CX Reference

[Edwin G. Pettis]

The point being, you're actually looking for a stable equilibrium in the environment around your circuit board, no air drafts in the enclosure.  The main reason that heater ratio is so sensitive is that there is a big power dissipation difference between the two resistors, a slight draft is going to wreck havoc.  The cheapest way to get stability in the heater divider is provide good equilibrium with no air drafts, hermetic may slow down the drift under the SAME conditions but eventually whatever is causing drift with regular resistors is going to get to the hermetic too, they are not immune to temperature variations.  The 3458A's Vref board doesn't use hermetic and you don't need to either unless you like to waste money on band aids.

[CalMachine]

After digesting all of the information put forth, going back over ap notes, and looking at already proven layouts, I think I am finally ready to get some boards made!  Any last minute quick changes that would behoove me to make, before boards are made, due to a mistake I've made?   Thanks for all of the advice thus far.

Key design changes/additions
~Finalized board size and first enclosure.  (PCB = 100mm x 50mm.  Enclusure = Hammond 1590B)
~Decided to hold off on 2x2 array until I can get a single resistor design down
~Added Ground Plane and +15V Plane
~Removed Ref_Drive connections (not driving anything)
~Added alternate footprints for Vishay Resistors
~Postive/Negative net star points, both, on front side of PCB


Notes
~Paid close attention to balance and equal out the trace lengths of the most important connections, coming to/from LTZ and star points
~Streamlined component placement
~Attempted to get most efficient layout of resistors around LTZ without copying, verbatim, off another reference layout.


I'm excited to have this project come to fruition and join the LTZ reference club.

[Andreas]

Hello,

some of my thoughts:
- where is the connection between GND and the negative star point?
- The usual stocked vishay resistors have 150 mils pin distance. The 200 mils are not so common.
- how do you mount the PCB within the housing?
- from where do you get the plastic cap? / will it also shield the soldering side of the PCB? Is there enough height in the housing?
- wouldnt it be better to place R4/R5 in direct thermal contact?
- I would put the voltage regulator within the housing (can be on a separate PCB) but D3 should be moved to the input of the voltage regulator.
  (I use LTC1763 as voltage regulators which have a built in reverse polarity protection but are for maximum 20V input voltage.)
- after 3 accidents on unbuffered references (each time loosing 2-5 ppm + a horrible drift the next 6 months),
   I appreciate the output buffer on my newer design.
- no EMI measures? (except the housing, but without line filtering)

with best regards

Andreas

[CalMachine]

Hello,

some of my thoughts:
- where is the connection between GND and the negative star point?
- The usual stocked vishay resistors have 150 mils pin distance. The 200 mils are not so common.
- how do you mount the PCB within the housing?
- from where do you get the plastic cap? / will it also shield the soldering side of the PCB? Is there enough height in the housing?
- wouldnt it be better to place R4/R5 in direct thermal contact?
- I would put the voltage regulator within the housing (can be on a separate PCB) but D3 should be moved to the input of the voltage regulator.
  (I use LTC1763 as voltage regulators which have a built in reverse polarity protection but are for maximum 20V input voltage.)
- after 3 accidents on unbuffered references (each time loosing 2-5 ppm + a horrible drift the next 6 months),
   I appreciate the output buffer on my newer design.
- no EMI measures? (except the housing, but without line filtering)

with best regards

Andreas

Thank you, Andreas!  Catching many of my mistakes

-The PCB will not be explicitly mounted.  It will sit snug (although freely) in foam/padding.
-Plastic cap will be 3D printed by fellow EEVBlog member and friend, MM.  The cap should have a little clearance within the enclosure.
-What advantage would using a voltage regulator over my current lay, bring?
-R4/R5 I was contemplating reworking to get them both vertical next to each other.

Can you elaborate on your output buffer setup?

What would you suggest for EMI measures?  I ran traces in tandem and tried to reduce loop area, atleast the ones I thought were necessary.


Fixed grounding net / start point issue and a few other minor tweaks.

