Hello Fred,
When looking at the cirquit of your reference the OP-Amps may be a source of
common mode error too.
With my LTZ1000 Reference I tested each Pin of each semiconductor with a
metallic
needle (held with my hand) if it is sensitive to common mode effects.
The resulting cirquit diagram is in the archive from 6. November 2010.
Usually Im cleaning the PCB after soldering with denatured alcohol
to remove the dirt which might cause leakage currents.
And finally: I would never put trimmers with several hundreds of ppm/degree
tempco
within the direct signal path.
If you look at the internal cirquit of a precision reference the external
trimpot is
always used as voltage divider where the tempco cancels nearly out.
The wiper of the trimmer is fed over a reference internal series resistor to
a voltage divider.
The series resistor and the voltage divider on the chip have again equal
tempco which cancels out.
So the trimming of the output voltage has only little influence on the
tempco of the output.
With best regards
Andreas
----- Original Message -----
From: "Fred Schneider" <pa4...@gmail.com>
To: "Discussion of precise voltage measurement" <volt-nuts@febo.com>
Sent: Saturday, October 20, 2012 7:34 PM
Subject: Re: [volt-nuts] Traveling Standards
So I have nothing to complain, after the first week changing " much" it
now goes up/ down a bit with temp and probably humidity. At 10V, 1ppm is
10 uV so it is changes 5 to 6 ppm over 5 degrees. Today it changed less
then 1 ppm over 3 degrees so I that leaves humidity. I do not have a
hygrometer so it becomes time I get one.
Thanks for the tips for the common mode, i will try it soon,
http://www.pa4tim.nl/wp-content/uploads/2011/06/lm399schema_1.jpg
This is the schematic ( completely my own design but I'm no EE so it
probably will not be to great) but I appriciate comments for improvement.
http://www.pa4tim.nl/?p=2531 pictures are at the bottom of the page, all
my reference experiments are here together.
Fred PA4TIM
Op 20 okt. 2012 om 17:41 heeft "Andreas Jahn" <andreas_-_j...@t-online.de>
het volgende geschreven:
Hello Fred,
It is now 24/7 on for about 5 weeks.
5 weeks is nothing in the life of a reference.
During the first year my 2 LM399 references drifted 500uV against each
other
The 2nd year it was only 220uV
now I know that LM399 #1 is drifting much more (200uV) than LM399 #2
(20uV) in the 2nd year.
In the 3rd year LM399#1 drifted around 140uV and LM399#3 less than 10uV.
So not all references seem to be usable for precision measurement.
Its shurely no fault to start with a set of several references and select
out the bad ones.
With best regards
Andreas
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