Best out-of-the-box 10V reference?

[Andreas]

Hello Lars,

of course you are right according to the SVR-T cirquit.
If you can choose the NTC value freely and adjust it to the T.C. compensation resistor (Trimpot) then you can set it accurately to the 80% rule.

I have not calculated the final values for R3. But it is shurely in the 1 Meg range.
So the 27K is already the 80% of the 33K with typical 0.3-0.4 deg C linearity in the 10-40 deg C range.

But after your 2nd explanation your current cirquit has much more charme.
I overlooked that you can trim +/- TC with one and the same cirquit. Chapeau!! 
So the current cirquit is more practical.

What are the typical values for R1 and R2?
I think you have to trim R1/R2 in a way that the output voltage without trimpots is slightly below 10V.

With best regards

Andreas

[lars]

What are the typical values for R1 and R2?
I think you have to trim R1/R2 in a way that the output voltage without trimpots is slightly below 10V.

R1 changes the output about -1700ppm/(R1 in Mohm + 0.13Mohm) for a typical AD587
R2 changes the output about +4200ppm/(R2 in Mohm + 0.13Mohm) for a typical AD587

So I should be prepared to have resistors in the E12 series from say 1.5M to 10M and maybe 15M and 22M. Of course you can both parallell and series connect resistors if you are missing a value.
I have used cheap thick film resistors like RC0805 or RC1206 (Digikey). They are not very stable but if the resistor moves the output less than 1000ppm, a 1000ppm change on the resistor will be less than 1ppm on the output.

Before you select R1 or R2 you have to adjust the trimpot for the TC as that moves the output by about 50ppm per ppm/C you adjust.
The trim range for the offset pot is about +-130ppm.

Lars

[pitagoras]

Adding a 7815 on the input can reduce the line regulation issues almost entirely.

Or bootstrap the reference using an operational amplifier and single transistor if necessary.

Bootstrap is something like this?

[tszaboo]

Adding a 7815 on the input can reduce the line regulation issues almost entirely.

Or bootstrap the reference using an operational amplifier and single transistor if necessary.

Bootstrap is something like this?

Though I have no idea why this would be better than a 7815 (or low noise equivalent). A good 7815 has a maximum 20mV line regulation, and the AD587 has some 100uV/V. This together would result a maximum 2 ppm error, if you change the input voltage from 18V to 30V. Typical results will be better, and why would you fiddle with the input voltage this much?

[pitagoras]

Adding a 7815 on the input can reduce the line regulation issues almost entirely.

Or bootstrap the reference using an operational amplifier and single transistor if necessary.

Bootstrap is something like this?

Though I have no idea why this would be better than a 7815 (or low noise equivalent). A good 7815 has a maximum 20mV line regulation, and the AD587 has some 100uV/V. This together would result a maximum 2 ppm error, if you change the input voltage from 18V to 30V. Typical results will be better, and why would you fiddle with the input voltage this much?

As it has been suggested by David Hess as an alternative I was motivated to understand the statement and posted the circuit I came up with.
Now... yes, the question is: why it is better (if it is).

[David Hess]

As NANDBlog points out, it is overkill in many applications.  On the other hand it is simple, lost cost, and high performance.  It is more commonly done with shunt references to control the input current but it can be applied to series references as needed.

And a 7815 has more like 150 millivolts of line regulation over temperature and input voltage.  Load regulation adds another 150 millivolts to that.  If you use a premium regulator, then it will cost more than a cascode made from a separate operational amplifier and transistor.

[pitagoras]

OK this is the circuit I'll be building.
trying to agree with everyone, the board will contemplate soldering both 7815 or (exclusive or!) bootstrap circuit.
It contemplates the NTC circuitry as per Lars suggestion.

[Andreas]

Hello,

you will need the test points from Lars cirquit for the adjustment. (at least they would help)
R12 = 1 Meg ?

What happens to the boot strap cirquit in case of a short on the output of the reference?

Bonus question: will the boot strap cirquit ever start up (depending on offset of the OP)?

with best regards

Andreas

[pitagoras]

Hello,

you will need the test points from Lars cirquit for the adjustment. (at least they would help)
R12 = 1 Meg ?

What happens to the boot strap cirquit in case of a short on the output of the reference?

Bonus question: will the boot strap cirquit ever start up (depending on offset of the OP)?

with best regards

Andreas

OK I will add the TPs, thanks!

