X100 Preamp for oscilloscope  - Page 1
Forum section: electronics / projects

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 08, 2025, 07:07:31 pm
content: I’m working on a low-noise power supply design and need to verify that its output noise and ripple are below 100µV RMS.However, my oscilloscope (Rigol 1202Z-E) has an input noise floor ~500µV RMS, which completely buries the signal I’m trying to measure. Even with averaging and bandwidth limiting, the scope’s own noise dominates.So after a little bit of digging and questioning a preamp suggested to me, I have never done this sort of circuit design so I could really use some feedback.Differential input, Two bipolar caps block DC, buffered to get high Z, gain of 100 achieved by differential amplifier, sallen-key low pass filter with cutoff frequency of 106KHz.All left to do is PCB design and getting a metal box.

author: moffy
author_rank: Super Contributor
timestamp: October 08, 2025, 09:19:48 pm
content: For low noise you want low impedance resistors, voltage noise increases with the square root of the resistor value, and you want as much gain in the first stage as possible so that only one stage is contributing to the noise not multiple stages. The THAT1512 is a reasonable choice if you can use low input impedance, if you need higher input impedance then use a low noise JFET opamp as current noise will dominate. 1/f noise at the low end is always an issue, and thermal noise from differential temps means you should let it stabilise before any measurements.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2025, 12:04:39 am
content: In this case the power supply output is very low impedance, so shorts out current noise from the LM833 through the input AC coupling capacitors. The 1 megohm resistors should still be lower value to keep input offset from input bias current to a reasonable value.The LM833 is a lot of fun, but there are modern inexpensive JFET and CMOS input parts with equally low noise, and much lower input current noise and input bias current which will make design easier. Alternatively there are bipolar parts with both lower noise and lower input bias current and input current noise.I have made this measurement a couple of times and just used a single non-inverting operational amplifier wired into the regulator circuit to provide an amplified noise output, or two stages if I needed higher bandwidth.

author: free_electron
author_rank: Super Contributor
timestamp: October 09, 2025, 12:52:20 am
content: https://www.analog.com/media/en/technical-documentation/technical-articles/s64_en-op_amps.pdfhttps://www.ti.com/lit/ds/symlink/opa2227-ep.pdf?ts=1759901140455Susumu makes very low noise resistors.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 09, 2025, 08:29:41 am
content: You can get better cmrr by using the standard "three opamp instrumentation amplifier" topology.Put all the gain into the first stage for best noise performance.A ready rolled diff amp will usually outperform a homebrew.Low noise audio parts often have very high input bias currents. 10nA!Audio mic amps are great but you have to watch out for Ib*Rsource offsets.With 1meg of input impedance that can lead to massive dc offsets at high gain. eg 10nA*1meg=10mV, mutiply by a gain of 100 and that 1V!A fet input amp will probably be the best choice. Dont be fooled by Vn specs, In can sneak up on you.BTW Vn/In equates to to the best input impedance match. You'd aim to get the closest match of Vn/In to Rsource, 1meg in your case.Look into "servoing out" some DC errors see attched>

