Author Topic: Building your own voltage reference - the JVR  (Read 133487 times)

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Offline Alex NikitinTopic starter

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Building your own voltage reference - the JVR
« on: October 04, 2015, 10:20:38 am »
With so many discussions and data on precision voltage references, I've decided to resurrect my old (about 1993) idea of building my own voltage reference from scratch. At the time I had no means to measure it performance accurately, and even now, my two Keithley 2015 are struggling. The circuit I've built yesterday on my bench is deceptively simple, essentially one JFET and one resistor. I've used one half of U440 dual FET with a rather high (about 4V) cut-off voltage, connected a multi-turn W/W pot in place or R1 and adjusted the pot for a near-zero tempco of Vref output by heating the JFET with a soldering iron and looking at the voltage changes as it cools down back to room temperature. Then I've measured the resistance of the pot and replaced it with a couple of metal film resistors (standard type, 100ppm/C, nothing special). The beauty of this circuit is that the resistor changes reflect on the output voltage at about 1/100 of a value, i.e. 100ppm change in the resistance only creates about 1ppm change in the voltage - and the same applies to the supply voltage, about 1/90 ratio I've measured. I've used LH0070 as the supply regulator to avoid a possible supply influence though. The reference voltage for this particular sample of U440 is 3.75308V at 23C.

I did not "tune" the reference further and the circuit was just sitting on the bench overnight. I've recorded the reference voltage for over 12 hours and for about 5C temperature change the voltage changed from 3.75308V down to 3.75301 at the lowest point and came back to 3.75308 again after the room warmed up back to 23C (making the tempco about 4ppm/C, nicely linear, so it can be tuned better). I've tried to see a thermal hysteresis by first freezing the JFET with a freezer spray and then heating it with a soldering iron for about 5 sec, however the voltage recovered nicely back to 3.75308V after the device cooled down.

So here it is. If somebody with a better than mine measuring capabilities can have a go at this simple arrangement, I will be happy to see the results. I can see one serious problem with this circuit - the voltage reference point would be unique for each JFET sample , so it is difficult to use such a circuit in a production equipment. I will eventially build a proper boxed version with a scaling buffer to get, say, exactly 10V out of it. When I'll do it I'll post an update here.

Cheers

Alex




« Last Edit: October 04, 2015, 12:05:05 pm by Alex Nikitin »
 
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Offline TiN

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Re: Building your own voltage reference - the JVR
« Reply #1 on: October 04, 2015, 10:52:30 am »
Voltage on LM399/LTZ's are also unique, but that's zero problems, as it's calibrated out in software/processing.
Unlucky to me, I don't have LH0070 or U440 to test your circuit idea though.  :-//
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #2 on: October 04, 2015, 10:58:21 am »
Voltage on LM399/LTZ's are also unique, but that's zero problems, as it's calibrated out in software/processing.
Unlucky to me, I don't have LH0070 or U440 to test your circuit idea though.  :-//

I've just used what was available in my lab  8) . You don't need specifically LH0070 - "any" voltage regulator with a better than 100ppm/C tempco will do nicely, and U440 I've chosen for the metal case, again, "any" JFET has a "zero tempco" point, it is just reasonable to use ones with a higher cut-off voltage, i.e J112 may work well for example, though it is in a plastic case.

Cheers

Alex
« Last Edit: October 04, 2015, 12:31:56 pm by Alex Nikitin »
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #3 on: October 04, 2015, 11:31:30 am »
Here is the JVR reference output voltage recorded by Keithley 2015 over ~6.5 hours from 10PM till about 4:30AM, the temperature dropped from 23C to 20C over that time with the heating switched off at midnight. Vertical scale is 2ppm/div.

Cheers

Alex

« Last Edit: October 04, 2015, 11:42:36 am by Alex Nikitin »
 

Offline poorchava

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Re: Building your own voltage reference - the JVR
« Reply #4 on: October 04, 2015, 11:52:55 am »
This thing begs for an oven :)

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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #5 on: October 04, 2015, 11:56:01 am »
This thing begs for an oven :)

Wys?ane z mojego HTC One M8s przy u?yciu Tapatalka

The tempco is easily adjustable to near-zero at a particular temperature. With a dual JFET like U440 I've used, the second FET's junction can be used as a temperature sensor  ;) .

Cheers

Alex
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #6 on: October 04, 2015, 12:15:51 pm »
Here is a 30 min time span "zoomed-in" at the flat part of the graph, with 1ppm/div vertical scale. This is on the bench in open-air, so the "slow" variations easily can be due to thermal effects. Even a simple enclosure may make it better. On the other hand my Keithley 2015 noise at 10V 10NPLC is about 0.6ppm p-p from 10V, or ~1.5ppm p-p at 3.75V, so a large part of variations could be the 2015  ::) .

Cheers

Alex

P.S. I need HP3458A  :-\

« Last Edit: October 04, 2015, 12:25:27 pm by Alex Nikitin »
 

Offline Andreas

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Re: Building your own voltage reference - the JVR
« Reply #7 on: October 04, 2015, 04:02:42 pm »
and adjusted the pot for a near-zero tempco of Vref output

I can see one serious problem with this circuit - the voltage reference point would be unique for each JFET sample

Hello,

interesting finding.

I already had the idea to use a BF245C as "zero quiescent current 5V" voltage regulator (e.g. for a real time clock IC) but never as voltage reference.

The other "problem" is that you will need a output buffer.
Every draw of current will change tempco + output voltage.

with best regards

Andreas
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #8 on: October 04, 2015, 06:00:14 pm »
The other "problem" is that you will need a output buffer.
Every draw of current will change tempco + output voltage.

Hi Andreas,

A buffer is required but it is not a difficult task to make one with a chopper opamp and, say, a foil potentiometer. These are just usual design details. What I like about that reference is that the tempco is adjusted "internally" to zero, and there is only one JFET makes the reference, the influence of other parts on the Vref output is easily manageable. The reference can be bootstrapped to the buffer output like in the circuit below (it is a simplified circuit, as one may need a start-up circuit to get this arrangement to work reliably on a single supply).

Cheers

Alex

« Last Edit: October 04, 2015, 06:02:01 pm by Alex Nikitin »
 

Offline ralphd

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Re: Building your own voltage reference - the JVR
« Reply #9 on: October 04, 2015, 10:49:03 pm »
I already had the idea to use a BF245C as "zero quiescent current 5V" voltage regulator (e.g. for a real time clock IC) but never as voltage reference.

That sounds interesting too.  Do you have a schematic?
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Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #10 on: October 05, 2015, 12:04:01 am »
I *do* know that buried Zener references have far less time-drift than band-gap references

XFET voltage references on the other hand have had some outlandish long term stability claims in the past, they are no longer present in the current datasheets ... but I'm not so sure they were too far off the mark.

Look at this complaint from Linear about the claimed performance. Yes, without burn in they measured quite a bit of drift. After 400 hours though it became pretty flat, except for the large amount of 1/f noise which they intentionally didn't remove. Pity there aren't any hermetic XFET references.
 

Offline Andreas

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Re: Building your own voltage reference - the JVR
« Reply #11 on: October 05, 2015, 05:37:37 am »
I already had the idea to use a BF245C as "zero quiescent current 5V" voltage regulator (e.g. for a real time clock IC) but never as voltage reference.

That sounds interesting too.  Do you have a schematic?

Hello, (sorry for off topic)

nothing special, I am using the pinch off voltage of the gate as voltage reference.
Drain is input source is output.
Gate is tied to GND.
As the cirquit is very high impedant you will need 2*100nF for stability.
A diode against reverse polarity.
And a protection resistor against transients; thats all.
Schematic + simulation is for 2N4119.


With a BF245C I typically get around 4.0V idle and 2.8-3V with a 1K resistor as load.
But usually a clock only draws some microamps.
Input voltage is up to 25V. (30V).

With best regards

Andreas

 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #12 on: October 05, 2015, 05:39:23 pm »
Would some foam and mylar be able to isolate the FETs well enough to use the second FET as the heating element? (If so you could just bias it with a source resistor and put a current limited voltage on the drain, it will have a single temperature equilibrium).
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #13 on: October 05, 2015, 05:47:15 pm »
Would some foam and mylar be able to isolate the FETs well enough to use the second FET as the heating element? (If so you could just bias it with a source resistor and put a current limited voltage on the drain, it will have a single temperature equilibrium).

Yes, I've looked at that option, however it is only possible for devices with a high enough drain current - not all of them are good in that respect. At the moment I would like to go further with the main reference circuit, the thermal stabilisation would come next. Today I've found an easy (and reasonably cheap) way to get standard voltages (5V and/or 10V) from this circuit. 10V is preferable because the output can be used to supply the reference  ;) . I'll test this idea soon and will update the thread with the results.

Cheers

Alex
 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #14 on: October 05, 2015, 06:05:12 pm »
Yes, I've looked at that option, however it is only possible for devices with a high enough drain current

No need to use Vg=0 here, a bit of drift from a resistor divider for the temperature setting isn't that critical, so IDSS isn't a limiting factor.
 

Offline ralphd

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Re: Building your own voltage reference - the JVR
« Reply #15 on: October 05, 2015, 06:54:35 pm »
I already had the idea to use a BF245C as "zero quiescent current 5V" voltage regulator (e.g. for a real time clock IC) but never as voltage reference.

That sounds interesting too.  Do you have a schematic?

nothing special, I am using the pinch off voltage of the gate as voltage reference.


Thanks.  So the regulated voltage really depends on the current and varies from part to part?

After looking at the prices for jfets (~20c ea), it seems like more of a curiosity than a practical application since LDOs like the AMS1117 and Torex 662K cost 3-5c ea.
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #16 on: October 05, 2015, 09:07:25 pm »
So, to scale the voltage up is easy, just needs one quality resistor in series with the reference JFET, like this:



The FET's current is as stable, as the reference Vgs voltage and it just scales up. R2 should have a low tempco though.

Here is the first 30min run's results (vertical scale is 0.5ppm/div  8) ):

 

One important change from the first prototype. This circuit is still hard-wired and in open air on my bench, however I've used <10ppm/C precision resistors for R1 and R2. From the start I did not expect to get into single ppm/C digits so I've used cheap resistors. However I've found that if I tune the circuit accurately, the largest drift component comes from the resistors! Now I'll leave the circuit on overnight and record the voltage for 7-8 hours.

Cheers

Alex

P.S. - it also lookls like a lot of LF noise was coming from the resistor. Now the noise on the graph is dominated by the noise of the Keithley 2015 I use to measure the voltage.
« Last Edit: October 05, 2015, 09:15:33 pm by Alex Nikitin »
 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #17 on: October 05, 2015, 09:56:57 pm »
Alex, but now you are back to having the resistor drift having an unattenuated effect on your output voltage again. Goodbye potential long term stability.
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #18 on: October 05, 2015, 10:27:09 pm »
Alex, but now you are back to having the resistor drift having an unattenuated effect on your output voltage again. Goodbye potential long term stability.

I am not planning to design a replacement for the LTZ1000  ::) . And if you need a certain standard voltage you have to use some resistors. At least here it is only one really stable resistor required (and an opamp buffer, but that is not a problem).

Cheers

Alex
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #19 on: October 06, 2015, 07:27:23 am »
Here is the overnight data, the vertical scale is 2ppm/div. For about the same three degrees C temperature drop the voltage change was ~12ppm, or ~4ppm/C, at 5V with the additional scaling resistor. I may yet try to tune it a bit better.

Cheers

Alex


« Last Edit: October 06, 2015, 07:56:22 am by Alex Nikitin »
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #20 on: October 06, 2015, 03:09:29 pm »
As U440 is not easily available, I've also tried a different (and very cheap) JFET - J112. The sample I've used has a near-zero tempco point at ~3.25mA drain current, ~1.75V Vgs. I've scaled it up to 5V with standard 100ppm/C metal film resistors and it again looks like these resistors are the main source of temperature-related voltage variations. The resulting drift, I would estimate at about 20-30ppm/C. Here is the result for a 30 min run in open air (2ppm/div vertical scale).

Cheers

Alex



 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #21 on: October 06, 2015, 03:15:29 pm »
Supposedly zero tempco for a JFET is near the pinch off voltage plus 0.63V.
 

Offline Kalvin

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Re: Building your own voltage reference - the JVR
« Reply #22 on: October 06, 2015, 03:28:20 pm »
Would placing a few J112-based voltage references in parallel give better results, what do you think?
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #23 on: October 06, 2015, 03:33:05 pm »
Would placing a few J112-based voltage references in parallel give better results, what do you think?

I think it would be more sensible just to use good low tempco resistors, and, if you need the best stability, a temperature control. By the way, a single device with the same chip inside as the U440 is J211, still made by Fairchild Semi (and available from DigiKey at 15p each).

Cheers

Alex
« Last Edit: October 06, 2015, 03:53:10 pm by Alex Nikitin »
 

Offline Bud

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Re: Building your own voltage reference - the JVR
« Reply #24 on: October 06, 2015, 04:44:25 pm »
By a coincidence i just went through "The art of electronics" 3rd edition reading on JFETs. The autors do not recommend using JFETs at high source-drain voltage (which your circuit is) because above a few volts gate current raises dramatically, by several orders of magnitude. I'd imagine this may have effect on this circuit out V drift, and make it dependent on the primary regulator output voltage which is the JFET input. But im not good enough with JFETs yet to say for sure. Anyway, something you may need to have a look at and perhaps to test.
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #25 on: October 06, 2015, 05:01:51 pm »
By a coincidence i just went through "The art of electronics" 3rd edition reading on JFETs. The autors do not recommend using JFETs at high source-drain voltage (which your circuit is) because above a few volts gate current raises dramatically, by several orders of magnitude. I'd imagine this may have effect on this circuit out V drift, and make it dependent on the primary regulator output voltage which is the JFET input. But im not good enough with JFETs yet to say for sure. Anyway, something you may need to have a look at and perhaps to test.

By a coincidence, I knew about that nuance of JFETs probably before the first edition of "The Art of Electronics" was published (was it what, 1980? - yes, I am that ancient  :( ). Also (not by a coincidence this time, but as a normal design practice), I tend to look at datasheets of devices I plan to use before actually using them. And in the datasheet for the U440 you may find the following diagram:



And I also have in my library an old catalogue from Siliconix where one can find the same graph for the J112 device (and the gate current for it  starts to increase substantially only for Vds over 15V .

Cheers

Alex

P.S. for 1ppm effect you would need the gate current at about 1-3nA ( for 1-3mA drain current), by the way. Look at the graphs. Even at 125C and 10V the gate current does not reach that level.

« Last Edit: October 06, 2015, 05:11:47 pm by Alex Nikitin »
 

Offline Kalvin

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Re: Building your own voltage reference - the JVR
« Reply #26 on: October 06, 2015, 05:13:22 pm »
How well this simple and very affordable JFet-based voltage reference compares to the LM399? Let's assume that one creates a simple "oven" by gluing an appropriate resistor to the J112 as an heater element and places tho whole thing into a styrofoam or similar insulator? Let's assume the constant current through the heater resistor will be good enough to keep the temperature within 1 degree of Celcius. Any educated guesses?
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #27 on: October 06, 2015, 05:27:42 pm »
How well this simple and very affordable JFet-based voltage reference compares to the LM399? Let's assume that one creates a simple "oven" by gluing an appropriate resistor to the J112 as an heater element and places tho whole thing into a styrofoam or similar insulator? Let's assume the constant current through the heater resistor will be good enough to keep the temperature within 1 degree of Celcius. Any educated guesses?

1) The main question would be the long time stability. It is an unknown. My guess would be that unless the junction is damaged by a static discharge, the stability of the JFET itself (especially in a metal can) should be very good to excellent. After all we are looking at a simple single structure, not a complex one as in the LM399 or any other IC reference.

2) It is reasonably easy to create a temperature control down to 0.1 degree, especially for a small device like that, using something like LM35 for a temperature sensing.

I am not sure that it is sensible to look at this circuit as a replacement for the LM399 before some long-term tests are done on it. My aim was to show a simple way of making a voltage reference in a rather unorthodox way, and frankly I am surprised at the level of performance actually available from this circuit.

Cheers

Alex
« Last Edit: October 06, 2015, 05:37:31 pm by Alex Nikitin »
 

Offline 6581

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Re: Building your own voltage reference - the JVR
« Reply #28 on: October 06, 2015, 05:39:13 pm »





C
 

Online Gyro

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Re: Building your own voltage reference - the JVR
« Reply #29 on: October 06, 2015, 06:04:04 pm »
I'm beginning to wonder if there is too much emphasis to oven implementations rather than temperature compensation / characterized predictable temperature coefficient in all of these references.

Surely long term drift in the oven temperature controller is as much of an error contribution as drift in the reference itself - particularly in a reference as simple as this one (as opposed to LM399 syndrome as Alex hinted). It just seems a bit 'brute force'.

Wouldn't putting the whole thing in an isothermal metal box, insulated to limit rate of temperature change and avoid thermal stresses work better in a home lab environment? Putting the effort into characterization and compensation instead.
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Offline Bud

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Re: Building your own voltage reference - the JVR
« Reply #30 on: October 06, 2015, 06:19:46 pm »
How well this simple and very affordable JFet-based voltage reference compares to the LM399

Perhaps the dealbreaker is in repeatability. For mass production you throw an integrated IC with tight tolerances and no worries every single unit of the product will be as good as the prototype. Whereas JFETs characteristics differ largely between manufacturers and even within the same batch. The datasheet for J112 says cutoff voltage can be anywhere from 1 to 5v. Having said that this circuit operates at near cut-off voltage on the gate, chances are each unit will need to be individually adjusted for the specified output voltage.
But if you only need one off for personal use then not a problem.
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #31 on: October 06, 2015, 06:27:29 pm »
How well this simple and very affordable JFet-based voltage reference compares to the LM399

Perhaps the dealbreaker is in repeatability. For mass production you throw an integrated IC with tight tolerances and no worries every single unit of the product will be as good as the prototype. Whereas JFETs characteristics differ largely between manufacturers and even within the same batch. The datasheet for J112 says cutoff voltage can be anywhere from 1 to 5v. Having said that this circuit operates at near cut-off voltage on the gate, chances are each unit will need to be individually adjusted for the specified output voltage.
But if you only need one off for personal use then not a problem.

 :-+

Exactly. If, however, the long term stability happens to be in the range of few ppm per year (and I have a sneaky suspicion that it might, given a metal case and a good attention to the construction and the resistors used), than it is a different ball game altogether  8) . For that one may even select the most suitable FETs out of a large batch (I've just bought 100 of J211 from DigiKey - it cost me less than one LTZ1000 would, even including UPS delivery from the US). But we would know that only in a year or so. Or at least in a 1kh after I'll get my hands on the HP3458A  ::) .

Cheers

Alex
« Last Edit: October 06, 2015, 06:30:47 pm by Alex Nikitin »
 

Online Gyro

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Re: Building your own voltage reference - the JVR
« Reply #32 on: October 06, 2015, 06:59:15 pm »
Quote
Perhaps the dealbreaker is in repeatability. For mass production you throw an integrated IC with tight tolerances and no worries every single unit of the product will be as good as the prototype. Whereas JFETs characteristics differ largely between manufacturers and even within the same batch. The datasheet for J112 says cutoff voltage can be anywhere from 1 to 5v. Having said that this circuit operates at near cut-off voltage on the gate, chances are each unit will need to be individually adjusted for the specified output voltage.
But if you only need one off for personal use then not a problem.

However neither of the 'superstars' ie. LTZ1000 and LM399 is tight initial tolerance. The have quite a large variation requiring adjustments on an individual basis. Stabilty and long term drift are excellent but initial tolerance is poor.
Best Regards, Chris
 

Offline branadic

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Re: Building your own voltage reference - the JVR
« Reply #33 on: October 06, 2015, 07:01:25 pm »
I might be wrong, but I found J112 to be TO92 and SOT23-3 only, no metal can. So humidity sensitivity can cause a big influence, right? A big advantage of your U440.
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #34 on: October 06, 2015, 07:08:40 pm »
I might be wrong, but I found J112 to be TO92 and SOT23-3 only, no metal can. So humidity sensitivity can cause a big influence, right? A big advantage of your U440.

