Live design flow of 'FX' 10V LTZ1000A-based reference

[nidlaX]

I've renamed this thread from "TiN livestreaming: layout of new LTZ1000A 10V reference module" per TiN's request since the layout is done and has been sent for manufacturing!

FX Live Test Data

Layout videos:

Part 1:

Part 2:

Part 3:

Assembly video:

Part 4:

Also check out his posts for different design stage PCB pictures:

First Batch
Second Batch
Third Batch
Fourth Batch (Assembled)

Final Schematic (as of 11-9-2017)

[dr.diesel]

He'll be working for Fluke before you know it!   

[Echo88]

I wanted to start designing my own LTZ1000A-board, but considering that digikey claims delay of delivery for the LTZ1000A till April next Year i still have much time for other stuff... 

[mimmus78]

OK I guess actually I look like my nephew when he watch mine-craft videos on YouTube.

 

[TiN]

Wow. It's not worthy a separate thread. Just a first try, I'm sure there are lots of issues to iron out first on this rev.

I'll try to get second part tomorrow.

[Dr. Frank]

Illya,
once again, you designed a really nice board!

You mentioned in the video, that you designed in a low pass between the LTZ reference and the AD OpAmp, to suppress the chopper spikes going backwards into the reference circuit.
Well, I doubt a bit that this effect is so important.
You also introduced a lot of solder junctions between the LTZ and the OpAmp, that may ruin your T.C., or you have to balance all these junctions, as it's been done in the 3458A DCI circuit, you remember?

Anyhow, in my in own Spartan single-sided design, I directly (copper-wise) connected the LTZ output to the LT1052 ChopAmp, accepting possible spike feedbacks, but avoiding thermo junctions..

We'll see in 1 year time, what will happen,... and I just started a noise / stability measurement on the 10.000V output, realized with these ordinary BMF from AE.. we'll see tomorrow.

Frank

[eurofox]

I wanted to start designing my own LTZ1000A-board, but considering that digikey claims delay of delivery for the LTZ1000A till April next Year i still have much time for other stuff... 

https://www.benl.ebay.be/itm/LTZ1000ACH-CAN-8-Precision-Reference/322563133347?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649

[TiN]

Illya,
once again, you designed a really nice board!

Will see on that after getting year worth of data .

LPF between chopper and reference also attempt to REF reduce noise little bit for quieter output. Likely not very effective.
I'm going to revisit that part, by removing R20,R28 and installing same package resistors complimentary to R18,R19, to equalize number of junctions.
I also don't like the grounds, might as well redo that part.

Also I messed up with guarding, guard trace potential is 0.44V so wrapping it around reference output do not good, should be around temperature divider node.
Consequences doing layout at 4am  .

We'll see in 1 year time, what will happen,... and I just started a noise / stability measurement on the 10.000V output, realized with these ordinary BMF from AE.. we'll see tomorrow.

Looking forward for your data.

I've scheduled finishing up part 2 for today, 10pm UTC+8.

[Zucca]

Preorders? 

[Kleinstein]

The LTZ circuit usually does not need guard traces, as it is mainly low impedance (e.g. 1 K). It is only part of the temperature regulator that uses high value resistors, but this is with rather high signal levels an not much sensitivity. Also the 70 K the collectors are at point that is not that sensitive.
Chances are the guards do more bad than good due to thermal effects - though I don't think this is that important.

Before doing much work on the layout it is a good idea to get the schematics right before. A first idea of the layout is Ok, to get an idea of the placement of the main parts, but no final layout before the circuit is finished. AFAIR there where still some points not finished about the output stage.

[TiN]

Already removed unwanted guards, as even if there would be any benefit, guard source need to be low impedance, not the way it was done. That was a flop.

PDF-schematics.

I've add provisions for compensation cap around output stage opamp, from our discussion before.

Animated GIF all layers (5 sec/layer):



Changelist today:

* Replaced input filter RCR to balanced network using MELF metal film resistors.
* Routed GND_Signal return as point-point signal, no shapes.
* Rearranged placement for output chopper for more balanced routing

I'll do some final checks and finetune tomorrow, and get board for fab . It's better to test any issues on practical setup, than spending pages on forums on theory. 

