Lowest noise OP amps for low frequency, low level AC coupled signals. - Page 1
Forum section: electronics / general

author: janaf
author_rank: Frequent Contributor
timestamp: February 21, 2019, 03:23:21 pm
content: I made this graph for some of the lowest noise OP amps available for- Low frequency i.e 0.1-10Hz- AC coupled signals- 1000x gain 10k & 10RThe noise is SPICE calculated.- LT6018- OPA828- OPA189LT6018: The new king of low noise bipolar land, after 30+ years with LT1028. Needs less than 100 ohms input impedance and a 22000 uF input capacitor it maintains around 6nVrms in 0.1-10Hz. There are a few others like the LT1007 that would be better around 1kOhm.OPA828: A JFET input amp with very respectable LF noise data. With 1Mohm and 2,2uF in, 50nVrms 0,1-10Hz.OPA189: An auto-zero (chopper) amp, less than 20nVrms 0.1-10Hz with 100K input impedance. I also plotted quad of these paralelled, 33K in via 67uF should give less than 10nVrms 0.1-10hz.The LT6018 was modelled with LTSpice. The OPAs where modelled in TINA which has a very handy tool for noise accumulated over frequency.Any others that I missed, that really challenge these?I hope you find the graph useful.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 21, 2019, 03:48:04 pm
content: How much can we trust SPICE modelling of noise compared to real world measurements?(This is not a criticism of SPICE which I love to use, I just don't understand how "good" it is on this front)

author: chuckb
author_rank: Frequent Contributor
timestamp: February 21, 2019, 04:12:46 pm
content: Does your noise model include the Johnson Noise (white noise) for R1? With a 1 Meg resistor the Johnson noise density will be something like 130nV/rt Hz. In a 10Hz BW would that be 400nV rms? So, for low noise you really want to stay with a lower resistance.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: February 21, 2019, 04:17:11 pm
content: QuoteADA4625 has lower noise for JFET input (0.15 μVp-p, 0.1 Hz to 10 Hz) than opa828...Also would be interesting to compare the ADA4522-1 which I think is comparable or superior to the opa189 in the chopper realm.Thanks!Randall

author: SilverSolder
author_rank: Super Contributor
timestamp: February 21, 2019, 04:45:16 pm
content: In tests done by @chuckb of this parish, the ADA4522-1 did not perform as well as OPA189:https://www.eevblog.com/forum/metrology/low-frequency-noise-of-zero-drift-amplifiers/msg2093356/#msg2093356

author: David Hess
author_rank: Super Contributor
timestamp: February 21, 2019, 04:59:58 pm
content: A spot noise graph would show the contribution of 1/f noise versus wideband noise. The higher 1/f noise of the FET input parts could be suppressed with a parallel chopper stabilized amplifier without introducing more wideband noise although I have only done this in DC coupled applications.Discrete bipolar and FET input stages are especially suitable for low noise AC coupled applications but there is a lot to be said for the simplicity of using an integrated part.Note that the non-inverting configuration produces additional distortion products from limited common mode rejection and non-linear input impedance which may be undesirable in low noise applications. Some operational amplifiers take special steps to avoid the especially pertinacious non-linear input capacitance problem like the LT1169 and parts like this may be desirable in some non-inverting applications despite higher noise. The alternative is changing the circuit topology to suppress these sources of error.

author: janaf
author_rank: Frequent Contributor
timestamp: February 21, 2019, 05:09:38 pm
content: I'll get back about that when I've done some measurements, for a point here and there.It would be very time consuming, long measurement times plus the requirement to measure with different cap/resistor values... There's also quite a spread in real world from device to device.... See the graph as ballpark values showing what is realistic to aim for.Quote from: SilverSolder on February 21, 2019, 03:48:04 pm

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: February 21, 2019, 05:13:27 pm
content: Quote from: SilverSolder on February 21, 2019, 04:45:16 pmSure, but chuck's testing (and this simulation) are done with a pretty specific test setup. It does not take much to flip the relative rankings around. So, in my view, best to keep all of the contenders in the mix. For example, the OPA189 has higher bias current than the ADA4522. So, if that is important, and it often is, you might find yourself choosing the ADA4522.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 21, 2019, 05:31:23 pm
content: Quote from: RandallMcRee on February 21, 2019, 05:13:27 pmUnderstood and agreed. I guess Chuck's testing was focused on ultimate low noise performance below the usual 0.1Hz test range, which seems interesting for DC measurement applications and is a spec that we don't normally see in datasheets. Having to deal with the 70pA bias current is definitely a minus for the OPA189, but it seems a pretty unique part if ultimate low frequency noise performance is important to you (as long as other factors don't outweigh this benefit, as you said).