Yes, that is correct. However the humidity sensitivity is usually due to the resistors on the chip, here we don't have any. It is possible that for a JFET even in a plastic case it should not be a problem.

 ::) Now I need an environmental chamber as well!

Cheers

Alex
« Last Edit: October 06, 2015, 07:11:31 pm by Alex Nikitin »
 

Offline Kalvin

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Re: Building your own voltage reference - the JVR
« Reply #35 on: October 06, 2015, 07:23:11 pm »
::) Now I need an environmental chamber as well!

I guess it is still not too late the sell those 100 J211s and forget the whole project, and buy the LTZ1000 :)
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #36 on: October 06, 2015, 07:25:53 pm »
::) Now I need an environmental chamber as well!

I guess it is still not too late the sell those 100 J211s and forget the whole project, and buy the LTZ1000 :)

Nah, that would be too easy. I am looking for fun, not for gain  8) .

Cheers

Alex

P.S. - on the other hand, the LTZ needs a very good board layout and a lot of expensive supporting electronics. Sort of fun too, but it is already a known road  ;) .
« Last Edit: October 06, 2015, 07:35:18 pm by Alex Nikitin »
 

Offline branadic

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Re: Building your own voltage reference - the JVR
« Reply #37 on: October 06, 2015, 08:02:12 pm »
Quote
however the humidity sensitivity is usually due to the resistors on the chip, here we don't have any. It is possible that for a JFET even in a plastic case it should not be a problem.

Are you sure about that? What about the Source-Drain-channel, not some kind of a resistor though? Not sensitive to mechanical stress?
Computers exist to solve problems that we wouldn't have without them. AI exists to answer questions, we wouldn't ask without it.
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #38 on: October 06, 2015, 08:10:21 pm »
Quote
however the humidity sensitivity is usually due to the resistors on the chip, here we don't have any. It is possible that for a JFET even in a plastic case it should not be a problem.

Are you sure about that? What about the Source-Drain-channel, not some kind of a resistor though? Not sensitive to mechanical stress?

The only way to be reasonably sure is to test thoroughly. I have no data on a single JFET package sensitivity in that respect. On the other hand, everything is sensitive to everything, the only question is to what degree  ;) .

Cheers

Alex
 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #39 on: October 06, 2015, 08:15:21 pm »
Are you sure about that? What about the Source-Drain-channel, not some kind of a resistor though? Not sensitive to mechanical stress?

Does a metal can maintain constant air pressure?
 

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Re: Building your own voltage reference - the JVR
« Reply #40 on: October 06, 2015, 08:19:36 pm »
Are you sure about that? What about the Source-Drain-channel, not some kind of a resistor though? Not sensitive to mechanical stress?

Does a metal can maintain constant air pressure?

AFAIK, strictly speaking, no, as it is though a metal package is hermetic, but not evacuated, and the internal gas pressure should change with temperature.

Cheers

Alex
 

Offline codeboy2k

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Re: Building your own voltage reference - the JVR
« Reply #41 on: October 07, 2015, 03:23:08 am »
Supposedly zero tempco for a JFET is near the pinch off voltage plus 0.63V.

There is a 1973 US patent (long expired now)  3,760,199  that details a manner for determining a compensating voltage for biasing any JFET into it's 0-tempco region.  The goal of the design in the patent was to determine Vp, the pinchoff voltage, at any temperature, and a compensating voltage Vc, such that Vgsz, the gate-source voltage needed to bias the JFET at its 0-tempco region was Vgsz = Vp + Vc.  Since Vp, the pinchoff voltage is temperature dependent, then he develops Vc = 0.64 + a temperature dependent part opposing the tempco of Vp. With proper placement of some PN junctions and some BJTs and resistors, and a little bit of KVL, the inventor develops a voltage loop involving Vp, Vc and 0.64 and places the second JFET's gate-source across the point where those voltages are summed, yielding Vgs = Vp + Vc + 0.64, without ever knowing what Vp was (so it can vary with the standard process variations).  The second JFET will now have this temperature compensated bias voltage applied to it's gate wrt source and develop a nearly constant drain current that is, for the most part, temperature independent.  Very Cool!

It's an easy patent to read and a good one too. I think it was intended for use at chip scale, so that all the elements are matched and isothermic.  It might not work with discrete parts if someone were to try it out. 

 
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Offline ralphd

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Re: Building your own voltage reference - the JVR
« Reply #42 on: October 07, 2015, 04:26:16 am »
I might be wrong, but I found J112 to be TO92 and SOT23-3 only, no metal can. So humidity sensitivity can cause a big influence, right? A big advantage of your U440.
Immerse it in mineral oil in a tictac container.  No humidity effect and more temperature stability.
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Offline jwm_

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Re: Building your own voltage reference - the JVR
« Reply #43 on: October 07, 2015, 04:44:36 am »
Now that I have my scavenged cryocooler getting to temperatures it can liquify air ( ) and keep high temp superconductors superconducting indefinitely, I thought a neat trick would be to be the ultimate volt-nut and combine it with my rubidium standard to make a Josephson junction voltage reference. First step is to get a SQUID working, it would at least show the effect is reproducible with my setup. I think it is unlikely I will succeed on my allotted budget, but I am sure it will be a fun ride anyway.

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #44 on: October 08, 2015, 07:09:46 pm »
Well, my 100 of J211 arrived from DigiKey and I've measured the cut-off voltage on the lot at 5V Vgd. Actually, a very consistent batch for JFETs, it looks like the processing did improve since 1980s. All hundred measured between 2.75V and 3.35V at ~3uA current (10M of Keithley 2015 on 100V range between the source and gate). My "experimental" U440 measures 4.6V in the same setup (and the zero tempco point is at ~3.74V, so the "theory" about 0.63 or 0.64V is not working  ;) ). One obvious bad point about plastic casing - there is some photo-sensitivity, so the device should not be exposed to light when used as a reference. Not much of a problem though. I am nearly finalised a complete reference circuit, which would take 12-24V supply and provide 10V output with up to 10mA current available and would contain only two FETs, an opamp and few resistors. Not temperature regulated - that would be the next step.

Cheers

Alex
« Last Edit: October 08, 2015, 07:18:08 pm by Alex Nikitin »
 

Online BravoV

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Re: Building your own voltage reference - the JVR
« Reply #45 on: October 08, 2015, 07:18:31 pm »
..... I am nearly finalised a complete reference circuit...

 :popcorn:

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Re: Building your own voltage reference - the JVR
« Reply #46 on: October 08, 2015, 07:43:14 pm »
:popcorn:

I am not going to publish a complete circuit any time soon, only measurement results  8) . I plan to include a temperature sweep from 0 to 40C and a 1000h stability test against LM399 (I don't have LTZ1000  :( ).

Cheers

Alex
 

Offline Bud

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Re: Building your own voltage reference - the JVR
« Reply #47 on: October 08, 2015, 08:13:14 pm »
 :popcorn: ... :popcorn:... :popcorn:
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Re: Building your own voltage reference - the JVR
« Reply #48 on: October 11, 2015, 02:37:55 pm »
OK, let's not get all this popcorn wasted  8) .

I've decided to try a very simple circuit to get 10V output and to use the J211 transistor as a reference. This is a "budget" solution, as it requires only two good quality resistors (R1 and R2 and I've used 15ppm metal film for that, "padded" to the required value by some standard 100ppm/C resistors, so I should not expect much better performance than this value), cheap J211 JFET and an inexpensive INA133 chip connected as a gain 2 buffer with low tempco (~2ppm/C). Altogether is less than £10 in parts (resistors R3-R6 are internal for the INA133).

Here is the circuit which I've assembled on an old piece of a pcb:



And here is a 30-min run on the bench (vertical scale is 1ppm/div):



My estimate is that the temco around 23C is indeed about 10-15ppm/C. The power supply voltage variations from 12V to 28V create less than 2ppm/V change.  The load regulation is better than 0.5ppm/mA (a conservative estimate, as a load current up to 5mA, sink or source, creates less than 1ppm voltage change). The supply current is about 2mA.

First R1 is selected for the lowest tempco and then R2 for 10V output voltage.

Cheers

Alex

« Last Edit: October 11, 2015, 03:42:55 pm by Alex Nikitin »
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #49 on: October 11, 2015, 06:53:15 pm »
I've seen this schematic some days ago in the LF355 datasheet, it made me think about you! Your idea is not new.

Thank you, I always thought that it was used before but couldn't find a confirmation!

 :-+

Cheers

Alex

P.S. - would be interesting to find if that circuit was used somewhere in practice!

P.P.S. - The 2N4118 has a very low drain current (around 100uA for near-zero tempco) - it should make it good for low power but fairly noisy as a reference. A higher current device would make it a very low noise option - much quieter than ~1ppm p-p LF noise of the LM399. 
« Last Edit: October 11, 2015, 07:23:46 pm by Alex Nikitin »
 

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Re: Building your own voltage reference - the JVR
« Reply #50 on: October 15, 2015, 09:22:04 pm »
Well, my two prototypes (5V with U440 and 10V with J211) are running nicely and manage to stay inside +/-10ppm range at a somewhat varying room temperature, without any obvious drift. I am waiting for some high stability wire-wound and bulk foil resistors to be delivered so I can start building a "superstable" version, where all influences except that only of a JFET itself would be below 1ppm/C. The aim is to get a stability better than +/-10ppm in 20-30C temperature range (or better than 2ppm/C) without temperature stabilisation and then check the long-term drift for 1000h with a temperature log.

Cheers

Alex

P.S. - here is a 30min run on the bench of a somewhat "fine tuned" version on J211 device. Vertical scale 1ppm/div, Keithley 2015 works with a 10x averaging filter at NPLC10 to reduce the noise.

« Last Edit: October 15, 2015, 10:30:37 pm by Alex Nikitin »
 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #51 on: October 16, 2015, 05:38:02 am »
Might it be possible to get a low chosen voltage (say 2.5V) at near zero tempco by using an opamp based "constant" current sink in parallel with the source resistor?
« Last Edit: October 16, 2015, 06:33:08 am by Marco »
 

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Re: Building your own voltage reference - the JVR
« Reply #52 on: October 16, 2015, 06:47:49 am »
Overnight (from 12am till 6am, in open air), 1ppm/div:



Cheers

Alex
« Last Edit: October 16, 2015, 07:49:10 am by Alex Nikitin »
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #53 on: October 16, 2015, 07:16:31 am »
Might it be possible to get a low chosen voltage (say 2.5V) at near zero tempco by using an opamp based "constant" current sink in parallel with the source resistor?

It is not difficult to get lower voltages - just by choosing a different JFET. The sample of J211 I'm using right now has the reference voltage around 2.6V and J112 I've tried earlier - even lower, about 1.7V. I suspect however that it could be better to have this voltage as high as possible for a low noise and better stability.

Cheers

Alex
« Last Edit: October 16, 2015, 07:50:50 am by Alex Nikitin »
 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #54 on: October 16, 2015, 08:11:07 am »
The point wasn't to get a lower voltage, the point was to get a given voltage. It would be nice to have the elegance of the first circuit (where variations from everything but the JFET on output voltage are highly attenuated) but at a specific voltage. If you have 2.5V you can make 5v out of it with a switched capacitor circuit.
 

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Re: Building your own voltage reference - the JVR
« Reply #55 on: October 16, 2015, 10:13:53 am »
The point wasn't to get a lower voltage, the point was to get a given voltage. It would be nice to have the elegance of the first circuit (where variations from everything but the JFET on output voltage are highly attenuated) but at a specific voltage. If you have 2.5V you can make 5v out of it with a switched capacitor circuit.

Well, in that case an additional current source should also have a very high stability (means at least some top quality resistors) and would complicate things. An additional resistor gives you the required voltage above the reference, nothing more is required.

Cheers

Alex
 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #56 on: October 16, 2015, 11:07:33 am »
I'd have to the math to be sure, but as far as I can see drift in the current source has an attenuated effect due to transconductance just like variation in the source resistor (but NOT the second resistor you are adding in your current circuits).

PS. doing even a small bit of math in my head showed me I was wrong, I was overestimating transconductance by many orders of magnitude.
« Last Edit: October 16, 2015, 11:31:55 am by Marco »
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #57 on: October 16, 2015, 11:13:39 am »
I'd have to the math to be sure, but as far as I can see drift in the current source has an attenuated effect due to transconductance just like variation in the source resistor (but NOT the second resistor you are adding in your current circuits).

Yes, however there is only one reference voltage point and you can not move it by adding a current, you only can adjust the voltage to that point by an external current if the source resistor has not quite correct value. It is easier IMHO, just to get the resistor right in the first place (or to use a trimmer to get the last 100ppm).

Cheers

Alex
 

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Re: Building your own voltage reference - the JVR
« Reply #58 on: October 16, 2015, 07:22:06 pm »
OK, Friday night - a torture night  >:D . As the J211 version runs rather well and I am still waiting for the bits to build a controlled temperature chamber, I've decided to torture the JVR on record, first with a freezer spray for 10sec (about -50C or 70 degrees drop), then with a soldering iron for 20 seconds flat on the plastic case of JFET (I would estimate it as at least 70 degrees rise). Unlike the U440 in a metal case, some hysteresis is clearly visible, about +20ppm after the cold session and about -30ppm after the hot one. Here I have to note that the freezer session was not the first and on the first session I didn't freeze the FET so deeply, however the hysteresis was more - about 50ppm. Clearly for a stable reference a metal case is much better. (A note - the "wave" on the second graph after 400 samples is just me adjusting the voltage back to 10.00000 Volts).

Otherwise it is clear from the graphs that in both cases the voltage drops about 0.2-0.25% or 2000-2500ppm for 70-75 degrees change, so at 23C the reference sits nicely on the very top of the curve and has the lowest possible tempco. With a metal case and the temperature stabilised around, say, 45-50C (and the tempco adjusted for zero at that temperature), the stability should be very good.

Cheers

Alex

Each record is about 6min long:




« Last Edit: October 16, 2015, 07:51:57 pm by Alex Nikitin »
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #59 on: October 17, 2015, 02:03:10 pm »
A quick estimate - the temperature coefficient for this version (10V with J211 and INA133) is about 5ppm/C with few degrees change near 23C and up to 30-40ppm/C at temperature extremes.

Cheers

Alex
 

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Re: Building your own voltage reference - the JVR
« Reply #60 on: October 17, 2015, 05:21:30 pm »
To compare my results with a quality band-gap reference, I've decided to repeat my torture tests on LT1019-2.5 in DIL8 I had in the lab (with a stated 5ppm/C typical and 20ppm/C maximum tempco). This chip did show however a lot of hysteresis - about +600ppm after a freeze and -400ppm after a soldering iron application, leaving a total permanent shift about +200ppm. Ouch!  :o

Cheers

Alex
« Last Edit: October 17, 2015, 06:09:50 pm by Alex Nikitin »
 

Offline Messtechniker

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Re: Building your own voltage reference - the JVR
« Reply #61 on: October 18, 2015, 11:16:12 am »
Since I am new to this game please bear with me.

Simply as a first test, I built your circuit without the amplifier and with the components at hand. FET was BF 246 C and 2N 3819 new old stock resting for some 40 years in the box. Resistors were all standard metal film ¼ W, 1%, 50 ppm. Stability of these FETs was pretty awful +/- 300 to 500 ppm over 1 hour. So I had a look at the power supply. This was a LM 317 set to 24 V (to power the amplifier at some later date). Load was 10 mA approx. (dropping resistor and LED) and fed with 30 V from a benchtop power supply. Surprisingly drift was around +/- 5 ppm at room temperature.

Then to investigate further I changed the output to around 10 V. Feed voltage now being 20 V. What I then got is for you to see.

At the beginning it’s the test rig without me in the room. The droop before the peak is me entering the room whereby the “air blast” seemed to cool the LM 317 a little. The big peak is due to the soldering iron placed at the LM 317 for a short while. Then left the LM 317 to settle with some hysteresis being apparent. 25 ppm approx.

So I’ll probably follow this route and put the thing in a metal box and heat it to some temperature. Since the TO-220 casing is flat, I plan to attach flush one BD 139 (TO 126) as the temperature sensor and two further BD 139s on the outsides as heaters bolted together as a single compact pack. Hope this makes for a fair voltage reference, and this with standard components only.

Yours Messtechniker.

P.S: This is great fun. :)
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Re: Building your own voltage reference - the JVR
« Reply #62 on: October 20, 2015, 03:13:08 pm »
P.S: This is great fun. :)

Oh, yes!

I've just bought the Fluke 731B and a lot of precision wire-wound resistors. I would like at least match the stability on the Fluke (and, as a side point, to be able to calibrate in-house the DC parameters of my two Keithley 2015).

Cheers

Alex
 
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #63 on: October 21, 2015, 06:20:29 pm »
While I am waiting for the Fluke and resistors, here is a 21 hours run of the same 10V version using J211. The Keithley 2015 runs with 40 averaging, vertical scale is 1ppm/div. I've noted the room temperature at some time points. It looks like the TC around 23C is ~2ppm/C  8) .

Cheers

Alex

 

Offline Andreas

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Re: Building your own voltage reference - the JVR
« Reply #64 on: October 22, 2015, 04:42:05 am »
I've noted the room temperature at some time points. It looks like the TC around 23C is ~2ppm/C  8) .

The TC of the Keithley (according to datasheet) or of the JVR?

With best regards

Andreas
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #65 on: October 22, 2015, 07:31:13 am »
I've noted the room temperature at some time points. It looks like the TC around 23C is ~2ppm/C  8) .

The TC of the Keithley (according to datasheet) or of the JVR?

With best regards

Andreas

Hi Andreas,

Your guess is as good as mine - the Keithley TC, noise and drift all included! The only possible clue is that the "fast" changes are more likely to be that of the JVR itself due to the air movements. As you may see there are variations in the evening when I was moving around the lab and in the morning hour when I was preparing to go to work, while at night and during the day there was very little air movement and the changes are more gradual and uniform. I hope that the Fluke 731B will provide me with a more stable reference than the 2015s (I'm running two in parallel though recording only one - and the difference between two 2015 varies from 4 to 6ppm). I plan to record the Fluke against 2015s after a good warm-up and then record the difference between the Fluke and the JVR. I also plan to build the "ultimate" 10V JVR using U440 and a Vishay Foil resistor divider for the 5 to 10V step up.

Cheers

Alex
« Last Edit: October 22, 2015, 07:41:16 am by Alex Nikitin »
 

Offline Andreas

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Re: Building your own voltage reference - the JVR
« Reply #66 on: October 22, 2015, 09:26:23 pm »

 the "fast" changes are more likely to be that of the JVR itself due to the air movements.


Hello Alex,

air movements?
or changes by parasytic capacitance?
have you checked what happens when your hand moves over the cirquit or touch the power supply and voltage output lines?

My "zero quiescent current voltage regulators" were very sensitive in this regard.
 (several volts output change without blocking capacitors).
I had to use the blocking capacitors to keep the noise away from the gate diode.

With best regards

Andreas
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #67 on: October 22, 2015, 09:40:53 pm »

 the "fast" changes are more likely to be that of the JVR itself due to the air movements.


Hello Alex,

air movements?
or changes by parasytic capacitance?
have you checked what happens when your hand moves over the cirquit or touch the power supply and voltage output lines?

My "zero quiescent current voltage regulators" were very sensitive in this regard.
 (several volts output change without blocking capacitors).
I had to use the blocking capacitors to keep the noise away from the gate diode.

With best regards

Andreas

Hi Andreas,

No, these "fast" changes are actually quite slow - minutes. And the reference is not sensitive to the external interference or capacitive coupling. I can touch it, move it, shake it and, as long as I don't transfer any heat from my hand to the circuit, it stays rock solid. I am now building a 0.1Hz-10Hz filter-amplifier, with the gain 10000 and self-noise around 100-150nV p-p in the pass-band, so I can accurately observe and measure the LF noise.

