Characterising low frequency noise? - Page 1
Forum section: electronics / general

author: alanambrose
author_rank: Frequent Contributor
timestamp: January 31, 2016, 04:25:06 pm
content: Hi,I thought it would be helpful to gather together a thread with the various pre-amp designs for measuring noise, plus the thinking and characterisation techniques focusing mainly on 0.1-10Hz voltage ref noise, but also bearing in mind ~10-20MHz PSU/regulator noise. This is a new area for me, so I feel only qualified enough to ask some of the dumb questionsI'm hoping to get people's thoughts on:+ the pluses and minuses of the various available pre-amp designs+ techniques for measuring / characterising noise+ instruments and techniquesAs always I'm hoping that we might sometimes wander off topic to relevant sub-topics. Following Rumsfeld, here are some things I think I know, but am pretty sure I don't understand well...Some of the various amplifiers / filter designs for noise measurement:Linear (Jim Williams) -http://cds.linear.com/docs/en/application-note/an124f.pdfTI -http://www.ti.com/tool/TIPD122Andreas -https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg834013/#msg834013(see last attachment in message)Cosylab -http://users.cosylab.com/~msekoranja/tmp/04447683.pdfBlackdog -https://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/msg684852/#msg684852c4757p -https://www.eevblog.com/forum/projects/op-amp-for-amplifiying-5uv-of-noise-to-200uv/msg365606/#msg365606(see png at end of message)tangentsoft -http://tangentsoft.net/elec/lnmp/misc/schematic3.pdfGerhard Hoffmann -http://www.hoffmann-hochfrequenz.de/downloads/lono.pdfLinear - Measuring 2nV/?Hz Noise and 120dB Supply Rejection on Linear Regulators -http://cds.linear.com/docs/en/application-note/an159fa.pdfSome noise measurement and low noise design techniques:Intersil -http://www.intersil.com/content/dam/Intersil/documents/an15/an1560.pdfElectronic Design -http://electronicdesign.com/analog/build-your-own-capacitor-free-high-pass-filterSome previous discussions:Measuring amplifier for 1 / f noise 0.1 - 10 Hzhttp://translate.google.com/translate?hl=&sl=de&tl=en&u=http%3A%2F%2Fwww.mikrocontroller.net%2Ftopic%2F207061&sandbox=1Low frequency, very low level, DC biased, noise measurementshttps://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/50/Op amp for amplifiying 5uV of noise to 200uV?https://www.eevblog.com/forum/projects/op-amp-for-amplifiying-5uv-of-noise-to-200uv/msg364788/#msg364788Question about preamp for measuring noise.https://www.eevblog.com/forum/beginners/question-about-preamp-for-measuring-noise/msg204478/#msg204478Filtering voltage ref noise:TI SBVA010 - "Improved Voltage Reference Filter Has Several Advantages"http://www.ti.com/lit/an/sbva010/sbva010.pdfWalt Jung - "Build An Ultra Low-Noise Voltage Reference"http://waltjung.org/PDFs/Build_Ultra_Low_Noise_Voltage_Reference.pdfCalculating op amp noise:Nat Semi - "Noise Specs Confusing?"http://www.electro.fisica.unlp.edu.ar/temas/pnolo/p1_AN-104.pdfAD MT-047 - "Op Amp Noise"http://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdfAD MT-048 - "Op Amp Noise Relationships: 1/f Noise, RMS Noise, and Equivalent Noise Bandwidth"http://www.analog.com/media/en/training-seminars/tutorials/MT-048.pdfReferences:Noise Reduction Techniques in Electronic Systems, 2nd Edition 2nd Edition by Henry W. Otthttp://www.amazon.com/Noise-Reduction-Techniques-Electronic-Systems/dp/0471850683/ref=tmm_hrd_swatch_0?_encoding=UTF8&qid=&sr=Low Level Measurements Handbook - 7th Editionhttp://www.tek.com/sites/tek.com/files/media/document/resources/LowLevelHandbook_7Ed.pdfAs always, cost-effectiveness is helpful. For instance, I read that a 'dynamic signal analyser' (which appears to be a lf spectrum analyser) is great for making 0.1-10Hz spectral noise plots - but this is not easily available technology. Personally, these questions are bugging me atm:+ what are the pluses and minuses of the various noise amplifier designs?+ what are the effective lowish-cost measurement techniques?+ what about <0.1Hz?+ is there a cost effective technique for spectrum plots?+ what is the good instrumentation / software for this use - analog scopes?, adcs? adc-style signal acquisition with 3458As? Tektronix 7A22? something else?+ why do all the commercially available amplifiers and dynamic signal analysers seem to be available only as old boat anchors - does nobody do this stuff these days, or do they use different techniques/instruments?I hope this is useful / interesting. As always, I also hope that some of our more experienced members will give us the benefit of their knowledge & thinking. Does anyone care to set the context by describing a 'map' of where we are now?Regards, Alan

author: Andreas
author_rank: Super Contributor
timestamp: January 31, 2016, 06:52:15 pm
content: Hello Alan,thanks for collecting all that info onto one site.The design depends very much on the purpose for which you want to use the pre-amplifier.There is no easy way which fits all.So what application do you want to do with this amplifer.Which noise levels and which DC-offsets do you want to cover?Most of the designs are only valid if you have no DC-bias into your cirquit.The input impedance is a design criteria.Most OP-amps with low voltage noise have a large current noise.So a low voltage noise needs a low input impedance.With a low input impedance you cannot measure directly a unbuffered LTZ1000.The charging pulse of the input capacitor+input impedance will degrade the LTZ1000 for many months,since the setpoint of the temperature controller goes to infinity.So my design is done with the constraint of being able to measure references with DC-Bias offset.A maximum reference differential resistance of up to about 10 ohms without much affect on the measurement.A noise << 1.2uVpp (< 0.2 uVpp) for a LTZ.This is possible when selecting the input resistance of around 1K.But during charging the input capacitor you will need a extra resistor in order not to kill the LTZ.Quote from: alanambrose on January 31, 2016, 04:25:06 pmFor 0.1 Hz you will need a minimum observation time of 10 sec.Below 0.1 Hz you have to be very patient.And how do you distinct thermal drift from noise below 0.1 Hz?with best regardsAndreas

