More than one reference all together? why? - Page 1
Forum section: electronics / general

author: nour
author_rank: Frequent Contributor
timestamp: May 25, 2017, 11:09:42 am
content: I want to build an LM399 Reference box ad I came across xDevs model designI have noticed there are 2 LM399 connected together, why?Would that help regarding the stability?because I have seen the other LTZ1000 diagram and there was only one, so why not two like LM399?

author: TiN
author_rank: Super Contributor
timestamp: May 25, 2017, 11:22:49 am
content: Some designs use four LTZ1000 or even ten LTZ1000, such as Fluke 7001 system or Datron 4910. More references provide lower noise (due to averaging) and better prediction uncertainty (due to statistics, it's less likely that all references drift very differently).Design with two LM399 is cheap (total BOM maybe 50-60$), but LTZ1000 5 times more expensive. Actually I use multiple LTZ1000 modules together as well.Some data. Also multiple references (doesn't matter eitherh LTZ or some cheaper LTC6655) can be compared to each other for better idea of stability. This way you don't have to worry about your DMM/voltmeter drift, as it's nulled out.Beware with that design, values on schematics are not for exact 10V or 20V output and need correction.

author: nour
author_rank: Frequent Contributor
timestamp: May 25, 2017, 11:31:39 am
content: Thanks, I somehow knew that you will be the first one to reply, anyway, the perfect person to do the first comment.Ok, so basically due to averaging the noise will become less, that is very interesting, so I have two questionsFirst, how many LM399 should we but togeather to make the noise lower by an order of magnitude?second, which is better, 2 LTZ1000A or 10 LM399AH regarding the noise and stability? or maybe those are two separate things, not quite sure about it really!

author: nour
author_rank: Frequent Contributor
timestamp: May 25, 2017, 11:40:55 am
content: Since you are here, I wanted to ask you about some details about your LM399 circuit!what is the EXT_IN+ and EXT_IN- for?on the inverting input of the LTC2057HV you have 4x 3.75K resistors connected in parallel and also 3x 466 connected in parallel and altogether connected on series, why? why not one for the first 4 and one for the other 3, actually why not only one resistor for all of them?

author: nour
author_rank: Frequent Contributor
timestamp: May 25, 2017, 11:48:32 am
content: Also, one more thing, what is the square in the red circle in the attachment image means?, I am using eagle and I haven't seen this before!Thanks

author: TiN
author_rank: Super Contributor
timestamp: May 25, 2017, 12:03:39 pm
content: Some voltnuts may drag me to the witch bonefire, but I'd say even 1 x LTZ1000 is more stable than 10 x LM399s. But LTZ is far more delicate circuit to begin, if you don't have high budget to spend. It's not enough to just buy parts and get it all together, testing the circuit stability (even LM399) require understanding and proper (read, expensive) equipment.I've used multiple resistor positions, so I can use different resistors to trim for specific value, or match resistors with opposite temperature coefficients for better stability. You can use just one position if your requirements are relaxed.EXT_IN+/- are there due to multiple purpose of the circuit. Alternative use was NOT to put LM399 modules there, but supply external voltage (for example 7.1366351 from one of LTZs) and use opamp + resistors on this board to generate different voltages.Square on schematics means guard trace inside the square with the box trace. Google term : high-impedance guarding.Here's interesting appnotefrom Analog Deviceswhich explains a little bit about using multiple parts for lower noise. It's different purpose there with using multiple opamps, but idea is close. In two words - momentary noise of each reference are not matching together, so once outputs summed opposite "spikes" are compensating each other, giving lower peak-peak amplitude signal. However measurement of this noise levels is not easy, very slow process

author: Kleinstein
author_rank: Super Contributor
timestamp: May 25, 2017, 12:11:35 pm
content: TIN's circuit used a lot of resistors in series / parallel connection and AFAIK also some for alternative use. So this is not a very good example, but more confusing.With equal references the noise (voltage) goes down like the square root. So to lower the noise by a factor of 10 it would take 100 units in parallel. So it work wo use several in parallel, but it is not very effective in larger numbers. Another point to take into account is that not all units of the LM399 will have the same noise - so in might be worth testing for noise and leave out the noisy ones. Ideally one could adjust the weights (resistors used for averaging), so that even higher noise units could give a small positive contribution.With stability, chances are that drift in the units will be similar, so the gain in stability tends to be even less. The use here is more to detect possible bad units. However this would need to look at the individual references and than decide by software which ones are good and which ones are likely bad. With 2 refs one could detect if they are drifting apart to much and especially the case of one shows a sudden jump / step - though no way to tell which one is bad. Same type refs tend to show a similar drift. Paralleling can not detect / reduce that common drift, but only the more random part that effects individual refs.AFAIK it takes about 20 LM399 to get a noise level comparable to the LTZ1000. The number can very, depending on the current and maybe temperature used in the LTZ1000 circuit. Also noise of LM399 is not all the same. The drift values are essentially never reaching that level.