Part 2 vid:

.

[dr.diesel]

Preorders? 

Once everything is finalized, perhaps a group order?

[Zucca]

Once everything is finalized, perhaps a group order?

Bare board will be cool but without Fluke 2464707 kind of sad business.

[TiN]

Maybe somebody is in friends with VPG to get a group buy on divider? That should be easy, compared to Fluke custom network  .
Same reason why as of now I see no point to making FX a group order device.

[Zucca]

Maybe somebody is in friends with VPG to get a group buy on divider?

That would be sooo soooo cool.  +1

[TiN]

PCB layout complete.

PDF-schematics.

Animated per-layer GIF PCB.



Changelist:

* Added R33 for equal amount of resistors in negative feedback.
* Added optional C48,C49 at the output force
* Redraw schematic symbol and changed trim connections on Fluke resistor network for clarity
* Added power LED D18,D30
* Added I2C U7 EEPROM, for storing module-specific DC output voltage, calibration values, tempco/drift values, configuration parameters, etc.
* Changed connector type to 20pin 0.5mm pitch XP2 for onboard temp sensor and I2C ROM. Power for sensor and ROM are 100% isolated from rest of the reference.
* Added extra C46 cap near heater NPN.

Part 3 video:

.

[branadic]

Illya,

What do you plan for the housing?
How do you fix the pcb inside the housing?
What connectors are you planing to use?

From a temperature point of view the LTZ connections are rather asymmetrical. I also wonder about the shape of the board. Just another fancy looking LTZ board like the KX? Sorry, but I'm sceptical that this approach does any good at all. But I appreciate that the ground layer is no longer fragmented.

-branadic-

[TiN]

branadic,

It will be box very similar to this 112 x 60mm Hammond, places next to battery compartment in Fluke 792A power pack.
There is no room to make it any bigger, so it will be all very tight fit.

PCB will be suspended inside the housing, without rigid mechanical coupling, as box eventually might see transportation, however it's not quite the travel standard due to weight.

There will be no connectors at the board, other than auxiliary 20 pin for digital interface and onboard temperature monitor.
Power input and reference output will be routed out of the box with copper shielded twinax, soldered to output port J3.

External world connectors will be Low Thermal 2758 or LEMO EGG, not decided on that part yet.

Yes, just another fancy looking board with some bling added to it. Board sent to fab, will see it in few weeks. Should be enough time to get some parts. I already have LT1013's, LTZ1000ACH's and VHD 20K/7.663K dividers.

[Kleinstein]

I had not look at the layout in detail before. The animated gif is kind of hard to watch if one want to look at details. For me it looks like the connections to R1 not so well made. My feeling is there is way to much trace resistance in series with the 120 Ohms. One might be able to fix this with extra wire in parallel, and / or a bridge.

A more minor point with the output amplifier is that C41 and D43 would be better towards ground instead of the the +- supply.

The filtering cap C36 might also be better differentially instead of toward ground - but this might be only a minor effect.

[TiN]

I'll post PDF-version later.

Connection to R1 is 40 mil wide trace, 1340 mils long from LTZ, and 1150 mils to GND star point.

[Andreas]

- buyer be cautious - most likely this is counterfeit part.

not only the datecode also the alignment of the marking versus package is fishy
so for me this is 100% counterfeit. (at least re-marked)

with best regards

Andreas

[mimmus78]

Hello Andreas - I observe 3 issues:

The metal case is glossy and should be matte

The TAB indicating pin 1 is at wrong position

The date code is not inline with LTC's marking system
[/list]

Best regards
Flintstone

Pin 1 is not in the wrong position, this is special version of LTZ1000 and must be mounted on the other side (back) of the PCB :-)

PS: I'm sure only the few will get it ...

[eurofox]

- buyer be cautious - most likely this is counterfeit part.

not only the datecode also the alignment of the marking versus package is fishy
so for me this is 100% counterfeit. (at least re-marked)

with best regards

Andreas

Many others are available but up to you and wait 6 months 
https://www.benl.ebay.be/sch/i.html?_from=R40&_sacat=0&_nkw=LTZ1000&rt=nc&LH_PrefLoc=2&_trksid=p2045573.m1684

[Edwin G. Pettis]

To my knowledge, there is no 'special' layout from LTC, I think what you are referring to was an incorrect board layout and he had to mount the LTZ on the backside of the PCB to make it work.  It wasn't discovered until after the boards were made, it happens.

