[volt-nuts] Best reference after LTZ1000

Marvin E. Gozum marvin.gozum at jefferson.edu
Wed Aug 25 00:52:58 UTC 2010

Hello Andrea,

WarrenS has just posted a dataset similar to what I am still 
collecting.  It has been done!  But his reflects special selection of 
the AD587 chip and a modification to it he describes in an old post 
at time-nuts.


FWIW, I am collecting preliminary data before connecting my PC and 
doing something similar to WarrenS, so I can get a feel for the 
response.  I make several thousand automated measurements daily, 
covering 75-80F, 35-50% Relative humidity, over the past 
week.  However, I am using a 3456A and have no GPIB adapter so I 
cannot download the data, so I am doing it the hard way, allowing the 
HP to do statistical analysis, and recording the variance for fixed 
time intervals, and recording temp and humidity manually.

My curve is similar to WarrenS but I can only resolve 10uV.  Just 
quickly, my stats as I type, my standard deviation is 15uV from a 
mean of 10.000 05 Vdc.  Thus, my 95% confidence interval is 10.000 00 
to 10.000 10, and this includes the internal error of the HP 3456a 
too.  This is a fairly consistent response over 1 week except the 
mean will travel as the temp deviates from the 73F when Geller 
calibrated the SVR.

It is fairly crude compared to WarrenS's results, but it has a 
similar response. I cannot be sure which contributes to drifting 
more, the HP 3456a, the Geller, or both!

But overall, as you see, the reference is not shabby, and now 1 week 
since purchase is still reading true.

At 06:43 PM 8/24/2010, Andrea Baldoni wrote:
> > measure it as soon as possible, lest it drift!  Sure enough, under the
> > same temperature conditions, its accurate to my calibrated 8456A,
> > reading 10.000 00 Vdc.  I am now gathering stats on its performance.
>You are doing an interesting work. Interesting also would be
>to buy a barebone chip and measure it untrimmed.

As WarrenS comments, a reason for getting the assembled board is a 
need to burn in the assembly and the chip for hours, I forget if 
Geller does 10 or 200 hours before calibration, then calibrate 
it.  The SVR calibration includes both trimmed and untrimmed 
calibration values, so if you think the pot has gone bad, you can 
remove a jumper to detach it from the circuit, and measure the native 
reference output directly.


Marv Gozum
Philadelphia, PA  

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