OP AMP CIRCUIT DESIGN QUESTION? - Page 1
Forum section: electronics / projects

author: GOHO
author_rank: Contributor
timestamp: February 14, 2017, 03:41:40 am
content: Hi All, The attached circuit is a metal detector. The circuit references the positive as ground but i want to change the differential voltage from +/-5v to +/-3.3V so i want to use a different dual voltage power supply that will reference negative as gnd but im having trouble getting the 5534 Op am to work properly when i try to use the negative/GND circuit.Any suggestions on how I can keep the basic design of this circuit but use a negative referenced to gnd instead of the positive reference to gnd.

author: trophosphere
author_rank: Frequent Contributor
timestamp: February 14, 2017, 06:05:15 am
content: Just be aware that the minimum supply voltage for the 5534 Op Amp is +/-5v . +/-3.3v will not be enough.

author: GOHO
author_rank: Contributor
timestamp: February 14, 2017, 03:31:14 pm
content: Thanks for pointing that out, My ultimate goal is to bring the op amp supply to +/-3.3V for using in a logic circuit but the largest hurdle i face at this moment is trying to use the basic circuit design that is attached but change the dual power supply to a smaller circuit. I found a nice design using Linear 3032 that supplies dual 3.3V or 5v from a single DC supply of 4.5 to 20V but it uses negative as GND like normal where as the attached circuit uses positive as GND. So in my quest to condense this circuit i need to shrink the +5V generator on the attached circuit to a much smaller surface area and finding a away to move the GND to the NEG so i can use a simpler dual voltage is a big step forward.

author: GOHO
author_rank: Contributor
timestamp: February 14, 2017, 04:36:26 pm
content: I went back to the Data sheet on the NE5534 OP AMP... The front sheet says, WIDE RANGE OF VOLTAGE 3V to 20V but in the specs it says min voltage 5V... Whats up with that?see attached pages 1 and 4...

author: Kleinstein
author_rank: Super Contributor
timestamp: February 14, 2017, 05:28:01 pm
content: The +-5 to +-15 V is the recommended supply range. It will start to work with +-3 V, but initially the common mode and output range will be rather limited. So when working with only +-3 V, one should look for a different OP, that is working better at low voltage. The NE5534 is low noise, but also takes quite some supply current.Changing this design to start from the positive supply would need a high side switch and thus likely a P_channel MOSFET. As N Channel MOSFETs are easier to get, it perfectly makes sense to start with the negative supply.With battery supply the LT3032 won't help very much, as it needs a +-xxV supply to start with. The more practical way might be a low noise boost converter or a charge pump like in the lined circuit.

author: GOHO
author_rank: Contributor
timestamp: February 14, 2017, 07:10:50 pm
content: Thanks Kleinstein for the info, The 3032 in conjunction with the LTC3261 create a dual supply single source power supply.So in order to use a negative gnd in this circuit i would need a P channel mosfet instead of the N channel in the schematic?

author: Zero999
author_rank: Super Contributor
timestamp: February 14, 2017, 08:27:30 pm
content: Why not use an op-amp which will reliably work from 3V such as the TS271?http://www.st.com/content/ccc/resource/technical/document/datasheet/5a/cf/9f/f2/8d/cf/48/90/CD00000494.pdf/files/CD00000494.pdf/jcr:content/translations/en.CD00000494.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: February 14, 2017, 08:43:02 pm
content: The circuit uses quite a lot of current to "charge" the coil / test pulse. This is why they use this power directly from the battery for this. It also helps to have a relatively high voltage for driving the coil - it might even help to use more than 9 V.What is wrong with having the battery positive side at "GND".What is the purpose of the new circuit ?The TS271 could be a problem due to high noise.

author: GOHO
author_rank: Contributor
timestamp: February 14, 2017, 08:54:50 pm
content: I want to bring the output of the 5534 or whatever op amp i end up using into an ADC the positive gnd causes issues for me trying to accomplish this... but i am new at electronics...

author: Kleinstein
author_rank: Super Contributor
timestamp: February 14, 2017, 09:30:57 pm
content: Depending on the ADC, one could still have the ADC working range centered around the positive battery. They supply current to the µC is likely only a relatively small part and cold thus go through an converter. The main power is usually needed to drive the coil.With an high side switch (e.g. P-MOSFET) one could have the amplifier relative to the negative GND, but I am not sure this would change very much, as the signal can be still positive and negative over time. The interesting part might be one sign, but I don't know. The relevant changes in the interesting part can be rather small so one might need a little more than just amplification and ADC. So DC level shifting would be only a small point.For the auxiliary negative of positive supply it could be a good idea to have it sync with the excitation of the detector coil, so maybe controlled from the µC.