author: free_electron
author_rank: Super Contributor
timestamp: October 09, 2025, 02:34:21 pm
content: interesting company ... They have their actual waferfab still in silicon valley on Concourse Drive in San JoseIt's a very small one (they occupy a corner of a office building that also houses a cabinet making company)4 inch wafers and a very old technology step. Then again, bigger transistors (bipolar) are better for low noise..And you can get their stuff through mouser.... i gotta order some.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2025, 05:44:37 pm
content: Quote from: Terry Bites on October 09, 2025, 08:29:41 amThe first stage only needs enough gain to overcome noise in the following stage, which is likely lower noise anyway because it can be lower impedance.The disadvantage of putting most of the gain in the first stage is limited bandwidth. Most of the likely parts have a maximum gain-bandwidth product in the 10 to 20 MHz range, so achieving say a 1 MHz bandwidth means limiting first stage gain to 10 or 20 or so. I would use 1 MHz bandwidth as the closest standard for lower frequency noise measurements.QuoteAn integrated instrumentation or difference amplifier is certainly much easier, but bandwidth tends to be low. In a higher performance design, I would likely use something like an AD8129 difference amplifier to maximize common mode rejection at higher frequencies as well as plenty of input common mode range and output drive for a double terminated 50 ohm coaxial transmission line. Its high gain-bandwidth product allow for plenty of second stage gain.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 09, 2025, 05:50:06 pm
content: You might find the Tektronix ADA400A interesting. They don't publish a schematic (AFAIK) but this barely readable one might be useful-https://www.reddit.com/r/electronics/comments/mtnsqi/reverse_engineering_the_tektronix_ada400a/#lightboxClick the right arrow on the image to get to it.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 09, 2025, 06:32:41 pm
content: Quote from: Conrad Hoffman on October 09, 2025, 05:50:06 pmFound a high resolution picture. expensive and complex!AD829 is an interesting opampGonna test it with NE5532, I don't need to measure lower than ~100uv and my scope noise ~500uVrms dominates that, as long as I can get out of scope's noise floor it is good enough.

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2025, 07:38:31 pm
content: Quote from: Terry Bites on October 09, 2025, 08:29:41 amTHAT makes some interesting parts with “instrumentation amplifier” differential circuits that are optimized for microphone amplification and other low-impedance sources, using BJT inputs. They do have high input bias current that can cause DC errors: the easiest way to fix that is capacitor-coupling the output, if DC response is not needed.Note that the ratio ofen/in, whereenandinare the noise densities in V/Hz1/2and A/Hz1/2, gives the optimum source resistance, as mentioned above, but adding a resistor in series or parallel with the source will degrade the SNR.This refers to the source resistance, not the input resistance of the amplifier.A series capacitor from the output of a power supply will present a small source resistance to the amplifier input at frequencies where the capacitive reactance is low.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2025, 11:57:57 pm
content: Quote from: TimFox on October 09, 2025, 07:38:31 pmOne thing I never tried was adjusting the tail current of the differential pair, which would also work with the common two transistor instrumentation amplifier, to minimize noise for a given source impedance. This is easy to do, but as the transconductance changes, the frequency compensation also changes so the frequency compensation needs to be adjustable also which is easy enough.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 10, 2025, 12:25:33 am
content: I've edited the schematic with your suggestions and decided to prototype one to see how it performs. the board is 5x3cm almost the exact size of a 9V battery.I know NE5532 is noisy for this application but right now it's the only opamp I've got.*AC coupling caps are bipolar.

author: free_electron
author_rank: Super Contributor
timestamp: October 10, 2025, 03:07:15 am
content: Panasonic and Susumu have trimmed/matched resistor networks. You can get them in ratios.remove tghe ground flood around the inputs of the opamps. you do not want stray capacitance

author: magic
author_rank: Super Contributor
timestamp: October 10, 2025, 06:50:21 am
content: Quote from: ElectronSurf on October 10, 2025, 12:25:33 amDoesn't look too bad. As for noise, 40nV/rtHz over 100kHz bandwidth is still less than 20μV RMS, so you could even get away with LM358. But NE5532 is better of course because LM358 doesn't have enough GBW to deliver the gain you want.The 10μF/1kΩ cutoff frequency is somewhat high at 160Hz.Quote from: ElectronSurf on October 10, 2025, 12:25:33 amDo they need to be? Only if you plan to measure both positive and negative rails.editThe usual solution is to have R3 and R4 connected with each other (or simply a single resistor) but not to ground. Not sure if your version has equally good CMRR?

