Measuring a signal in the μV range - Page 1
Forum section: electronics / beginners

author: tigrou
author_rank: Regular Contributor
timestamp: April 16, 2025, 08:26:45 am
content: Hello,I'm trying to measure a signal that is quite weak, in the μV-mV range. The impedance is about 5 ohms and frequency is below 25KHz.I use two lm358 in cascade, each giving a 40 gain (thus 1600 gain in total). There is a peak detector at the output (using a diode and another lm358).I learned (the hard way) that op amps have aninput voltage offset(about 2mV for the lm358) making it hard to detect anything around 1mv.2mV x 1600 (rough approximation) is about 3.3V ! (close to the output range of 0-5V)Questions:- I am planning to use a low input offset voltage op amp, is there any popular one (eg: easy to find, reasonable price) you would recommend for this task ? I made a search and there isso manyof them. There is some expensive ones ($10-20$ piece) for audio, not sure it's needed there.- I read about instrumentation amplifiers (eg: such as INA128), should I use one of those instead ? They give a gain of 1000-10000 max (and I need more than that), does it make sense to put two of them in cascade ? It seems it's useless to try to create an instrumentation amplifier with lm358 as it does not improve input offset voltage, only CMRR (also add high impedance on inputs). Additionally, they require high precision resistors which I don't have.- Should I take care of the noise characteristic of the op amp (usually measured in nV/√Hz) ? I don't really care about distortion. It's not for audio, I need to measure signal strength mostly.

author: tggzzz
author_rank: Super Contributor
timestamp: April 16, 2025, 08:58:03 am
content: There are quite a few different effects and cures that can arise in such circuits. "The Art of Electronics" (and TAoE X-Chapters) is an excellent introduction to the theoretical and practical problems and solutions. It includes tables of opamps that will make your choice significantly easier.I will note that measuring DC signals is different to measuring AC signals (1/f noise, drift, seebeck effect), and that you deal with thermal noise, shot noise and EMI noise in different ways.

author: tigrou
author_rank: Regular Contributor
timestamp: April 16, 2025, 09:42:27 am
content: "The Art of Electronics": You are probably talking aboutChapter 5 Precision Circuits. Thanks, I will have a look.After some tinkering :I think adding an instrumentation amplifier (eg:INA128) in first stage (eg: 1000x amplification), then a precision op amp as second stage (eg:OPA227, or even cheaperOPA277with slightly more noise) might do the job.

author: xvr
author_rank: Frequent Contributor
timestamp: April 16, 2025, 11:07:28 am
content: INA128 has up to 50uV input offset. It is in your input voltage range.You don't need instrumentation amplifier if your signal is relative to ground. It usefull only if you have input differential signal or you have to compensate voltage drop in input wires.

author: ivo
author_rank: Regular Contributor
timestamp: April 16, 2025, 11:27:05 am
content: If you feel like it, you can get two single OP27ss, and make use of their offset trim pins to get no offset.Otherwise there are many parts, like MAX44246, OPA2189, MAX4239. If you want good signal at 25khz, you have to make sure the opamps have enough bandwidth. To get their roll off point, you need to divide your decided on gain into their bandwidth spec. So 1Mhz / 40 = 25khz, the minimum GBW spec you'd need is at least a 1Mhz opamp. There's heaps of chopper opamps but if you see lower-power ones that would usually mean they've sacrificed on speed but are still good at DC applications. Higher power consumption will also usually get you better noise specs.

author: tigrou
author_rank: Regular Contributor
timestamp: April 16, 2025, 02:19:15 pm
content: Quote from: ivo on April 16, 2025, 11:27:05 amIn case of OP27, it's to go from 10 μV to something like 2-3 μV right ? Or it this to reach 10 μV ?Also I'm confused about those the offset trim pins: is this something you fine tune once for good based on measurements in lab (as it depends of the factory process mostly) or can does it require adjustment based on what you are gonna do with it (gain, input current, bandwidth, ...)The datasheet (section 7.3.2) doesn't say much about it.chopper opamps :I just read in "The Art of Electronics" that while they achieve incredible performance they also add lot of noise at the inputs and outputs due to the switching.

author: jwet
author_rank: Frequent Contributor
timestamp: April 16, 2025, 04:20:51 pm
content: You might consider looking at how ECG and EKG signals are measured. They generally use INA type amps with low offset often with autozero/chopper techniques."Chopper" is a very inexact term. Very early amps (Philbrick) modulated the input signal to make it AC, amplified it and then demodulated it- this is noisy but also archaic it gave choppers this bad rap. Other techniques switched the input at low frequency and inverted the gain of the amp in sync to cancel offset- also noisy.Modern amps do some very clever things usually off line to null offsets and create no noise. Look at Maxim MAX44250- the spec that's likely going to dominate is low frequency noise- this part is about 100 nV from 1-10 Hz. Maxim makes many others but the 44250 is exceptional. The MAX4238 that Dave Jones uses in his micro current is very good but considerable lower bandwidth.Don't try to get your offset down below your signal- you'll be chasing thermocouple effects all day. Get down to where you can gain up the signal without saturating and then remove DC downstream in hardware of software. The low noise chapters of A of E is really excellent. Having a source with a low output impedance makes things much easier in all ways.

