Author Topic: Ultra Precision Reference LTZ1000  (Read 1333352 times)

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Offline cellularmitosis

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Re: Ultra Precision Reference LTZ1000
« Reply #2050 on: December 16, 2017, 11:39:58 am »
I recreated Dr. Frank's board layout (https://www.eevblog.com/forum/metrology/mx-reference/msg1297126/#msg1297126) in kicad and published the design on OSHPark, so that others could order a copy of the board if they want recreate his setup.  (Dr. Frank, I hope you don't mind!)

I imported the jpg of his board into kicad and tried to match it as closely as possible.  I hope I got it right!

The board dimensions are just shy of 80x50mm.

Modifications:

- The precision resistor footprints include 0.15" (Vishay) and 0.2" (AE) lead spacing.

Edit: OSHPark order link: https://oshpark.com/shared_projects/aF98MB4w

Edit2: github link: https://github.com/pepaslabs/dr-frank-ltz1000

Edit3: Hmm, Frank's design actually has the copper on the back-side.  oops...

Edit4: Moved the copper traces to the back side, and used copper traces for the two jumper links.  New OSHPark link: https://oshpark.com/shared_projects/RFzngdpy

Edit5: Omitted D3 (thanks Frank!), corrected value of R5 (thanks CalMachine!).  New OSHPark link: https://oshpark.com/shared_projects/gl2aXmlh

Edit6: Just a reminder: BC639 has a different footprint (emitter-collector-base) than 2N3904 (emitter-base-collector).

Edit7: Correcting the silkscreen to show LTC1052 rather than MAX420.  Also using larger pads for the TO-99 footprint.  OSHPark: https://oshpark.com/shared_projects/dGYcQAgn
« Last Edit: April 30, 2018, 02:22:43 am by cellularmitosis »
LTZs: KX FX MX CX PX Frank A9 QX
 
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Offline dr.diesel

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Re: Ultra Precision Reference LTZ1000
« Reply #2051 on: December 16, 2017, 12:20:32 pm »
Humidity should not be a problem so close to the LTZ chip, as the temperature is higher than the normal air temperature. It is more that the high temperature could be a problem for the foam, accelerating aging. Quite a lot of foam foam from the 1960's does not look that good today and at something like 60 C aging could be 10 times faster than normal.

I was thinking in more of an on/off situation like many's 3458A, but you're right on kj7e's case with it hot 24-7.

But I think the foam aging is a better point, as it ages it will slowly drop strength/push/compression against the LTZ can.

Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #2052 on: December 16, 2017, 01:49:00 pm »
I recreated Dr. Frank's board layout (https://www.eevblog.com/forum/metrology/mx-reference/msg1297126/#msg1297126) in kicad and published the design on OSHPark, so that others could order a copy of the board if they want recreate his setup.  (Dr. Frank, I hope you don't mind!)

I imported the jpg of his board into kicad and tried to match it as closely as possible.  I hope I got it right!

The board dimensions are just shy of 80x50mm.

Modifications:

- The precision resistor footprints include 0.15" (Vishay) and 0.2" (AE) lead spacing.

edit: OSHPark order link: https://oshpark.com/shared_projects/aF98MB4w

edit2: github link: https://github.com/pepaslabs/dr-frank-ltz1000

edit3: hmm, Frank's design actually has the copper on the back-side.  oops...

edit4: moved the copper traces to the back side, and used copper traces for the two jumper links.  new OSHPark link: https://oshpark.com/shared_projects/RFzngdpy

Really cute!
If you like to improve the circuit  and simplify the PCB a bit, then you might skip D3, as it's not necessary to have two diodes in series.
That might even disturb the T.C. compensation, so I did not use that either.

Frank
 
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Offline kj7e

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Re: Ultra Precision Reference LTZ1000
« Reply #2053 on: December 16, 2017, 03:02:30 pm »
Humidity should not be a problem so close to the LTZ chip, as the temperature is higher than the normal air temperature. It is more that the high temperature could be a problem for the foam, accelerating aging. Quite a lot of foam foam from the 1960's does not look that good today and at something like 60 C aging could be 10 times faster than normal.

I was thinking in more of an on/off situation like many's 3458A, but you're right on kj7e's case with it hot 24-7.