author: nour
author_rank: Frequent Contributor
timestamp: May 25, 2017, 12:13:20 pm
content: Thank you very much, I really appreciate your replies the are very valuable to me, I do understand your emphasis on the expensive equipment point, yes I know those parts on their own are not much to benefit from them unless I have the proper equipment to properly measure all necessary parameters which are very unfortunate for me!

author: nour
author_rank: Frequent Contributor
timestamp: May 25, 2017, 12:20:18 pm
content: Is there a cost friendly way to reduce the noise of the LM399 without adding more units to the design?like for example adding active filtering stages with very low noise floor without gain to the output?

author: MK
author_rank: Regular Contributor
timestamp: May 25, 2017, 02:46:19 pm
content: The frequency of the noise is very very low, and there are the intermittent steps and orientation changes to consider. Knocking out the mains hum etc is feasible, but filtering the VLF noise is impossible. There is noise with a period of days, weeks and months.

author: David Hess
author_rank: Super Contributor
timestamp: May 25, 2017, 02:54:29 pm
content: Quote from: nour on May 25, 2017, 12:20:18 pmThe most cost effective way to reduce the noise is to use an integrating converter and ratiometric design which rejects the noise. Otherwise the best way to reduce 1/f noise is to use a quieter source.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 25, 2017, 03:49:15 pm
content: Noise can be addressed as described above but I'd also make the case for several references that you can inter-compare, three or four. That way if some circuit flaw occurs you know which one is bad, and can even reset it after repair. I'm also a fan of having several different types of reference circuit, reducing the chances of all the references having similar long term drift.

author: nour
author_rank: Frequent Contributor
timestamp: May 25, 2017, 11:30:52 pm
content: Quote from: nour on May 25, 2017, 12:20:18 pmThe reason I said "without adding more units" because I have seen several teardowns for high-end benchtop multimeters, 6.5 to be specific that have only one LM399 reference, so I thought there is no need to have extra LM399 and maybe there are alternative ways to just reduce the noise with maybe just active filtering or something similar!And one more thing, I think for DC measurements, the noise that would make a concern would be under 100 Hz noise, right, wrong? or maybe between 0.1 and 10 Hz?Quote from: MK on May 25, 2017, 02:46:19 pmI really didn't understand what do you mean by noise with period of day, weeks or months

author: timb
author_rank: Super Contributor
timestamp: May 26, 2017, 12:10:54 am
content: Quote from: nour on May 25, 2017, 11:30:52 pmThink noise with a frequency in the uHz to pHz range.

author: nour
author_rank: Frequent Contributor
timestamp: May 26, 2017, 01:37:42 am
content: Quote from: timb on May 26, 2017, 12:10:54 amYea, I know it meant this, but I meant to ask about how is this happening, why it is happening, is it normal for all references to have this kind of weird noise(is it actually weird?)

author: CalMachine
author_rank: Frequent Contributor
timestamp: May 26, 2017, 01:42:24 am
content: Quote from: nour on May 26, 2017, 01:37:42 amhttps://en.wikipedia.org/wiki/Pink_noise

author: David Hess
author_rank: Super Contributor
timestamp: May 26, 2017, 02:45:41 am
content: Quote from: nour on May 26, 2017, 01:37:42 amIn MOSFETs it is caused by charge trapping in the gate dielectric which shifts the threshold voltage. In PN junctions it is caused by defects which alter the gain. For the simplest case where only one charge trap or defect is present, pink noise becomes popcorn/telegraph/burst noise. When many charge traps or defects are present with different time constants, then the aggregate popcorn/telegraph/burst noise becomes what we see as pink/flicker/1/f noise.It can be filtered only with difficulty because of the long time constants required so it is better to avoid generating it or use a design which inherently rejects it.

author: MK
author_rank: Regular Contributor
timestamp: May 26, 2017, 04:46:01 am
content: Hi,Read this and you will see that there is a very low frequency noise component present in even the very best units available:http://vmetrix.com/ZenerP.pdf

author: Andreas
author_rank: Super Contributor
timestamp: May 26, 2017, 10:08:56 am
content: Quote from: nour on May 25, 2017, 11:30:52 pmHello,its not active filtering but selection.filtering is difficult at frequencies near / below 0.1 Hz.You would need large (electrolytic) capacitors which have(not constant over time) leakage currents that also create noise.And you do not want a "stinker" LM399 with > 7uVpp in a precision instrument.And also if using averaging: a LM399 with 7uVpp will spoil a group of references with 3-4uVpp.Attached noise measurements of LM399#2 (7.2uVpp) and LM399#1 (3.6 uVpp)y-axis 1 uV/divx-axis 1 sec/divAnd you also want to sort out references with "popcorn" noise(jumps of several uV or even ppm´s for several seconds or even minutes)Attached a example of a MAX6350C reference over several minutes with jumps up to 4-5 ppm.Quote from: MK on May 26, 2017, 04:46:01 amI would not call the seasonal changes as noise.Since you can correlate it to the environment.So it is more a drift. (temperature / humidity / pressure dependant).Noise is something statistical that cannot be predicted.with best regardsAndreas

