CX Reference

[CalMachine]

I know what you're probably thinking...  Another damn X"X" reference?!

But, after many of poking and prodding from fellow community volt-nutters, I come here with the beginning stages voltage reference project! 

CalMachine EXperimental
Main design choices I wanted to include:
~Implement Main LTZ Reference Circuit with few tweaks.
~Use mostly THT/DIP components
~LTZ1000 non-A ~LTZ1000A
~2x2 Resistor arrays (PWW technology)

End Goals:
~A more stable reference than the original circuit.
~Eventually design PWM DAC implementing this as the primary reference

Current schematic iteration and initial PCB component placement tweaking is attached. 
I am new to, and very inexperienced, with KiCad..  There will be many amateur mistakes made here, so bear with me

This is just 1 of the many VRef ideas I want to pursue.  The thought behind using 2x2 resistor arrays is to cut down on self heating and power through the resistors, reducing their lifetime stress and aging.  I know the drift of resistors are attenuated by many factors... but it is still something I want to try out.  The main reasoning behind using a non-A LTZ is so I can run at a lower setpoint, also decreasing long-term aging.  And also as a side-thought, lower temp setpoint will induce lower thermal gradient on the PCB and in the enclosure.

Comments/questions are welcome as this project is in its infancy.

[ManateeMafia]

Thanks for posting. There's always something to learn from doing it differently.

Have you considered interleaving R4 and R5?

[mimmus78]

With four resistors you are starting playing with big numbers ... so you will get less melons but also less outsiders.


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[chuckb]

Good luck with the design!
I would make D3 a 1N400X series diode to handle the surge current for the electrolytic caps.
U1 needs a 0.1ufd cap directly across it's power pins.
As I mentioned in the main LTZ thread a few days ago, installing a zener in the collector of Q1 can help to prevent excess stress on the chip and to prevent damage.
You will want to minimize the distance (copper resistance) between the M2 and the M3 nodes.

Will this be for you lab use only or will it be transported?

[Kleinstein]

I don't think it is worth using 4 resistors each in stead of a single one. This drives up the costs, but with very little advantage. There is not much to improve one the standard LTZ1000 circuit. The important point is to get the layout right so that not too much copper resistance is included and to keep RF noise out. A suitable stable thermal layout can also be important. If at all one might consider the modification that allows more capacitance at the output and thus more filtering there.

Doubling might be an option for the heater transistor: having a symmetric layout with 2 transistors could be used to reduce the possible temperature gradients due to the heater transistor(s). If well distributed, there heat could even be considered a partial temperature stabilization for the whole circuit. Another point is a limitation of the heater power to prevent damage - just in case.

[CalMachine]

Thanks for posting. There's always something to learn from doing it differently.

Have you considered interleaving R4 and R5?

I have not!  I could possibly see that being beneficial..  To help with thermal coupling of the resistors?

Good luck with the design!
I would make D3 a 1N400X series diode to handle the surge current for the electrolytic caps.
U1 needs a 0.1ufd cap directly across it's power pins.
As I mentioned in the main LTZ thread a few days ago, installing a zener in the collector of Q1 can help to prevent excess stress on the chip and to prevent damage.
You will want to minimize the distance (copper resistance) between the M2 and the M3 nodes.

Will this be for you lab use only or will it be transported?

Thank you for the advice!   These changes should make their way into the next revision.

This will mostly sit in the standards lab on the DP scanner and not move.  For transporting, I've got 2 732Bs in the lab.  Once of which is my personal unit (which I got fairly recently) and the other is the lab's main transfer reference that we've had for many years now.

[Edwin G. Pettis]

There will be little to gain from quadrupling the resistor count, particularly with PWW, the power levels are very low and will have no significant effect on them as far as aging goes.  In the case of film/foil, having a very small mass for a resistor element, the effects, however small, would be multiplied due to the very small mass.  In either case, once the unit has achieved equilibrium and stays at a relative constant temperature, everything will settle down to a minimum drift with time, all it takes is patience, it will not happen in months, it takes years for it to happen.  Even accelerated aging has its drawbacks.