But I think the foam aging is a better point, as it ages it will slowly drop strength/push/compression against the LTZ can.

Yeah, I'm going to radius the inside edge so there is minimal contact with the can itself.  I'm finding out (as I'm sure may of the old players already know) simple seems to work the best.
 

Offline cellularmitosis

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Re: Ultra Precision Reference LTZ1000
« Reply #2054 on: December 16, 2017, 05:50:19 pm »
If you like to improve the circuit  and simplify the PCB a bit, then you might skip D3, as it's not necessary to have two diodes in series.
That might even disturb the T.C. compensation, so I did not use that either.

Thanks!  I've removed D3.

Also thanks to CalMachine for pointing out that both R4 and R5 were labeled 12k!
LTZs: KX FX MX CX PX Frank A9 QX
 

Offline cellularmitosis

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Re: Ultra Precision Reference LTZ1000
« Reply #2055 on: December 17, 2017, 09:51:42 am »
I decided to round out this bunch of boards with a clone of the A9 reference board used in the HP 3458A.

I reproduced this just from some of TiN's hi-res photos, so some of the dimensions might not line up perfectly (I'm pretty confident about the placement of the components (which are on grid), but I'm unsure of the exact diameter and position of the mounting holes.  If TiN could weigh in with some official dimensions, I'll update the design). -- This has been fixed in v2 of the board.

A huge thanks to TiN for the incredible depth of information he posts to xdevs.com!

Modifications:

HP included some footprints for non-Vishay versions of the 15k, 1k, and 111R resistors, but they're never used.  I eliminated those to allow a bit more room to accommodate hermetically sealed versions of these resistors, as well as a footprint for a Vishay divider (e.g. VHD200).

This board is slightly shorter than the official board, which allows it to fit within a 50x50mm area (the cheapest pricing tier for many PCB manufacturers).

OSHPark link: https://oshpark.com/shared_projects/fXEO68Tb

github link: https://github.com/pepaslabs/hp-03458-66509-clone

edit: the through-board connectors are Molex 22-17-2052: https://www.mouser.com/ProductDetail/Molex/22-17-2052/?qs=sGAEpiMZZMs%252bGHln7q6pm%252bS0pk2Wo0XxllCQVBT5EJg%3D

edit2: v2 of the board has various alignment fixes and hermetic options for R413 and R415.  OSHPark link: https://oshpark.com/shared_projects/jP5xkfJy
« Last Edit: December 18, 2017, 06:15:03 am by cellularmitosis »
LTZs: KX FX MX CX PX Frank A9 QX
 
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Offline cellularmitosis

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Re: Ultra Precision Reference LTZ1000
« Reply #2056 on: December 17, 2017, 09:53:36 am »
For comparison, here are some of TiN's reference photos of the A9 board.

edit: Thanks again to TiN for publishing these!  See https://xdevs.com/article/volt_xfer/ and https://xdevs.com/fix/hp3458a/#a9pcba
« Last Edit: December 18, 2017, 06:22:56 am by cellularmitosis »
LTZs: KX FX MX CX PX Frank A9 QX
 

Offline TiN

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Re: Ultra Precision Reference LTZ1000
« Reply #2057 on: December 17, 2017, 02:36:44 pm »
Keeping links to related articles would be nice too.  :P

I think will be much faster for someone else who has A9 to take measurements for mount hole dimensions, as I'm currently on vacation in the windy city. :)
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Offline cellularmitosis

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Re: Ultra Precision Reference LTZ1000
« Reply #2058 on: December 17, 2017, 06:49:22 pm »
Ah, sorry, I should have thought to link to the articles.  I'll update the post.  Enjoy your vacation!
LTZs: KX FX MX CX PX Frank A9 QX
 
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Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #2059 on: December 17, 2017, 09:50:27 pm »

There is a disagreement going on about R418 (a 2K67 resistor from -15V to ground)-- and what is it's purpose.  I say it's not needed, others say "oh yes it is".  If anyone can, please test your meter with and without this to see if it makes any difference-- if it doesn't, (test temp tracking also)-- then leave it out.


It's R419, and its purpose is quite obvious.. it draws -5.6mA out of GND, exactly compensating all currents into GND from the reference circuit.
 