author: GOHO
author_rank: Contributor
timestamp: February 15, 2017, 02:09:50 am
content: Ive been successful at replacing IC3 and IC4 which are multi-vibrators with a micro-controller in order to change the timing gates to try and get better detection on small gold items but the detector is tuned to about the best it can get without digitizing the signal and using advanced signal processing on it. After the pulse, the critical part of the detection signal is only 30uS before it starts to flatten out but the basic circuit as is opens the detection gate @ 42uS from the end of the pulse and another one 100 uS or so to compare the two. If i can look at the signal from the first return to the 30uS mark this detector would be twice as sensitive as it is now and be able to detect small items that right now is impossible for it to detect.So, my big task is to bring the op amp out and into an ADC and then an FPGA or fast Microcontroller...... my chosen ADC is 25 msps...any advice or criticism is great appreciated!

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2017, 05:20:15 pm
content: I also see that using a digitized version might help to get a better sensitivity. I don't think you will need a 25 MSPS ADC, as the amplifier is way slower than that. So I would guess more like a 1 -5 MSPS ADC should be fast enough. So this might be still in reach of a 12 bit ADC found inside some ARM µCs.One might still need a kind of circuit to get much of the DC offset from the time after the pulse.Still it might be a good idea to have an ADC that is faster than the NE5534 and maybe even lower noise. Something like the OPA209 might be a good idea.Another point might be the excitation, more power (e.g. voltage) would help, but there are limits of cause. Also one might have to look at the relatively high induction voltage on switch off - just using the avalanche mode of the MOSFET might not be the best way. At least the choice of MOSFET and maybe the switching speed might also be a factor. I would keep the way with a N-MOSFET and just have an appropriate supply for the OP/µC. This could be a separate battery - this might be simpler than a low noise switcher/charge pump.

author: GOHO
author_rank: Contributor
timestamp: February 15, 2017, 06:17:18 pm
content: A slower ADC would work and i do have a micro controller that is fast enough for a basic detector but my goal is to be able to identify different metals and in order to do that i need to look at the return signal with a microscope! a 25 msps ADC will give my roughly 600 samples across the 30 uS time period and with that amount of samples i should be able to see the decay rate of the eddy currents in the target and determine the metal composition. Each type of metal will decay at a different rate.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 15, 2017, 07:55:09 pm
content: A fast ADC only makes sense if the amplifier and signal source is that fast too. So more than 5 MSPS would make sense if the amplifier is good for more than 1-2 MHz. The plan with the NE5534 is using gain of 1000 if one ignores the source impedance and thus would have a bandwidth in the 10 kHz range. It could be lower gain and thus faster if the signal impedance is included. Also for reasonable size parts the speed of decay is more in the 10-100µs range, not in the 100 ns range.I just realized the coil might not be such a low impedance signal source at the relevant signal frequency (e.g. 100 kHz range). So it might be better to use a JFET based amplifier instead, possibly even an discrete JFET instead of an OP.

author: GOHO
author_rank: Contributor
timestamp: February 20, 2017, 10:04:53 pm
content: I appreciate all the advice but im a little confused by what you said about the frequency of the op amp and the ADC.My O scope has no issue looking at the signal and its 100 mhz 1 gsps scope. I attached 2 pics of the signal i want to digitize with my 25 msps ADC. The first pulse is the TX pulse and the second curve is the induced current. As you can see, it doesnt last long, by 30uS its gone. I need an ADC fast enough to look at the induced current as if its under a microscope then i would be able to ID different metals.I appreciate any and all advice on this project.Thanks

author: StillTrying
author_rank: Super Contributor
timestamp: February 20, 2017, 11:13:11 pm
content: Quote from: GOHO on February 15, 2017, 06:17:18 pmTimes 360 pulses per second, you don't have to collect all your data from a single pulse. You could almost get away with just one moving 1us sample windowper pulse.

author: GOHO
author_rank: Contributor
timestamp: February 20, 2017, 11:47:50 pm
content: I guess you could use a configuration like that but in my opinion the entire sample needs to be digitized and analyzed with each pulse. The circuit pulses at around 192 HZ but the detector coil is in motion so it needs to respond to a target as fast as possible. Also, in order to identify the type of metal thats being detected you need higher precision than 1us sample. I think around 10x that resolution is need at very least to separate the decay rate of each type of metal as each type will decay at a different rate.

