Author Topic: two LTZ1000 in "series" and attenuate to 10V?  (Read 13591 times)

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aronake

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two LTZ1000 in "series" and attenuate to 10V?
« on: March 28, 2024, 01:16:09 pm »
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

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #1 on: March 28, 2024, 01:50:45 pm »
"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

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #2 on: March 28, 2024, 02:06:08 pm »
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

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #3 on: March 28, 2024, 08:59:13 pm »
..
Has anyone tried a design like this?

Thoughts?
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
« Last Edit: March 28, 2024, 09:00:52 pm by dietert1 »

Kleinstein

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #4 on: March 28, 2024, 09:09:32 pm »
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.

KT88

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #5 on: March 29, 2024, 12:35:18 am »
Summing two references in series should have the same effect as paralleling them - noisewise...
As the other comments hint - it comes with some complexity and pitfalls.
A slight advantage could be expected from the input referred noise of the Opamps not being amplified but attenuated - given an ingenious circuit could leverage that advantage.
However, the gain in performance would be disappointing as the Vz remains the major source of noise and in sum the reduction of noise in comparison to paralleling them would be negligible. (rule of thumb: a source with 1/3 of the aplitude of the main source only contributes 1/10 to the overall noise).
The seebeck related errors might still sum up...

dietert1

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #6 on: March 29, 2024, 11:09:42 am »
If somebody makes an array with more parallel zeners in order to suppress zener noise, they will be approaching a situation where the amplifier deficiencies matter more.
Those 4x references like Fluke 734 or that Wavetek 7004N currently for sale at ebay for US$20 490 support averaging (parallel array mode). Don't know whether they can be used to generate 40 V. I remember in professional analog audio they used differential high level signals of up to +/-15 V, about a factor 100 more than the usual consumer audio. Ideally a 4x reference should support 10 V, 20 V (2 x 2) and 40 V array modes. Regards, Dieter alm • Super Contributor • Posts: 2903 • Country: Re: two LTZ1000 in "series" and attenuate to 10V? « Reply #7 on: March 29, 2024, 11:35:11 am » If somebody makes an array with more parallel zeners in order to suppress zener noise, they will be approaching a situation where the amplifier deficiencies matter more. Those 4x references like Fluke 734 or that Wavetek 7004N currently for sale at ebay for US$ 20 490 support averaging (parallel array mode).
I'm pretty sure the Fluke 734 is just a mechanical enclosure with the only electrical connection being mains power. But the Wavetek/Fluke 7004N and Datron 4910 can indeed do averaging using resistors (the Datron 4910 also provides a buffered average output). I believe the Wavetek/Fluke 7010N can even average 10 cells. But they average after amplification to 10V. So the op-amps are also in parallel.

Ideally a 4x reference should support 10 V, 20 V (2 x 2) and 40 V array modes.
The old Fluke 730A quad voltage standard from the early seventies had buttons to put up to four cells in parallel or series. I'm not aware of any later standards supporting this, although nothing prevents you from making those connections yourself.

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #8 on: March 29, 2024, 11:37:01 am »
In theory Wavetek/Fluke 7004 should support 10, 20, 30 and 40 V as all references are floating, similar to F730A, where you can choose between average (parallel) or series connection.

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Arhigos

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #9 on: March 30, 2024, 11:33:59 am »
I know that fluke 57xx series are using two LTFLU(aka SZA263) chips.
But I have no idea why there s two of them

dietert1

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #10 on: March 30, 2024, 12:12:48 pm »
When i made a LM399 10 V reference with time division (PWM) for the 10 to 7 V divider, i had some difficulties, since the PWM low pass filter was in the 10 V regulation loop. I got the loop stable after some iterations, but the next version had a 2x5 LM399 array with 14 V output. Then there is no regulation loop. It's just the PWM division.

Regards, Dieter
« Last Edit: March 30, 2024, 12:14:48 pm by dietert1 »

Dr. Frank

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #11 on: March 30, 2024, 07:20:03 pm »
I know that fluke 57xx series are using two LTFLU(aka SZA263) chips.
But I have no idea why there s two of them

Hi, that's also used already in the Fluke 5440/5442.
From these 13V reference you can easily use a PWM, instead of first amplifying from 6.9V to 11V
the 5440 and 57x0 use ranges of 11, 22, 275 and 1100V, I.e. need D/A outputs of 0 .. 11V in each range.

