Author Topic: The LTFLU (aka SZA263) reference zener diode circuit  (Read 52435 times)

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Offline janaf

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The LTFLU (aka SZA263) reference zener diode circuit
« on: February 14, 2015, 03:59:57 AM »
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 LTFLU
The 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. http://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.
my2C
Jan
 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #1 on: February 14, 2015, 04:40:55 AM »
I attach a copy of the LTFLU section from the 732A datasheet.

I have started cad-ing the schematics of the reference board.
my2C
Jan
 

Offline DiligentMinds.com

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #2 on: February 14, 2015, 05:08:24 AM »
By the way, I don't know if the LTFLU and SZA263 are identical or not.

The SZA263 is a "Ref-Amp" that was made by Motorola, which discontinued making the part due to lack of interest.  This forced Fluke to find a partner to design a replacement for the SZA263, because Motorola would not sell the design files to anyone.  Fluke ended up partnering with Linear Technology, and they designed the LTFLU-1AH [there was never a non-'A' version].  This was to be pin and function compatible with the SZA263, and they did a pretty good job, with one exception: the LTFLU-1AH uses a different material for the interconnects [aluminum] than Motorola did [a proprietary alloy], and this causes the SZA263 to drift *UP* over time, while the LTFLU-1AH [and the LTZ1000(A) and the LM199/299/399] will drift *DOWN* over time.

If you have a bank of (4) 732A's and a bank of recent 732B's that you get calibrated once a year, you will clearly see these drift patterns.

To make things extra confusing, Fluke had some SZA263's left over when they switched to the 732B design, and so some early 732B's will drift *UP* just like the 732A does.  After some point [and I'm not privy to what serial number it starts], they switched to using the LTFLU-1AH in all 732B's.

This long term [time] drift in these reference IC's is caused mainly by the parasitic resistances of the interconnects and the interconnect/silicon interface-- the "buried" [subsurface] Zener has very little drift over time unless it is damaged during die separation from the wafer.  The damaged units are easy to spot-- they have accentuated low frequency noise over the "average" value.  I'm pretty certain that Linear Technology tests for this, and destroys any parts not meeting the LF-noise ["1/f noise"] spec, so if you buy an LTZ1000, LTZ1000A, LM399H, or LM399AH, this issue should never come up.
« Last Edit: February 14, 2015, 05:40:30 AM by DiligentMinds.com »
 
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Offline quarks

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #3 on: February 14, 2015, 05:34:38 AM »
Bookmark

Can you share were you bought them. I just searched ebay and could not find any (incl. completed listings)
« Last Edit: February 14, 2015, 05:40:03 AM by quarks »
 

Offline ManateeMafia

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #4 on: February 14, 2015, 05:42:52 AM »
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.

http://www.ebay.com/itm/Fluke-8842A-5-1-2-5-5-digit-Digital-Multimeter-Calibrated-with-NIST-Cert-LTFLU-/321575961800?pt=LH_DefaultDomain_0&hash=item4adf6bc4c8
 
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Offline DiligentMinds.com

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #5 on: February 14, 2015, 06:09:14 AM »
I didn't realize the 8842A had a LTFLU-1AH in them!  That's really *overkill* to the maximum for a 5.5-digit meter.  The LTFLU-1A is capable of such stability that it can be used in 8.5-digit DMM's [and in fact *may* be used in later models of the Fluke 8508A-- not the LTZ1000A like everyone thinks]...
 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #6 on: February 14, 2015, 08:43:44 AM »
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.)
 
Bookmark

Can you share were you bought them. I just searched ebay and could not find any (incl. completed listings)
my2C
Jan
 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #7 on: February 14, 2015, 09:09:42 AM »
First part of the schematics around the LTFLU on the 732A. Not completed! Working on it now and then.

In the Fluke schematics, E23, E24, E24 are all marked 10V Hi while E26, E27, E28 are marked Lo. As far as I understand:
E23, Supply Hi
E24, 10V guard
E25, 10V hi
E26, low for front panel stuff
E27, 10V low
E28, ground

E24-E25 are interconnected via 51R R26 and E27-E28 via 51R R64.
Several other components are somewhat exotic / obsolete.

Almost all resistors on this schematic are selected / matched / marked "Return board to Fluke for repairs", no specs and sometimes no value given.
my2C
Jan
 

Offline Andreas

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #8 on: February 14, 2015, 09:40:22 AM »
Did you buy a complete "RefAmp-Set" including the trimmed resistors or only the zener.
I think without appropriate resistors you will have much fun to find out how to trim the tempco.