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2016, 07:52:55 pm
content: If you've got AoE3, there's a lot in there about noise and measurement, too.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: January 31, 2016, 09:40:33 pm
content: The instruments one could use these days would be a reasonably quality bench DMM, likely a low noise amplifier and software on the PC. The software could be just something like excel or a package to do FFT. There should also be some free versions available. The main part of interest for the DMM would be a mode to get continuous 1PLC readings with more than about 3-4 digit resolution - so nothing really that demanding. Higher resolution might be helpful if the amplifier does not have fine adjustment. Alternatively something like a 12 -16 bit ADC card for the PC should work too. Sound-cards are usually missing the DC and LF part - otherwise they would be an option.Digital scopes can be tricky and have too much low frequency-noise, and 8 Bit resolution might need a limited frequency-band. Some USB scopes with higher resolution converters might be a very good option.50 Hz readings should be just enough to get the noise up to 10 Hz - the lower limit is possibly set by the amplifier or the time used to take date. Also thermal fluctuations get more imports at very low frequencies, especially with amplifiers based on discrete transistors.For very low values the classical 2 channel method with 2 FET based amplifiers , ADCs and than correlation in software is also worth looking at. It may just take some time to find suitable software. The nice thing is that one could trade in longer measurement times for lower noise relatively easy. It's somewhat an advantage as thermal fluctuations are not that reproducible to allow accurate subtraction of single channel measurements - as a first approximation this still works.For amplifier one has the choice of BJT based (high current noise), low noise AZ OPs (possibly high current noise, essentially no 1/f noise) and JFET based amplifiers (often relatively high 1/f noise and thermal effects). As far as I know, there is no clear winner here. The low current noise of JFET based versions can be essential even at low frequencies. Its just expensive to make them work well with low impedance. Usually modern amplifiers are good enough for all but extreme cases (e.g. resistors at low temperature).Very low frequencies (e.g. < 0.1 Hz) might need DC compensation instead of AC coupling at the input.

author: TiN
author_rank: Super Contributor
timestamp: February 01, 2016, 04:20:03 am
content: Nice idea, Alan, subbed to thread.I don't have anything to fill in this thread as of this moment, but I thinkKeithley's 7th Edition Low Level Measurements Handbookis relevant.Also while ago one of EEVBlog members startedthreadabout Keithley 1801 nV preamp for 2001/2002 DMMs. I had finish design of preamp supply plugin board for it with isolated PP SMPS and ultralow noise LDO's, but did not had much time to play with that setup and EM A10 preamp head yet. Thread owner and few other volt-nuts have that board too.Hopefully I'll have something worthy to show here later on.

author: alanambrose
author_rank: Frequent Contributor
timestamp: February 01, 2016, 07:47:25 pm
content: >>> If you've got AoE3Good point, just ordered, I was being cheapskate over $70.>>> So what application do you want to do with this amplifer. Which noise levels and which DC-offsets do you want to cover?For me, low impedance voltage refs and 'low noise' voltage PSUs/regulators/sources - all <25V. I don't yet understand what anyone uses low noisecurrentsources for, I wouldliketo know thoughMy interest is OCD/nuts motivated.>>> Below 0.1 Hz you have to be very patient. And how do you distinct thermal drift from noise below 0.1 Hz?Can 'drift' with time be thought of as vlf noise? 'Thermal drift' I'm hoping to correlate out by regressing general drift vs temperature and then compensating for it (which begs the question 'temperature of what).>>> 50 Hz readings should be just enough to get the noise up to 10 HzIs there a general formula for sampling 'random walks'? My guesstimate was 2 x 10Hz (i.e. 2.5 NPLC in 50Hz land) plus a bit for luck, so I've been using 2NPLC.>>> For very low values the classical 2 channel method with 2 FET based amplifiers , ADCs and than correlation in softwareApologies, but I have not come across that technique before - do you have a reference/link anywhere?>>> Keithley 1801 nV / EM A10Ah interesting, I hadn't seen the 1801 and you just reminded me about the the EM A10. More choicesHo ho ho, you've already given me several weeks of homeworkI'll undertake to update the first post so it has a ready reference set of links. There must be a better wayAlan

author: Andreas
author_rank: Super Contributor
timestamp: February 01, 2016, 08:28:40 pm
content: Hello,I mentioned the "current noise" of a pre-amplifier.Which is a design criteria.You cannot have both: low voltage noise and low current noise.So you have to optimize for a certain input impedance.See also DN140, DN6 and DN3 of Linear Technology.With best regardsAndreas

author: Kleinstein
author_rank: Super Contributor
timestamp: February 01, 2016, 09:02:41 pm
content: The two channel method is a classical method from old times when amplifier noise was a real problem and FFT was not jet available. As far as I know it is still the standard method of choice for very low noise level, e.g. with DUTs with lower noise than the amplifiers used. Could not find a really good free source, here a few links, with the 1st beeing likely the best:http://tf.nist.gov/general/pdf/1133.pdfhttp://www.felicecrupi.it/wp-content/uploads/2011/04/IMTC07_A.pdfhttp://home.deib.polimi.it/sampietr/spectrum/SPECTRUM1.htmlThe main idea is, that the power spectrum is equal to the fourier-transformed auto-correlation function. The correlation of the output of two amplifiers that measure the same source gives the auto-correlation plus the cross correlation of the noise of the two amplifiers. As the noise of separate amplifiers is usually not correlated the amplifiers noise is ideally eliminated. Especially longer averaging can reduce the background very effectively.

author: branadic
author_rank: Super Contributor
timestamp: February 01, 2016, 09:03:06 pm
content: There are two very good german threads for two different types of noise amps:0.1 - 10Hz:http://www.mikrocontroller.net/topic/207061#204603410Hz - 100kHz:http://www.mikrocontroller.net/topic/250656#2573926QuoteNo, not as far as I know. But modified allan deviation or overlapping allan deviation is a nice tool to indicate at what point your values are stable and what noise types are present in your system.