Cheers

Alex
 

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Re: Building your own voltage reference - the JVR
« Reply #68 on: October 27, 2015, 08:09:45 pm »
Well, the Fluke 731B arrived yesterday (no battery though, so I've disconnected R30 to avoid the indicator overload and added noise). It was last calibrated by Fluke in 2009 and agrees with both of my Keithley 2015s within 10ppm on the 10V output. I've run it overnight and it did show about -1ppm/C TC  - see the graph below, taken with NPLC10 and averaging over 100 samples. On the previous night I did run my 10V JVR with J211 using the same settings on the 2015. Below both graphs using the same scale. Again, I've noted the room temperature at some points. Next step would be to put the JVR in a suitable enclosure as the "faster" variations are most likely due to the air movements in the room.

Cheers

Alex






« Last Edit: October 27, 2015, 08:15:54 pm by Alex Nikitin »
 

Offline Messtechniker

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Re: Building your own voltage reference - the JVR
« Reply #69 on: October 28, 2015, 08:08:56 am »
Well, here now the next development step of my poor man’s 10 V reference based on the venerable LM 317 with its internal bandgap reference. The LM 317 is now heated by two BD 139 trannies on each side with a third BD 139 serving as a temperature sensor sandwiched in between making a single compact block bolted together with a M3 bolt. The arrangement is now insensitive to air movements although not yet contained in a metal box.

The next development steps will be:
1.)   Replace the metal M3 bolt and nut of the block with a plastic M3 bolt and nut.
2.)   Put the LM 317 block with the heating circuit on an new and clean hole dot matrix epoxy pcb. (instead of the currently used hard paper hole dot matrix pcb. material and avoiding flux residues) in a small metal box.
3.)   Provide separate power supplies for the LM 317 block (+15 V) and the heater (+15 V, -15 V and - 5 V)
4.)   Do some extended burning-in and subsequent ppm measurements. Watch this space.

By the way, my Keysight 34465A DMM set to the 10 V range and with its input shorted produces an output fluctuating at about 0.2 ppm (as measured over 1 hour). So my measurement limit, i.e the contribution from the 34465A in the enclosed graph will be about 10 times that at 2 ppm re. 10 V.

My aim is to produce a voltage reference circuit specc’d for around +/- 10 ppm (20 ppm total) perhaps, using cheap off-the-shelf components only.

What do you think?

Yours – Messtechniker

P.S. Still having fun with this. :)
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Online BravoV

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Re: Building your own voltage reference - the JVR
« Reply #70 on: October 28, 2015, 12:01:11 pm »
Messtechniker, schematic and photos please, tia.

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #71 on: October 28, 2015, 12:05:31 pm »

What do you think?

Yours – Messtechniker

P.S. Still having fun with this. :)

Hi,

Where are the voltage setting resistors placed? You need to keep these at a stable temperature too if you are looking at <20ppm stability.

Cheers

Alex
 

Offline Messtechniker

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Re: Building your own voltage reference - the JVR
« Reply #72 on: October 28, 2015, 01:25:02 pm »
Messtechniker, schematic and photos please, tia.

Circuit is still at a very early stage (see remark below). Will provide it as soon as I (hopefully) get improved results.

Where are the voltage setting resistors placed? You need to keep these at a stable temperature too if you are looking at <20ppm stability.

The voltage setting resistors are currently placed close to the heated LM 317 but horizontally on the pcb. Based on your remark I'll put them upright and against each side of the LM 317 in thermal contact. Will look messy, but will probably be more effective. Thanks for the hint.

Yours - Messtechniker
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Re: Building your own voltage reference - the JVR
« Reply #73 on: October 28, 2015, 09:12:19 pm »
Well, here now the next development step of my poor man’s 10 V reference based on the venerable LM 317

My aim is to produce a voltage reference circuit specc’d for around +/- 10 ppm (20 ppm total) perhaps, using cheap off-the-shelf components only.

What do you think?


I think you could get better results when using a LM723.
the LM317 has alone a "typical" noise of 30 ppm RMS = 2mVpp.

with some clever design you could use the current limiting transistor
as chip temperature sensing diode for the oven.

with best regards

Andreas
 

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Re: Building your own voltage reference - the JVR
« Reply #74 on: October 28, 2015, 11:25:31 pm »
Two more graphs, taken at NPLC1, no filtering, for 30 minutes. The room temperature was about 24.5C, the JVR is still in open air on the bench. I was hoping to compare the noise, however the noise in the graphs is completely dominated by the noise of Keithley 2015  :( .

Cheers

Alex



 

Offline TiN

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Re: Building your own voltage reference - the JVR
« Reply #75 on: October 29, 2015, 01:16:36 am »
Why NPLC1? Run NPLC5 ;).
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Re: Building your own voltage reference - the JVR
« Reply #76 on: October 29, 2015, 09:53:06 am »
Why NPLC1? Run NPLC5 ;).

OK, I'll try. I am building an amplifier with 0.1-10Hz filter to measure the p-p noise on a scope, that should give a much more accurate result.

Cheers

Alex
 

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Re: Building your own voltage reference - the JVR
« Reply #77 on: October 29, 2015, 08:52:37 pm »

OK, I'll try. I am building an amplifier with 0.1-10Hz filter to measure the p-p noise on a scope, that should give a much more accurate result.

Which of the cirquits / application notes do you use for this?

With best regards

Andreas
 

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Re: Building your own voltage reference - the JVR
« Reply #78 on: October 29, 2015, 11:00:50 pm »

OK, I'll try. I am building an amplifier with 0.1-10Hz filter to measure the p-p noise on a scope, that should give a much more accurate result.

Which of the cirquits / application notes do you use for this?

With best regards

Andreas

I took the response graph from the OP27 datasheet and simulated a suitable circuit using the OPA2227 opamp with a couple of 9v batteries for the P/S.

Cheers

Alex
 

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Re: Building your own voltage reference - the JVR
« Reply #79 on: October 30, 2015, 08:08:15 am »


I took the response graph from the OP27 datasheet and simulated a suitable circuit using the OPA2227 opamp with a couple of 9v batteries for the P/S.

Cheers

Alex

Hello,
we are talking of figure 31 in the AD datasheet?
http://www.analog.com/media/en/technical-documentation/data-sheets/OP27.pdf

With this cirquit the input impedance is 10 Ohms.
The DUT has to be below 0.1 Ohms without having too much errors.
(And the output voltage has to be very close to 0 Volts without saturation).

A standard Zener has around 10 Ohms. Your cirquit will have several 10s of Ohms.

Have a look on AN124
http://cds.linear.com/docs/en/application-note/an124f.pdf

Similar results can be get by a 2 stage design with LT1037 and LT1012 + selected (low leakage current)
standard electrolytic capacitors.

And dont forget the (metal) cookies box (when using LT Parts) or the (metal) paint can (when using TI parts).

With best regards

Andreas

 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #80 on: October 30, 2015, 08:42:04 am »


I took the response graph from the OP27 datasheet and simulated a suitable circuit using the OPA2227 opamp with a couple of 9v batteries for the P/S.

Cheers

Alex

Hello,
we are talking of figure 31 in the AD datasheet?
http://www.analog.com/media/en/technical-documentation/data-sheets/OP27.pdf

With this cirquit the input impedance is 10 Ohms.
The DUT has to be below 0.1 Ohms without having too much errors.
(And the output voltage has to be very close to 0 Volts without saturation).

A standard Zener has around 10 Ohms. Your cirquit will have several 10s of Ohms.

Have a look on AN124
http://cds.linear.com/docs/en/application-note/an124f.pdf

Similar results can be get by a 2 stage design with LT1037 and LT1012 + selected (low leakage current)
standard electrolytic capacitors.

And dont forget the (metal) cookies box (when using LT Parts) or the (metal) paint can (when using TI parts).

With best regards

Andreas

Hi Andreas,

I only took the graph as a reference, the circuit there is not suitable. I also have an option to adapt one of my FET phono stages circuits for this filter-amplifier, the simulation shows that I can get a gain of 1000 with the right frequency response, however I am not sure how quiet the input FET will be at 0.1-10Hz. I'll probably try it anyway, as I have plenty of nice pcbs with an excellent low signal level behaviour and even some good 24V DC power supplies (including some rechargeable batteries) left from the time I was building and selling these plus these fit perfectly into an aluminium Hammond enclosure. If I can get <100nV p-p self-noise, I'll be quite happy.

Cheers

Alex
 
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #81 on: October 31, 2015, 09:10:45 pm »
I've received some Vishay Bulk Foil 5K+5K resistor dividers - I plan to use these for 5V to 10V amplifier in the "ultimate" version of the JVR. I've decided to check how accurate they are. With 0.05% stated tolerance I was not expecting a much better performance, however the measurements show that the resistor pairs are matched very closely - the worst one out of ten is about 15ppm off, and the best two were good to 1ppm or less, as that was the limit how accurate I could measure the error of the divider in my simple improvised DC bridge, using Keithley 2015 as a null-meter (the inconsistency in the results with plugging/unplugging the resistors and repeated measurements were about 1ppm).

Cheers

Alex



« Last Edit: October 31, 2015, 09:35:13 pm by Alex Nikitin »
 

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Re: Building your own voltage reference - the JVR
« Reply #82 on: November 13, 2015, 07:48:00 am »
Will 2N4392 TO-18 (datasheet) work decently ? Our local store carries it.

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #83 on: November 13, 2015, 08:48:50 am »
Will 2N4392 TO-18 (datasheet) work decently ? Our local store carries it.

Yes, it looks like a very suitable device, with a good Vgs(off) between 2 and 5V., and a metal case. Another possible option available is 2N4857A/2N4856A.

Cheers

Alex

P.S. I've ordered some of 2N4392 and 2N4391 from Mouser, it looks like the cheapest TO-18 metal can JFET available new right now. Thank you for the pointer.
« Last Edit: November 13, 2015, 01:06:51 pm by Alex Nikitin »
 

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Re: Building your own voltage reference - the JVR
« Reply #84 on: November 17, 2015, 10:22:27 am »
1) I've received 2N4391 and 2N4392. The 4391 devices look really interesting, as the zero tempco point is very high (from 6V to 8V) and the current at that point is low (1-1.5mA), that makes for a very low internal dissipation (only 2 to 6mW with 10V supply) and very good stability. The gate is connected to the case and is grounded in the circuit, which is also good. I will build one JVR sample with the 2N4391 now, it looks very promising.

2) I've done a small mod on the Fluke 731B. As I am running it without a battery, the needle meter on the front panel is essentially useless. I've installed a temperature sensor (LM35) and wired the meter so it shows the temperature of the internal screen around the reference board. It did not require any significant changes, only a cut in one of the power supply board tracks and a couple of resistors. Now I have an accurate internal thermometer with the scale roughly from 18C to 27C (the green sector is from 20C to 24C) which gives me a very good idea of the output voltage deviation.

Cheers

Alex
 

Offline ltz2000

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Re: Building your own voltage reference - the JVR
« Reply #85 on: November 17, 2015, 01:24:09 pm »
I've done a small mod on the Fluke 731B.

Another mofication worth doing, which actually improves the stability, is replacing the crappy pre-regulator. I used the good (enough) old LM723.
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #86 on: November 20, 2015, 09:03:25 pm »
I've built the first 10V prototype with the 2N4391. The reference voltage was around 7.228V, so I've used OPA227 and some wire-wound resistors to scale it up to 10V, the same type wire wound resistors were used for the reference source as well. Without much tuning I've got around 2ppm/C and very low noise level. This is the first time I've measured the JVR against the Fluke 731B, using Keithley 2015 on 100mV range. The variations are around 1ppm, the noise is well below 1ppm p-p.

This is a step towards a properly built reference - I've used a good aluminium case, good resistors and Tellurium Copper terminals, though the circuit is built on a prototyping board.

Cheers

Alex

10V JVR on 2N4391 measured for 30 min against the Fluke 731B by Keithley 2015, 100mV range, NPLC1, no filtering, 1ppm per division:

 

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Re: Building your own voltage reference - the JVR
« Reply #87 on: November 21, 2015, 10:45:10 pm »
Here is another graph of the 2N4391 JVR v Fluke 731B. This time I've used a better cable (a screened pure copper twisted pair with copper plugs which I've just received), a foam cup on the 2N4391 FET and NPLC10 on the Keithley 2015. 30min run, 1ppm per division.

Cheers

Alex

« Last Edit: November 21, 2015, 10:48:20 pm by Alex Nikitin »
 

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Re: Building your own voltage reference - the JVR
« Reply #88 on: November 21, 2015, 11:11:12 pm »
Hello,

ist the lesser noise mainly from NPLC10 or also from the additional shielding?

with best regards

Andreas
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #89 on: November 21, 2015, 11:19:15 pm »
Hello,

ist the lesser noise mainly from NPLC10 or also from the additional shielding?

with best regards

Andreas

The noise is less because of NPLC10, however the slow voltage variations are less due most likely to smaller thermal effects from pure copper cabling and connectors. I've used just standard cables previously and at least some of the drift was due to thermoelectric effects.

Cheers

Alex
 

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Re: Building your own voltage reference - the JVR
« Reply #90 on: November 22, 2015, 10:53:49 am »
Here is another graph, this time for an overnight measurement, where the lab temperature varied from 22.5C in the evening down to 19C early in the morning before the heating was switched on. Vertical scale is 1ppm/div, so the tempco is about 1ppm/C, on par with the Fluke 731B. I may try to tweak it a bit more to get even lower figures. The circuit allows for a two pot trimming, one for the tempco and another for the voltage, without affecting the tempco, however as there are drifts in the other parts of the circuit, some iterations are required to get closer to zero in a 5-6 degrees C range. I may try to do it later today.

Cheers

Alex

« Last Edit: November 22, 2015, 08:02:42 pm by Alex Nikitin »
 

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Re: Building your own voltage reference - the JVR
« Reply #91 on: November 24, 2015, 08:54:38 pm »
It looks like I've got the tempco under 1ppm/C, though it is still positive and combined with a bit over 1ppm/C negative from the Fluke 731B the tempco of the differential voltage between two units is about 2ppm/C, on the first graph. The combined noise is, however, very low, a small fraction of a ppm, as visible on the first 24h graph (2ppm/division). On the second 24h graph the JVR is measured by Keithley 2015 and the noise of the meter dominates, however the tempco is just under 1ppm/C. The 2N4391 costs about $2 from mouser.com and probably the cheapest new metal can JFET you can buy - and quite suitable to use in the JVR. A very simple and cheap reference with potentially a good long-term stability, plus low and adjustable tempco can be built with this JFET, a quality opamp (i.e. OP27) and few good resistors (actually, you only need three resistors with the right values, but as JFETs parameters have a large spread it is necessary to build up the right values from several standard ones).

Cheers

Alex



« Last Edit: November 24, 2015, 11:04:34 pm by Alex Nikitin »
 

Offline Macbeth

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Re: Building your own voltage reference - the JVR
« Reply #92 on: November 24, 2015, 10:22:25 pm »
731B? meh, we need a 734A at least when measuring a cheap FET standard, providing a JJA is not immediately at hand  :palm: (ignore me, just bookmarking!  ;) )
 

Offline uski

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Re: Building your own voltage reference - the JVR
« Reply #93 on: November 25, 2015, 12:14:47 am »
:popcorn: keep going, I've been looking for a cheap reference good enough to roughly calibrate my 5 1/2 digits multimeter
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #94 on: November 27, 2015, 10:48:02 pm »
The weather was warmer today and I've pushed the room temperature up to 24-25C. Interestingly, it looks like my current version of the JVR with 2N4391 has a near-zero tempco between 22C and 24C - the voltage changes are less than 1ppm in that range, and at 25C the voltage starts to drop, so the tempco changes to negative. The Fluke 731B voltage changes about 4ppm in the same 3 degrees range, still negative.

Cheers

Alex

P.S. - this version runs untouched for about 150 hours and there is no voltage change at the same room temperature I can reliably detect (~below 2ppm).
« Last Edit: November 27, 2015, 11:03:07 pm by Alex Nikitin »
 

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Re: Building your own voltage reference - the JVR
« Reply #95 on: January 12, 2016, 10:11:59 pm »
Here is a graph of the JVR voltage v room temperature over one month (from 12th of December till today). As I've recorded the voltage mostly from the display of Kethley 2015, there is at least 1ppm uncertainty of that readout. When the display did flicker between two points, I've recorded a half-ppm point. It looks like the JVR + Keithley combination drifted not more than 2-3ppm over that month. Either both are quite stable or there is a remarkable similarity in the drift character of two units  :-// .

Cheers

Alex

« Last Edit: January 12, 2016, 10:39:41 pm by Alex Nikitin »
 

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Re: Building your own voltage reference - the JVR
« Reply #96 on: February 19, 2016, 04:53:45 pm »
I've measured the same 10V JVR unit with HP3458A over three and half hours today. The temperature in the lab was between 21-22C all that time. Vertical scale is 0.5ppm/div .

Cheers

Alex

 

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Re: Building your own voltage reference - the JVR
« Reply #97 on: February 24, 2016, 04:34:11 pm »
More measurements with HP3458A - this time for 29 hours. At night time temperature in the lab drops steadily to about 18C in the morning, and during the day the temperature is mostly reasonably stable around 21-23C. The day to day stability looks very good, even with the unit travelling with me and so experiencing a much colder conditions (around 3-7C). 1ppm/div vertical scale.

Cheers

Alex

 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #98 on: February 24, 2016, 05:24:40 pm »
For such a low current reference the performance looks really good.

With the HP3458 measurements there might also be some contribution from it's internal drift. With changing temperature you are supposed to use ACAL routine from time to time.
 

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Re: Building your own voltage reference - the JVR
« Reply #99 on: February 24, 2016, 06:09:22 pm »
For such a low current reference the performance looks really good.

With the HP3458 measurements there might also be some contribution from it's internal drift. With changing temperature you are supposed to use ACAL routine from time to time.

Thank you. I've just left the setup running unattended. The HP3458 has option 002 though, so I hope the internal drift (especially on 10V range) should be minimal (ACAL gave me a difference below 0.5ppm when I've run it at different temperatures, i.e 18C and 23C.

Cheers

Alex
 

Offline Vgkid

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Re: Building your own voltage reference - the JVR
« Reply #100 on: February 24, 2016, 06:22:20 pm »
Thanks for the updates, especially the drift performance.
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Re: Building your own voltage reference - the JVR
« Reply #101 on: February 27, 2016, 03:27:59 pm »
Odd bit of synchronicity here. This thread came back to life a few days ago and today I was hunting around for an old microphone preamp design of mine. In the process I tripped across the circuit for the AKG C451E microphone's internal preamp. This is phantom powered and lurking inside it is a JFET acting as a voltage reference for the on-board voltage regulator. It's T5 in the diagram below. Note that R15 is marked as select-on-test.

Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

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Re: Building your own voltage reference - the JVR
« Reply #102 on: February 29, 2016, 10:36:30 am »
That is one of the best 12h runs I've had with this reference. It was taken on Saturday with the room closed and no heating, so the temperature was about 18C all day from 7:30 till 19:30. Vertical scale is 0.5ppm/div.

Cheers

Alex
« Last Edit: February 29, 2016, 10:38:40 am by Alex Nikitin »
 

Offline zlymex

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Re: Building your own voltage reference - the JVR
« Reply #103 on: March 05, 2016, 03:48:05 am »
Anyone heard of the related story? Here is goes.

1. For a very long time, only LT's LTZ1000 claims to have 2uV drift @1kHr(@65C, roughly 0.3ppm @1kHr)

2. In late 90's, Analog annouced its XFET based ADR293 5V Vref claimed to have drift of only 0.2ppm @1kHr @25C (better than LTZ1000!)
http://pdf1.alldatasheet.com/datasheet-pdf/view/48847/AD/ADR293.html

3. There are people, without extended test, written an article in 1999 introducing this 'remarkable' device.
http://www.ti.com/lit/an/slyt183/slyt183.pdf

4. LT were getting mad at this and written a design note in 2000 to criticize this using phrase like 'THIS IS A DELIBERATE LIE!'
http://cds.linear.com/docs/en/design-note/dn229f.pdf

5. Analog made modifications to the datasheet from 0.2ppm to 50ppm, test conditions also modified
http://www.analog.com/media/en/technical-documentation/data-sheets/ADR293.pdf

6. Although this ADR293 is still in production, its not recommended for new design. Rather, Analog gives an alternative ADR3450, which is CMOS based and with much larger noise.
http://www.analog.com/media/en/technical-documentation/data-sheets/ADR3412_ADR3420_ADR3425_ADR3430_ADR3433_ADR3440_ADR3450.pdf
 
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Offline Macbeth

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Re: Building your own voltage reference - the JVR
« Reply #104 on: March 05, 2016, 04:59:14 am »
@zlymex - Thanks for that.

