PX Reference

[Andreas]

is it better to use, maybe 4s 18650 unregulated (shurely observing the discharge level) as to use this variable heatsource regulation down to 11V? In my subjektive practise, I see no negative psrr impact with unregulated 4s over the whole entire discharge voltage range. But maybe I miss something?

It all depends also on your overall thermal design:

Think of the heater transistor and how he will behave with variable power supply.

In the new 34470a design the heater transistor is non SMD outside the LTZ-cap
(most probably within the air draft of the fan of the instrument).

My personal strategy is:
keep the power dissipation as constant as possible (at constant temperature).
And thermally isolate any variable heat sources.

Your strategy may differ.
At the end only the final result counts.

with best regards

Andreas

[cellularmitosis]

Hello,

I have designed a small LT1763 board.

OSHPark: https://oshpark.com/shared_projects/uVr4yDgz

Github: https://github.com/pepaslabs/lt1763

[cellularmitosis]

The board has provisions for an RLC input filter.  Here's my estimate of performance for a few values, combined with the LT1763's own ripple rejection.

I used this web tool: http://sim.okawa-denshi.jp/en/RLCtool.php

Note that a 10R resistor will add 300mV of dropout when passing 30mA.

[hwj-d]

Hi cellular,

Thanks again, for your nice work.

Seems to me, that 10V isn't that stable as with 12V (- shottky voltage drop). My measurements at the last days are with min 12V, 4s 18650, to prevent discharge limit under 3V per cell.

After your post, I set the voltage to 10V. The buffered voltage is a bit erratic after this. Will see, whether the voltage is still stabilizing the next few hours. Maybe 9.7V is is something at the lower limit.

First half in the picture is 12V with slightly ambient temperature rise of 0.4 °C, second half is 10V (- shottky voltage drop).

I would prefer 12V (€: 11.5V), that's 3k9 for R3, if you allow. 

[cellularmitosis]

Thanks so much, hwj-d!

Perhaps I should have performed some tests before ordering the LT1763 boards

[Andreas]

Here's my estimate of performance for a few values, combined with the LT1763's own ripple rejection.

Hello,

The values above 1 MHz in the bode diagram for the filter dampening are somewhat optimistic.
The self resonant frequency of a 1uF foil capacitor is in the region of 1 MHz so above this frequency the capacitor gets inductive and ripple rejection will be less than estimated.
Also the self resonant frequency of your inductors is somewhere in the 2-5MHz range. Above this frequency the inductor is shorted by the winding capacitance. Reducing further the ripple rejection.

with best regards

Andreas

[cellularmitosis]

Here is an overnight (~8 hour) run to test the supply voltage coefficient of the LTZ circuit.

I measured LTZ #3 vs #4 (back-to-back), powering one from a lipo into an LM7815, and the other a lipo into a programmable LM317, controlled by a laptop.  (see https://github.com/pepaslabs/ProgrammableLM317 ).

I ramped the LM317 from 15V to 9V (up and down) in 0.1V steps, with 2 minutes per step.

It looks like the output swung by about 11 microvolts over a 6V supply change, so supply voltage coefficient is about -0.25ppm/V.

Edit: github link: https://github.com/cellularmitosis/logs/tree/master/20180225-ltz-voltco

Edit 2: Looks like Andreas board has better thermal management https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg875274/#msg875274

Without slot on LTZ#3 the voltage output change was around 5uV / 4.5V (0.15ppm/V) as regression curve.
With slot the change is reduced to 2.9uV / 4.5V (0.09ppm/V).

On LTZ#5 I have 2.1uV/4.5V (0.06 ppm/V) with the slot.

[hwj-d]

Thanks again, to figure that out so precisely.

My observations, that a voltage coefficient is not recognizable for me, referred to directly connect 4s 18650 3400mAh NCR18650B (measured capacity 3200mAh) to the reference. So it tooks about 5 days, to go from 16.5V down to 13V - 12V, (not 9V as a difference), before i took them out to replace them with a freshly loaded set. With PX, I did it for 5 weeks. Before that some weeks with KX. With it i have more trouble with not so constant results, maybe because this is not an LTZ-A type, don't know at the moment.

But with both i didn't see any impact, that I could attribute to the declining voltage. In particular the buffered px shows a little tempco in the expected frame and meanwhile about 4µV voltagedrop, but can't and couldn't see this 11µV difference, that correlates to this 6V supply change.

I didn't made this precise measurements. So i think, the interpretation error lies with me. Maybe some tempdifferences (measured between 21 - 22°C) overlapped, or my PX react differently, don't know. But I believe you, there must be any supply voltage coefficient.

The time brings it to me.

Thanks for your effort.

[TiN]

My observations, that a voltage coefficient is not recognizable for me

Yes, I agree with this one, I was not able to see any correlation to Vout/Supply voltage, given that there are no ground loops in measurement system!
I was trying range 10.2V to 18V, nada, below measurement noise/temperature coefficient on both KX, HP A9 and FX designs.

[Andreas]

Hello,

if not already done:
I would try to do a better thermal isolation around the legs of the LTZ1000
either some micro fiber cloth on both sides of the PCB or some "cotton pads" (the micro fiber version).

with best regards

Andreas

[Andreas]

Yes, I agree with this one, I was not able to see any correlation to Vout/Supply voltage, given that there are no ground loops in measurement system!

