LM399 based 10 V reference

[Kleinstein]

The LM399 is fine with less current (e.g. series resistor or current limit from regulator) during start up. It would just take a little longer (e.g. another 30 seconds) to reach the stable temperature. A weak regulator with not much eat sink could still be effected a little from the temperature rise causing a little voltage drift. So good cooling for the regulator may still help. I have not done the math how much the effect really is.

The TC of the averaging resistors are as important as the difference in the voltages is compared to the 10 V. So it depends on the actual ref. voltages and may be attenuated by about a factor of 100. So the resistors should be resonable matched if there is a significant difference. It is similar to a divider from the 10.11 V to 10 V, just with equal resistors and this way a bit easier to get good matching.

[Birb]

Hmm it seems like another LT5400 can be used, though I’d assume that there is attenuation on the actual tempo of the resistors, so possibly even 25ppm/K may be ok (numerical testing is ~2ppm/K).
Though I wonder: Should there be another buffer on the averaged output?
(But that adds another few uV/K which is not that desirable)

Edit: Possibly some buffer can be used like the LT1010?

[Andreas]

The LT1010 has too much input bias current. You need a additional driver for it.
But why not use a low noise chopper like ADA4522 which delivers enough current?

Take care on your cirquit.
The ADR1399 needs more than 1 mA for the zener.
At the LM317 the resistors are not according to data sheet (minimum output current + voltage divider interchanged).
I am not shure wether the LM317 survives back powering by the battery when the input voltage is off / shorted to gnd.
But in any case the battery will be discharged by the voltage divider. (which should carry 5 mA according to data sheet).

with best regards

Andreas

[Birb]

yeah, the battery circuit is not particularly good and needs changing, i’ll fix it when i can.
(Though one issue is the battery voltage, 18V is not as easy compared to 12 or 24V of gel batteries which are usually easier to use.)
(Perhaps NiMh trickle charge?)
As for the output buffer, something like the ADA4523 might be good, but i also want to know if filtering should be added in the feedback loop, similar to that in the datasheet of the ADA4523?
thanks!

[dietert1]

For our ADR1399 setup there is a 21 V 2 Ah Li-Ion battery pack i got from Aldi (meant for battery tools). The power supply it buffers runs at 20 V (2x 10 Vref). Pass transistor and battery are outside of the oven while op-amp and 2:1 voltage divider are inside the oven.

Regards, Dieter

[Kleinstein]

As a LM399/ADR1399 does not need to be powered 24/7 a separate battery pack like the power tool battery makes sense. The alternative is than direct power from mains for the initial ageing and less critical use.

The ADA4523 is overkill. It is anyway a rather extreme part with limited use: very low voltage noise, but high current noise and high power consumption that makes it less suitable for high precision (may still be OK with a 5 V supply). The OP-amps to consider are more OPA205, ADA4522, LTC2057, MCP6V51. The question there is if one needs an extra in loop driver to reduce the loading to the OP-amps.

[iMo]

I've been using mains+trafo+uA723 w/ aprox 15V for the 1001. And an external post for a 21V 1AH LiIon battery pack when transferred the reference to the metrology offices. BTW., both sources could be wired at the same time (ie. the battery wired via a simple diode with a "trickle charging" resistor in parallel, wired to the input of the 723). Simple and it works fine.