With best regards

Andreas
« Last Edit: February 14, 2015, 09:45:47 AM by Andreas »
 

Offline Jay_Diddy_B

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #9 on: February 14, 2015, 10:49:18 AM »
Hi group,

I can confirm that the LTFLU was used in the 8842A. Here is a picture of the board showing the LTFLU:



And here is the reference schematic form the 8842A manual. The adjustment is done by supply the reference and the resistor network as a matched set.






Regards,

Jay_Diddy_B


 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #10 on: February 14, 2015, 10:50:34 AM »
Nope, no complete set, just the LTFLU ICs.

Even with the Amp set, which contains two resistors R5 and R9, there seems to be a ton of trimming to be done.
I'm sure these will keep me busy for a while.
my2C
Jan
 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #11 on: February 14, 2015, 11:00:37 AM »
Jay_Diddy_B, thanks a million! This seems a lot more straight forward than the 10V implementation of the 732.

If you happen to have access, it would be great to have some values for the unmarked resistors R701 and two Z701. Would be a lot of help!

I wonder what they mean by -0.05V near Z702 / 3K2 on the base of U701?
my2C
Jan
 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #12 on: February 14, 2015, 11:12:20 AM »
I just downloaded the 8842A manual. It has the BOM and schematic, but of course no values indicated for the ref set resistors. Are any of those exotic divider resistor chip with two resistors physically "interwoven" to ensure ratio tracking? 
my2C
Jan
 
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Offline Jay_Diddy_B

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #13 on: February 14, 2015, 12:09:46 PM »
Hi,

I measured the values of the unknown resistors, But I think that there is more to these resistors than shown on the schematic. I suspect that the top of Z701 may be a divider.

I measured 5.081K from pins 1 to 2 and 0.7983K from pins 2 to 3. But I do not get the correct operation in my LTspice model. If I use 0.07983K then I measure the same voltage in LTspice as I do in the unit.



The results:





I have attached the LTspice model if you want to play along.

Regards,

Jay_Diddy_B
« Last Edit: February 14, 2015, 12:17:43 PM by Jay_Diddy_B »
 

Offline Andreas

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #14 on: February 14, 2015, 03:54:06 PM »
Hello Jay_Diddy_B,

Im missing the 4.445 K resistor in your model.

And the LF411/412 are J-FET amplifiers with low input bias (e.g. like LT1022 but not LT1013).
(dont know if this matters).

With best regards

Andreas
« Last Edit: February 14, 2015, 04:14:57 PM by Andreas »
 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #15 on: February 14, 2015, 08:29:12 PM »
With the 4.445K resistor and changing R7 to 20K I get near +/-7V at room temperature. I've also used the 6.2V zerer which may of course be wrong.


I'm wondering what the 20K divider is. The ratio would have to be "ppm perfect" and stable as anything.
« Last Edit: February 14, 2015, 10:07:22 PM by janaf »
my2C
Jan
 

Offline Jay_Diddy_B

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #16 on: February 14, 2015, 11:35:57 PM »
With the 4.445K resistor and changing R7 to 20K I get near +/-7V at room temperature. I've also used the 6.2V zerer which may of course be wrong.


I'm wondering what the 20K divider is. The ratio would have to be "ppm perfect" and stable as anything.

Jan and the group,

If you look at this picture:



The 20K resistors are part of the larger resistor module(Z702). The smaller resistor module is the Z701. Fluke engineers placed these so they are in thermal contact.

I may have a 'parts unit' 8842a that I am unable to repair. I will look for it later.

Regards,

Jay_Diddy_B

 

Offline Jay_Diddy_B

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #17 on: February 15, 2015, 12:17:50 AM »
Hi,

A little more research.

I have attached a pdf with the Fluke 431B schematic. It uses a reference similar or the same as the LTFLU.

Here is the reference section from the 335D voltage standard:



The manual says that R13 is selected to set the tempco. R9 trims the output voltage. This gives a clue as to how the LTFLU chips should be trimmed.

This is a heated reference.
The heater is on the die, similar to a LM399. The heater is fed with constant voltage and the heater is a non-linear PTC that controls the temperature. A1 is a 723 voltage regulator.

