Measuring less than 1mV noise/signals on an oscilloscope - Page 1
Forum section: electronics / general

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 10, 2025, 06:03:20 pm
content: This idea partially relates to theLFLNA-80(amplifying 0.1Hz-10Hz noise).I've been experimenting with building stable constant-current sources and I've managed to build one with oscillations below 1mV using an LM324...But I want to actually know + measure how stable my constant-current source is, and compare it with other circuits + dedicated IC's (ex. LM334, LT3092). The problem is that most oscilloscopes cannot easily measure 1mV and lower signals.Design goals:- 0.1Hz-100KHz amplifier (utilizing high-pass filter)- 1000x amplification (1mV = 1V)- Low-costTo achieve these design goals + specifications, it seems that the ubiquitous NE5532 may be suitable, paralleling multiple to achieve lower noise.(My math may be wrong here)but the 10MHz GBW + 1000x gain limits a single NE5532 to operate at a max output frequency of around 10KHz. If I instead cascade (series, 31.62x gain each) 2x NE5532's, it seems like I can achieve the 100KHz design goal (around 3.2MHz GBW required).The problem:- Attempting to simulate the DC-servo design (to eliminate DC offset voltage) of the LFLNA-80 inFalstadseems to causing problems. I assume the DC-servo is actually introducing oscillations, and not eliminating the DC offset?- Modifying theFalstadcircuit to eliminate the DC servo and utilize a 1KHz input frequency, I am still unable to attain the 1000x gain.- Cascading 2x op-amps (31.62x gain each) @ 100KHz inFalstad, it again does not achieve the 1V expected output.Question:- Should I avoid using a DC-servo for this circuit, and instead simply use multiple high-pass filters (including between the cascaded op-amps)?- How can this circuit be made to work? Are my high-pass capacitors too large? Is it the Falstad LM741 simulation causing problem, even at lower frequencies when GBW shouldn't be applicable?- Is there maybe a better way to measure less-than 1mV noise @ 100KHz, with an oscilloscope?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2025, 06:38:35 pm
content: The DC servo part is not really needed with a scope. So a more classic AC coupling is easier.Most scopes are not that bad with noise and would not need a gain of 1000. Likely a gain of 10, maybe 20 or 50 would be sufficient.The NE5532 is a rather old cheap part, not super low noise.The AC coupling at the input for 0.1 Hz can still make things a bit tricky. A BJT based amplifier has quite some current noise and this requires a rather large coupling capacitor, maybe in the 1000 µF range and thus outside what is practical with film capacitors.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 10, 2025, 07:08:12 pm
content: Quote from: Kleinstein on September 10, 2025, 06:38:35 pmThat's a good point and didn't think about this. I guess I can easily exclude the DC servo, as well as eliminate the output high-pass filter.Quote from: Kleinstein on September 10, 2025, 06:38:35 pmI'm aware that a gain of 1000x is unlikely to be needed, but since I'm currently unaware of how "stable" the circuits are, a 100x gain may be insufficient (ex. AC noise of 100µV only gets amplified to 10mV, which is pretty close to my scopes noise floor of 1mV).Quote from: Kleinstein on September 10, 2025, 06:38:35 pmWhile it is old, it has a fairly decent CMRR of 100dB (typical), 16nV noise @ 10Hz, and 6nV noise beyond 100Hz. It of course can't compare to the new ADA4528 and some other chopper amps, but it seems like for this specific application it'll be fine. However, maybe I'm missing a significant flaw/limitation of the NE5532 which would limit its use in this application?Quote from: Kleinstein on September 10, 2025, 06:38:35 pmBased on the research done in the LFLNA-80 project, I actually plan to use a low leakage electrolytic capacitor as it seems like it'll act as an appropriate replacement for this application, instead of a film capacitor. Again, feel free to clarify or provide your greater insight if I'm missing a significant flaw/limitation in this decision.

author: David Hess
author_rank: Super Contributor
timestamp: September 10, 2025, 07:09:21 pm
content: They used to make oscilloscope amplifiers for that sort of measurement, like the Tektronix AM502. You might study the manual to get an idea for your own simplified design.https://w140.com/tekwiki/wiki/AM502

author: mawyatt
author_rank: Super Contributor
timestamp: September 10, 2025, 07:09:54 pm
content: Some of the modern 12bit entry level DSOs have aTrue500uV/div and good measurement statistics, these might prove useful. On most DSOs input AC coupling have the High Pass corner between 1~5Hz, so you may need to use DC coupling and a DSO with a large "input offset range".100KHz at 1000 gain is a lot to ask of a single op-amp, better to split this up between two. There has been a few tests with large electrolyic caps, and some have surprisingly low leakage, but you still must deal with the DA. As mentioned above a film input cap (PP) is better but requires a very low level of current noise, so JFET or CMOS input op-amps are preferred.A DC feedback loop can be stable, the LFLNA-80 is an example, and another example is in that thread.https://www.eevblog.com/forum/metrology/lflna-80/msg6014593/#msg6014593Best

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 10, 2025, 07:21:46 pm
content: Quote from: mawyatt on September 10, 2025, 07:09:54 pmThat's the plan as mentioned in the original post; I'm specifically thinking 3x paralleled + 2x cascade (therefore 6x op-amps total, or SOP8 3x packages) to achieve lower noise + higher bandwidth.Quote from: mawyatt on September 10, 2025, 07:09:54 pmThe bias current of the NE5532 isn't something I had actually considered, and is a good point and could be problematic. If it does end up posing as a significant problem, I could trial either the TL07* series parts (which have higher noise), or add a jFET input to the NE5532 (but then adds extra cost to the circuit). Since this would be a one-off PCB and not mass-produced, I'm not too concerned about pulling a pair of used jFET's from some old PCB's. Trying to avoid purchasing brand-new parts for this one-off PCB, and avoid using some of my more expensive/niche components.The dielectric absorption (DA) is again something I hadn't thought of... I guess I may actually need to find the largest film caps that I have in my stock and use them, and compensate for the high-pass filter by adjusting the resistor values...

author: mawyatt
author_rank: Super Contributor
timestamp: September 10, 2025, 07:48:28 pm
content: Quote from: LooseJunkHater on September 10, 2025, 07:21:46 pmThere's no need to parallel 2 op-amps in the 2nd stage cascade, the first stage 3 paralleled with enough gain isolates the second stage cascade section noise-wise, so a single op-amp there (2nd stage) is all that's needed.Check out this thread by Curtis, this might yield the best overall solution IMO.https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg6026257/#msg6026257Also, should mention with the DSOs with high sensitivity a preamp gain of ~ 100 might prove sufficient.Best

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 10, 2025, 08:51:57 pm
content: Quote from: mawyatt on September 10, 2025, 07:48:28 pmThat's another brilliant point that I had not even thought of; I can eliminate one of the SOP8 NE5532 packages and remove quite a few components this way.Quote from: mawyatt on September 10, 2025, 07:48:28 pmI'll give this a look tomorrow, thanks!Considering there's also been multiple people to suggest the 100x gain instead of my initial idea of 1000x, I'll trust all of your (more experienced) judgments and create the circuit instead with 100x gain.

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2025, 02:14:47 am
content: Quote from: mawyatt on September 10, 2025, 07:09:54 pmOne division on an 8-bit DSO is 20 or 25 counts, while noise is about 250 microvolts peak-to-peak, so added resolution is wasted. On analog oscilloscopes this was more clear when they linked bandwidth limiting to higher sensitivity because the higher sensitivity was useless without lowering noise. (1) Some entry-level DSOs allow DSP bandwidth limiting and scaling to produce a usable higher sensitivity trace.As far as the original poster's question, the only dilemma is whether a single ended design will work; sensitive measurements may have to be differential despite added noise, which explains why high sensitivity amplifiers like the AM502 that I mentioned have differential inputs.Use a low noise low input bias current fast amplifier for the first stage, followed by faster low impedance gain stages. I might use an OPA140 for the first stage operating at a gain of 5 (2 MHz) or 10 (1 MHz), followed by whatever low cost fast amplifier is popular now. Low input bias current relaxes the requirements for input AC coupling.(1) This is why most old Tektronix oscilloscopes only went down to 2 millivolts per division; they knew noise limited useful sensitivity, and their higher bandwidth instruments were limited to 5 millivolts per division because their higher bandwidth resulted in higher noise. The Tektronix 7A13 vertical amplifier is the exception that proves the rule; it has a 1 millivolt per division sensitivity, but with several times the noise because of a complex input stage to support high common mode range. The reason this was justified is that it also has a switchable 5 MHz bandwidth limit; even 20 MHz was too much bandwidth for its 1 millivolt per division sensitivity to make sense.