author: nour
author_rank: Frequent Contributor
timestamp: May 26, 2017, 11:19:06 am
content: So basically, you are saying I should buy some of them and go through the process of categorizing them according to their noise and choose the best onemaybe I will just buy a lower noise reference like LTZ1000A and use it, right?Hope you correct me if I am wrong about it!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 26, 2017, 11:33:07 am
content: Filtering on the LM399 can be useful, but this is usually only for the higher frequency range, like higher than 10 or 100 Hz. For a DMM the noise frequencies of interest are more in the 0.1 mHz - 1 Hz range. Normal analog filtering is not practical in this range.In some applications it is possible to use a kind of filtering, by using different references for different frequency ranges. So one could use a lower noise, but higher drift reference in combination with a low drift, but higher noise reference (to cover the really long times scales). This is kind of using a digital filtering scheme which can work well for very low frequencies too. The Keithley 2001 7.5 digit DMM is using such a 2 reference scheme, though I am not sure the implementation is made as well as it could be.Choosing the best out of several LM399 is a viable way. So is averaging a few (like 2 or 3) of them, with checking there individual quality and if needed leave out the really bad ones or adjust the averaging weight. With a suitable low weight even a noise reference could help a little to reduce the overall noise. There is little sense in using more than about 5 of the LM399, as then you start to get to the price range of the LTZ1000 with even lower noise.

author: Andreas
author_rank: Super Contributor
timestamp: May 26, 2017, 12:40:41 pm
content: Hello,also a LTZ1000 can have popcorn noise.https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg833804/#msg833804So you have to check this also with a LTZ.With best regardsAndreas

author: MK
author_rank: Regular Contributor
timestamp: May 26, 2017, 02:05:58 pm
content: Quote from: Andreas on May 26, 2017, 10:08:56 amseasonal shifts were only one paragraph in that paper, Fig 1 is the most telling diagram.

author: David Hess
author_rank: Super Contributor
timestamp: May 26, 2017, 03:58:46 pm
content: Quote from: nour on May 25, 2017, 11:30:52 pmGood multimeters benefit from using analog to digital converters which integrate the input and by necessity the reference. This produces a sin(x)/x response which low pass filters the input and the reference and tracks the desired resolution by reducing the noise from both.Even with this though, pink or 1/f noise can be seen in many cases in the display where the last digit "flickers" between values in excess of what would be expected from quantization error and other noise sources hence the name "flicker noise". Unfortunately because flicker noise increases at lower frequencies, the longer integration times which produce a lower cutoff frequency also integrate more of the flicker noise which increases at low frequencies.Burst noise sounds like popcorn or morse from a telegraph hence these other names for the same thing. It is caused by contamination of the semiconductor fabrication processes and has been a solved problem for decades now which is fortunate because it can be very difficult to screen for. I suspect low noise zener references suffer from it more simply because it is more noticeable with their inherently lower noise levels unlike higher noise references where the burst noise is covered up.Below is an example of popcorn noise which took me weeks to track down in the vertical amplifier chain of a Tektronix 2230 DSO. The fact that it looks so digital made me initially search for a problem in the digital logic but it was really in a linear transconductance amplifier.

author: Muxr
author_rank: Super Contributor
timestamp: May 28, 2017, 01:22:00 am
content: Consequently I am working on a non metrology related project which requires an ample amount of Zener noise. I just actually bought like 20 different zeners to characterize the noise with, and pick the one with "best noise".

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 28, 2017, 12:57:45 pm
content: Drifting OT, but last time I built an RF noise bridge, I used the emitter-base breakdown voltage of a small transistor. That seemed to have a better noise spectrum than a zener. Might have been a 2N2222, but try several different types.

author: MK
author_rank: Regular Contributor
timestamp: May 29, 2017, 10:53:37 am
content: Quote from: Muxr on May 28, 2017, 01:22:00 amOT, but you might like this set of links:http://www.ciphersbyritter.com/he is going the opposite of Volt nuttery, he wants noisy zeners by running at as low a current as is practical.

author: chuckb
author_rank: Frequent Contributor
timestamp: June 03, 2017, 12:35:18 am
content: Other people have discussed and explained flicker noise so here is a visual to support the understand of flicker noise floor limitations in Voltage Standard measurements.I used a K2182 Nanovoltmeter to monitor the 10V output voltage of two Fluke 732A zener references. This is similar to what you would do to calibrate a known Voltage reference to an unknown reference. For a real calibration the interconnect leads would be reversed to average the thermal EMFs. During this test the temperature was stable and I did not see any correlation between error voltage and temperature.The raw data was sampled every 30 seconds and it has a 2.4 uVpp noise level. If that data is post processed with a 10 minute moving averagethe noise level only drops to 1.8 uVpp. With an extreme 2 hour moving average the noise is reduce to 1 uVpp. At these time levels the drift of the zeners starts to add to the noise. You can see a slight downward slope to the traces after 100 hours.With 2 or 4 independent references you can get some noise reduction and you reduce the influence of one bad device.