[Kleinstein]

The possible lower temperature in the non A version is a two sided thing: Aging is slower, but this also means it takes longer to get a "stable" state. The difference is not that large anyway. With an additional about 300 K/W and a typical power dissipation of the LTZ1000 (without the heater) of some 30 mW it's about a 10 K higher temperature that is needed for the A version. An extra 10 K will speed up aging by something like a factor of 2 to 4. With a relatively high temperature there is a chance to get the fast processes settled after 1 year or so, so that after that mainly a single slow process will remain active and thus drift gets predictable and allows for extrapolation.

The A version allows a lower power consumption, despite of the higher temperature set-point, unless very good thermal isolation around the non A version is used - but this tends to reduce the performance of the temperature regulation.

The temperature gradients on the board are more due to the power level. With the A version more of the temperature gradient is inside the case and less outside. So the A version can help to build a more compact version.
The logical way to fight temperature gradients (especially those than can change) would be a metal case all around the board and a rather compact design. The natural place for the heater controlling transistor would than be mounted to the case.  An always slightly elevated temperature can also help with humidity effects, especially if the unit is always on.

[CalMachine]

The possible lower temperature in the non A version is a two sided thing: Aging is slower, but this also means it takes longer to get a "stable" state. The difference is not that large anyway. With an additional about 300 K/W and a typical power dissipation of the LTZ1000 (without the heater) of some 30 mW it's about a 10 K higher temperature that is needed for the A version. An extra 10 K will speed up aging by something like a factor of 2 to 4. With a relatively high temperature there is a chance to get the fast processes settled after 1 year or so, so that after that mainly a single slow process will remain active and thus drift gets predictable and allows for extrapolation.

The A version allows a lower power consumption, despite of the higher temperature set-point, unless very good thermal isolation around the non A version is used - but this tends to reduce the performance of the temperature regulation.

The temperature gradients on the board are more due to the power level. With the A version more of the temperature gradient is inside the case and less outside. So the A version can help to build a more compact version.
The logical way to fight temperature gradients (especially those than can change) would be a metal case all around the board and a rather compact design. The natural place for the heater controlling transistor would than be mounted to the case.  An always slightly elevated temperature can also help with humidity effects, especially if the unit is always on.

Thanks for the valuable information    Always a pleasure reading your posts.  I kind of like the Q1 mounted to case idea.  How compact would you say 'compact' is?  The current board I have is 150x100mm and it's the nearly the exact size of one of the aluminum enclosures I have.   Thermal gradients would better be tackled with a smaller enclosure?

I don't think it is worth using 4 resistors each in stead of a single one. This drives up the costs, but with very little advantage. There is not much to improve one the standard LTZ1000 circuit. The important point is to get the layout right so that not too much copper resistance is included and to keep RF noise out. A suitable stable thermal layout can also be important. If at all one might consider the modification that allows more capacitance at the output and thus more filtering there.

Doubling might be an option for the heater transistor: having a symmetric layout with 2 transistors could be used to reduce the possible temperature gradients due to the heater transistor(s). If well distributed, there heat could even be considered a partial temperature stabilization for the whole circuit. Another point is a limitation of the heater power to prevent damage - just in case.

I realize a design with 2x2 resistor arrays is going to drive up cost.  I don't care.  I know there is not much to be improved.  Hence, why I'm mostly sticking with the basic reference circuit and very minimal changes. 

I think where the added difficulty from these tweaks come when I delve more into the board layout.  I'm intrigued by the double transistor idea.  I'll look into that more. 

LTZ1000 non-A
... The main reasoning behind using a non-A LTZ is so I can run at a lower setpoint, also decreasing long-term aging.  And also as a side-thought, lower temp setpoint will induce lower thermal gradient on the PCB and in the enclosure.
Comments/questions are welcome as this project is in its infancy.

Hello CalMachine - the LTZ1000ACH has a much better thermal insulation - 400K/W vs. 80 K/W for old LTZ1000CH part - this means you need to pump in less energy in system for -A part vs. non -A to reach same oven setpoint ... ? Are you sure about your above statement in bolt ?

cheers
Butterfly

I'm fairly certain I'm right in that train of thought.  I could always be wrong though. 

Here's what the datasheet says.

Because higher temperatures accelerate aging and decrease
long-termstability, the lowesttemperature consistentwith
the operating environment should be used. The LTZ1000A
should be set about 10°C higher than the LTZ1000. This
is because normal operating power dissipation in the
LTZ1000A causes a temperature rise of about 10°C. Of
course both types of devices should be insulated from
ambient. Several minutes of warm-up is usual.