[TiN]

I'm bit lost how any of this fake related to the thread. 

[mimmus78]

To my knowledge, there is no 'special' layout from LTC, I think what you are referring to was an incorrect board layout and he had to mount the LTZ on the backside of the PCB to make it work.  It wasn't discovered until after the boards were made, it happens.

Yes, he wasn't the only one ...

Inviato dal mio Nexus 6P utilizzando Tapatalk

[pitagoras]

The boards are back from manufacturing and the saga continues with livestreaming the assembly of the pcb

[TiN]

First FX board is alive.

After few hurdles due to late night assembly (wrong resistor value, 1KOhm instead of 75Kohm, lol), LTZ section is working.
I couldn't be bothered to wait for initial few hundred hours warm-up, so started tempco curve right away.

Resistors used for test: AE 1K + AE 15K, 220 ohm AE + 330 ohm S102J + 900 ohm S102J (115 ohm), 2 x 75Kohm PTF56 <5ppm/K (tested).

.

This is without 400K TC compensation resistor, so good start I'd say.

Two data lines are since I connected reference 7V output to both 3458A for better confidence.

[TiN]

Assembled two more modules, for total 3 units.

First, comparison to well known HP 3458A A9 DC Reference module and my old KX public version LTZ1000(A) module.



I'm missing opamp, resistor network and MELF resistors for the 7V->10V output amplifier section, so that will be assembled after parts arrive. Currently I can test tempco and establish intermediate 7V output, and get a glimpse on initial stability, to make sure no funny business happenin.



Little section with DFN chip is low-noise Linear LT3042 +11V linear regulator, that powers LT1013A and LTZ1000A chip.
Output amp powered directly from input power entry (power source for this module is +12/-12V VRLA battery array).
There are just TVS at the input and few bulk tantalum capacitors.

20-pin connector provide isolated +3.3V to calibration data EEPROM (I2C 24128, U7) and +5V supply to MAX6610 (U4 in center).



Configurations:

FX 001 : LTZ1000ACH, 51 week 2016, 1K AE YB BMF, 20K S102J + 34.8K S102K, 330R + 300R + 522.2R S102, 2 x PTF56 75K (tested <-6ppm/K).
FX 002 : LTZ1000ACH, 51 week 2016, 1K AE YB BMF, 28.5K S102J + 28.5K S102K, 120R Edwin PWW, 2 x Edwin PWW 70K
FX 003 : LTZ1000ACH, 51 week 2016, 1K AE XB BMF, 15K AE XB BMF, two S102 in parallel to get 132R

[Echo88]

Nice! 

[nidlaX]

Very nice, updated the first post.

[TiN]

Got Digikey parts today and continued to assemble FX module, this time output stage to get 10V.

Livestream.

There was a hiccup (no wonder, 2am not best time to debug analog precision circuits), and only after video I discovered my mistake and got proper 10V output.
Now it's already running 30 minutes, hovering around 9.999960 V, with std-dev ~1.8uV (K2182).

P.S. Sorry about low sound level.

[Kleinstein]

I don't think the 2182 is good enough to directly measure the noise. Normally the noise of the 10 V signal should be lower than even the HP3458 in the 10 V range. So those 1.8 µV of SD are likely something like 1.6 µV from the 2182 and maybe 0.5 µV from the 10 V reference.

It would be only as a difference to a second reference or with AC coupling that one can get an idea of the noise.

[TiN]

I don't think the 2182 is good enough to directly measure the noise.

I thought so too, (3458s were busy measuring resistor's tempco yesterday) but today data shows that 2182 on 10V is quite a performer, not that far off 3458A or 2002.

Attached graph capture, no sample averaging applied.

Settings:
Blue and green : 3458A, NPLC200, 10VDC, AZER ON
Red: 2002, NPLC50, 20VDC, AZER SYNC, DFILT MOV 10, LSYN ON
Urple: 2182 (non-A), NPLC5, AZER ON, LSYNC ON, FAZ ON, LPAS ON, DFILT ON, 10VDC, Channel 1

Reference output stage is powered from +12.6V (K2306) and -12.6V (K2400). 7V Zener section is powered by onboard LT3042 +11.0V, regulated from +12.6V rail.