author: TimFox
author_rank: Super Contributor
timestamp: April 16, 2025, 04:30:38 pm
content: Your signal bandwidth is < 25 kHz.Is the DC component important to your measurement?If not, capacitors in the signal path will remove it: amplify the signal before the capacitor until the amplitude is large compared with the offset of the remaining chain.The noise of the op amp, both the voltage noise in nV/rt-Hz and current noise in pA/rt-Hz are important for the first amplifier in the chain; since your source impedance is only 5\$\Omega\$, the current noise is probably not important.Good BJTs ordinarily have lower voltage noise than JFETs (with higher bias current and noise current), and can have lower temperature co-efficient of the offset voltage.There are extreme measures to reduce voltage noise from low source impedance, such as using multiple BJTs in parallel at the input: consult the low-noise literature for examples.The classic text on low-noise amplifiers is Motchenbacher and ConnellyLow Noise Electronic System Design, Wiley 1993.Pirated:https://pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Motchenbacher_Connelly/Low-noise_Electronic_Design.pdf

author: tigrou
author_rank: Regular Contributor
timestamp: April 17, 2025, 08:15:24 am
content: Something really weird happened yesterday :I have refactored my circuit, going from 2 x 8DIP to only 1 (for the gain stage).The lm358 I have contains 2 of them per package. So same circuit but one less physical chip.After testing the circuit I couldn't believe my eyes : I could measure way lower voltages than before (with far less offset)I found out that one lm358 (I have 5 in total) stands out of the rest. It's the one used now solely for amplification. That one is much better than the others (which they have far more voltage offset). The other 4 ones give similar results to before (they "saturate" pretty quickly at high gain, due to voltage offset). So either that one is "gifted" or all of them are pretty bad and it's the only one working correctly. Before that circuit change, I used 2 opamps for the gain stage, so there was always a "bad" one.I ordered that pack on Amazon (yes, I know) from popesq.de seller, not sure what it worth. I should probably have ordered from mouser or digikey (as I do usually).The plan is to retest all lm358 (and maybe throw them away if they are not trustable). From a quick test yesterday they all seems to "work" (not sure if they are within specs), no DOA.Funny thing is there is one lm358 in the package with a TI logo on it (but is it really a TI one ?). That one is not the good one. The 4 other (including the good one) are similar.Additionally : I did some tests yesterday with a lower input signal frequency. I used 460 Hz (which should be close to resonant frequency according to the capacitor installed at that time). It indeed boost the signal (eg: vs 300Hz or 500Hz). Then I tried a different frequency (as there might be some measurement error and resonant frequency might be different from theory). I found out that at 453 Hz the output signal (after peak detector) was indeed stronger, but now it became very slow sine wave at 1 or 2Hz (a LED at the output would pulse slowly). Really weird. Is this expected ? I can't explain what is happening. Slightly outside that very specific frequency, it will pulse faster and faster, to finally became DC with some slight noise (as expected because of the high gains and peak detector).

author: RoGeorge
author_rank: Super Contributor
timestamp: April 17, 2025, 08:57:12 am
content: Quote from: tigrou on April 16, 2025, 08:26:45 amuV level, 5ohms and less than 25kHz make me think it's a speaker used as a microphone. If so, then only the AC voltage matters, and the two amplifier stages can be separated in DC (pass the signal from one stage to another through a capacitor). The amplified offset won't go from one stage to another, so it might be possible to use modest/inexpensive op-amps like LM358.Depending on what exactly are you trying to measure, other techniques to extract the signal from noise may be applicable, for example averaging, or a lock-in amplifier. What to use for best performance, better opamps or better signal processing, depends of the signal you want to measure.What exactly is the signal you want to measure?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 17, 2025, 09:01:31 am
content: For the amplifier choice one would need more info, like the supply and signal voltages. Also the frequency range of interest can matter. One may not need that much DC precision if one can use AC coupling at least between 2 stages.The offset specs are more for the worst case or show how much the values scatter. There is still a chance to also find a LM358 with a much lower offset. The LM358 is about the cheapest one to get and the chances of getting fakes is not that high. So the amazon source may not be such an issue here.With a resonant system one can get thing like a beat frequency from the excitation and actual resonance. Chances are this is more a thing of the resoant system and not the amplifier. The reset of the peak detector may also make a difference and could cause issues.