author: alanambrose
author_rank: Frequent Contributor
timestamp: February 02, 2016, 04:43:18 pm
content: Hi,>>> There are two very good german threadsThe 1st I've seen but the 2nd is new to me, thanks. Quite hard going if you're a Google translate user but patience and persistence is rewarded (apparently)>>> The two channel methodVery interesting, I think I get it - and it looks quite doable.>>> modified allan deviationAhhhh, Allen deviation for noise measurement, very interesting. Would there be some way of comparing standard and Allen deviation on the same data and therefore isolating the noise term from the drift term?What would be a good way of setting up an Allen deviation measurement in the 0.1-10Hz voltage noise domain? Something like: noise amp -> ADC at say 1 NPLC -> analysis in R? Should we worry about the 1 NPLC integration time not picking up the full P-P data?p.s. would it be reasonable try and automate the switching in and out of the big input capacitor in most of the pre-amp designs? Or maybe DC couple and null out the DC offset with a DAC - using say a battery as the ref for the DAC i.e. a kind of roll-your-own chopper with a variable null point?Alan

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2016, 07:14:33 pm
content: Using a 1 PLC integration still gets most of the 10 Hz signal. So this will still be able to capture the xx-10 Hz peak-peak noise. This definition also assumes a low pass filter at 10 Hz - so there is some attenuation in this range to. If you really want it exact like definition one might need to adjust this filter a little or use a faster sampling. To really look at noise it the spectrum (from FFT) is often more useful than peak-peak values.In a certain way (auto-) correlation is similar to the Allan deviation. It both comparing values a slight time apart.Input coupling for sources with DC offset is a difficult part. And might be limiting, especially below 0.1 Hz or with low noise sources. Switching in and out of capacitors might not really help (there is still current noise of the amplifiers acting on the caps) and cause extra spikes. So I don't think this a really viable path.Using batteries as a compensation for DC offset is more viable: even with only 1.2 V steps (NiCd Cells) one could use some DC coupled amplification. A DAC might need to be really low noise, so possible, but not easy. For very low frequencies DC coupling is essentially the only option. Higher resolution ADCs could allow measurements even with quite some DC offset at the ADC. So with DC coupling it really might make sense to use something like a 6 digit DMM or 24 Bit ADC, to ease on the DC compensation.

author: alanambrose
author_rank: Frequent Contributor
timestamp: February 09, 2016, 12:37:47 pm
content: I'm having a very nice time reading the new AoE, luckily SWIMBO is pretty relaxed about it. These already raised my interest:+ dc-coupled auto-nulling amplifier for picking off a small signal riding on a DC level - Fig 5.3 / Page 298+ op amp noise/frequency curves - Fig 5.54 / Page 338+ output noise filter for voltage refs - Fig 9.95 / Page 683+ 'capacitor multiplier' for reducing power supply noise - 8.15.1 / Fig 8.92 / Page 578+ noise spectrum of common bench power supplies - Fig 8.123 / Page 580Any thoughts on the 1st item above - the auto-nulling amp?Alan

author: alanambrose
author_rank: Frequent Contributor
timestamp: February 17, 2016, 12:14:27 pm
content: OK I'm going to keep documenting what I find for others tracking the same path.+ picking small signals off drifty dc levels is often encountered in bio-medical applications e.g. EEG. There are some clever cmos implementations of very low frequency low pass filters.+ I didn't realise but Dave has done some teardowns on old Agilent DSAs e.g. 35660A and 35670A.+ the reason that dynamic signal analysers (i.e. low frequency spectrum analysers) are largely discountinued old boat anchors is that the processing has moved to dsp and pc-based solutions. With these low frequencies (unlike at GHz) an fft can be done very easily on any old processor. e.g.http://www.crystalinstruments.com/dynamic-signal-analyzers/. You could see any old adc plus pc combination with enough dynamic range being used in this fashion (e.g. maybe a 3458A with a preamp).+ DSAs are used in lots of mech eng applications e.g. vibration, cars, strain gauges, seismic etc+ alternatives to conventional low noise/low impedance 0.1Hz coupling (e.g. 1,500uF wet tantalum / 1.2K low noise resistor) appear to be:- switched capacitor filters- DSP filters- ADC/processor/DAC methods,see e.g. this conversationhttp://www.electronicspoint.com/threads/ultra-low-frequency-low-pass-filters.26609/