author: mawyatt
author_rank: Super Contributor
timestamp: September 11, 2025, 03:18:03 am
content: Quote from: David Hess on September 11, 2025, 02:14:47 amThe mentioned modern 12bit entry level DSOs come with built-in DSP filtering, examples are the Rigol and Siglent 800 HD series. With the Siglent it has an array of user definable FIR DSP filters available, however we often employ ERES as a LPF which is directly linked to the ADC effective sample rate. Very powerful instruments indeed, at a very modest costEdit: Check out whatiMois doing here with the entry level DSO:https://www.eevblog.com/forum/metrology/simple-noise-analyser-for-scope-owners-who-do-not-have-that-option/Btw we've used all the Tektronix "plug-ins" and even kept an old Tek mainframe with "plug-ins" around way back in our company lab just for special measurements like these, never got used much tho, just sat in the lab corner along with the other Tek carts. When they closed the lab down, one of the techs took it home (was going to be scraped along with the other Tek cart base scopes, all the techs got "dibs" on all the old lab stuff that was to be scraped before anyone knew!!).Best

author: mikeg
author_rank: Newbie
timestamp: September 11, 2025, 11:50:40 am
content: I have a Tek 7904 with a pair of 7A22 differential modules for this very reason.

author: Simmed
author_rank: Frequent Contributor
timestamp: September 11, 2025, 01:01:17 pm
content: have you seen this ?https://www.analog.com/en/resources/app-notes/an-159.htmlhttps://www.eevblog.com/forum/projects/bode-plot-(-1hz-to-10-mhz)/?allhttps://www.eevblog.com/forum/projects/shield-connections-for-low-noise-measurement-(an-159)/

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 11, 2025, 04:37:59 pm
content: Quote from: David Hess on September 11, 2025, 02:14:47 amHonestly I kind of like the idea of going differential and splitting up the gain a bit more between stages. I did come acrossthisieee research paper, which has a simple circuit to the one I was planning on building (except it's differential input with input trimming).Quote from: Simmed on September 11, 2025, 01:01:17 pmI am familiar with the construction of a magnetic + electrical shielded enclosure, as I did actually make a post similar to that recently. Regarding the attached schematic, at this time I don't feel the need nor require measurements in the nV range as discussed in AN-159, so I don't need intend to build that more complex design, which also doesn't necessarily fit the "low cost" idea. I am partially at-fault for not specifying what "low-cost" truly is in the design goal.__________I think at this point, I'll need to compare some more of the op-amps that I have laying around (including some low-cost choppers), and additionally focus on:- Low input bias currents- Differential input- 100x gain (instead of 1000x)- Maintaining the 0.1Hz-100KHz bandwidth- Selecting appropriate low-noise caps (potentially avoiding electrolytic, due to the dielectric absorption) + resistors- Excluding the DC servo aspect of the circuit- Simulating the circuitI do still have some unanswered questions:- For some general guidance, should I implement a "general" rule of avoiding resistors above 100Kohm (on the preamp stage), or will 100K introduce noise in the uV range?- I'm still learning about high-pass filters, but is it problematic in any way (for this circuit) to utilize high capacitances (ex. 1000uF)?- For pairing resistors, is 1% suitable for the differential design, or should I aim for closer to 0.1% pairs?

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2025, 06:15:59 pm
content: Quote from: LooseJunkHater on September 11, 2025, 04:37:59 pmThat is along the lines of what I was thinking, with two operational amplifiers operating as a differential amplifier with high input impedance, and a difference amplifier of one kind or another to do the differential to single ended conversion. Common mode rejection of the OPA140 that I might use falls to 50dB at 1 MHz and ... Hmm, Analog Devices removed bandwidth from their selection guides for their difference and instrumentation amplifiers.

author: mawyatt
author_rank: Super Contributor
timestamp: September 11, 2025, 06:37:07 pm
content: A potential issue with the "Differential Input" is the 2 capacitors need to be well matched, especially at the lower frequency ranges.Best

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2025, 07:03:08 pm
content: Quote from: mawyatt on September 11, 2025, 06:37:07 pmThe alternative is to DC couple the inputs and then AC couple through the instrumentation amplifier, but this requires the input to handle the full input voltage range.The differential preamplifier shown only has a common mode gain of 1, so represents a good compromise if this is enough. With +/-15 volt operational amplifiers, this would allow a +/-10 volt input range.The AM502 uses a 0.1 microfarad coupling capacitor, so into 1 megohm, this provides for a low frequency cutoff of 1.6 Hz. A custom design could extend this to 0.16 Hz pretty easily.

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2025, 08:45:35 pm
content: I have two PAR differential preamplifiers, from eBay, that have AC/DC input coupling and discrete-frequency HPF and LPF.Here's an example from current eBay listings:https://www.ebay.com/itm/186034739227?_skw=PAR+preamp&itmmeta=01K4X89GVTTW6QVEQ40ZTDR2RP&hash=item2b5088d81b:g:7R4AAOSwGaNk2hah&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1cJhExREDRuyhaB%2BlUBQYOTgQshXF6dC0jb5VyE8nuV7Ym5toUXst1poQyJzKh2EIoH5dZq4I6KHPd3ZxeTiOPnopWj8F2TlaLdnSlXLAdNPPto6OhWhUJfmuyAbSE6Pli8uYh13d8Rcthgbej%2Bga4kBHoAG3AtxtYTY23LkgpYzm4itrq%2FnH4nPSbwC43bwTz0MDugCVVkmHxnjyLnE2gg9vnnpP5OBl4EkUWGelzLJ%2BoMf3jw%2FV%2BvtZ6L3kDVssSG%2BKQRhXv%2BTMs5C4cMkJEy9eFbFjjt%2FSHxyWuMrjxk5Q%3D%3D%7Ctkp%3ABk9SR4KOpqinZg

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 12, 2025, 05:01:46 pm
content: Quote from: mawyatt on September 11, 2025, 06:37:07 pmThis had honestly not even come to my mind but makes sense, considering the resistors would also need to be well-matched within the instrumentation amplifier. Honestly, I'm not sure what characteristics I'd need to match (just ESR + capacitance? Or does DC leakage and other aspects also need to be measured?) but I think this likely eliminates the possibility of going differential.Based on much of the feedback on this post, I've modified myFalstad circuit, going back to the single-ended design.Additional noteworthy changes:- 100x gain, with fine-tune gain trimming- 50ohm resistor on output op-amp- 1k resistors on input + output of most op-amps(unsure if this is required???)- There is some "rounding"/distortion of the square wave, which I think is primarily related to the use of the LM741 (0.6MHz GBW) in simulation, instead of a higher bandwidth part. It's unlikely I'll encounter actual square waves at this low of a measurement range (will primarily see sine waves and noise), so I think this may not be a problem?A major problem:- The high pass filter capacitor; my largest film capacitor is a (used) 3.3uF capacitor. I can parallel a few and get ~13.2uF before it takes up excessive space. I have brand new film caps, but they're 220nF.Questions:- I'm aware that ceramics have piezoelectric noise, especially X7R-type, however is it likely that in this measurement range, they'll pose as problematic?I'm thinking that without any compensation circuitry for the dielectric absorption of electrolytic capacitors (as is done in the LFLNA80, using the DC-servo), electrolytic caps aren't feasible to use in this design?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 12, 2025, 05:09:50 pm
content: Minor thought:I think I can actually remove the high-pass filter between the 9x gain stage + 11x gain stage. Even if the first stage creates a 10mV DC offset, with 11x gain, it only becomes 110mV, which will be filtered out by the oscilloscopes AC coupling stage.I've therefore just updated theFalstad circuitto exclude that high pass filter.Edit:Another problem; I just noticed that the Falstad sim is showingthe output voltage amplitude as slowly decreasing with time, while the input stays the same. I have absolutely no idea why this may be occurring.

author: iMo
author_rank: Super Contributor
timestamp: September 12, 2025, 05:33:23 pm
content: A hint: do start to work with LTspice, publish your .asc and people may help you then, perhaps.Also for observing sub 1mV noise in 100kHz BW a single ADA4523 with A=50 will do the job..