author: tigrou
author_rank: Regular Contributor
timestamp: April 17, 2025, 09:10:42 am
content: Quote from: RoGeorge on April 17, 2025, 08:57:12 amCapacitor : do you mean in series ? (but then AC, will pass, am I right ?)Or connected to ground ? (as in a low pass filter)Quote from: RoGeorge on April 17, 2025, 08:57:12 amIt's a perfect sine wave, send from a coil to another coil (both isolated). Only peak detection is important

author: RoGeorge
author_rank: Super Contributor
timestamp: April 17, 2025, 09:17:36 am
content: Coupling capacitors between the opamps, like this:https://learningaboutelectronics.com/Articles/What-is-a-coupling-capacitor

author: voltsandjolts
author_rank: Unknown
timestamp: April 17, 2025, 09:31:09 am
content: Quote from: tigrou on April 17, 2025, 09:10:42 amI guess there is no option for circuitry on the signal source side, before coil, as some 'pre-processing' there would make life easier?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 17, 2025, 09:34:33 am
content: Unless it's not a secret, the nature of the signal is important. What is that signal?Typical solution from intercom installations (where a speaker was used as a microphone, so a 4-8 ohms coil generating uV of audio signal) was a BJT (Bipolar Junction Transistor) first stage with the transistor as a common-base amplifierhttps://electronics-course.com/bjt-cb-amp, followed by another generic audio amplifier. From there, you can attach a peak detector.For a coupled coil, however, resonance and coupling phenomena might significantly alter your signal, might be necessary to consider that, too, for a proper measurement. Again, other more complicated phenomena might occur, for example line splitting:That is why asking what are you trying to measure. Knowing that will be of great help for you in getting the proper design advice.

author: tggzzz
author_rank: Super Contributor
timestamp: April 17, 2025, 09:42:16 am
content: Quote from: tigrou on April 17, 2025, 09:10:42 amSo, it is an AC signal, not a DC signal. That isconsiderablyeasier.An AC coupling capacitor in series forms a high pass filter. See any text book for the frequency response and select a suitable corner frequency for your signal.With AC coupling the DC offset due to an opamps input offset voltage is much less critical. Since each stage is isolated from the others, each opamps Vio can be analysed separately; the first Vio is not amplified by later stages, since it is removed by the high pass filter. This in your first post, Voo=40*Vio, not 1600*Vio.If the excitation sinewave is also under your control, it might be possible to use a "lock-in" amplifier to detect really low signal levels. While that might be beneficial, you have not mentioned anything that makes thatnecessary.You might benefit from simulating your circuit in a Spice, e.g. LTSpice. Just make sure the opamps models include input offset voltage and any other parameters that might affect your design.

author: tigrou
author_rank: Regular Contributor
timestamp: April 17, 2025, 10:19:34 am
content: Quote from: RoGeorge on April 17, 2025, 09:34:33 amNo, there is nothing secret thereSorry if I miss your question.AC Signal is generated using a Hifi System (I plan to use something better in the future). It's a pure sine wave (should be between 500Hz-20000Hz). Voltage should be below 1V (I don't remember exactly).That signal goes straight into a small inductor (2200uH, 8 omhs).I am trying to capture and amplify that signal using another coil (12mH, 5 omhs). The two coils are electrically completely isolated (no common ground or anything).There is some distance (air gap) between the two coils.With "bad" lm358 best I can get between the two coils is 40cm. With "good" lm358, it's at least one meter.Goals are : to at lest detect signal (is it there or not) and trying to evaluate distance (based on signal amplitude)

author: RoGeorge
author_rank: Super Contributor
timestamp: April 17, 2025, 10:33:38 am
content: I see, so you want to sense from a distance. My guess is your receiving coil is acting like a magnetic antenna, since it can still receive from 1 meter away.In that case, you may want to look at VLF (Very Long Frequency) receivers, for schematics examples and sensing coil (antenna) advice. Usually a ferrite core helps a lot for a VLF antenna (for your signal sensing coil).

author: RoGeorge
author_rank: Super Contributor
timestamp: April 17, 2025, 10:54:13 am
content: Here's a fun one based on opamps (TL082). Covers both the ELF and VLF bands, apparently can even sense insect's wings flap (that's mentioned in the article).https://www.radio-astronomy.org/pdf/qex/whistler-proof.pdfhttps://www.aavso.org/sites/default/files/solar/GYRATOR-III-DI-G-Di-Filippo.doc.pdf

author: tigrou
author_rank: Regular Contributor
timestamp: April 17, 2025, 11:17:30 am
content: Quote from: tggzzz on April 17, 2025, 09:42:16 amI use falstad. Here is mycircuit.It's probably not as good as other simulators as you can't put specific models (eg: all models are generic) but it does the job.About the circuit : the voltage sources across the op-amps positive input are there to simulate the 2mV offset.The DC separation between the two op amps stages helps a lot ! ThanksRoGeorgefor the suggestion.I am not sure if the second DC separation (between 2nd stage and peak detector) is necessary and there is no amplification afterwards (so 2mV offset shouldn't create trouble)I am eager to test this for real.EDIT :I tried to reproduce the LC tank and it seems to have the same slow oscillating behavior I mentioned earlier :