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2019, 05:48:17 pm
content: For the low frequency part the OPA827 is still a little better than the OPA828. Another option is the OPA140.For the low current noise JFET based OPs there is the option to use a few of them in parallel.From some point on the AZ OPs can be lower noise, especially if the bias current is not a real issue in a pure AC application.The BJT based OPs are low voltage noise, but the current noise also usually goes up with an 1/f contribution. Quite often the 1/f cross over for the current noise is higher than for the voltage noise. So the optimum source impedance is even lower in the low frequency range. So in the LF range even 100 Ohms is still on the high side for the LT1028.The AD8676 seems to be a relatively good BJT based OP, with not that much low frequency current noise.However one problem with the data for the current noise is that there are two noise currents that are partially correlated. So tests with the same resistor at both inputs may give results that can be misleading in cases where the high impedance is only at one input.AZ OPs noise, especially the current noise may depend on the capacitance at the input.

author: janaf
author_rank: Frequent Contributor
timestamp: February 21, 2019, 05:53:58 pm
content: I did do the ADA4522, ADA4622 and they ended up a bit higher than OPA189. I didn't plot all devices I calculated, only the best in this particular setup.

author: janaf
author_rank: Frequent Contributor
timestamp: February 21, 2019, 06:00:19 pm
content: Quote from: David Hess on February 21, 2019, 04:59:58 pmThat's pretty much why I made this graph. As far as I know some discrete input AC coupled JFET amp can achieve sub 10nvrms 0.1-10Hz while having (edit) 100Meg+ (edit) input impedance (Levinzon and others) But quite a lot of work, calibration, stability issues......would go into that.

author: David Hess
author_rank: Super Contributor
timestamp: February 21, 2019, 06:41:06 pm
content: Quote from: Kleinstein on February 21, 2019, 05:48:17 pmI should have mentioned that for bipolar input parts. Low noise operational amplifiers intended for audio applications do not include input bias current cancellation partially because of the noise it adds. So even under ideal conditions in an AC coupled application, a discrete bipolar design can achieve lower noise than an integrated part which includes bias current cancellation.Is there an equivalent to any of the lowest noise bipolar precision parts which does *not* include bias current cancellation? I have looked a couple times but never found any. I suspect that market is too small so discrete designs are used for that kind of performance.

author: janaf
author_rank: Frequent Contributor
timestamp: February 21, 2019, 07:19:48 pm
content: Quote from: Kleinstein on February 21, 2019, 05:48:17 pmYou are right, OPA827 and OPA140 are both a little better than OPA828 in this setup, see pic.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2019, 07:36:47 pm
content: Quote from: David Hess on February 21, 2019, 06:41:06 pmFor the precision parts the bias cancellation is kind of a requirement. The extra circuit is not a big deal anymore - so hard to find some.Even the low noise audio amplifiers (e.g. OPA1611) tend to include bias cancellation as otherwise the bias would be rather large.Too keep the current noise reasonable it may take super beta transistors, and these usually require a kind of bootstrapped cascode to use them. So not that attractive in discrete form.It looks like even in the low frequency range good JFETs are better than BJTs, when it comes to high impedance - so there is likely not that much market for discrete BJT solutions and such BJT pairs. So if discrete it's more like JFETs anyway.With lot's of effort a discrete build copper amplifier may also be an option, as it can use a JFET based amplifier instead of the usually CMOS based in AZ OP chips. So no more need for a high chopper frequency to get low noise, especially if a low BW is OK.

author: janaf
author_rank: Frequent Contributor
timestamp: February 21, 2019, 10:51:12 pm
content: Added OPA27 which has input bias cancellation and it performs very well here! Interesting, didn't know about this....OPA1611 was mentioned, I had it in there originally but removed it as it wasn't top notch in _this_ application. However it seems one of the very best in terms of THD. I also saw somewhere that it's virtually identical to OPA211. Anyway, I put the OPA1611 back in th plot.