CSV-data LIVE.

[Kleinstein]

From the board pictures, the 2182 ADC / reference part looks rather similar to the ADC in the 2010. The 2010 is known to be rather low noise, despite using only a LM399 reference. As a  nV meter the input amplifier of the 2182 is expected to be very low noise, so the performance should be good in the other ranges too, not just the 10 V. It's probably even more impressive in the 100 mV and 10 mV ranges.

The comparison is a little tricky, as the digital filtering can be a little misleading.

[TiN]

Initial testing for tempco started, using 10V output stage.



I need to move one of the meters to 7V output before the stage, so we can see impact on tempco from gain stage alone. 

EDIT: And complete run:



Clear to see when we ran out of LTZ oven margin (output skyrocketed +140ppm).

[VintageNut]

I agree. If the LTZ is 0.0x ppm / deg c, then the resistors and/or opamp(s) around the LTZ are causing the 10V to change with temp.

For these these modules to be really useful, they have to be better than a 731B which I think mandates that the entire board be in a temp controlled oven.

From a cost standpoint, a 732B has such a high price that spending a few more hundreds of dollars on a temp controlled oven enclosing your reference is a good idea. If the entire ovenized reference is less than $1,000, then that is a very successful DIY design. I would want one of those. Without the oven, the reference is no more stable than my qty of $250 731B.

Please correct me if I have made any incorrect points above.

[TiN]

For these these modules to be really useful, they have to be better than a 731B which I think mandates that the entire board be in a temp controlled oven.
snip..
Please correct me if I have made any incorrect points above.

Not to nitpick, but what is the tempco of your 731B? Per instruction manual it is specified 1ppm/K. Do you have measurements data?
I'll surely try to get best stability possible out of the module, without external oven. 

[Kleinstein]

The 7 V reference module is good even without an extra oven. However the 7 to 10 V stage is only as good as the resistors. In this case it might actually be cheaper to use an oven to stabilize the resistors than hunting for resistors in the 0.1 ppm/K range.

The resistors don't produce much heat, and the power is constant - thus thermal stabilization is not that difficult and does not need high power, if thermal insulation is good. However one still needs long time stable resistors, it is only the TC that can be suppressed to a large part and the humidity effect might be reduced a little, as the hot part will be lower humidity.
In addition to the 7 to 10 V amplifier one might as well stabilize some of the resistors of the LTZ circuit as well, especially the 1 K / 13 K divider.
I kind of miss a switch to change between the raw 7 V and scaled 10 V reference.

The critical parameter of such a reference is usually not the TC, but more like the long time stability. So for quite a few cases just a good temperature sensor could be enough, if the temperature dependence is known for a certain range (e.g. 18 C to 30 C).

[TiN]

There is test point footprint that allow to fanout 7V output from LTZ, so one could add additional pair of binding posts to get raw 7V output, together with 10V output stage output.
Thermal coupling to case is one of the reason to ease on tempco issues, one could stabilize then whole can if desired. 

[Dr. Frank]

I agree. If the LTZ is 0.0x ppm / deg c, then the resistors and/or opamp(s) around the LTZ are causing the 10V to change with temp.

For these these modules to be really useful, they have to be better than a 731B which I think mandates that the entire board be in a temp controlled oven.

From a cost standpoint, a 732B has such a high price that spending a few more hundreds of dollars on a temp controlled oven enclosing your reference is a good idea. If the entire ovenized reference is less than $1,000, then that is a very successful DIY design. I would want one of those. Without the oven, the reference is no more stable than my qty of $250 731B.

Please correct me if I have made any incorrect points above.

The 732A/B use an oven for the whole board, because the SZA263/LTFLU is NOT ovenized itself.

Therefore, for a LTZ1000 based reference, which already had been trimmed to around 0.02ppm/K, it would suffice, to build another oven for the 10/7 resistor only, to get <<0.1ppm/K for the 10V output also.
To additionally ovenize any of the LTZ1000 circuit components would probably create problems on the stability of the circuit.