Jay_Diddy_B
 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #18 on: February 15, 2015, 01:00:12 AM »
As we know that, with your numbering, point nr 3 is GND, nr 1 is -7V and nr 2 approx -0.1V, the measured resistors values don't add up, unless the base is reverse biased, current flowing out of the base of the amp transistor  :-\

I measured 5.081K from pins 1 to 2 and 0.7983K from pins 2 to 3. But I do not get the correct operation in my LTspice model. If I use 0.07983K then I measure the same voltage in LTspice as I do in the unit.
my2C
Jan
 

Offline fmaimon

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #19 on: February 15, 2015, 01:12:19 AM »
My fluke 8840 also has a SZA263.

 

Offline Jay_Diddy_B

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #20 on: February 15, 2015, 04:35:18 AM »
My fluke 8840 also has a SZA263.



The difference between the 8840A and the higher specification  8842A is the quality of the resistor used in the reference circuit. The catalogue talks about higher accuracy and longer calibration cycles.

The blue resistors in your 8840a versus the hermetically-sealed, white resistors in the 8842A.

Regards,

Jay_Diddy_B
 

Offline DiligentMinds.com

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #21 on: February 15, 2015, 05:32:40 AM »
Quite fascinating.  It seems that Fluke abandoned their love for Ref-Amps in 5.5 and 6.5 digit DMMs, because they used a [burned in and selected] LM399 in my Tektronix DMM4050 [which is a re-badge of the Fluke 8846A].  I can attest to the stability of that meter, it is simply amazing.  It also has a big resistor network in it that is in a hermetic ceramic package with a quartz window-- and they [undoubtedly] trim those resistors with a LASER during production.  A similar quartz-windowed resistor network is used inside the 732B, and also on the reference board for the 8508A [which *does* use a LTFLU-1ACH Ref-Amp, and *not* an LTZ1000A according to my inside source].

The only reason that these voltage references are ovenized is that is the easy way to get temperature stability, but you *can* [with great effort] use temperature compensation, and some exotic support circuitry to get temperature stability, with the added benefit that the reference will have a little bit less time-drift than a heated reference might have.  BUT-- the effort to get this level of temperature stability is enormous-- your not just going to put the reference in an oven, and the twist a pot to set the TempCo-- at sub-ppm stability, the adjustment procedure can take weeks, and then must be checked for weeks after the adjustment.  This drives up the cost.  With a Ref-Amp, there are a *lot* of "moving parts"-- and they all interact with each other, so adjustment can be your worst nightmare on steroids...

The great thing about the LTZ1000 or LTZ1000A, is that you can just build it, burn it in, make your final voltage adjustment, and then enjoy extremely good stability after that.  With a little bit of effort, it is easy to temperature stabilize an LTZ1000 [not perfectly, but better than the "raw" +50ppm/K], then when you turn on the heater circuit, you will enjoy super-stable temperature behavior.

As for Ref-Amps, they are better left to big companies that can afford to mess around with them, and that also have in-house precision wire wound resistor manufacturing [because "custom" resistors will be needed that are super stable with temperature and time, and you can't wait for VPG to build a resistor for you].
 

Offline blackdog

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #22 on: February 15, 2015, 08:07:47 AM »
Hi,

Here a verry happy owner of two Fluke 8840A DMM's.
My first fluke 8804A multimeter I bought in 1986.
I had to do al lot of overtime, to buy him  :D
The 8840A is incredibly stable...

This is the 8840A from 1987 He never been calibrated after 1987.
Its standing on the shoulders of my 3458A, he is in good company...


Jumpt one digit, i switched of my voltage reference and it was not completely finished after turning again.


Kind regarts,
Blackdog
 

Offline Andreas

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #23 on: February 15, 2015, 08:37:52 AM »

This is the 8840A from 1987 He never been calibrated after 1987.


So it will definitively go wrong: 1990 the definition of Volt has changed by 9.264 ppm.
http://www.nist.gov/calibrations/upload/tn1263.pdf

With best regards

Andreas

 

Offline janaf

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #24 on: February 15, 2015, 09:12:58 AM »
Obviously Fluke made very different implementations with the LTFLU with the 8840A DMM's, 731B, 732, 8508A etc. No doubt it takes a LOT to achieve top performance like for the 732B. Not aiming there as mentioned in my first post...

One thing; the parts I have bought are four-pin, so there can't be any heater in the can like shown in the 335D schamatics?



   
my2C
Jan
 


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