So the XFET based ADR293 has a 1000 hour long term stability of just 0.2ppm @ 25C - but the latest datasheet has it at 50ppm @ 125C !!? ... and they recommend this for battery powered devices? Did they really burn this in at 125C? why? :palm:

ETA: Design Note 229 explains it!  :rant:
« Last Edit: March 05, 2016, 05:07:31 am by Macbeth »
 

Offline zlymex

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Re: Building your own voltage reference - the JVR
« Reply #105 on: March 05, 2016, 08:05:56 am »
@zlymex - Thanks for that.

So the XFET based ADR293 has a 1000 hour long term stability of just 0.2ppm @ 25C - but the latest datasheet has it at 50ppm @ 125C !!? ... and they recommend this for battery powered devices? Did they really burn this in at 125C? why? :palm:

ETA: Design Note 229 explains it!  :rant:
That's right. Analog must have tested some devices at 125 deg C for 1000hrs that they typically show 50 ppm drift. And they derived at first that the drift at 25 deg C to be 0.2ppm @1000hrs.
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #106 on: March 07, 2016, 12:19:52 pm »
Well, back to the topic  ;) . Here are the measurements on HP3458A for my 10V JVR unit for two weeks combined, 1ppm/division and with some room temperature markers. There are some gaps in the timeline.
Cheers

Alex

« Last Edit: March 07, 2016, 12:23:21 pm by Alex Nikitin »
 

Offline Squantor

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Re: Building your own voltage reference - the JVR
« Reply #107 on: March 08, 2016, 07:51:37 am »
Just to share my experience.

When I was matching BF862's on VGS using the setup of gate connected to source and measuring the voltage drop at a certain current. The Vgs usually climbs as the device is heating up, this was prevalent on the higher side of the IDSS current range (10 to 25mA). But I noticed that devices around 15mA did not have this effect at all, and below this current it actually was the opposite.

When I have some time I will perform some tests with the BF862. Problem is that these are SMT devices and interchanging is not as easy. I do have a socket from my old matching setup that I can reuse, but dont know about the thermal EMF quality of this socket
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #108 on: March 14, 2016, 04:51:47 pm »
I've checked the load regulation on the 10V unit today. 10K (1mA) load directly on the output connectors drops the voltage by 2uV or 0.2ppm, so the output impedance is about 2mOhm. There is no visible additional voltage shift for over 30min under that load.

Cheers

Alex
« Last Edit: March 14, 2016, 04:54:56 pm by Alex Nikitin »
 

Offline Andreas

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Re: Building your own voltage reference - the JVR
« Reply #109 on: March 14, 2016, 10:05:36 pm »
I've checked the load regulation on the 10V unit today. 10K (1mA) load directly on the output connectors drops the voltage by 2uV or 0.2ppm, so the output impedance is about 2mOhm.

Hello Alex,

that is unbelievable for me.
I would expect a impedance at least 3 orders of magnitude higer (> 2-20 Ohms).
Are you shure that the cirquit is not oscillating?
(I had some problems with my FET voltage regulator until I put some bypass capacitors on input+output).

With best regards

Andreas
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #110 on: March 14, 2016, 10:42:44 pm »
I've checked the load regulation on the 10V unit today. 10K (1mA) load directly on the output connectors drops the voltage by 2uV or 0.2ppm, so the output impedance is about 2mOhm.

Hello Alex,

that is unbelievable for me.
I would expect a impedance at least 3 orders of magnitude higer (> 2-20 Ohms).
Are you shure that the cirquit is not oscillating?
(I had some problems with my FET voltage regulator until I put some bypass capacitors on input+output).

With best regards

Andreas

With correctly designed output stage it is not difficult to get very low output impedance (at DC) and good stability. I have 220nF capacitor connected across the output and the circuit is completely stable. Here is the "live" recording of the loading experiment (unfortunately, at the usual NPLC100). You can see the points where the load connected and disconnected by the transient "spikes".

Cheers

Alex



« Last Edit: March 14, 2016, 10:53:32 pm by Alex Nikitin »
 

Offline Andreas

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Re: Building your own voltage reference - the JVR
« Reply #111 on: March 15, 2016, 05:22:38 am »
Hello,

so its the (modified?) cirquit variant with the INA133 buffer from page 2
measured at the output of the INA and not directly at the FET?

with best regards

Andreas
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #112 on: March 15, 2016, 11:07:30 am »
Hello,

so its the (modified?) cirquit variant with the INA133 buffer from page 2
measured at the output of the INA and not directly at the FET?

with best regards

Andreas

Hi Andreas,

No, all my latest data are for the version with 2N4391 JFET and Ultrohm resistors, from the end of November 2015. I've used OPA227 (actually, a dual OPA2227) for the 7V to 10V amplifier and output buffer. So the output impedance is for the buffer stage.

Cheers

Alex
 

Online The Soulman

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Re: Building your own voltage reference - the JVR
« Reply #113 on: March 21, 2016, 10:26:17 pm »
Hello,

so its the (modified?) cirquit variant with the INA133 buffer from page 2
measured at the output of the INA and not directly at the FET?

with best regards

Andreas

Hi Andreas,

No, all my latest data are for the version with 2N4391 JFET and Ultrohm resistors, from the end of November 2015. I've used OPA227 (actually, a dual OPA2227) for the 7V to 10V amplifier and output buffer. So the output impedance is for the buffer stage.

Cheers

Alex

Hi Alex, did you or maybe someone else did measurements on the relationship between vgs(off) and the current (and therefore resistor) required for optimum (as close to zero) tempco?
Could do it the hard (on the device..) way as in the first post using temperature extremes and a variable pot but would prefer a more scientific approach, at least to get in the ballpark.  :-+

 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #114 on: March 22, 2016, 11:52:01 am »
Hi Alex, did you or maybe someone else did measurements on the relationship between vgs(off) and the current (and therefore resistor) required for optimum (as close to zero) tempco?
Could do it the hard (on the device..) way as in the first post using temperature extremes and a variable pot but would prefer a more scientific approach, at least to get in the ballpark.  :-+

There is no single formulae - the position of the zero tempco point depends on the JFET geometry, junction temperature and the Vds, so it is possible to make a rough prediction where that point should be only for a particular type/make JFET . For the lowest tempco at a certain temperature you need to adjust it at that temperature +/- few degrees. I will collect some data for 2N4391 but don't have any real statistics yet.

One really attractive point of the JVR is that the tempco can be adjusted directly for the main reference device and so tempcos of the rest of the circuit can be largely compensated for, around the temperature of interest - or even used to "flatten" the curve in 5-10 degrees range.

Cheers

Alex
 

Offline amspire

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Re: Building your own voltage reference - the JVR
« Reply #115 on: March 23, 2016, 06:44:54 am »
The old Natsemi Discrete semiconductor book is pretty useful - it has diagrams of the chip layout and characteristic data for each family of device.

http://www.introni.it/pdf/NatSemi%20-%20Discrete%20Databook%201978.pdf

The 2N4391 is a type 51 process.


This layout changed between the 1974 version of this data book and the 1978 version, so there may be different versions of the 2N4391 that behave sightly differently.

I wonder if a type 58 process FET would be lower noise - it has a much bigger area and lower resistance.

The 2N5432 (Type 58) has a pinch-off between 4 and 10V and a channel resistance 6 times lower then then the 2N4391. The package is a TO52 that is similar to the TO5 metal can.

A bit harder to get and more expensive. More likely to find them at a smaller distributor or on Aliexpress then the major distributors. I can get them for about US$6 here in Australia.

Richard
« Last Edit: March 23, 2016, 07:00:54 am by amspire »
 

Offline zlymex

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Re: Building your own voltage reference - the JVR
« Reply #116 on: April 07, 2016, 07:35:34 am »
Hi Alex, did you or maybe someone else did measurements on the relationship between vgs(off) and the current (and therefore resistor) required for optimum (as close to zero) tempco?
Could do it the hard (on the device..) way as in the first post using temperature extremes and a variable pot but would prefer a more scientific approach, at least to get in the ballpark.  :-+
I have an old 491-page Chinese book(in pdf) called "High Stability Power Supply"where on page 121 the author derived a formula:
Vg0=Vp-0.66V
which means the zero tempco Vg is the Vp(Vgs off) minus 0.66V. And the corresponding current is
Id0 = Idss * (0.66/Vp)^2
I applied this to my DIY 100V voltage reference and it works fine.
« Last Edit: April 07, 2016, 07:45:38 am by zlymex »
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #117 on: April 11, 2016, 09:13:31 pm »
I have an old 491-page Chinese book(in pdf) called "High Stability Power Supply"where on page 121 the author derived a formula:
Vg0=Vp-0.66V
which means the zero tempco Vg is the Vp(Vgs off) minus 0.66V. And the corresponding current is
Id0 = Idss * (0.66/Vp)^2
I applied this to my DIY 100V voltage reference and it works fine.

It is not a very accurate formula, even on the same make and type of JFETs the voltage difference between the cut-off voltage and "zero tempco" voltage varies according to my measurements (at the same temperature) and on top of that the cut-off voltage does vary with temperature, so this difference is temperature dependent as well!

Cheers

Alex
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #118 on: April 11, 2016, 09:33:49 pm »
I am struggling to measure the drift on this sample of the JVR. In three months it is less than I can reliably measure with the HP3458A (opt 002) and Fluke 731B as the second reference. The difference between the Fluke and JVR stays about the same for the same temperature (at 24C it is about 90-110 uV) and it is the same absolute value (+/- 1ppm) when measured with the HP after auto-cal over these three months (at the same room temperature), despite several moves between my home lab and my work lab. Here are today's measurements at home of the difference between the JVR and the Fluke, done by the Keithley 2015 with the upgraded input opamp used as a null meter at 100mV range and NPLC 10 with 10 averages.

Cheers

Alex

« Last Edit: April 11, 2016, 10:00:49 pm by Alex Nikitin »
 

Online The Soulman

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Re: Building your own voltage reference - the JVR
« Reply #119 on: April 11, 2016, 10:04:27 pm »
I have an old 491-page Chinese book(in pdf) called "High Stability Power Supply"where on page 121 the author derived a formula:
Vg0=Vp-0.66V
which means the zero tempco Vg is the Vp(Vgs off) minus 0.66V. And the corresponding current is
Id0 = Idss * (0.66/Vp)^2
I applied this to my DIY 100V voltage reference and it works fine.

It is not a very accurate formula, even on the same make and type of JFETs the voltage difference between the cut-off voltage and "zero tempco" voltage varies according to my measurements (at the same temperature) and on top of that the cut-off voltage does vary with temperature, so this difference is temperature dependent as well!

Cheers

Alex

In the Vishay journal AN103 (http://www.vishay.com/docs/70596/70596.pdf) they use nearly the same number 0.65volt (VGS(0TC)  VGS(off) – 0.65 V) that would probably get one in the ballpark,
however what Alex is saying does make sense and testing and trimming is the way to go for optimum thermal stability.

I need to get some parts ordered and start building one!
 

Offline quarks

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Re: Building your own voltage reference - the JVR
« Reply #120 on: April 12, 2016, 04:52:41 am »
Very interesting.
Now bookmarked.
 

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Re: Building your own voltage reference - the JVR
« Reply #121 on: April 19, 2016, 10:28:54 am »
Another update, this time ~200 hours run in my home lab, with the top temperature about 24C and some night time drops. The Keithley 2015 measures the difference between 10V JVR and Fluke 731B, 100mV range, 10NPLC + 10 averages, so the 100NPLC equivalent. 1ppm/div vertical scale. Again, this JVR build is quite stable in time.

Cheers

Alex

 

Offline amspire

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Re: Building your own voltage reference - the JVR
« Reply #122 on: April 26, 2016, 08:22:19 am »
I finally got around to a few tests myself. I had some MPF102 (plastic) so I decided to test them. The zero temp coefficient point was only 1.2V at 1mA, but this can be useful if you need a reference that can run from 5V.

The voltage sensitivity with a 5V supply was 0.003V/Vsupply. At 15V it reduced to 0.0013V/Vsupply. I am pretty happy with both numbers. As long as you maintain the voltage across the reference fet to 100mV, the voltage sensitivity even with a 5V supply is negligible.

Resistance sensivity is worse. For both 5V supply and 15V supply, the output varied by about 1/3 the resistor variation. This does not include the fact that as the resistor drifts, the FET current drifts and so it gets a temperature coefficient. The TO92 package is not good for temperature tests as you can get a positive or negative change depending on where the heat is applied to the package.

There are a number of 3ppm to 5ppm reference ICs for around $3. To equal this, I will need 10-15ppm resistors.

So on this score, it may not be worth the effort.

The other issue is the long term stability and temperature hysteresis. For reference ICs, it is not unusual to see a 50ppm drift spec  for 1st 1000 hour drift and another 50ppm drift if the chip is temperature cycled from 25degC to the min and max temperatures and back to 25 deg C. It would be interesting to know what kind of drifts and temp hysteresis you get with the JVR reference. It is certainly cheaper and easier getting a hermetically sealed JFET then getting a hermetically sealed voltage reference IC, but of course, the resistor needed for the JVR is probably not going to be hermetically sealed.

If it turns out the JVR is low drift/low hysteresis, it is interesting for custom jobs, especially if the JVR is put in an oven.

Anyone have some suggestions for economical and stable resistors in the 5ppm to 15ppm range?

 

Offline Cerebus

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Re: Building your own voltage reference - the JVR
« Reply #123 on: April 26, 2016, 03:43:13 pm »
Anyone have some suggestions for economical and stable resistors in the 5ppm to 15ppm range?

For surface mount, the Panasonic ERA series has 10 and 15ppm resistors at relatively low cost. They' range from 20p to a pound (GBP) each in small quantities [RS pricing, packs of 5 or packs of 100, 10k ohms 0.05% 10ppm to 0.1% 15ppm, 0603 and 0805 sizes].
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline Vgkid

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Re: Building your own voltage reference - the JVR
« Reply #124 on: April 26, 2016, 04:51:55 pm »
Anyone have some suggestions for economical and stable resistors in the 5ppm to 15ppm range?
For through hole, TE Connectivity Neohm- YR1 series offer 15ppm tc, and are cheap.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline bingo600

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Re: Building your own voltage reference - the JVR
« Reply #125 on: April 26, 2016, 05:34:43 pm »
Anyone have some suggestions for economical and stable resistors in the 5ppm to 15ppm range?
For through hole, TE Connectivity Neohm- YR1 series offer 15ppm tc, and are cheap.

Ask Edwin for a quote - https://www.eevblog.com/forum/profile/?u=96921

/Bingo
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #126 on: April 26, 2016, 07:24:46 pm »
Anyone have some suggestions for economical and stable resistors in the 5ppm to 15ppm range?
For through hole, TE Connectivity Neohm- YR1 series offer 15ppm tc, and are cheap.

Ask Edwin for a quote - https://www.eevblog.com/forum/profile/?u=96921

/Bingo

That is the best suggestion  :-+ . With his resistors and using a JFET in a metal case with high Vgs (like the 2N4391 in my current prototype) it should be possible to make a reference with less than 1ppm/C tempco in 5-6 degrees range and a very good long term stability (my sample has drifted less than 5ppm for last 3 months - at least that is the limit I can measure it against HP3458A Opt002 and Fluke 731B) .

Cheers

Alex
 

Offline amspire

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Re: Building your own voltage reference - the JVR
« Reply #127 on: April 26, 2016, 11:53:14 pm »
Alex, have you done any thermal hysteresis tests? I am really interested in that one. It seems to be the test that affects the IC's signifigantly. Say

room temp ->70degC -> 0 deg C (whatever your refrigerator freezer does) -> room temp.

If the JRV does not have hysteresis, it becomes very interesting.
 

Offline amspire

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Re: Building your own voltage reference - the JVR
« Reply #128 on: April 27, 2016, 12:18:28 am »
Anyone have some suggestions for economical and stable resistors in the 5ppm to 15ppm range?
For through hole, TE Connectivity Neohm- YR1 series offer 15ppm tc, and are cheap.
Mouser stock these - around about 25c a resistor. Not bad. The specs do not mention 1000hr drift, so that needs testing.

http://www.mouser.com/ds/2/418/NG_DS_1773265_A-722435.pdf

Not sure about the higher voltage zero-TC FETS, but the 1.2V MPF102 I tested (it was the one I had) would get 5ppm/C with a 15ppm/C resistor. I guess you could find the zero TC point of the resistor and FET combined and get a lower TC over a limited range. Get a combined tempco of 1ppm, and then put it in an oven with 0.1 degC stability and it should be a nice reference.
« Last Edit: April 27, 2016, 02:09:55 am by amspire »
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #129 on: April 27, 2016, 01:35:56 pm »
The TC of the resistor might not be such a big problem, as one could find the point for combined zero TC. The trouble is that you need a different resistor value for each FET. So individual matching / adjustment is needed. So when a pot / digi-pot is used for adjustment two resistors are needed. Otherwise you may need odd individual values - e.g. specially made for the FET or combinations of 2 or more.

The more important point than TC  is stability of the resistor and possibly the FET also. If stable, even a high TC might be acceptable - though I would not go for a PT1000.
 

Offline Squantor

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Re: Building your own voltage reference - the JVR
« Reply #130 on: May 17, 2016, 09:51:38 am »
Hello All, I am a member of the tekscopes group and I read this interesting bit of information on finding the zero tempco point of a JFET with a curve tracer:

The 577 D1 also allows you to find the ideal bias point (zero temperature
coefficient) of a FET. You can do this with a 7CT1N and a storage 7000 scope
as well. Just display a set of curves and apply heat and/or cold to the case
of the FET. The curves above the ideal bias point will drift up (and get
stored as a smear). The curves below the ideal bias point will move down
(and also get stored as a smear). The ideal bias point curve will not move
so there will be no smear. It pops right out at you.

Original post here: https://groups.yahoo.com/neo/groups/TekScopes/conversations/messages/129217
It has a few nice curvetracer tricks.
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #131 on: August 02, 2016, 09:22:38 am »
Just a quick update, I've re-measured the JVR after 4 month with the same HP3458A opt 002 (ACAL DCV done in all cases just before measurements) and the result is about -4...-5ppm over that time at the same temperature (24C). As it is in the ballpark of the meter stability, I've checked also the JVR against my Fluke 731B at the same temperature and it shows about -3ppm drift and the Fluke's drift over the same period of time, measured on the HP3458A is about -1ppm at the same 24C. So it all sort of adds up nicely (whatever the real drift is in all three  ::) ). I might now do a hysteresis test to, say, +60C unpowered (we have an oven that runs at that temperature) and back to 24C.

Cheers

Alex
« Last Edit: August 02, 2016, 11:22:33 am by Alex Nikitin »
 

Offline CJay

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Re: Building your own voltage reference - the JVR
« Reply #132 on: August 02, 2016, 11:36:52 am »
Well, my 100 of J211 arrived from DigiKey and I've measured the cut-off voltage on the lot at 5V Vgd. Actually, a very consistent batch for JFETs, it looks like the processing did improve since 1980s. All hundred measured between 2.75V and 3.35V at ~3uA current (10M of Keithley 2015 on 100V range between the source and gate). My "experimental" U440 measures 4.6V in the same setup (and the zero tempco point is at ~3.74V, so the "theory" about 0.63 or 0.64V is not working  ;) ). One obvious bad point about plastic casing - there is some photo-sensitivity, so the device should not be exposed to light when used as a reference. Not much of a problem though. I am nearly finalised a complete reference circuit, which would take 12-24V supply and provide 10V output with up to 10mA current available and would contain only two FETs, an opamp and few resistors. Not temperature regulated - that would be the next step.