[nikonoid]

Cool design, CM. Not that know a lot about the stuff, but let me through few ideas in anyways.

You are trying to thermally insulate reference from the rest of environment. Out of all materials we are dealing with the air is actually the best thermal insulator. With that in mind you probably want to make sure the foam is not touching the cap. Additionally with cap in place the most of thermal exchange between reference and outside world will be the PCB material. I would route a channel around the reference just inside of the cap or just outside of it. Having less of PCB material there should minimize thermal exchange.
Let me know if this makes sense or if I am completely off base.

Additionally have anyone considered making a dual zone temperature controlled unit? In a other words this board that you are designing would have its own heater/regulator to keep it at 40-50C then foam would go on the outside of it (not inside), to be enclosed by another metal box. This also could allow the use of resistors that have slightly inferior tempco, but possibly other good qualities, like long term stability, noise, price, etc.

[Andreas]

-What advantage would using a voltage regulator over my current lay, bring?
Can you elaborate on your output buffer setup?

What would you suggest for EMI measures?  I ran traces in tandem and tried to reduce loop area, atleast the ones I thought were necessary.

Hello,

PSRR measured on 2 of my units is -0.3ppm/V without voltage regulator.

Possible output buffer attached. (other possibility see DATRON reference cirquit).
R23 is not populated in a 7V reference.
R25,C24 will be added in my next revision to improve behaviour in case of a short cirquit on the output of the buffer.
(be aware that many precision OPs have diodes between their inputs: so above 0.7-1.4V they are shorted against each other).

I use capacitors to do EMI filtering: see LTZ1000 thread
 (but you would also need slight changes against the datasheet cirquit to maintain stability).

Dr. Pyta uses ferrite beads.  (I would add at least one additional into the negative power supply).
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg941288/#msg941288

Reducing loop area helps against magnetic pick up. But not against common mode noise from the mains line.

with best regards

Andreas

[Kleinstein]

Air is a good isolator, but if the air space is too large, there tend to be thermally driven convection. Convection will depend on the orientation, air pressure and will make a kind thermal noise from fluctuations in the temperature if the air current starts to get turbulent. So it is a good idea to a thermal setup where convection currents are not significant and at least not turbulent. As a rule of thump this mean avoiding air spaces with significant temperature differences that are larger than a few millimeters.

For the board usually the copper traces are more important than the pure FR4. The purpose of cuts is more to reduce mechanical stress. The TO99 case is not very sensitive to board stress though. So one can likely get away without cuts.

A very good insulation the the reference is not needed and not even wanted - the regulator circuit needs some heat flow to work well. The main task is to avoid convective air currents and drafts in a system with a fan.

EMI can be an important point, especially with mobile phones everywhere. However in the GHz range EMI tolerant design can be tricky - down to the black magic level for those of us not used the RF signals. Ferrite beads and feed-through caps are likely a good idea. However the output signal is kind of sensitive to capacitive load, which makes EMI filtering even more tricky.

[CalMachine]

~* Update *~

I sent OSH park my latest revision and received it back a few weeks ago.  I've been waiting to receive resistors from Hifi.  Hooray snail mail from China!   Without realizing the PCB thickness would be reduced, I sprung for OSH park's extra copper fill (2 oz instead of 1 oz).
These boards are very similar to the boards Datron used for their reference modules.

Attached are some pictures of the unpopulated board, populated board in it's housing, and a 1 hour graph monitoring output and ambient temperature.  Blue line is Voltage measurement, Yellow line is temperature measurement via BME280 within 15 cm of the CX box

I don't currently have a cap on the LTZ, so I think the ~0.2ppm/ºC I'm observing is not too bad.

[CalMachine]

I was able to snap a few thermal images with a Fluke Ti9 that is currently here. 

I know not all of the materials here have the same emissivity, so there isn't a whole lot of concrete data that can be drawn.  But, you can still locate hotter/colder spots on the board.

