Author Topic: Idea: binned, salvaged LM399's from taobao for forum members  (Read 1693 times)

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Offline Cerebus

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Re: Idea: binned, salvaged LM399's from taobao for forum members
« Reply #25 on: March 24, 2018, 02:32:47 pm »
Throw a super-low-noise-regulator like the LT3042 at the battery and the battery related noise doesnt matter anymore. But in this case batteries are superior when it comes to available isolated measurement time indeed.

Go back and look at the noise spectral density for the NiCd battery 70pV/rtHz, if you add a regulator you'll add noise, much much more noise than from the battery. The best figure for the LT3042 is 2nV/rtHz, that's about 30 times noisier than the bare battery. You do not want to add a regulator.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline Andreas

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Re: Idea: binned, salvaged LM399's from taobao for forum members
« Reply #26 on: March 24, 2018, 03:44:49 pm »
. You do not want to add a regulator.
Hello,

Usually PSRR is limited and the references have a much larger noise than the batteries.
At the LM399 you have also to stabilize the heater voltage to get a stable output.
So you really want a regulator near the LM399.

With 90nV/sqrt(Hz) for the LM399 I would not worry to spend a low noise regulator.
Even the LT1763 with 20nV/sqrt(Hz) is below that figure.

And usually as calibrator the 0.1 ... 10 Hz noise is the more interesting since broadband noise is integrated away by the DMM.

Noise is also what limits resolution if you use of a 100 mV range instead of the 10V range of the instrument.
And of course every voltage range has its individual divider/amplification resistors in the DMM.
Usually the native (3 or 10V range) is the most stable. The stability in the 20mV or 100mV range may differ.
So practically you do not get a factor 100 improvement by this. Its more like a factor of 10.
And of course it does not help if all references drift in the same direction. (Which is very likely).

It was a big surprise for me when I did the first re-calibration of my LTZ#1 and LTZ#2.
I told our calibration department that I have a difference drift of 12uV between both.
And LTZ#2 has a zero drift against my best ADC at that time.
But the calibrator showed that both had drifted down. One by 8 the other by 19 uV.
So the ADC had drifted also nearly 3 ppm.

Edit: if you have a 34401A it might be better to use the ratio mode for relative measurements.
I really should do some stability measurements.

with best regard

Andreas
« Last Edit: March 24, 2018, 03:51:08 pm by Andreas »
 
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Offline Magnificent Bastard

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Re: Idea: binned, salvaged LM399's from taobao for forum members
« Reply #27 on: March 24, 2018, 04:23:48 pm »
For a few more $ you can get parts directly from Linear Tech (until the end of the month anyway when the web site will be taken down), and these have unquestionable pedigrees.

Wait, what! I'm getting to like ordering from Linear.com, especially since Mouser barely carries Linear. What did I miss? (Sorry to derail, a pointer would be fine).

Analog Devices bought Linear Technology, and they are going to get rid of the (excellent) Linear Technology site-- and you will be redirected to the "new and improved" (and really crappy) Analog Devices website.  I do not know what will happen to the Linear Technology store, as Analog Devices has their own, and I think it is going to be a nightmare until everything gets integrated.  I suspect that this is just the beginning-- Analog Devices says that they are "going to leave Linear Tech alone", and I say BULL$HIT-- they are going to screw it all up-- you will see.  If you need to order something from Linear Tech you better get it done fast as they are closing everything by the end of this month.  So, pretty soon, when you are looking for data on Linear Tech devices, you will need to (often) "click here to see more" (which is for smart-phones-- engineers use big screen workstations, so this is some kind of bull$hit to please executives).
 
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Offline 3roomlab

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Re: Idea: binned, salvaged LM399's from taobao for forum members
« Reply #28 on: March 24, 2018, 07:36:12 pm »
So, for a budget-minded volt-nut to track the volt, he needs access to cheap LM399's, and they need to be binned pretty tightly.

what about ALEX's JVR ?
http://www.eevblog.com/forum/metrology/building-your-own-voltage-reference-the-jvr/?all
buying a new JFET and new resistors eliminates the uncertainty of used 399s. and the extra budget can be shifted to the op-amps and better resistors (maybe PTF56 is already sufficient for this, but i dont know, ALEX n others will need to chime in, ANDREAS? any thoughts?).

it seem like this JVR doesnt care about temp drifts and has no heaters (very low power consumption). likely an added oven will make it even better?  :-//

i feel like suggesting JVR because, it looks like a one up DIY jellybean project that is cheaper (likely better?) than using a used 399 from a scrapyard source. why not just leave the 399 in the scrapyard? i think is worth re-using a 399, when it is not removed from its PCB, like a battered old fluke.
« Last Edit: March 24, 2018, 07:44:21 pm by 3roomlab »
[What Do You Care What Other People Think?] ... remember to brush your teeth before you read it ... *tchk tchk tchk* ...
 

Offline Andreas

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Re: Idea: binned, salvaged LM399's from taobao for forum members
« Reply #29 on: March 25, 2018, 03:44:29 am »
i feel like suggesting JVR because, it looks like a one up DIY jellybean project that is cheaper (likely better?)

Hello,

I think that Alex has spent very much time in adjusting his reference.
You will also need some equipment to do this.
So I would not call it "cheap" when you count in your time.

A naked LM399 reference (without the 10V transfer) is the cheapest project (price/performance ratio) that you can have.
You need only a voltage regulator 2 100nF caps and a pull up resistor.
Of course you can use a bootstrap cirquit, but I fear the most of us will not find any difference with their equipment.
Wait half a year until it stabilises and then you have only to find a possibility to calibrate it.
At best you build 3 or 4 of them so you can determine (and sort out) the one with largest drift.
You then have a robust working horse which you can use for many other things.

If you need stable 10V then it gets complicated.
You either need a temperature stabilized housing (like ap with his DVR1 device)
(which is clever when you want to sell the device)
or much time to characterize + adjust the device.
(which is fun but no one wants to pay your time).

with best regards

Andreas
 
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Offline cellularmitosis

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Re: Idea: binned, salvaged LM399's from taobao for forum members
« Reply #30 on: March 25, 2018, 05:05:25 am »
A bit off topic, but I jumped on the 'bay and got lucky.  Found an old Keithley scanner full of low EMF cards.  Wether I end up selling Vrefs or not, time to start recording data!

LTZs: KX FX MX CX PX Frank A9 QX
 


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