[volt-nuts] Low noise reference

[jasonpepas at gmail.com]

Apologies, the last message went out before I was finished drafting it. 

See these links:

https://www.eevblog.com/forum/metrology/ultra-low-noise-reference-2dw232-2dw233-2dw23x/

https://www.eevblog.com/forum/metrology/factory-2dw23x-order-aggregation-thread/

Note that these zeners are quite cheap. You could parallel a number of them for ever further noise reduction, yet still be cheaper than an LTZ. 



Sent from my iPhone

> On Feb 17, 2018, at 7:24 PM, jasonpepas at gmail.com wrote:
> 
> Hello,
> 
> Are you familiar with the 2DW232, a Chinese zener?  There is one particular factory in China which is somehow producing exceptionally low noise units (much lower noise than the LTZ1000). 
> 
> There is an EEVBlog forum member who lives in China and has graciously volunteered to perform group buys from this factory and then distribute the pets to the individual buyers. 
> 
> 
> 
> Sent from my iPhone
> 
>> On Feb 17, 2018, at 9:07 AM, Randy Evans <randyevans2688 at gmail.com> wrote:
>> 
>> Thanks for the feedback.  I don't think I am having problems with leakage
>> in the test setup, thermal, or shielding issues.  I used aluminum cast
>> boxes (Pomona 2391) which have BNC M and F connectors, which use teflon
>> insulators.  The cast boxes have enough thermal mass and not subject to
>> moving air currents, so it is unlikely that the wide voltage extremes, over
>> periods of a few to 10's of milliseconds, I am seeing are due to thermal
>> changes.  Also, I had a typo in the original message (I said pA when I
>> meant to type nA) in that the last sentence in the second paragraph should
>> read:  "and around 1 nA at 0.1V, but with widely varying leakage current of
>> 0.5 to 1.5 nA, with occasional peaks of -0.5 to 2 nA.  This would equate to
>> about +/- 2 uV voltage variation across R1, making a 10 V 0.1ppm stable
>> voltage reference of questionable value."  the first sentence in the fourth
>> paragraph should also be referencing nA's.  Sorry about the brain lapse.
>> 
>> The wide variations in current through the 100uF cap-1Kohm resistor are my
>> main concern since I can't explain it.  It is absolutely not present in the
>> 100 Mohm cal resistor in the same type aluminum cast box and is completely
>> stable.  I originally suspected interference but the cap-resistor and
>> calibration resistor are mounted in identical shielded boxes but the 100
>> Mohm cal resistor is clean and stable.
>> 
>> I suppose I need to bite the bullet and build the circuit and see how
>> stable it is.  I can check it with my two Fluke 732As and two HP-3458As.
>> 
>> Rrandall Evans
>> 
>> 
>>> On Sat, Feb 17, 2018 at 4:38 AM, Andre <Andre at lanoe.net> wrote:
>>> 
>>> also see https://workmanship.nasa.gov/lib/insp/2%20books/links/
>>> sections/407%20Splices.html
>>> 
>>> -Andre
>>> 
>>> ________________________________________
>>> From: volt-nuts <volt-nuts-bounces at febo.com> on behalf of Randy Evans <
>>> randyevans2688 at gmail.com>
>>> Sent: 16 February 2018 18:39
>>> To: Discussion of precise voltage measurement
>>> Subject: [volt-nuts] Low noise reference
>>> 
>>> I have a question for the group.  I was looking at an article for building
>>> an ultra-low noise voltage reference by Walt Jung, published in Electronic
>>> Design June 24, 1993 and a URL to the article is below.  I want to filter
>>> the output of an LTZ1000 based 10V reference I am building and this circuit
>>> has a very low freq corner of 1.6 Hz.  I was concerned about the leakage
>>> through R1-C1.  If C1 had as little as 1ua leakage, it would drop the
>>> voltage through R1 by 1 mV.  The spec on 100 uF electrolytic and tantalum
>>> capacitors show a leakage of 20 ua  at rated voltage so this could be of
>>> great concern.  However, at the low few tenths of a volt that should be
>>> across C1, the capacitor should have a much lower leakage amount, which is
>>> the theme of the article.
>>> 
>>> To get a better appreciation of the issue, I connected a precision 0 to 10
>>> V source (100uV resolution steps) to a series combination of a 1 Kohm
>>> resistor and a 100 uF electrolytic and, later, another 47uF tantalum and a
>>> 47 uF electrolytic capacitor.  In all cases the leakage, as measured with a
>>> Keithley 414 picoammmeter, showed a leakage or around 0.08 uA at 10V and
>>> varying 0.04 to 0.12 uA, around .1uA at 1V and varying , and around 1 pA at
>>> 0.1V, but with widely varying leakage current of 0.5 to 1.5 pA, with
>>> occasional peaks of -0.5 to 2 pA.  This would equate to about +/- 2 uV
>>> voltage variation across R1, making a 10 V 0.1ppm stable voltage reference
>>> of questionable value.
>>> 
>>> I also tried a 0.68 uF polystyrene capacitor and also saw leakage current
>>> variations, although much less than the electrolytic and tantalum
>>> capacitors, as one would expect.
>>> 
>>> Thinking the problem might be the the picoammeter, I put a 100 megohm 0.1%
>>> precision resistor in place of the capacitor across the precision voltage
>>> source set for 0.1 V and measured the current through the resistor at a
>>> very stable 0.9 pA on the Keithley 414 (sb 1pA but accurate enough for my
>>> measurements - the resistor shielded box likely has some sub pA leakage
>>> also).  Note that I used shielded cables for all measurements, and the
>>> resistor and capacitor were in a shielded box, as well as the 100 Mohm
>>> calibration resistor.  Touching the cables or boxes did not change the
>>> picoammeter reading at all, indicating to me that the shielding was
>>> reasonable.
>>> 
>>> I suppose the best approach is to build it and characterize it, but it's
>>> not fruitful if someone has already done this. So my question is: has
>>> anyone built this circuit and characterized it, particularly over
>>> temperature for stability at the sub ppm level?
>>> 
>>> Thanks,
>>> 
>>> Randall Evans
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>>> http://waltjung.org/PDFs/Build_Ultra_Low_Noise_Voltage_Reference.pdf
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