author: Inverted18650
author_rank: Frequent Contributor
timestamp: February 22, 2019, 02:22:06 am
content: Quote from: janaf on February 21, 2019, 05:09:38 pmIt may take a full year (or more) to test these all in real world environments, with all the variables, but that would be a great paper!I am retired and almost interested enough to give this a go. Seems the TI and Analog Devices clans are split pretty evenly among their respective sides. Only real world data will due. The question then becomes, what is a fair and practical test that can provide repeatable data within for each DUT?edit*Adding: we need a basic building block circuit, with say ten (10) variations (for caps, etc), that we can plug and play each of the 20 or so DUTs and record the data for accurate comparisons. If this were the case, I believe I would love to be involved.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 22, 2019, 03:16:07 am
content: Perhaps a board with reed relays to choose between various fixed caps and resistors, so the testing could be automated if desired?That way it would be possible to subject each amp to the same GPIB battery of tests...

author: Andreas
author_rank: Super Contributor
timestamp: February 22, 2019, 05:40:34 am
content: Quote from: janaf on February 21, 2019, 03:23:21 pmHmm,the question for me is: how usefull are source impedances below 1K Ohm. (when building a LNA for noise measurements).Typical Zeners (1N829) have 10-20 Ohms zener impedance.The unbuffered LTZ1000 is also in the some Ohms range.So if you load such a zener by 100 Ohms you already measure up to 20 % lower noise than actual available.So typical I want to stay at 1KOhms as input impedance which is the domain of LT1007/LT1037/OP27/OP37.A large part of the LNA noise floor is also contributed to (the AC-part of) leakage currents of the input coupling capacitor.(if not using foil capacitors with thousands of uF).with best regardsAndreas

author: Inverted18650
author_rank: Frequent Contributor
timestamp: February 22, 2019, 06:05:10 am
content: Very nice catch.As we move foward designing the test circuit, our guidelines will include a minimum 1k input imp. to ensure noise floor tolerance for every DUT.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2019, 08:53:33 am
content: Some BJT based OPs need a low source impedance to get a very low noise level or more accurate there best noise figure. However this does not mean that the amplifier is loading the source with this impedance. The input impedance of the amplifier circuit can be higher.Ideally the resistor to ground after the input capacitor is larger by something like a factor of 10 than needed to get the low frequency limit at 0.1 Hz. This reduces the loading to the source and also avoids much of the resistor noise in the transition region. The exact low frequency cut of that has to be set in a later stage, behind the initial amplification. This could also be in the digital domain, if the signal goes to an ADC anyway.Still it is problematic to have OPs that need a low impedance source, as it needs a huge capacitor, that can represent there own problems.Using Spice models to compare OP performance is a little tricky - not all models are really accurate when it comes to details like current noise.Usually the data-sheets are still a little more reliable. From the DS performance the OPA27 is not that good.Testing low noise in real life is not that bad. There is another parallel thread on such tests on AZ OPs. It's mainly setting up an amplifier for 2 input resistors (one near zero and one suitable for the OP, so that current noise dominates). The behavior with a different source impedance can than be calculated to a good approximation. Only AZ OP may need extra test with small capacity to ground, as there can be effects from short input current spikes, that makes them possibly less predictable. The noise test can be done with DC coupling, so no need to wait for a coupling cap to settle at the input.To get noise data down to 0.1 Hz one might want to take something like 10 times the minium 10 second interval to average over some random parts like popcorn noise. Still this would be something like a few minutes to measure and maybe 30 minutes for thermal settling. Just looking directly for peak to peak noise is not very effective. The better way today is more like an FFT to get the noise spectrum.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 22, 2019, 09:36:03 am
content: The 0.1Hz lower limit of noise testing seems too high a frequency, somehow... one gets the sense that this limit was chosen because it was possible to measure with reasonable equipment, rather than what we really need to know.For example, if you have an instrument that you want to stay stable for hours or days on end, you really need to know the noise performance down to the milliHertz or even microHertz range.

author: janaf
author_rank: Frequent Contributor
timestamp: February 22, 2019, 10:00:15 am
content: Quote from: Inverted18650 on February 22, 2019, 02:22:06 amGlad this caught on!To provide comparison, I made a new plot with a line I called the "0110 Axel line".It's a spline on log-log diagram, with just three points, preliminary values at100R 4nVrms100K 10nVrms1G 500nVrmsThe 0110 Axel could be a baseline to relate to and plot results against. Op-amp results below the line would be Breaking News, or errorsI also extended the OPA868 and at very high input resistor values, it beats the OPA827 by about as much as the OPA827 is better at low resistances. So the OPA828 remains in the plot.