Cheers

Alex

Light sensitive?

now there's interesting, I wonder if it would be possible to build a 'photonic oven' to compensate for temperature drift by exposing it to varying intensity light....

Or I may just be nuts...
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #133 on: August 02, 2016, 11:55:03 am »
First results on the hysteresis (from 24C up to 60C for an hour, then back to 25C - in that hour the temperature in the lab gone up 1 degree) - about +1.5ppm and even that is likely to be  mostly a temperature change effect. That result is for the complete unit, including the 7 to 10V amplifier and buffers. The shift at ~54C was about -2mV or -200ppm, indicating the tempco of about -7ppm/C box. Near 25C the tempco is about 1ppm/C .

Cheers

Alex

P.S. - the voltage measurements graph is added - 1ppm per vertical division scale. While the JVR was in the oven, the Fluke 731B was connected and slowly drifted down with the lab temperature increase.
« Last Edit: August 02, 2016, 12:49:37 pm by Alex Nikitin »
 
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #134 on: August 02, 2016, 05:46:17 pm »
Here is the graph of the hysteresis after 60 min in a fridge (temperature of the unit went down to about 12C) . There was quite a bit of condensation on the unit what I've got it back to the lab. I left for home while the voltage was still rising so the complete data will be available only tomorrow. Here is what I have so far (the "shelf" at 9.999974 is again the Fluke connected in place of the JVR while it was in the fridge):



It looks like the voltage should be back within 2ppm at least.

Cheers

Alex
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #135 on: August 03, 2016, 09:20:26 am »
Here is the complete graph. It looks like between 12C and 60C I can not reliably detect the hysteresis - if it is there, it is below 2ppm.

Cheers

Alex
 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #136 on: August 03, 2016, 10:51:57 am »
Anyone have some suggestions for economical and stable resistors in the 5ppm to 15ppm range?

Susumu RG for less than 15 ppm and Stackpole RNCS for 15 ppm? Both passivated with an inorganic layer instead of the usual resin.
 

Offline montemcguire

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Re: Building your own voltage reference - the JVR
« Reply #137 on: August 03, 2016, 06:27:54 pm »
Agreed - the Susumu resistors are really nice, far better than typical NiCr resistors. They're also trimmed by laser through the glass layer, after it has been deposited, so there's a lot less mechanical strain left on the part to cause drift. The only issue is that they're tiny SMD chips, but they can be attached to larger thermal masses easily.

Edit: The RG series are nice, but the new URG series are even higher precision, along with a higher price.
« Last Edit: August 03, 2016, 06:30:24 pm by montemcguire »
 

Offline montemcguire

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Re: Building your own voltage reference - the JVR
« Reply #138 on: August 07, 2016, 05:00:13 pm »
The drawback to foil is when you're using the resistors with low frequency AC signals. The low TC of a foil resistor is done by bonding the foil to an engineered ceramic that has a complementary expansion coefficient. For DC, the current induced foil heating and the substrate will stabilize thermally, and the resulting TC is low. For very high frequencies, there's also no problem, since the waveform changes so quickly that the foil heating is constant over all parts of the waveform.

However, with low frequencies, the foil and the substrate will not heat uniformly over the entire waveform, and the foil can expand and contract differently at different parts of the waveform, causing the resistance to modulate with the signal voltage. What's worse is that the thermal time constants can cause the substrate to expand and contract out of phase with the foil, actually worsening the thermal effects of the signal current foil heating. This creates third harmonic distortion, and at just the wrong frequency, the effect is worse than simply using a 'normal' higher TC resistor that doesn't try to cancel its TC with an engineered ceramic substrate.

So, for audio or other LF AC uses where distortion is important, foil resistors can perform worse than traditional NiCr films like the Susumus, and can generate more distortion than the amplifiers around them. As far as I know, the Susumus have a low TC by tweaking the alloy and the processing, and not by using a trick substrate ceramic, so they'll have better LF distortion performance than a foil.
 
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Offline guenthert

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Re: Building your own voltage reference - the JVR
« Reply #139 on: August 15, 2016, 06:44:50 am »
I've seen this schematic some days ago in the LF355 datasheet, it made me think about you! Your idea is not new.

Thank you, I always thought that it was used before but couldn't find a confirmation!

 :-+

Cheers

Alex

P.S. - would be interesting to find if that circuit was used somewhere in practice!

P.P.S. - The 2N4118 has a very low drain current (around 100uA for near-zero tempco) - it should make it good for low power but fairly noisy as a reference. A higher current device would make it a very low noise option - much quieter than ~1ppm p-p LF noise of the LM399.

Not a practical application, but the idea (as far as I understand it) is also discussed here: http://www.analog.com/media/en/training-seminars/tutorials/MT-087.pdf
(p. 9). Which is not to say that it wouldn't be worthwhile to explore this further.
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #140 on: August 15, 2016, 04:14:36 pm »
That marketing document is OK, but it contains some inaccuracies that are most likely due the fact that ADI wants to sell you on the idea of an XFET reference.  Reader beware, and check your facts.  For example in figure 10, they show that an XFET reference is somehow "better" than a buried Zener reference for long term stability.  Sorry ADI, but this is just NOT TRUE.  I think that their XFET reference might be OK if you need low power, but I think they should be more honest.  This is not the first time ADI has made false assertions about it's voltage references.

It is an old paper (and not really relevant to the topic, as these XFET references use a "band-gap" type 2-FET structure). A single JFET reference would not have that good tempco over a wide temperature range, but can be adjusted for a near-zero tempco at a particular temperature and appears (providing a metal can device is used) very stable in time.

Cheers

Alex
 

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Re: Building your own voltage reference - the JVR
« Reply #141 on: November 14, 2016, 11:37:00 pm »
Another measurements before I take the JVR to the newly calibrated HP3458A. Taken with two Keithley 2015 and Keithley 617 as a thermometer (measuring the output of LM35D, 200mV = 20C, 10mV/degree, right scale). So far both JVR and Fluke 731B drifted less than 5ppm over the last year (even with all my attempts to measure a hysteresis). Interesting that the JVR has a low tempco between 22 and 26C

Cheers

Alex

« Last Edit: November 14, 2016, 11:40:40 pm by Alex Nikitin »
 

Offline TiN

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Re: Building your own voltage reference - the JVR
« Reply #142 on: November 15, 2016, 02:39:30 am »
It is tempco of 2015, -0.9ppm/K? It's very high out of spec for 731B alone. :)
EDIT: I was thinking about 732B, sorry.
« Last Edit: November 15, 2016, 02:42:29 am by TiN »
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Re: Building your own voltage reference - the JVR
« Reply #143 on: November 15, 2016, 08:36:02 am »
That marketing document is OK, but it contains some inaccuracies that are most likely due the fact that ADI wants to sell you on the idea of an XFET reference.  Reader beware, and check your facts.  For example in figure 10, they show that an XFET reference is somehow "better" than a buried Zener reference for long term stability.  Sorry ADI, but this is just NOT TRUE.  I think that their XFET reference might be OK if you need low power, but I think they should be more honest.  This is not the first time ADI has made false assertions about it's voltage references.

Analog Devices has neatly solved this problem by buying Linear Technology.
 

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Re: Building your own voltage reference - the JVR
« Reply #144 on: November 15, 2016, 11:45:43 am »
Well, the result from the JVR measured by HP3458A at 25C lab temperature (after 1.5 hours in my backpack and 3 hours warm-up in the lab) is essentially the same as three and half months ago (see my posts from the 2nd/3rd of August) - 10.000055V with about 0.5 ppm variations. The unit was almost continously powered up all this time (except when it was transported from my home lab to my work lab and back).

Cheers

Alex
« Last Edit: November 15, 2016, 11:48:18 am by Alex Nikitin »
 

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Re: Building your own voltage reference - the JVR
« Reply #145 on: June 08, 2017, 11:37:32 am »
As the HP3458A just came back from a repair and calibration at Keysight UK, I've brought the 10V JVR unit back to the lab and measured it after a good warm-up. Here is the result for 1 hour, at NPLC100 , combined with the similar one hour measurements at the same 25C temperature in August and December last year. I need to build few more of these and get some statistics ;) .

Cheers

Alex
« Last Edit: June 08, 2017, 11:50:59 am by Alex Nikitin »
 
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Offline TiN

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Re: Building your own voltage reference - the JVR
« Reply #146 on: June 08, 2017, 01:11:35 pm »
I wish I'd have backpack big enough for the 3458A :D
Quote
by HP3458A at 25C lab temperature (after 1.5 hours in my backpack and 3 hours warm-up in the lab)
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Re: Building your own voltage reference - the JVR
« Reply #147 on: June 08, 2017, 01:15:23 pm »
I wish I'd have backpack big enough for the 3458A :D
Quote
by HP3458A at 25C lab temperature (after 1.5 hours in my backpack and 3 hours warm-up in the lab)

I wish I'd have my own HP3458A. I would find a backpack large enough to put it in, no problem   ::) .

Cheers

Alex
 

Offline guenthert

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Re: Building your own voltage reference - the JVR
« Reply #148 on: June 20, 2017, 10:13:59 am »
I need to build few more of these and get some statistics ;) .
That looks very promising.  Build many more and sell them and thereby distribute the work of collecting statistics!
 

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Re: Building your own voltage reference - the JVR
« Reply #149 on: June 20, 2017, 10:36:16 am »
Just wanted to mention that I'm planning on building a few of these as well. I have some of the parts, still waiting on others. I'm also in the middle of setting up my meters and fixing some scanners so I can automate measuring of everything.  :-DMM

Looking forward to it having some fun!  :popcorn:

Ash.
 

Offline Vtile

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Re: Building your own voltage reference - the JVR
« Reply #150 on: June 20, 2017, 07:00:33 pm »
Yes, I've looked at that option, however it is only possible for devices with a high enough drain current

No need to use Vg=0 here, a bit of drift from a resistor divider for the temperature setting isn't that critical, so IDSS isn't a limiting factor.
Have anyone used the temp.co difference in the divider resistors to cancel out the reference tempco?? In theory (as two brain cycles without math) it should work.

PS. How is the long term spability now or is the prototybe trashed along the years.
« Last Edit: June 20, 2017, 07:05:25 pm by Vtile »
 

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Re: Building your own voltage reference - the JVR
« Reply #151 on: June 20, 2017, 07:23:58 pm »
PS. How is the long term spability now or is the prototybe trashed along the years.

Look on the previous page. Essentially the voltage is quite stable, using the 3458A meter the difference is few ppm over about a year, certainly less than 20ppm drift for a complete reference including the 7 to 10 volts amplifier stage.

Cheers

Alex
 

Offline Vtile

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Re: Building your own voltage reference - the JVR
« Reply #152 on: June 20, 2017, 07:25:29 pm »
Sounds nice.. Silly me I'm still on page 4, it seems also that the resistor? zero tempco point is discussed there.  :-[  :palm:
 

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Re: Building your own voltage reference - the JVR
« Reply #153 on: June 20, 2017, 07:27:20 pm »
PS. How is the long term spability now or is the prototybe trashed along the years.

Look on the previous page. Essentially the voltage is quite stable, using the 3458A meter the difference is few ppm over about a year, certainly less than 20ppm drift for a complete reference including the 7 to 10 volts amplifier stage.

Cheers

Alex

very good results. have you tried cascading the fet's like in the 3-fet design paper?

regards and keep up the good work.
 

Online IconicPCB

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Re: Building your own voltage reference - the JVR
« Reply #154 on: June 21, 2017, 10:09:13 pm »
3 FET design?

 

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Re: Building your own voltage reference - the JVR
« Reply #155 on: June 21, 2017, 10:16:25 pm »
 
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Re: Building your own voltage reference - the JVR
« Reply #156 on: June 22, 2017, 07:44:30 am »

very good results. have you tried cascading the fet's like in the 3-fet design paper?

regards and keep up the good work.

Thank you. For a voltage reference it is not necessary, as long as the supply voltage is steady enough (in my unit it is bootstrapped from the reference voltage, so no problems there). Introducing the second and the third FETs helps to increase the current source output impedance but in turn, adds new temperature dependencies of the second and the third order, so the FETs should be thermally coupled and ideally temperature stabilized as in the paper you've found.

Cheers

Alex
 
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Offline Ash

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Re: Building your own voltage reference - the JVR
« Reply #157 on: June 29, 2017, 12:07:35 pm »
Hi Everyone,

I just did a run to characterise the 2N4391 JFETs I have. They are all from Central Semiconductor with a date code of 1620.

I lashed up a circuit with a resistance decade box as the resistor between S and G / GND, and supplied 10V from an Aglient E3640A power supply. I measured the voltage at the source pin on my 6.5 digit Keithley 2015 (uncalibrated). Note that the resistance box is not hugely accurate, but it is sufficient to get me in the ballpark and I'll trim the operating point of each circuit from these values.

Basic process was to put the device in to a machine pin socket at room temperature (about 23.5C), and adjust the resistance box so that grabbing the metal can with my fingers, increasing the temperature didn't move the last digit of the voltage much (at most 1 or 2 counts).

FETResistanceVoltage
A87708.17287
B76007.71515
C86508.10104
D78707.83454
E83007.97806
F77007.77651
G69007.37061
H83708.06369
I65507.20610
J104008.56925

So the I(ds) at the 0 TC point for room temperature is floating around the 1mA point.

I'm also planning on testing these same FETs at an elevated temperature (say 50C) as I want to build ovenised references to combat the temperature swings in my office. In Brisbane it can get to 40C on bad days in summer... I'll add a precision OpAmp as a buffer, also in the oven. I'm then considering my options for 10V generation. I'm leaning towards PWM + another OpAmp with a 2x gain and a filter... Then it is programmable..

Ash.
 
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Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #158 on: June 29, 2017, 02:32:37 pm »
If you go the PWM route then given the negligible hysteresis wouldn't it make more sense to digitally compensate for temperature than ovenise?
 

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Re: Building your own voltage reference - the JVR
« Reply #160 on: June 30, 2017, 01:16:37 am »
If you go the PWM route then given the negligible hysteresis wouldn't it make more sense to digitally compensate for temperature than ovenise?

Hi Marco,

The thought did cross my mind and I've done similar software based temperature compensation for things before, but I had access to a temperature chamber then and I don't now :)

Maybe in the future, but for the initial version, I'll probably ovenise the critical sections, possibly the whole lot. I'll then perturb the oven temperature (say +/-5 deg C) and use that to determine and set the overall system 0 TC point as I should be able to (at least to first order) use the JFET to offset any other system TC.

I haven't yet put much thought into the 10V generation, and it may prove too much trouble, but I'd like to be able to generate an arbitrary voltage in a range (0-12V?), basically like a calibrator. Will have to see how that plays out also with the overall temperature compensation as well. However first I have to get some confidence and understanding of the reference itself. I'll also have to think about the noise implications as well...

I'm also going for something that can be built and calibrated with minimum of equipment (basically a good DMM and/or null against a +10v reference). I'm not making any quantity so I'm happy to spend the time tweaking the TC of each unit. I should be able to do this by setting up PWM for approximately 10V, then monitoring the output voltage as I slew the oven temperature. Once the TC is tamed, then I would allow the over the settle mid-range for a while and then calibrate the 10V PWM against a suitable reference.

http://shop.kuhne-electronic.de/kuhne/en/shop/amateur-radio/accessoires/crystal-heater/Precision+crystal+heater+40%C2%B0+QH40A/?card=724

That might help stabilise the JVR, as it is quite small.

Hi MK, thanks for the link. Interesting. I'll probably end up with a bit larger of an oven than that is for. But saved for future reference - thanks!

Ash.
 

Offline 0.01C

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Re: Building your own voltage reference - the JVR
« Reply #161 on: June 30, 2017, 01:52:35 am »

[/quote]

Hi MK, thanks for the link. Interesting. I'll probably end up with a bit larger of an oven than that is for. But saved for future reference - thanks!

Ash.
[/quote]

A Chinese forum  bbs.38hot.net   ,some people selling their diy oven and reference board , just CNY 200+
0.01℃
 

Online Marco

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Re: Building your own voltage reference - the JVR
« Reply #162 on: June 30, 2017, 02:59:13 pm »
The thought did cross my mind and I've done similar software based temperature compensation for things before, but I had access to a temperature chamber then and I don't now :)

Even without temperature control you still want all the critical components to be in a predictable environment which quickly gets to steady state, which is to say inside a high thermal conductivity housing. The same as with an oven, it just doesn't need insulation. Put some high wattage resistors on it and make your own temperature chamber.
« Last Edit: June 30, 2017, 03:01:54 pm by Marco »
 

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Re: Building your own voltage reference - the JVR
« Reply #163 on: July 05, 2017, 03:42:05 pm »
Would a heater without any insulation not result in a large temperature gradients, something even worse than temperature fluctuations?

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Re: Building your own voltage reference - the JVR
« Reply #164 on: August 30, 2017, 04:16:46 pm »
Another measurements before I take the JVR to the newly calibrated HP3458A. Taken with two Keithley 2015 and Keithley 617 as a thermometer (measuring the output of LM35D, 200mV = 20C, 10mV/degree, right scale). So far both JVR and Fluke 731B drifted less than 5ppm over the last year (even with all my attempts to measure a hysteresis). Interesting that the JVR has a low tempco between 22 and 26C

Cheers

Alex



Using  the same jfet in plastic (to-92) case how worse could be the TC and long-term stability?

Could be a good idea to consider power jfet with lower on resistance then small jfet?
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #165 on: August 30, 2017, 04:31:24 pm »
A plastic case JFET can have much higher long time drift due the humidity in the plastics. As this reference also needs a rather stable (and thus maybe more expensive than the FET) resistor, I see little sense in saving a little one the FET. The 2N4391-3 are still available in TO18 case.

A higher power FET could result in lower noise, just like using several in parallel.
 
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #166 on: October 03, 2017, 10:28:01 am »
Tomorrow will be two years since I've started this thread. The 10V JVR unit is back in the lab and connected to the HP3458A. The reference was not powered for two weeks in September when I was away  for a holiday in Canadian Rockies and Vancouver. It was not affected much (if at all) over this time and came back to the exactly the same voltage as it was before the power interruption. The measured voltage today after 2h warm-up is 10.00008 V at 24C (previous measurements were taken at 25C). Overall, the drift of a complete reference (including the 7V to 10V amplifier part) appears to be less than 5ppm per year (+ whatever the meters uncertainty and drift is, so in total probably better than 10ppm/year ), measured by HP3458A Opt 002 and also compared with the Fluke 731B which looks very stable.
Cheers

Alex
« Last Edit: October 03, 2017, 11:28:15 am by Alex Nikitin »
 
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Offline BNElecEng

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Re: Building your own voltage reference - the JVR
« Reply #167 on: November 02, 2017, 05:20:51 pm »
Hi Alex,

I've read through the entire thread of this topic. You've got an excellent low-cost voltage reference design by the looks of it. I'm hunting down the parts to make my own now.

I was curious if you had had the time to make multiple copies of the same design to use as references against each other, or are you only using your fluke 731B for this purpose?

Thanks in advance
 

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Re: Building your own voltage reference - the JVR
« Reply #168 on: November 02, 2017, 05:30:00 pm »
Hi Alex,

I've read through the entire thread of this topic. You've got an excellent low-cost voltage reference design by the looks of it. I'm hunting down the parts to make my own now.

I was curious if you had had the time to make multiple copies of the same design to use as references against each other, or are you only using your fluke 731B for this purpose?

Thanks in advance

I'm using 731B, HP3458A and an LTZ1000 based reference for cross-checks. The HP3458A Opt 002 (4ppm/year) was calibrated in May this year.

I plan to make a proper pcb which would accept either a JFET or LM399 or LTZ1000, and will slide in a standard Hammond case.

