I am new to all this and not in any way an expert. What I have gleaned reading lots of posts the two critical areas of the circuit are the divider used to set the heater temp and the resistor used to set the Zener current. For the divider, the ratio rather than the resistor values is the important parameter, it needs to have the best TC and long term drift characteristics. There is some debate as to exactly what ratio to use, seems to depend on your view of long term drift of the device vs max environmental usage temp. The
Forget the useless debate.
It is the maximum environmental usage temp that puts a floor under the operating temperature. If you want the reference to be stable with varying ambient temperature the heat regulation has to work properly. Give it some headroom by fixing the regulator temperature adequately above. Even with the heater disconnected, you have a temperature increase. In my case of the "non-A"-version it was like 3-7 degrees celsius if I remember correctly.
There is a really helpful description how to measure that temperature posted by doc Frank somewhere.
That description discloses the real operating temperature of your reference.
The precision of that measurement depends on two things:
1. Your LTZ, being off, having adapted ambient temperature.
2. Your multimeter being fast when measuring the initial Ube-voltage relating to ambient temperature.
Once you found that temperature figure you can calculate how to change the operating temperature taking into account the individual Ube of your LTZ in question.
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Not looked too much at the WW resistors but have looked at the Vishay Z201 and VSMP smt metal foils. Their sweet spot seems to be in .01%, they don't offer much at lower precision.
In case of the VSMP the table in the datasheet looks like the absolute resistor value determines the possible precision and TC.
If you need 120 Ohm you have no choice and the resistor does not get better if you order it in 0.005%.
I have the impression that everybody is using through-the-hole resistors for the divider instead of SMD ones.
Your possible options are:
1. Using simple metal film resistors (p.e. Yageo MF).
I used them in my prototype and was rewarded with a shaky output of a couple of ppms up and down upon temp variations of +- 5 degrees celsius in my room.
2. Using something slightly better, like PTF56. At least the spec sheet shows better figures than the standard Yageo metall film resistors. I replaced the 120 Ohm and the divider and my prototype looked stable then.
3. Use metal foil or WW resistors. Examples: Vishay S102, GR's "econistor" sold by Rhopoint or the often quoted "Ultrohm Plus" manufactured by fellow member Edwin Pettis.
4. Use something special with good tracking ratios:
a) poor man's tracking solution: try to couple your resistors thermally with copper foil (see pictures from Andreas, p.e.)
b) LT-5400 can be used for a certain divider ratio
c) Vishay 300144 + 300145 and their Z-variant offer you a divider product manufactured according to your specs
5. Expensive solution with high impress factor: Order your resistors or your divider in an hermetically closed housing. (see pictures from tin). Doc Frank did the calulation, conclusion: This luxury does not improve the performance. But if it makes you sleep better, why not?
A normal .1% film resistor is going to have way worse TC and drift than the Vishay foil resistors. Going to .005% or .001% adds a lot to the cost. Still need to look at WW resistors. A custom Vishay .01% VSMP runs about $13 so it's not a killer for the three critical resistors. Vishay can make almost any value by programming shunts included on the raw stock material.
Yes, but the precision (.1%) is not the determining factor. You get Vishay S102 in 1% and all standard foil properties apply regardless of precision. The only difference with 1% is that the laser cutting the resistor templates stops cutting earlier.
One of the trimmers is used for the 7-10V converter -- didn't want to select resistors. The other trimmer is on the heater divider to play with the temps but will be fixed in the final design. The board uses normal 1% resistors and not built to be an accurate reference, it's just for exploration and burn-in.
Craig
Out of the data sheet for a simple trimmer in 10K:
General offer ±250 ppm/℃ (±100ppm/℃ is available if => 250ppm through the wiper!
Pettis recommends Bourns 3250 and 3290 (expensive, but. mil-grade)
Fellow member branadic used one of those Vishay Models: 533, 534 or 535 (still expensive)
Regards
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