That gives more stability and a less noisy reference voltage.
Frank

Andreas

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #12 on: March 31, 2024, 06:04:01 am »
Hello,

and I am pretty shure they used two with a slightly different zero TC temperature
to spread the zero TC temperature over a wider temperature range.

with best regards

Andreas

Mickle T.

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #13 on: March 31, 2024, 06:16:07 am »
AFAIR, the highest voltage reference voltage source is used in the Datron 4000A calibrator, in which two chains of 4 zener diodes (and a resistor divider) provide an output voltage of 20.5 V for a 0+-20 V hybrid PWM DAC. But it is also inferior to 8-1/2-digit voltmeter-calibrator Impulse V2-41/1, where 4 zener diodes form a voltage of 25.6 V for a multiphase PWM DAC.

KT88

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #14 on: March 31, 2024, 09:09:14 pm »
I would think that possible advantages from putting heated zeners in series, probably won‘t justify the effort. Unheated ones would be a different story… It would require only one control circuit and the seebeck effect could be mitigated far more easily as the temperature gradients would be lower compared to heated ones.
A higher reference voltage in a system like a calibrator would help to reduce interferences if the reference voltage has to be distributed to multiple subsystems.

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #15 on: March 31, 2024, 09:18:10 pm »
It is at least sort of easy with LMx99 and hence with ADR1399 too.

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KT88

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #16 on: March 31, 2024, 10:13:14 pm »
Neat circuit. I agree that this is easy to implement. It should work up to 4 in series….
The reason for that is the integrated heater control and the straight zener…
The LTZ/ADR1000 would need individual heater circuits. The rest of the circuit won‘t be straight forward as well…

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YARE

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #17 on: May 20, 2024, 08:12:45 am »
Hello all, this is my first post.

The classic Datron 4000 did employ multiple series connected zeners in a complex 4S 2P (two parallel connected banks of four series zeners), but these were standard 1N829A low temperature coefficient zener diodes.

Unfortunately modern buried-zener references are NOT simple zeners, but rather integrated transistor-zener devices designed to lower the dynamic output impedance and improve temperature stability.  In the the AD1399 this has been carried to such an extreme degree, that I doubt that you will be able to get away with connecting even 2 in series without oscillation. Even if such a series chain doesn't oscillate, the loop stability issues could still cause gain peaking such that the noise gets worse instead of better.

So let's go back over some of the other options ...

Other than wiring references in series, there are basically three commonly used methods to scale reference voltages:

1) Integer ratio scaling using a switched-capacitor charge transfer device like the LTC1043.

2) Arbitrary ratio scaling using expensive high-stability ultra-precision resistors.

3) Arbitrary ratio scaling using Pulse Width Modulation.

1) The LTC1043 is fairly low cost, and has amazingly stable performance, but is limited to integer ratios.

2) Precision resistors are a proven technology, but the needed high-precision wire-wound or metal-foil resistors are expensive and may have long lead times.

3) PWM circuitry can give good performance, but there can still be some slight temperature related drift as switching rise times and RDSon change over temperature. Also, most micro-controller PWM blocks lack the needed resolution to achieve PPM levels of voltage resolution without resorting to high-range, low-range, hacks that can add additional error sources and drift.

In the last few months I have been tinkering with an alternate fourth option that I think has a lot of promise because it has basically ALL of the advantages listed above and none of the disadvantages. Like Andreas, I am rather enamored with the LTC1043, and stocked up a while back when they were only a few dollars each, and I am tickled to find that there is a way to apply them not only for integer multiplication, but ALSO in ultra-high-precision NON-INTEGER-MULTIPLICATION.

4) This fourth option is based on running a pair of LTC1043 switched capacitor devices in a variation of the "Frequency-Controlled Gain Amplifier" circuit shown on page 14 of the current LTC1043 datasheet. (Note: Two LTC1043 are required because, although a single LTC1043 has two sections, they can not be independently clocked).

