Ian Johnston | 26 Nov 13:39 2015

LM399 & LM399AH

Hi all,

Just wondered if anyone has experience of the A and non-A versions of the 
LM399. I.E. 0.3 - 1ppm max., versus 0.3 - 2ppm max.

In reality, is it worth the extra cost for the 1ppm max. 
version.................or in normal typical circumstances are they 
generally going to perform not as far apart?



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Ian Johnston | 25 Nov 23:29 2015

LM399 selection test PCB

Hi all,

Thought you guys might be interested in a small PCB I made in order I 
could test, select & check pre-age (if there is such a thing!) my LM399's.

10 channel.
Each channel has it's own chopper op-amp (self bias the LM399),
2ppm/degC tempco resistors throughout,
2x2 sockets for the LM399's (same footprint).
Test points for each channel.
Jumper link on the power rail to disable each channel individually 
(op-amp and heater power).
Holes around each LM399 footprint to help isolate any thermal stress.


So far it's working out great.......i've already discovered one of my 
LM399AH's (sitting in ch.4 in the photo) is somehow unstable. It's 
output jumps around by about 10uV's. Need to investigate........

The idea is that I can test, select and pre-age LM399AH's before 
soldering them into my target PCB's. I understand that soldering 
can/will reset any aging.......but I figured it might help.

I did a pre-test on those Ebay LM399's I bought and they seem to work 
ok.......still to properly evaluate.....but I do notice that the new 
ones's I bought from Digikey are all giving over 7Vdc, the Ebay ones all 
well under 7Vdc.

(Continue reading)

Charles Steinmetz | 25 Nov 11:34 2015

Re: Low-cost voltage reference questions

Adrian wrote:

>I guess the reference to the "standards" means those sold on ebay 
>US. If so, I would think it is a false expectation this would meet 
>3ppm acc. guaranteed within a year.

I didn't make any claim as to the ebay units, and don't know anything 
about them except what is stated by the seller.  I just said that 
their claims were "in the ballpark" of what I considered standards of 
interest to this group.  That was in relation to the OP's proposal of 
a 0.1% standard (1000 ppm) -- a standard could miss what I defined as 
interesting by quite a lot and still be in the ballpark compared to 1000 ppm.

Best regards,


Mark Sims | 25 Nov 07:08 2015

Low-cost voltage reference questions

Egad,  I just read through that EEVBLOG thread...   now I need a mind enema... 		 	   		  
Charles Steinmetz | 23 Nov 23:26 2015

Re: Low-cost voltage reference questions

Russ wrote:

>What is considered the break-over point of precision with low uncertainty
>versus cost to a group like this? Is there a rule-of-thumb for the cost of
>each additional digit of precision after N digits?

One person's opinion:

To a group like this, I'd be inclined to say that interest begins at 
a room-temperature (say, 20C +/- 3C) accuracy of 3ppm (i.e., 
guaranteed to remain within 3ppm from 18-22C for at least one year 
after purchase).  3 ppm is 0.0003%.  There is at least one 10v 
reference with specifications in this ballpark available at an asking 
price under $130 (I'm told the seller has accepted offers 
significantly lower than this).

>If I sell someone a reference
>that I've ascertained is 2.50163v  <at> 70.3 F with a calculated uncertainty, is
>it valuable as a 0.1% reference even though the error may be much less,
>like +/- 0.08%?

I, for one, do not consider 0.08% to be "much less" than 0.1%.  One 
sneeze and it's out of spec.  Indeed, I would consider a claim of 
0.1% accuracy to be bordering on fraudulent based on a calibrated 
measurement at 0.08%, unless the spec was qualified as "within 0.1% 
at [temperature within 0.1C] as is, where is -- no claim as to 
accuracy after it has been shipped to the buyer."

Speaking as someone with substantial commercial design experience, I 
would never offer a voltage reference for sale as a claimed "0.1% 
(Continue reading)

Russ Ramirez | 23 Nov 22:33 2015

Low-cost voltage reference questions

I am comparing designs centered around the MAX6126-25 and the TL431B (0.4%
grade). I have an HP 3457 that is in cal from Agilent and a NI DAQ unit
that is more useful at seeing the noise (unshielded) on the output (which
is +/- 600 uVs or so) and logging the warm-up time to a (+/- 1 mV) stable
output. I also have a multi-channel Keithley 740 thermometer scanner so I
know the ambient at which readings are being made.

What is considered the break-over point of precision with low uncertainty
versus cost to a group like this? Is there a rule-of-thumb for the cost of
each additional digit of precision after N digits?

I mean these as simple, don't get into all of the uncertainty contributions
and general traps of metrology, questions. If I sell someone a reference
that I've ascertained is 2.50163v  <at> 70.3 F with a calculated uncertainty, is
it valuable as a 0.1% reference even though the error may be much less,
like +/- 0.08%?

Thanks for your time and feedback.
Mark Sims | 23 Nov 20:06 2015

Differential amplifier >=1Mhz

Maybe a Tektronix AM502 module and the single slot TM501 mainframe? 		 	   		  
Andrea Baldoni | 23 Nov 11:44 2015

Differential amplifier >=1Mhz


Someone knows if it's possible to find on the market at a reasonable price
differential amplifiers instruments (packaged with power supply, connectors,
gain knobs, etc.)?
Nothing extreme, 1MHz BW, decent Voffset but not autozero... just to use them
in the lab to condition signals, to save time before making ad-hoc circuits on
the breadboard.

I found plenty of cheap old HP and TEK plug in modules for oscilloscopes (but
then you must add an enclosure with power supply and the thing start to be
heavy to ship to EU) or very expensive special purpose devices...

Best regards,
Andrea Baldoni
Andrea Baldoni | 22 Nov 12:00 2015

Keithley 181, 220, 617 ROM dumps

In the process of cleaning and controlling the instruments (some capacitors
need to be replaced and so on) I made a dump of the ROMs of Keithley 181 nV,
220 Is and 617 electrometer.

If someone need them, just ask.
By the way, they have very little firmware for actual standards.

Best regards,
Andrea Baldoni
Andrea Baldoni | 19 Nov 15:12 2015

Keithley triax cables, 6011 and 1506/1507


I will soon acquire a nVmeter, a current source and an electrometer made by
Keithley. They will come without any cable and manuals and in unknown working

One of the first steps is obtaining the cables.
I saw some of them for sale on Ebay, but I can roll my own, if it's possible
to do it without recurring to poisonous cadmium solder and the like (for the
nVmeter; the electrometer too will advantage from low EMF cables but maybe it's
not so important, or not? More important would be the cable insulation and

Someone already did it? What components (connectors, cable) do you suggest?

Best regards,
Andrea Baldoni
Andrea Baldoni | 13 Nov 17:15 2015

nA advice


I would like to measure insulation resistance of some defective load cells.
The measurement should be done with no more than 50V and the full scale
reading should be >=5Gohm. Accuracy of +-10% is enough (in fact it could
suffice just to check if it's >=5Gohm or not).

I have not been able to find a ready made instrument that could do this within
a reasonable price (100 EUR); the normal insulation resistance testers go much
high with the probing voltage (on the purpose, but this also lighten much the
requirements on the sensitivity of the meter).

It could be as simple as a led indicator "pass/fail", but at this point, why
not to build a good frontend for a multimeter, so it could be much more useful?

There are many way to do this and also many ready circuits (like the
null detector in the mini metrology lab by Conrad Hoffmann) or in the AoE,
but instead of making it from scratch, someone knows if there is something
already made (or a kit)?

Best regards
Andrea Baldoni