PX Reference

[chuckb]

Just a quick check. Your graph is labeled as 17 uV (and the noise looks way to small). On another post the DVM in the background is displaying 17 mV. Should the graph be mV?
Thanks for all the posts and have a great new year!

[cellularmitosis]

Just a quick check. Your graph is labeled as 17 uV (and the noise looks way to small). On another post the DVM in the background is displaying 17 mV. Should the graph be mV?
Thanks for all the posts and have a great new year!

Yeah, it is 17mV.  If you zoom way in on that image, that’s a comma after the 17, i.e. 17500 uV.

Edit: I just realized it is a bit ambiguous!  If I were in Europe, a comma would have a different meaning.

[cellularmitosis]

When the two references (and the connections) settle down, for the 10-second period (or is that minutes?) between 33.20 and 33.30, you can see the Vpp is about 1uV-- which is phenomenal!  RMS noise is (about) 6X less than that.  Even after the X1.4 boost to 10V, it is still very low.

Now, build some high isolation power supplies for these, and let them "cook" in your lab for a few months (measure them once a week or so)-- and then you will see them settle down even more!

Congratulations!  What does Dave say?  "Winner, winner, chicken dinner!"

-Ken

Thanks!  And now it is time for this hobby to teach me the virtue of patience

[gamalot]

There is an example of using LTC2057 as a buffer in the LTC2508-32's datasheet.

[zhtoor]

well done sir! 

best wishes and regards.

-zia

[cellularmitosis]

40 hours of data, starting shortly after the last graph ended.

Looks like I have some sort of periodic spike I need to figure out.  Also, there is an almost sine-wave like behavior to the voltage, which shows more clearly on the 100-point rolling average.  Typically, I would assume this is from the air conditioning cycling on and off, but in this case I forgot to switch my A/C from cool to heat before leaving, so the temperature is currently not controlled (you can see it falling).  You can also see when I took a shower just before leaving for the airport (in the humidity graph).

[3roomlab]

100-point rolling average.

should it not be repeating average? as in each 100 point data averages to 1 output, and none are re-used in next count? as opposed to next output using 99 old data point and 1 fresh?

[mimmus78]

40 hours of data, starting shortly after the last graph ended.

Looks like I have some sort of periodic spike I need to figure out.  Also, there is an almost sine-wave like behavior to the voltage, which shows more clearly on the 100-point rolling average.  Typically, I would assume this is from the air conditioning cycling on and off, but in this case I forgot to switch my A/C from cool to heat before leaving, so the temperature is currently not controlled (you can see it falling).  You can also see when I took a shower just before leaving for the airport (in the humidity graph).

Looks like you found another little jumper ... fortunately is not inside a 3458a this time. I still have to decide when make transplant of mine beating inside my 3458a.

[Kleinstein]

There is still quite some "noise" or EMI crap present. So it is hard to tell it the one jump seen at about the middle of the curve is a jumpy LTZ or if this is just from the not so good setup.

Connecting the com side of the DMM to one reference adds quite some load to it and might be a path to couple EMI. So at least that one reference should have a buffer.

[mimmus78]

There is still quite some "noise" or EMI crap present. So it is hard to tell it the one jump seen at about the middle of the curve is a jumpy LTZ or if this is just from the not so good setup.
Connecting the com side of the DMM to one reference adds quite some load to it and might be a path to couple EMI. So at least that one reference should have a buffer.

Yes the one big peak in the middle of the chart can be from any source ... but the 1uV periodic look like my jumpy reference inside 3458a. The side by side chart few posts ago look like many of my charts.

I think cellularmitosis need a third one reference ... or better a fluke 732a (noo just kidding) to check what happening.

[Kleinstein]

The more periodic part looks odd - kind of going up and down, more like a triangle and with a spike at the top. I would not be surprised if this is some EMI crap. The low frequency could be something like the line frequency off from the nominal value / sampling rate of the DMM.

[hwj-d]

Last but not least check it with batteries.

[cellularmitosis]

Back in town, and the behavior looks similar over the past 7 days, but with a few even larger spikes.

(currently working on writing a x-y scatter plot script to get a look at the tempco)

[cellularmitosis]

I also just threw the two LTZ references on 3S lipo batteries for an hour.  I no longer see the "spikes", but I now see a couple of bands of 20uV noise.  Hmm...  I will definitely be trying this again.

[cellularmitosis]

The more periodic part looks odd - kind of going up and down, more like a triangle and with a spike at the top. I would not be surprised if this is some EMI crap. The low frequency could be something like the line frequency off from the nominal value / sampling rate of the DMM.

