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Author Topic: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.  (Read 30568 times)

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Online TiN

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #325 on: April 21, 2017, 12:14:31 AM »
I'll play with compensation resistor. Best case would be after this to leave this reference be, and use it as comparison vs rest of the references.
But that will be next month, as all my gear is tied up now, running other python snakes, and I have few more items to clear out first.
For time being it's attached as is to my scanner setup (2x3458A/K7168) with rest of the LTZ crowd for logging.

If we get tempco fixed (<0.05ppm/K), then VK5RC could use this known tempco ref to test his own 3458A tempco, so he could later "transfer" better stability into rest of his LTZ's.
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Offline Edwin G. Pettis

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #326 on: April 21, 2017, 03:42:44 AM »
@echo88,

I remember this article, while there is some reduction in 1/f noise from this filter, the application of it with a LTZ reference is questionable.  First, the LTZ has significantly lower noise than any of the Maxim Vref chips, the lowest appears to be about 4uV P-P typical vs. the LTZ's 1.2uV P-P.  In the case of the LTZ, the filter would also increase the DC drift of the output noticeably, there are always trade offs to be made and frankly I doubt this filter will reduce the 1/f noise of the LTZ all that much.  In this case, the trade offs of perhaps slightly less 1/f noise vs increased DC drift is likely a washout at best.

I also question the measurements being done with a DVM, it is a sampling/averaging device by nature and therefore is not the best instrument for measuring P-P waveforms, an analog o'scope or strip chart is much better or perhaps a spectrum analyzer.  1/f noise averaged will not produce an accurate result and that is all a DVM can do.  Of course, comparing a DVM's averaged noise measurement has some small indication of 'total' noise but nothing more.  You simply cannot average out 1/f noise.

The best noise reduction technique is to use the lowest noise components in the circuit, since the LTZ represents the lowest in noise available, that leaves the other components, the LT1013 is intended to be used with the LTZ, it was designed with that in mind and the lowest resistor noise components are wire wound.  The next level of noise reduction is the circuit layout and power supply noise.  There are always trade offs in circuit designs and you have to pick the places in the circuit where you can reasonably reduce the noise, unfortunately filters in this case are very difficult to implement when you're working at such low frequencies and the DC output is also very important.
 

Offline Kleinstein

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #327 on: April 21, 2017, 04:46:39 AM »
The main source of 1/f noise in the LTZ reference it the LTZ1000 itself. So there is not much to gain from using super high quality OP or resistors. So the obvious choice would be a slightly higher current for the LTZ1000 or two of the references in parallel. Also an other, lower noise reference and slow compensation of the long term drift of the low noise reference might be an option.
 
This filter circuit adds twice the noise of the OPs. So for a low noise reference it needs lower noise OPs, which will cause more current noise and possible drift. The listed MAX4238 is specified at 1.5 µV_pp für 0.1 - 10 Hz. So you trade in a reduction of the 1.2 µV_pp by an additional about 2.1 µV_pp from the 2 OPs. So the OPs uses should be much lower noise, more like LTC2057.

The 0.1 -10 Hz frequency range is used in datasheets, because it can be measured in reasonable time and with analog AC coupling. However in many applications of a stable reference the actually interesting noise could be the 0.001 -0.1 Hz range. Filtering is even less practical here.

Using a DMM to measure the LF noise is not that bad. However one has to include the averaging of the meter as part of the filter that sets the bandwidth. With fast enough readings (e.g. 1 PLC) this should not be an issue. The more important point could be noise of the input amplifier - so it might need an extra amplifier stage or a low level meter like the 34420.
 

Offline MisterDiodes

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #328 on: April 21, 2017, 01:47:03 PM »
@ MisterDiodes: Always a pleasure to read about your experiences, especially since i have the 2508-32-Evalboard sitting in a corner!

Maybe you can have a look at the proposed voltage-reference-filter-design mentioned here and share your opinion: http://electronicdesign.com/energy/filter-trims-ultra-precision-voltage-reference ?

Also: do you know any good ref-filter-design for a LTZ1000 regardless of the stabilizing-time?

Edit: Since the LTZ1000/Ref-Amps are the best voltage references out there apart from the superior JJA and companies like Fluke/Keithley surely like to improve their stuff: Do you know of any successful projects which improve stability/noise of those references or does it just not make sense and the only good way is to handselect and parallel those references like it is done in the Datron 4910?