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 12, 2025, 06:28:41 pm
content: Quote from: iMo on September 12, 2025, 05:33:23 pmI've tried learning LTSpice before. It's incredibly thorough and I like it, but I haven't been able to invest the time to learn it in-depth, especially when Falstad often provides enough accuracy + details for my basic needs.Quote from: iMo on September 12, 2025, 05:33:23 pmI'm aware about that a single ADA4523 would do this jobeasily, but they're decently expensive opamps, I don't have any stocked (therefore I'd incur a shipping fee), and the ADA4523 is likely overkill for this application.I'm attempting to complete this project using components that I have on-hand, as I have a decent stock of op-amps, caps, and resistors. While the ADA4523 would simply the design, I see no reason for going out to purchase it when it seems like the NE5532, TL071, or anMCP6v86(which I have stocked). At this point, I'm thinking of using the MCP6v86 (5MHz chopper, +- 50pA bias current, low noise) on the front end, and either the NE5532, TL071, or MCP6v86 as the second amplifier.At this point, I'm primarily asking for help on the input/output resistors for the opamp, choosing a capacitor for the high-pass filter based on what I have stocked, and ideas on what may be causing the output voltage amplitude to decrease over time.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 13, 2025, 03:31:10 pm
content: Small update; been comparing some new and used electrolytic caps that I have, using the following method, as discussed inthis EEVblog post. With 10M input impedence of my multimeter, that gives a resolution of 10mV/1nA.Sadly, most of theusedelectrolytics that I've tested (1000uF and larger) have quite bad leakage, exceeding 1V. I do have ~15xused3.3uF film caps and I did measure one of them to have 0.06 nA (60pA) of leakage (may be lower in reality, as I was using a switching power supply). By paralleling 10x of the best film caps, I'll get 33uF, and with a 120k resistor, I'll get the 0.04Hz high pass cutoff that I'm targeting. There will be a bit of noise introduced by the high resistance, but perthiscalculator, at 120k ohm, 23C, 100KHz bandwidth, it'll create 14uV of noise, or 1.4mV after the 100x amplification, which is close to my oscilloscopes noise floor.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2025, 04:21:04 pm
content: The resistor to ground is actually not introducing much noise. Inside the range of interest the capacitor is lower impedance and effectively in parallel. For less resistor noise a larger resistor and lower cross over would be even better.Amplifier current noise together with the impedance from the input capacitor may be a relevant noise source. This would be the case with a BJT based amplifier like NE5532 and to a lesser degree also an ADA4523.A large enough capacitance may be needed because of current noise from the amplifier. With a JFET amplifier even a single 3.3 µF capacitor would not that bad.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 13, 2025, 04:56:10 pm
content: I'm still learning more about high-pass filters and have been doing some research on the side, but can you clarify a few points from your comment?- The resistor to ground is actually not introducing much noise.[ Is this because the resistor isn't in the "current path" to the amplifier? ]- For less resistor noise a larger resistor and lower cross over would be even better.[ No idea what any of this means, most-specifically the "cross over" aspect ]- A large enough capacitance may be needed because of current noise from the amplifier. With a JFET amplifier even a single 3.3 µF capacitor would not that bad.[ I was trying to research whether a smaller capacitor and larger resistor to compensate would effect the circuit, and all I found was that a larger resistor would introduce more thermal noise. How would a significantly-larger capacitor + lower-value resistor effect the circuit otherwise? Is this related to opamp bias currents?]

author: MiDi
author_rank: Frequent Contributor
timestamp: September 13, 2025, 05:12:59 pm
content: The capacitor shunts the resistor (decreasing impedance over f) down to the output impedance of the DUT.Noise of modern electrolytics or film capacitors is very low.Example of LPF noise plot attached (LTSpice).Info on kTC noise in section "Thermal noise on capacitors"

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2025, 05:25:21 pm
content: In the frequency range of interest the main current path from the amplifier current noise is via the capacitor and DUT. The resistor only contributes a little in the transition region (lower end of the frequency range). The cross over is the lower frequency limit.One can largely avoid the resistor noise with the lower frequency limit for the input AC coupling something like 3 times lower than the frequency range of interest and than do the final high pass filtering after the gain.A large capacitor helps with a BJT based amplifiers (e.g. NE5532, OP27, OPA209) with there high current noise. This is about the only positive point.One the downside the very large capacitors are usually electrolytic and these have high dielectric absorbtion (roughtly 10 times higher than polysester and 100 times higher than PP) and this causes a long settling time when the DC voltage changes. The lower resistor to ground gives a lower input resistance / more loading to the DUT. This could be an issue when going well higher than some 5000 µF for really low voltage noise amplifiers like LT1028.The large capacitor can store quite some charge and present a pulse load that can effect some sensitive DUTs. Many of the LTZ1000 circuit can already be effected by some 100 nF and may need extra care when connecting (e.g. extra series resistor initially or not so hard clamping at the input).

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 16, 2025, 08:45:24 pm
content: Minor update to the post:I have mostly-finished creating the circuit on some perfboard as a proof-of-concept. I'm expecting random signal oscillations due to capacitance and imperfect trace routing, but I'm hoping for it to at least work somewhat.Here is my mostly-final Falstad schematic.Component choices:- All 3x op-amps are Microchip MCP6v86u (chopper op-amp)- All resistors are metal film MELF resistors from Multicomp Pro MCFRFTD-series (max ±100ppm/°C tempco) or Yageo MMF25SFR-series (±50ppm/°C tempco)- No inputs protection (risky)What is still pending:- Building the initial high-pass filter (still testing electrolytics + film caps for leakage)- Haven't chosen nor soldered a capacitor for the DC-servo op-ampWhat's likely to get changed:- The 470 ohm resistor on the "precise gain trimming" section of the circuit- The high-pass filter capacitance-resistance ratio, maintaining the same high-pass frequency level- Will be trialling paralleled ceramic caps (DC bias and piezoelectric effects may cause problems), or a DIY bipolar electrolytic using two same-model capacitors (as discussed here on the EEVBlog) for the DC servoCurrent observations(with input shorted + DC servo disconnected):- Powering the circuit with 4x rechargeable AA batteries (+2.4v, -2.4v, center-ground), the circuit is at the noise-floor of my oscilloscope (which is good)- It amplifies the mains frequency to ~1v PP, even with the input shorted, and being a meter away from AC lines (therefore likely requiring a shielded enclosure that I already started designing, as discussedhere on the EEVblog)

author: mawyatt
author_rank: Super Contributor
timestamp: September 17, 2025, 01:41:22 pm
content: Here's a highly simplified analysis of the DC or Leak Compensation Feedback some folks (students) might find useful, see our earlier posts for more details here:https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg6013765/#msg6013765A detailed proper analysis of the entire amplifier chain should be performed with all the frequency dependent effects, but we'll leave that for the interested reader (student)This simple model has a forward ideal gain of G, ideal feedback integrator of -1/as, where "a" is the op-amp based RC time constant, K is the feedback gain, R is the input shunt resistance, C is the input coupling "high pass" capacitor, Ri is the source impedance, and Vi the input source.This produces a 2nd order function inswhich checks out ats= 0, ats= infinity and K = 0 as shown on page 5477. Adding a post amp high pass shown in 5478 creates a 3rd order function and the numerator and denominator coefficients are shown on page 5479.Including the active low pass characteristics and adding additional frequency dependent terms {like making G a LPF function {G(s) = G/(ds+1)} allows better analysis fidelity with simulations and actual hardware implementations.Additionally one can create a desired overall transfer function (example 2nd or 3rd Order Butterworth or Bessel approximations) of the 1st amplifier or the entire low noise amplifier chain with these "tools" in hand and can optimize for dynamic range, offset containment and study component sensitivities!!BTW way back in ~2000 when we were an adjunct created and taught an advanced graduate level course in RFIC design, we advised grad students to create simple modelsbeforejumping to the simulator (Cadence). This allowed some rudimentary understanding of what was going on and allowed better interpretation of the simulations (they often lied!!!). Anyway a few listened, most didn't, and we hired the best ones that listenedWe should mention a good technique to understand more complexsdomain Transfer Functions is to evaluate ats= 0 and infinity. Here one can establish the "expected" result and compare as a "sanity" check. For example the 3rd Order TF shown in 5478 and 5479 evaluates ats= 0 to zero as expected (after all this is an overall high pass filter function),and ats= infinity as:TF ats= inf = ((RC/K)a*b)/(((C(R+Ri)a*b)/GK), which evaluates to simply G(R/(R+Ri)) as expectedBest