[cellularmitosis]

Possible output buffer attached. (other possibility see DATRON reference cirquit).
R23 is not populated in a 7V reference.
R25,C24 will be added in my next revision to improve behaviour in case of a short cirquit on the output of the buffer.
(be aware that many precision OPs have diodes between their inputs: so above 0.7-1.4V they are shorted against each other).

I use capacitors to do EMI filtering: see LTZ1000 thread

Andreas, I'm looking to add a similar output buffer to my design and wanted to understand your design.

I started with this (first attachment).

The 1k resistors were added because of https://electronics.stackexchange.com/a/56736

Considering your comment about op amp input diodes, the 1k isn't ideal (7V through 1k into a short is 7mA, and we probably don't want to load the LTZ that heavily) -- is that why you went with 10k?

Can you talk about C24?  I see it forms a low-pass filter with R25, but is it a concern that its energy would be dumped into the op amps input diodes if the op amp output were shorted?

Is R21 there because op amps don't like capacitive outputs (C23)?  And is C23 there to absorb EMI in the measurement leads?

What is C22 for?  To slow down the loop?  Or is that for EMI as well?

Thanks!

[Kleinstein]

An important function of the resistor is some additional filtering. This is higher frequency noise of the reference circuit and those high speed current spikes from the chopper amplifier. So an additional resistor to ground at the OP is likely a good idea.

With the LTZ reference circuit load current comes from the OP, not the reference itself. The LT1013 should be OK with 7 mA worst case. The trouble with too much load is that if the voltage drops, the temperature regulation will go to a higher temperature, possibly too high. So it might be a good idea to have a power limit for the heater, just in case and startup.

[Andreas]

Hello,

I would not load the LTZ output with more than 2 mA.
So the sum of the feedback and +Input resistor should be around 4K in sum at least.
1K for both is lower than that.
The 10K value is mainly for having a possibilty of a 7->10V transfer together with the 25K Z201 Resistors that I already have.
And both resistors should be equal to equalize the bias current errors.
C24 is intended as low pass and EMI reduction. Its energy into the diodes is limited by R22.

Yes R21 is the standard measure together with C22 to isolate capacitive loads. (see any old analog data book).
(Adapt the values when you want to use different amplifiers).

C23 is against EMI. Without it I had several tens of uV difference between unbuffered and buffered output.
(the offset of the LT2057 is much lower).

C22: see above.

@Kleinstein: try the 7mA with your own reference before writing (and do not tempt others to do silly things).

with best regards

Andreas

[CalMachine]

I think it's about time I've posted some measurement results of the first spin of CX Reference.  Finally have the equipment to allow me to properly test CX at home! 

There was a few weeks there of headache due to wild TC's and instability because of the series zener I attempted to try to limit the power the heater would see at turn-on.  That zener was removed and the circuit fell back more in line with how it should perform.  In hindsight, I think I chose the wrong zener and the heater was being starved of the much needed current to properly ovenize the buried zener.

I'm observing ~0.15 ppm/ºC.  You can also see the TC of my measurement setup rearing its ugly head when my heat kicks on!  Interesting stuff     Next will be TC adjustment attempts

Plot 1: ~8hr run of Vref vs. Time

Plot 2: Same run Vref vs. Box Temp

[cellularmitosis]

There was a few weeks there of headache due to wild TC's and instability because of the series zener I attempted to try to limit the power the heater would see at turn-on.  That zener was removed and the circuit fell back more in line with how it should perform.  In hindsight, I think I chose the wrong zener and the heater was being starved of the much needed current to properly ovenize the buried zener.

Since I've implemented the heater zener as an option on my board as well, I'm interested in which zener you saw problems with.  I initially went with a 3.3V zener on a 15V VCC, which is pretty conservative.

[CalMachine]

There was a few weeks there of headache due to wild TC's and instability because of the series zener I attempted to try to limit the power the heater would see at turn-on.  That zener was removed and the circuit fell back more in line with how it should perform.  In hindsight, I think I chose the wrong zener and the heater was being starved of the much needed current to properly ovenize the buried zener.

