Electronics > Metrology

two LTZ1000 in "series" and attenuate to 10V?

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aronake:
Hi,

The usual LTZ1000 10V design is to have one LTZ1000 and amplify output signal to 10V.

I am thinking of making a design with two LTZ1000 in series to get around 14V, then attenuate that down to 10V with OP amp and resistors.

My thinking is that there then would be noise from two LTZ1000 would be a bit higher than from one, but then the attenuation largely scale down the noise linearly with attenuation.

Theoretically would the noise be less than the regular multiple LTZ1000 design which seems to be to average output then scale up to 10V?

Has anyone tried a design like this?

Thoughts?

Alex Nikitin:
"Theoretically" the relative to the full scale random noise will go down when using multiple zeners in parallel or in series in exactly the same proportion of the square root to the devices number. So if you have 2 devices output summed in parallel or in series the noise will be lower by 3dB (1.41 times). Unfortunately the LTZ1000 is rather more difficult to arrange in series if you plan to use the internal heater and temperature control, so a parallel connection makes more sense. The LM399 on the other hand, with 3 in series for ~20V, and than divided by 2, could be an interesting arrangement... .

Cheers

Alex

tszaboo:
You can also put 72 in series and divide it down with a 1:100 divider!
The reason they don't do it is simple. It's not "just a zener" it has a heating circuit, where the current has to go somewhere, and the top heater current would need to go through the bottom zener. Or you isolate the circuit, which opens up a lot of other issues about noise. Oh, and because it costs like 40 EUR per IC.

dietert1:

--- Quote from: aronake on March 28, 2024, 01:16:09 pm ---..
Has anyone tried a design like this?

Thoughts?

--- End quote ---
I am using a double LTZ1000 reference for my DIY voltmeters. Supply voltage is 18 V, reference output voltage is 14.xx V. The two heater circuits are in series, so each one can get up to 9 V. The lower one has the notorious diode in the heater circuit in order to avoid substrate currents. And the 9 V is the output of an opamp that provides/absorbs the heater current difference. The circuit worked well from the very beginning. The board runs inside an outer oven, so one can use the LTZ heaters at rather low power. See thread about "Prema BK7 derived multimeter".
Noise will be about 30 % lower, similar to a double zener array (1/sqrt(2)).

Regards, Dieter

Kleinstein:
Noise wise and for the sensitivty to the resistors there is not much difference between the 2 paths ( 2 in series and 14 ot 10 V or average from 2 and than 7 to 10).   The step from 7 to 10 is rather similar to the step from 14 to 10, just the other direction. It could be a little easier to start with 14 V and go down, if a PWM type divider / DAC is used and not resistors.

Haveing 2 x LTZ1000 in series is extra effort and complications. Having 2 x Ref in parallel makes it possible to measure the difference of the references as an added self test and there is also the option to use only 1 reference with little change or as a first start.

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