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 25, 2025, 04:57:13 pm
content: Just anothersmall updateon this project before a release alarger update(which I'm working on) on successes and  problems of the project:Quote from: LooseJunkHater on September 16, 2025, 08:45:24 pmThis quote of mine from earlier is INCORRECT. The reason I was measuring mains frequency is because I had actually missed a soldering correction between the cascaded op-amps. The mains frequency interference disappeared after making this connection (as shown in the attached image).Additionally, running some more simulations, I can actually eliminate the output DC offset simply, without needing my oscilloscopes AC coupling mode (which actually caused various problems for me at frequencies below 10Hz; will discuss in a future update) nor another high pass filter, simply by changing the location of the DC servo voltage input, as I show in theupdated Falstad circuit. (Ignore the 1-ohm DC servo resistor + 1A output power of the DC servo op-amp, as it simply speeds up the simulation). After letting the simulation run for about 1-2seconds, the output swings between around +-75mV. I will be testing this change in the near future, but the real-life circuit does have about +100mV DC offset on the output currently.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 30, 2025, 08:48:07 pm
content: Bigger Update:The circuit works with the high-pass filter + DC servo! But not without its flaws.Problems:1. There is quite a bit of noise when amplifying a 2mV signal by 100x gain (picture 1; 100KHz, 2mV PP, 9V DC Offset, Square Wave). The square wave should have much tighter transitions between high-and-low, but they're clearly rounding. At lower frequencies, no rounding is present, but the noise remains. The noise is also present with a sine wave.2. Using 16.5uF film caps + 220k resistor, there's significant signal attenuation from the high-pass filter, up until ~10Hz (picture 2). I'm not 100% certain of the cause, but (based on my limited knowledge) I'd assume it's from op-amp bias currents or leakage.I'm unsure if both the noise + square wave signal rounding + significantly different high-pass behaviour is due to the opamp's (bias currents, limited 5MHz GBWP), stray PCB capacitances & leakages, or something else, but will be experimenting further.To test in the near-future:- Significantly larger DC servo capacitor (only using 820uF, but reverse-paired, so about 410uF capacitance, and they're aluminum polymer caps with unknown leakage currents)- Cleaning the PCB once again- Larger high-pass resistor value, OR remove the high-pass filter resistor and measure where the roll-off occurs- Metal enclosure, to reduce external noise interference (as with the input shorted, I get about 20mV noise on the output)- A 1000x resistor divider (needs to be within an enclosure, as without an enclosure, it picks up too much RF around 1.43MHz (per FFT measurement), as shown inpicture 3 + 4), to see how low of a signal measurement is reasonably visible. My function generator (UNI-T UTG932) cannot output lower than 2mV PP.To test in the far-future(may never get done):- Improved circuit layout + custom PCB (to greatly decrease stray capacitances, leakages, and poor signal routing)Traps for young players:- The "AC Coupling" feature of my Siglent SDS1204X-E oscilloscope initially caused a LOT of confusion. At frequencies less than 10Hz, it leads to strange measurements of square waves (picture 5), and therefore, SHOULD NOT BE USED, and instead, DC Coupling is a requirement if measuring lower frequencies (below 10Hz).- RF noise can cause a lot of problems when amplifying low voltages with high gains. The resistor divider introducing significant noise really stumped me, until I used the FFT function of my oscilloscope.

author: Mahagam
author_rank: Regular Contributor
timestamp: September 30, 2025, 08:59:49 pm
content: Quote from: LooseJunkHater on September 30, 2025, 08:48:07 pmExplain please, what do you mean?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 30, 2025, 09:05:05 pm
content: Quote from: Mahagam on September 30, 2025, 08:59:49 pmMentioned a bit earlier and stated> Will be trialling paralleled ceramic caps (DC bias and piezoelectric effects may cause problems), or a DIY bipolar electrolytic using two same-model capacitors (as discussed here on the EEVBlog) for the DC servoI did trial paralleling many 0805 ceramic capacitors, attaining about 200uF, but they took up a significant amount of space, so I abandoned this idea. The rationale for the DIY bipolar electrolytics is because the DC servo may lead to positive or negative voltages across the capacitor. By going bipolar, I don't need to worry about reverse-polarizing the capacitor. Additionally, I had hoped that 410uF was going to be enough capacitance, even though simulations said 410uF would create this exact problem. The reason for wanting to use the smaller capacitance is because I have many 820uF 2.5v aluminum polymer caps, they're small, and neatly fit onto the PCB. Sadly, they are definitely not a large enough capacitance, hence needing to now use much larger capacitors, located somewhat off the PCB, but connected to the PCB via wires.

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2025, 09:31:38 pm
content: Have you measured the amp BW?What DSO probes are you using for the 100KHz squarewave, were they set to 10X and calibrated?Do you knowIFthe AWG is outputting a crisp squarewave? What is the high frequency loading as "seen" for the AWG output? Maybe a 100X or 1000X resistive attenuator should be considered with known scaling and BW.DSO AC coupling at that low frequency is common, this is differentiator at a few Hz!! Us DC coupling and the Offset to center the display waveform (Siglent DSOs usually have a very large input offset range).You might want to wait for the High Z input amp that was being developed, recall it used small (10~20uF) PP film input caps.Best

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 30, 2025, 10:05:16 pm
content: Quote from: mawyatt on September 30, 2025, 09:31:38 pmI have not, and it's likely a good idea to do this to evaluate what may be causing the rounding of my square wave. The datasheet only lists a nominal 5MHz GBWP, but not min/max. I'm not sure yet how I'd measure the opamps bandwidth, but I'll look into it.Quote from: mawyatt on September 30, 2025, 09:31:38 pmCorrect, 10x, calibrated. Outputing 1v 100KHz squarewave from the AWG to the scope probes shows a clean signal. Trying the 1000x resistive attenutator, as I mentioned in my"Bigger Update", will require a metal shielding enclosure.Quote from: LooseJunkHater on September 30, 2025, 08:48:07 pmQuote from: mawyatt on September 30, 2025, 09:31:38 pmCan you clarify on this, I'm unsure what you mean by this? Wait for what to be developed?

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2025, 10:46:00 pm
content: Quote from: LooseJunkHater on September 30, 2025, 10:05:16 pmHere's the link.https://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg6026257/#msg6026257Also, not talking about the Op-Amp BW but the entire amplifier stage BW (all components involved). Squarewave edge rounding could be due to the high frequency bypass on the overall input as "seen" by the AWG. This means if the AWG "sees" a high frequency bypass for example this can shunt the high frequency components of the squarewave due to the AWG output impedance. Please remember it's not the squarewave frequency that matters it's the edge speed which can extend well beyond the normal amp 3dB bandwidth. You will experience the same "edge rounding effects" even at very low AWG input squarewave frequencies if you look. Once you get a handle on this, then go to a simulation for more detail and verification.Best

author: dietert1
author_rank: Super Contributor
timestamp: September 30, 2025, 11:39:15 pm
content: I'd like to confirm the advice above that a modern 12 bit scope might do the job without additional amplifier. I was showing it here:https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg5951267/#msg5951267.The screen dump is at 200 uV/display unit. The additional very low frequency noise i mentioned in the text may be 100 uVss or so.Regards, Dieter

author: KrudyZ
author_rank: Frequent Contributor
timestamp: October 01, 2025, 02:57:40 am
content: If you're into boat anchors, there is the Yokogawa DL3120B that has a few tricks up its old sleeves.200 uV / div, built-in selectable analog front end filters and differential inputs.Since it's a 12 bit scope that is 400 LSBs per division for 0.05 uV resolution. Of course that would be mostly useful for looking at drift in DC signals with lots of filtering.It samples at 25 Msps for a 10 MHz bw, which is reduced to 5 MHz for the higher gain settings.And if you find that signal of intelligence in your SETI scan you can print it out on the built-in thermal printer!

author: zlymex
author_rank: Frequent Contributor
timestamp: October 01, 2025, 05:19:50 am
content: I've been using this LNA meter from JOY.He has newer versions, one of which ishere.

author: 2N3055
author_rank: Super Contributor
timestamp: October 01, 2025, 07:12:16 am
content: Quote from: KrudyZ on October 01, 2025, 02:57:40 amIn attachment Picoscope 4262. 1mV P-P and 300µV P-P. Triggering rock solid.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: October 02, 2025, 03:20:39 am
content: Quote from: 2N3055 on October 01, 2025, 07:12:16 amSure, but Yokogawa built that scope 33 years ago in 1992...

author: EC8010
author_rank: Frequent Contributor
timestamp: October 02, 2025, 12:19:33 pm
content: As long ago as that? That is seriously impressive.

author: 2N3055
author_rank: Super Contributor
timestamp: October 02, 2025, 01:32:13 pm
content: Quote from: KrudyZ on October 02, 2025, 03:20:39 amAbsolutely impressive, but I don't understand the comment.What is it relevant to? Was this some kind of competition?I have shown something you can buy now, new, and that works very well. That is all. If someone finds it useful, good. If not, I contributed data point. That is all.Best,

author: dietert1
author_rank: Super Contributor
timestamp: October 02, 2025, 08:12:07 pm
content: The Yokogawa hint can be taken more general: There may be "data recorders" more appropriate for this application than a modern 12 bit digital scope. The Picoscope 4262 = 16-bit at 16 Msps is one example of a "data recorder" and at a very moderate price.Regards, Dieter

author: David Hess
author_rank: Super Contributor
timestamp: October 02, 2025, 10:13:20 pm
content: The whole discussion makes me wonder if there is demand for a 20 MHz bandwidth oscilloscope high impedance low noise preamplifier. 5 microvolts RMS noise should be feasible with essentially no drift.I have a couple of oscilloscopes which support 1mV/div or better sensitivity at reduced bandwidths, but they are all either 1 MHz or 5 MHz bandwidth, which is annoying when 20 MHz is the standard for spot noise measurements.