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: October 10, 2025, 08:45:23 am
content: Quote from: David Hess on October 09, 2025, 11:57:57 pmgm rises with the 2nd root of the drain current, voltage noise shrinks with the 2nd root of gm, so rising the current is not VERY attractive. (4th root improvement).More transistors improve gm directly, without the root.I'd like to play with the Widlar follower-Ersatz. That would take care of the high imput impedance and the exact gain of 1.000 nourishes hope for good offset, too. Also, BJTs have friendlier1/f noise specs than FETs. The steep rise here is probably an RC effect. The 1.000 gain is a drawback, but then the noise is still quite ok if we leave the voltage gain to the 2nd stage.ed.: typos

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 10, 2025, 09:28:03 am
content: Quote from: free_electron on October 10, 2025, 03:07:15 amThank you for mentioning this. Not only did I maintain the clearance, but I also moved the ground plane entirely to the bottom. This made the PCB design single-sided and simpler. By removing the vias to the top ground plane, the components now have solid ground connections through thick spikes.Is there anything else I should be aware of?

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 10, 2025, 09:38:16 am
content: Quote from: magic on October 10, 2025, 06:50:21 am10uF and 1K cutoff frequency is 16Hz.Bipolar electrolytic caps are better than polarized electrolytics and X7R ceramics, they handle AC signals in both directions and have lower noise, lower dielectric absorption, and less microphonic effect, which keeps the preamp cleaner.

author: TimFox
author_rank: Super Contributor
timestamp: October 10, 2025, 02:40:47 pm
content: 10 uF can be found in TH film caps. Polypropylene is better, but PET might be good enough and is smaller.I invested a few years ago in a cache of surplus 10 uF, 100V PPs that are roughly 30 mm square. I use 400 or 600 V larger ones with tube circuits.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 11, 2025, 10:38:13 am
content: Say the NE5532 has an Ib of 100nA, that will cause a DC offset of 0.1volt using 1meg input resistors.At a gain of 40dB the NE5532 response will be ~3dB down at 100KHz.I've attched a possible compromise solution for this opamp. 40db, a few mV of offset and a BW~500KHz. A big non-polarised cap is needed....Some CMRR trimming is probably needed. Using the diffamp as the filter will screw this circuit up unless you use precision components and a lot of trim range.Anyhow, stick it in a simulator and see what can be achieved.How noisy is it, see attched app note from Renesas.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 11, 2025, 01:17:22 pm
content: Quote from: Terry Bites on October 11, 2025, 10:38:13 amI've changed the schematic a bit (a few posts earlier👆), changed the biasing resistors to 1K.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 12, 2025, 12:17:15 pm
content: Here's an idea that lets you have the input coupling components of choice without DC offset worries.The integrator provides servo feedback that neutralises input offset potentials.The servo action will allow several volts of DC common mode voltage at the inputs and still provide the deisred AC and DC differential performance.R3 and R4 limit the over voltage fault current flowing in the opamp inputs.You could get rid of the coupling caps if you like but I'd keep 'em though.Add a DPST switch or a reed relay and keep your options open.-20db input attenuator, I would.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 12, 2025, 06:26:14 pm
content: Quote from: Terry Bites on October 12, 2025, 12:17:15 pmI like how the servo takes care of the DC offset without messing with the signal path, it’s really smart. Thanks a lot for sharing it.Doesn't R1 and R2 change the input impedance? if yes what's the point of buffering the signal?This is an interesting read.

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2025, 09:42:02 pm
content: Something else you can do is implement AC coupling at the difference or instrumentation amplifier, which improves common mode rejection in the transition band:https://www.ti.com/lit/an/sboa242a/sboa242a.pdf?ts=1760266801223The input still has to have enough common mode range to avoid overload however. This is why a high performance differential input may be much more complex to handle a high input voltage range without AC coupling.