author: janaf
author_rank: Frequent Contributor
timestamp: February 22, 2019, 10:22:27 am
content: On a parallell note, any leakage through the input cap would destroy the results. For high impedance inputs, tiny cap values require v-e-r-y low leakage while for the lowest noise amps, low input impedance requires large caps. Which E-lythsare good enough?I once stared doing measurements on various caps but results where not very clear due to- leakage through the voltmeter, even it has 1 Gohm input, that was way too much.- temperature variations on the cap made results very shaky.I am thinking of setting this up again. would use- Mechanical relay MUX- A fA leakage input amp buffer (LMC660?) for the DMM, repeatedly checked vs a reference.- A temperature controlled box.Attaching a pic of a good 200uF polypro. It's BIG.

author: TiN
author_rank: Super Contributor
timestamp: February 22, 2019, 12:32:37 pm
content: I have measured few wet-slug electrolytic caps, same as Jim used in his preamp.They shown leakage <4 nA at 10V bias. Only problem is the cost.

author: chuckb
author_rank: Frequent Contributor
timestamp: February 22, 2019, 02:41:16 pm
content: Quote from: janaf on February 22, 2019, 10:00:15 amShould the 0110 Axel line be raised by by a factor of 3.16 because you are looking at the noise over a 10Hz bandwidth?The X axis of the graph is labeled "Input Impedance R1 ohm". Is this assuming a source impedance of 0 ohms?If the source had a 1k impedance the noise in a 10 Hz bandwidth would be 12nV rms.If the source had a 100k impedance the noise in a 10 Hz bandwidth would be 120nV rms.Linear Tech sorted their opamps based on input Z in the attached Design Note. The graph is based on a 1Hz bandwidth. You are looking for noise in a 10Hz BW so your results will be 3.16 higher that the Design note values.Good luck!

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2019, 04:24:16 pm
content: Quote from: janaf on February 21, 2019, 10:51:12 pmBeware that early input bias current cancellation circuits caused distortion in audio applications. As I recall, this applies to the OP-27 and related parts and was one of the things that Linear Technology corrected in their LT1027. The Burr-Brown OPA27 dates back to at least 1984 so very likely suffers from this but I would need to see the detailed schematic to know.Quote from: Kleinstein on February 22, 2019, 08:53:33 amBe careful doing this if you want to make comparisons to datasheet specifications for noise. Limiting the measurement to 10 seconds as is commonly done adds another pole into the frequency response. Making the measurement over a longer time period results in a different filter shape factor complicating comparisons with other sources.QuoteThis is what I would prefer down to 0.01 or 0.001 Hz which requires a 100 or 1000 second measurement and there is no reason to use AC coupling in the test circuit then.On the other hand, the standard 0.1 to 10 Hz peak-to-peak or rms noise measurement with the proper filter shape is much easier to produce and more directly comparable to the datasheet specification. Note that operational amplifiers are not tested in production for this specification except in special cases like the customer paying a premium for it. Instead, a spot noise measurement at a higher frequency is made which correlates well enough with low frequency noise.Quote from: SilverSolder on February 22, 2019, 09:36:03 amAC coupled applications tend not to extend below 0.1Hz.Quote from: TiN on February 22, 2019, 12:32:37 pmQuote from: janaf on February 22, 2019, 10:22:27 amSome low leakage high voltage aluminum electrolytic capacitors can achieve similar performance but must be tested for burst noise.For shunt capacitors in a filter, two capacitors can be used in series with the middle node bootstrapped with 10 times the input capacitance. This diverts the leakage to the output and minimizes the voltage across the top capacitor lowering its leakage. This is a common configuration for filters on precision references where excessive leakage would compromise DC accuracy.In some applications, a DC accurate filter using an AC coupled operation amplifier is used.QuoteThe LMC6081 which is the lowest noise and highest precision of that series was the natural choice years ago but there may be some better ones now. The LMC6001 is a low bias current tested LMC6081 but practically all of the LMC6081s are 10s of femtoamps at worst and testing them is pretty easy with the operational amplifier configured as an integrator to integrate its own input bias current.

