Author Topic: The LTFLU (aka SZA263) reference zener diode circuit  (Read 135879 times)

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Offline imo

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #500 on: January 24, 2021, 04:42:34 pm »
@dietert1: I've made a table based on a lecture here



taking into account walls, edges and corners of the Box. There are the edge and corner "coefs" which depend on a ratio of the box sizes to the wall thickness, however.

Added the heat transfer via copper.

Added the xls file.

Maybe a topic for a separate thread..


« Last Edit: January 25, 2021, 09:37:21 am by imo »
 
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Offline dietert1

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #501 on: May 21, 2021, 08:22:09 pm »
Here i have a log of two LTFLU references that i made last year (schematic and images above). Nominal oven temperatures are 17.4 °C and 27.69 °C. The voltage difference was recorded with a HP 3456A. Each blue dot represents the average of about 17 000 measurements with 100 PLC and Autozero.
There is a one month gap between 5000 and 6000 hours, when i cleaned the air filter of the HP3456A and used it for other measurements. I also removed three days of data after an unnoticed oven shutdown, when the 17.4 °C oven went up to 28 °C for more than 24 hours. Added an exponential decay fit. Since this was a first crude setup i can't tell which process this may be. There are no significant correlations with ambient temperature, nor humidity, nor barometric pressure.
Next time i want to log multiple references, including a 5x LM399 device and a revision of the LTFLU shown above.

Regards, Dieter
 
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Offline dietert1

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #502 on: June 05, 2021, 06:21:44 pm »
These are 10 V references both including a gain stage inside the same oven. In the 17.4 °C reference the gain stage has 10x UPF25 100R resistors, in the other one the divider is made from three Dale RS5 wirewound resistors of 750R and 150R plus 180R that i found in a drawer. Impressed by the stability of those wirewound resistors i ordered several lots of Vishay Dale RS02 750R and 330R resistors that are similar except smaller. Those resistors need selection if you want TC below 5 ppm/K, but then i found several with very low TC in the interesting temperature region around 25 to 30 °C.
I am running the reference ovens at relatively low temperatures as i want battery backup. In fact those LTFLU references have seen power outages and oven shutdowns without "jumping around". The 17.4 °C oven was the first serious attempt and it works well, but that TEC takes 1 W or more, while the other one at 27 °C runs from 50 to 100 mW due to self heating.
Another alternative to make a low TC gain stage is a PWM circuit as i recently showed in the LM399 thread. Something similar can be nice with a LTZ1000 reference.

Regards, Dieter
 

Offline branadic

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #503 on: June 05, 2021, 07:23:43 pm »
I guess you mentioned yourself, that your PWM approach might be good for an LM399, but as far as I understood isn't for an LTZ or LTFLU, due to:

"My PWM prototype has an offset of 11 ppm (max of log). The PWM ratio remained unchanged after implementation. In my opinion a more perfect PWM divider would be more accurate, i mean without tuning or "calibrating" it. Accuracy depends on the symmetry of the PWM switches: resistance and/or timing difference. At 10 KHz a 1 nsec timing asymmetry makes 10 ppm. IC muxes have delays of about 50 nsec that can easily contribute an asymmetry of 1 nsec..."

On the other hand the approach using a TOMC1603 resistor network including parallel resistors for trimming the gain as I did on my LFTLU  reference (even a TDP1603 would do) worked quite good, as it is part of the ovenized aluminum board with the LTFLU. This is though a totally different story, when the gain stage including the parallel resistors for trimming are exposed to ambient temperature changes.

An approach not discussed yet is the gain stage used in W/F7000, that can now be discussed thanks to the reverse engineering by chekhov. Is uses resistor networks for a coarse amplification and adds additional voltage for fine trimming provided by parallel DACs, so no clocking being involved.

-branadic-
Computers exist to solve problems that we wouldn't have without them. AI exists to answers questions, we wouldn't ask without it.
 

Offline Kleinstein

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #504 on: June 05, 2021, 08:07:34 pm »
The on resistor matching is mainly a thing with linearity and only to a small part with temperature drift. An offset due to delays and charge injection is likely quite stable. For a gain stage we don't need good linearity or a very low offset. The main point is long term drift. The more problematic point may be EMI. Chances are a lower PWM frequency could help - however there is a slight complication: in the feedback path it is tricky to use more than a 1 st order filter. So residual ripply could become a problem with a much lower frequency, unless one could reduce the ripple in a different way (e.g. the compensation with an opposing signal).

For a stable ration there would also be the option to use a charge pump (e.g. LTC1043) for the coarse part (Faktor 1.5) and than use resistors for the fine part only.

Anyway it gets a bit off topic from the LTFLU ref., though the ref and gain are closer coupled with the LTFLU - at least in the normal circuit.
 

Offline dietert1

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #505 on: June 05, 2021, 09:13:32 pm »
My plan outlined in the LM399 thread is to reduce PWM frequency from 10 KHz to 1 KHz (or 1200 Hz  = multiple of 50 and 60 Hz). For this i implemented ripple compensation as shown in the schematic. Then you have a 2nd order filter that behaves better than a third order filter as ripple compensation helps on PWM input and output side. This change gives me a factor 8 or 10, so the 11 ppm becomes about 1 ppm.
Then you want another factor ten of precision. This happens by the implementation of a discrete PWM output stage as we see them in Fluke/Wavetek/Datron calibrators. I already posted a study/proposal for a discrete PWM output stage. By combining everything one should arrive at a PWM with about 0.1 ppm absolute accuracy. If PWM stability will be another factor ten better than absolute accuracy as observed with the current circuit then that PWM gain stage will be good enough for scaling a group of LTZ1000s to 10 V.
Currently i am rebuilding my metering setup so i can check long term stability at a 100 nV level or below. I want to integrate four references: The two LTFLUs with analog gain stage inside oven, the 5x LM399 with improved PWM gain stage and a 10 V double JFET reference without gain stage. And of course there should be at least one unused port for testing other references..