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2025, 10:16:35 pm
content: Quote from: ElectronSurf on October 12, 2025, 06:26:14 pmSo long as any high-pass filter passes the low-frequency ripple/noise components of your power supply, the input impedance should not matter.If the power supply voltage is high enough, the input coupling capacitors are still required.Also note that the input noise current of the amplifier will pass through the coupling capacitor, so the capacitor must be large enough that the resulting noise voltage is less than the inherent noise voltage.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 13, 2025, 07:20:29 am
content: Quote from: David Hess on October 12, 2025, 09:42:02 pmThat won’t work, the very first stage must be AC-coupled, since we’re measuring the output noise of power supply units, and the DC voltage can exceed the input limits of the op-amps.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 13, 2025, 07:25:23 am
content: Quote from: TimFox on October 12, 2025, 10:16:35 pmI don’t think adding a servo is a good idea because it can complicate things, it adds extra components and a slow integrator that might saturate or oscillate (at very low frequency). It also introduces long transient recovery after connecting or disconnecting the probe. Removing it can improve linearity since the buffer isn’t fighting a slow feedback loop.

author: MariuszD
author_rank: Regular Contributor
timestamp: October 13, 2025, 09:50:47 am
content: Quote from: ElectronSurf on October 08, 2025, 07:07:31 pmHave you seen Walt Jung's articles? There's a comparison of several low-noise regulators, measurement methods are provided, and there's an amplifier.https://pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Jung_W/Regs_for_High_Perf_Audio_1-4.pdf

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 13, 2025, 01:01:32 pm
content: Maybe I missed it, but without knowing the required lower and upper cutoff frequencies it's very hard to give specific suggestions.Also need the required input-referred-noise level (either in RMS volts or spectral density). Getting something with lower noise than the scope should be trivial. But depending on the DUT, you might require far better performance in order to properly characterize it.Also, why bother with differential input? Is there some reason the DUT cannot be earthed during measurement?

author: mawyatt
author_rank: Super Contributor
timestamp: October 13, 2025, 04:19:59 pm
content: Quote from: MariuszD on October 13, 2025, 09:50:47 amGreat resource, Walt certainly knows his stuffHe's responsible for alerting us back in the day of Harrison's book on Current Sources & Voltage References:https://www.eevblog.com/forum/beginners/current-mirror-explanation/msg5686305/#msg5686305Best

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 13, 2025, 04:29:47 pm
content: Quote from: mtwieg on October 13, 2025, 01:01:32 pmTypical 10Hz to 100KHz, it may sound weird but the building I'm living in lacks an earth connection XD.

author: magic
author_rank: Super Contributor
timestamp: October 13, 2025, 05:17:25 pm
content: You could use a single-ended amplifier grounded to the PSU under test if the amplifier is powered from batteries to avoid ground loops.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 13, 2025, 06:10:55 pm
content: Quote from: ElectronSurf on October 13, 2025, 04:29:47 pmOkay, what about target noise level?QuoteThat's weird, but doesn't mean you need a differential amp.To put the question in a more general sense, is there any reason the DUT can't share the same ground as the preamp and scope?

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 13, 2025, 08:38:19 pm
content: Quote from: mtwieg on October 13, 2025, 06:10:55 pmSomewhere below ~30 nV/√Hz across 10 Hz–100 kHz should be fine for my measurements. not aiming high tbh.The DUT can share the same ground as the preamp and scope if needed, but right now I’m concerned about avoiding ground loops since the building doesn’t have a proper earth connection. That’s why I initially considered differential input, but it may not be strictly necessary.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 13, 2025, 09:22:16 pm
content: Quote from: ElectronSurf on October 13, 2025, 08:38:19 pmYou should be able to meet these requirements with an off-the-shelf INA, like the AD8429. Could be used either differential or single ended.Should make sure the input bias tee has a low enough impedance such that input noise current doesn't severely increase the input referred noise. Maybe use a low-leakage electrolytic or tantalum capacitor of 10-100uF.Also make sure to consider input overdrive conditions. Will likely need some clamping diodes on the input, after the bias tee, and maybe some resistance intentionally added on the input to reduce the surge current when connecting to a DUT.

author: Someone
author_rank: Super Contributor
timestamp: October 13, 2025, 10:24:26 pm
content: Quote from: mtwieg on October 13, 2025, 01:01:32 pmObserving low level noise from power supplies (OPs stated aim) is much easier with a differential probe, as soon as power is involved there will be ground bounce/gradients/"common" components.