Does the bootstrap even start?
Well I *think* that even if opamp output is 0V, Q1 is biased and collector is around 1/4 Vsupply. Might be this is not enough for the Ad587 to start?

[tszaboo]

As NANDBlog points out, it is overkill in many applications.  On the other hand it is simple, lost cost, and high performance.  It is more commonly done with shunt references to control the input current but it can be applied to series references as needed.

And a 7815 has more like 150 millivolts of line regulation over temperature and input voltage.  Load regulation adds another 150 millivolts to that.  If you use a premium regulator, then it will cost more than a cascode made from a separate operational amplifier and transistor.

Though, load regulation is insignificant, unless you are planning to go up up to an 1A load on your reference. Anyway, I just like the LM78Nxx parts, they come in nice SO8 packages, can source 100mA, work with ceramic caps and behave as you expect them to behave. Not a lot of power.
I think we already spent too much time on this.

[lars]

As I said above I like the LP2951 with a 40V 1A schottky diode in series.
Some reasons:
-   Around 1USD at Digikey for both. Both available in through hole and surface mount.
-   Can handle +-30V inputs.
-   Voltage drop less than 0.5V for both in series. For the AD587 I set the output of the LP2951 to around 14V so reasonable good margin for 15V supplies. For the LT1021 it can probably be set for an even lower voltage.
-   Only about 0.1mA current extra with a 2mA load gives longer battery life time and less heating. 78 series regulators normally have many milliamperes of quiescent current
-   Line regulation and stability together with AD587 and LT1021 are more than enough. Using two 9V batteries will not be a problem.
-   Noise from the regulator in the 0.1-10Hz region is not a problem as far as I can see. (So I see no need for a low noise regulator)

Lars

[lars]

As Andreas says 1Mohm in series with the two 680k NTC’s gives more linear TC (temperature coefficient) compensation than 1.5Mohm for the AD587. For small TC (< +-2ppm/C) it doesn´t matter a lot and for positive TC it is a benefit to have the slight nonlinearity you get with the 1.5Mohm. For negative TC, I should select 1Mohm or even less if I want to compensate some of the nonlinear TC of the AD587. I have had many AD587 with second order TC of about -0.02 to -0.05ppm/(C*C) but Andreas seem to have more like -0.01ppm/(C*C). But as has been stated before the non-linearity of the compensation probably will be less than other errors.

If you use 2x680kNTC +1.5Mohm the change on the wiper (TP) will change the TC about 4ppm/C per Volt. With 2x680k+1M it will be slightly above 5ppm/C per Volt instead.

Lars

[pitagoras]

Ok just a quick update.
I finally decided to follow Lars about the LP2951.
I attach the schematic and the PCB...


PS: Do you think I should share the project (kicad) on github? Do I have the agreement of the "owners"? With due credit to this thread of course.

[Gyro]

While you're going to the effort, wouldn't you be better using an AD587JQ CERDIP part? Using an SO8 package seems like too much of a compromise in humidity sensitivity and long term stability.


P.S. I missed your post earlier in the thread about already having some AD587KR, but even so...  Maybe you could dual footprint?

[pitagoras]

 

While you're going to the effort, wouldn't you be better using an AD587JQ CERDIP part? Using an SO8 package seems like too much of a compromise in humidity sensitivity and long term stability.


P.S. I missed your post earlier in the thread about already having some AD587KR, but even so...  Maybe you could dual footprint?

Yes, now that you mention that... you're right.  I could also make the PCB for CERDIP, avoid the milling around  the 587, an mount the SOIC as Kleinstein suggested (with wires).

[Muxr]

As I said above I like the LP2951 with a 40V 1A schottky diode in series.

I like the BAT86 Schottkys (easy name to remember).

[David Hess]

I attach the schematic and the PCB...

Wouldn't it be better to have the reference and sensitive parts on a little peninsula even if that was in the middle of the board with a U "moat" routed around it instead of those opposed corner attachments?  Your layout reminds me of a shear beam load cell.

[Muxr]

Thanks to the great advice in this thread I designed a reference board. Since I live in the tropics I wanted something to have fairly good moisture and humidity resistance. I used Conrad Hofman's MML ltc1027 reference as a basis with some additional protection and the temp sensor, also the LTC1027 noise reduction mylar cap. Decided to skip the NTC temp compensation for now, until I can characterise it and see how it goes.