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2016, 04:43:03 pm
content: If you want to use correlation of two inputs, it is essential to have input amplifiers with low current noise (e.g. JFET) . In this case one can use film capacitors (50 µF range) to rather low frequencies, well below 0.1 Hz. Discrete JFETs (e.g. SK369) can be rather low noise, not much higher than good BJT OPs.Even if only the 0.1 Hz - 10 Hz noise is of interest, it is a good idea to have the input AC coupling with a much lower frequency limit, with a much higher resistor. The resistor to ground does not directly adds voltage noise. The contribution of the resistor is more a kind of current noise (thus a higher resistor is better). So the initial AC coupling could be something like 20 µF (Film type) and 1-10 M Ohms, with possibly a switch for initially lower values to speed up settling. The final 0.1 Hz filtering is better done in a later analog stage, or digitally after the ADC. High resolution ADCs have plenty of room for extra LF signal, so it won't matter to have the analog part down to lower frequencies.The other option I see is rough compensation of the DC offset with a series of batteries and/or a DAC (very low noise !) and than have DC coupled amplification (e.g. 10 times) for the first stage.The DAC to compensate does not have to be high resolution or linear or fast, it's just to keep the input stage from saturation. The main requirement is ultra low noise - so normal DACs won't really work. If the initial DC coupled amplifier can be build to handle a +-0.8 V signal one could get away with just a string of batteries and relays. After that the final high pass filter is far less critical, as the signal is low impedance and noise levels can be higher.I would consider planing with moderate quality amplifier(s) first, and plan with the option to have an external amplifier that fits special needs in case the standard one is not sufficient.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2016, 05:57:50 pm
content: Quote from: Kleinstein on February 17, 2016, 04:43:03 pmWhy?How is voltage noise also not a concern?The lowest noise (as noise figure) amplifiers seem to be BJT, assuming your signal source impedance is low enough to match. JFETs are most suitable for higher impedances (kohms+).In principle, you can use a lossless transformer to achieve a constant noise figure for any arbitrary source impedance. Good transformers are hard to make, so it's usually better to design your circuit around the source impedance in the first place. This dictates the choice of device: not current or voltage noise alone, but the ratio.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: February 17, 2016, 08:49:42 pm
content: When doing the correlation type measurement, the voltage noise of the amplifiers still matters, but it can be suppressed by choosing a longer measurement time. The current noise is more critical, as current noise of the two amplifiers will add at the source source resistance and thus produces correlated noise, just like the noise of the source itself. So current noise is suppressed by correlation.The second reason current noise is nasty is that with AC coupling capacitors this will cause quite some noise voltage, as the impedance of the coupling caps may not be so low. At 0.1 Hz, even a 1000 µF capacitor has about 1.5 K of impedance.Voltage noise of BJTs is generally lower than that of FETs. This is especially true for OPs. However this comes with a high current noise and an 1/f component in the current noise. So the low noise of BJTs is usable only for low impedance sources.As the 1/f corner for the current noise is generally higher than for the voltage noise, this is especially important in the low frequency range. The optimum input impedance can be lower by something like a factor of 10-100 in the LF range.Good, large area discrete JFETs also can reach noise levels below 1 nV/Sqrt(Hz) and even the 1/f noise is not that bad with good ones. As the current noise with FETs is very low, its a question of price on how many JFETs or how large ones one can use to achieve low voltage noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2016, 10:36:27 pm
content: I don't get the "cross correlation FFT" either: surely, this simply shows a peak near zero, because the two amplifiers have similar delays? (Referring to thehttp://tf.nist.gov/general/pdf/1133.pdfdiagram.) How does that give you a spectrum?Surely the only useful processing would be to average the signals together, which can be done in analog or digital, and therefore is equivalent to simply connecting the amplifiers in parallel (thus reducing noise as 1/sqrt(N) as usual)?Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: February 18, 2016, 05:32:24 pm
content: The Fourier transformed of the auto-correlation function gives the power spectrum of a signal. With a bandwidth limited noise or 1/f noise, the ACF will be more than just a peak at zero delay, but will show a more or less slow decay and possibly "ringing" for pink noise. For computational reasons one might still compute the ACF via a method using FFT - thus the other way around: doing FFT of the signal, "square" and than FFT back to get the ACF.With two amplifiers sensing the same signal, one can use cross - correlation to calculate the auto-correlation of the signal. This can suppress the voltage noise of the amplifiers with longer averaging times. Measuring of the correlation function can be done over long times without the need to store all the data - just the data for the delay window (gives lower frequency limit) need to be stored. So it's not just the 1/sqrt(2) reduction from two amplifiers in parallel, but another reduction from doing one long measurement. It's more convenient and less prone to drift (e.g. temperature dependence of noise) / nonlinear effects than measuring the noise of the amplifier separately and than subtract it from the noise of the system + amplifier.

author: branadic
author_rank: Super Contributor
timestamp: February 18, 2016, 08:09:50 pm
content: AN159 by Linear Technology is worth reading.www.linear.com/docs/47682

author: alanambrose
author_rank: Frequent Contributor
timestamp: March 21, 2016, 09:09:59 pm
content: Yes v interesting although only >10Hz.Attached is a link to a pdf of all the LNA schematics I could dig up. I'm planning to build something based on Gerhard Hoffmann's 2014 design to begin with.Alanhttp://anagram.net/nuts/LNA/Noise%20Amplifier%20Designs%20Summary.pdf

author: splin
author_rank: Frequent Contributor
timestamp: March 21, 2016, 11:00:00 pm
content: Quote from: alanambrose on March 21, 2016, 09:09:59 pmDon't use Hoffmann's preamp as is - he cocked it up by forgetting, or miscalculating the noise resulting from the opamp input current noise multiplied by the source impedance - especially the input decoupling capacitor. You'd either need to use opamps with much lower input noise current or use a much larger input capacitor. See reply #27 here:https://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/25/ corrected link from email to url

author: alanambrose
author_rank: Frequent Contributor
timestamp: April 17, 2016, 11:59:59 am
content: OK thanks splin - it's taken me a while to get round to chewing through thatCan I ask some dumb-ish questions? - I can't quite replicate your calcs atm - I'm probably missing something...Firstly, where does the sqrt(20) factor come from?Also is there some easily available arithmetic which approximates total rms/p-p noise at the 1/f end given a couple of points on the 1/f graph, a bandwidth (or the upper end of the bandwidth anyway), and an assumption that the line is straight-ish? Or alternatively could you get the noise results like this out of spice?TIA, Alanp.s. I've added links in the 1st post to voltage ref noise filtering and op amp noise calcs.>>>He measured his amplifiers's voltage noise density to be 220 pV rms/sqrt(Hz) at 1kHz, rising to 1nV rms/sqrt(Hz) at .1Hz. The current noise will be rather high though with sqrt(20) x the noise of one op-amp (1/f characteristic – 2.4pA/sqrt(Hz) at 1kHz rising to 10pA/sqrt(Hz) at 1Hz) so only suitable for low impedance sources as he notes in his article. However I believe his amplifier to be seriously flawed – I calculated that the noise voltage generated by the current noise flowing through the 10k//160uF input filter will be around 45nV rms/sqrt(Hz) at 1Hz rising to 800nV at .1Hz!...Note that his article has a chart showing the very respectable noise floor with the amplifier input grounded; however the short is applied after his 160uF, 10kohm DC blocking filter so only the voltage noise is shown. The significant current noise can be seen in the same chart showing the noise floor with the input grounded via a 50 ohm resistor – again after the input filter. At .1Hz this is 18nV/sqrt(Hz) compared to 10nV for4 the voltage noise alone; this implies the noise due to the current noise in the 50 ohm resistor is sqrt(18nV^2 – 10nV^2 – 1nV^2), or 14.7nV/sqrt(Hz). The DC filter has a resistance of 7kohms at .1Hz so the current noise totally dominates.He’s not alone though – looking at the data sheet for the ADA4898-2, on page 14 Analog Devices seem to have made exactly the same mistake! Their test circuit shows an AD743 used to amplify the noise x1000 using an input filter of 1uF/1612kohms!! The current noise of the AD743 will swamp the ADA4898-1’s noise - as shown in their 500nV pk-pk result shown in Figure 45.The real value .1 to 10Hz AD4898-1 noise I calculate to be around 82nV pk-pk. Since AD generally know what they are doing, this may be a simple mistake showing the wrong values in the schematic etc. but I’m not convinced given that their 500nV pk-pk noise is almost exactly what I calculate it would be from the AD743 current noise spec into the 1uF//1612k input resistance. Don’t believe everything you see in the datasheets.<<<

