Mhm,

im missing a 100nF at the output of the 7815.
and a 100 nF directly at the heater input  of the LM399.
Also at the MAX6350 a 100nF is missing.

It is essential that these capacitors are located directly
at the pins of the respective ICs.

I have seen a 3,3V Voltage Regulator shifting output voltage by
several 100mV with a 3V/m electric field strength.
(which is quite low compared with a wireles phone of 30V/m or a mobile with
up to 200V/m).
And there was already a 10uF Tantal at the output.
The voltage shift could be elimininated with a 100nF directly
at input and output.

With best regards

Andreas

Hi Bob,

I have been monitoring your thread since it got started.  Personally, you would
be better off spending your money on a used Fluke 731B off of eBay.  You would be
way ahead of the game compared to where you are at with your current concept.

First, the biggest issue is not having your circuit set to a "cardinal" voltage
point; i.e., 10 volts.  By having some other level the measuring party would have
to ratio against his standard to report a value.  That adds an unnecessary level
of complexity and increases the error limits in reporting the uncertainty.

Second, you need to provide separate power supply regulators for any heating
system and the reference voltage circuitry.

Third, your choice of a power supply regulator is very poor.  You should consider
something more exotic like the Linear Technology LT-3080 or the LT-3083.  In
particular, pay attention to the noise reduction considerations in the
specification sheet.  Preferably a two stage affair using a LT-3083 (handles 3
amps) as a pre regulator set to 18 volts and then the separate LT-3080 (handles 1
amp) devices to their respective circuitry.

Fourth, you need to pay particular attention to ground loops and your ground
plane layout.  Think along the lines of a star pattern, i.e., single point of
return.

Fifth, to do it right you should provide four (4) separate 10 volt references
with their own outputs in the package.  This would provide for independent
analysis of the "bank mean" and allows for determining if any individual
reference shifts outside of a nominal "bank mean."

Sixth, in designing the 10 volt reference, the output loading should provide the
highest possible isolation as to affecting the actual reference cell.  The output
circuit should provide a minimum of 10 milliamps without a any change in the
output level.

Seventh, if any adjustable elements are used in the design, they should be of the
smallest possible size as to resistance value.  This will help to minimize
temperature affects.  A "cermet" potentiometer type should be used with values
under 50 Ohms.  For example a 89pr10 (or 20 etc.).

Eighth, if it is going to be powered from the mains voltage, then a faraday
shielded transformer should be used.  Proper EMI & RFI shielding is also needed.

Just food for thought and, by the way, having done all of the above, you just
constructed a 731 if you just do one reference.

Bill....WB6BNQ

Bob Smither wrote:

WB6BNQ wrote:

Hi Bill,

Thanks for taking the time to provide your very thoughtful feedback on
this project.

The last 731B I saw on EBay was ~$800.00.  Unless I had it independently
calibrated (another expense) I would end up with another instrument in
my lab that I could not trust.  Even if calibrated, after 30 days it
would have 10 ppm uncertainly and after a year 30 ppm.

My objective with this project is to obtain a stability of 10 ppm,
verified by having several independent readings made by other volt-nuts
members.  If this objective can be realized, another advantage is the
unit can be passed around to help other list members.

Establishing a cardinal value such as 10 Volts would require an
adjustment.  I am hoping to avoid that uncertainty.  On a 10 volt scale
the ~7 Volt output of the LM199A provides 1.4 ppm resolution on a 6
digit VM - adequate for my objective of 10 ppm.

Good point!  I did overlook this.  The heater of the LM199AH draws about
18 mA after it stabilizes and is fairly constant.  It will certainly
change less than 5 mA over the expected temperature range of a lab.  The
load regulation of the LM78L15 is (worst case) 1.5 mV / mA resulting in
an error at the LM199AH of 0.13 ppm.  Although small, this can be
reduced further by powering the heater of the LM199AH directly from the
24 Volt supply and I will do that.

Although not shown directly on the posted schematic, the design does
provide two stage regulation.  As detailed in my notes on the design
page ( http://www.c-c-i.com/ts ), there is a separate external 24 volt
supply.  The specs on that supply are !% line regulation and .02%/C TC.
At the LM199AH, after secondary regulation from the LM78L15, these
errors translate to .06 ppm and .0011 ppm/C respectively.

The LT3080 is a great part.  I want to keep the package as small as
possible, but I don't do surface mount.  The through hole version of the
LT3080 is large compared to the LM78L15 (TO-92) that I am using.

The LM78L15 drifts at about 1 mV/C resulting in .018 ppm/C at the
LM199AH.  If we assume that labs will be between 20 and 30 C this adds
.2 ppm.

I have an idea to substantially reduce this error source and will try it
when I get time.

My instructions will be to measure only one output at a time.  Each
output (from the LM199AH, the MAX6350E, and the LM34) has a separate
return terminal and the return current is brought back to the return pin
of each device, effectively removing the effect of other loads in the
ground wiring.

The intent with this unit is that it will be measured using a voltmeter
or in a nulling arrangement.  I am assuming that the input impedance of
the measuring instrument will be high.  A 10Mohm load (.7 uA) on the
LM199AH introduces .1 ppm.

The external 24 Volt supply I picked has 240 mV p-p of noise.  After the
LM78L15 this is reduced to 3 mV p-p or .018 ppm at the LM199AH.

To recap - my objective is a shared standard that can be inexpensively
shipped to other volt-nuts.  After several of us have measured the unit
it will hopefully provide a reference that can be trusted to calibrate
other instruments, if only to the 10 ppm level.

Many thanks for your thoughts!

Best regards,

Bob Smither, PhD                                Circuit Concepts, Inc.

---=====
"We are over in Iraq trying to devise a new constitution for them.
Why don't we just give them ours?  We're not using it any more."
--Jay Leno

---=====
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Andreas Jahn wrote:

Thanks for catching that!

I will have to adjust my packaging concept, but have added those parts.
See the updated schematic here:

http://www.c-c-i.com/ts

I tried with a hand held walkie talkie - big effect.  More notes to add
to the instructions that will go with the traveling standard.  My cell
phone did not cause a measurable disturbance.

Thanks again,

Bob Smither, Ph.D.                                    Smither@c-c-i.com

---======
"The most potent weapon of the oppressor is the mind of the
oppressed." - Steven Biko

---======
Circuit Concepts, Inc.                                      281-331-2744

Bob,
Is it not better to let it age a bit befor it starts its journey ? I have build several standards a few months ago. I have not used them since but then I meaured them during two weeks or so and yesterday I put them on again. They still are not perfect stable. The short time drift took a day or two to get smaller but the long time drift still is not perfect. ( i am talking about 40-60 uV for a 1V and a 2.5 V standard.)

Also test it mechanical. See if it reacts on shocks, temp differences ect.

Fred PA4TIM

Op 6 sep. 2011 om 19:04 heeft Bob Smither smither@c-c-i.com het volgende geschreven:

Fred Schneider wrote:

Thanks Fred.  Both references (the LM119AH and the MAX6350) have several
hundred hours on them.  I have left them on for all the time since I put
together the first bread board.  When I was not working on them, they
have been powered.  They will accumulate an additional hundred hours or
so before I actually ship them.

I will do some mechanical tests as well.  The last thing I want to do is
"cook" the TS for a day or so at perhaps 100C and then see what has changed.

Thanks for the suggestions.

--
Bob Smither, PhD                                Circuit Concepts, Inc.

---=====
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