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2019, 04:52:16 pm
content: Quote from: Kleinstein on February 21, 2019, 07:36:47 pmIntegrated Linear Systems sells a super beta NPN pair but good luck finding it; they have one of the worst web sites ever.Input bias cancellation is kind of weird. On one hand, it makes the need for balanced source resistance unnecessary so there is no added noise from the balancing resistance. On the other hand, correlated input bias current cancellation *increases* noise with unbalance source impedance. *runs in circles waving arms wildly*QuoteThe low noise JFETs have higher input capacitance which adds its own complications. Still, they are awfully good now.QuoteLow noise low drift discrete JFET input stages are still feasible albeit at a high cost. A more economical option is to chopper stabilize a low noise JFET input stage, discrete or integrated, with a parallel CMOS chopper stabilized amplifier and get the best of both including suppressed 1/f noise and suppressed charge injection. The only drawback is again high JFET input capacitance although I think this can be suppressed with a cascode.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 22, 2019, 04:56:17 pm
content: Quote from: David Hess on February 22, 2019, 04:24:16 pmI was thinking DC applications - and acknowledging that due to 1/f phenomena, DC is the new AC!

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2019, 05:35:34 pm
content: Quote from: David Hess on February 22, 2019, 04:24:16 pmThe 0.1 Hz to 10 Hz range is kind of used as a standard, but AFAIK there is no real standard on what type of filter to use exactly. Defining an effective noise bandwidth is not accurate if there are different amounts of 1/f noise. AFIAK the common choice is a 2 nd order HP and LP each. However the AC coupling and similar might add some modifications to the ideal filter curve, especially at the lower end, even if that extra cross over is below 0.1 Hz.The 10 seconds window is another option for the low end and would especially well fit modern implementations. Longer data sets could still be looked at with 10 s sections at a time. Especially for the direct peak to peak reading it would take many 10 second intervals and than some average of the peak to peak values from different intervals. The peak to peak readings show quite some scattering, when just looking at single intervals.So there might be a value to using the spice models instead of datasheet values that might use slightly differing definitions of the 0.1-10 Hz band.Another way to show low frequency noise, especially well below 0.1 Hz is using Allan deviation plots. Here the upper end (corresponds to lowest frequencies) is scattering by definition. So the good data only extend to something like 1/5 the measurement time.

author: MiDi
author_rank: Frequent Contributor
timestamp: February 22, 2019, 06:34:57 pm
content: Quote from: Andreas on February 22, 2019, 05:40:34 amHow did you measure that?My LNA did not show any significant difference between input shorted and 33mF@8V ~30nA leakage (10min):Quote from: janaf on February 22, 2019, 10:22:27 amA two stage design rejects the leakage induced offset.Take a look at the threaddiy-low-frenquency-noise-meter.I measured twoPanasonic2.23.3mF M Typ A <4nA@10V.Quote from: TiN on February 22, 2019, 12:32:37 pmWet-slugs might be better at high temperatures, for normal lab environment selected el-caps can get in the same range.For selecting in the nA range there is no need for special equipment, a dmm with 1-10MOhm input resistance in series with cap will do in conjunction with reasonable linear psu.It takes a couple of days to soak/settle.Further details e.g. in Threadmeasuring-capacitor-leakage

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2019, 08:37:42 pm
content: Quote from: Kleinstein on February 22, 2019, 05:35:34 pmThat is one reason I really liked Linear Technology. They were very specific about how they measured 0.1 to 10Hz noise in their datasheets so customers could reproduce their results. Back when I was messing with LT1128s and LT1150s, I got results exactly in line with what their datasheets said. It was actually weird when the math, theory, measurements, and datasheet specifications all matched up. It was even weirder being able to measure a 50 ohm change in source resistance to better than 2% by noise alone.QuoteIf I was automating noise measurements, I would certainly do that. My own experience is that if the peak-to-peak measurement over 10 seconds is inconsistent, then something is wrong. It is worth doing the RMS measurement as well in parallel as a consistency check.QuoteAs pointed out earlier, I would not blindly trust the SPICE model noise results. I would only use them for tuning after having verified them in the real world.QuoteBut my math already uses integrated noise density and that is what is in the datasheets.

