Electronics > Metrology

LM399 based 10 V reference

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Birb:
When it’s stated the NOMCT has better passivation does this apply to the NiCr NOMCTs or does it mean the TaN NOMCTs?
Thanks!

Kleinstein:
The difference seems to be from TaN based resistors to NiCr based resistors. From what I understand it is not so much the passivation, but more the substrate roughness and film thickness. TaN tends to be thinner and NiCr is easier to etch to a fine line pattern.

With only a LM399 reference the extra 1/f noise from a bit noisy TaN resistors in a 7 to 10 V gain stage would be hardly noticable (maybe comparable to an OP07 OP-amp).  It is still easy to choose the lower noise NiCr version.

Birb:
The (probably not so good) artifact 10V reference:
Should achieve 10V with relatively ok accuract: +- a few mV
Stability is hopefully good?
Tempco is probably very low because LT5400s are used instead of NOMCT (Ratio drift is lower)
No clue about op amps though (LT1012 has a drift of ~0.2uV/K)
Also not exactly sure the best way to design a battery charger, so there is a very naive implementation - probably need to change that
(And the battery voltage has to be ~18V to accomidate the LDO regs to work at 15V (The LM317 is there to regulate to around 16V in case a 24V battery is used, at the expense of wasted power!)
Entire thing is best separated into 4 boards (3 references + 1 battery charger), but I show it as a complete schematic.

(this entire thing could be not very good (or cost effective!) but it's an idea.)
(Not shown: AC input capabilities, transformer)

Kleinstein:
There is no need for separate regulators for the references. A single reference for mupliple units should be fine.

The idea with mixing different gain setting to get closer to 10 V only works if the references are relatively close to nominal. So one would have to select the refenrence with a suitable voltage. The actual voltage of the reference would determine which type of circuit is more suitable. For the fine trim one would very likely still need some extra adjustment points.

With 3 reference in parallel one also needs to take care at the ground side. The ADR1399 ref. current should be more like 2 mA and this may already add a bit drop at copper traces, especially if one would use separate PCBs.

The LM399 and likely also the ADR1399 usually has little hysteresis and no real need to have it powered 24/7. So battery power would only be for a realatively short time when actually needed. So no need for hot shipping and similar things as with the really high end reference.

Birb:
Thanks for the advice.
I added three regulators mainly because of the initial inrush current of ~100mA, which is quite a lot (And 100ma is the upper limit of the LT1761)
On that note, how can this current be easily delivered during startup? Or does it not really matter and even a lower current will startup just fine given sufficient time?
(Do I need a startup resistor? Are there methods that affect this less? Thanks)

As for the battery thing, I guess it's just "best practise" or something, though I might try it in a future project when I need to.
On that note, how important are the three resistors that do the averaging?
(Actually i can just do the math - but i'm too lazy so desmos it is)
(Based on a numerical estimate, assuming the tempco of only one of the 10.111V reference's resistor is different, the total tempco of the ratio is some 0.0689 of that)
(I.e. if one had a tempco of ~500ppm/K and all the others at 0ppm/K the total tempco would be ~35ppm)
(Thus it seems that even with mediocre matching the difference might be almost immeasurable with what I have)

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