I also think, that such an extended reference could be realized for less than 200 $/€, if the additional oven and a VHD200 may add another 60..80$ to the about 120$/€ for the LTZ1000 board alone.

In the end, my idea for frequent re-calibration of these 10V, is to use any 6 1/2 digits DMM with ratio function, which might give < 2ppm uncertainty due to linearity error, when the raw LTZ1000 is available on the front panel also.

Frank

[TiN]

Reran tempco test, this time with K2002 monitoring direct 7V output before the output stage input. Also this time temperature ramp is +16 to +50C.



Data reveals that nearly all of the temperature coefficient actually derived from the LTZ section, not the output stage.
This was a bit of surprise, considering that I trimmed this output prior to adding output stage to <-0.03ppm/K. 
Output opamp stage changed thermals and loading to the LTZ output, causing more negative TC?

[VintageNut]

My temp measurements are not from a calibrated system. Just an inexpensive temp display.


deg V  Voltage                   V /  deg C
21.5     9.999954         
22.4     9.9999574      3.77778E-06   
23.1     9.9999614      5.71429E-06   
23.9     9.9999648      4.25E-06   

A bit better than 1 ppm / deg C.  Not consistent over temp. Maybe a better temp system would give a better characterization.

If the entire 731B could be placed into a temp controlled oven, that would be a nice reference. All of the resistors are decades old.

[TiN]

.. 1 ppm / deg C.  Not consistent over temp. Maybe a better temp system would give a better characterization.
If the entire 731B could be placed into a temp controlled oven, that would be a nice reference. All of the resistors are decades old.

Hm, seem to be no much difficulty, beating those numbers by factor of 10.
Currently running with 226K compensation resistor. .

[Kleinstein]

If you want to measure a small TC over a small temperature variation, this gives very minute changes. Thus the meter for the measurements could be a problem too. A better temperature reading is likely not yet that important.

Old resistors may help a little for stability. The true value of an old reference comes if there is a good calibration history - this somewhat reflects the price of some $500 for every point.

[TiN]

More on tempco saga..



First time I see myself a positive tempco LTZ output .
Also "waves" around +30c likely due to me, moving stuff in lab around and turning on/off random gear.

Maybe just a liiiiiitle less compensation and we are in business for some aging and some x00-ish long logging.

If somebody forget, red dashed line is +0.05ppm/K threshold, blue is -0.05ppm/K.

[kj7e]

First time I see myself a positive tempco LTZ output .
Also "waves" around +30c likely due to me, moving stuff in lab around and turning on/off random gear.

What did you change (please include value)?

Funny enough, I see a small bump (about 2uV) right at 30 Deg C transition on my LTZ1000A's as well.

[TiN]

I don't think value would be much of help, as it is specific for the LTZ used, and each reference need own value for "sweet-spot".

Anyhow, got second module in the action, and ended up with somewhat confusing data.

Everything looked fine and dandy till +32C on positive ramp, and then LTZ direct voltage took off at rate ~+0.03 ppm/K, while 10V output barely changed?
Then after sitting for an hour at +50C flat 10V output started to decline.

Maybe it's the tempco of ESI DB52 used for second-order trim, as ambient temperature changed about -0.7C from same +32c point. But that shouldn't have that much impact. 



Installed fixed resistor in the ref now, and restarted the test again.

Moral of the story: always test ramp up AND ramp down, that can reveal unwanted behaviors like this or hysteresis offsets easily.
Also step response tests do no good for such suddle changes. 

[TiN]

Retest update with slower 0.05C/minute speed ramp up completed and fixed trim resistor instead of decade.

.

Tempco anyone?   

Also attached pre-trim 7V only tempco test from last week, on this same reference.

Voltage versus Box temperature graph:

[TiN]

I've received 3D printed caps from ManateeMafia:



They fit rather nice, just minor tweaking required.



Later this week will finally test output stage performance and trimming options with resistor network.

[ManateeMafia]

Any streaming video today of final assembly and testing?

[TiN]

No, maybe next weekend. I need figure out resistor situation and rearrange lab first.

[TiN]

BTW, I'm streaming now assembly of 4th prototype with LTZ socket.

.

