready for it.
That's why I recommend: If you want to give your LTZ the best chance at slower aging : Use an RC between your chopper amp and LTZ to isolate those current spikes from the die.
Mister Diodes,
I appreciate many of your contributions as good brain food, but in the end I have the same problems with your recent statements as well.
You do not present any evidence, like measurements, calculations, schematics, or else, for your claims about irreversibly affecting the LTZ 1000 chip by either chopper or EMC spikes.
You explained, that 'the effects are small', but there's no idea, nor any numbers about how small or how big these effects really are, or if they can be totally be neglected, compared to the thermally activated drift, of about -0.8ppm/year @ 45°C, which was
measured by Spreadbury et.al., and
described also by Pickering in some of his papers, and
specified for his DATRON 7001 reference.
Latter documents and the underlying experiments are simply an evidence, so no one has just to 'believe' something.
The T.I. paper you're citing, also does not support your claims, first because the LTZ zener, but also the whole circuit is relatively low-impedance, as you yourself already mentioned, and 2nd, as the 100nF output capacitor in the circuit by Andreas will dominantly absorb these chopper spikes in first place.
Your proposed measurement with and w/o RC filter, on several different LTZ1000, over 3-5years, is very unpractical, at best.
That's more an unrealistic approach, due to the diversity of any of two LTZ1000s, and due to the obviously very small expected magnitude of the effect.
Being noticeable only on a 5 or even 20 years timescale, as you already stated in another thread, seems to me to be completely irrelevant for our application here.
This eevblog forum contains a lot of open source material, like freely available designs, measurements and real facts from many other engineers and scientists.
Therefore I have a general credibility problem with all other contributors, who are not willing, or are not able to openly publish profound substance, and are just arguing in a hand-waving manner.
This way, you also withdraw yourself from critical peer reviews, which is a striking aspect of any scientific / engineering publication of this kind.
In that sense, and to get a real value from your posts, I would like to see from you at least a concrete description, and specification of your LTZ1000 application, in which environment they are located, i.e. if they are sitting isolated inside a double shielded box (as in the 3458A), or if the output is exposed to external disturbances, as in voltage references. Concrete figures for external / internal temperature, drift and noise immunity would also be helpful.
Thanks in advance
Frank
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