author: TimFox
author_rank: Super Contributor
timestamp: October 02, 2025, 10:33:25 pm
content: 5 uV noise in 20 MHz bandwidth is 1.1 nV/Hz1/2, comparable to a 50\$\Omega\$ resistor (0.9 nV/Hz1/2) which is probably difficult to achieve at a high input impedance.JFETs can be around 2 nV/Hz1/2at 100 kHz; BJTs can be quieter in voltage, but have higher noise current that interfere with high-impedance applications.My classic -hp- low-frequency two-channel differential-input analog oscilloscope goes down to 0.1 mV/division, with 500 kHz or (switched) 50 kHz bandwidth.The original noise specification is 20 uV, measured tangentially, at full bandwidth (JFET input amplifier), or 15 uV at 50 kHz bandwidth (all referred to input).Input impedance is 1 M\$\Omega\$ in parallel with 45 pF at each input.I also have a couple of classic Princeton Applied Research differential preamplifiers (with variable hpf and lpf). These were originally sold to work with lock-in amplifiers, and similar devices were built as plug-in modules.The apparent current successor to these are from Ametek/Signal Recoveryhttps://www.ameteksi.com/-/media/ameteksi/download_links/documentations/5113/model_5113a.pdf?revision=f4e5f61f-47b9-43dd-bd1f-36a7c1430f25The model 5113A cited there has a maximum bandwidth of 1 MHz and a typical noise density of 4 nV/Hz1/2at 1 kHz.To minimize hum and interference, it can be run from internal batteries and with the simple microprocessor shut off.

author: David Hess
author_rank: Super Contributor
timestamp: October 03, 2025, 10:25:40 am
content: Quote from: TimFox on October 02, 2025, 10:33:25 pm1 nV/SqrtHz it is. 20 MHz bandwidth means a much larger JFET or MOSFET can be used, and then bootstrapping can be used to reduce the larger input capacitance which can still be somewhat greater than 20 picofarads and still be compatible with common probe compensation. (1) I might start with several LSK489s in parallel, but would also investigate small vertical MOSFETs to see if any have enough transconductance combined with low enough capacitance to achieve the needed bandwidth.(1) I have used modern inexpensive probes which can compensate to at least the 47 picofarads of my 1 MHz 7A22.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 03, 2025, 01:43:41 pm
content: Quote from: TimFox on October 02, 2025, 10:33:25 pmIf you want to be a cheapskate (and not worry when the front end is destroyed) then J112 at 1.2nV per root Hz does nicely. Noise corner is at 13Hz. You can definitely afford to wire a few in parallel to achieve a differential pair having 1nV per root Hz noise.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 03, 2025, 03:33:35 pm
content: Quote from: David Hess on October 02, 2025, 10:13:20 pmAbsolutely. There's a lot of noise and problems that can effect metrology measurements below 1mV, but it's difficult to see it on an oscilloscope without specialized/custom low noise amplifiers. This whole thread + project was inspired by theLFLNA-80. However, with a bandwidth of 20MHz, stray RF would create a lot of problems, like I was experiencing with the 1000x resistor divider (here).If someone actually does manage to produce a much better circuit than my own, with lower noise, more stability, and open-sources it, I'd be so happy and jump on that. One of my main design goals was design goals "low-cost" (I've maybe spent $5-$10USD on parts for everything, since much of my components were used/pulled), and I'd imagine utilizing jFET's for the front end would really help meet the low-cost, low-noise, and stability. I did previously consider it (and still am), but I didn't pursued it due to lack of knowledge of jFET's, and because I'm not sure it would be feasible/worth it on the prototype boards that I'm using.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 03, 2025, 03:51:49 pm
content: Quote from: EC8010 on October 03, 2025, 01:43:41 pmWhen I briefly considered using jFET's for this project, J112's (by OnSemi) were the part I considered using. Couldn't find anything comparable to their low noise, for the price.

author: TimFox
author_rank: Super Contributor
timestamp: October 03, 2025, 04:42:40 pm
content: When working on "low to medium-field" MRI systems, operating around 5 to 15 MHz, ca. 1990, I used the late, lamented Sony 2SK152 JFET, which had a better-than-the-average-FET ratio ofYfs/Ciss.Unfortunately, it was intended for low-noise vidicon preamplifiers, which market disappeared. A specialty manufacturer, InterFet, produced a similar part for replacement.The original Sony part claimed 1.2 nV/Hz1/2(typ) at 1 kHz, when operated at 10 mA. The highest-current bin, 2SK152-4 (27.4 to 42 mA) had higher transconductance, with slightly lower noise voltage.The Sony data sheet shows the noise voltage flattish up to 1 MHz. At 100 MHz, in a tuned, noise-matched circuit, they claimed 1.8 dB noise figure (typ) at 100 MHz, at 10 mA.https://www.datasheets360.com/pdf/5688259163679467641

author: mawyatt
author_rank: Super Contributor
timestamp: October 03, 2025, 06:01:00 pm
content: Quote from: TimFox on October 03, 2025, 04:42:40 pmSony had great speciality parts back then, and one the best bipolar processes. They made a bipolar DAC used in their studio monitors for RGB generation that was better for RF than any other DAC around. We often used those but kept quiet about it!! One of our customers would tell their favorite contractor (not us) everything we did, so needed to be careful around them!!!Best

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 04, 2025, 12:11:07 pm
content: Quote from: TimFox on October 03, 2025, 04:42:40 pmUnfortunately, the InterFET IFN152 has a significantly larger input capacitance, around 15pF (the original 2SK152 had 8pF) so the equivalent charge noise is double that of the original SONY device. Fortunately, there are more recent devices that are even better than the original 2SK152 in this respect, for example the classic (now also obsolete though I have some) BF862 from NXP with 0.8nV/rtHz at 100kHz and only 10pF input capacitance, and some current low noise JFETs from ON Semi, for example the CPH3910 with ~1nV/rtHz noise and only 6pF input capacitance, about twice as good as the original SONY device in the charge noise respect.CheersAlex

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 04, 2025, 03:19:55 pm
content: Was building a simple EMI enclosure yesterday from a steel cookie tin + aluminum tape (to see if it'll help decrease the circuit noise + RF interference) and realized that the aluminum tape isn't using a conductive adhesiveNow none of the aluminum tape strips are electrically connected together.Any suggestions for electrically connecting the individual tape strips + enclosure?I'm thinking of either purchasing some proper copper tape which is conductive on both sides, or solder paste. Conductive adhesives/glues appear to be $10USD+ for only 0.2mL...My fear of using solder paste is some of the microscopic solder balls dislodging and causing a short in the circuit or elsewhereorjust simply not being conductive after a certain period of time, when the flux dries...

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2025, 03:52:13 pm
content: The good 3M 1181 conductive-back copper tape works well, but you should use new stuff each time.Not all copper tape has this type of adhesive.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 04, 2025, 03:57:29 pm
content: https://uk.farnell.com/c/tools-production-supplies/tape/electronic-tape/copper-aluminium-tapeCheersAlex

author: EC8010
author_rank: Frequent Contributor
timestamp: October 04, 2025, 04:25:50 pm
content: What were you hoping to achieve by lining the tin with aluminium tape?The self-adhesive copper tape sold to gardeners for deterring slugs almost certainly doesn't have conductive adhesive. But does it matter? If you use it to bridge the seam between tin and lid (for example), the adhesive is so thin that the capacitance between the tape and whatever it is stuck to is pretty high and a short circuit at RF. I very much doubt that you can see any difference between tape with conductive adhesive and without, except in price. If in doubt, you can always run a seam of solder along the edge.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 04, 2025, 04:35:04 pm
content: Quote from: EC8010 on October 04, 2025, 04:25:50 pmImproved RF (electrical interference) blocking, similar to what I discussedhere on the EEVblog, with the main difference being that I'd be doing only the tin outer (for weak MI) --> Aluminum Foil (for EI) --> Air Gap --> Copper Tape (for EI) --> Plastic Enclosure + PCBQuote from: EC8010 on October 04, 2025, 04:25:50 pmI'll see if I can manage to get solder to stick to the steel enclosure + aluminum tape, but I don't have strong hope. To be continued...

