Looking for a low noise amplifier (<1nV / rt. Hz) - Page 1
Forum section: electronics / projects

author: jbb
author_rank: Super Contributor
timestamp: November 13, 2018, 07:42:07 pm
content: I’m working on a wide band current sense system (DC - 1MHz) and I’m hunting for a better amplifier.I checked Analog Devices (and this LT) and Texas Instruments and came up with the ADA4895-1, which will run off my +- 3.3V analog rails and has about 1nV/rt.Hz ven.The LT1028 is a tiny bit better but consumes a lot more current and needs +-5V rails.Can anyone suggest another supplier which might have something even better?

author: coppercone2
author_rank: Super Contributor
timestamp: November 13, 2018, 07:50:12 pm
content: I don't think you will get much better then those without parallel devices.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 13, 2018, 09:37:03 pm
content: I have used the OPA211/OPA2211 which is 1.1 nV/rtHz and would work on +/- 3.3 V.At this level, remember that the noise density is usually specified at 1 kHz. If you want to compare 0.9 nV, 1.0 nV and 1.1 nV, you really need to look at the performance at the frequency you care about. This means including 1/f noise at low frequency and including the effect of input capacitance on noise gain at high frequency.Its really hard to do much better than 1 nV/rtHz, especially at low frequency. Some HEMT amplifiers do a factor of 2-3 better than that but only at high frequency, pretty much anything beyond that requires cryogenic cooling. Most current shunts are terribly matched to transistor amplifiers which have noise resistances in the hundreds or thousands of ohms. If you could live with AC coupling a transformer would really help. As an illustration, look at the SR554 which gets a noise voltage of 0.1 nV/rtHz by using a 100:1 transformer.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 14, 2018, 01:25:12 am
content: Maybe the LT6200: 0.95nV/√Hz (100kHz)A lot more power hungry.

author: jbb
author_rank: Super Contributor
timestamp: November 14, 2018, 01:25:13 am
content: Thanks allI had a look at paralleling 2 op-amps, and it made some improvement (approx 20% drop in total system noise). I don’t think that’s worth it on my 1st prototype where I will likely have some other mistakes to fix.Thank you for mentioning the SR554, ejeffrey. Right now I’m looking for DC - 200 kHz (or 1 MHz) bandwidth, so a straight up transformer input won’t hit the spot. However, in version 2 or 3 I could have a look at a LF + HF path to get a wide band transformer in there.

author: Marco
author_rank: Super Contributor
timestamp: November 14, 2018, 02:37:32 am
content: A discrete differential pair front end with say 9 BJTs on each leg?

author: drussell
author_rank: Super Contributor
timestamp: November 14, 2018, 03:08:25 am
content: Does Linear Systems make anything suitable to adapt a circuit around? I know they make all sorts of very low noise stuff, certainly lots of JFETs and I remember seeing some article in some print journal that I'm sure I still have around here somewhere years ago about using some LSK parts for an op-amp front end to get a low noise configuration but I think they were mostly going for very high input impedance rather than ultra low noise.I honestly don't recall. Hmmm... Think think think...

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 14, 2018, 03:20:25 am
content: I've pushed it to 220 pV/rt Hz with 10 pairs of ada4898-2, but they also need +-5V.The 4898-2 is interesting because you get 2 op amps for the price of an AD797.<http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf>My preamp has been redesigned in the mean time with some costly components :-(such as RS-170 relays and a wet tantalum cap in the input.There is also a differential preamp in Art of Electronics, ed 3. page 506.. I first wanted to buildthe differential version, but since I needed that ugly coupling cap anyway, I settled onthe single ended version. It features 70 pV/rt Hz and needs only 1/4th the transistors.regards,Gerhard

author: Marco
author_rank: Super Contributor
timestamp: November 14, 2018, 03:53:51 am
content: Need the differential (ie. composite opamp) for DC since he doesn't want a two path amplifier.

author: jbb
author_rank: Super Contributor
timestamp: November 14, 2018, 07:10:04 am
content: Quote from: Marco on November 14, 2018, 03:53:51 amYes, I'm looking at composite amplifier for (at least) the first go. Future versions - if I decide to keep going - can go further afield.So I had a bit of a poke about online and found thisbeastfrom the good people at ETH Zurich. 100nV/day drift, 100fA max input leakage, Peltier coolingand an oven. Quote of the paper:QuoteDecoded: "we spent ####ing ages binning components and most of them weren't good enough."

author: cat87
author_rank: Regular Contributor
timestamp: November 14, 2018, 07:34:45 am
content: I've come across some designs until now that might be what you need. Bust they don't come in a nice SO package.There's this amp using the 2SK117, claims 1.4 nv/sqrtHz @ 1kHz:http://techlib.com/electronics/audioamps.htmlThen there's Glenn's design:http://www.glensstuff.com/ulnma/ulnma.htmAnd the eevblog discussion thread on this one is:https://www.eevblog.com/forum/projects/low-noise-amplifier/For Glenn's design, Rs still has some of those BFxxx Jfets in stock.Hope these help.Later edit:Just remembered there's also a nice design in LT's application note 159. The design there is mainly for low frequency stuff but it can go up to 1Mhz and more