Ha! That circuit's been floating around a while for sure, in various forms.  Nothing to see there, and a poor ad for Maxim - another capacitance multiplier.  A great teaching moment in 1/f noise and how hard it is to get rid of.

Just try building a LP filter with cutoff down in the 10's of mHz - watch what happens when you turn it on.  You'll be waiting a while... <Grin>  You'll still have noise over longer time spans now, and you'll be kicking yourself for blowing that much on those $65 caps, etc.

You don't need the "Teflon standoff's" either if you understand guard circuits - and you sure don't need that extra shunt regulator.  That was there just because those goofy Maxims were only 5V parts - typical of the early crop of AZ amps.  Noisy buggers also, compared to later offerings. Maxim seemed to always miss the mark there.

Not making noise in the first place is the concept, and paralleling Vref's (up to a point) is probably the better bet.  Probably 9 or 10 Vrefs is a practical limit and point of real diminishing returns. 4 Vrefs might be even a better upper limit.  Only if the customer is willing to pay though.

If you build this stuff for a living, you'd better make sure there's a practical sense of profit also.  If you chase low PPM's for no real reason and no profit - you'll be out of business.

At some point you have to tackle the problem the other way and decide what noise you can live with...and what is "good enough" if you have multiple measuring systems.  The 3458a has great linearity and has good resolution for relative measurements over shorter time spans.  As good as it is you still have 1hr, 10hr and 24hr uncertainties that are unavoidable - and that's why the most practical approach is to average across multiple systems (Vref and ADC or whole DMM's) to help deal with 1/f noise.

Better yet is to spread your measurements across different devices and techniques - so that why we have 3458a's as well as 732's, KVD's and null meter sets.  That way you get a better spread of drift rates and noise, and more likely to fence in an accurate estimate of a DC voltage.  What you don't want is everything drifting together with similar 1/f noise characteristics.

Remember: It's always only an estimate measure when you're in low PPM territory.  Even a JJ-Array has uncertainty.  Most of the time trying to spend an exponential amount of effort trying to shave another half-ppm of uncertainty is a major waste of time AND money.
« Last Edit: April 21, 2017, 01:54:13 PM by MisterDiodes »
 
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Online TiN

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #329 on: April 21, 2017, 02:18:10 PM »
LP filter with cutoff down in the 10's of mHz - watch what happens when you turn it on.  You'll be waiting a while... <Grin>  You'll still have noise over longer time spans now, and you'll be kicking yourself for blowing that much on those $65 caps, etc.
$65? Cheapskate! How about $707? :)


Quote
that's why the most practical approach is to average across multiple systems (Vref and ADC or whole DMM's) to help deal with 1/f noise.
Hm, you got me an interesting idea for that old DMM noise test experiment. Sample 4 DMMs (2x3458 + 2x2002's) in sync together and average their common result minus the offset, to get total noise figure. That could show if noise of such rig lower than single DMM.
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Offline MisterDiodes

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #330 on: April 22, 2017, 02:44:17 AM »
Again:  Learn from the app notes where 1ppm-level values are being tossed around.  You'll never see just one DMM being used, it will be usually 3 or 5 units at least.

TiN: Using a $700 cap is unfortunate- let alone a $65 X 2 in that rip-off Maxim LP filter circuit (Burr-Brown was doing that in the 80s, and I think they ripped it off the idea from maybe National, if I remember right).  The original idea dates back to mechanical chopper / tube days.  If you're having to buy those caps, there is -probably- a better way; or you're not really selling the circuit for a living.  The real companies that use wet tant caps won't be paying $700 a pop, guaranteed.

Here's the rule:  If you don't like the price of a component, you're not buying enough per year.

Just some other random thoughts here...Even more stuff that has nothing to do with "Story of Little Jumper"

Now class, looking at that Maxim circuit (Which is doubtful it was ever used, it's just an ad for a trade rag) - someone tell me how long it takes to charge that 10uF Teflon cap C1 via 3M at the input to let's say a 10V Vref to within 0.1ppm of Vref (where you'd like it to be to start stabilizing output).  Now look at how long it takes to charge that second cap via 1M (actually, what is the net charge across second 10uF Teflon cap?).  Like I said, you'll be waiting a while.  You'd like to use the circuit in some reasonable amount of time.  "Reasonable Time" might be open to interpretation here - but this is something you have to calculate if you're designing a real project.  Maybe the device is left on all the time...or maybe the device has to be battery powered, in which case you don't want to wait a long time after power up before you can even measure anything.