[branadic]

What does Edwin G. Pettis think about your resistor lead origami?

-branadic-

[Edwin G. Pettis]

For the first board about 55 minutes in, those bends are a bit tight, while they probably won't have any problems, a better way would be to use a needle nose next to the body and then bend the lead, more like TiN did at about 1:45:00 minutes on the second board I seen,  Sorry don't have time to watch it all right now.

The leads are 'anchored' into the bobbin and held in place by the epoxy sealant so mechanically it is normally not a problem, however, if soldering (or multiple soldering) takes too long it can damage the epoxy allowing the lead bend stress to migrate into the bobbin and in some instances, causing the weld to be broken by excess strain.  It doesn't happen very often but it can, even Bob Pease managed to mangle some resistors trying to remove them from a double-sided PCB.

[MisterDiodes]

Correct.  For "young players" not familiar with assembling precision analog circuits:  The -minimum- bend radius on component leads is typically 2X the lead diameter, and on PWW  / Foil resistors that's more like 3X - when you kink the leads that develops a cold-worked alloy concentrator zone that can eventually cause problems over time.  This can even show up on low value resistors as increased apparent TC.

Also keep the start of the bend at least some distance (2X diameter minimum) from the body to help prevent any mechanical strain making it into the wire weld area - this applies to any precision resistor, PWW or otherwise.

When bending leads, always support the leads so you're not "jacking" the lead against the body (i.e. pulling it out of the body).

The component should fit into the mounting holes before soldering with zero stress on the leads - for production we'll have a zero-stress bending jig system setup for the various parts. 

That "Origami" look is basically what you don't want. The circuit will still work, but might not pass a precision assembly inspection.

[babysitter]

When bending leads, always support the leads so you're not "jacking" the lead against the body (i.e. pulling it out of the body).

A coworker (M.Sc. in physics, might play a role?) just arrived at that with a resistor from Edwin, i was barely able to believe it. I think I posted a photo on the E.L.T. I still don't know how he has done it, its his nasty secret.

Hendrik

[Andreas]

This one?
https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg1184489/#msg1184489

[babysitter]

Yes, that one, I thought it was in the Epic LTZ thread.

[TiN]

Today's update:

Sorted LTZ chips by voltage output, using socket board. Now need to map-match Fluke networks for each chip.
LTZ resistor setup: Edwin's PWW 13K/1K, 120R, 2x70K.



 

[TiN]

Bit of progress of target device, why FX reference actually created... Conversion of 792A Power Pack into 10VDC battery-powered reference.
Main aluminum frame of 792A transfer box and power pack is actually exactly the same, only front face plate is different and guard inner shield does not have holes for shaft and terminals.
So off we go, drill missing holes for binding posts.

I need 4 posts, because FX output is 4-wire, force and sense wires are separate, so I can use it to drive resistive loads (to act as current source).



Looks somewhat decent, eh? I'll have to drill now holes in guard plate, and likely to lift PCB a bit further from shield, to avoid interference of binding posts and PCB.



Need to find out way to make white text marking on black background to add labels.



LEMO power supply output port still works as original design, providing regulated isolated +11/-11, in case I need connection to real 792A transfer unit.

[Pipelie]

Maybe you can try the sublimation transfer, that's what I'm going to try out.

[TiN]

Since February I am using one of these modules in the modified Fluke 792A power pack, that provides battery power backup and triple shielding to the reference. Module that went into the box was trimmed to 10V +/-2ppm vs all my other current references and meters, and stays there since.

Here are some more details about this design, this time thermal image map, captured by Fluke Ti32:



Darker area : warmer temperature. You can see most of heat stay near the oven drive transistor, LDO and surprise - indicator LEDs  .



My layout efforts seem to pay off well, no uneven thermal gradients around output stage amp (ADA4522-2 in this case) or output pass transistors. I'll need to test this again during max loading test (40mA load to 10V output).



This reference is using 12.5K/1K VHD200 divider, 120R VHP202, 70K VHP and 71.5K RNC90 resistors. As can see on closeup on bottom image, thermal gradients around LT1013 and resistors are less than 0.5C in worst case.

Also small update on long awaited VHP resistors - so far ETA is ~July for all sets.