Offline Macbeth

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Re: Ultra Precision Reference LTZ1000
« Reply #2060 on: December 17, 2017, 10:00:44 pm »
There is a disagreement going on about R418 (a 2K67 resistor from -15V to ground)-- and what is it's purpose.  I say it's not needed, others say "oh yes it is".  If anyone can, please test your meter with and without this to see if it makes any difference-- if it doesn't, (test temp tracking also)-- then leave it out.
I think you mean R419 - and I can't see any purpose for this resistor on the unused -15V rail on the A9 ref board. I believe it may have been used on the factory test rigs used to calibrate or pass/fail the A9 boards for some purpose or other. But perhaps just a stupid afterthought left in the design...
 

Online Kleinstein

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Re: Ultra Precision Reference LTZ1000
« Reply #2061 on: December 17, 2017, 10:27:39 pm »
Having a resistor in series with the zener at the positive side is a two sided thing. The idea of such a resistor is to reduce the raw TC (without temperature stabilization) of the voltage reference part. However this will also make the resistor and the current setting resistor more important. Sensitivity of the current setting resistor will be up by about a factor of 3 and the extra resistor should be similar critical. So the added resistor only works if really good resistors are used. More for the understanding of the residual TC is might be interesting to see of the added resistor would make a big change to the compensation resistor - this might give hints if the residual TC is more due to not so good temperature control or effects of the pins / layout.

There is somewhat limited gain from spending a lot of money on the resistors - at some point, using a second LTZ reference might be the more efficient way, maybe even used with just PTF type resistors. Inside a meter or with access to a good meter, it might even be worth having access to the divided voltage for the temperature set-point. In theory drift of the divider could be corrected if measured without too much distortion (this might not be easy). With access to the temperature set-point one could consider a controlled way of startup procedure to reduce thermal hysteresis.

Only about 0.5% of the OPs drift and low frequency noise will be at the output. Thus the OP should not be that critical, and there is not that much choices of better OPs. There may be option if single supply is not needed, but than the startup sequence of the positive and negative reference might become important.  The 1 M resistor setting the response speed for the temperature control is not at all critical, it's working in combination with a cap of usually high tolerance and the regulator speed is not that tight adjusted. LT suggest the same control for the LTZ1000A and no adaption is made to the actual thermal setup or the nonlinear heater - so who cares about a +-10% or even more tolerance here.

I agree that the nominal 400 K resistor for TC compensation is a kind of bodge: The residual TC before correction seems to be about constant (e.g. the effect is linear in temperature) . However the correction follows about a square root of the set temperature - actual temperature - self heating. Thus it will get kind of nonlinear if the temperature is too close to the set point. So the compensation will not work that well with a low temperature setting. It might need individual adjustments, depending on the layout and thermal setting and maybe individual units.
 
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Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #2062 on: December 17, 2017, 11:31:27 pm »
It's R419, and its purpose is quite obvious.. it draws -5.6mA out of GND, exactly compensating all currents into GND from the reference circuit.

OK, you got me!  Thanks for the correction-- yes, when I zoom in on my very low resolution copy of the schematic, it does say R419, and not R418.  Oops!

Well, this is one of the theories that I have heard as to it's purpose, but I have never seen any experimental results proving such a claim.  If this is true (and R419 is indeed necessary), then the value would have to be adjusted if we choose a better op-amp that might have a different quiescent current than the LT1013.

Experimental result?
Instead of measuring the currents, you can do a simple calculation beforehand, using typical or even maximum current values:
Zener current ~ 0.40V (@90°C) /111Ohm = 3.6mA
collector currents ~ 2x(7.2V (typ.) -0.4V) / 74.25k = 183µA
divider current ~ 7.2V /16k =450µA
LT1013 supply = 2 x 0.5mA (max. value) = 1mA

Sum~5.2mA versus 15V/2.67kOhm = 5.6mA

Evidently, these currents nearly cancel, leaving the analogue ground supply 'A', pin J400 (4) with some 100µA, depending on sample variation. I can't overlook, where on the main analogue board, or on the A/D board the Ref GND is married with analogue GND, but very probably, it's not buffered, in contrast to the +7V Ref signal.
On the A/D board, at least there's R168 (?), a 10k2 resistor, sinking -1.2mA out of Ref GND (it's labelled ZR_LO2 there).
And on the analogue board, the potential of Ref_GND can be measured by the DCV input multiplexer also..