I cannot agree with that. I have battery supply on the one side
and kelvin sensed output directly from the zener.
So there is no ground loop in the system.
Despite of that I have a voltage coefficient (individually for every device).

Perhaps in your case the multilayer helps keeping the feets at the same temperature.

with best regards

Andreas

[cellularmitosis]

I can try again tonight by inserting a serial-TTL opto-isolator inbetween the laptop and the programmable LM317.  Currently, this laptop connection is causing the LTZ's to sit on ground, rather than float on batteries.

[TiN]

HP A9 board does not have multilayer design, so PSRR has barely anything to do with thermal equilibrium theory.

[TiN]

I agree, however miracles or not, maybe I'm just lucky to have A9 (STD) that does not show PSU voltage impact (beyond measurement ability of quad 8.5d dmms).
My refs in both 3458s are good too, but they are pimped with 100k for lower temp SV.

I got 6581T, so now I can test its ref module too (which is more "proper" traditional design).

[MisterDiodes]

Digilent:  You should inform the 45ea. 3458a's running in one room that I just walked out of that they have drifty references.  Some old, some new, all of which are well under 8ppm yr drift, and around 18 standard units that are running to HFL spec. in terms of yearly drift.  Oh...those are some of the oldest units too.

1 unit is on the sideline waiting for a new VFD, and one unit went to sick bay for a new power supply cap, and one unit has a squeaky fan.  1 unit had Vref board replaced a year ago but has been fine since.  That's a typical sort of repair list for year or two.

That IS a shining example of making the product work well.  Out of all the equipment in the building, the 3458a's have a very good profit margin - and Vref failures are not too common, but happen now and then.

Again:  None of those units happened by "miracle", and no units need to have Vref pimped, etc.  All running within spec.

[cellularmitosis]

Just to make sure my understanding is correct, if you increase the supply voltage, the biggest difference in circuit behavior is that the 2N3904 burns more power in order to keep the same current going through the heater, is that correct?  This is why Andreas saw an improvement with adding a slot to his PCB?

Perhaps a very small secondary effect is that the LT1013 burns a tiny amount of additional quiescent power?

Edit: Also, I'm not concerned with attempting to correct for the supply voltage coefficient -- I'll soon be including LT1763's with these refs, so the supply voltage won't be varying.

[Andreas]

Hello,

good that you mention it: of course you are right.
on my PCB the LT1763 is the only variable source of heat since it stabilizes all to 14.0 V
On a standard cirquit you have the 2N3904 and the current output of the LT1013 as main variable heat sources.

By the way: in my "heat gun" experiment I got the impression that variable heat sources which are in the "south" of the LTZ (below pin 8 ) on my board have lower influence than the other directions.

https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg861416/#msg861416

with best regards

Andreas

[MisterDiodes]

Cellular:
We get better results using TO-220 (or similar) pak heater switch transistor...  Get that warmer away from the board and into it's own private airspace, then supply voltage changes should have a smaller effect - The whole system will come to a quieter thermal equilibrium.  Otherwise the transistor and LTZ heater can interact. 

You can even play with a heat sink on the TO-220 although usually not needed.

TO-92's tend to drive heat down into the board.  SMD heater switches can cause issues for sure depending on where they are located relative to LTZ.

Everything around an LTZ circuit is treated as a thermal "system", and the quieter that is, the more stable your output.

[cellularmitosis]

Hmm, well after introducing the opto-isolator between the laptop and the programmable LM317, I don't see such an obvious pattern.

I also see a ~70uV offset from the previous run.

Perhaps I did have some sort of ground-loop.

I also changed up the parameters a bit: the ramp was from 18.0V to 9.0V in 0.1V steps every 60 seconds.  The run was ~8 hours (overnight).

I assume the large noise at the very start of these graphs is either me leaving the room, or the display of the laptop shutting off.

csv, etc: https://github.com/cellularmitosis/logs/tree/master/20180225-ltz-voltco/run4-1519715191

[hwj-d]

Much better...

Your PX is better, as you thought 

Moment, 17500 to 17520 -> 20µV?

[cellularmitosis]

Moment, 17500 to 17520 -> 20µV?

yeah, it was a pretty noisy run, especially compared to previous battery-powered runs.

[hwj-d]

In the moment I've shot some fotos from quick manual going through some Voltages with my 832 PS from 18V down to 10V and up again, that are all in the 5µV area. Wait a moment for posting...





That's going from 12V up to 16V down to 10V again up to 12V and then to 18V and back to 12V. All with approximately 5 min settletime and negative coefficient.

[hwj-d]

My second PX shows in about the same characteristic, but is in the 3µV Area. Probably the reason why I haven't noticed the voltage influence as such one.

The voltagedrop, that destroys the statistic, is from setting the voltage to 9V. Minus the schottky drop that is too little. So, i set it to 9,6V as a minimum after that.

[cellularmitosis]

I forgot to mention that these tests were with the reverse-protection diode bypassed (as well as the heater zener).  I did a test run to see when the LTZ circuit dropped out of regulation, and it was something like 8.9V if I recall.

[cellularmitosis]

One more run, while I was at work today.

This time I used a "sleep 120" command before starting the logs, which gave me two minutes to leave the room and for the laptop's display to sleep.  This cleared up the leading noise at the beginning of the charts.

You can see when the aircon kicked on towards the afternoon.