Hi John,

The LTZ1000 can be run much harder, but the LTZ1000a can only be run upto about 40-45 mA before the chip temp is up to 125C where the dift is going to be much higher.

I am assuming a 20C lab environment for homebrew kit with decent cooling. But to stop thermal drift you really have to give the chip and pcb poor cooling or dip in parrafin oil.

Regards,

M K

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@devereux.me.uk
To: volt-nuts@febo.com
Sent: Wednesday, October 19, 2011 3:14 PM
Subject: [volt-nuts] LTZ1000 at higher currents

Hi,

Yes I am assuming a LTZ1000 (not A) and 20C lab environment.

Could you clarify your remarks about cooling, I can't tell if you mean
poor cooling (good insulation) or good cooling (heatsink somehow)?

I was wondering about oil immersion too.

Thanks,

John

m k m1k3k1@hotmail.com writes:

--

John Devereux

Hi Andreas,

Thank you for your thoughts!

I think you only have to increase the setpoint if there is a risk of
exceeding it during operation. So I could run at 20mA with a 10C extra
temperature rise. So as long as the set point is >>10C above my board
temperature this should be OK.

The patent I meant does change the current (as I understand it). The
point of it is to allow (intermittent) high-current operation while
keeping the temperature set point low. It is US 6342780.

http://www.google.co.uk/patents?id=Ec8KAAAAEBAJ

I suppose it does allow control of the temperature too but I thought the
benefit is the ability to run the diode at higher current than would
otherwise be possible.

By the way, I am not sure about a statement in the patent that "the
typical thermal time constant is many tens of seconds". It seems to me
this may be true of the entire device but the chip itself must have much
faster thermal time constants in the order of milliseconds. So with the
method described, the temperature will be changing even as the voltage
is being sampled.

Yes I suppose it would be on the datasheet too. Perhaps it improves the
noise but worsens the drift. But I gather (from this list) that Fluke
used the "Pickering Patent" in their recent references.

Thanks,

John

"Andreas Jahn" Andreas_-_Jahn@t-online.de writes:

--

John Devereux

Hi John,
The cooling comment is because when you reduce the airflow enough to reduce the thermocouple effects on the pcb you may well raise the temp just outside the LTZ1000 can to significantly above 20c.
For a homebrew oil immersion will significantly reduce thermal effects.
Currently I am wondering how much the pickering patent is useful for a homebrew as one then has just transferred the stability issues onto the sample and hold circuit, as that requires sub ppm accuracy there.
We know that at 95/97C that the HP reference is set to gives higher drift. So that sets an upper limit on the current to run through the zener. I am also wondering why they give 5mA as the upper limit? Is it possible that the zener is so small that above 5mA one has current coming out through the shoulder of the end of the zener? By design the diffusions give the lowest breakdown voltage at the planar end portion, and the shoulders have progressively higher breakdown to keep the current away from the surface. BUT, if that was so the pickering patent method would not work. The zener tempco will be higher at higher currents and it will be more sensitive to orientation at higher temperatures too, so that would limit my enthusiasm to run it too hard.
I am still pondering all that while I try and burn in my LTZ sample.
M K