Hello Fred,
I also had large common mode effects on the LM399 until I spent 2 100nF
ceramic multilayer capacitors to the heater and the zener
See C6 and C8 on Bobs cirquit.
http://c-c-i.com/sites/default/files/reference_voltage.pdf
Further you could try using batteries during measurement for the standard.
And charging only when no measurement is made.
Without these capacitors there was a difference up to 100uV depending where
I put my hand on the wiring.
(either near the Reference or directly to the NiMH-Batteries of one of the
references).
The voltage was measured as difference of 2 references to get improved
resolution.
The common mode effect came from the switch mode supply of my personal
computer.
With these capacitors the common mode effect was reduced to
My first ADCs did not have photocouplers and where galvanically coupled to
the PC.
With the decoupling capacitors the common mode effect was below 2uV.
Maybe also this were resulting thermocouple effects.
Now I have galvanic decoupling between PC and ADCs further reducing effects.
Common mode problems are only visible when I forget to disconnect the
switchmode chargers for my batteries.
With best regards
Andreas
----- Original Message -----
From: "Fred Schneider" <[email protected]>
To: "Discussion of precise voltage measurement" <[email protected]>
Sent: Saturday, October 20, 2012 10:47 AM
Subject: Re: [volt-nuts] Traveling Standards
I have made a LM399 standard, dead bug build, at 10V. A uA723 for
powersupply at 15V. In a metalhousing and shielded internal extra using
double sided pcb.
It is now 24/7 on for about 5 weeks.
I modified my solartron 7601 so it stays at 38C plus/minus 1 degree
internal. ( mounted two heatsincs on top, one above the transformer, a ntc
inside, and a speedregulated fan on the main heatsinc. Thanks to your post
i started monitoring it. So since a few days that meter is also on 24/7. A
few times a day Zi take readings ( i am digital disabled so logging
probably always will stay a dream) Room temp changes from 19 to 24
degrees.
I made a 10 turn potentiometer on the front that gives me some adjusting.
Still more as needed but until it is aged enough I keep it like this.
Resistors are all 0.02% or 0.01%
One whole turn of the multiturn is around 100 uV.
I adjusted it to 10.000,000 Volt at 19 degrees Celcius. This was a cold
mornig and a lot of rain. According to my bones humidity was high. Later
that day it became 21 degrees ( heath from instruments and light spots)
It was the 10.000,030V, yesterday a dry and warm day it ( 21 to 24
degrees) raised to 10.000,052) this moring 20 degrees, rain outside, it
was 10.000,058
The ten turn pot has a reading ( a dial with two arms like a clock) and i
made the standard so that the pot at 10.000,000 V was at the 5.00
position. Until that, I had to turn it between 4.45 and 5.15 to get it
back at 10V. So it still has a total drift around 50 to 100 uV ( monitored
it als a few days three weeks back using two meters and they showed the
same trend.
But i think I have a problem regarding magnetic fields or other sources
because I have strange issues while meauring using the 332 and KV deviders
or my LM399.
I looked to it with a friend and he thinks it is some magnetic field or
common mode thing.
I had powered everything off, even the lights. Then both 7,5 digit meters
showed the same values, turning the polarity did not matter. Both KVs were
lineair upto 1 uV. Using my 399 or Fluke 332, also tested the 332 divider
dircect.
Then with the lights on severa instruments ect. A big difference if i
change polarity, both meters gave different readings, both KV dividers
gave excact the same non lineairity. I did this several days and up to
about 1 mV !! between 100 mV and 10V but an verage of a few hundered mV)
if I power all stuff down exept the 332 and 7106 all is perfect again. All
intruments are grounded. I use shielded cables to the meters and from 332
to KVs from Beats me :-(
So I think your standard performs well if it is this stable while tossing
around, i wish my 332 and LM399 was that stable ( or more, the environment
was more standard friendly becaus all powered down things are much more
stable)
Fred PA4TIM
Op 20 okt. 2012 om 04:27 heeft Bob Smither <[email protected]> het
volgende geschreven:
On 10/18/2012 02:05 PM, Andreas Jahn wrote:
Hello Bob,
whats the matter with you. Infected by precision virus like me?
You wanted to have a standard with about 10ppm and now you blame a
3-4ppm drift.
:-) - afraid so Andreas!
The LM199A is hermetically sealed.
The PCB, the 8K Resistor and the voltage Regulator are not.
Although a small sample, the two references appear to be similarly
affected by
whatever caused the drift - similar range of drift, similar time
constant.
On the first view I would blame it on the meter.
It is very unusual that the drift of 2 different references has nearly
exact the
same amount of ppm and direction.
But on the other side you state that there are several HP3458A which
recorded
the drift.
It is not probable that all came freshly from calibration of a other
location.
Any ideas about what could cause the drift we are seeing?
From time constant it could be the humidity change.
This is my best guess.
My 2 LT1027CCN8-5 references which ara mounted only with 1 Pin
to the PCB have time constants in the range of 4-7 days.
The epoxy material of a PCB should lie in the same ball park area.
The LT1027 are influenced by around 0.5 ppm per percent humidity change.
For the hermetically sealed brand new references LT1236AILS they state
in their
new product catalog
a humidity change of less than 10ppm for 25% humidity change. (page 36)
http://cds.linear.com/docs/Product%20Info/NPC.pdf
This is most interesting - so even "hermetically sealed" units are
influenced by
humidity!
I asked them whether this is from mechanical stress from the PCB and
they
confirmed to me
that with a dead bug mounting the influence of humidity will be
virtually
unmeasurable.
So they will delete the parameter from the data sheet.
Mine are not "dead bug" mounted. The 'PCB' is in fact a Radio Shack perf
board
- certainly not the best substrate to mount them on - I don't think it is
FR-4
material.
So for the LM399 it might be mechanical stress introduced by the PCB.
When looking at your cirquit there are several points to mention:
One common failure source will be the LM78L15. A output voltage change
will
influence the supply of MAX6350 and the reference current of LM399.
PSRR of MAX6350 is about 2-5 ppm/V above 10V supply. (without self
heating
effects).
LM78L15 spec is 1mV/C. This would result in .02 ppm/C on the LM199A
(operated
at 1 mA with 8K resistor providing the current).
For the MAX6350 1mV/C and 5 ppm/V => .005 ppm/C
The LM399 resistor will give a current change of about 10% per Volt
(100uA)
resulting with 0.5 Ohm impedance in about 50uV/V or 7ppm/V
see note above.
Other weak points of the cirquit are:
The LM399 heater voltage is not stabilized. this will give about
0.5ppm/V
It is stabilized by the power supply - a 24 volt, .02%/C unit => 5mV/C =>
.0025
ppm/C.
And finally: was the LM399 always in the same orientation during
measurements?
(will be difficult with a cylindrical housing).
My LM399 drift 3-4 ppm by tilting orientation.
Not sure - but from the consistent results (consistent drift and apparent
settling) it likely was.
The above still leaves humidity induced PCB changes causing mechanical
stress as
likely. I am going to look for some better board material for when I
construct TS2.
Thanks Andreas!
--
"As we act, let us not become the evil that we deplore."
<smither.vcf>
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