[volt-nuts] LTZ1000 at higher currents

[Andreas Jahn]

Just some thoughts:

if you increase the zener current you have
to increase temperature setpoint of the heater.
Otherwise the temperature regulation will
be bad. You need to have a temperature stability of
0.001 K on the zener to get the 0.05ppm/K
temperature gradient.

The "Pickering patent" changes the temperature
setpoint of the heater not the zener current.

Although there is no direct prohibition,
all diagrams are ending at 5mA.
And if there would be a significant improvement
at higher currents then HP would have used this
in his 3458A. The only thing they did is changeing
the 120 Ohms to 111 Ohms. On my reference
I have about 3.8 mA with the 120 Ohms.

With best regards

Andreas



----- Original Message ----- 
From: "John Devereux" <john at devereux.me.uk>
To: <volt-nuts at febo.com>
Sent: Wednesday, October 19, 2011 3:14 PM
Subject: [volt-nuts] LTZ1000 at higher currents


> Hello everyone,
>
> Is there some reason why LTZ1000s are not run at a lot higher current
> than 5mA?
>
> Thermal resistance is 80'C/W, or ~0.5'C/mA. So it would seem OK to run
> it at 10, 20, 30mA before seeing serious problems due to raising the
> temperature too high.
>
> The datasheet noise graph shows a big fall in noise level between 1 and
> 5mA, so this ought to reduce the noise further. I am not sure how far
> down in frequency this improvement continues. On the datasheet the
> "noise gap" widens more and more with lower frequency, does this extend
> to frequencies so low they would be regarded as long term drift?
>
> There is no current limit for the zener that I can see (of course there
> will be one eventually).
>
> The "Pickering patent" uses pulsing to achieve a high current in what is
> obviously a LTZ1000, the stated reason being to minimise "VLF and long
> term instability".
>
> But it seems like even a higher DC current could be of benefit.
>
> -- 
>
> John Devereux
>
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