In the resistor reference current circuit, you'll also find a similar compensation resistor, that's R311 (9k09) which sinks 2mA from analogue ground, balancing the reference currents through resistors RP300A .. C..


Back to the LTZ circuit... a current of 5.6mA over GND would create a potential difference of about 100µV over only 20mOhm, or on the order of 15ppm.. So if the connector would change its resistance by that amount, that would create a well measurable error.

I bet, that removing this 2k67 resistor would shift the calibration remarkably. (Would be the experimental proof, that I won't do for obvious reasons)

Fluke uses active current cancellation circuits in their 732B, and as far as I remember, in their reference circuits for their calibrators also. The purpose is the same, very obviously.


By the way, my 3458A showed a clearly visible -0.5ppm shift, after returning it from a volt-nuts meeting two years ago, that's been 2x200km ride by car, with some vibration, although I got a very comfortable French car, with smooth suspension.
 
I directly thougt that the connector of the reference had changed its contact resistance towards smaller values.. and I also thought, that sending these instruments to hp for calibration, that it would always give some shift by the transport itself.

Frank
« Last Edit: December 18, 2017, 09:37:21 am by Dr. Frank »
 
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Offline chuckb

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Re: Ultra Precision Reference LTZ1000
« Reply #2063 on: December 17, 2017, 11:50:08 pm »
You get a bunch of engineers together and they have to start redesigning things. What Fun! Here is my contribution.

I think the A9 PCB layout could be better. TiN took several IR images of the electronics in the 3458A.
https://www.eevblog.com/forum/repair/repair-old-rusty-hp-3458a/msg808569/#msg808569

The last image shows that the R419 and the heater transistor are the hottest things on the PCB. And they are located right next to the Kovar pins of the LTZ1000A. The Image even shows a strong thermal gradient right across the pins. 

These high temperature parts will contribute to warm up drift and increase the temp co of the reference. As ambient temperature changes the pass transistor temperature changes so the thermal gradient across the LTZ1000A pins will change so the Temf of the Kovar pins will change.

These parts could be arranged better. They should be on the opposite end of the pcb from the LTZ1000A.



 

Offline chuckb

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Re: Ultra Precision Reference LTZ1000
« Reply #2064 on: December 18, 2017, 02:22:05 am »
Here is my guess for all the current balancing resistors.

The offset voltage caused by the ground current could simply be nulled out during calibration. However, if it is caused by the voltage drop in a copper trace on the PCB then that offset voltage has a 4000 ppm / deg C drift. So you have to do what you can to minimize the offsets caused by current flowing in copper traces.

You also have the problem of unstable resistances in connectors.
 
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Offline pitagoras

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Re: Ultra Precision Reference LTZ1000
« Reply #2065 on: December 18, 2017, 03:20:09 am »
I've shared my board at OSHPARK too, if anyone is interested.

https://oshpark.com/shared_projects/DmMENXtg

It was going to be PX but it has been taken  :)

So it has become QX, much better for the logo  :P 
 
 
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Offline MisterDiodes

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Re: Ultra Precision Reference LTZ1000
« Reply #2066 on: December 18, 2017, 05:09:44 am »
Digilent:

It doesn't matter if you believe me or not re: '2057 input chopper noise.  You're welcome to pay my client for the documentation - otherwise just pay no attention.  Consider the free advice clue as worth exactly what you paid for it. That's as far as I can go on a public forum.

What does matter is LT specifically recommends against a '2057 for various reasons, unnecessary excess noise across the Zener is one of them.  These are the people that manufacture the part.  I would guess they have some knowledge of what works and what doesn't, and they handle a lot more design feedback from volume manufactures.  They have about zero economic reason to even sell LTZ's in small orders, so be glad they even do that.  The fact that they even discuss design know-how is an added bonus.

For the gazillionth time:  It is true that other amps will work as the LTZ current driver, but the "1013 is hands down the best amp to use for the right mix of current and voltage noise.  It is tailor- made to be the LTZ's companion.  That is the part to use, and that is the part they will stand behind on datasheet specs.  When you talk to LT apps engineering, that's what they tell you - at least that's what I've gotten on more than one occasion when I ask about "upgrading" to a chopper amp.  There is just no reason to do that.  Remember you're using the amp as a CURRENT driver, and the zener + transistor stability will overwhelm whatever small Voffset you've got on the '1013.