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 04, 2025, 04:36:02 pm
content: Quote from: TimFox on October 04, 2025, 03:52:13 pmHonestly was thinking that most/all aluminum & copper tapes would have a conductive adhesive backing, but clearly not. The more I know! Live and learn.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 04, 2025, 06:02:21 pm
content: Quote from: LooseJunkHater on October 04, 2025, 04:35:04 pmWell, I did it; I managed to solder to the aluminum tape. Tried plenty of flux but was unsuccessful,howevera random comment on YouTube suggested motor/engine oil (I used bike chain oil), apply it to the site, scrape the aluminum & apply solder. Eventually, the aluminum oxide layer is scratched away, I assume the oil prevents quick reoxidation of the aluminum, and then the solder was able to stick. The joints look fairly weak, but they should be "good enough", as I'm only connecting all of the aluminum pieces together.

author: smaultre
author_rank: Regular Contributor
timestamp: October 04, 2025, 07:00:33 pm
content: Take a look on Tektronix ADA400 Differential Preamplifierhttps://w140.com/tekwiki/wiki/ADA400can be connected via Tektronix 1103 power supply to any modern hi-res scopes.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 04, 2025, 07:27:56 pm
content: Quote from: smaultre on October 04, 2025, 07:00:33 pmMostly-correct schematicof the ADA400.Significantly more complicated than my current design or the LFLNA-80, (likely?) requires matched initial high-pass filter capacitor, a lot of internal trimmers, unnecessary gain controls.If I can find a 100% complete schematic (as there's a lot of things that the Github poster is unsure about), I may consider simulating it, seeing how well it works, what can be removed, and how to speed it up. The device is over $300USD on EBay, so I definitely won't be purchasing one.Early on I did consider a differential input amplifier, but I quickly became dissuaded by the requirement for matching input high-pass filter capacitorsEdit:Looking at this image, I could likely remove the bandwidth switch, gain selector switch, input selection (AC, DC, GND) + associated circuitry. Hmm, maybe I will consider looking more into this design...

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 04, 2025, 08:08:13 pm
content: Quote from: LooseJunkHater on October 04, 2025, 07:27:56 pmSome notes on the schematic:- Uses a +15V & -5V for DC offset voltage correction, instead of a DC servo (in my design) or high-pass filtered output (of LFLNA-80)- DC offset voltage correction is fed into an instrumentation amplifier, which also appears to be the main preamplifier with a super-low input bias current of 20pA (max).- No idea what Q1 & Q2 do/are for- AD829 amplifies the signal further for the output, and uses an external compensation capacitor for stability (per datasheet, appears to be required for gains less than 20x)- Simple overload detection circuitThe trimmers: (= I have no idea what it's doing, as I'm not smart enough, but I do take a guess)- RV1 ()- C2 ()- C3 (. For reducing noise?)- C6 (. Not enabled for 100KHz/Full bandwidth? Can be removed?)- C7 (. Not enabled for 100KHz/Full bandwidth? Can be removed?)- RV5 (. Not enabled for 100KHz/Full bandwidth? Can be removed?)- RV6 (gain fine-tuning?)- RV8 (gain fine-tuning?)- RV2 (External pot: coarse DC offset voltage correction)- RV3 (External pot: fine DC offset voltage correction)- RV4 (another DC offset voltage correction trimmer)- RV7 (DC offset voltage correction)Without knowing the purpose for all of the trimmers, I don't think I'd really be able to replicate the circuit.

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2025, 08:19:37 pm
content: Q1 and Q2 are JFETs connected as diodes (gate to channel), used as part of the input protection circuit.Small JFETs are often used thus, exploiting the very low leakage current at low voltages, compared with signal diodes (e.g. 1N4148).

author: dietert1
author_rank: Super Contributor
timestamp: October 04, 2025, 08:38:50 pm
content: Q1 and Q2 are low leakage current input protection diodes.And yes, the circuit becomes more complicated if you include the second input channel and one needs matched filters etc. Maybe the 300 € ebay price is justified. The device supports capacitive coupling (in case of larger DC offsets) and i guess one could increase the two capacitors C1- and C1+ in order to implement a lower bandwidth limit of 0.1 Hz if required. Differential input is nice to have for low level measurements if the DUT is more than a little test PCB.Regards, Dieter

author: voltsandjolts
author_rank: Unknown
timestamp: October 04, 2025, 09:16:39 pm
content: BAV199 might be agood substitutefor the Q1, Q2 diode connected jfets.

author: David Hess
author_rank: Super Contributor
timestamp: October 05, 2025, 07:46:20 am
content: I always skipped the J111 to J113 because it lacks the needed specifications, but these could be read from the curves.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2025, 08:35:51 am
content: The 1st amplifier is the INA111. So this sets the input bias, noise and drift performance. It is a good FET INA, but could still be limiting when it comes to super low noise. The ADA400 is mainly a differential probe, not a super low noise amplifier.The trimmers are not that complicated to see what they are good for:RV1 and C2 are for the input impedance per channel -- this may be the slightly tricky part to trim, RV1 may use a fixed resistorC3 to trim the higher frequency CMRRC6 frequency compensation for setting with 1:100 divider at the frontC7 input capacitance for setting with 1:100 divider at the frontRV5 symmetry in the 1:100 dividers / DC CMRR for 1:100 settingRV6 overall gainRV8 gain for 0.1 gain - there may be something wrong in the gain switching part

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 05, 2025, 03:40:20 pm
content: I ended up spending a bit of time simplifying the Tektronix ADA400A schematic that was posted to Github, to improve my understanding of it (image attached).Hopefully I didn't mess any of it up.Removed:- Gain selection + Trimpot RV8 (now it's a constant 100x gain)- Bandwidth selection (now it's a constant "full" bandwidth; above 100KHz)- Overload detection/Overload range LED- 5v Relay (was associated with gain selection)I'm a bit confused as to what "Scale_Factor_Coding" is for, as it doesn't seem to connect to anything on the circuit and attaches to the scope? Maybe it's just for automatic scaling change on the oscilloscope?https://raw.githubusercontent.com/mattico/ADA400A-Reversing/refs/heads/master/Images/ada400a_top_annotated.jpgAdditionally not entirely sure how the In+ circuit is wired up into the circuit (related to "To + Channel" being mentioned twice in the schematic), so I may try to figure this out + simulate the circuit._____________________________________________________________________________________From simplifying the schematic + Kleinstein's explanation of the trimpots, I have a better understanding of what's going on in this circuit, the purpose of the trimpots, and how it's similar & different from the circuit I made.

author: TimFox
author_rank: Super Contributor
timestamp: October 05, 2025, 04:35:10 pm
content: I have used differential-input oscilloscopes, ‘scope plug-ins, and external preamps over the years.The variable bandwidth (LPF and sometimes HPF) feature is very useful when dealing with small signals.I would retain at least the low-pass filter feature.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 05, 2025, 05:01:22 pm
content: Quote from: TimFox on October 05, 2025, 04:35:10 pmHow do the low pass + high pass filter settings help when viewing small signals? Would it be comparable to the 20M bandwidth limit on oscilloscopes, removing extraneous noise?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2025, 05:20:38 pm
content: The scale factor coding should be a way to tell the scope what gain is set. This is likely a tektronic specific probe interface. So this is a part to leave out in most cases.The +in part should see essentially a copy of the input circuit. It is just the other side of the differential probe.The filters are helping in removing noise, especially higher frequency noise. For the highest gain one may really want a reduced BW. 100 kHz instead of 20 MHz from the scope alread could give around 1/14 the higher frequency noise.As shown in the cut down schematics the 1:100 divider is in place. For a gain of 1 one would remode R25, RV5 and related parts and reduce R16. The R16 value would be a compromise between noise and the level of protection.

author: TimFox
author_rank: Super Contributor
timestamp: October 05, 2025, 05:58:43 pm
content: Specifically, if looking for 60 or 120 Hz interference with a 20 MHz or greater bandwidth ‘scope, an extreme LPF of 10 kHz will make the hum components more visible, especially with a “line” trigger source.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 05, 2025, 06:10:59 pm
content: Considering the #1 goal of this thread (and project) is about "Measuring less than 1mV noise/signals on an oscilloscope" (with lesser important, but still important design goals of 0.1Hz-100KHz measurement + low-cost), I see no rationale for incorporating any gains less than 100x for the circuit, increasing complexity + price. My oscilloscope and many other modern scopes can easily see single-ended AC signals around 10mV with high input impedance's. For differential signals above 10mV, I could use the XY mode of the scope & two probes, or build/buy a dedicated differential probe.The dedicated low-pass and high-pass filter switches I can support the idea behind; being able to remove signals below 500Hz (AC + harmonics) & above 100KHz (RF) would be nice, as both have been causing me problems in my own circuit (as I discussed earlier in the thread + showed in the oscilloscope shots). I'll consider incorporating them into the simulated circuit and/or future builds.