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2019, 08:39:02 pm
content: Quote from: SilverSolder on February 22, 2019, 04:56:17 pmSome operational amplifiers, especially modern fast CMOS devices, have incredibly high 1/f noise but it tends not to be a problem for low frequency precision devices above 0.1Hz. Chopper stabilized devices dominate below 0.1Hz which is why their informal noise specification goes down to 0.01Hz. From 0.1 to 10Hz, non-chopper devices are competitive if not better.Newer chopper stabilized operational amplifiers are better or say they are but I have not used them.

author: Andreas
author_rank: Super Contributor
timestamp: February 22, 2019, 08:48:56 pm
content: Quote from: MiDi on February 22, 2019, 06:34:57 pmHello,I have that usually when I did forget to put a bias voltage on the input of the LNA before storing the LNA.So I have to bias the input capacitor for ~2 days before I can make useful measurements.Otherwise the noise floor is increased above 200uVpp (low frequent) where it should be around 120uVpp.at ~8V there are less problems with noise floor. But if I increase to ~13V bias (10*NiMH) I have even large excursions of the noise floor.But I also have to admit that one of both input capacitors is rated only for 16V. (so somewhat at the limit).with best regardsAndreas

author: janaf
author_rank: Frequent Contributor
timestamp: February 22, 2019, 09:36:05 pm
content: Quote from: chuckb on February 22, 2019, 02:41:16 pmThe results are TOTAL noise 0.1-10Hz, not per sqrtHz. This because the 1/F noise at (low frequency) has nothing like a constant per sqrthz noise, as it usually has at higher frequency. And yes, it's calculated for zero source impedance as that would be application specific.

author: MiDi
author_rank: Frequent Contributor
timestamp: February 22, 2019, 09:38:09 pm
content: Hi Andreas,I did not used my LNA for couple of weeks - or more like months and left it input shorted.When measuring my LTZ1000 first time, I let the input soak for ~15min on 9V battery and ~15min with 7V output, so first measurement was taken after ~30min - see results in ltz thread.10V was measured afterwards and before every measurement series I did quick sanity check on noisefloor with input shorted - with matching results to earlier measurements.Maybe the 0815 25V cap used is pretty good in terms of settling and leakage

author: janaf
author_rank: Frequent Contributor
timestamp: February 22, 2019, 09:50:03 pm
content: Quote from: chuckb on February 22, 2019, 02:41:16 pmThis is because there's no zero on a log scale, I stopped at 1. Where the noise line in these plots becomes horizontal, it's in reality equivalent to what you get if you short the non-inverting input to ground (with zero or 1 ohm). This is also simple to verify by measurement; Set the (non inverting) gain to some very high value, 1000 or more, short the non inverting input to ground and measure the noise on the output. The resistor values should be low, and can be as the output will only be some microvolt. That measured value, divided by the gain, should be equivalent to the datasheet rock bottom noise value for the device.

author: Andreas
author_rank: Super Contributor
timestamp: February 22, 2019, 10:21:55 pm
content: Hello,I fear I have to excange the input caps in my LNA if you can even short the input during storage.Last time I had forgotten to bias the input capacitor (for several weeks) I had following results:100nVpp noise floor at 7.6V NiMH3.8uVpp noise floor at 11V NiMHso completely unusable for a 10V reference.I never measure noise floor with shorted input.with best regardsAndreas

author: janaf
author_rank: Frequent Contributor
timestamp: February 22, 2019, 10:34:39 pm
content: Quote from: Andreas on February 22, 2019, 05:40:34 amYes. Measuring on unbuffered zeners with 1K in would. The graph was made to give a first view of what noise level can be achieved for a wanted input impedance (and thereby also what capacitor size is needed).For a zener with something like 200nVrms 0.1-10Hz (1.2uV ptp) i'd want something like a factor 5 or 10 lower noise from the analyzer amp, ie 20-40nVrms. Depending on if the DUT is buffered, like the LTZ1000 standard circuit, or un-buffered, one can see what opamp solutions are possible to use. A quick look at the graph will tell that at 20nVrms(0.1-10), the best devices would require some 100k input or less, while if we accept 40nVrms(0.1-10) almost 1Mohm would be usable the best suited opamps... One can easily see on these graphs why they where struggling to measure the 2.5V LT6655, which has around 600nVptp/100nVrms(0.1-10), with some decent safety margin.

author: janaf
author_rank: Frequent Contributor
timestamp: February 22, 2019, 10:43:06 pm
content: Sorry if I wasn't clear. I meant shorting inputs on the DUT, DC coupled inputs to analyzer, not trashing your months aged bias capacitor...

