[volt-nuts] Low noise reference

[Randy Evans]

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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