Confusing OpAmp circuit (used in a metal detector) - Page 1
Forum section: electronics / beginners

author: Rigolon
author_rank: Regular Contributor
timestamp: February 08, 2018, 12:46:47 pm
content: Hi guys,I've been studying OpAmp (and electronics) not for a long time. I found that trying to understand a complete device/project is the best way for me to learn.I got my hands on a metal detector circuit and I think I've got the basics (even bought some books on metal detectors).But I'm having problems understanding part of the reception. I won't post the whole circuit for now, because I believe I got a good understanding of what is going on with the rest.Before this part there is an OpAmp that filters and amplify the the received signal. After this part it has a few more OpAmp's and others circuitries. From what I found out about this metal detector, it's a walkthrough with 4kHz frequency of operation.Now for the part I'm having problems. Here is what I understood (or think that I do):- It's a differential amplifier with unity gain- In one input has a slow RC time and the other input have a faster RC time.But I'm not sure what is the purpose of this stage. (If it helps, when a metal is detect it should change the amplitude and/or phase depending on the target)Perhaps I will know the purpose if I understand what is acctually happening. So if anyone could help me explaining what is going with each part, like what C4 does.Thanks in advanceI tried to simulate and changed values of the components, changed the frequency, but still not sure what is going on.Edit: Input signal 4kHz about 200mV between R1 and R2. Output is the OpAmp output. Will fix the image as soon I get my internet problems solved.Edit2: Fixed the schematic and also attached it.

author: hiperco
author_rank: Contributor
timestamp: February 08, 2018, 05:39:26 pm
content: Can you post the schematic?

author: Twistedsnail
author_rank: Contributor
timestamp: February 08, 2018, 09:22:38 pm
content: Where is the input signal connected?Where is the output being measured?

author: StillTrying
author_rank: Super Contributor
timestamp: February 08, 2018, 10:09:12 pm
content: I would think there has to be errors in that schem. No DC path to bias the op amp, and the caps connected directly to the output.

author: bson
author_rank: Unknown
timestamp: February 08, 2018, 10:26:09 pm
content: A circuit without inputs and outputs?

author: hiperco
author_rank: Contributor
timestamp: February 08, 2018, 10:31:02 pm
content: What are you guys looking at? Am I missing the schematic somewhere?

author: Rigolon
author_rank: Regular Contributor
timestamp: February 08, 2018, 11:24:37 pm
content: Sorry guys, i forgot to put the input/output, i'm having some internet problems, tomorrow i will fix the schematic.But as I said the input is a 4kHz signal, the tech guy that got me this circuit said he measuered once with the coils working and at this point it has about a couple hundreds mV. He also said that the signal has a negative DC offset.The input is right between R1 and R2. (Here is why i get confused, it's basically the same input for both OpAmp inputs Only saw this on current sensing circuits.)The output is at the OpAmp output.QuoteThe schematic is at the post. Tomorrow i will attach the fixed one just to be sure.QuoteIt probably has, I found a capacitor with both pins grounded at some point. But the thing is, the circuit works. The company that made them closed because they got into problems by being part of a cartel. But the tech guy told me that this circuit works quite well, he usually installed them or changed parts/components in the circuits working in banks. He doesn't know much about how it works either, but he saw it working quite a few times.

author: Rigolon
author_rank: Regular Contributor
timestamp: February 09, 2018, 12:18:57 pm
content: I found that this circuit actually works as a filter also. I simulate a 4kHz 1V multiplied with some white noise (about 1V amplitude). (See graph attached).The output (red trace) is actually quite better than the input (green trace).But how does this circuit filters this signal. How the components are calculate?

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 09, 2018, 02:28:20 pm
content: Quote from: Rigolon on February 09, 2018, 12:18:57 pmNothing attached.Are you driving it at the junction of R1-R2 with an ideal voltage source? If so, R1 and C1 have no effect.The capacitor on the output has no effect either (1st order). >>I just ran a sim...the load cap does come in to play at pb edge<<The configuration is the classic Sallen-Key filter.Go here to see the transfer functionhttps://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topologyThen do the algebra to see where the poles and zeros lie. Were it not for the R4C4 combination, you could find the analysis done for you and all you do is plug in values for your components.

author: floobydust
author_rank: Super Contributor
timestamp: February 09, 2018, 05:18:32 pm
content: C3 is in the wrong location, OP can you check your schematic.

author: Rigolon
author_rank: Regular Contributor
timestamp: February 09, 2018, 06:08:39 pm
content: QuoteSorry about that. As I've said, I'm having some problems with my internet connection and stuff like this keeps happening. Will try to attach when it gets a little better.QuoteIn my simulations I was. But as I said, this is from the reception circuit of an operating wlakthrough metal detector. I have TX and RX circuits, but i don't have the coils. The guy that gave me this circuit didn't have the coils. So all I can do for now is to simulate and try to understand the concepts within the circuit.QuoteI guess it probably has something to do with the first part of the reception. I guess i will upload the full circuit.QuoteI saw some resemblances with Sallen-key too, but all Sallen-key the I saw had positive feedback which is not the case here. I guess i will look a bit more for Sallen-Key topologies. Thx for helping with this.QuoteYou are right, I guess C3 should be at the output from the OpAmp. I probably made this mistake because I was drawing the schematic with the PCB layout in mind.

author: Audioguru
author_rank: Super Contributor
timestamp: February 09, 2018, 06:47:34 pm
content: The schematic is drawn wrong and maybe the opamp is biased by the input signal circuit.Where are the "red and green traces"?

author: Rigolon
author_rank: Regular Contributor
timestamp: February 09, 2018, 07:17:38 pm
content: QuoteAs I've said in my last reply, C3 should be problably part of the output.(There is a lot more to this circuit as stated on original post, I will post the full circuit when my internet gets better, right now I'm using phone to post, and it's very bad so i can't upload images)But even so, the connections are just as in the pcb, and the circuit works, It was from a company that sold them to public and private banks.Not saying that is a good circuit (especially this part), but at least it worked quite well from what I've been told.QuoteThe image didn't get attached correctly, will try upload again when my internet comes back to life.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 09, 2018, 07:28:14 pm
content: QuoteHere is the general Sallen-Key configuration. Negative feedback.

author: Rigolon
author_rank: Regular Contributor
timestamp: February 10, 2018, 10:24:20 am
content: Thanks all of you.I get what I did wrong and learned a valueble lesson. Drawing the schematics in the right way could help a lot to understand or find a known configuration a lot faster. Even though I saw some resemblances with Sallen-Key I focused too much attention on R1C1 thinking they would be part of the configuration and disregarded that really was Sallen-Key.I was also giving too much attention to the positive feedback to see that the negative feedback is also there. Especially because most material I read on Sallen-Key focused on how the positive feedback helps get a larger Q.Anyway, Thx again. If you guys want me to post the full schematic from the circuit I got, leave a reply. As soon I get my internet running good I will upload. (I live in a small town, just 1 internet provider and it was a company that almost got bankrupt last year, so you can imagine my desperation with internet here xD)

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 10, 2018, 03:11:27 pm
content: Quote from: Rigolon on February 10, 2018, 10:24:20 amYES...this is critical. Here are a few of my rules:-- Positive supply on top, ground on bottom (or ground in middle and negative supply on bottom for dual supply)-- Signal flow left to right where possible.-- NEVER NEVER use four-way connections with a dot (see image)QuoteYes

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 10, 2018, 08:37:38 pm
content: R1 and C1 are the impedance matching for the coils. The output would probably be DC coupled to keep C3 from doing naughty things.