Check your specs - we've never seen an hfe on the the LTZ transistors as low as 50.  Typical is 200, the range is 80 to 450.  A small Voffset on the current driver amp is not going to result in significant Vref change.

There are lots and lots of older "standard" 3458a's that work just fine to "002" high stability option specs - we have several that don't drift a ppm per year.  That means the board design is not the main contributor to drift - it's the LTZ die itself (and scope of normal drift variation part to part), and that is far beyond anyone's "hand tuned" control.

Go ahead, try it - remove the ground current cancellation resistor on that 3458a Vref board and let me know what happens as the ambient temperature changes....and let me know if it's not pretty smart to use a sub-penny carbon resistor (fancier types not required here) to reduce ground current at the module header pins....  I really don't see designers flailing around: I see a smart, well thought out design that produced the most successful DMM in history - and a DMM that has outsold and out-lasted every competitor design over time.  Am I wrong?

If there were a better way to do it we'd have it after 30+ years.  You're more than welcome to produce a better Vref replacement that works in production.  Hopefully Keysight will be able to at some point...
« Last Edit: December 18, 2017, 06:01:26 am by MisterDiodes »
 
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Offline cellularmitosis

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Re: Ultra Precision Reference LTZ1000
« Reply #2067 on: December 18, 2017, 06:08:32 am »
Update the HP A9 reference board clone (03458-66509):

I figured out how to scale and align TiN's images (see https://xdevs.com/article/volt_xfer/) on top of kicad screenshots (using gimp) and dialed in all of the dimensions.  They match up pretty closely!  (see attached overlays).

Also, I've managed to squeeze in room for hermetic footprints on R413 and R415.  The footprints are backwards compatible, so you can populate this board with original HP components, or upgrade the critical  resistors to hermetic packages.

This version of the board is intended to mimic the original board as closely as possible, with the option of upgrading the resistors, but in a future version I'd like to explore diverging from the original design a bit, perhaps with the "R60" mentioned above, etc.

I was a bit surprised to find that the LTZ isn't perfectly centered between the two mounting holes.  I decided to err on the side of reproducing HP's error.

v2 OSHPark link: https://oshpark.com/shared_projects/jP5xkfJy

github: https://github.com/pepaslabs/hp-03458-66509-clone
« Last Edit: December 18, 2017, 06:10:39 am by cellularmitosis »
LTZs: KX FX MX CX PX Frank A9 QX
 
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Online Dr. Frank

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Re: Ultra Precision Reference LTZ1000
« Reply #2068 on: December 18, 2017, 07:44:02 am »


Well, if this is being done for the reasons that you state-- and what you are saying sounds reasonable, then why are they bothering when it's an offset, and as long as the offset does not change then it doesn't matter?  Why in the heck would they use a very noisy carbon-film type resistor for this.  There is really something goofy going on here-- I think somebody was desperately plastering band-aids all over this design-- with the hope of being able to "fix" a number of issues-- instead of just re-engineering the whole circuit and layout.  The whole design looks as though they were flailing about without really understanding what they were doing.  Those engineers are no longer at HP (or Agilent, or Keysight) anymore-- so we may never know what the hell they were up to.  I guarantee you this-- I could design a much better Vref replacement for the 3458A than factory; and if I can do it, then why don't THEY do it?


Hu, Carbon?
In the BOM, R419 is specified TF (usually Thin Film), TC100.

And on my A9 board, it really looks like a Thin Film resistor, not at all like a carbon type.
(and it's definitely 1%, brown ring, also visible on other boards. Carbon resistors are 10% at best)

Again, the whole reference circuit is not directly fed into the A/D circuit, but is routed over at least three connectors, which might well change their resistance.

In the Fluke 5440 and 5720, they treat this GND star point and supply current problem much nicer, just lay both schematics side-by-side.

That's also the reason, why I  designed a big star point into my latest design as a stand-alone 7 / 10V reference, but I would also have some trouble with GND, if I would use my modules inside another circuit.