author: bobaruni
author_rank: Regular Contributor
timestamp: February 21, 2017, 03:05:34 am
content: I don't think you will need to sample more than a few times during the decay.Take 3 very accurate samples then work out 2 vectors using the 3 samples and all the information you need to discriminate materials will be in those vectors and samples.You should probably take some samples during a very "quiet" period in order to create ground balance and magnetic component vectors.Also, using only a single pulse duration will have a comb effect on the target and will almost completely miss gold nuggets of certain sizes.To avoid this, use at least 2 pulse durations that are not harmonically related to create a composite output that doesn't have any gaps in the target response.Also, take care with your coil, driver and amplifier to minimize capacitance as the whole system must be critically damped (Q = 0.5) or the signal of interest will be swamped.Regarding op-amps, pick one that is extremely low noise at 0.1 to 10Hz at the source impedance you are using, ones that come to mind are LT1028, LME49990 (discontinued), OPA1612 and OPA2211.As others have mentioned, FET inputs might be better suited at your source impedance/Freq, the latest high end PI metal detectors use discrete FET input amplifiers to minimize noise as front end noise is what limits the overall sensitivity once ground balancing and magnetic components have been dealt with.

author: GOHO
author_rank: Contributor
timestamp: February 21, 2017, 11:40:08 am
content: Thanks for the great advice! I disagree with you on the amount of samples it would take to id different metals only because i have used high in detectors such as the Geonics EM63 which has 26 timing gates across the decay current and still its not enough to accuratly predict the type of metal thats being detected.I am very curious about the discrete FET that has been suggested, i will research that componet more..Thank you!

author: bobaruni
author_rank: Regular Contributor
timestamp: February 21, 2017, 12:09:06 pm
content: Quote from: GOHO on February 21, 2017, 11:40:08 amSo far, nothing exists that is capable of identifying the target material accurately 100% of the time in a hand held detector, a coin detected flat will have different properties when detected edge on and will fool many detectors.The decay can be almost perfectly described with only a few points, how you use those few points to reconstruct the data IMO matters more.Timing gates and sample values shouldn't be directly compared as gates are an average of a window which are then eventually weighted then summed linearly whereas you can apply any formula you can think of once you have the instantaneous vectors in the digital domain.Having said this, the more data you can gather, the higher your S/N ratio will be, this is one area that gated windows have an advantage over instantaneous sampling in terms of noise.If you can find a fast enough sigma-delta ASC then you have the best of both worlds.Re FET preamplifiers, BF862 differential pair connected to an OPA have been successfully used in the front ends of PI detectors (at least in Australia by Minelab).In fact there is a thread on this forum about a low noise amplifier that might be suitable.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2017, 12:15:25 pm
content: Very fast sampling only makes sense, if the amplifier is very fast too. As the amplifier is in a high gain configuration (e.g. gain of 100-1000) the OP needs a really high GBW. So the NE5534 configured at a gain of 100 would only have a bandwidth of about 100 kHz. At this frequency the sampling at 1 MHz is well sufficient - more samples do not really help. It still makes sense to use more than just a few samples - the idea should be to capture most of the decay, down to where the signal disappears in the noise. So something like 50-100 samples at 1 MSPS are well enough to measure the decay constant and signal amplitude. Having 1000 samples at a higher rate would not help much: in this case the first step would be a kind of low pass filtering / averaging to reduce the number of samples to something like 20-80.Unless you are after really small particles like gold nuggets, the decay is not that fast. The main frequencies of interest are more like 1-50 kHz. You actually don't want much energy at higher frequencies, not to get trouble with the FCC. Intentionally slowing down the speed the puls is turned off might help in reducing unwanted emissions and this way allow higher driving power or a higher pulse frequency.With a BJT based amplifier one is limited by the impedance of the signal source (in the shown circuit the is a little over 1 K Ohms, higher impedance is possible at higher frequencies). So the very low noise BJT based OP (like the LT1028/OPA211) might already be too high in noise current as they are made for something like a 100 Ohms source. JFET amplifiers have a chance to reach a better noise figure, especially with higher impedance signal sources. A suitable FET would be something like an BF862. However the difference should not be that large.I don't think the very low frequency noise at less than 10 Hz would be that relevant, the clean part after the pulse is more like from 100-200 µs. There could be a magnetic background from moving/rotating the coil - so much later / earlier data might not be relevant. Frequencies below the pulse frequency should not really matter.