author: jmelson
author_rank: Super Contributor
timestamp: February 22, 2019, 11:02:55 pm
content: Quote from: janaf on February 21, 2019, 03:23:21 pmAbout 10 years ago I had an urgent need for a low 1/f noise op-amp for a voltage regulator circuit. I found the AD706 which had a guaranteed spec on .001Hz noise, individually tested. It really solved the problem in a circuit that was VERY sensitive to steps in the power supply. We could VERY easily see the result of tiny, micorvolt-level steps in the supply upsetting the circuit. Putting the AD706 in made the jumps disappear.Jon

author: Andreas
author_rank: Super Contributor
timestamp: February 23, 2019, 04:32:46 pm
content: Quote from: janaf on February 22, 2019, 10:43:06 pmIf you mean my answer: this was related to the post of MiDi:Quote from: MiDi on February 22, 2019, 09:38:09 pm@MiDi: I was a bit confused that on your scope readings the RMS-noise is in the same ball park as the PP-Noise.I usually use the "AC-RMS" on my scope to remove the scope offset from the RMS calculation.with best regardsAndreas

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2019, 10:58:25 pm
content: Quote from: jmelson on February 22, 2019, 11:02:55 pmA couple of times I have resorted to my own variation of the circuit shown below but using an OP27/LT1007 class of operational amplifier to make low noise regulator. It is a little weird to have a regulator capable of putting out an amp of current with the noise of a reference and a load regulation of less than 10 microvolts from 0 to 1 amp.

author: iMo
author_rank: Super Contributor
timestamp: February 24, 2019, 10:54:01 am
content: OPA388 (chopper)?Quote

author: Andreas
author_rank: Super Contributor
timestamp: February 24, 2019, 11:08:45 am
content: Quote from: imo on February 24, 2019, 10:54:01 amFrom the web-page:Single supply: 2.5 V to 5.5 Vso as most choppers not so interesting for a 10V system.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: February 24, 2019, 11:30:04 am
content: OK, >>5V Vcc requirement limits the choices.PS: I would be "extremely careful" when comparing opamp's "special" params like noise, TC of their offsets/drifts, etc. based on SPICE simulations..Most (or, better, I would say "all") opamp's models use only a few "behavioral voltage and current sources" (behavioral source is a source where its value is given as a "function of any parameters/variables used in the circuit") inside the model.They do not use the actual opamp's internal wiring/schematics with the specific models for each transistor/diode/R/C used on the Si chip/die..

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2019, 12:01:31 pm
content: The OPA388 is just another 5 V supply chopper. So limited in use if higher voltage is needed. Depending on the circuit is may still be useful.For the input AC coupling I see mainly 2 options:1) a foil cap that is available / practical to some 100 µF. This requires an OP that works well with high impedance.This essentially only leaves good JFET (e.g. OPA140, ADA4625,..) or chopper based OPs (e.g. OPA189, ADA4522,...). One may be able to use a few in parallel.2) a relatively large selected electrolytic cap. The input impedance kind of sets an upper limit to the use full capacitance: The larger the capacitor, the longer it takes for settling, as the resistor ground is kind of limited to some 10 K as an lower bound. DA related settling inside the cap can also be a problem - though not much effected by the size. This would alow for BJT based OPs. So far the good candidates are LT1007 (or LT1037) and AD8675. The LT6018, LT1028 or similar would need unrealistic size caps. I kind of doubt the OPA27 curve shown - the datasheet suggests more noise, more like the LT1007.In some cases there could be an alternative to AC coupling at the input: a separate DC removal loop could be used. This would also need an OP reading an RC element, but not connected to the input. So there could be a little more freedom to use a lower impedance / very large cap.Just comparing the spice models can be tricky, as the models may not be accurate. As slight advantage they get a round possibly different definitions of the 0.1-10 Hz interval and if lucky also handle to correlation of current noise between the inputs. But this about the only advantage. Especially low frequency noise can be different between samples / batches.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 24, 2019, 01:33:40 pm
content: Quote from: Kleinstein on February 24, 2019, 12:01:31 pmEvery time I start thinking down that path, I get a nagging feeling that a DC servo will introduce its own noise, which may or may not be possible to subtract from the signal we are interested in?I am increasingly convinced that DC doesn't actually exist... it is an ideal only, a simplification of the complexities of very low frequency AC!

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2019, 01:50:29 pm
content: At least for the circuits I know, the DC servo will introduce some noise. Unless the DC to compensate is rather small the noise can end up essentially as high as the input amplifier and needs an amplifier to work with a rather similar RC combination it tries to replace. If the DC voltage to compensate is only small (e.g. only a few10 mV of the offset of an FET amplifier) the noise due to the DC loop can be rather low. It still may need quite some power to avoid resistor noise.