Cheers

Alex



 
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Offline RandallMcRee

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Re: Building your own voltage reference - the JVR
« Reply #169 on: November 02, 2017, 05:44:22 pm »

Sounds like a winning pcb. I've been following this too, and have some jfets ready to go. Can you make the pcb available on (for example) osh?

Per the ltz1000 I think you should stick to the datasheet circuit as opposed to the more elaborate KX one (my vote--obviously do what you like).

Thanks!
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #170 on: November 02, 2017, 06:11:42 pm »

Per the ltz1000 I think you should stick to the datasheet circuit as opposed to the more elaborate KX one (my vote--obviously do what you like).


I have my own ideas  ;) .

Cheers

Alex
 

Offline iainwhite

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Re: Building your own voltage reference - the JVR
« Reply #171 on: November 06, 2017, 07:40:15 pm »
Is the schematic for the latest reference the same as the one in Reply #49 on Page 2, except with a 2N4391 JFET and an OPA227 OpAmp?
I would like to try building it.

Thanks for an interesting thread.
 
 

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Re: Building your own voltage reference - the JVR
« Reply #172 on: November 06, 2017, 10:17:20 pm »
Here is the latest schematics (for the unit running for 2 years now). R1 is selected for the zero tempco point, R5 and R6 for 10V output.

Cheers

Alex

« Last Edit: November 06, 2017, 10:19:02 pm by Alex Nikitin »
 
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Offline BNElecEng

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Re: Building your own voltage reference - the JVR
« Reply #173 on: November 06, 2017, 10:26:22 pm »
Hi Alex,

Is J2 used to start up the circuit? I'm assuming it pulls up the input of the op amp when power is applied, in order to generate a voltage to allow J1 to begin regulating.

Thanks in advance
 

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Re: Building your own voltage reference - the JVR
« Reply #174 on: November 06, 2017, 10:32:08 pm »
Hi Alex,

Is J2 used to start up the circuit? I'm assuming it pulls up the input of the op amp when power is applied, in order to generate a voltage to allow J1 to begin regulating.

Thanks in advance

Yes.

Cheers

Alex
 
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Offline Cerebus

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Re: Building your own voltage reference - the JVR
« Reply #175 on: November 06, 2017, 10:46:35 pm »
Hi Alex,

Is J2 used to start up the circuit? I'm assuming it pulls up the input of the op amp when power is applied, in order to generate a voltage to allow J1 to begin regulating.

Thanks in advance

Ha! You beat me to it. I was just about to type out the very same question.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline BNElecEng

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Re: Building your own voltage reference - the JVR
« Reply #176 on: November 06, 2017, 10:48:14 pm »
  :) you know what they say about great minds
 

Offline iainwhite

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Re: Building your own voltage reference - the JVR
« Reply #177 on: November 07, 2017, 02:44:53 pm »
Here is the latest schematics (for the unit running for 2 years now).

I appreciate you making this available - Thank you.
 

Offline Cerebus

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Re: Building your own voltage reference - the JVR
« Reply #178 on: November 07, 2017, 03:02:09 pm »
  :) you know what they say about great minds

Yeah, and also, sadly, about fools.  :)
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline vindoline

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Re: Building your own voltage reference - the JVR
« Reply #179 on: March 09, 2018, 02:33:27 pm »
Here is the latest schematics (for the unit running for 2 years now). R1 is selected for the zero tempco point, R5 and R6 for 10V output.

Cheers

Alex



Hi Alex, thanks for posting your schematic. I'm thinking of building one up and I have a question. R3 on the output is marked "k12" I'm assuming this is a typo and it should be 1k2? Thanks!
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #180 on: March 09, 2018, 02:38:24 pm »
Hi Alex, thanks for posting your schematic. I'm thinking of building one up and I have a question. R3 on the output is marked "k12" I'm assuming this is a typo and it should be 1k2? Thanks!

R3 is 120 Ohm .

Cheers

Alex
 
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Offline stijena1973

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Re: Building your own voltage reference - the JVR
« Reply #181 on: January 05, 2020, 04:37:20 pm »
Hi Alex, thanks for posting your schematic. I'm thinking of building one up and I have a question. R3 on the output is marked "k12" I'm assuming this is a typo and it should be 1k2? Thanks!

R3 is 120 Ohm .

Cheers

Alex



Any new developments? Statistics maybe?
 

Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #182 on: January 06, 2020, 10:10:54 am »
Any new developments? Statistics maybe?

Not much , the unit is still running 24/7 and didn't drift more than 5ppm over last four years.

Cheers

Alex
 
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Online BravoV

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Re: Building your own voltage reference - the JVR
« Reply #183 on: January 06, 2020, 12:13:38 pm »
Any new developments? Statistics maybe?

Not much , the unit is still running 24/7 and didn't drift more than 5ppm over last four years.

Cheers

Alex

Looking at the price/performace, I call it boringly good.  ;D

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #184 on: January 06, 2020, 03:05:43 pm »
Any new developments? Statistics maybe?

Not much , the unit is still running 24/7 and didn't drift more than 5ppm over last four years.

Cheers

Alex

Looking at the price/performace, I call it boringly good.  ;D
The long time performance very much depends on the stability of the source side resistor. A very high stability resistors can be quite expensive.
In addition to being stable the resistor value has to be right for the given FET. So it may take more than just 1 resistor.
If it just comes to short time stability / noise the JFET version can still be good and cheap.
 

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Re: Building your own voltage reference - the JVR
« Reply #185 on: January 06, 2020, 03:27:26 pm »
The long time performance very much depends on the stability of the source side resistor. A very high stability resistors can be quite expensive.
In addition to being stable the resistor value has to be right for the given FET. So it may take more than just 1 resistor.
If it just comes to short time stability / noise the JFET version can still be good and cheap.

The changes in the source resistor are attenuated by a factor of ~30, so a 100ppm change in the resistor value creates ~3ppm change in the output voltage, essentially even a quality metal film like RC55 series may be sufficient as a source resistor. As usual, the main problem is with the 7V to 10V amplifier, as resistor changes are not attenuated much. In my prototype old wire-wound resistors are used and holding surprisingly well as the <5ppm change in 4 years is for the 10V output (the change looks smaller but 5ppm is the reasonable limit that I can measure it to).

Cheers

Alex
« Last Edit: January 06, 2020, 03:30:53 pm by Alex Nikitin »
 

Offline guenthert

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Re: Building your own voltage reference - the JVR
« Reply #186 on: January 06, 2020, 04:49:16 pm »

The changes in the source resistor are attenuated by a factor of ~30, so a 100ppm change in the resistor value creates ~3ppm change in the output voltage, essentially even a quality metal film like RC55 series may be sufficient as a source resistor.
  While it might not be nearly as tolerant as a LM399 (with its impressive 0.1Ohm dynamic resistance), contrasting this with, say, an old temperature stabilized Zener like the 1N829 with r=15Ohm, fed from self-referenced 10V supply via a ~515Ohm resistor at about 7.5mA for which  a 100ppm change in the current setting resistor value causes a voltage change of about 10ppm, I'd say that's a very good value.  :-+

  So, when can we buy it?  ^-^

 
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Offline Alex NikitinTopic starter

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Re: Building your own voltage reference - the JVR
« Reply #187 on: January 08, 2020, 03:12:54 pm »
  So, when can we buy it?  ^-^

I have no intention to produce it for sale. As the thread title says: build your own. It is not that difficult  ;) .

Cheers

Alex
 
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Offline TERRA Operative

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Re: Building your own voltage reference - the JVR
« Reply #188 on: December 26, 2020, 02:17:21 pm »
Have the schematics been taken down? I can't see them anywhere.
Where does all this test equipment keep coming from?!?

https://www.youtube.com/NearFarMedia/
 


Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #190 on: December 26, 2020, 05:03:09 pm »
The circuit with the better choice 2N4391 is just some 12 post before:
https://www.eevblog.com/forum/metrology/building-your-own-voltage-reference-the-jvr/msg1343014/#msg1343014

Jaromir linked the old one with an J211.
 

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Re: Building your own voltage reference - the JVR
« Reply #191 on: December 26, 2020, 06:00:02 pm »
Hmm, well that's weird. I can't see any attachments at all...

I'll see what's up with my browser etc.

[EDIT] Dove into the HTML stuffs and found the link. Got it now, all good. :)
« Last Edit: December 26, 2020, 06:10:48 pm by TERRA Operative »
Where does all this test equipment keep coming from?!?

https://www.youtube.com/NearFarMedia/
 

Online BravoV

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Re: Building your own voltage reference - the JVR
« Reply #192 on: December 26, 2020, 06:10:54 pm »
No problem here what so ever, all images are fine at Firefox.

Offline exe

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Re: Building your own voltage reference - the JVR
« Reply #193 on: December 26, 2020, 06:31:24 pm »
I don't see the image. Inspecting the element, I see computed image width and height are both zero. I suspect onload="pagespeed.CriticalImages.checkImageForCriticality(this);" for some reason doesn't work. May be it doesn't interract well with one of my browser extensions (disabling uBlock Origin didn't help).
 

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Re: Building your own voltage reference - the JVR
« Reply #194 on: December 27, 2020, 01:07:04 am »
I figured out the issue in my case, if I disable the HTTPS-Only mode if Firefox settings, the images work.

Seems like the OP doesn't have any form of HTTPS enabled on his website so Firefox just says a big nope to any crosslinks from there if HTTPS-Only mode is enabled.

Here's a direct link to the schematic on the website.
http://www.ant-audio.co.uk/Test_Gear/10JVR_2N4391_001.gif
(R3 = 120 ohm)
Where does all this test equipment keep coming from?!?

https://www.youtube.com/NearFarMedia/
 

Offline SilverSolder

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Re: Building your own voltage reference - the JVR
« Reply #195 on: December 27, 2020, 01:52:14 am »
Reposted circuit as a local image, to reduce dependencies:

 
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Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #196 on: December 27, 2020, 09:33:53 am »
For a practical implementation the resistor R1 would likely be a combination of a few resistors, like  4.7 K in series with 300 Ohms and than maybe some larger ones in parallel to the 300 Ohms to do the fine trim. This way the resistors for the fine trim don't need to be highest stability and one does not need to order a  highly stable resistor at a specific value (it may be practical in cases - e.g. custom wire would ones)

The OPs can be lower grade: for a start the good old OP07 should be OK.
 

Offline exe

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Re: Building your own voltage reference - the JVR
« Reply #197 on: December 27, 2020, 11:24:01 am »
As I understand, J1 is the jfet that generates reference current. What is the purpose of J2?
 

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Re: Building your own voltage reference - the JVR
« Reply #198 on: December 27, 2020, 12:55:08 pm »
As I understand, J1 is the jfet that generates reference current. What is the purpose of J2?

To provide bias for reliable startup (J1 is biased from the opamp output).
Best Regards, Chris
 
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Offline exe

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Re: Building your own voltage reference - the JVR
« Reply #199 on: December 27, 2020, 01:25:21 pm »
Ah, got it, it stops conducting when the circuit is active.
 

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Re: Building your own voltage reference - the JVR
« Reply #200 on: December 27, 2020, 04:22:05 pm »
What's the significance of the asterisks alongside R1, R5 and R6?
Where does all this test equipment keep coming from?!?

https://www.youtube.com/NearFarMedia/
 

Offline exe

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Re: Building your own voltage reference - the JVR
« Reply #201 on: December 27, 2020, 04:49:01 pm »
What's the significance of the asterisks alongside R1, R5 and R6?

These resistors need to be adjusted. R1 needs to be trimmed for zero tempco, R5 and R6 define voltage gain. We need to adjust gain because voltage reference J1 has loose tolerance.

I'd myself wouldn't try to trim all three resistors and once. I'd first figured out the value of R1, and supplied J1 with the desired final output voltage (what we expect to have at U2 out, typically 10V). After adjustment of R1 is done, I'd trim R5 and R6 (I think only one of them needs to be trimmed) so that final output matches what we expect.
« Last Edit: December 27, 2020, 04:50:43 pm by exe »
 
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Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #202 on: December 27, 2020, 06:23:57 pm »
R1 needs to be trimmed for a low TC. I have not checked the numbers very accurate, but expect some 1-10ppm/K of TC for 1% deviation of the value. The initial trim part, down to some 50 ppm/K is relatively easy, the fine tuning can take quite some time, as one than starts to get the square part of the temperature dependence too. So one needs to wait for the JFET too cool down - not to get a low TC at the wrong temperature.  It really helps to have a good DMM (e.g. 6 digits, ideally with a graph) for the adjustment, ideally also a temperature sensor at the reference.

The final voltage setting with R5 and R6 should not effect the TC very much.

The voltage can vary quite a bit (e.g. some 4 - 9.5 V for the 2N4391). So the value for R1 can also vary quite a bit (even a little more, as low voltage FETs also tend to like more current). So one may want to first do a test with lower grade resistors and check for noise and only than order the final resistors, with still a little room for a fine tune.

I did a quick test with a J111 (essentially the TO92 case version of the 2N4391): it kind of works OK on the bread board and the low frequency noise looks promising, e.g slightly better than LM329.
 
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Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #203 on: December 28, 2020, 10:37:03 am »
I checked ten J105 parts left from a previous repair and found pinch-off voltages between 6.8 and 7.5 V at 10 V input. This was at about 1 uA source current (DVM input impedance = 10 MOhm). Then i used two different source resistors 4K7 and 4K7+6K8=11K5 to measure output impedance to be between 84 and 113 Ohm. Source resistor attenuation would be 60- to 80-fold.

With one of the FETs the pinch-off voltage was 7.35 V. Tried to find a low TC point starting with 4K7 as source resistor and stopped after trying various values down to 1.3 KOhm, always with strong positive TC. In the end the FET had more than 5 mA of source current and was operating well below the recommended gate voltage = pinch-off - 0.65 V.

Then i tried TC compensation with a thermistor in the source circuit. I used two 4K7 MF resistors in series and the 10K thermistor parallel to one of them. Now source current and TC are both small. Output voltage is 6.905 V - similar to a TC compensated zener reference. Next step would be fine tuning in the oven, but before that i will get and try a TO18 2N4391 as recommended above.

Regards, Dieter
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #204 on: January 06, 2021, 08:25:29 pm »
Meanwhile i got 10x 2N4391 and found a pair of them that gives 10.041 V at 1 mA when stacked upon each other. That appears to work very well and is much easier than finding a 2N4391 with 10 V pinch off voltage. The maximum i found was 7.1 V and it dropped to 6.3 V at 1 mA. So better combine two of them.

I think this is a way to make an excellent metrology reference:

- TC of this 10 V reference is positive and about 12 times less than the usual 6.2 V zener, so you don't compensate 300 ppm/K but 25 ppm/K down to a fraction of a ppm/K. A very similar compensation scheme can be used: based on a fraction of a pn junction CTAT.

- Noise is also expected to be lower (no zener noise, no gain stage with voltage divider).

- In a divider from 10.041 to 10 V we only need precision for 41 mV. No problem with TC of some ppm, it will be attenuated a factor 200.

- If you think about the long term stability of wire bonds the small current through the reference appears an advantage.

Regards, Dieter
« Last Edit: January 06, 2021, 09:09:04 pm by dietert1 »
 
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Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #205 on: January 07, 2021, 08:34:39 am »
For trimming the TC, the obvious way is to trim the current. With the right resistance value the linear TC can be trimmed to near zero - a little like it was done with the old 1N829 compensated Zeners.  It is not very convenient for mass production, but for a single unit as a hobby project this is very feasible. One should get below 5 ppm/K without too much effort.

Ideally one would have some kind of simple oven and temperature reading, to aid in the adjustment, especially at the fine end.
A simple oven regulation would also be effective against the higher order TC and with a well trimmed linear TC the demand on the oven is no longer that high.

A difficulty is the large scattering in the voltages.  Chances are a batch of fets would be relatively close - so one may not have much to choose from and ordering 10 or 20 pieces from a single source would not extend the voltage range a lot.  Getting 2 FETs to give close to 10 V is more like a lucky find - I would not count on this.

As far as the data-sheets tell, the zero TC current would change a little with voltage. So stacking 2 FETs with significant different voltage would likely result in slightly positive TC for one fet and a slightly negative TC for the other. So temperature differences could cause an error. A resistor in parallel to the lower FET (with higher threshold) could compensate, but is extra effort.
 
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Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #206 on: January 07, 2021, 10:52:42 am »
The reason why i am not using current to compensate TC is in one of your previous posts: The difficulty to adjust, since the source resistor should be a precision resistor. In my test setup i am using a 5K UPW50 that we had. The lower JFET gives 5.46 V of which i am using 0.55 V for the diode, so i get almost exactly 1 mA. Parallel to the diode i have a voltage divider to drive the lower FET gate with the small compensation signal. I think that divider can be made with standard MF resistors.

To keep temperature differences between the two FETs and the diode as small as possible, i soldered them one next to each other into a small raster board. Thermal coupling can be improved by potting that part of the board and finally using an aluminum enclosure as oven. Or use plastic FETs that can be glued into a common piece of copper.

Concerning the "lucky find" objection: Of the ten JFETs i had two got destroyed during experimentation (pinout confusion). From the remaining eight i found that pair with 10.041 V, but there are 3 pairs between 10 and 10.3 V. Statistics helps if you start with 100 parts.

Regards, Dieter

 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #207 on: January 07, 2021, 12:28:08 pm »
Using a diode in series with the resistor will change the TC, even if not tapping of a part of the diode. It should shift the zero TC current more towards a lower current.  In addition it would effect the 2nd order temperature effect , though I don't know if it would make things better or worse.
For adjusting the resistor, there is still the option to combine a precision resistor with an smaller series one or larger parallel one. If the good resistor is not too far off, the additional resistor would be not as critical. Not sure if the combination with a diode and resistors for a trim is more stable. It probably depends on how good the initial guess is.

When starting with a bag of JFETs near 5 V it is easy to find a pair for 10 V. However it is well possible to have bag of 100 FETs all higher than 6 V. Variations inside a batch can be limited. Still even with 2 x 6 V = 12 V or with some 7.5 V from one Fet one would have a better start for a transfer to 10 V than from the usual 6.9-7.2 V.  7 V is about as bad as it can be:  2 in series and a divider down is about as bad as the direct amplification. So every thing away from 7 V would be an improvement for scaling to 10 V.
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #208 on: January 07, 2021, 04:36:25 pm »
An incubator test 20 °C to 40°C to see whether the diode compensation signal is linear was done before, but that was using a single JFET and a HP 3478A meter. In that test  deviations from linear were within resolution = 20 ppm, with no indication of a nonlinear term. From that test i learned that for perfect compensation one has to take into account the source follower gain which is slightly less than one.

Will repeat that with a more complete circuit including reference buffer and 10 V to 15 V gain stage as intermediate supply. And with more precision to look at ppms and below. Maybe using 14 V as intermediate supply it can work with the 15 V supplies we have.

Regards, Dieter

PS: The 2N4391 are Motorola NOS and i already ordered another batch from somewhere else.
« Last Edit: January 07, 2021, 04:57:05 pm by dietert1 »
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #209 on: January 08, 2021, 05:12:14 pm »
I did a quite test for the temperature dependence with an 2N4391.  In my case the voltage is at around 6.8 V - so kind of comparable with the classical 7 V ref.  I currently have a combination of simple resistors, but heating the JFET only. As a funtion of temperature the voltage shows a maximum.
Having a diode as part of the source resistor would increse the current with higher temperature this would cause a more downward slope. So the diode would increase the 2nd order effect.

The split in the up/down part is likely due to not so good thermal coupling to the sensor (diode).
 
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Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #210 on: January 08, 2021, 06:54:46 pm »
Today arrived  another batch of 10x 2N4391 (mixed Motorola and Philips) and now i got six pairs with 10.024 V, 10.041 V, 10.041 V, 10.054 V, 10.088 V and 10.195 V, a nice result. For both batches i spent about as much money as one LTZ1000ACH - plus several hours of characterization and selection. Until now a bit of try and error.

Kleinstein, i observed that 2N4391 source voltage TC at  1mA and room temperature is positive and lower for higher pinch-off voltage parts. The part you are using probably has a pinch-off of about 7.6 V. The test i mentioned above was with a 2N4391 at 1 mA and 5.5 V Ugs. It showed positive TC up to 40 °C .