author: bobaruni
author_rank: Regular Contributor
timestamp: February 21, 2017, 12:33:27 pm
content: Quote from: Kleinstein on February 21, 2017, 12:15:25 pmAgreed, better to use more stages with less gain and therefore higher bandwidth.It also wouldn't hurt to use a couple of signal diodes in the feedback loop to limit swing and speed up recovery during large signal transients.Quote from: Kleinstein on February 21, 2017, 12:15:25 pmYou're right, this stuff shouldn't matter but image you are amplifying the signal 1000x, some of it still leaks through even a brick wall bandpass filter.Also there is some other mechanism where the LF noise causes modulation of the wanted signal...possibly jitter, I'm not sure as I haven't identified it but maybe others have?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2017, 12:46:27 pm
content: The low frequency noise can be an issue in an analog implementation, like circuit from the start. It should not be a problem in a digital readout configuration. Anyway the main candidates like the BF862 or low noise BJT based OPs do have a low LF noise anyway.For looking at the data, the method of choice would be something like a least square fit of an exponential decay, not looking at the signal at a few discrete times.

author: bobaruni
author_rank: Regular Contributor
timestamp: February 21, 2017, 12:54:59 pm
content: Quote from: Kleinstein on February 21, 2017, 12:46:27 pmThis is why i proposed creating a few vectors which can then be used to recreate the decay using a spline.

author: GOHO
author_rank: Contributor
timestamp: February 21, 2017, 01:22:24 pm
content: How many samples do you think would be sufficient ? 3, 5?If i was only going to take 3 samples, maybe i could get away with a microcontrollers ADC instead of the seperate ADC i intended to use. Thinking maybe of prototyping using a Teensy 3.6.Any suggestions?

author: GOHO
author_rank: Contributor
timestamp: February 21, 2017, 01:38:23 pm
content: Is this the pre amp suggested?http://docs.google.com/viewer?url=http://www.mouser.com/ds/2/302/BF862-840156.pdfAny examples on how that would get implemented into the schmatic above?Thx!

author: bobaruni
author_rank: Regular Contributor
timestamp: February 21, 2017, 02:21:41 pm
content: IMO, as a minimum, I would take 1 vector at the very beginning of the decay (1 vector being 2 consecutive samples, a tail and a head) for the very small metallic component, another vector a small time after this as the first vector will be swamped or saturated by large metallic components. another vector in the middle of the decay and one towards the end for the ferrous and magnetic components.You can also take another vector (or at least a single sample) in a very quiet period for ground noise and magnetic field cancellation but as Kleinstein has stated, you can probably get away without the last vector.That makes it a total of 6 to 8 samples per decay pulse.I think even a Teensy 3.0 can do it in terms of processing power and ADC speed but do not expect the full 16 bits of useful data until you use a separate ADC.If you want a faster ADC (80 MSPS) but only 12 bits, take a look at LPC4370, with some good analog front end variable gain amplifier VGA it could also work.A bit hard to use the LPC as it's BGA so maybe take a look at a board called LPC-Link 2 (can be had for under $30) but not so easy to implement flash based boot on this board as it runs user firmware in RAM but there might be a way to inject your code into flash?Teensy would be my first try if you have one laying around and if it's not good enough, add an external ADC or go the LPC with a VGA.I'm not sure about the Teensy 3.6, I don't think this can do 16 bit conversions?

author: GOHO
author_rank: Contributor
timestamp: February 21, 2017, 02:36:47 pm
content: Thanks For the advice, I will give it a shot with my new Teensy 3.6...But now im back to my original problem of keeping the output of the opAMP to 3.3V MAX.. Should I keep the circuit positive gnd or switch to P channel mosfet and flip the gnd to neg? is one way more sensitive than the other?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2017, 02:56:21 pm
content: N-Channel MOSFETs are usually better in having less capacitance and more types to choose from. So there is a good reason to use a kind of positive ground, or negative supply to drive the coil. For keeping the signal within the 3.3 V voltage range, one option would be to have the last amplifier stage powered by those 3.3 V and limit the signal before that stage with diodes. This could be something like a times 50 from a FET+BJT based stage, followed by a times 10 with an OP powered from 3.3 V.The linked BF862 should be good choice. However I would not use the inverting amplifier, as this usually has higher noise.For taking the data something like 10 points in the decay region would be probably enough. For the final "DC" values a few more point from the 100-200 µs window. So something like a 500 kHz sampling rate would be fast enough. My choice would be a something like 1 MSPS. A slightly faster sampling could compensate for lower resolution - I would expect 12 Bits to be good enough. With switchable gain, even 8 Bits could be good enough.