author: ivo
author_rank: Regular Contributor
timestamp: April 18, 2025, 02:11:11 pm
content: Quote from: tigrou on April 16, 2025, 02:19:15 pmYes, or even approaching 0.5 (into its quiescent noise), with enough finessing.QuoteYes, it is usually something you set once with a ten-turn trimmer pot. You have to be careful about what you are trimming: you're not trimming out the offset of the whole "system" signal from end to end, but only purely the particular output signal of that opamp given its exact input signals at the pins are 0.000. In general, this feature was "given up" in modern opamps because even getting a factory worker to spend 30 seconds twiddling a knob at the factory once for each product on the assembly line, is way too much in labour costs, except for probably an audiophile product companyThe commercial companies simply figured out digital and post-manufacture auto-calibration ways to get around the offset (or just, buy even better opamp). It's extremely doable for hobbiests working on their one-time projects, though.QuoteMost modern chopper opamps are chopping above 100khz. Some at a set frequency, some also use spread-spectrum techniques. If they were adding lots of noise to their signal they would have quickly become completely non-viable as a product category.

author: MrAl
author_rank: Super Contributor
timestamp: April 19, 2025, 02:08:17 pm
content: Quote from: tigrou on April 16, 2025, 08:26:45 amHi,For DC and low frequency AC you can look into a chopper stabilized op amp. These have the very lowest input offset but their bandwidth is limited.You may also need to set up with an adjustment so you can null the input offset.For something like this you would also want to do a calibration. That's where you get an instrument that is already capable of the measurements and compare that to what you get from your design. This is an important step because it shows just how good your circuit is and if you need any adjustment. That's if you need really good accuracy. This might mean renting or buying some expensive equipment of course. If you were a company that did this on a daily basis you would already have that equipment on hand.A general rule is if you want to measure a signal with a certain resolution then you should design for that divided by 10, which gives you one more digit. So for example if you want to measure signals that can differ by a max of 10mv, then you want 1mv resolution. So instead of reading say 1.01v you would read 1.010v (that extra digit). Of course there may be times when you can't get it that good, and then of course you have to settle for what you can actually get. The least would be one of two states for that last digit: either 5 or 0. So if you go to measure 1.01v then you should have the ability to measure either 1.015v or 1.000v, and that means you won't ever see 1.011v or 1.012v or 1.013v, ..., or 1.016v or 1.017v, etc.In meters that can not even do that, a rough measurement would be to see the digit bounce between 0 and 5 but nothing in between 0 and 5 or between 5 and the next digit up. So we'd be able to measure 1.01v, but if we see it bounce between 1.01v and 1.00v, then that means it's probably closer to 1.015v.If you are using a microcontroller then there is also the idea of oversampling. This can be a bit tricky though so you'd have to read up on that a lot. This works with the idea of the always present noise on the input that can actually help to increase resolution.

author: TimFox
author_rank: Super Contributor
timestamp: April 19, 2025, 02:39:57 pm
content: In a later post above, the OP states that his signal is sinusoidal, down to 500 Hz.Absolutely no need for AZ or chopper op amps, a simple series capacitor in the chain will pass the signal and totally reflect any DC offset or drift.

author: tigrou
author_rank: Regular Contributor
timestamp: April 19, 2025, 09:20:20 pm
content: Hello, I have been playing a lot with this lately (many hours). Here is some update :(TL;DR I wasn't able to achieve DC separation between amplification stagesand I'm looking for better op amps than LM358)1) All 5 LM358 have been tested and they all work fine. I used thiscircuit. This is not a complete test (there might be other defects) but if it one of them was completely fried, it should have show up.There is still one op amp that stand out of the rest (being able to detect signal as weak as 30uV). It's extremely noisy and unstable with that much gain but this is something other LM358s I have can't do. It's unclear why.2) Despite lot of attemps, I haven't been able to use a capacitor to "DC separate" the two stages of amplification. What happen is usually the signal is passed between the two stages (LED is on) then it turn off as there would be an open circuit between the two stages. It looks like the capacitor is charging (allowing current to passthrough) then, once charged, it block the current (as it would be powered by DC). I have tried different capacitances (10p, 10uF, 100uF) and it's all the same. I have also tried to pull capacitor output to ground with a resistor to discharge it with time (as in a high pass filter) but it does not help.3) The issue from above might tell me there is another issue : the opamps are powered with 0V / 5V instead of -5V / 5V, meaning the output is not a full sine wave.To improve this, I have setup a smallcircuitto provide -2.5V and 2.5V to the opamps (the rest receiving 0V-2.5V).Now the LED will light up at 50% most of the time (even without signal) and merely blink according to the signal (it's really weak). Need more investigation.4) The way I connect the coil is to attach one side to the ground and the other side to the "+" terminal of the 1st stage opamp (the "-" terminal being connected to resistors for setting the gain). Between each side of the coil there is a capacitor (tuned to frequency) which really helps. Is there a better way to connect it ?Next steps :- Implementing DC separation properly (as my previous attempts failed)- Buying better opamp :1) a precision op amp (eg:OPA277) (low noise and low voltage offset)2) a chopper / auto zero op amp (eg:LTC1050) (very low voltage offset)3) an instrumentation amplifier (eg:AD620) (high CMRR, high input impedance). Probably not needed as somebody here mentioned.There is a inexpensive and popular module that already include that IC, along with a L7660 to provide negative rail.- Buying a scope. I love my aneng 8008 but it show it's limits. A scope would really help here. I just learned there is huge difference between simulator and practice.