Frank
« Last Edit: December 18, 2017, 10:04:53 am by Dr. Frank »
 
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Online Echo88

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Re: Ultra Precision Reference LTZ1000
« Reply #2069 on: December 18, 2017, 10:15:19 am »
Man, so many new open source LTZ1000-boards, but no LTZ1000A available....  :-DD
Maybe we can gather all open source LTZ1000-boards in a new thread/post?

Special thanks to cellularmitosis, who apparently needs no sleep and produces a new board every few days.  ;D
« Last Edit: December 18, 2017, 10:19:52 am by Echo88 »
 
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Online BU508A

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Re: Ultra Precision Reference LTZ1000
« Reply #2070 on: December 18, 2017, 11:04:58 am »
A seller on AliExpress is offering this LTZ1000 board (no affiliation):

https://www.aliexpress.com/item/Ltz1000-ltz1000a-PCB-circuit-board-thickness-1-0mm-size-62mm-62mm/32829991789.html



And there is a seller which is selling LTZ1000ACH, but I'm not sure if I would buy them (no affiliation):

https://www.aliexpress.com/item/5PCS-LOT-LTZ1000ACH-LTZ1000/32757679028.html
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Offline mimmus78

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Re: Ultra Precision Reference LTZ1000
« Reply #2071 on: December 19, 2017, 12:38:21 am »
Update the HP A9 reference board clone (03458-66509):

I figured out how to scale and align TiN's images (see https://xdevs.com/article/volt_xfer/) on top of kicad screenshots (using gimp) and dialed in all of the dimensions.  They match up pretty closely!  (see attached overlays).

Also, I've managed to squeeze in room for hermetic footprints on R413 and R415.  The footprints are backwards compatible, so you can populate this board with original HP components, or upgrade the critical  resistors to hermetic packages.

This version of the board is intended to mimic the original board as closely as possible, with the option of upgrading the resistors, but in a future version I'd like to explore diverging from the original design a bit, perhaps with the "R60" mentioned above, etc.

I was a bit surprised to find that the LTZ isn't perfectly centered between the two mounting holes.  I decided to err on the side of reproducing HP's error.

v2 OSHPark link: https://oshpark.com/shared_projects/jP5xkfJy

github: https://github.com/pepaslabs/hp-03458-66509-clone

Nice I think I will use it to build and age a A9 replacement for my 3458a. The one actually inside is friend of TiN little jumpy.
« Last Edit: December 19, 2017, 12:44:23 am by mimmus78 »
 
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Offline 3roomlab

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Re: Five new kids on the block - Ultra Precision Reference LTZ1000
« Reply #2072 on: December 19, 2017, 01:33:39 am »
I have assembled and tested my 5 new LTZ1000 reference boards.

They all are based on Andreas schematic, providing additional EMI filtering for great immunity against RF and mains disturbances.

ive been playing with eagle, i wonder, has anyone been succesful in totally laying this entirely on 1 side? i must say i have no success.
« Last Edit: December 22, 2017, 02:00:08 am by 3roomlab »
 

Offline Macbeth

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Re: Ultra Precision Reference LTZ1000
« Reply #2073 on: December 19, 2017, 01:52:12 am »
And there is a seller which is selling LTZ1000ACH, but I'm not sure if I would buy them (no affiliation):

https://www.aliexpress.com/item/5PCS-LOT-LTZ1000ACH-LTZ1000/32757679028.html

I wouldn't touch them with a bargepole. Straight off the bat no manufacturers logo and no date code.
 

Offline cellularmitosis

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Re: Five new kids on the block - Ultra Precision Reference LTZ1000
« Reply #2074 on: December 19, 2017, 05:32:44 am »
I have assembled and tested my 5 new LTZ1000 reference boards.

They all are based on Andreas schematic, providing additional EMI filtering for great immunity against RF and mains disturbances.

ive been playing with eagle, i wonder, has anyone been succesful in totally laying this entirely on 1 side? i must say i have no success.

Dr. Frank has!  https://www.eevblog.com/forum/metrology/mx-reference/msg1297126/#msg1297126

edit: actually he used two jumper wires, so *almost* entirely on one side.  I put a copy of his design on OSHPark: https://oshpark.com/shared_projects/gl2aXmlh
« Last Edit: December 19, 2017, 05:35:00 am by cellularmitosis »
LTZs: KX FX MX CX PX Frank A9 QX
 


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