author: bobaruni
author_rank: Regular Contributor
timestamp: February 21, 2017, 03:21:53 pm
content: And something not mentioned is that BJT OPAs will be more susceptible to interference from AM radio stations causing random signals in the passband giving false signals through modulation and rectification effects in the OPA.A fet input stage will be less susceptible to this phenomenon but even then some care must be taken (LPF) to make sure you don't have too much RF pick up and amplification above 500Khz.Also, another trick to improve sensitivity to small metal objects is to increase the resonant frequency of the coil by reducing the capacitance on the coil by place a high speed high current low cap diode in series with the MOSFET and the coil.Then the MOSFET's capacitance has much less effect on the tuned frequency thereby allowing you to sample earlier on in the decay.But with only 3.3V supply, this is probably not feasible.Also remember to drive the MOSFET gate with a low impedance source as the speed or slew of the off transition will help with sensitivity by sampling as early as possible not to mention the reduction in heating and loss by making sure the gate is driven hard enough to ensure minimum On resistance at high frequencies.What is your battery voltage? Do you have more than 3.3V available at high current?

author: StillTrying
author_rank: Super Contributor
timestamp: February 21, 2017, 09:35:16 pm
content: I can't see any any schematic of the pulse amp in this thread, - if there is one.But, going by the scope traces in reply#14,https://www.eevblog.com/forum/projects/op-amp-circuit-design-question/msg1141449/#msg1141449and taking many things into account, I can't figure why the amp detects BOTH the energizing voltage AND the decay voltage as BOTH positive going. What (possibly obvious) thing am I missing?

author: GOHO
author_rank: Contributor
timestamp: February 21, 2017, 11:22:18 pm
content: There is no pulse amp, just a 555 that triggers the mosfet.

author: bobaruni
author_rank: Regular Contributor
timestamp: February 22, 2017, 12:57:09 am
content: Quote from: StillTrying on February 21, 2017, 09:35:16 pmYes, good observation, they decay signal has overshot and is the wrong polarity.Either the coil is not critically damped (Q = 0.5) causing overshoot or the Op amp stages are too slow or maybe some other problem with OPA?Try reducing the value of R1 to see if the decay will invert then try increasing the BW of the gain stages.

author: GOHO
author_rank: Contributor
timestamp: February 22, 2017, 04:04:23 am
content: That is the normal signal from that circuit. I have looked at a dozen or more different detectors that use this circuit and they all have the same exact signiture on the scope. Even a Brand new one right from the factory.

author: GOHO
author_rank: Contributor
timestamp: February 22, 2017, 04:18:56 am
content: The orignal circuit uses 9.8v-10.3v nimh but i am replacing with an 11.1v lipo 20c 5400mah.

author: bobaruni
author_rank: Regular Contributor
timestamp: February 22, 2017, 01:39:29 pm
content: Quote from: GOHO on February 22, 2017, 04:04:23 amAhhhm, OK well then I'm confident that with the information here that you will better the dozen or more detectors out there as they are not damping the coil enough to achieve excellent sensitivity.I have run some simulation and by setting the coil damping resistor to 390R, the result is underdamped, by setting it to 330R, the result is damped well enough to be the correct polarity.See attached LTspice simulation and output image, the green trace is the desired polarity using 330R, the blue trace is underdamped using 390R.

author: GOHO
author_rank: Contributor
timestamp: February 22, 2017, 01:45:07 pm
content: Thats interesting... ill test it out.The search coil resistance is 1.6R and inductance is 27uH.

author: bobaruni
author_rank: Regular Contributor
timestamp: February 22, 2017, 02:05:03 pm
content: 27uH is very very low, 300uH is the norm for high end PI detectors, at least of the Minelab variety and starting price is about 3K on those.Do you know the coil capacitance and cable capacitance or at least the search head resonant freq?Plug those values in the sim and have a fiddle.With such a fast coil, you will be chasing very small shallow nuggets, down here we are chasing very large but deep nuggets in heavily mineralised soil and not many prospectors use discrimination when looking for gold.What are you going to use it on, coins/relics/gold?