author: TimFox
author_rank: Super Contributor
timestamp: April 19, 2025, 09:58:29 pm
content: If you connect the output of one amplifier through a capacitor to the input of the next amplifier and a resistor from that input to ground or other DC voltage, the DC (quiescent) voltage at the next amplifier's input will be the voltage at the other end of the resistor (plus a shift due to the input current of the next amplifier flowing through that resistor).For example, if you use 0.1 uF and 10 k\$\Omega\$, that forms a high-pass filter passing frequencies above 160 Hz (at -3 dB).However, if you do not use that resistor, the DC voltage will be the same as if you had no connection to the next amplifier's input, determined by a small current into an unknown resistance.If you are worried about the DC quiescent voltages (which should be zero with respect to something), you are better to use a "split" supply, such as +/- 5 V.The total voltage between positive and negative supplies must be within the manufacturer's recommended range for supply voltage.For the LM358, that recommended range of total supply voltage is between 3 V and 36 V (above 40 V may damage the device).If you need 3.3 V rms (9.33 V pk-pk) at the output, you need more than 10 V supply voltage; if 3.3 V peak, then only 7 V: check the datasheet for the amplifier you choose to see what is required for your voltage swing.Notice that the LM358 can swing its output up to about only 1.5 V from the positive rail, and down to maybe 1 V from the negative rail (depending on output load, closer to the negative rail if you add a "pull-down" resistor).All this is in the datasheet.

author: tigrou
author_rank: Regular Contributor
timestamp: April 20, 2025, 12:46:17 am
content: @TimFox : I was aware that high pass require a resistor but I think I had this mental idea "it could work without" after I saw thispageI think the reason why the capacitor does not work is the "half sine" wave: look at thisexampleThe LM358 probably need a lot more volts to work properly as well (as you suggested), the -2.5V / 2.5V is probably too weak.At 41x gain there is more DC due to offset (31mV) than AC (16.3mV). So this need to be filtered (or to get a more precise op amp)

author: RoGeorge
author_rank: Super Contributor
timestamp: April 20, 2025, 04:39:44 am
content: Attach a picture of your complete schematic, can be a hand drawn schematic, doesn't matter.Otherwise it is not clear what schematic you tried, and why some of the tries didn't work.A picture with the physical circuit will help too, because even for a perfect schematic, there is a lot of things that could go wrong when building that in practice.

author: MrAl
author_rank: Super Contributor
timestamp: April 20, 2025, 07:54:53 am
content: Quote from: TimFox on April 19, 2025, 02:39:57 pm...as long as there is at least a capacitor at the output of the last amplifier.

author: tigrou
author_rank: Regular Contributor
timestamp: April 20, 2025, 05:21:11 pm
content: SchematicDifferences between schematic and picture :- The high pass filters are not shown, as well the "negative rail generator" circuit.- There is a transformer and a sine generator on schematic to simulate a coil being energized.- I have only polarized caps in the μF range. What you see on the picture is 2 x 10μF polarized capacitors (sharing their negative side) to make a single non-polarized 5μF one.Additional notes:- The coil is on a ferrite rod (12mH, 5 Ω). One side is grounded, the other goes to the "+" side of 1st stage opamp.- the high pass filter did not work probably because the signal is a half sine (positive only). A full sine would probably make it work. This requires op amps to receive a negative voltage.- the negative voltage circuit did not worked, probably because the opamps voltage is too weak (eg: require more than 2.5V). When I tested the "negative rail", all the grounds of the circuit would go to the new 0V "virtual ground" (including one side of the coil), with the exception of the opamps, having their "-" side being connected to -2.5V.- When receiving 5V, the opamps gives max 3.5V output. This makes opamp in the peak detector useless (as it can only boost up to 3.5V).My next step is to buy a chopper (or a precision opamp) and see if it improves precision (and probably to buy non polarized caps as well).I will not buy a instrumental amplifier as it just cost too much ($15-20$) and probably not necessary.

author: TimFox
author_rank: Super Contributor
timestamp: April 20, 2025, 05:43:01 pm
content: Why not just use two 9 V batteries to run decent op amps (5534 or TL072, for example) at +/- 9 V rails, with a 0.47 uF polypropylene capacitor in series with the output (with 10 k\$\Omega\$ load)?