All critical components are hardened against humidity, ltc1027 LS8 package for instance, capacitors and resistors are moisture resistant. The only thing that isn't is the trimpot, well and non critical power regulation components. I kept the costs fairly low as well. Most expensive being a few wirewound resistors and the reference itself. Haven't done the math on it yet but I think in total around $50 worth of components (I usually order in multiples).



The weird J connectors on the edges are for the copper bridges for easy hookup and measurement with grabber clips. That weird orange cap it's just an aluminum electrolytic.. couldn't find the right footprint for some reason, but the holes match. Done in KiCad.

Anyways thanks to: Conrad Hoffman, pitagoras, lars, Kleinstein, David Hess, Andreas and others I forget to mention in this thread!

[Andreas]

All critical components are hardened against humidity, ltc1027 LS8 package for instance,

Hello,

you forget that the PCB itself is not hardened against humidity.
So this will have some influence.
see also comparison of a soldered and a dead bug style mounted LT1027 in LS8 package.

mounting:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1003827/#msg1003827

resulting hysteresis:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1011371/#msg1011371

with best regards

Andreas

[Muxr]

Hello,

you forget that the PCB itself is not hardened against humidity.
So this will have some influence.
see also comparison of a soldered and a dead bug style mounted LT1027 in LS8 package.

mounting:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1003827/#msg1003827

resulting hysteresis:
https://www.eevblog.com/forum/metrology/t-c-hysteresis-measurements-on-brand-new-lt1027dcls8-5-voltage-reference/msg1011371/#msg1011371

with best regards

Andreas

Hi Andreas, thank you! I did in fact think about doing the dead bug style. However I wanted to have a baseline for comparison before I started doing anything extra. So that I can compare say, NTC compensation and maybe dead bug style.

Have you tried conformal coating in your experiments by any chance? Something like this for instance: https://www.amazon.com/MG-Chemicals-Silicone-Conformal-Coating/dp/B004SPJOKK

[pitagoras]

Thanks Muxr for sharing.
I see you used a grounding technique different as mine; and I was not sure in my case if having a layer with the full ground was a good idea.
Can you comment in your strategy?

[Muxr]

Thanks Muxr for sharing.
I see you used a grounding technique different as mine; and I was not sure in my case if having a layer with the full ground was a good idea.
Can you comment in your strategy?

Sure, I only used the ground plane on the linear regulator side as you can see (both top and bottom layers are the same in this regard). This is to help dissipate the regulator heat. It's only present on that section of the PCB.. the rest of the PCB has no grounding plane. I even went as far to make a cut out right next to the regulator to force the heat to the top and bottom sides of the board away from the critical reference components.

I don't know if any of this will matter in practice, but I wanted to minimise the influence of different voltage (higher voltage more heat generated by the regulator) being supplied to the reference board.

[Andreas]

I don't know if any of this will matter in practice, but I wanted to minimise the influence of different voltage (higher voltage more heat generated by the regulator) being supplied to the reference board.

Hello,

this will matter.
Even on my LTZ1000 board you can measure the influence of the voltage regulator.
A slot improved this by a factor of 2.
So in the re-design I will even put the voltage regulator on the other side of the battery pack so that the distance between regulator and reference increases from 3 to about 8 cm.

Have you tried conformal coating in your experiments by any chance? Something like this for instance:

No I did not try this. Silicones are not vapor tight.
I saw photos with water between PCB and silicone after damp tests.
So the maximum what you can expect from conformal coating is a increased time constant for humidity influences.

with best regards

Andreas

[Muxr]

Finished my LTZ1027 reference (inspired by Conrad Hoffman's MML reference and this thread). For the first go around I just used some decent thin film 10ppm resistors (non matched).

One of the trimmer resistors was not the right value so I just bodged it for now with 3x 50K resistors wired in parallel (of 15ppm variety which I have tons of for now). I didn't even solder them on, they are just plugged into the PCB for now, until I get the right value resistor.



Aside from the above mentioned caveats, the aging process begins. I am actually quite pleased with what I am seeing so far (although probably too early to tell due to all the hysterisis going on):

[Andreas]

Hello,

Mhm, there should be also a 5 V output to distinguish ageing of the reference and ageing of the rest of the cirquit.
I guess that that drift what you see is not the ageing of the reference.

With best regards

Andreas