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2025, 11:16:31 pm
content: Quote from: mtwieg on October 13, 2025, 01:01:32 pmThere are many reasons:At low noise levels, conducted noise through the oscilloscope's ground path becomes a problem. The ground connection for a single ended probe has a low impedance so even small voltage differences can cause considerable current.The increased noise from making a differential measurement is almost always more than made up for by the differential probe's common mode rejection. Common mode rejection can allow measurements which are very difficult or impossible with a single ended probe.The low impedance of the ground connection may interfere with the measurement, as in the ground bounce example.A differential measurement is safer because if ground in the DUT is lost, there will be no low impedance connection through the oscilloscope. I once had a probe's ground lead turn white hot and melt.

author: jonpaul
author_rank: Super Contributor
timestamp: October 14, 2025, 09:29:12 am
content: see TEK circuit concepts booksVertical Amplifiershttps://davmar.org/TE/TekConcepts/TekVertAmpCircuits.pdfj

author: mawyatt
author_rank: Super Contributor
timestamp: October 14, 2025, 02:08:48 pm
content: Back then Tek housed more practical amplifier knowledge than the entire world combined, which included Bell Labs and arch rival HP, and thisis a detailed published collection of such!! Must read for anyone interesting in core amplifiers and their use!!They didn't hide stuff (expect maybe the Ross T-Coil solution for awhile), neither did HP, those were Glory Days of Electronics long since passedWith the famous Ft Doubler and Batties/Ross T-Coils, this gave Tek a 4X BW advantage that took HP a decade to catch up!! The pure genius of Batties T-Coils, followed by Ross's brilliant use of Wang's obscure algebra to solve something unsolvable, but they didn't care if it was unsolvable as they built these T-Coils and they worked!!Also the creation of the Gaussian Response Amplifier by cascading multiple 1st Order amps, the Spiral Transmission Line, and the Tampered Lossy Transmission Line for Probes, seems Tek knew more about waveform delay, transmission lines and terminations than just about anybody as well!!Its sad to see the state Tek is in today, same for HP (Keysight), those Glory Days have long since passedAnyway, thanks for posting and hopefully others will spend some time to discover the absolute brilliance of Tek back then with these marvelous analog amplifiers, techniques and oscilloscopes.Best

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 15, 2025, 06:19:04 am
content: Why sallen-key low pass filter starts attenuating the signal at 10KHz while the cutoff frequency is at 159KHz?it seems it's the caps in NE5532 model that causing early attenuation and phase lag?Code:***** NE5532 Source: Texas Instruments NE5534* C2 added to simulate compensated frequency response (Uwe Beis)* NE5532 OPERATIONAL AMPLIFIER "MACROMODEL" SUBCIRCUIT* CREATED USING NE5534 model from Texas InstrumentsAT 12:41* (REV N/A) SUPPLY VOLTAGE: +/-15V* CONNECTIONS: NON-INVERTING INPUT* | INVERTING INPUT* | | POSITIVE POWER SUPPLY* | | | NEGATIVE POWER SUPPLY* | | | | OUTPUT* | | | | |.SUBCKT NE5532 1 2 3 4 5*C1 11 12 7.703E-12C2 6 7 23.500E-12DC 5 53 DXDE 54 5 DXDLP 90 91 DXDLN 92 90 DXDP 4 3 DXEGND 99 0 POLY(2) (3,0) (4,0) 0 .5 .5FB 7 99 POLY(5) VB VC VE VLP VLN 0 2.893E6 -3E6 3E6 3E6 -3E6GA 6 0 11 12 1.382E-3GCM 0 6 10 99 13.82E-9IEE 10 4 DC 133.0E-6HLIM 90 0 VLIM 1KQ1 11 2 13 QXQ2 12 1 14 QXR2 6 9 100.0E3RC1 3 11 723.3RC2 3 12 723.3RE1 13 10 329RE2 14 10 329REE 10 99 1.504E6RO1 8 5 50RO2 7 99 25RP 3 4 7.757E3VB 9 0 DC 0VC 3 53 DC 2.700VE 54 4 DC 2.700VLIM 7 8 DC 0VLP 91 0 DC 38VLN 0 92 DC 38.MODEL DX D(IS=800.0E-18).MODEL QX NPN(IS=800.0E-18 BF=132).ENDSI've tried to reducing C1 and C2 in the model to almost zero and didn't have any effect...