There will be little to gain from quadrupling the resistor count, particularly with PWW, the power levels are very low and will have no significant effect on them as far as aging goes.  In the case of film/foil, having a very small mass for a resistor element, the effects, however small, would be multiplied due to the very small mass.  In either case, once the unit has achieved equilibrium and stays at a relative constant temperature, everything will settle down to a minimum drift with time, all it takes is patience, it will not happen in months, it takes years for it to happen.  Even accelerated aging has its drawbacks.

You can just call me the Ronco Showtime Rotisserie Oven..  I'm just going to set it and forget it 

[mimmus78]

You can just call me the Ronco Showtime Rotisserie Oven..  I'm just going to set it and forget it 

You know it never ends this way, aren't you? :-)

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[Kleinstein]

With the A version the extra temperature gradient will be inside the case. Despite the higher set point the case of the ltz1000A can already be at a lower temperature than with the non A version. The higher internal temperature gradient seems to be no problem at all. The A version seem to have an even lower TC and thus does not need the 400 K external compensation resistor.

I would consider something like the HP ref in the 3458 or the Datron LTZ module (kind of cheating with a special resistor hybrid) relatively compact. If not so large resistors (like the Vishey foil types) and cap form factors are used something like 40x80 mm should be possible. With a cover over the LTZ there should be no need for a really large distance. Not sure if you even need extra slots of holes for the thermal design, but they might help. I would be more afraid of large pockets of air, as this is where convection currents can develop.

One of the few point's where it standard LTZ circuit might be improved would be heater control at low power. With linearization of the power it should be possible to get higher gain and thus possibly better regulation at the low heater power end. This can help a little with performance when the heater is nearly off and thus could extend the working range a little to higher temperatures. I am not sure if it is worth it.  To see how big the possible effect is, one could do a test with a standard circuit: close the upper environment temperature limit, the low frequency noise and TC is expected to get worse. One could check how large that range is where the temperature control is not working well enough. A minimal version could be just limiting the heater power (limited voltage) and based on the limit and maybe the individual heater resistance increase the loop gain (e.g. increase the 1 M resistor and reduce the series cap). The next step would be something like resistor + LED(s) from the heater transistors base to ground, as a very crude approximation of a square root law.

[CalMachine]

Thank you everyone for your valuable feedback thus far!

I've taken into account a lot of the feedback I have received.  As something that can be added relatively easy and doesn't alter the main reference circuit terribly... I've decided to throw in a series zener on the heater side, in an effort to reduce excess stress.  In my next reference design, I would like to tackle the efficiency of the heater side of the circuit, in more depth.

I've also decided to go with the 'A' verson of the LTZ, hoping that it will radiate out less heat (outside of the TO-5 can) and cause a smaller delta from ambient inside the enclosure.  I'm hoping this will create a more stable, more uniform temp inside the enclosure/oven.  I left the temp compensating resistor R9, in case I wanted to throw a non-A reference in.

I've attached my most recent schematic and PCB revision.  Further comments/questions/advice is welcome

[Andreas]

I've attached my most recent schematic and PCB revision.  Further comments/questions/advice is welcome

Hello,

if you really use the BC639 (and not the 2N3904) be aware that the wiring on your PCB is wrong.
(the BASE is not the middle pin on BC639)

with best regards

Andreas

[CalMachine]

I've attached my most recent schematic and PCB revision.  Further comments/questions/advice is welcome

Hello,

if you really use the BC639 (and not the 2N3904) be aware that the wiring on your PCB is wrong.
(the BASE is not the middle pin on BC639)

with best regards

Andreas

Thank you for catching that!  I'll go back to a 2N3904 for now.

[Kleinstein]

It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.

The PCB lines are quite thin in areas where they don't need to, this can add to trace resistance, especially around the 120 Ohms. The sensing lines to the reference usually go right up to the chip. With the 4 mA reference current it's mOhms that can matter.

The temperature sensor should be placed where it matters, so more like at the 13K/1 K divider, but not so close to the LT1013.

The 13 K / 1 K divider is rather close to the heater controlling transistors - this might not be such a good idea, as the transistor is a variable heat source. One can not use the heat from the transistor to compensate for environmental temperature change, as the heat from the transistor and the zener is approximately  proportional to the current and the current is more like changing like a square root law with temperature. So for the transistors there are two good options: one is far away from anything critical - that is what most circuits use. The other good option would be mounted to a metal case that surrounds the whole circuit or at least the LTZ1000 part - in this case the transistor would be a secondary heater for the outer shell. However the contribution of the transistor would not be very large, so not sure it is really worth the effort.

