10V precision voltage reference

[fcb]

I've just released our first batch of LT1021 based 10V references, they're on our website (www.FisherInstruments.com) and I've put some up on eBay also.
These have been developed from an in-house prototype we built a while back and I've been testing a couple of batches of (50 units) for a couple of months now.

Ben

[RandallMcRee]

Looks interesting. Something seems to be lost between English and American here:

When I was young there was a sign on the wall in my grandfather’s film cutting room, it went something like this “Fred was a wheeltapper – one year Fred changed 387 wheels before he realised his hammer had a crack in it.”

Having experienced ‘wheeltappers syndrome’ a few times in the past (including with a venerable Fluke multimeter) we embarked on developing our own precision voltage reference.

What is wheeltapper's syndrome?

I'm guessing your multimeter was out of calibration and you did not realize, but still not getting the wheeltapper thing...ie. 

[fcb]

In the old days, a man with a large hammer would go round steam-trains and hit the wheels.  By listening to the sound (ringing) they made he could tell if there was a crack in the wheel.

Essentially Fred was using a defective hammer, so he ended up declaring perfectly good wheels faulty.  So it's been known in my family as "wheeltappers".

[mimmus78]

In the old days, a man with a large hammer would go round steam-trains and hit the wheels.  By listening to the sound (ringing) they made he could tell if there was a crack in the wheel.

Essentially Fred was using a defective hammer, so he ended up declaring perfectly good wheels faulty.  So it's been known in my family as "wheeltappers".

Well the problem arises when you begin to use two hammers ...

Inviato dal mio ONEPLUS A5010 utilizzando Tapatalk

[David Hess]

Wheeltapping is a go or no-go test though.

A man with one multimeter knows the voltage.  A man with two multimeters is never sure.

[The Soulman]

In the old days, a man with a large hammer would go round steam-trains and hit the wheels.  By listening to the sound (ringing) they made he could tell if there was a crack in the wheel.

Essentially Fred was using a defective hammer, so he ended up declaring perfectly good wheels faulty.  So it's been known in my family as "wheeltappers".

Huhmm, tapping a steel wheel with a normal or a cracked hammer wouldn't make it ring any different..

Otherwise nice story. 

[fcb]

Huhmm, tapping a steel wheel with a normal or a cracked hammer wouldn't make it ring any different..

Otherwise nice story. 

Probably so.  I've spent to much time today disappearing down the rabbit hole of wheeltapping.

[Andreas]

Hello,

All non-heated buried zeners (LT1021, LT1236, AD586, AD587, MAX6350) have around 0.6 ppmpp 0.1-10Hz noise.
(this is due to the fact as all are operated at below 1 mA for the zener itself)
So a 10V zener has 6 uVpp according to this rule (if it is not defective).

So a noise measurement should be done to sort out defective parts or parts with execssive popcorn noise.

Also a good LM399 has around 4uVpp (but the stray here is much larger).
So what do you expect as performance for metrology here?
Pure 1/f noise (gaussian distribution) can be filtered out by longer integration times.

And I would expect that all buried zeners have similar ageing drift for the chip itself.
So the most influence for metrology usability has the package and the die attach.

With best regards
 
Andreas

[ap]

LT gives an overview of the noise of their voltages references in 'How to choose a voltage reference'. As can be seen, the burried zeners (these are the most stable and required for precision applications) vary between 1ppm and 0.17ppm, with the LM399 being the worst, and the LTZ1000 being the best, the others are at 0.6ppm. Typical data. Amplifier stages may add, filters or averaging may reduce that, to a certain extent.

Hello Ben - can you add a DSO screendump how the 0.1 - 10Hz noise looks like ... - this is often one of the key parameters that causes the difference in classification of such instrument - is it made for the hobby industry (5 1/2 digit) or is the performance as good that it enters the arena of the metrology industry ?

It must be noted that the noise is not the most critical factor for a voltage reference's usability for precision (6 digit or potentially more) applications. Meters use integrating A/Ds and also averaging, and so measurement noise during calibration is filtered out more or less. E.g. a 3458A, when doing NPLC100 measurements, makes 10 NPLC10 measurements and averages these. Other meters do something similar, e.g. by applying a slow mode.
What is more critical for a voltage standard to be used in precision applications is stability (temperature, aging) and calibration uncertainty.  So a 0.6ppm pp noise reference is typically not a problem with a 6 digit DMM. But you do have a problem if the temperature drift is even only 1ppm/K, because your environment will have a few degres variation, and to keep the related change at 0.6ppm you would need +/-0.3C stability. Essentially impossible in a normal lab. And only IF your reference is 1ppm/K. And let alone aging drift. And let allone calibration uncertainty. A 34401A has a 90day spec in the 10V range of 25ppm (plus cal. uncertainty), with a test uncertainty ratio (TUR) of 4 or more (if you dont want / cannot to use guardbanding), the absolute uncertainty of the 10V reference used should be no worse than around 6ppm (including all aging/temp drift and calibration uncertainty).

[David Hess]

You can calculate the expected shot noise from the zener current and it is proportional to the square root of the current.  Or you can do the calculation in reverse to find the zener current after measuring the spot noise above the flicker noise corner frequency.

The flicker noise depends on device construction and has to do with charge carriers getting stuck for variable amounts of time.  Since this is also process dependent, some devices will be better than others and if everything is working correctly, this is what you grade for because the shot noise mentioned above should be constant for a given device type.  The name comes from the "flicker" it produces in the least significant digits of the ADC.

Filtering flicker noise is really annoying because its increase at lower frequencies so you can never get rid of all of it in a reasonable amount of time.  Chopping limits it very effectively in amplifiers.  The sin(x)/x response of an integrating ADC can be used to help filter it at the expense of measurement time but if you had less flicker noise, you could increase the sample rate and get the same useful resolution.  The best way to avoid it is to not generate it in the first place.

Popcorn noise is a special case of flicker noise where one time constant dominates.  If you could lower the amount of flicker noise enough, then it would become popcorn noise although at a very low level.  It should only show up with bad processing.  It is not that buried zeners have more popcorn noise but that they have less flicker noise making any popcorn noise more apparent.  The name comes from the sound it makes if you listen to it.

At low frequencies, the distinction between noise and drift loses meaning.

[fcb]

Hello Ben - can you add a DSO screendump how the 0.1 - 10Hz noise looks like ... - this is often one of the key parameters that causes the difference in classification of such instrument - is it made for the hobby industry (5 1/2 digit) or is the performance as good that it enters the arena of the metrology industry ?

Thanks
Butterfly

0.1Hz - 10Hz noise - I'll look into publishing this after I've researched an appropriate way of doing this (basically I need to document the amplifier used between the reference and the 'scope).

We also do the basic adjustment at 10 NPLC and then 100 NPLC, so 1/f noise is somewhat ameliorated and it's effect is very small compared to the 30ppm we quote for the product.

Neither the 389-1 and 389-2 are aimed at metrology, and are priced accordingly.

We have several research side projects on-going here into precision references (including digital temperature compensation of the LT1021), and we're collecting long term data from a group of 389-2's. So I guess our ultimate aim with the reference's is to make useful metrology grade device.

[eurofox]

Got one little bad boy from this company and check it with my 8 1/2 digit multimeter.

Calibration is spot on and only the 2 last digits "are moving"

For calibrating or check up to 6 1/2 digit multimeter it is certainly a winner.

eurofox

[Andreas]

Hello,

would be interesting how much the actual T.C. is (typical below 2.5 ppm/K according to data sheet).
Also interesting would be if there is any seasonal (humidity) influence or if the coating that they use actually protects against humidity.

with best regards

Andreas