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 14, 2018, 09:15:41 am
content: Quote from: jbb on November 14, 2018, 07:10:04 amExactly. These here are the Id vs. Vgs plots for the IF3602. Don't expect them to be matched in the sensewe use that word. But at least you don't get opposite outliers in one can for €55 per piece. (Mouser)<https://www.flickr.com/photos/137684711@N07/37321004540/in/album-72157662535945536/lightbox/>You also get a few hundred pF input capacitance per transistor, so cascoding is definitely required for your MHz.The ADA4898 does not share that problem. 1 MHz is easy.In that FLICKR album are also measurements of some depletion FETs that can be used as a cascode withoutgate voltage dividers. That eases bootstrapping the JFET's drain if required.On Semi has some new JFETs that can replace the EOLed BF862.I have not yet read this ETH article, but in JFETs Id and Vgs have opposite TCs, so there is anoperating point that has TC = 0, and one may get by with less oven ado.If you don't want to count single electrons, you will probably have a low impedance shunt resistor,so better go bipolar. (A sub-nV/rtHz preamp fed from a 1K source is futile a priori.)In the Mat-02, 03, 04 data sheets are also examples for multiple transistor pairs in front ofop27-ish op amps, but parallel op amps are probably more painless.cheers, Gerhard

author: Marco
author_rank: Super Contributor
timestamp: November 14, 2018, 09:27:21 am
content: If you can use a wideband transformer, you don't need FETs ... your source impedance will be very low. If your source impedance is very low you can use BJTs.

author: Marco
author_rank: Super Contributor
timestamp: November 14, 2018, 02:56:17 pm
content: BTW, has anyone ever made a low noise amplifier out of boost converter? I've never seen it, but it should be possible.

author: Echo88
author_rank: Frequent Contributor
timestamp: November 15, 2018, 11:37:21 am
content: @Marco: Wouldnt the input noise be massively changed trough the inductor?@Gerhard: Why do you use wet tantalums now? Too much leakage current/leakage current noise from the foil caps that you use?Thanks for the paper jbb, very interesting!

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 15, 2018, 04:40:45 pm
content: That's because I underestimated the need for a large input capacitor.A 100 or 200 uF foil capacitor would have been enough from a frequencyresponse point of view, but that resulted in a rise of noise at the bottom endthat is much stronger than 1/f.Sub-nV/rtHz amplifiers make only sense when their input signal source issmaller than, say, 60 Ohms. 60 Ohms delivers 1nV/rt Hz thermal noise atroom temperature and your complete system could never be better than that.OTOH the input impedance of the preamp itself is high and determinedmostly by the 10K bias resistor. But these 10K also create noise like a 10Kresistor. That completely spoils the good noise behavior of the preamp itself.Here the low signal source impedance can help. AC-wise, it is in parallel tothe bias resistor, but the coupling C is in series to it. That works nicely in themiddle of the passband of the amplifier.But at the lower -3dB point, Xc is equal to Rbias, so there is not much shortingleft. From a noise viewpoint, Xc must be small against the signal source impedanceand not only small against Rbias.That is very unpleasant. You no longer need 100 uF but 5000 uF.That cannot be made with foil caps. I already had a dozen or two of WIMA10u/50V in parallel. The largest foil caps I could get and already costly.So i used the wet slug tantals. 5000u cost about €70 if you are very lucky.Could be more than twice that. The cheapest I have found was AVX from Digikey.Vishay was MUCH more than twice.The wet tantalums still have a guaranteed ESR of 400 mOhm. Quite a lot, IMHO.I choose the wet slug tantal after reading Jim Williams' article on measuring the ref noiseof the LT6655.BUT, even in the 70 pV amplifier in the Art-of-Electronics-III-style i could not findan obvious drawback using good quality aluminium electrolytics by Panasonic. YMMV.That design was more a proof-of-concept for a chopper amplifier. At the 500 KHzchopping frequency, the capacitors are harmless.In FET amplifiers you may see gate resistors in the 100 Meg range and one istempted to see that as a drawback, noise voltage-wise. But that's not true.The 100 Meg are easily ac-shorted.Pic: chopper amp, might feature 100 pV rt Hz at << 100 mHz, but I cannot yet verifythat because of the 1/f noise of my 89411A FFT analyzer. The CPLD creates thechop clock and the delayed unchop-clock after the 500 KHz filter.

author: Echo88
author_rank: Frequent Contributor
timestamp: November 16, 2018, 11:35:15 am
content: Interesting details, thanks. Didnt know you design discrete chopper amps yourself. Did you consider using two LNAs and cross-correlation to further reduce your LNA-noise? Like the battery noise measurement at NIST for example.I have a cookie-box with two 0.1-10Hz-LNAs and would like to employ the cross-correlation via Python, but lack the knowledge/documents to do so.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: November 16, 2018, 12:01:51 pm
content: I have played shortly with the cross correlation in my Agilent 89441A.The effect was so strong that I had doubts about the results. Thatmachine has so many things to set up that you need a programfor it (or at least a checklist).I have just married a LT2500-32 ADC to a Beaglebone Black; thathas probably much better 1/f behavior than the 89441A. I'm justgetting first time series; still a lot of software to do. Eventually I'll docross correlation also, maybe over xmas.It seems that numpy can do cross correlation, but my programsare in C.cheers, Gerhard

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2018, 04:06:14 am
content: Quote from: Marco on November 14, 2018, 02:37:32 amIt does not even require parallel devices to do this. Large area audio driver transistors like the BC327/BC337 can operate below 1nV/SqrtHz assuming that the relatively high bias current is acceptable. A precision amplifier is used in parallel to remove DC offset. If low noise at low frequencies is necessary, then a chopper stabilized amplifier will be needed to remove 1/f noise.The BC327/BC337 is not used for its power or current handling capability. The large area is a way to lower the base spreading resistance which limits low noise operation. Using lots of smaller transistors in parallel is more complicated because emitter ballasting cannot be used without raising the noise level. This points to the problem JFETs have achieving low noise with their lower transconductance from their higher source resistance.