Regards, Dieter
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #211 on: January 08, 2021, 08:48:38 pm »
One gets a positive TC with a resistor that is larger than optimal. I get about 15 µV/K more for another 100 Ohms extra. So to get the TC down to 1 ppm/K one would need to adjust the resistor to some 50 Ohms.

The Drain voltage has quite some effect on the voltage and suitable resistor. So it absolutely makes sense to get this from the amplified ref voltage. If this voltage is only used for the supply, one could do the fine trim of the TC also here, at least for a small range. To avoid extra 1/f noise one usually wants a relatively low drain- source voltage (e.g. < 5 V).

The 2nd order TC at some -0.19 ppm/K² seems to be comparable to what one gets with compensated zeners. So for a really stable reference one would need either a stable temperature or a compensation for the 2nd order effect.
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #212 on: January 09, 2021, 02:25:33 pm »
In a first determination of curvature for one of the JFET pairs i also got a flat max of the temperature curve, with -0.054 ppm/K² curvature. Since i compensate the linear term, the max temperature is in the middle of the temperature sweep (20 °C .. 40 °C). You can also see that this time i need 0,36 of the diode correction signal.
Next i need to improve and automate my setup to look at it below ppm level.

Regards, Dieter
 

Offline SilverSolder

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Re: Building your own voltage reference - the JVR
« Reply #213 on: January 09, 2021, 04:23:27 pm »

I have a question on temperature correction:   Is the temperature dependence repeatable? - e.g. so if you know the temperature, you also know the offset?

I'm thinking, you could measure the specific device and create a lookup table in a small micro that compensates the reference (either mathematically after the fact, or via a D/A converter feedback to the output voltage).  Workable?
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #214 on: January 09, 2021, 05:00:20 pm »
The temperature dependece is rather repeatable. The slope depends on the current / resistor used , but for give device it should be rather stable.
A feedback from a DA may be possible. It may still be good idea to first adjust the lineat TC to a low value (e.g. bring the temperature of the maximum to something like 10 - 40 C), so that the deviation range is small and thus not too much correction term. To keep the low noise level I would expect that DA steps would have to be a little below 100 nV - so some 10-11 Bit to corret up too 100 µV.

The alternaive would be a temerature regulation to a litte above normal operating temperature. With a well trimmed linear TC the redulation would not have to be so accurate.
 

Offline Cerebus

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Re: Building your own voltage reference - the JVR
« Reply #215 on: January 09, 2021, 05:03:37 pm »

I have a question on temperature correction:   Is the temperature dependence repeatable? - e.g. so if you know the temperature, you also know the offset?

I'm thinking, you could measure the specific device and create a lookup table in a small micro that compensates the reference (either mathematically after the fact, or via a D/A converter feedback to the output voltage).  Workable?

Reasonably predictable. The temperature dependence curve is subject to hysteresis, so you have to know the temperature history, it isn't just a straight temperature lookup. The amount of hysteresis is also dependent on how far out the temperature has wandered. So, it isn't simple. The more history you have, the better a temperature dependence model you can build, but it's a model you need, not just a simple lookup table.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #216 on: January 09, 2021, 06:01:44 pm »
We have two LTFLU references that run in hermetic TEC ovens at about 17.4 °C and 27.7 °C and i can tell that no significant shifts (more than  0.03 ppm) have been observed after temperature cycles. Those were accidental oven shutdowns or during reconfiguration. I'd suspect that temperature hysteresis is more relevant for high temperature ovens like the LM399 or the LTZ1000, but not at lower temperature. As far as i know most of the experts here do not implement the Pickering anti-hysteris cycling in their LTZ1000 references.

Regards, Dieter
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #217 on: January 09, 2021, 06:15:27 pm »
Hystersis is more a thing of larger excursions, much less of small excursions. The absolute temperature would not make that much of a difference - this more like changes the time scale for thermally activated effects.

 When the temperature is set relatively close to room temperature there are usually no large excursions. The ref. chips are build to have low hysteris - we don't yet know how bad it is with the JFETs that are not specially made to have low hysteresis.

I did see a little hysteresis, but not much and this could very well be from the not so ideally places temperature sensor and from contacts (I still have the FET in a socket).
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #218 on: January 09, 2021, 07:22:36 pm »
No, you didn't see hysteresis. You don't know what you saw.

Regards, Dieter
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #219 on: January 11, 2021, 12:22:09 pm »
I did a quick test on the noise levl of the JRV refernce. As the noise is relatively low, this is not so easy.
The main idea is to build kind of 2 JRV type reference (2N4391 + 6.18K + stable drain voltage) and measure the amplified difference.

For the amplification one of the references is modified, so that the gate is used for feedback. So the circuit is the same as the old days combination of 2 JFETs as source follower before an BJT based OP.
The drain voltge is stabilized from the 1st unmodified ref. part.

Attached is the first test, so there is still quite some temperature rise with time - which is kind of positive.
The resistors are such that the idividual temperature drift is relativly small, though not perfectly. For the differnce one has a temperature dirft of some 20 µV/K - so not good for an amplifier, but not so bad for a crude hacked togetger reference at some 7.1 V.

The curve shows the change in voltage difference over some 12 minutes plotted versus a diode voltage to measure the temperature change. The temperature rise is still relatively linear in time.

The noise is surprisingly low with some slight signs of pop-corn like jumps.  Some of the noise may still be from the crude setup and simple resistors (may still be thick film, though not sure).  So there is a chance to get a really low noise refernce.


edit: add circuit diagram for the test
« Last Edit: January 11, 2021, 12:38:09 pm by Kleinstein »
 
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Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #220 on: January 11, 2021, 03:09:12 pm »
I just realized the drain voltgate part was not really working - so some of the noise / drift could be from that end.
I repeated the part with working drain voltage part and changing the amplier OP to an OP27  (much of the higher frequency noise was from the OP07 - and the OP27 still conributes).
The plot is this time versus time and not "temperature" and a little more zoomed in.

So it looks like the  10 seconds peap to peak noise can reach the 1 µV region - so about comparable with LTZ1000 / LTFLU.
 

Offline exe

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Re: Building your own voltage reference - the JVR
« Reply #221 on: January 11, 2021, 03:26:41 pm »
Kleinstein, on your schematic both Q1 and Q2 have same source resistor. It means that at least one of them not trimmed for zero tempco. So, can be part of change in voltage difference is due to temperature change?
 

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Re: Building your own voltage reference - the JVR
« Reply #222 on: January 11, 2021, 04:48:01 pm »
The tempco is not trimmed very well, but the residual TC for the difference is also not so large (20 µV/K range).  For 1 of the JETs the TC is reasonable well trimmed. The general slope in the plot versus temperature is very likely the temperature effect. The noise may be a littel better with better adjusted resistors, but I would not expect that much improvement.

The noise part is more like small jumps of some 0.5 µV - this is definitely not a temperature effect.  The short time noise bewteen the jumps has parts from the OP, especially in the first curve.
The points are 20 ms integration and thus 25 Hz noise BW. The data are 1 point about every 81 ms. The "missing" points are not expect to contribute much to the noise. So the data from 10s windows should about reflect the 0.1 Hz to some 10 Hz noise.  The exact upper end of the frequency band does not make much of a difference in this case, as there is little noise between some 5 and 25 Hz.   
 
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Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #223 on: January 14, 2021, 09:47:10 pm »
Meanwhile i automated my dual JFET reference setup.  I am using 2x Fluke 8502A (A and B). To check for hysteresis i tested temperature dependence at various fixed temperatures - no scan because of sensor lag. This test shows no indication of hysteresis above a limit of maybe 0.5 ppm. Only the first point at about 20 °C shows more deviation, but i know that the DVMs drift a little in the morning while warming up after the night.
This time the diode voltage is a bit higher, since i omitted the 1K resistor parallel to the diode (placeholder for TC divider). I was hoping that curvature would reduce. But results are the same as before (-0.060 ppm/K²).

Regards, Dieter
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #224 on: January 14, 2021, 10:08:48 pm »
If I understand it right, a diode in series with the source resistor would add to the curvatutre, though not very much. With increasing temperature the current for the reference could go up, and a high current gives a more negative TC. So the curvature gets larger.

A diode at the drain side could help a little to reduce the curvature: lower drain voltage reduces the reference voltage and current. The ideal zero TC current would also drop a little, but not as much. So one could get a little compensation for the curvature. However from my estimates the effect would be rather small. So I don't think it would be practical to fully compensate the curvature this way. With the 2 JFET version the diodes could be between the FETs: the DS voltage should ideally not be very large (e.g  > 5 V) anyway.
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #225 on: January 14, 2021, 10:47:15 pm »
Yes, the TC of a diode in the drain path arrives at the reference output with some attenuation, which is very interesting. In order to get the required fraction of 0.36 i would need several diodes, maybe 5 or 10. To get the TC sign correct, the diodes will not be in the drain path, but in the intermediate supply divider. Will try that later.

Regards, Dieter
« Last Edit: January 14, 2021, 10:51:26 pm by dietert1 »
 

Offline DeltaSigmaD

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Re: Building your own voltage reference - the JVR
« Reply #226 on: January 26, 2021, 12:12:39 pm »
The JVR reference circuit is known at least since early 70s, but it wasn't used in measurement equipment because its initial reference voltage has huge tolerances. However, it is reported in this forum that the stability of the JVR can be amazingly good. The pressure to find better references is high, so the JVR should be analysed as low power, low noise reference in this new light of stability.

Kleinstein and dietert1 discussed the temperature dependency of the JVR. I performed some simulation experiments with LTSpice. The first diagram Fig.1 shows a temperature compensation for a JVR, so that the residual drift has a forth order shape. The reference voltage deviates less than 4 ppm from -20°C to 57°C. It is possible to move the minima closer together in order to get an optimally flat zone, see Fig.2. However, this theoretical result is too good to be true: as the precision of R1 and R2 shows, a perfect matching of the characteristics of J1 and Q1 is essential to obtain this low temperature dependency. I assume that such tight matching can be hardly realised in practical circuits, or it will need days or weeks of work. Assuming this adjustment can be obtained, it will be complicated to ensure that J1 and Q1 chips have a temperature tracking better than 3 mK. The reference voltage is defined by the combination of 3 large currents, and good long term stability cannot be expected therefore. Other simulated circuits with third order residuals are not better in this aspect. 

If the allowable working temperature range is not too large (f.e. with battery-operated hardware), the temperature of the reference JFET can be measured and its drift is corrected by software, so that drift compensation by analog hardware can be omitted. Long-term stability is the remaining problem. Fig.3 shows two 10V JVR reference circuits without compensation. The lower circuit uses an OP amplifier to get the 10V value, while the upper circuit uses JFET J3 for amplification and the OP only as buffer. The simulation shows that the noise of the reference alone is much lower than the noise of the OP. Therefore, the upper ref should be applied for low noise, even if its adjustment (drift maximum and voltage) is more complicated. C4 reduces the high fequency noise to the noise value without amplification. 
 
Assuming the LTSpice JFET models are correct, the JFET reference has very low noise. The buried Zener references have a sharp break-down, while the JVR has a soft working point, even softer than of bandgap references. The depletion zone of the JFET has a large volume, while a Zener reference relies on a limited set of hard punctual microplasmas, which must be very sensitive to interstitials, voids, impurity clusters etc.. Please see Zener microplasma action at Richi's Lab - unbelievable microscope pictures there. 

The essential point of the circuit Fig.3 top is that only 2 parts must have utmost precision. Since no heater is applied, there will be no temperature hysteresis with power failure and the temperature gradients around the reference JFET can be kept very low (Kovar feed-throughs at hermetic case!). Is it possible to obtain long-term stability with a soft working point? How large is the temperature hysteresis of different JFET types? The next step is testing of real JVR hardware, which includes the JFET case temperature measurement.
 
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Offline Cerebus

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Re: Building your own voltage reference - the JVR
« Reply #227 on: January 26, 2021, 01:14:40 pm »
Assuming the LTSpice JFET models are correct, the JFET reference has very low noise.

Sadly this isn't the case. It isn't LTSpice, or even spice in general that is to blame, it's the models. I don't think I've yet encountered a single BJT or JFET model that actually properly populates the model parameters that allow spice to model \$\frac{1}{f}\$ noise properly. With those parameters at their defaults you always get optimistically good results for \$\frac{1}{f}\$ noise.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 
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Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #228 on: January 26, 2021, 01:40:26 pm »
The 2 nd order TC is relatively large. So the numerical correction for the square part is not that easy, if the range gets larger.
The analog compensation is tricky, as this often also give quite some linear contribution and thus needs to be quite stable.
I compared J111 (similar to 2n4391) and J113 (similar to 2N4393) they both showed a comparable 2nd order effect at around 3 µV/K². So inprinciple one could use 2 similar JFETs with different theshold and only use the difference - so kind of a discrete version of what AD called the XREF referencce. The drain voltage seems to have some effect also one the 2nd order TC, though not enough for a full compensation.

For ultimate stability, temperature stabilization is likely the way to go. Even only a crude temperature stabilization is quite effective against the square effect. Trimming the linear TC to some 5 ppm/K is not that difficult, so that the requirements for the oven can be moderate.
Trimming the 2 nd order compensation can be quite trick, it can be done, as shown with the Solartron 7071 , but it is quite some effort.

The large scattering in voltage level is a problem. Using resistor gain is tricky, if the adjstment range is larger. This does not get much better when the adjustment is done like in Fig3 combined with the Ref. itself. It saves 1 resistor, but the demand on the resistor that needs adjustment also goes up.

For a DMM or high resolution ADC a variable reference votlage to start with may not be that bad. Some designs measure the ref. voltage directly by the ADC to adjust / check the ADC gain factor anyway. This step can tolerat a large range, as long as it is well within the ADC range. It would be only 1 gain stage to set the working ref. voltage to the ADC. I am actually considering a JRV ref. for my ADC circuit  - availabilty of LM399 and LTZ1000 is currently not the best.

The simulated noise is not especially accurate, epsecially not for the 1/f part.

However the measured noise also looked good, at least for the 2N4391 and J111 that I checked.
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #229 on: January 26, 2021, 05:10:44 pm »
Here a little report of my recent tests with a JFET pair prototype.

Regards, Dieter
 
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Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #230 on: January 26, 2021, 06:15:16 pm »
The circuit looks a little odd at 2 details: the capacitor C3 is quite some capacitive loading to the OP IC1 this does not look right.
The ouput amplifier looks like it can tolerate some capacitance, but having already 10 µF (C11) allready looks like pushing it close to the limit.
In my test I see very little higher frequency noise - so there is likely no need for most of the caps, at least not that large. The datasheets also suggest vers little higher frequency noise.

The OPA140 is a really good JFET OP, but it can still have quite some 1/f noise by itself. The more suitable choice for the amplifier would be more like precison BJT baded ones. So more like the classic OP27 or OP07.  Modern higher performance ones (e.g. OPA207 or AD8676) are also possible, but likely not needed.
The ouput impendace (some 150 Ohms+R2) should be low enough to drive a BJT based OP with no problem.


 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #231 on: January 26, 2021, 06:32:22 pm »
Yes, there is a more complete schematic as well. The schematic i showed is "schematic" and was meant to show my variant in contrast to the above LTSPICE models. Not meant for copy-kids. Meanwhile i think i will make a board and then use a chopper amplifier like OPA2189. Also the diode signal should not be brought out on a wire, but with a buffer with some gain (bridge circuit).

Regards, Dieter

PS: The divider for the TC correction signal can be made temperature sensitive, e.g. by using one resistor with a certain TC. That becomes a multiplier for the TC correction signal to generate a second order correction term. Just an idea.
« Last Edit: January 27, 2021, 06:32:53 pm by dietert1 »
 

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Re: Building your own voltage reference - the JVR
« Reply #232 on: January 29, 2021, 11:20:23 pm »
Dieter,

Thank You for the report.
For some reason I am under the impression that in the two fet cascode like structure the fets should have different pinch off voltage ratings.

Specifically the upper fet should have pinch off voltage which is typically twice the pinch off voltage of the bottom fet.

Expected device selection upper fet 2N4391 lower fet 2n4392.

Please consider
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #233 on: January 30, 2021, 07:56:19 am »
Yes, it appears like a cascode, but the upper JFET has an elevated gate voltage. In my prototype the lower FET runs at Ugs = -5.6 V and Uds = 4.4 V. The upper JFET runs at Ugs =  -4.4 V and Uds = 3.3 V. If i exchange them it should still work, except with lower current.
This proposal was meant as method to overcome the large uncertainty in pinch-off voltages by selecting pairs of JFETs. Think of two 5 V zeners. Imagine you could  only get bad zeners with a specification of 5 +/- 2 V. Then you could combine a 4 V one with a 6 V one to get 10 V without throwing away zeners.
Somewhere in this thread Kleinstein proposed to search for JFETs with 10 V or near 10 V pinch-off, but i haven't found any. I even ordered a batch of BF247C which are specified for up to 14 V pinch-off, but again nothing like 10 V.

Regards, Dieter
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #234 on: January 30, 2021, 09:42:41 am »
The circuit is essentially 2 of the JRV references in series. 2 in series give more choices with the voltage.
The more unusul part is the diode use to compensate the residual TC  - the usual way is by adjusting the resistor to set the current. The diode may give a little more flexibility with the voltage, though also a high 2nd order TC.

One could combine 2N4391 and 2n4392 to get a special target votlage like 10 V, though there is a tendency that the 2N4392 would need a little more current to get near zero TC. So one may want some extra resistor to get both FETs in the range of low TC.

10 V from a single FET would be rather extreme - this would be more like a possible target if special fets would be designed for this purpose. The largest I have found so far is one J111 with a little over 8 V. The voltage can vary between batches and also somewhat (e.g. +-500 mV) inside a batch.
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #235 on: January 30, 2021, 11:09:47 am »
No, 8 V may the pinch-off voltage, but if you increase current to get near zero TC, you will have more like  6 or 7 V. Getting 10 V with one JFET was fantasy.

I am using a diode for TC compensation because for me adjusting TC by JFET current didn't work. Above i reported my experiments with J105 JFETs that did not reach zero TC at more than 5 mA. And from my discrete bandgap reference experiments with diode chains i learned that currents in reference elements should be small. Otherwise there will be the bond wire weakness. If you assume that a bond wire is 10 milliOhm, then 1 mA contributes 1 ppm. A common 1 mil bond wire has about 50 milliohm per mm of length. Recently i found a 2N3055 with redundant bond wires to the emitter - both completely open.

Kleinsteins statement about "high second order TC" is fantasy as well. Please look up the numbers and diagrams above. The temperature sweep in my little report shows less than 1 ppm curvature between 20 and 28 °C. So this is as good as any temperature compensated zener, except with a much smaller linear compensation in case of the JFET.

Regards, Dieter
« Last Edit: January 30, 2021, 11:23:40 am by dietert1 »
 

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Re: Building your own voltage reference - the JVR
« Reply #236 on: February 03, 2021, 10:11:36 pm »

Finally got a step further with an oven for the JRV reference. At least at the sensor used for regulation the temperature is quite stable (some 10 mK_pp). 
The oven is some 32x32x20 mm with 2 mm (0.6 mm one side) aluminum at the outside, where the heater is attached. There is some isolation with bubble-warp, just for the first tests.
The set temperature is relatively low, at some 30 C or so. I may want a little more isolation and a slightly higher set temperature.


The curve shows the measurend voltage and heater current (15 V voltage).  At some 2300 s the whole oven is covered with some paper - this likely caused the power slowly going down.

The noise and medium time stability looks good - as far as one can tell in comparison to a LM399 ref.
A little of the higher frequency noise is from the ADC (some 1 µV_pp for average over 12 conversions at 1 PLC). The pop-corn like jumps (~2 µV) are larger than with the 2 JRV ref. test before (~0.5 µV). Still not sure if they are from the LM399, as the jumps there often looked larger (e.g. 4 µV). Still the overall noise looks good, given that it includes an LM399 (filtered to some 5 Hz - but this should not help very much).

