Electronics > Metrology

The LTFLU (aka SZA263) reference zener diode circuit

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I bought a couple used LTFLU from ebay thinking that if they are good enough for the Fluke, they are good enough for me :) They should arrive within a week.

Now I have been searching to see what I can find out about the LTFLU reference zener. As some know I'ts a custom Fluke component from LT and used in some of the very best voltage standards like the Fluke 732B. The part was never sold to end users.

While digging in to the documentation, mostly the for the 732A, where there is a schematic and BOM, I realize a few things.

* While the LTZ1000 docs are not the best, for the LTFLU they are almost non-existant.
* Some DIYers in china have implemented boxes with the LTFLUThe main differences between the actual LTFLU IC and the LTZ1000 are:

* The  LTFLU has no heater built in while the LTZ1000 has
* The  LTFLU has the temperature compensating transistor in series with the diode, not parallel as in the LTZ1000.

* Therefore the  LTFLU circuit needs a lot more temperature and current compensation than LTZ1000 circuits do
* i.e the LTFLU is more complicated to implement than the LTZ1000
* The  LTFLU is in a 4-pin can, the LTZ1000 in an 8-pin. They are far from drop-in compatible.
A schematic for the LTFLU circuit is available in the 732A instruction manual, page 8-11 (93), figure 85, part U2. Available for download on the net. I don't know if it's identical to the 732B, I have not found a schematic for the reference board of the 732B.

Replicating / cloning the 732A or 732B is definitely out of the question. Those boxes contain several advanced custom components and, as far as seen in a tear-down thread on this forum, also advanced thermal engineering &hardware. https://www.eevblog.com/forum/testgear/fluke-732b-dc-standard-teardown/

As far as I know the 732B was improved over the A, by placing more of the critical components in an oven and the B model also has better current compensation for the zener.

IMO, there are two main problems with making a voltage standard circuit.

* To make a stable enough reference voltage
* To amplify and trim the reference to 10.000000V and keeping it stable.The second is much more difficult than the first.

* With the LTFLU and 732, the zener an 10V circuits are as far as I can see, integrated into one loop
* With the LTZ1000 you typically have one 7.1V zener circuit and another amplifies to 10V (even if it's possible to combine them!) I'ts now obvious to me that it will be more complicated to get good results with the LTFLU than with the LTZ1000, so why bother? I'll quote what I heard in an interview with a guy who's interest in life was old English sports cars, on why he liked tinkering with them: "They give you such interesting problems to solve!". I guess it's the same with me.

I guess there could also be interest from 732 owners on understanding the inners of this circuit.

As soon as the mail arrives I will throw the parts on a good SMU and get some basic characteristics. Does anybody have a pin-pout?

Yes, I should have searched the info before buying the components.  :palm:

By the way, I don't know if the LTFLU and SZA263 are identical or not.

I attach a copy of the LTFLU section from the 732A datasheet.

I have started cad-ing the schematics of the reference board.


Can you share were you bought them. I just searched ebay and could not find any (incl. completed listings)

There is an auction that shows a LTFLU-1 installed in a Fluke 8842A. I noticed this a while ago but did not think it was the same part. It would be interesting if a member here had one that could verify that this is authentic. I haven't seen anyone mention this previously. The date code appears to be 1989 and the other surrounding parts are from 1991.

It could be that these meters are the source of some of these LTFLU references.


Sorry I bought the last ones  8)

I think he also had some SZA263 but it seems the shop is closed now, vacations in China.
It's the guy who also sells salvaged foil resistors; hifi-szjxic

(I stay away from the resistors, if they have been subject to high temperatures, they may be at end-of-life where they deteriorate faster and faster.)

--- Quote from: quarks on February 13, 2015, 06:34:38 pm ---Bookmark

Can you share were you bought them. I just searched ebay and could not find any (incl. completed listings)

--- End quote ---


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