Since I've implemented the heater zener as an option on my board as well, I'm interested in which zener you saw problems with.  I initially went with a 3.3V zener on a 15V VCC, which is pretty conservative.

I had it populated with an NZX2V4.  Don't ask me why I chose that one back when I was putting the circuit together... 

https://assets.nexperia.com/documents/data-sheet/NZX_SER.pdf

[cellularmitosis]

That's only a 2.4V zener, so that shouldn't be a problem unless you are trying to run the circuit on a fairly low VCC.

You can measure the voltage drop across the 2N3904 transistor, and if it is greater than 2.4 + 0.6, then the zener isn't the problem.

Edit: oops, I meant to say that if you short the zener, and can then measure more than 2.4 + 0.6 across the transistor, then the zener isn't a problem.  Alternatively, if you measure more than 0.6V across the transistor with the zener still in-circuit, then the zener isn't the problem.

[CalMachine]

I'm sorry by the time you suggested the measurements, the diode had been removed. 

As of now, I've tweaked the TC of the board to <0.1 PPM/ºC.  I was observing a few orders of magnitude greater TC with the diode in place.  I had played with input supply voltage up to 15V while testing the board in ambient.  I, however, did not test input voltage levels during TC tests.

Data to come soon!

[CalMachine]

I've done some TC tweaking to the first revision of the CX layout.   This board has a normal LTZ1000 populated and started off with 402k for my R9.  Calculator value in screen caps is a ~ppm/ºC estimate.   As I increased the R9 value, my TC slowly dropped.  I ended up stopping at ~ 0.08 ppm/ºC.


For all of the graphs.  Ambient Temp, RH, Box Temp and 3458A temp are on the right secondary Y-axis.  Voltage measurements on the main Y-axis

Nominal R9 @ 402k


@ 470k


@ 560 k


@ 1 M


@ 2 M


@ 3.3 M




Rev 2 of CX reference is currently undergoing TC tweaking.  Here is a screen cap of TC data  with R9 value @ 150k.  Observing some funky jumps and hard to determine cause due to little apparent TC.




Still have Cells 1 and 2 of rev2 to tweak the TC on while I'm currently working on the layout of rev3 (which includes LDO and 10V booster)

[cellularmitosis]

Fantastic!  I think this is the first post I've come across with this much detail on fiddling with the 400k resistor.

Any chance I could trouble you to render those graphs using the same Y scale?

[kj7e]

At 3.3M you had the best results, did you test it with out the resistor populated?

[CalMachine]

Fantastic!  I think this is the first post I've come across with this much detail on fiddling with the 400k resistor.

Any chance I could trouble you to render those graphs using the same Y scale?

How about I throw you the CSVs?  It takes awhile to do these large data sets on my  laptop, here.

At 3.3M you had the best results, did you test it with out the resistor populated?

I did not.  I figured I was getting pretty close to open that ~0.08 ppm for a first rev board was pretty good..  and I wanted to get testing on the rev2 boards

[cellularmitosis]

Oh, nice, I see the CSV's here: https://xdevs.com/cm/teckit_test/

[CalMachine]

Oh, nice, I see the CSV's here: https://xdevs.com/cm/teckit_test/

You found my little stash

Here are pics of my tecbox setup, as well!  For those that are interested.

https://xdevs.com/cm/TECBOX/

[Pipelie]

the funky jumps look like have something to do with the room temperature and the TC of 3458.
did you turn on your AC?

[CalMachine]

the funky jumps look like have something to do with the room temperature and the TC of 3458.
did you turn on your AC?

The small oscillations are definitely coming from the TC of the 3458 in the test setup.  It's been in the single digit ºC here, so I've got my heat on.  Some of the abnormal offset jumps could be from my dishwasher or clothes dryer turning being used during the test.  I've not had the setup going long enough to really weed out my environmental disturbances.  Also, the RPi time stamp is off by a few hours.