author: TimFox
author_rank: Super Contributor
timestamp: October 05, 2025, 06:49:44 pm
content: On a traditional analog CRO, the first thing you see when on a sensitive mV/cm range and you apply the low-pass filter is that the trace width narrows considerably due to reduction of high-frequency noise in the reduced bandwidth.

author: dietert1
author_rank: Super Contributor
timestamp: October 05, 2025, 07:32:12 pm
content: That simplified circuit has a gain of about 1, as there are two 10x gain stages and a 1:100 input divider (990K+10K). As mentioned before by Kleinstein the instrumentation amplifier INA111 may be detrimental to noise performance and unnecessary if you go for a single ended input.I would also keep some low and high pass filter option in case you need that later on.Regards, Dieter

author: 2N3055
author_rank: Super Contributor
timestamp: October 05, 2025, 07:57:05 pm
content: When measuring very low level signals there is a problem of probing.Differential preamp is sometimes big help. Even if you have very low noise in preamp itself, that is of little use if you going to pick up lot of stuff from elsewhere.Having an simplified ADA 400A with only 1x 10x and 100x amplification, and filtering, would be useful because of differential probing despite noise levels being just slightly better to what nowadays scopes have...

author: EC8010
author_rank: Frequent Contributor
timestamp: October 05, 2025, 09:49:01 pm
content: Those apparently useless gain settings on the ADA 400A are all about making the instrument useable. Agreed, when everything is set up perfectly, there's little point in having less than x100 gain. But before you reach ideal setup, it is extremely useful to be able to flip a few switches and quickly find out in which direction you should be heading. And those scale factors are another part of making it easy to use; they tell the oscilloscope (via the Tekprobe interface) what the sensitivity is at the input of the pre-amplifier so that automated measurements etc come out correctly. Yes, we require good technical performance, but we also require good ergonomics that allow good measurements. With all those factors in mind, I've just ordered an ADA 400A to plug into my TDS3032 oscilloscope (it has Tekprobe interface). That 100dB low frequency CMRR will come in very useful...

author: mawyatt
author_rank: Super Contributor
timestamp: October 06, 2025, 01:39:19 am
content: Quote from: LooseJunkHater on October 05, 2025, 05:01:22 pmThese limit/reduce the "noise" superimposed on the signal of interest, and some of the later entry level DSOs have selectable built-in filters, these are digital types and usually somewhat configurable. If a DSO has ERES, this can effectively be utilized as a digital LPF, check the specific DSO manual for details on the filtering effect.Best

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 07, 2025, 05:09:36 pm
content: Quote from: LooseJunkHater on October 04, 2025, 07:27:56 pmEnded up creating a simplersimulation in Falstad(excludes JFET protection, gas discharge tube, and various other unnecessary parts) and it doesn't immediately crash, which is good. However,the circuit doesn't work correctly, whichI assume is related to the resistor values. I'm not sure if the person hosting the github repo wrote "562" and "475" to represent 562ohms or 5.6k, and 475 ohms or 4.7M. Just quickly made this and will try getting it working later, but sharing right now in case anyone can identify any other obvious errors.I'm additionally confused bythis potentiometer(pointed to with a red arrow in the attached image) from the Github repo; it's only located on the negative side and I have 0 idea what it's for. It's clearly not RV2, RV3, RV4.Is it possibly RV1, with the positive side having "+ Channel Trim" with just a trim cap?

author: EC8010
author_rank: Frequent Contributor
timestamp: October 07, 2025, 05:56:34 pm
content: In your circuit, the input amplifier is configured for A = 1000. The differential stage is A = 1. The output stage is A = 82. I would expect an amplifier with that much gain to saturate at even the slightest input offset. So the resistor values must be wrong. To get decent bandwidth out of the first stage will require a lower gain that 1000, so I'd start checking there. Perhaps that 562 is really 5k6. The github circuit shows 5620 ohms. That would make more sense. An output stage with a gain of 82 sounds wrong, too. Maybe, 10, but less is likely. The github circuit shows a gain of 10 (ish) with a 1210 ohm resistor, not 121 ohm. You have introduced errors.No way is that series 4M7 resistor correct. Maybe 47 ohm or 475 ohm, but definitely not 4M7. And the following 1.2nF sounds far too high as well.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2025, 05:59:47 pm
content: For RV1 the + input has a fixed resistor. So 10 K for the + input and some 5K and the trimmer for the -input. So a fixed resitor value instead of the trimmer.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 10, 2025, 11:17:39 am
content: My ADA 400A has arrived and has a fault on the "fine offset" control, so I've been looking through the github documentation and comparing to what I have.Mine has LT1027-5 voltage references; which are pin-compatible with LT1021-5 but reduced tempcoRV1 is 20k on mine, not 2kRV8 is 5k (unspecified on github)I came to the same conclusions as Kleinstein for the functions of the trimmers. Unfortunately, the github documentation didn't identify the variable resistors on the photograph, so I've been pinning those down with a view to labelling the internal cover with the function of each tweak.EDIT: I finally found the xdevs page:https://web.archive.org/web/20250319171400/https://xdevs.com/fix/ada400a/. In it, I found this alarming note:This preamplifier have limited compatibility with newer Tektronix TekVPI oscilloscopes, such as our DPO7104C. Adapter TPA-BNC must be used, and gain selection must be done ONLY before plugging probe into scope. Switching gain while connected likely to damage TPA-BNC adapter’s +5V regulator due to excessive current spike!Although I started with the ADA 400A plugged into a 3000 series oscilloscope, I would have used it (via TPA-BNC) on another. I won't now...FURTHER EDIT: The offset problem is due to a failed LT1027-5 that is producing a noisy -4.02V instead of -5V. Interestingly, the xdevs.com ADA 400A also had a failed LT1027-5, although theirs was the +5V reference.FINAL EDIT: Replacing the LT1027-5 (I had a metal can version in stock) fixed the problem.

author: 1audio
author_rank: Frequent Contributor
timestamp: October 10, 2025, 09:00:02 pm
content: The basic circuit of the ADA400 can be traced back to the 1A7A plugin but with opamps replacing a lot of parts.https://w140.com/tekwiki/images/4/48/070-0782-00.pdfThere was an earlier tube version with less capability. The 1A7A and the 7A22 both have a lot of precision caps for the bandwidth control.I was looking for a low noise high gain differential preamp late last year and found this on eBay:Its an interesting circuit that works pretty well. Seems to have fewer parts to do the job. I have attached a partial schematic in LTspice I reverse engineered to get it running. Its targeted at biomedical so safety is important.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 10, 2025, 09:54:31 pm
content: Quite a few years ago, I bought a Keithley 6517B electrometer. Then I learned how to design and build better performing (but less versatile) electrometers. The point being that the 6517B was essential to my education. I suspect the same will be true for the ADA 400A. I have previously made differential amplifiers, but perhaps not with as much thought as should have been applied. My next differential preamplifier will be significantly better than the ADA 400A. Less versatile, but at a cost that allows me to tailor it to a specific need. And to make a slightly different version if another need arises.I shall have a look at the partial circuit."Targeted at biomedical" means huge profit margin because the buyer is unknowledgeable and only too happy to pass enormous costs on because it makes their own charges look more reasonable. I visited dentist a few years ago and admired her magnifying overglasses with built in LED lights; effectively an upmarket Optivisor plus a pair of 1W LEDs and battery. I asked about them and she said they were quite expensive. I pressed her and she said £2k (presumably without VAT). £2k for that? And she didn't seem horrified by the price, suggesting that it was in line with everything else they bought; overpriced.

author: 1audio
author_rank: Frequent Contributor
timestamp: October 10, 2025, 11:22:02 pm
content: Biomedical- Pro, safety, certfications, con, Protected market due to barriers to entry, High to very high profit marginsn many cases monopoly profits.The WMI runs for some time on two small batteries which makes it quite nice for low level measurements. It has the essential band limit filters. The form factor is clumsy but it was $40 on eBay. . .