author: tigrou
author_rank: Regular Contributor
timestamp: April 20, 2025, 05:57:06 pm
content: @TimFox : That's a great idea !I already use a 9V battery to power this (the module on the left lower it down to 5V).Would it work to keep this, and add another 9V battery in series :- The "+" side of the new battery will be connected to the "-" side of the actual battery.- The "+" side of the actual battery would go to the "+" side of the opamps.- The "-" side of the new battery would go to the "-" side of the opamps.-The rest of the circuit (actually only the LED) would receive still 5V. EDIT : no need, everything can run on 9V, if the LED is using a 2 x 220 Ω resistor.The potentiometer and coil would be grounded to the "new virtual ground" (the common node between the two batteries)Your RC filter should has a cutoff frequency of 33 Hz, shouldn't it be higher ? (to attenuate the 500-600Hz signal a little bit more)

author: TimFox
author_rank: Super Contributor
timestamp: April 20, 2025, 06:22:37 pm
content: That connection of two batteries is correct, and their connection point is your ground reference.The term “virtual ground” refers to a node (such as the inverting input of an op amp) that is forced to zero by feedback, but is not literally grounded. When that node must be at a non-zero voltage (as in a single supply circuit), there may be a reference voltage instead of 0 V, which may be called the virtual ground reference.A series capacitor into a load resistor is a high-pass filter. The attenuation decreases with increasing frequency compared to the “cutoff” frequency value.0.47 uF and 10 kis -3 dB at 34 Hz, but < 0.02 dB down at 500 Hz.If you can’t tolerate that small loss, you need to lower the cutoff frequency by increasingRC, not increase the cutoff.

author: tigrou
author_rank: Regular Contributor
timestamp: April 21, 2025, 01:13:07 pm
content: I'm going to buy new op amps. I have see that their max input voltage rating is 0.7V (for the OPA277) and even 0.3V (for the LTC1050 chopper). This is different from the LM358 which pretty much accept everything within supply voltage.The coil I have can generate more voltage than this.How can I protect the opamp ?EDIT : it can generate 200mV at best using closest distance I usually made the tests (a few cms).Putting the coil next to the generator coil (almost touching, I never do that) can generate 900mV.Here is two solutions I have in mind :1) putting two diodes across the "+" and "-" terminals (one in each direction). Since diodes have a forward voltage of 0.7V that should do the trick (at least for the OPA277)2) put a big resistor (eg : 1KΩ) in series of the "+" terminal (so between the opamp and the coil). Would this work ?

author: TimFox
author_rank: Super Contributor
timestamp: April 21, 2025, 02:20:44 pm
content: Protection diodes between + and - inputs is normal.Negative feedback reduces that differential voltage to near zero, until you drive the amplifier too hard and exceed the useable output voltage or slew rate, at which point you may need to protect the input from over voltage.What is the noise of your resistor in series with the input signal (integrated over your system bandwidth)?If you use series capacitor(s) to eliminate the DC component, why use a chopper amplifier?What is the noise voltage spec for the op amps you are considering?What is the noise gain for your feedback amplifiers?

author: TimFox
author_rank: Super Contributor
timestamp: April 22, 2025, 05:55:09 pm
content: Above, I strongly recommended the book by Motchenbacher and Connelly and cited a link to a .pdf.Looking again at that classic text, I refer you especially to Chap. 7 (see section 7.5 on LC resonant circuits) and to the example amplifier circuits in Chap. 14 (14.3 shows two AD745 op amps with two 9 V batteries and series capacitor high-pass filters). Cheaper ICs can be used, and the capacitors remove the need for auto-zero or chopper amplifiers for your requirements. The AD745 is particularly low-noise for a JFET op amp.

author: tigrou
author_rank: Regular Contributor
timestamp: April 24, 2025, 04:57:02 pm
content: I just received two new opamps today but it's a huge disappointmentI tripled checked everything before (to make sure nothing is wrong) and after (to find what's wrong) but it does not work as expected.Original circuit that works (with good old LM358) : LED blink at 1Hz interval, according to signal ON/OFF pulse. There is a 1M resistor to ground on the input signal, the cap in parallel with the coil is missing.There is 2 configurable 0-100 gains and a peak detector (already discussed, there is a link to circuit in mypreviouspost)OPA2277(precision op amp) : LED stay ON all the time. Like it's friedThe OPA2277 can be put instead of a LM358 (direct replacement) because pinout is the same.LTC1050(chopper) : LED stay OFF all the time (0mv output).The LTC1050 require circuit to be adapted (positive rail and output are not on the same pins).If I disconnect the coil and remove the 1M resistor to ground (leaving the input floating) it goes 100% ON. If I put a 1M resistor between "+" input and the coil and apply a huge gain (x50), I can capture a very weak signal : the LED barely blink while it should be full almost 100% on/off. If I put another smaller resistor (eg: 100K) it goes OFF again. At least that IC seems to be alive.Notes :- I don't have any mistake to confess (or at least none I'm aware of).- I always put the ICs in the right direction (the notch on the right). Polarity was never reversed (this is something I triple check every time).- It shouldn't have received too much voltage on the inputs due to the coil. The coil generate at best 200mv. Furthermore, the OPA277 has two 1K resistors and diode protection on the inputs (page 15 ofdatasheet).- They can be powered with 36V/18V max respectively (no worry there, I feed them with 5V from a 9V battery).- Adjusting potentiometers does not help.Either:- I made a mistake or missed something that prevent them working correctly.- I fried them (then the mistake should be visible on the pictures as I never did any other wiring)- I got 2 defective chips (unlikely). They are from DigiKey (not a random Amazon seller).

