Microphone waveform is not centered, need help please. - Page 1
Forum section: electronics / beginners

author: alexg
author_rank: Regular Contributor
timestamp: November 11, 2016, 05:35:07 am
content: Hi All, I have tried everything, by now I probably spend over 16 hours and countless combinations of every prescription I found on net, trying to figure it out and I am just stuck, need push in right direction.I am learning microphone circuits and I trying to connect simple capsule mic to read its output with oscilloscope and I am getting weird results. Here are the details:The circuit that I am building is as simple as a very basic one out there, see attached illustration #1, power supply is 3V, resistor from power to positive pin is 2k and capacitor from positive pin to output currently is 1uF but I have tried many others 0.1uF, 4.7uF, 2uF, 10uF (ceramic capacitors)When I clap or make other somewhat loud-ish sound, waveform is offset up, see illustration #2 and #3When I connect microphone from Adafruit, waveform is nice and symmetric as you see in illustration #4I have tried everything, DC coupling, AC coupling on oscilloscope and waveform is still offset except with DC coupling it is half supply voltage up of course, I tried running this through inverting op amp and still same result of course inverted. I tried different capacitors as already mentioned, different supply voltage 4.5v and 3v, directly from batterys or via voltage regulator, I am running out of ideas what I am doing wrong, can you please help?

author: ataradov
author_rank: Super Contributor
timestamp: November 11, 2016, 05:59:57 am
content: Do you just have the capacitor open like that going into high impedance input on the scope? You need to load it a bit. - connect a resistor to ground. Something like 10K should work.

author: alexg
author_rank: Regular Contributor
timestamp: November 11, 2016, 06:05:23 am
content: Yes, just scope probe on capacitor's pin, you mean I need to add a resistor in here (see attach)?--EDIT--Oops, sorry, somehow did not see entire message where you suggested 10k to ground, now I see it and adding res in right place.--EDIT--

author: ataradov
author_rank: Super Contributor
timestamp: November 11, 2016, 06:06:29 am
content: No, to the ground. Leave your probe point like it is right now, but add resistor from the capacitor to ground.

author: Zero999
author_rank: Super Contributor
timestamp: November 11, 2016, 08:57:50 am
content: You could also try using a x1 probe, rather than x10 and leaving it connected up for awhile until it reaches steady state. If the voltage never gets to within 1mV of 0V, then you probably have a faulty capacitor.

author: alexg
author_rank: Regular Contributor
timestamp: November 11, 2016, 03:17:01 pm
content: Quote from: ataradov on November 11, 2016, 06:06:29 am10k resistor did not work, but 1k seems to work and waveform is better centered, I will try different combinations or R C to see which one hits the spot.Another thing is that I also have MEMS microphone, from Adafruit and that one I just can't get rid of the same problem, I've countless combinations of resistors and capacitors and nothign seems to fix the issue, some R C combinations seem to do better, some seem to completely clip bottom of it, if you can suggest anything about that I will greatly appreciate that, I just can't understand mechanism behind this. From what I understand that resistor from cap to GND is what controls output impedance and helps discharge capacitor faster when it passes AC signal, but with MEMS I thought it would be the same mechanism and it keeps failing on me. Thanks again.

author: ataradov
author_rank: Super Contributor
timestamp: November 11, 2016, 04:58:30 pm
content: Quote from: alexg on November 11, 2016, 03:17:01 pmWhat microphone is that? MEMS microphones tend to have real amplifiers in them.Quote from: alexg on November 11, 2016, 03:17:01 pmYou need to understand how that RC thing works and actually calculate requited values instead of randomly guessing.What do you what to do with that signal afterwards anyway?

author: alexg
author_rank: Regular Contributor
timestamp: November 11, 2016, 06:03:10 pm
content: Quote from: ataradov on November 11, 2016, 04:58:30 pmYeah, I figured ..., I just did not know it would be that sensitive to exact values, I thought I could just be lucky with mix-matching caps and resistors, I honestly still new to this and while I learned alot lately some thing still new to me but I will get them.About the signal, nothing really, I am playing just to learn it, right now I simply want to make sure I know how to get perfect waveform out of it and second goal is probably to replicate dB meter ive got off ebay for $19 but that is far from now because I don't even know what dB really is and how to translate mic output voltage to dBs.

author: ataradov
author_rank: Super Contributor
timestamp: November 11, 2016, 06:16:25 pm
content: In real life you rarely want this thing "centered" around 0 anyway. You need to figure what you want to do with that signal and work on conditioning the signal accordingly.That MEMS microphone has 100 mVpp output centered around 0.6V. This is perfect for interfacing with MCU ADCs.

author: StillTrying
author_rank: Super Contributor
timestamp: November 12, 2016, 10:57:55 pm
content: For good audio, I consider the output level of these things to be about 10mVpp, if your getting 1V and 700 mVpp it's much too saturated, changing the capacitor wont work because it's the capsule itself that's saturated.And if hit with a LF air blast such as from a hand clap it's common for the output to be offset and take a while to recover.Expect a smaller output 100 mVpp ? and make a sound that doesn't cause an air blast such as clicking two pens or pencils together, and the waveform's mid level should stay centered.