Yeah, I took a closer look at this and it appears the smaller spikes are almost exactly 30 minutes apart.  My current guess as to the cause would be my refrigerator.  The triangle shape is definitely a mystery -- I'll take a longer battery-powered measurement and see if that still shows up.

[hwj-d]

As you will buy fresh 18650's, beware of protected cells. These could be also very noisy, of course.

[cellularmitosis]

Ok, another round with 3S lipo batteries (no regulator, with the negative terminals of the lipo's bridged with a thick wire).  4.5 hours of logging and most of the data is within a 5uV band -- not bad!

I do still see some small spikes on a 30 minute interval, so it appears those must be some sort of EMI (I also realized I forgot to bodge in C15 on the second board -- perhaps that will help?).

I think next I'll have to try altering the temperature of just one of the refs and see if I can get a good look at tempco.

Edit: I waited 10 minutes before starting the logging, to exclude the "settling" from the log.

[cellularmitosis]

I've created a "b" variant of the board which uses two discrete resistors for R4/R5, rather than a divider.  This variant of the board is intended for use with "hifi's" vishay resistors from ebay.

The R4/R5 resistor footprints are adjacent so that they can be thermally bonded.

https://oshpark.com/shared_projects/S1rjgxed

https://github.com/pepaslabs/px-ref/tree/master/kicad/releases/v2.2b

edit: hifi's resistors: http://stores.ebay.com/HIFI-AUDIO-IC/Metal-Foil-Resistors-/_i.html?_fsub=2634142012 and my daily updated indexes of his resistors: https://ssl.pepas.com/hifi/

[hwj-d]

Build two PX-References these days. I used V2.2 boards with ltz1000a and hifi resistors, they are RNC90Y types of 13k, 1k, 2*75k, 120R. For the R4/R5 divider, simply but careful i combined them together with shrinking tube and a little thermal grease between, and bend solder feets together without stress them to much at the place they go into the resistor.

One Reference has ltz1000a 9636 from hifii. As i can see, it is very stable after the first burn-in about 60h, although the can looks a little bit damaged and has no gilded pins, but luckily they are long enough. For this one, i have a Hammond 1590B, but not drilled holes for the Pomona binding-posts in the moment. (edit: in the moment no pictures from this one)

For the other one with new ltz date 1750, i used a TEKO372 steel sheet housing from Reichelt TEKO-372. Normaly this housing, dimension 83 x 50 x 26 mm, is too little for binding posts at two sides and this little PX-PCB with components, but it would be nice if it worked anyway. Without binding posts, it even fits inside the Hammond 1590B for better steel sheet shielding and nevertheless reasonably airtightness, as we can see on one of the pictures.

Because the PX-pcb doesn't really fit inside the TEKO-372, i cut 2 mm at a suitable edge of the pcb, carefully without damaging any traces.
Sorry cellular, now your github url isn't visible anymore...   
But the result is awesome!

Underneath the pcb i put one of this Digi-Key typical silicat-sacs, let's see if that helps a bit, and above one cotton pad before i closed the lid. I haven't had such little, precise, and really low-noise reference before.
More will follow...

Thanks cellular. 

[hwj-d]

Some more Pictures









Edit: brings that pictures to front

[cellularmitosis]

Very cool!  Thank you for the pictures and the report!

Thanks also for providing a link to that case.  Dr Frank has used a similar one, but nothing came up when I googled for “tuner box”.  I like that the steel will give you a bit of magnetic shielding.

Hmm, perhaps v2.3 will have to be a few mm shorter

[hwj-d]

Those chinese binding posts are really crap! I don't want to use them anymore. So, i need an idea, what to use without instead, that uses minimal space inside this box. The inner dimenson left to right was in reality 47mm, the material thickness is .75mm.

[hwj-d]

Another idea is, maybe to make it a little elongated, to have space for four another resistors and maybe one poti, to use the buffer 2057 also to bring the reference to 10V like showing here: Spread sheet aided design of compensation for 7V to 10V step up resistor set?
But now I am getting rude...   

[cellularmitosis]

I've been resisting the temptation to make a 7-10V circuit, because it is so much easier to make a stable 7V circuit, but then I realized that I can only compare it to a 3458A, and not to e.g. a 732B...  Adding 7-10V is probably inevitable at this point 

I have a pile of those banana jacks as well.  They are convenient as a cheap through-board banana option (and the can be made to work for manhattan-style board as well).  They are OK for non-precision stuff.

Edit: also, thanks so much for the link to that tempco compensation post, I hadn’t seen that before!  The idea of winding copper wire around an existing resistor is very clever.

[hwj-d]

That's blog synergies, we all learn from one another. I wish, that would be so everywhere in the world.