author: tigrou
author_rank: Regular Contributor
timestamp: April 24, 2025, 07:42:11 pm
content: Quote from: TimFox on April 22, 2025, 05:55:09 pmI had already ordered the opamps when you wrote this.Quote from: TimFox on April 22, 2025, 05:55:09 pmLooks very similar to what I have. It's unclear to me why they have different pairs of resistors for doing twice the same 32x gain.Also, i'm uncertain about the 0.03Hz and 0.3Hz why such low values ?By the way, there is something I learn about RC filters. C should be as low as possible because then you have possibilty to use ceramic or film capacitors which are very stable (more stable than the electrolytic equivalent). The capacitors I just ordered are so much better for such filter (compared to before).

author: TimFox
author_rank: Super Contributor
timestamp: April 24, 2025, 07:54:58 pm
content: Quote from: tigrou on April 24, 2025, 07:42:11 pmAs discussed in the book, it's a good idea to use low-value resistors for the feedback network on the first stage, since the thermal noise of the resistor network adds to the noise of the input amplifier.For the second stage, it's a good idea to use higher values, since the voltage across them is greater than in the first stage (due to the high gain of the first stage), but their noise is negligible compared with the output voltage of the first stage.The very low frequency cut-offs were used by the author to measure low-frequency noise in circuits being tested.Capacitors: high-value film capacitors (polypropylene, not polyester) are a good idea. For filter circuits, only C0G/NP0 ceramic should be used (not X7R). Since you don't need 0.03 Hz, the required capacitor values for, say, 100 Hz are practicable. Relatively low capacitance implies relatively high resistors: it's a trade-off.For low-noise input stages, if you use a very low series capacitor before the amplifier, the input noise current of the amplifier will develop a voltage across the high reactance at low frequencies, and increase the low-frequency noise. With no capacitor or a very large one, the noise current develops an unavoidable voltage across the source impedance.

author: tigrou
author_rank: Regular Contributor
timestamp: April 24, 2025, 08:34:49 pm
content: Quote from: TimFox on April 24, 2025, 07:54:58 pmThanks for the info. Did you saw my message with the different opamps configuration I tried, is there something that you think might be wrong?I am unsure how to troubleshoot it.

author: TimFox
author_rank: Super Contributor
timestamp: April 24, 2025, 08:54:03 pm
content: With the 2277 installed, what waveforms do you see?Specifically, is the amplifier oscillating when there is no input signal, both with the inductor circuit connected, and with the amplifier chain input shorted to ground?Or possibly, is the circuit picking up an external magnetic field at RF or AC-line frequency?A quick look at the data sheet seems to indicate that the 277 series is a reasonable modern substitute for the AD745.However, I worry that for high-gain amplifiers (such as the two AD745s), there may be nasty coupling between two op-amps on the same chip (leading to oscillation): two OPA277s might be better than one OPA2277.Important: I don't see "bypass capacitors", such as the 0.1 uF capacitors on each of the two AD745s, in the wiring diagram you posted above. These are mandatory to deter oscillation, especially in high-gain circuits.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 25, 2025, 07:06:48 am
content: Quote from: tigrou on April 24, 2025, 07:42:11 pmThere are 4 of 0.1uF bypass capacitors in the schematic. Those have to be placed near the IC power pins. In simulation they do not matter, but in practice it is important to have them. Add them on the breadboard. Without them might work, or might not work (might self oscillate or other strange effects), so to be sure, always add them in the physical circuit.I didn't check all the breadboard wires, but to debug a circuit, first thing to do is to verify if it is powered correctly. Measure with a DMM on DC volts. The voltage between GND (black probe of the DMM) and pin 7 (red probe) should be +9V or close, and between GND (black probe) and pin 4 should be -9v or so.Then, I would connect Vi to GND directly (disconnect the coil), and measure the DC voltage between pin 6 and GND. It should be close to zero (a few mV). Same for the second opamp. Repeat the measurements with the DMM set to measure AC volts, it should show zero at pin 6.If all good so far, add an input signal at Vi (no more than a few mV, either with a signal generator, or with the soundcard (or an audio mp3 player/mobile/etc.). You may want to use a voltage divider to reduce the max 1V line out audio to only a few mV.Then with an oscilloscope (or with the line-in of an audio card, or with an audio amplifier) listen to the amplified audio signal at pin 6. The DMM (this time set it on AC volts, not DC) should tell the exact amplitude at pin 6.An oscilloscope would help to visualize the signals, but if you don't have, a sound card may be used as an oscilloscope (with certain measures taken to protect its inputs).Asking because the debugging procedure might differ, depending on the available measuring tools. What instruments do you have (generator, oscilloscope, DMM, etc)?

author: TimFox
author_rank: Super Contributor
timestamp: April 25, 2025, 02:52:05 pm
content: Clarification: the circuit with two AD745s is from the textbook I cited, and includes proper bypass capacitors.The circuit drawing without bypass caps uses a OPA2277, and was posted by tigrou.