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: October 15, 2025, 11:13:12 am
content: By these TEK heroes:<https://www.amazon.de/Wideband-Amplifiers-Peter-Staric/dp/0387283404>and<https://www.amazon.de/Wideband-Amplifier-Materials-Circuits-Devices/dp/1891121510/ref=sr_1_2?__mk_de_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dib=eyJ2IjoiMSJ9.VVSctK8rhN3IFxrfDUJM_xA3I3trcjfCuma8cuP8pCH9vzjR-N-xpYN727NAih1M0kiHc64Co8n_4LpNLu3bsHSibvQl7G8JESD_kKeO_4Gt4fSXZUAcSM0LFvgl8gqiRYBbZSINNWsVfVkrjaWNBBN8_kF1s4gzwRXQ0mwyz2-oAjv8OBv3U-ELSwKndQ_ld622nphtdptZOaxJPrWSyLm7XoTFyy8XO-Fg0iJlf1E.mB_eMOj4Zb47ky-FXMJIi8rOov9cIQmMUKXy5Rr5AK4&dib_tag=se&keywords=wideband+amplifiers&qid=1760525726&s=books&sr=1-2>I could not disagree more with that Charles Clark opinion on Amazon.cheers, Gerhard

author: 2N3055
author_rank: Super Contributor
timestamp: October 15, 2025, 11:34:55 am
content: Quote from: Gerhard_dk4xp on October 15, 2025, 11:13:12 amThat is THE book on the subject. Fun fact: Erik Margan is from my hometown. No relations, and unfortunately not even a fraction as smart as him because of this trivia..

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 15, 2025, 11:46:44 am
content: Quote from: ElectronSurf on October 15, 2025, 06:19:04 amAre you sure it's actually attenuation? Looks like a shift in DC offset to me...

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 15, 2025, 12:31:48 pm
content: Quote from: mtwieg on October 15, 2025, 11:46:44 amYeah, it gets worse at 20KHz and above.

author: mawyatt
author_rank: Super Contributor
timestamp: October 15, 2025, 12:54:00 pm
content: Quote from: 2N3055 on October 15, 2025, 11:34:55 amExactly "The Book" on the subjectThis helped us immensely way back with the BW extension of a SiGe Differential Cherry-Hooper (with Caprio Cell) Wide Band Amplifier/Mixer.Best

author: mawyatt
author_rank: Super Contributor
timestamp: October 15, 2025, 01:16:48 pm
content: Quote from: ElectronSurf on October 15, 2025, 06:19:04 amA couple notes on the circuit.1) You are relying on the GBWP of the input amps to match for HF CMR with a gain of 10X, these likely will not match well being separate packaged amps with layout differences. Consider applying BW limiting feedback on each amp to help reduce this effect.2) The Differential to Single Ended Amp may need to have a LF and HF trim for the best LF and HF CMR.3) Watch out for HF Sallen-Key feedthru by means of the feedback cap, search for some notes we made on this subject.Also, make your input source have a ground reference path rather than pure floating. This is easy with dual like sources (one inverted for the - side) for the + and - inputs, then another source to ground in the middle. This middle source now becomes the source to simulate CMR effects (DC and AC) as it propagates to both amp inputs equally.Best

author: Terry Bites
author_rank: Super Contributor
timestamp: October 15, 2025, 03:40:15 pm
content: As a rule of thumb the GBW of an opamp filter needs to be >=10x the cutoff freq.There are ways to compensate for this.....