Similar, if no extra driver transistor is used the heat from the LT1013 can be more of a possible problem.

 

[zhtoor]

It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.

series parallel combination of resistor enhances the accuracy spec.
if you connect 100 0.1% resistors in parallel, you get a resistor which accurate to .01%

moreover, series-parallel combination can also help in tempco adjustment of the composite.
ie; one leg having a +ve tempco and the other having -ve tempco balances out.

regards.

[The Soulman]

It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.

series parallel combination of resistor enhances the accuracy spec.
if you connect 100 0.1% resistors in parallel, you get a resistor which accurate to .01%

moreover, series-parallel combination can also help in tempco adjustment of the composite.
ie; one leg having a +ve tempco and the other having -ve tempco balances out.

regards.

Accuracy won't be enhanced by default, only after characterization of each resistor and matching those with opposite properties.

[zhtoor]

It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.

series parallel combination of resistor enhances the accuracy spec.
if you connect 100 0.1% resistors in parallel, you get a resistor which accurate to .01%

moreover, series-parallel combination can also help in tempco adjustment of the composite.
ie; one leg having a +ve tempco and the other having -ve tempco balances out.

regards.

Accuracy won't be enhanced by default, only after characterization of each resistor and matching those with opposite properties.

assuming a usual gaussian curve distribution of resistor tolerances, say 100 of 0.1% each, i would say
the composite (parelelled) would be pretty near 0.01%

and of course, tempco balancing would need characterization, at least 2 bins +ve and -ve tempco.

regards.

[Echo88]

Resistor-accuracy doesnt matter in this application, only long-term-drift, absolute tempco and matching-tempco of some resistors. Noise is sufficiently low due to resistor technology (pww or foil).

[Kleinstein]

The resistor noise also does not matter in this application. It would need rather tiny (e.g. 0201 SMD maybe)  or carbon resistors to get enough noise to only have chance to see the noise. A relative resistor change is attenuated by something like a factor of 100 (can be even more) at the output. The reason for using PWW, foil or good thin resistors is mainly long term stability and to small part the low TC.

[CalMachine]

Resistor-accuracy doesnt matter in this application, only long-term-drift, absolute tempco and matching-tempco of some resistors. Noise is sufficiently low due to resistor technology (pww or foil).

The 2x2 array is not being implemented to achieve a more accurate nominal value of each resistor. 

The first and biggest reason behind the 2x2 arrays is to average the drift of the critical resistors across 4 resistors.  In turn lowering the drift of the equivalent single resistor.  The lower power draw and less self heating result, was more of an after thought.  I understand now that reducing power consumption for each resistor won't make any improvements really.

It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.

The PCB lines are quite thin in areas where they don't need to, this can add to trace resistance, especially around the 120 Ohms. The sensing lines to the reference usually go right up to the chip. With the 4 mA reference current it's mOhms that can matter.

The temperature sensor should be placed where it matters, so more like at the 13K/1 K divider, but not so close to the LT1013.

The 13 K / 1 K divider is rather close to the heater controlling transistors - this might not be such a good idea, as the transistor is a variable heat source. One can not use the heat from the transistor to compensate for environmental temperature change, as the heat from the transistor and the zener is approximately  proportional to the current and the current is more like changing like a square root law with temperature. So for the transistors there are two good options: one is far away from anything critical - that is what most circuits use. The other good option would be mounted to a metal case that surrounds the whole circuit or at least the LTZ1000 part - in this case the transistor would be a secondary heater for the outer shell. However the contribution of the transistor would not be very large, so not sure it is really worth the effort.

Similar, if no extra driver transistor is used the heat from the LT1013 can be more of a possible problem.

 

Thank you for the pointers!  Keep them coming, I appreciate it.  I can see that using the 2x2 arrays is throwing a big wrench in the layout design, lol.  I'm enjoying the process though

How do you feel about the idea of making a cap, like for the LTZ1000, but for the heater transistor?

