LM399 based 10 V reference

[dietert1]

Looks good. I'd have five proposals:
- Select a metal enclosure and make board to fit the mounting holes
- LM7818 instead of LM7812 for better LM399 oven performance, mount to enclosure
- Solder or screw terminals instead of supply socket
- Add separate output Gnd terminal
- If PTC heater won't be used, move temperature sensor closer to the 10 V to 7 V divider resistors

Regards, Dieter

[Birb]

Thanks for your feedback!
Perhaps I was slightly unclear - the PTC heater will remain in use, but I do not need the 5V regulator to be placed together with it.
However, using a higher voltage might cause the 5V regulator to dissipate a larger amount of energy and that may cause heating issues - I'll see.
Actually, the temperature sensor will have some wires to extend it to be inside the enclosure for the resistors (I have some insulating fiber/ foam material)
As for a metal case, I will probably use an extruded aluminium one.
The ground terminal is likely connected through external wires.

The socket was chosen because some 24V lithium battery packs, as well as most power plugs use this type of connector to connect.
I could consider to break it out and use wires directly, I suppose.
Though I haven't figured out a good way to keep it on continuously.

[iMo]

It has been discussed here many times - the voltage regulator inside the box or close to the reference circuitry is not the best idea in case your input voltage will vary (ie a battery or mains transformer). That is because the heat dissipation of your 7812/18 will vary with the input voltage changes..

[Birb]

Then perhaps I will separate the PCB into two, to try and isolate the heat better.
Or I may need to figure out a different power solution?
Though using a higher voltage rated PTC may be able to allow me to continue to heat the resistors without adding the 12V to 5V conversion, at the expense of higher temps for the resistors and current draw.

[iMo]

There are two options (when using linear regulators, what is usually recommended with Vrefs):

1. to leave your voltage regulator on the Vref board as-is in your design, AND to add the second one in front of it and have it mounted outside the Vref board
2. to have a single voltage regulator outside the Vref board (thus not thermally coupled to the Vref circuitry).

Mind the chip temperature of ANY linear voltage regulator is aprox:

Tchip = (Vin-Vout)*Iout*Rthja+Tambient, where the Rthja is the thermal resistivity of the junction to ambient (a constant).

[Birb]

Thanks for the feedback.
By second one, do you mean the 12V to 5V linear regulator? Thanks.
At 12V the temperature is around 60C
I haven't figured out how to effectively reduce the setpoint, but it may include adding a layer of insulation between the PTCs and the resistors, so the resistors are kept at ~50C or lower (preferably)
Is this more optimal?

[iMo]

Thanks for the feedback.
By second one, do you mean the 12V to 5V linear regulator? Thanks.

The second option means your "input to 12V" regulator (the 7812) should be off of the voltage reference board.
Also the 7805 powering your PTC will change its temperature as well - its in/out voltages will be "constant" but the current will change.
Simply look at the above equation and try to apply it to your design accordingly.. 

[Birb]

Random new question / idea / whatever:
I just realized that it is possible to use one single octal resistor array to achieve the necessary resistances:
Using 6 resistors, it's possible to make a 1:2 1/3 resistor ratio, equivalent to a 3/7 divider ratio.
Then, using the two remaining resistors, if chosen correctly, we can have an approximate 3k resistance to set the zener current.
8x 5k resistors work for this, as two 5ks in parallel give 2.5k, which would mean a zener current of 1.2mA.
The question is, how much impact does the 0.2mA have on the LM399?

However, something like this is inheritly non-trimmable, but it would probably have a tiny footprint and be relatively stable.
One more thing: Is it advisable to insulate the resistor array together with the LM399 to achieve better thermal stability for both, or would this be detrimental to the resistors, due to the high oven temperature of the LM399?
Thanks!

[Andreas]

The question is, how much impact does the 0.2mA have on the LM399?