Your end application makes all the difference here.

Back to Maxim circuit:
WHOops...Now check the influence of that op-amp input DC and noise current and overall effect of amp switcher spikes, and how does that affect charge time and accuracy?  With a 3M resistor is place and the op-amp running, what is the net effect of switcher current spikes when they are not exactly bipolar / cancelling in nature?  (That's the other dirty secret of AZ amps ).  Will that cap C1 gain or lose more charge over time, and what noise effect does that add to the output?

WHOoops...What happens when you power off or glitch the power rails (maybe another device fails on the power rail)?  That 10V stored on the cap C1 has to go somewhere, and that input pin and internal surge diode on the amp is the only real path.  Now you find out how fragile that diode in the amp is.  You learn to look for this in amp circuits whenever you see a larger cap hanging off an input pin directly, with no limiter resistor.  That's a red flag right there nobody really used that circuit in practice - or never designed for reliability.

And on and on and on.

That capacitance multiplier filter isn't a very good approach, and its doubtful it was ever put into real practice beyond a trade mag advertising blurb.  For the cost of that filter you could add another paralleled LTZ and resistors at least.

 
« Last Edit: April 22, 2017, 03:25:29 AM by MisterDiodes »
 

Offline Kleinstein

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #331 on: April 22, 2017, 03:28:32 AM »
The filter circuit with the OPs as shown would have a background noise level of around 45 nV/Sqrt(Hz) (more at low frequency) from the OPs and the additional high noise from the AZ switching (in the 10s of kHz range). So about the noise level one could get from 2 LM329 in the kHz and above range.  So if you start with a good reference like the LTZ1000, noise at the output would be higher than at the input side.

The bias current and current noise should not be such a big problem here: the input impedance seen at higher frequencies is relatively low and the bias current is still low for the modern AZ OPs. Even lower noise AZ OPs are often below 200 pA, and this bias is relatively constant over temperature. The offset of A1 is not critical at all (maybe 1 mV DC at the filter cap), and A1 sees a 10 K input impedance. Thus only 1 µV from a 100 pA input bias.

Still the circuit should have a second passive filter stage (before A2) to get rid of much of the higher frequency noise of A1.

One possible nasty part hidden in this circuit is dielectric absorption in the C1 capacitor. The RC time constant is something in the 300 s range, thus something like 1 hour waiting to get it stable - however dielectric absorption can be a really slow process, depending on the capacitor.
 

Online TiN

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #332 on: April 22, 2017, 02:00:50 PM »
MisterDiodes, little jumper is working happily since, other than jumpy, it's actually stable. And to be fair, those wet slug caps are not for any kind of filter, they are bought for far less than mouser list price for AN124 style low-noise preamp.
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Offline MisterDiodes

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #333 on: April 23, 2017, 02:33:10 AM »
CORRECTION to my previous post:

The capacitors mentioned in the Electronic Design mag article above - 935C1W10K - are not $65 as I wrote above, but I see TTI has some for $19, generally around $20~$25ea. elsewhere.  Somehow I was guesstimating the BOM cost of that LP filter circuit with newer AZ amps, and that number stuck in my head.  It will still take a very long time to settle, important if you're building some sort of battery powered application.

Sorry about that.  My gift is I know everything there is to know about everything, my bane is I just don't know it all at the same time  :)



 

Offline ap

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #334 on: April 23, 2017, 04:25:36 AM »
There are other PP capacitors available at a much lower cost per uF.
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Offline Echo88

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Re: [FT] xDevs.com KX LTZ1000A fairy-tale, or the story of little jumper.
« Reply #335 on: April 23, 2017, 08:20:05 AM »
Thanks MisterDiodes and Kleinstein for dissecting the circuit. Since theres no real cost-effective solution to improve those LTZ1000-references i will stick with temp-stabilizing my reference and be done with it.
 


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