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2016, 12:33:26 pm
content: The input current noise is exactly why Hoffmanns circuit is not good and only of limited use with very low impedance sources.At low frequencies the input filter will not be useful - but at higher frequencies (e.g. > 100 Hz) it can work. The problem is not the 10 K resistor but to small a capacitor.I can't find the circuit in the AD4898 datasheet (maybe newer version), but usually the amplifier under test is used to do a first amplification (e.g. 100 or 1000 fold). So the following stage is far less critical. The AD743 is well suited for such a filter with 1 µF and 1.5 M, as it has the low current noise of a FET amplifier: 7 fA at 1.2 M (which is about the impedance at the crossover frequency) give some 8.5 nV /Sqrt(Hz) of noise at that low frequency. At higher frequency it gets less. Already at 3 times the lower limit the current noise is less critical than the voltage noise. One could have used a larger cap though.With an amplifier like the AD743 it does make sense to use a few in parallel. It still does not need low impedance sources.

author: alanambrose
author_rank: Frequent Contributor
timestamp: April 17, 2016, 09:00:33 pm
content: OK I think I'm beginning to understand. Below is a spice analysis with an OP27 and a 10mHz (that is 750uF (planned to be 16 x 47uF 25V MLCC) // 20K) HP filter. Gives 17nV RMS total from 10mHz to 10Hz. The application is for voltage ref noise measurement < 10V.What do you think?BTW I did run some stats on the AF743 but I have not been able to figure out how to get non-Linear Tech parts into LTspice yet. I'm also thinking that the 10Hz-10MHz and 0.1Hz-10Hz requirements are different enough that they need to be different designs (or at least different front-ends).Alan

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2016, 09:31:49 pm
content: The spice models for many OPs do not include noise. So the noise seems to be from the resistors only.The OP27 like other BJT based low noise OPs has quite some current noise. This really gets a problem at low frequencies. So it is likely not a good choice here.So I would think about a difference stage based on discrete JFETs like Sk170, BF862.

author: splin
author_rank: Frequent Contributor
timestamp: April 17, 2016, 11:15:52 pm
content: Quote from: alanambrose on April 17, 2016, 11:59:59 amWell you've probably worked it out by now but he paralleled 20 ADA4898s to get the voltage noise down to 0.201 nV/sqrt(Hz), from page 3 ofhttp://www.hoffmann-hochfrequenz.de/downloads/lono.pdfHe clearly knows the disadvantages but must have miscalculated the i/p capacitor impedance:That does not come for free: at the same time, current noise gets worse by the same factor. Thislimits the usefulness of this amplifier to low impedance signal sources such as power supplies,voltage references or control voltages from op amp outputs etc.QuoteTake a look at AoE 8.13.2 to 8.13.4. The latter describes how to calculate the 1/f noise corner from a couple of datasheet noise density values, one above and another below the corner frequency.Or take a look at AD's Tutorial MT-048 'Op Amp Noise Relationships: 1/f Noise, RMS Noise, and Equivalent Noise Bandwidth'

author: alanambrose
author_rank: Frequent Contributor
timestamp: April 18, 2016, 07:41:33 am
content: >>> he paralleled 20 ADA4898sAh yes, duh.Thanks splin for the links, very useful. More studyingKleinstein: Yeah agree you are dependent on the model. I believe the ones I've been using do - as you can change the input filter capacitor and see a substantial effect on total input-referred voltage noise due to current noise. Having said that I must have done something wrong in that run (picked up the noise number from the wrong point I think). I've updated the previous graphic.Alan

author: splin
author_rank: Frequent Contributor
timestamp: April 18, 2016, 02:41:53 pm
content: Quote from: Kleinstein on April 17, 2016, 12:33:26 pmAD have indeed changed the datasheet by removing the offending section from rev E in response to someone pointing out the error on their community discussion board (not me - it was before I spotted it). I've attached the relevant section. As you can see the UUT, the ADA4898-1 is run at unity gain hence the problem.It's a bit disappointing that they simply removed the section rather than measuring the .1 to 10Hz noise properly for a device touted for its low noise characteristics but I guess they are short of resources and the datasheet does include the voltage noise graph so you can at least work out the integrated noise yourself.Splin. Added unity gain statement

author: alanambrose
author_rank: Frequent Contributor
timestamp: April 22, 2016, 04:58:00 pm
content: Are these simulations believable? C/R/Op Amp variations in LT Spice with 10mHz-10Hz and 100mHz-10Hz bandwidths.Alan

author: Kleinstein
author_rank: Super Contributor
timestamp: April 22, 2016, 06:05:18 pm
content: The values look plausible. There might be a small amount of 1/f current noise with AZ OPs like the LTC2057. So for the very low frequencies and small caps the noise might be slightly higher.