Mhm 0.2 mA with max 1 Ohm dynamic resistance gives (constant) 0.2 mV higher output than nominal.
Besides this the ageing rate will sligthly increase (ageing parameter is given based on 1.000 mA)

However, something like this is inheritly non-trimmable,

No, look e.g. at the AD587 datasheet how external trimming can be done by a pot and a additional "RT" resistor.

One more thing: Is it advisable to insulate the resistor array together with the LM399 to achieve better thermal stability for both, or would this be detrimental to the resistors, due to the high oven temperature of the LM399?

I personally would isolate heat sources like voltage regulators and LM399 heaters from the rest of the precision cirquit. (OP-Amp, voltage divider)
Otherwise you have large heat gradients depending on the distance to the heat source.
I would probably also use an extra resistor for the Zener diode to remove heating from the voltage divider.
And would use less than 10% of the specified power rating.

The voltage divider itself should be a high stable one (e.g. thin film resistors like TDP1603 series).
From my ADR1000#01 I know that it needs about 2 years until the resistor array gets stable.
See diagram with divider drift here:
https://www.eevblog.com/forum/metrology/lowest-drift-lowest-noise-voltage-reference/msg5617061/#msg5617061

with best regards

Andreas

[iMo]

The question is, how much impact does the 0.2mA have on the LM399?

Mhm 0.2 mA with max 1 Ohm dynamic resistance gives (constant) 0.2 mV higher output than nominal.
Besides this the ageing rate will sligthly increase (ageing parameter is given based on 1.000 mA)
..

The zener current itself in the 399 is set fixed to 250uA afaik, it had been discussed here many times in past. The same with 1399 (but different current). With higher currents (like 2mA) the 1.75mA is diverted off the zener (plus minus)..
Thus you cannot set the zener current in 399 or 1399. The small differences you see while measuring the voltage vs. current come from a nonlinear characteristics of the circuitry around the zener (see the schematics in the datasheet).

[Alex Nikitin]

The small differences you see while measuring the voltage vs. current come from a nonlinear characteristics of the circuitry around the zener (see the schematics in the datasheet).

Not as the result of a nonlinearity, only of a limited open loop gain.

Cheers

Alex

[Birb]

Hmm, I think my real question should be about how the 0.2mA affects drift.
Is it really a big difference?
Also, how does it affect settling times of the references? Thanks
I mainly want to try to avoid any extra resistors as this can keep the entire setup simple and component count minimal.

Also, is it better to have multiple references in parallel or put the LM399s themselves in parallel with each other?
(The advantage of small units is several could be placed together to form a more accurate(?) unit.)
(Though I note the ratio of 3:7 is really idealized and as far as my "testing" is the voltage is more like 7.04ish, so that would be a bit higher than wanted.)

[iMo]

Hmm, I think my real question should be about how the 0.2mA affects drift.
Is it really a big difference?
Also, how does it affect settling times of the references? Thanks
I mainly want to try to avoid any extra resistors as this can keep the entire setup simple and component count minimal.

Also, is it better to have multiple references in parallel or put the LM399s themselves in parallel with each other?
(The advantage of small units is several could be placed together to form a more accurate(?) unit.)
(Though I note the ratio of 3:7 is really idealized and as far as my "testing" is the voltage is more like 7.04ish, so that would be a bit higher than wanted.)

1. No, 2. No, it does not affect,  3. you may put N 399s in "parallel" (it does not mean the zeners are all parallel, they have separate zener cathode rezistors) via N summing resistors into a single common 7->10V amplifier (if required), 4. the output noise will drop sqrt(N) times down, where the N is number of "paralleled" refs (like 4 refs will do aprox 2x less noise).

[Kleinstein]

Instead of in parallel, I would consider the new ADR1399 for about half the noise, comparable to 4 x LM399 in parallel.

The actual voltage of the LM399 can vary quite a bit between units. They can be above or below 7 V. A parallel version would need extra resistors for averaging and separate paths for the current. One could do the averaging at the 7 V level or the 10 V level to also average over amplifiers and resistors.