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 15, 2025, 04:21:42 pm
content: Quote from: mawyatt on October 15, 2025, 01:16:48 pmI've tried everything, even removed sallen-key filter and used passive RC filter, also removed the first stage and connected input directly to second stage.The signal attenuates at 10KHz and higher, I think something is wrong with the model or ltspice itself.

author: mawyatt
author_rank: Super Contributor
timestamp: October 15, 2025, 04:27:12 pm
content: Try a differentversionof the Op-Amp model, or a different op-amp model.Best

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 15, 2025, 06:44:26 pm
content: Quote from: ElectronSurf on October 15, 2025, 04:21:42 pmI ran your simulation, even recreating the sallen key filter as shown in your screenshot. I don't see any problem. At 10kHz, the AC simulation shows gain of 39.963dB at the lowpass filter output, just -0.037dB from the passband. Transient simulation shows 39.854dB at the lowpass output, down just -0.146dB from the passband. Is this unexpected, or are you seeing different numbers?

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 15, 2025, 06:50:31 pm
content: Quote from: mawyatt on October 15, 2025, 04:27:12 pmI tried it withLT1358, which is way superior to puny 5532, produced almost the same result. maybe I'm doing something wrong?Quote from: mtwieg on October 15, 2025, 06:44:26 pmDid you go up to 100KHz? increasing the cutoff frequency solves the problem of early attenuation, but it doesn't look like a solution.I have to add that I just know the basics of ltspice, so not a pro!

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 15, 2025, 07:47:09 pm
content: Quote from: ElectronSurf on October 15, 2025, 06:50:31 pmYou said there was a problem at 10kHz.At 100kHz, AC sim shows gain of 37.06dB. Transient sim shows 37.016dB. Again, not surprising at all. If I replace it with an ideal opamp model, the results do not change significantly (except for the small offset, that is).Again, what are youexpectingthe circuit to do?

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 17, 2025 05:35:34 am
content: Quote from: mtwieg on October 15, 2025, 07:47:09 pmI was expecting the filter to start attenuating the signal after ~50KHz, but attenuation starts at ~20KHz.

author: Someone
author_rank: Super Contributor
timestamp: October 17, 2025 05:55:18 am
content: Quote from: ElectronSurf onYesterdayat 05:35:34 amThis is what just the ideal low pass filter you show will do.

author: MariuszD
author_rank: Regular Contributor
timestamp: October 17, 2025 06:05:26 am
content: Where the attenuation starts depends on how much you zoom inBy changing the capacitor value, you can obtain a Butterworth filter with the maximally flat characteristic (Va).

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 17, 2025 06:12:02 am
content: Quote from: MariuszD onYesterdayat 06:05:26 amIt's not a zooming problem. and cutoff frequency is not even 100KHz, it's 159KHz.

author: jonpaul
author_rank: Super Contributor
timestamp: October 17, 2025 06:54:29 am
content: Myawatt, Electron surf: Many thanks,The TEK Circuits Concepts of 1980s TEK field eng gave me a full set.The Circuits Concepts describe the circuits and designThe Measurement series teack how to use..Highly reccomended !Have an Absolutely Fantastic DayJonPS: my latest 2465B restoration, blown up PSU (RIFA) ,A5 control SMD leak corrosion, NVRAM deadNow in in CAL within 1%with 2 mv/div VERT, no preamp needed!

author: MariuszD
author_rank: Regular Contributor
timestamp: October 17, 2025 06:58:10 am
content: I don't see significant differences between the characteristics of your circuit and a Sallen-Key filter built with an ideal buffer.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 17, 2025 07:13:11 am
content: The noise is an order of magnitude better than I was expecting (100uV RMS):Term | Value----------------------------------------------------------|---------------------Preamp gain | 100Preamp noise density | 1.07 µV/√HzScope noise floor | 500 µV RMSInput referred scope noise | 5 µV RMSPreamp input referred noise (100 kHz BW) | 3.38 µV RMSEffective total input noise floor | ≈6 µV RMSMinimum measurable PSU noise (reasonable SNR) | ≳10 µV RMSBut how much of this am I going to be able to achieve in real world?