author: splin
author_rank: Frequent Contributor
timestamp: April 24, 2016, 12:36:30 am
content: It depends on whether you believe the datasheets - if they are wrong the SPICE models are unlikely to be correct. Take a look at AoE 8.13.6, page 569:QuoteThey accuse the datasheets for some bias-cancelled op-amps of specifying the current noise calculated from the bias-cancelled rather than the uncancelled bias current values with results 10X too low - see 5.10.8, page 327. They cite the LTC1012 as an example with a spec of 6fA/sqrt(Hz) at 1kHz; they measured 55fA/sqrt(Hz).They found the discrepancies between published and measured current noise for many auto-zero op-amps were much worse - for example, the LTC1049 measured noise being 50X spec and the OPA2188 100X!Worst was the MCP6V06 which specifies 0.6fA/sqrt(Hz); it doesn't state at what frequency but the datasheet claims it has no 1/f noise. AoE figure 5.52, page 336 shows the measured current noise density to be 230fA/sqrt(Hz) from 100Hz to 4k with large peaks (up to 10000fA/sqrt(Hz) above 4kHz, then reducing to 38fA/sqrt(Hz) at 50kHz!The attached shows a few of their measurements - hopefully not enough to infringe copyright. There may an issue with exactly how they made their measurements but I have no reason to doubt them.Unfortunately they haven't measured the LTC2057 - anyone set up to measure one?

author: alanambrose
author_rank: Frequent Contributor
timestamp: April 26, 2016, 10:43:30 am
content: Ah yes, very good point - many of the datasheet current noise numbers are nonsense. OK that makes the problem a little harder - I need to build an op amp noise measurement set-up first- then screen the likely op amps...Alan

author: alanambrose
author_rank: Frequent Contributor
timestamp: May 08, 2016, 01:41:28 pm
content: OK some more simulation results and a schematic for your thoughts. I'm targeting this as a first base solution here without pushing the envelope. This is based on Herr Hoffman's design with (apparently) lower noise op amps and an RC filter which should give a lower noise contribution from current noise. I don't want to start on a FET front end or any other more creative solution at this point - but would like to just have a baseline to check against. I'm planning to use MLCCs and live with any piezo drawbacks also. This design is targeted at low impedance sources and low frequency noise e.g. voltage refs. Oh, and I'm planing to take a flyer on LiPo batteries as a power source.Apologies if this is old news but I just listened to the 9-part TI video series on opamp noise - I thought it was a great complement to the AoE noise chapter.Alan

author: Kleinstein
author_rank: Super Contributor
timestamp: May 08, 2016, 04:18:23 pm
content: The power supply is not that critical. At low frequency the PSRR of OPs is really good, so not problem there.However using BJT based OPs in parallel and AC coupling at the input might be tricky. The main limit is current noise and even a single OP27 might need a really big capacitor to give good results at 0.1 Hz. So there is rather little use in paralleling more BJT OPs unless you go for several mF of electrolytic caps. I doubt it gets viable to use even a single LT1028 at less than 0.1 Hz with AC coupling.Paralleling OPs might be an Option for high quality JFET based OPs, as they have low noise current and the noise current is not going up as 1/f. So 10 JFET based OPs might be more viable - here things get better with more in parallel. So it might be worth using sockets for the input OPs, so one can relatively easy change and test the OPs in circuit.MLCC caps should not be the lowest grade / highest capacitance - they could have a equivalent to Barkhausen noise (domain jumps) as they can have ferroelectric parts. So it could be difficult to get the really high capacitance.The input AC coupling should not set the lower frequency limit - so the resistor should be larger and an extra filter after amplification used to set the lower frequency limit. This is lower noise and more flexible. The filter could be digital in software as well. One likely needs several minutes for settling anyway for thermal reasons.The amplifier circuit still misses input protection ! this could be something like 2 anti parallel diodes at the OPs input to GND or the inv. input. Otherwise one could easily break the OPs when connecting to a low impedance source. Similar there should be a way to charge the input capacitor in a slow controlled way without overloading the source.I would consider less than +-15 V for the supply (e.g. +-10.8 V (9-12,6 V) = 3 Li Cells each). This means less heat and no need for regulation. At least the input part does not need that much amplitude and not many signal destinations (Scope, ADC cards, Soundcard, DMM) need a high voltage. I am also not sure about so much amplification.

author: Andreas
author_rank: Super Contributor
timestamp: May 08, 2016, 05:05:50 pm
content: Hello,some annotations:The LT1007 is marginal at 1 MHz and amplification = 10 (5-8 MHz bandwidth amplification product).Perhaps the LT1037 would be a better choice.But in my personal opinion it is a better idea to have 2 different amplifiers for <10 Hz and >10 Hz.So you can select the optimum OP-Amp for the task.I am also unshure wether parallelling four LT1007/LT1037 is a good idea together with the 1K resistor.Above 1.5K there are better choices for OP-Amps:http://cds.linear.com/docs/en/design-note/dn140f.pdfAnd 4 LT1037 in parallel would give a maximum resistor of 375 Ohms if chosing optimal input impedance.The noise simulation seems also to give a too high value for the 0.1 - 10 Hz noise.I get below 200uVpp (around 24 uVeff) noise floor with 1K + single LT1037 input stage when measuring a NiMH battery over 100s measurement time.(with the typical 10s measurement time its usually in the 100-150uVpp range).By the way: you should connect all resistors properly in your cirquit diagram (R27-R30).with best regardsAndreas

author: alanambrose
author_rank: Frequent Contributor
timestamp: May 08, 2016, 05:56:49 pm
content: Thanks guys,Lot's of sound advice as everLet me mull over those points and do some more thinking and maybe some more simulations.Duh, point taken about paralleling BJTs.>>> The input AC coupling should not set the lower frequency limitThis occurred to me this later in the afternoon - I did that because lots of other designs seem to>>> By the way: you should connect all resistors properly in your cirquit diagram (R27-R30).Good pointAh that's curious - this is Diptrace and it passes the erc without those connected !Alan

author: alanambrose
author_rank: Frequent Contributor
timestamp: May 30, 2016, 05:53:55 pm
content: Errr,Does this THAT300 front-end sound plausible? The model gives 0.036uV RMS for 0.01-10Hz and 0.032uV RMS for 0.1-10Hz. This is using the 'THAT 300-Series Noise Performance Optimized' model from here:http://www.thatcorp.com/Device_Models.shtmlMy spice file is here if anyone is interested:http://anagram.net/nuts/LNA/THAT300%20front%20end.zipRegards, Alanp.s. I calculate R1 should be 16K for correct ENBW rather than the 10K6 shown in this model. Doesn't make a huge difference to the noise though - 37nV RMS 0.01-10Hz.