What makes this circuit truly ideal is that it works by charge transfer, with the charges transferred being directly proportional to the frequency ratio which can be VERY precisely controlled. Drift in the capacitors can also effect the output voltage, but this is easily managed.  To achieve PPM levels of accuracy all we have to do is make the two LTC1043 charge transfer capacitors identical low temperature-coefficient, low-leakage parts (polystyrene 0.01uf).

I have the circuit wired for the non-inverting configuration (as noted below the schematic), which is really nice because it allows simple self-calibration to manage initial capacitor mismatch error (and future re-calibration to cancel any long term differential drift of capacitor values).

The self-calibration uses the second precision zero drift op amp in the ADA4522-2 as a comparator (I know, op-amps are not recommended as comparators, but it works just fine with the 10k resistors recommended in the datasheet). The calibration circuit is simply this comparator wired across between the input reference voltage input and the multiplied output voltage output. Then to calibrate the Frequency-to-Gain factor, we slowly sweep the frequency-ratio through the range which should give unity gain and note the precise frequency ratio at which the comparator toggles. Then to reset the calibration, we just adjust the applied in-service ratio as needed based on the unity-gain ratio - simple!

This means that ten years from now, even if one of the capacitors were to drift up by 10%, and the other down by 10% (causing a huge uncorrected error of 20% !), simply running the above calibration would still instantly restore the corrected post-calibrated frequency-to-gain transfer function error to less than one PPM!

I don't want to post my prototype schematic because it's an ugly lashed up proof-of-concept kludge based on a AD9854 DDS running off an internally multiplied 30MHz source, with a discrete 12 bit ttl divider chain from the same 30MHz clock giving the other frequency (7324.21875 Hz). The varible DDS frequency and fixed 7.32kHz frequency feed through level shifters into the two LTC1043's.

The DDS works well, and gives me parts-per-billion frequency ratio tweaking capability (corresponding to nanovolt voltage resolution), but it's overkill in this application and I am working to replace it with a simple \$4 Si5351 clock generator module.

I would be interested to hear from Andreas and some of the other forum regulars to see if anyone has tried this approach before.

Y.A.R.E

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Kleinstein

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #18 on: May 20, 2024, 08:30:34 am »
The ADR1399 is internally complex, but with the 1 µF 5 ohm RC in parallel to each unit as suggested from the datasheet there should be no problem having 2 or more of the references in series.
There is one more option no mentioned: have 1 ref. as positive and the other as a negative side circuit. If needed to reference could than have a seprate floating supply.

The frequency ratio circuit as shown by LT is not very stable. The voltage also depends on the capacitor ratio. The capacitance would include parasitic capacitance, which could be relevant especially with the rather small 100 pF shown in the circuit.

To get from some 7 V to 10 V the more obvious use of the LT1043 would be to do a capacitve x 1.5 muliplier to get to some 10.5 V and than use a resistive divider for the small rest. Here the resistors are less critical (about 10 x) than with a direct gain stage.

MiDi

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #19 on: May 20, 2024, 09:01:24 am »
Additionally to 1) there is the option to use (statistical) resistor arrays e.g. TDP1602 (like in F7000-series).
As Kleinstein already mentioned it boils down to the stability (TC & LTD) of the Frequency-Controlled Gain Amplifier.
That may be mitigated by frequent internal cal, but the downside is that the output during that period is invalid.

Andreas

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Re: two LTZ1000 in "series" and attenuate to 10V?
« Reply #20 on: May 20, 2024, 09:40:41 am »

I would be interested to hear from Andreas and some of the other forum regulars to see if anyone has tried this approach before.

Hello,

interesting idea. Have not tried it yet.
So if I have understood it right it depends on a capacitor ratio and a frequency ratio. (and the stability of charge compensation of LTC1043).
I would fear that the stability (temperature, humidity) of the capacitors require frequent calibration.
But I have no clue how stable they are compared with a statistical resistor divider.

Do you have some numbers how linear the frequency ratio reflects the voltage ratio after calibration?
I guess there is also a optimum for the working frequency and capacitor values with regard to the charge compensation of the LTC1043.
(At least I have recognized that there seems to be some sweet spot with 1uF and ~400Hz for the 2:1 divider with the LTC1043).

with best regards

Andreas

Smf