author: GOHO
author_rank: Contributor
timestamp: February 22, 2017, 02:32:28 pm
content: when i replace the 470R damping resistor with a 330R this is what i get... i loose the induce current signature.This detector is a coin and relic detector but i would like to increase the sensitivity to small gold items.

author: bobaruni
author_rank: Regular Contributor
timestamp: February 22, 2017, 02:51:47 pm
content: What you are now getting is a good starting point, just add your MCU, I wouldn't worry about the FET OPA as your coil is very low impedance if it is really 27uH.To get better sensitivity to small items, you need to sample as early as possible in the decay period.To be able to sample earlier, you need to reduce the capacitance on the coil, try adding a high voltage low capacitance high current diode in series with the MOSFET like in the sim.Your MOSFET has 170pF of output capacitance, with a series connected diode, this could drop to about 20pF increasing the resonant freq substantially.Also, the MOSFET gate should be driven really hard to speed up recovery, definitely more than an MCU I/O port can do but should still have a gate stopper resistor.Use a pair of BC327 / BC337 in an unbiased push pull voltage follower arrangement or a proper MOSFET driver IC like the TC4427A family.

author: GOHO
author_rank: Contributor
timestamp: February 22, 2017, 10:13:49 pm
content: I agree that in order to detect small gold i need to sample higher up the curve but this circuit wont allow that . I have been successful replacing the mulitvibrators with a micro controller and i used to reprogram the sampling locations but once i got on the curve i couldn't balance out the detector, im sure it could be modified but i was unable to get it to work better than original..., well, maybe slightly better..Here are the original factory sample gates, as you can see they are way down the line, to far to detect small gold but when i have it on my scope, i can detect small gold items by watching the curve move.

author: GOHO
author_rank: Contributor
timestamp: February 23, 2017, 12:15:46 am
content: sorry, i miss quoted the inductance... its .27mH not uH!

author: bobaruni
author_rank: Regular Contributor
timestamp: February 23, 2017, 12:31:51 am
content: 0.27mH or 270uH will work very well.I wouldn't expect to be able to balance it with modified timings.That's the beauty of doing it in an MCU, you can experiment with different ways of getting it sensitive and balanced.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2017, 08:43:00 am
content: Right after the current is turned off, there is a background from search coil itself too. If well done this background should decay rather fast and it should stay constant over time. Still it will limit the time on how close one should go to the pulse. To close to the start there will be a lot of background (that can also drift). So the starting point is compromise between the signal (that is highest at the start) and the background that is also highest there. But hopefully the background decays faster than the signal.The old sampling gates look rather late, but this is likely without an object in range. So the initial pulse is likely just background you don't want anyway. Still strange that the second background sampling window is also so short. More time for the background might give slightly better SNR.The advantage with a µC is that you get not just those 2 sampling windows, but can look at the whole curve. The logical probe background to zero out the detector is the decay curve you get in a clean environment. It is after subtracting that background one can test methods to get a more simple (e.g. single number) value from the curve.

author: GOHO
author_rank: Contributor
timestamp: February 23, 2017, 05:00:24 pm
content: I have a question about a brd design for a detector. Would it cause noise to have a GND plane?, For the schematic i attached above would it affect the the signal noise if the +9V/gnd was a plane on the circuit brd top and bottom layer? Is it better to design a detector brd without power or gnd planes?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2017, 05:37:06 pm
content: The electronics board would be detected by the circuit as well. So the usual ways is to have the electronics and battery just at the other end of the pole to get the highest possible distance. A GND plane would give same relatively slow background, just like a piece of copper so it might not be such a good idea. It could be OK if it is really small. Also make sure current loops are small.A thing to avoid is a metal enclosure with contacts that are variable - like aluminum with not so perfect screws. The background from the electronics part itself is not that bad, as long as it stays constant. The worst thing is a background that changes with movement, like wires flapping around or a flimsy battery holder.

author: bson
author_rank: Unknown
timestamp: February 23, 2017, 07:44:50 pm
content: Quote from: Kleinstein on February 21, 2017, 12:15:25 pmWell, it still produces output, only it's attenuated past -3dB, so instead of +40dB you get +37dB. At 1MHz you get a -30dB signal out of it, which is only +10dB. This might be perfectly acceptable for waveform recognition, as long as the loss is part of the waveform pattern or template.But I agree, why go to such great lengths - working with distorted waveforms - just to use such acrappy oldlimited vintage opamp? There are much better devices around today.