Regards, Dieter
 

Offline Kleinstein

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #506 on: June 06, 2021, 06:20:43 am »
For testing the reference and gain stages it would make sense to not only compare the scaled 10 V level, but also check drift of the raw reference voltage before scaling. The dirft of the 2 parts can be comparable.

For the PWM switches I would not count much on better performance from discrete switches compared to modern CMOS switch chips. The discrete solution may have more EMI issues and more charge injection.
The ripple compensation with the capacitive coupled inverted signal acts like an addional filter stage for the PMW part, but not adding much phase shift to the loop. So the ripple reduction is likely a good idea.
Another possible option would be 3 phase PWM, so 3 stages with 120 deg. phase shift, which would cancel the main ripple at 1/3 and 2/3 PWM setting and still reduced ripple close to those values.
More switches make discrete switches less attractive.
 

Offline dietert1

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #507 on: June 06, 2021, 02:45:26 pm »
I did my work and later wrote about a discrete implementation of a precision PWM output stage. My schematic in the LM399 thread shows a circuit i built and tested to some degree. As always there may be other solutions and i will be interested in other substantial proposals.
In my prototype i used a FDC6561AN dual mosfet with Rdson = 0.1 Ohm and about 0.03 Ohm match. If somebody wants to propose an IC switch with lower Rdson and/or better Rdson matching, you are welcome. In a totem-pole circuit with optimized dead time you can forget about charge injection.

Regards, Dieter
 

Offline Kleinstein

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #508 on: June 06, 2021, 04:50:01 pm »
The  FDC6561AN are interesting in that they allow a relatively high gate voltage. Many other modern FETs have the protection start engaging at some 10-15 V already.
 
However I don't think these rather large FETs would be a good choice for precision PWM. The gate capacitance is quite large and this leads to quite some current spikes at the driving side and the supply / gate driver. CMOS switch chips have a much smaller current spike to the input side driver. The charge injection would be a measure for the current pulse to expect. I am afraid that by avoiding the switch resistance effect by all costs, the larger spike towards the driver would cause trouble that is difficult to solve.
It also helps to have the supply separate from the switch connection. So some of the gate driver chips are not a good option, if they have a common supply for the output stage and internal driver.

Instead of the brute force very low resistance way, I would prefer compensation of the switch resistance, like in the Fluke 57xx series. So have a 2nd pair of switches with a slightly higher votlage to  provide essentially all the current. With integrated switches using 2 channels is no that bad.
Anyway for just a 7 to 10 V stage I would not worry so much about a little offset / nonlinearity. One can usually adjust the PWM ratio to get extact 10 V. There is no need to get it from the upfront calculated ratio.

My current favorite for a 7 to 10 V step is 3 phase PMW with 3 x DG419LE  (vishay  - maxim seems higher resistance) going to a common filter with relatively high resistance (e.g. 100 K range each). R_on is at some 15 ohms with matching in the 1 Ohms range. Over some 10 C temperature change this would change by something like 50 mOhms or some 2 ppm change relative to the 100 K filter resistance.
For capacitor leakage and amplifier bias the 3 channels are in parallel so that the overall resistance is not that high.
With relativly low charge injection the spikes effecting the drive should not be that bad. With a PWM ratio close to 2/3 the residual ripple to start with would be small, similar to better than with the coupled opposite phase compensation (which act like a separate filter stage).

 

Offline dietert1

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #509 on: June 06, 2021, 06:02:16 pm »
No, don't agree. My experience tells me that absolute accuracy is THE criterion. If you start fine tuning and adjusting at the 10 ppm level, you will never realize a circuit better than 0.02 ppm as required with a LTZ1000 (let's assume 200 nV). Just read the PWM discussion of branadic and Andreas in the LM399 thread.
The discrete PWM output stage i proposed recently is one way of solving the Mosfet drive problem you were speculating about. Your discussion does not replace a demonstration. Then you will learn to solve problems instead of inventing objections.

Regards, Dieter
 

Offline dietert1

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Re: The LTFLU (aka SZA263) reference zener diode circuit
« Reply #510 on: June 20, 2021, 08:04:55 pm »
Recently there was a discussion about "reference oven gain".  Today i have a result for our LTFLU ovens (image above).
These ovens have two temperature sensors: One NTC that is kept at constant temperature by the heater. Another temperature sensor is on-chip, derived from the TC of the transistor Ube. The setup also includes a JVR in a similar oven, with a diode as DUT temperature sensor (not on chip but close). The setup also includes two ambient temperature sensors (SHT = humidity and temperature, BMP = pressure and temperature). The first plot shows a log of the two ambient sensors (left y axis) and of the three DUT temperatures (righ y axis = deviation from average). The other diagram shows the correlation. The three line fits give values of about 0.02. That means the three oven gains are about 50. This is the suppression of ambient temperature changes by the oven.
Next step is fine control of the set temperature of each oven as a function of ambient temperature (pre) and DUT temperature (loop).

Regards, Dieter
 
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