author: tigrou
author_rank: Regular Contributor
timestamp: April 26, 2025, 11:51:15 pm
content: Hello,Some feedback : first I read all your comments. I might not reply to them directly but I will always try what has been suggested.Having very little tools to do measurements, I have decided to do some simulations using LTSpice. I used falstad early in the process (before doing anything for real) but I should have switched to LTSpice.LTSpice is so good. I have learned LOT of things in the process :1) The RC filter indeed works and help to reduce the DC bias caused by input offset. All needed is to choose the R and C values accordingly. I usually used too high R value (eg: 1M or 100K).With proper values (and a negative rail) it works finally ! (both in LTSpice and in real circuit).You can see here the offset being nullified (before and after RC filter)EDIT : another way to use the RC filter without a negative supply is to have the input oscillating around V/2 (and not around 0).This can be done by create a V/2 with a resistor divider and instead of putting one side of the coil to ground putting it to V/2 (thus coil will oscillate around V/2).2) The opamps in this circuit works somehow better with a negative supply. I have throw away the circuit that use a opamp for such purpose (it use a resistor divider that then goes into an opamp that act as a buffer). Instead, I use two 9V batteries as someone already suggested. They are not charged the same way, which create some imbalance but that's OK.3) While testing the LTC1050 and OP227 in LCSpice I found out they only work with a negative supply ! Otherwise they have similar behavior as before : always ON or always OFF.So they were not dead or fried, they just need a negative supply. This has been tested in a real circuit and indeed, it worksIn the end they work quite good. They have considerably less input offset. Without RC filters, they can at least reach performance of the LM358s with RC filters.The "gifted" LM358 does almost as much good as them (not quite). I suspect this is a rebadged unidentified opamp (using JFETs ?). This is probably not a LM358 because it just outperform the rest.Decoupling capacitors : they are indeed missing. I have decided to add them. Most circuits with precision opamps I see have them. They are not in the LTSpice circuit as it increase computation a lot.Oscilloscope : I wanted to buy a cheap one (those portable oscilloscopes usually available in the 50-150 EUR range), but after seeing they have at best a 10-50mV sensitivity, I just forget about it.During testing I found two unexcepted stuff :1) With high gain, I have to put the coil to some distance of the circuit as it will "self" activate itself (the circuit generating enough EMI to interact with the coil)2) Touching the common ground with my fingers might sometimes help to remove some DC bias on the coil (especially when I make measurements with the DMM). I'm not sure why. Is the coil picking up some other EMI in the room and being grounded helps ?I kept the topic open as there is still work to do. My goal is to measure something in near 0.050mV which is still not quite there. My cheap Aneng DMM can, so this circuit should be able to as well.

author: tigrou
author_rank: Regular Contributor
timestamp: May 08, 2025, 10:38:25 pm
content: Note: I don't wanna reopen the topic but to list some "lesson learned" points that might be useful for somebody else (especially beginners).1) It's important to supply a negative voltage to the op amps to make sure signal is processed fully. Otherwise you got some sort of clamped signal that make lot of things difficult or impossible after (eg: filtering). Some opamps don't work properly unless a negative supply is provided.A simple way to do this is to use a resistor divider followed by an opamp (so it stay balanced). Some IC (with charge pump) can also do that more efficiently (eg:ICL7660).If a negative voltage cannot be used, it's possible to create some virtual ground (eg: 2.5V for 0-5V supply) and make sure the signal is offset by that value.Here is an example (with 9V) :2) Most op amps are not rail to rail, and so might not output -5V/+5V but rather something lower (eg: 3V max)This might be an issue if you are trying to rectify the signal with diode + opamp (as typically used in a peak detector circuit).Rail to rail op amp exists but they are not the most common.3) if you use an IC that contains multiple opamps and some are not used, it's better to "disable" them by connecting the output and inverting terminal together and the non-inverting terminal to the ground (preventing them amplifying noise and potentially causing trouble to the other op amps in use)4) It's better to use an op amp in non inverting mode if you need high impedance as there is no current flowing in that mode, unlike the inverting mode (current will flow through the resistors divider). High input impedance might also be achieved using a instrumentation amplifier (eg:AD620or INA219)5) If you get some high DC offset at high gains (as I got) it might be useful to place a capacitor between the resistor divider and the ground (in non-inverting mode). It will act like a high pass filter. The schematic above contains such filters. Additionally, high pass filters between the opamp stages might be added.6) Using a microcontroller and the ADC input might be really useful for processing of the signal. It can detect mV range easily, do some scaling (based on min and max), adjusting the offset, do filtering (to some degree), ... Be careful that some ADC do not support negative voltages, so it should be shifted before measurement. It can be done using a voltage divider (with +5V at the top of the resistors and the signal to measure (-5V to 5V) at the bottom, the mid section will output 0V-5V). 5V to 3.3V shifter (eg: Raspberry nano) can be achieved with 2K and 1K resistors (voltage divider).