 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #237 on: February 07, 2021, 05:32:01 pm »
Comparing the JRV to a meter with LM399 reference is not really sufficient to judge the noise at this level. So I added a 2 nd JRV type reference to the same ofen (found just enough space to fit) using the same drain voltage and without buffer.

The curve shows the voltage of the 2 reference measured in parallel with the same ADC / ref. The points in the curve are for the average of 24 conversions at 1PLC in a fast sequence. So not exactly at the same time but very close and overlapping. Each point represents about 2 seconds.

In the initial part the temperature is likely not yet full stabilized.

The curve shows the noise from JRV reference plus the LM399 (seems to be a rather low noise one with only rare large popocon type jumps) at the ADC. There is some correlation in the noise, but only partially. Much of the noise seems to come from the JRV references and not the ADC or LM399. The ADC contribution should be essentially white noise of some 0.8 µV_pp.

The noise of the 2 curves looks a little different from what I saw in the direct difference on the bread board. So I checked for excess noise of the resistor type (Susumu RR - thin film 0805 size 25, ppm/K specs) used for th source resistors and the amplification for the drain voltage. The test showed low noise (better than -50 dBi noise index, which is about the limit for my simple test setup - DC bridge with NiCd batterie for floating bridge drive and AZ OP for amplification). So there should be essentially no excess noise from the resistors. It would take rather noisy resistors (e.g. -10 dBi range) to see the resistor excess noise at the JRV output as the resisor noise is attenuated about 30 times.
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #238 on: February 07, 2021, 07:33:23 pm »
Would you tell at what source currents the JFETs run and the temperature of your oven? Can you run a difference measurement, so the LM399 noise is out?

Until now i have only one ovenized JVR and my estimate would be about 2 uVpp. Will also make a prototype with two JVRs to get the difference measurement. Also with two of them, the average should have less noise..

Regards, Dieter
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #239 on: February 07, 2021, 08:20:18 pm »
Both FETs use a source resistor of 6.191 K nominally. For both this is a resonable good trim to low TC, though I still need to measure the voltage versus temperature curve.
So the current is at 1.1 mA  resp. 1.13 mA. The  drain voltage is at 4/3 of 6827 mV thus 9.1 V.

The temperature schould be at around 25-30 C, so a little above room temperaure, but not very much. The temperature is stable and like with the curve before there is no real correlation between heater current and ref. voltage.  For the initial tests on the bread board I also had a noise level more like 2 µV_pp, though for a shorter time frame.

A direct difference reading is a good idea. I can at least plot the difference from the same data file (see atachment). So still both conversions, but at least without much of the lower frequencey noise of the LM399. At the ADC there is some low pass filtering (2.2 Hz cross over) for the LM399 reference, so the higher frequency part of the LM399 noise is suppressed quite a bit. A direct difference measurement would be slightly better, but needs a new run. For the resistor noise measurement I have a difference amplifier anyway - so this would be relatively easy, just needs time, so maybe tomorrow.

edit : had the wrong curves with difference to LM399.
        There seems to be something slightly wrong with the scale factor of one of the curves.  The 2nd ref should be more like 6927 and not 6977 mV. It does not change much with the dift or noise.

The noise is still a little higher - but given the relatively long time (some 3 hours)  the difference is not that large. The timescale for the curves is a bit longer than the usual 0.1 - 10 Hz windown often shown in data-sheets.
« Last Edit: February 07, 2021, 08:31:17 pm by Kleinstein »
 

Offline SilverSolder

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Re: Building your own voltage reference - the JVR
« Reply #240 on: February 08, 2021, 02:16:46 am »

How does the noise compare to LTZ1000?
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #241 on: February 08, 2021, 08:56:41 am »

How does the noise compare to LTZ1000?

Over short times (e.g. 10-100 s windows)  I would consider the noise comparable to the LTZ 1000, for the white noise part the noise is even considerably lower (some 5 nV compared to 40 nV). However for the longer run (e.g. 1 hour) / lower frequencies there is higher noise (some 3-4 µV_pp compared to maybe 1.5 to  2 µV für the LTZ).

For most refrence applications the longer time scale is relevant, not so much the very short time. So the usual 0.1 - 10 Hz  (or peak to peak noise over a 10 s window) specs for low frequency noise are more like convenient to measure and not that close to real world use. For real world it would be more like 0.1 -10 mHz.

For the long time scale I am still not sure if the oven is good enough, but chances are it is. The LTZ starts with a relatively high TC (some 50 ppm/K) and thus need a very stable temperature control. The JRV can be adjusted to a low TC like 1 ppm/ K range to start with and than get away with a less stable oven. So different from the LTZ the set point for the oven is not so critial - it is the resistor at the source that is critical for the long term drift. Also the amplification for the drain voltage seems to need good resistors - my inital crude meassurements do no match up the datashett numbers as I understand them. This could be in part because the Fets are really high threshold ones, not the more average ones.
 
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Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #242 on: February 08, 2021, 07:59:28 pm »
I did the test with a direct difference measurement.  The difference signal is amplified by an AZ amplifier in a way that the ADC switches between +21.8 times the signal and -21.8 times the signal. This gives an AZ mode with near 100% sampling and 2 PLC data with 25 reading per second. I used the same amplifier for resistors noise testing an it was low noise and low drift there. So the ADC and amplifier noise should be only a very small part. The buffered reference is use for the "ground (low impedance terminal of the amplifier)" the other ref. is  not buffered and has some 68 pF+100ohms for filtering at the AZ OP.

It took quite some time for warm up - so that curve starts at 4000 seconds. It seems the TC adjustment is not that good as I thought.
The initial data look good, but later there is quite some nasty up and down. It still looks more like Popcorn type noise with relatively fast jumps, not really like a more slow drift or smooth variations as expected for a thermal effect. So at least one of the 2 reference is not behaving well. Over some short parts it still looks OK, but as a whole this does not look good. The shorter term part is still comparable to the indirect difference measurement reading both voltage separate. So it does not like the amplifier or oven is the problem.
 
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Offline DeltaSigmaD

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Re: Building your own voltage reference - the JVR
« Reply #243 on: February 23, 2021, 02:10:30 pm »
The 3rd order compensation of the temperature dependency of a JVR (as described Jan 26th) was verified in hardware. The compensation circuit was largely improved. A first prototype yielded a 5V voltage with 3.5ppm stability from 20°C to 32°C, see attached diagram. The 1 day drift is also within a 3.5 ppm box. More details can be found in the attached paper. Next, the stability over longer time will be measured. Note: the Keysight 34470A is used for reference voltage measurement.
 
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Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #244 on: February 23, 2021, 03:49:09 pm »
Nice experiment, thanks for the report. As far as i understand the transistor circuit works similar to a bridge circuit that allows control of the curvature of the base-emitter diode temperature curve. I mean the real temperature sensor is the base-emitter diode of Q1. Is that correct?
Did you think about giving Q1 a Miller capacitor to calm down it's operation point?
Also it appears to me that your FET is running at about 2.5 V of Ugs. The gain inherent in R7 R19 R8 should be a little less than 2. Did you try to find a FET with a higher Ugs? This would also help with noise.

Regards, Dieter
« Last Edit: February 23, 2021, 03:51:56 pm by dietert1 »
 

Offline DeltaSigmaD

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Re: Building your own voltage reference - the JVR
« Reply #245 on: February 24, 2021, 07:23:15 am »
The base-emitter diode of the bipolar transistor is working as nonlinear temperature sensor. The zero TC working point of the JFET is shifted to a particular position so that an agreement of 1st, 2nd, and 3rd order can be obtained at the working temperature. That is the reason why there is a power-on drift of the reference voltage.
The miller capacitor may improve the high frequency noise. But, if two capacitors --> lowpasses are in the feedback loop, there might be oscillation. The cap values must differ by more than one order of magnitude. I will place a Miller cap next time.
There is no free choice of Vgs, I could not order the JFETs I would prefer. The reference voltage gain by R7 R19 R8 has no effect on noise above about 10 Hz. Of course, a higher Vgs would be better. Stacking of JFETs is questionable: stacking is better if we are in the white noise region, but this noise contribution is filtered out anyway. If we have 1/f-noise or even random telegraph noise, stacking is not helpful since noise is added (sum, not sqrt(sum)) - correct me if this is wrong.
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #246 on: February 24, 2021, 09:05:16 am »
Even with RTN type noise using more FETs helps. The jumps are non correlated and with "averaging" the individual contributions one gets an even linear reduction in the step size. So with 2 refs in series the jumps don't get larger, just more frequent. So using more FETs would also help in the RTN regime range. For a certain time range the peak to peak noise may not go down, but the RMS noise still should give the the normal square root N reduction in noise, as the parts are uncorrelated. More frequent smaller steps may be even preferred over rare large steps.
Stacking can make the TC compensation more tricky though.

The fets have quite some scattering  ( I got 2N4391 with around 7 V ref voltage, while Dietert got some more around 5V), but there is still a choice in the types.  The 2N4391 should be mainly be in the 4-8 V range. 2N4392 would be more like 2-5  (have not checked the exact numbers). So one has some choice.
The 2N4416 is a more lower threshold fet and also lower current and thus maybe better for battery operation.
It is also one of the few more affordable ones in a hermetic case.

It may be possible that the TC correction does not work as well with a high threshold FET. At least the rather high (7 V range) threshold fets may behave a bid different from the usual models. There may be also a slight different between different types: the process 50 of the 4416 is more like made for RF use, while the process 51 for the 2n4391 is more made for switches. These types may differ in series resistance in addition to the actual FET and this can be part of the temperature dependence.

There may be even some advantage in not having too high a voltage per FET: at least for the normal amplifier operation there are curves in data-sheets that show that the low frequency noise goes up if the drain source voltage goes up, especially higher than some 5 V. For me it is not clear if the drain source or maybe he drain gate voltage is the relevant voltage. In my test I have not seen a large difference between J113 and J111 / 2N4391 , but RTN noise is not that reproducible and can vary between units.
 

Online IconicPCB

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Re: Building your own voltage reference - the JVR
« Reply #247 on: February 24, 2021, 10:53:46 am »
I think process 52 is better suited to the JVR needs.

 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #248 on: February 24, 2021, 01:00:27 pm »
Can you tell us how you arrived at this opinion?

Just found this description of a JFET selection setup and results: http://www.geofex.com/Article_Folders/fetmatch/fetmatch.htm
I think he is correct in not looking at the pinch-off voltage, but at a useful operation point.

@Kleinstein: I looked at four 10x batches of JFETs: J105, 2 batches of 2N4391 from different sources and a batch of BF247C.

Regards, Dieter
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #249 on: February 24, 2021, 02:08:53 pm »
The process 52 looks good, as it is used for relatively low noise audio use. So one can expect a low noise. The other important paramterer is the current range where to expect low TC.
From the typical graphs in Fairchild AN6609 one would expect them to get low TC at some 0.2 to 0.5 mA , though it is not accurate to tell from the graph. There is also more than just the mask  - the doping can also effect the properties.

The problem may however be to get suitable ones with a not too low Ugs, especially if one wants a hermetic case.

The process 51 (e.g. 2N4391) can also provide low noise, though at a higher current. Still some 1-1.3 mA is not that bad. It may not be the best choice for battery operation, but higher current helps with low noise. The nice thing is that the 2N4391 is readily avialable at a relatively good price.

Higher current fets (e.g. J105) may need to run at so much current that self heating gets a real problem.

For amplifier use there seem to be a preference for a relatively low threshold at some 0.5 to 1 V. So especially the lower noise types are usually not a vailable with a higher threshold more useful for JVR.
 

Offline Vtile

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Re: Building your own voltage reference - the JVR
« Reply #250 on: February 24, 2021, 02:22:19 pm »
The 2N4416 is a more lower threshold fet and also lower current and thus maybe better for battery operation.
It is also one of the few more affordable ones in a hermetic case.
I just did "run to order a affordable hermetic FET" to get a few before they all are obsolete -> 5.5€ to 11€  :scared: are these hermetic packages hand made by grey bearded virgins. they are so expensive, cheapest cellphones are almost on that price range.  :(

How this JVR actually differs from the old constant current FET arrangement. The resistor is obviously hand-picked and low ppm variety? Also how this is different from 1N829A based solution, it is doing basically the same feeding a constant current through a selected component and voltage is used as reference. Is it for better aging (hypothetical) or noise or both?
 

Offline SilverSolder

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Re: Building your own voltage reference - the JVR
« Reply #251 on: February 24, 2021, 02:36:32 pm »
You can get a current regulating FET "diode", e.g. 1N5297 for 1mA.  They have quite a hefty temperature coefficient though, and I'm not sure about other parameters e.g. noise.

« Last Edit: February 24, 2021, 03:06:21 pm by SilverSolder »
 

Offline Vtile

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Re: Building your own voltage reference - the JVR
« Reply #252 on: February 24, 2021, 02:41:46 pm »

You can get a current regulating FET "diode", e.g. 1N5297 for 1mA.  They have quite a hefty temperature coefficient though, and I'm not sure about other parameters e.g. noise.
I did refer to this arrangement seen here and there for "ages", dubbed just as 'FET current limiter' etc. https://www.vishay.com/docs/70596/70596.pdf
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #253 on: February 24, 2021, 02:53:22 pm »
In the end the idea is using the nonlinear FET behaviour as a low noise alternative to a zener diode. Zener diodes are inherently more noisy, there is enough literature to explain about the avalanche process. Forum member noopy showed those nice images of light emission from the processes in a zener diode.
Of course a FET is much more of an artifact than for example the pn junctions of a bandgap reference. Diode forward voltage relies on physical properties that change very little, so normal silicon diodes all exhibit very similar forward voltage. Yet the near zero temperature drift of the FET reference is a big advantage and a good reason to look closer.

Regards, Dieter
 
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Offline Cerebus

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Re: Building your own voltage reference - the JVR
« Reply #254 on: February 24, 2021, 03:05:54 pm »
Of course a FET is much more of an artifact than for example the pn junctions of a bandgap reference.

There are two senses in which one can use the word "artifact" in English, one of which is always formulated "artifact of <something>"  and the other just means "made by man". Neither of them make sense in that sentence. Translation issues?

Quote from: dictionary
artefact | ˈɑːtɪfakt | (US artifact)
noun
1 an object made by a human being, typically one of cultural or historical interest: gold and silver artefacts.
2 something observed in a scientific investigation or experiment that is not naturally present but occurs as a result of the preparative or investigative procedure: the curvature of the surface is an artefact of the wide-angle view.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 
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Offline SilverSolder

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Re: Building your own voltage reference - the JVR
« Reply #255 on: February 24, 2021, 03:07:23 pm »

There are some 8.5 digit multimeters using 1N829A, so they must be "not bad".   They are not all the same, though.  I tested a few different ones for upgrading an EDC voltage standard - it took a few of them before I got one that behaved well in terms of noise.  The one I liked, that I bought in desperation, cost $15...   and came with a ton of military type documentation.  I'm pretty sure it must have been hand tested and selected.  It is a beautiful part, though, almost no noise or temperature coefficient at the right current.  It made a big different to the performance of the EDC!

Bottom line, I think you have to pay for the nicest "classic" analog parts - I'm beginning to think of them like fine wines, or something like that! :D
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #256 on: February 24, 2021, 03:23:57 pm »
@ cerebus:

In a pn junction the forward voltage depends mostly on the physical properties of silicon and the elements used as dopants. This comes close to what a metrologist really wants (not artifact but physical constants). In a JFET the pinch-off voltage depends on the geometry inside the device. That's the reason why FETs vary so much. This is less desirable as it leads to considerations about aging, drift of dopants and the like. This is what i meant.

Using a zener for a high resolution DVM means acquisition times like 20 to 50 seconds to get the noise down.The best zeners can do it within some seconds. There is research for better alternatives.

Regards, Dieter
« Last Edit: February 24, 2021, 03:32:31 pm by dietert1 »
 
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Offline Vtile

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Re: Building your own voltage reference - the JVR
« Reply #257 on: February 24, 2021, 03:32:58 pm »
Bottom line, I think you have to pay for the nicest "classic" analog parts - I'm beginning to think of them like fine wines, or something like that! :D
Yes you can say that something like OPA111 in a canned form with golden legs and teflon stand-off is almost an art itself (any TO-99).
 

Offline Cerebus

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Re: Building your own voltage reference - the JVR
« Reply #258 on: February 24, 2021, 04:18:08 pm »
@ cerebus:

In a pn junction the forward voltage depends mostly on the physical properties of silicon and the elements used as dopants. This comes close to what a metrologist really wants (not artifact but physical constants). In a JFET the pinch-off voltage depends on the geometry inside the device. That's the reason why FETs vary so much. This is less desirable as it leads to considerations about aging, drift of dopants and the like. This is what i meant.

Using a zener for a high resolution DVM means acquisition times like 20 to 50 seconds to get the noise down.The best zeners can do it within some seconds. There is research for better alternatives.

Regards, Dieter

Yeah, I know how it works, I just couldn't figure out what you were trying to say. All the devices you talk about are artefacts (man made), and you haven't described the artefact of the measurement process that is the other usage of "artefact", so I assumed you were grabbing an English word that is a "false cousin" of some German word that carried the meaning that you were trying to get across. Not meaning to be rude, but I suggest you avoid the word as your most recent usage of it still isn't idiomatic and, while I can figure out what you mean in your most recent use of the word, it still wasn't clear at first reading. A better usage in your original sentence would have been something like: "FET parameters are much more of an artifact of the semiconductor manufacturing process than for example the pn junctions of a bandgap reference". I hope that's clear now.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline dietert1

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Re: Building your own voltage reference - the JVR
« Reply #259 on: February 24, 2021, 04:25:14 pm »
What is your contribution besides being picky? This isn't an language course but a discussion of electronics devices.
 

Offline Vtile

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Re: Building your own voltage reference - the JVR
« Reply #260 on: February 24, 2021, 04:33:29 pm »
No fighting please, I was interested the differences as I'm not that deep on the electronics or physics behind the devices.  :-+
 

Online Kleinstein

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Re: Building your own voltage reference - the JVR
« Reply #261 on: February 24, 2021, 04:34:04 pm »
The TO18 parts got pretty expensive. In part this can be caused by RoHs: some of the older hermetic cases used lead solder and also the glas at the seals may have contained lead or similar. The is not that much demand for those metal cases any more.  It is not just the JFETs that got expensice: also normal 2N2222 got relative expensive.

The JFETs may react relatively sensitive to dopant diffusion, but so can zener diodes.  Like with a burried zener the relavant part of the JFET should be away from the surface, so there is a chance to to get away from surface effects, like defects at the oxide interface. Not so sure that drift is more of a problem with the JFETs than with a zener diode. At least classical planar zeners have additional surface effects and the choice of burried zeners is limited.

The higher frequency noise looks really good with the JFets, it is just the not so nice popcorn noise, that is a problem. In this respect they are somewhat similar to some of the zeners. An interestuing point could be that there are some modern fets with rather good low frequency noise - so in principle there would be a chance for really good noise performance.  Detailed noise specs for the FETs are rare and I would not take them to serious. Especially the popcorn noise may vary between units and modern processes got cleaner. Some JFET based OPs can provider good LF noise - so in principle it could be possible to find really low noise JFETs.

With the TC the JFET solution is a bit similar to the compensated zeners: one can use the current to adjust the TC. The 1N829 seem to be special in that they can also have a relatively low higher order TC, though at the price of a rather high current (7.5 mA). With selected samples and individually adjusted current one can get a low TC over a reasonable large range. Still with the now relatively high price this can also get pricy to select for both low noise and low higher order TC. With the relatively high power, an oven for the zener would also need relatively high power (e.g. something like 2 times the power).  The chinese 2DW232 may be powe noise, but also relatively high 2nd order TC and large scattering (e.g. more like 1N821). So this also needs some luck (a good batch), selection and likely an oven with relatively high power or a tricky compensation circuit.

I have not tested a 1N82x, so not sure about their typical noise level. The reports a scattering a lot.
 
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Offline Cerebus

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