author: JohanH
author_rank: Frequent Contributor
timestamp: October 17, 2025 07:23:14 am
content: Just FYI, I built the one mentioned here a couple of years ago:https://www.eevblog.com/forum/testgear/preamplifier-for-noise-measurements/msg3077861/#msg3077861

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 17, 2025 11:54:38 am
content: Quote from: ElectronSurf onYesterdayat 05:35:34 amNo filter has a perfectly flat passband. Depending on how you look at it, it's equally valid to say it "starts attenuating" at 1Hz.Quote from: ElectronSurf onYesterdayat 06:12:02 amI think you're misinterpreting what the filter calculator website is saying. It says its cutoff frequency is 159kHz, and this is indeed the frequency of its two poles. This means at 159kHz its gain will be -6dB, not -3dB. And this is exactly what the simulation shows.There's nothing wrong with LTspice or the opamp model.

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 17, 2025 03:01:04 pm
content: Quote from: mtwieg onYesterdayat 11:54:38 amThe error was on my side, thank you.I made a few minor changes to the schematic and redesigned the PCB to have shorter traces. I also moved the power rail to the top side and made it double-sided.As for the capacitors I ordered, one is from Maxicap and the other is from Rubycon (though the Rubycon name isn’t printed on it). I couldn’t find any datasheets for them, so I’m not sure which one to use, or if it even matters.I also have 10 µF SMD tantalum capacitors. Should I use tantalum instead? GPT mentioned that bipolar capacitors offer better signal fidelity.

author: TimFox
author_rank: Super Contributor
timestamp: October 17, 2025 05:15:06 pm
content: On your circuit drawing, are there two batteries with a common 0 V (ground) connection, i.e. +V, -V, and 0?

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 17, 2025 05:55:06 pm
content: Quote from: TimFox onYesterdayat 05:15:06 pmYeah two 9V batteries are powering the opamps.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: October 17, 2025 05:56:25 pm
content: Wouldn’t you require some protection at the input to prevent damage to the opamps?

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 17, 2025 06:05:24 pm
content: Quote from: UnijunctionTransistor onYesterdayat 05:56:25 pmYou usually need them, but this preamp will only be used when the power supply noise is below 1 mV RMS, so it’s safe under these conditions.Since the input is AC-coupled, there’s no cause for concern.

author: mawyatt
author_rank: Super Contributor
timestamp: October 17, 2025 08:06:00 pm
content: Quote from: ElectronSurf onYesterdayat 06:05:24 pmThink again!!If you connect to a source of any kind this can create a step function of thepeak source voltageacross the op-amp input, this includes DC, AC and AC/DC sources.Best to put some input protection across the op-amp inputs and outputs.Best

author: ElectronSurf
author_rank: Regular Contributor
timestamp: October 17, 2025 09:00:30 pm
content: Quote from: mawyatt onYesterdayat 08:06:00 pmYep, I'm wrong again. gonna add two B2B diodes for each input.

author: mtwieg
author_rank: Frequent Contributor
timestamp: «Reply #67 on:Todayat 12:13:24 pm »
content: Quote from: ElectronSurf onYesterdayat 09:00:30 pmShould read the opamp datasheet carefully when considering this. For example, the NE5532 datasheet says the absolute maximum input voltage range is exactly the supply voltage, with no margin for overvoltage at all. I can't say I've ever seen that on an opamp datasheet before... If you clamp to GND, it should be okay (if the amp is powered, anyways).Like I mentioned earlier, it's a good idea to also add some series resistance on the inputs so the surge current won't kill the protection diodes. 1Kohm on each input should be enough, and won't add significant noise to the measurement.