 

[The Soulman]

It is still odd using 4 resistors each for the precision ones. It usually does not help much in this application. It might if you need higher power rating to lower self heating, but this is not the case here as the critical resistors all see an essentially constant power.

series parallel combination of resistor enhances the accuracy spec.
if you connect 100 0.1% resistors in parallel, you get a resistor which accurate to .01%

moreover, series-parallel combination can also help in tempco adjustment of the composite.
ie; one leg having a +ve tempco and the other having -ve tempco balances out.

regards.

Accuracy won't be enhanced by default, only after characterization of each resistor and matching those with opposite properties.

assuming a usual gaussian curve distribution of resistor tolerances, say 100 of 0.1% each, i would say
the composite (parelelled) would be pretty near 0.01%

and of course, tempco balancing would need characterization, at least 2 bins +ve and -ve tempco.

regards.

The center of the Gaussian bell shape doesn't have to be "on target".
I.e. if the resistors on average are 0,05% low then also the bell is 0,05% shifted low.

[Kleinstein]

....
How do you feel about the idea of making a cap, like for the LTZ1000, but for the heater transistor?

An isolating cap just for the transistor would not help - the power is fixed, so all it would do is to make the transistor hotter.

What would make sense it to add a heat sink to distribute the power a little. The zener in series is a little like this, as some power will be from the zener too. A more distributed heat source would cause less gradients. Still the best place would be away from the resistors. HP put the transistor right at the far edge, with an extra isolation toward the rest of the circuit.

The other extreme would be a metal cap around the LTZ  (or the whole circuit) and mount the transistor (e.g. BD135 in this case) on the metal cap.

[MisterDiodes]

Just some general head's up - and nothing is cast in stone here, just some friendly experienced design tips keep in mind.  They are worth at least what you paid for them.

Hmmm... The quad resistors I really don't think are going to get you anything - as been shown over and over, and in the LTZ datasheet, the resistor drift is -not- driving force behind overall output drift.  You don't want absolute cheap-ass noise-generator SMT thick films for resistors (like a 34470a Vref module), but they really don't have to be ultra-precision as long as you get some reasonable quality TC.  That's the whole point and beauty of the circuit - the resistor values are forgiving, and absolute ohmic value is not too important!!  The most important is the heater resistor -ratio TC- but again absolute value is not too important; you do want to see some good stability and repeatably on those two, any problems on the heater ratio TC will certainly show up on the Vref output.

Everything that is critical and the biggest contributors to over drift are already hermetically sealed in the LTZ can.  The main contributors of overall noise and drift is your luck of the draw on each LTZ die, and it's inherent stress in the crustal lattice at the moment it was born as a single die.  That is beyond your control.

But it's your experiment and path to discovery to enjoy.  You might find out something interesting!  It all depends on your expected output noise and drift rate requirement vs. the practical limits of how every unique LTZ die is going to stabilize over time.

What you do have to watch out for (and I can guarantee can be a problem based on my own bad discovery experience) is making the board bigger than required  and adding unnecessary noise injection antennas.   PWW resistors are excellent low-noise components (And the preferred LTZ datasheet recommended choice of course) but realize the even the PWW reverse-wound winding techniques don't cancel out their self inductance 100%, so you still do want a shielded enclosure (you'll do that anyway for serious work, shielding is another key to low-noise success).

For best chance of success I would go with something similar to the proven, compact board design around the size and general layout like the 3458a module - remember you're after small temperature gradients across the board, and to give that the best chance you want the board smaller, not larger.  I wouldn't make the board so small that you can't lay the PWW down on the board - Trying to install an axial PWW vertically with that one lead waving around loose in the breeze is not the best construction, but it an work in a pinch if no other way.

Pay close attention to keeping the every current loop enclosed area as small as possible - because every current loop is also an antenna for whatever interference comes your way, and every PN junction on a current loop is considered a potential demodulator than can result in Vref noise.

Of course keep in mind Star Ref points, etc.

For all those reasons:  I wouldn't make the board any bigger than it needs to be - you want to go for an optimized, compact design.  At least that's how we do it, and that's what leads to proven success for what we need.  Your needs might be different.

[kj7e]

... The most important is the heater resistor -ratio TC- but again absolute value is not too important; you do want to see some good stability and repeatably on those two, any problems on the heater ratio TC will certainly show up on the Vref output. ...

In an attempt to identify the small TC drift I see in my two LTZ1000A based references, I was using precision heating and cooling of individual components.  I found the heater ratio resistors to be the most sensitive components leading to TC drift.  Seriously considering ordering a VHP200 1K/13K .001% match network.