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2025, 02:59:38 pm
content: Quote from: EC8010 on October 10, 2025, 11:17:39 amWere the two pots + different vRef the only part/component differences between the Github schematic and your own?How do you also find the performance of the device? I assume you don't get the same level of noise that I was getting with my DIY circuit, feeding in a 100KHz Square Wave, 2mV PP, 9V DC Offset signal, that I previously discussedhere?

author: EC8010
author_rank: Frequent Contributor
timestamp: October 11, 2025, 05:35:40 pm
content: Quote from: LooseJunkHater on October 11, 2025, 02:59:38 pmI'm afraid I was rather lazy. Once I'd spotted the faulty LT1027-5 and replaced it, I didn't go looking to trace the full circuit. The way I've always traced circuits is to consider how I would design such a thing, then look to see if someone else thought the same way. The thing that makes the ADA 400A complex is the gain and bandwidth switching and communicating its gain to the Tekprobe interface. Other than that, it's pretty standard stuff.It's noisier than my home-made x500 preamplifier. But to be fair, my home-made uses much later (and quieter) op-amps (OPA1641) and is configured for only 1kHz bandwidth. But my CMRR isn't as good as the ADA 400A (I should have used a dedicated instrumentation amplifier having on-board matched resistors rather than adding resistors around an OPA1641. Also, my home-made has zero protection. Abuse it and you need to fix it. I expect to use the ADA 400A when I don't know exactly what I'm looking at and need flexibility rather than ultimate performance. For me, the convenience of it being able to plug directly into my 'scope and not need any extra power was a powerful argument. Later, I will make a new home-made using a proper instrumentation amplifier to improve CMRR, but probably retaining the (quiet) OPA1641 input amplifiers. An alternative to each OPA1641 is to add a 2SK2145 differential pair front end to an NE5534A and configure it for A = 1000.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2025, 07:30:44 pm
content: Quote from: EC8010 on October 07, 2025, 05:56:34 pmThe 4.7M resistor definitely seemed wrong to me. I've fixed all of the "562" resistors to 562ohms, "475" resistors to 475ohms, "5620" resistors to 5.62k, and "1210" resistors to 1.21k. Additionally, temporarily removed the 1.2nF capacitor on the output of the instrumentation amplifier, as it does indeed sound too high. I do wonder whether the 1.2nF mica cap on the input may also be too high?TheFalstad circuitis now working better.With these changes, it looks like instrumentation amplifier (INA111) gain is 9.89x, and the AD829 gain is around 9.2x (dependant on the RV6, gain trim pot), bringing the total system gain to around 90x.I do wonder whether the 9.2x gain of the AD829 is actually correct, considering it's a high speed (120 MHz bandwidth, 230 V/uS) op-amp, unless it was primarily chosen for the offset nul + external compensation capacitor support? Additionally for the AD829, it's odd to me that gain isn't set by adjusting an rF or rIn resistor (like is commonly done with an Non-Inverting Op Amp circuits), but instead the total series resistance entering the In+ line.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2025, 07:40:06 pm
content: Now I noticed that in the Falstad circuit, the 1mV differential input voltage is seemingly only becoming 10uV at the input of the instrumentation amplifier? What is happening to the signal?

author: dietert1
author_rank: Super Contributor
timestamp: October 11, 2025, 11:30:00 pm
content: Above i commented that there is a 0.01 input divider (990K, 10K). So 10 uV at instrumentation amplifier is expected with 1 mV input. With the two gain stages of about 10x each overall gain is about 1. Not really what you wanted.Regards, Dieter

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 11, 2025, 11:53:24 pm
content: Quote from: dietert1 on October 05, 2025, 07:32:12 pmCompletely missed your earlier mention of 1:100 input divider. Aside from CMRR (mentioned byKleinsteinearlier), what else would be the reason for this divider & RV5 trimmer? If I decrease the input division (ex. to 1:10), would that impact the ability to trim CMRR?Quote from: dietert1 on October 05, 2025, 07:32:12 pmI'm exploring this differential input design at this time mostly out of curiosity + to learn, even if it isn't the most practical for my use, and is more difficult to implement. I have 0-intention to use an INA111, and would likely create a discrete instrumentation amplifier using (hand-picked) resistors + MCP6v86 op-amps.I've also been thinking about + simulating how to implement additional filtering in my design. Since RF appears to be causing me some problems, I was thinking of either changing the input to a passive bandpass, or incorporate an active low-pass filter after the initial preamplifier. The LFLNA-80 uses a 4th order Sallen-key low-pass filter, and I know a bit about the magnitude response of Butterworth and type 2 Chebyshev filters, but I lack the knowledge to know which design to go with.

author: EC8010
author_rank: Frequent Contributor
timestamp: October 12, 2025, 09:08:15 am
content: Quote from: dietert1 on October 11, 2025, 11:30:00 pmAnd yet the actual ADA 400A has a maximum gain of x100. Something is not quite right. An input divider would make sense because it would reduce input capacitance and allow the variable offset to have a range larger than the internal supply voltage of +/-15V (which it has, but perhaps not on all gain settings). Looking at the INA111 data sheet, it has >1MHz bandwidth at A = 10, which is consistent with the ADA 400A data sheet and there is no suggestion that INA111 gain is varied. I think that 100:1 divider is switched in to enable x1 gain but is not permanently in circuit. That would also explain the ADA 400A's rather high input capacitance of 55pF. I'm not in the least surprised that the github drawing does not make the gain settings entirely clear - tracing that through would be a nightmare.I was happy to go inside my ADA 400A to fix it, but every time you interfere with stuff, you risk damage and the wires from the Tekprobe umbilical are extremely slender. In short, I'm not going inside again unless it's to fix a fault.Any input divider degrades the final amplifier's noise, so if you're concerned about noise, you don't want a fixed input divider unless you need a common-mode range greater than the amplifier's supply rails. I think you have to make a firm decision about what your differential preamplifier is for. If you are after low noise, then you cannot have a fixed input divider and you can't have a large DC between the inputs. But if you don't need to be able to measure down to DC, you can get round that by AC coupling. The ADA 400A has 2.2uF 63V input coupling capacitors into 1M, giving f-3dB = 0.1Hz, and that's a usefully low frequency. I used 14uF 900V capacitors as input coupling into 1M to achieve 10mHz, but that brings its own problems because the input transient can destroy the following amplifier, so I get round that with input shorting switches (but have to remember to use them appropriately), and the physically large size of those capacitors increases input capacitance. The ADA 400A has plenty of input protection instead (which is a better solution). In short, although it's easy to design something that will perform the task of a differential preamplifier and have low noise, designing it to be robust, versatile, and easy to use makes it more complex, which accounts for the ADA 400A's apparently unnecessary complexity.Beware that an active RF filter is too late. You have to stop the RF getting in to begin with. That means ferrites and shunt capacitors to ground at the input. That filter in the LFLNA-80 is to define noise bandwidth not to protect against RF.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: October 12, 2025, 04:20:14 pm
content: Quote from: EC8010 on October 12, 2025, 09:08:15 amCan't remember if I mentioned it in this thread before, but in the past I did measure some 1.43MHz (AM radio?) noise with the FFT mode of my scope before, on my amplifier circuit. Yesterday I did a bit more experimenting + testing with my circuit, andI think you're right that I need RF filters before the signal gets to the preamplifier. I have no experience in using ferrite beads for RF suppression, but I'll need to look into this.I noticed that there's some even-higher frequency noise being picked up within the FM radio spectrum (measured around 91MHz to 105MHz). Part of the problem is definitely that I'm using jumper-connections to enable/disable the DC servo aspect of the circuit; these jumper-connections appear to either be picking up noise or introducing noise for me? If there isn't a firm-connection with the jumpers, I noticed a lot more noise/problems in my amplifier. I was also comparing noise with & without the DC servo, and it doesn't seems to introduce any noise in itself, as the square wave waveform appeared nearly identical, visually and with FFT.Additionally, in the MCP6v86u datasheet, it seemed to recommend using a 10k resistor on the output to "maintain stability" of output loads. I tried adding and removing a 10k resistor to the output, and it seemed to have no effect on the actual circuit noise, instead causing the 100KHz square-wave to become more rounded (ie. slowing the square wave response).Before I go any farther with trying to reverse-engineer + simulate the Tektronix ADA400 (or any differential design),I still need to do the following with my circuit:- Complete + install PCB within an EMI enclosure (almost done)- Measure the true GBW of the op-amps- Trial input ferrite beads/pF capacitors for RF suppression- Clean the PCB for a second time- Remove DC servo jumpers + increase DC servo capacitors- Remove the input high-pass resistor + increase the high-pass input filter capacitance beyond 16.5uF (for improved low-frequency response)

author: EC8010
author_rank: Frequent Contributor
timestamp: October 12, 2025, 08:24:09 pm
content: I should say at the outset that I'm not experienced at deliberately tranmitting or receiving RF. But I have had to get kit through EMC compliance testing that subjected my darling to frequencies up to 1GHz at 10V/m. Nasty. I learned that gaps in conductive cases must be covered with copper tape and that ferrites were really useful when wires came into the enclosure. But what I most learned was that detail is everything. You really have to pay attention to everything. It is not good enough to do sloppy work and say, "That should be alright." It has to be definitely right. A wire >1mm long is an inductor and should probably be replaced by wide copper foil.