author: zlymex
author_rank: Frequent Contributor
timestamp: May 31, 2016, 03:17:03 am
content: Quote from: alanambrose on May 30, 2016, 05:53:55 pmIt can well be true but not very low. The noise of my meter is 0.015uV RMS for 0.1-10Hz by using ADA4528-2(two opamps in parallel).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2016, 12:35:24 pm
content: It looks like the noise current might be quite high for the 1500 µF input capacitance. It also depends on the 1/f noise of the transistors - could not find a note in the datasheet.The discrete transistor circuit has the same limitation on input current noise. It might be slightly better than some OPs (like LT1007) as there is no bias canceling circuit, but there is still the inherent limitation for BJTs. The THAT transistors are not special in that way. They don't even have a high h_fe which would allow a slighly lower current noise.So the amplifier circuit might be as good (low voltage noise) and bad (high current noise) as an LT1028 OP. So no really suitable for an AC coupled input at low frequencies.A good AZ OP might be the better option - just have to find one with relatively low current noise. The lower one goes in frequency, the more it's in favor of an AZ amplifier, as there is no 1/f part.

author: acbern
author_rank: Frequent Contributor
timestamp: May 31, 2016, 02:48:14 pm
content: Using BJT discrete input stages by far delivers the best noise level (MUCH better than any opamp and also better than JFET discrete amplifiers if properly selected). getting to 0.5nV/sqrt hz is certainly possible with a single diff. transistor stage, with paralleling even less. You pay this by relatively high current consumption and also you need to take care of drifts (precision low drift resistors, tight thermal coupling...). And only works with low souce impedance, and their input impedance is low.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2016, 04:16:30 pm
content: The problem is the required low input impedance. The amplifier circuit looks like it is made for something like a 50 Ohms input, but the 1500 µF input capacitor gives about an 1 KOhms impedance at 0.1 Hz and more at lower frequencies. So below about 1 Hz the main noise source would be input noise current time the impedance of the capacitor. Better values would be possible with using only one pair if transistors or reducing the current to a lower value. Just like with BJT based OPs, paralleling BJT inputs only makes sense for very low input resistance.At low frequencies current noise is even more important, as the 1/f cross over for the current noise is usually higher than for the voltage noise. So at frequencies the suitable input impedance is lower by something like a factor of about 10. So even 50 Ohms might be to high for the amplifier above.Not all BJT spice models include all noise effects. I have a little doubt that the noise is that low. This would require very low 1/f noise levels. Especially the only marginally higher noise for the 10 mHz lower limit is suspicious for a non AZ amplifier. Normally I would expect about So it would be interesting to see the noise spectrum, not just the integrated value.

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 12, 2016, 02:03:17 pm
content: Hi,FFS I just wrote a long post and the web browser lost it. Pah, start again. This is the abbreviated version:OK this just went to FAB:http://anagram.net/nuts/LNA/NoiseAmp3.pdfNow includes input protection, input current limiting, capacitor charging, peak measurement etc. It draws heavily on the designs from Hoffmann, AN159, zlymex.Alan

author: TiN
author_rank: Super Contributor
timestamp: June 12, 2016, 02:35:49 pm
content: Hm, I'm interested far enough to design and make a PCB for it, if ETD time within before August is OKHow about that?

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 12, 2016, 06:15:31 pm
content: Ah thanks TiN - my longer message (before the browser lost it) said I have actually received the PCBs back already, and ordered the parts, but I'm away for a week so won't have time to make it up for a bit. I'll have a few spare so happy to send out. You might want to wait until I have verified the design thoughAlan

author: TiN
author_rank: Super Contributor
timestamp: June 12, 2016, 06:46:40 pm
content: Well, we can go in parallel and I'd route fixed version of your design with all caveats emptied already.I wanted to do own preamp from long ago, but have too many unclear questions on which way to go, while your design seem to be a good in between solution and I have most of LT parts handy.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 12, 2016, 06:52:36 pm
content: I have no checked the exact data, but to me it looks like the current for the BJT amplifier stage is too high - thus very low voltage noise, but a rather high current noise. This is especially true for the 10 mHz lower limit -here the current should be considerably lower (e.g. a factor of 3-10) than for the 100 mHz lower limit as the effective input impedance (e.g. the coupling cap) goes up.The exact value of the best current also depends on the quality of the transistors, especially the 1/f part of the current noise. I am not sure the transistors are really good ones for this purpose. The models used for simulations might not be valid for the 1/f region - so the simulation might be way to optimistic. The only marginal higher noise with the 10 mHz lower limit suggests this very much. Normally I would expect something like 1.2 to 3 times the noise with the lower limit, not just 12% more.The 2 series resistors for input protection also add quite some extra voltage noise - this might be a problem at the upper frequency limit (10 Hz). So possibly more noise from these resistors than from the amplifier itself. R10 does not really help very much at that position, so I would leave it out.I don't think you will need D1 and D2. The two diodes to GND should give enough protection.R30 and R31 (and thus R32 too) should be higher values, e.g. 100 - 220 Ohms. The noise level here is not that critical anymore and 30 Ohms might be too much load to the OPs.The second 10 mHz cut off should be switchable to 100 mHz as well - at least if the final filtering is not done in the digital domain.In most aspects the circuit zlymex showed should give better performance, especially if the used AZ op is really that good or at least meets it's specs on current noise. This is especially true for the 10 mHz range.

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 14, 2016, 12:00:05 pm
content: Interesting points.(edit: corrected FET to BJT)I didn't check that the operating point for the BJT front-end is optimal (I meant to but forgot). But the board layout is the same, of course, so the values can be adjusted later. The model I used was the THAT300 spice model for noise so presumably it is close - how close I guess we'll find out. The close 10mHz and 100mHz noise values suggest to me that something more could be optimised, but I have not checked atm.Re the R8/R10 series resistors you may be right, I didn't include in the SPICE model. I posted my model earlier if anyone feels inclined to adapt it.Agree D1 and D2 are overkill but over-engineering is fun no?After modelling a dozen or so op amp-only front ends in SPICE (although not I think zlymex 's choice) i came to the conclusion that I should try a BJT front end which seems to give an order of magnitude improvement without much work.At this point, I'm aiming for a good shot at a workable noise amp here rather than the ultimate solution. I expect to do a bit more work when it comes time to test and characterise. I'm thinking I have a workable board for this even if I have to make a few modifications.Regards, Alan

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2016, 01:19:20 pm
content: The THAT300 is a BJT array - not JFETs. So expect similar problems with current noise as with other BJT based amplifiers. For die simulation it would be interesting to show the noise density and not only the integrated RMS value. This will show if 1/f noise is at least included (could still be to low). There is the option to use essentially the same circuit with low noise JFET pairs (e.g. LSK389) as well. Depending on the FETs, offset adjustment might be a good idea.D1 and D2 also add to the current noise, though not that much with really low leakage diodes. So at least with a JFET input stage leaving them out helps.

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 14, 2016, 06:16:05 pm
content: >>> The THAT300 is a BJT arrayYep my mistake, have changed my previous message so it is less confusingAnd you are right that the 2x500R protection resistors are not helpful for noise.A.p.s. THAT300 noise density below (2 parallel BJT pairs but without protection stuff)

author: Kleinstein
author_rank: Super Contributor
timestamp: June 15, 2016, 03:41:38 pm
content: The curve shows a very low 1/f noise crossover - something like 0.1 Hz. So either an exceptionally good transistor or poor model. If this is for the amplifier with input capacitor, the 1/f part could be just from the capacitor - so maybe the model is missing the 1/f part all together.

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 16, 2016, 12:29:08 pm
content: Well I have not tested the BJTs myself so you might be right ... but I'm working on the theory that Analog selected them in AN159 so they are most likely nothorribleparts and also THAT corp provide two spice models for the THAT300 array - 'rf' and 'noise' models and I'm using the noise model here. Of course, that's only a 'balance of probabilities' argument, 'the proof will be in the pudding' as they say (I wonder why they say that?)Regarding your earlier points - I tried modifying the operating point for the BJTs and it looks reasonably optimal for noise purposes - it might be possible to reduce the current by a factor of two or so without a great penalty - I'm still investigating. Also unless the protection resistor is brought down to about 50 ohms it hurts the noise performance a lot. Lastly, adjusting R30/R31/R32/C25 by a factor of 10 doesn't impact the noise much, so you're right, that's a good change.This all assumes the spice models are reasonably accurate...Alanp.s.https://en.wiktionary.org/wiki/the_proof_of_the_pudding_is_in_the_eating'you will see it when you fry the eggs'!

author: Kleinstein
author_rank: Super Contributor
timestamp: June 16, 2016, 04:44:13 pm
content: Changing the current of the BJTs does influence the noise. Good BJTs are close to the ideal behavior: With a higher current the voltage noise goes down with the square root and the current noise goes up about the same way. So the ration of voltage to current noise, which gives the source impedance for the best noise figure, depends on the current. The other way around the optimum current depends on the source impedance - higher current for a low impedance source and less current for a high impedance source.The input capacitor (not the resistor to GND) limits the source impedance a low frequencies - this is why the 10-100 mHz range might want a lower current. A 1.5 to 15 K input impedance is not that low and thus should have a low current (more like 100 µA instead of 1.5 mA).Form National Semiconductor LB52 the optimum current is at 26 mV*sqrt(hfe)/Z_source. This is without 1/f noise. As 1/f noise if often higher in the current, this should shift to even lower currents at low frequencies.Good quality BJTs can have a 1/f cross over in the 1 Hz range, but especially at the highest level the performance can be different from sample to sample. I am just a little skeptical in expecting leading etch performance for a part that does not even have 1/f noise specs in the data-sheet. The measurements will show how good the transistors really are.Looking at the Datasheet of the LM194 transistor pair might give a good idea of what to expect from good BJTs. So performance might be acceptable at much lower currents (e.g. < 100 µA).

author: alanambrose
author_rank: Frequent Contributor
timestamp: June 24, 2016, 08:52:13 am
content: Ah, many thanks for your clear description of optimum operating points, voltage and current noise vs. source impedance.>>> I am just a little skeptical in expecting leading etch performance for a part that does not even have 1/f noise specs in the data-sheet.Yes I understand and it seems odd that THAT corp is seemingly too lazy to document and test the noise specs of something they market for 'low noise front ends'.I had guessed that, for AN159, that Linear must have measured the noise of a number of a BJTs and selected the best for their application. Presumably, not more that a day or so of work for a junior engineer if you have the right equipment sitting on the bench. If that was the case, I was simply piggy-backing on their work.However, I noticed a couple of days ago (reading AoE on a flight) that tables 8.1a and 8.1b have H&H's own BJT noise measurements. For multiple parts (table 8.1b), the THAT300 is shown as 0.9nV/sqrt(Hz) at 1mA i.e. well towards the good end of the table. So, Linearmighthave duplicated H&H's work or they might have simply read the numbers in the AoE table.Alan

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2016, 09:57:27 am
content: Something like 0.9 nV/Sqrt(Hz) at 1 mA is about what many BJTs get if they are not to small. The big question is, where is the 1/f corner. Here the simulation looks to good to be true, likely they just omitted the 1/f noise as it is not that relevant for audio applications. Also 1/f noise can change quite a lot between wafers - so not all parts are the same in this respect. Not specifying 1/f noise also makes testing easier, as low frequency noise tests are time consuming and thus expensive.AN159 even shows 1/f noise - here the corner is at about 100 Hz, which is much more realistic.So that amplifier circuit is good for the 10 Hz - 100 kHz range, but not really below 10 Hz.For the low frequency end, the input capacitor is rather high impedance (e.g. 1-10 K range) so an BJT based amplifier should be more using a lower working current, more like 100 µA. Also high gain transistors have an advantage. So maybe even a relatively high gain BC548C could be a better choice. If you really want to go BJT, I would consider something like a MBT6429DW1 or similar, maybe a few of them in parallel. So a high gain transistor, so one can use higher collector current.