What could be the possible reason behind curved signal?

Hi,

This question is not directly related to MATLAB, but if someone can help me, I would really appreciate.

I have a MEMS microphone connected to the microcontroller on a PCB. The microphone collects the data and sends the analog data to the microcontroller in real-time. On the microcontroller, this data is converted to digital form and then I extract this digital data from the microcontroller. But when I plot the signal, I get the signal as a curved one. It looks like the DC offset is not constant. All the recordings have this issue.

However, for ex., I record a song and when I play the file (after converting them to .wav files using MATLAB with specs such as sampling freq: 40KHz, Bits/sample:16), I don't see any problem in the audio files, I am able to hear the song properly. I have attached the plot as well as the audio file for reference.

Can anybody suggest what could be the issue here or what can I try to get rid of this? Thanks.

This is audio file: Audio file

[Walter Roberson]

What happens if you have "silence" for the microphone input? Do you still get a drift?

What happens if you disconnect the actual microphone input so that the microcontroller literally picks up nothing (not just ambient noise) ?

[Star Strider]

Just guessing here.

The microphone (and everything else in the system including the computer) may need to have a common ground. What you’re seeing could be a ‘ground fault’.

[Giggs B.]

Hi, thank you for your inputs.

@Star Strider, everything is in same ground.

@Walter Roberson, If there is silence, there doesn't seem to be any drift, or if it is there...it is very low. That means when there is sound, there is this drift. What could be behind this? Below image is from one of the tests I did (for silence).

I cannot disconnect the microphone as I am using an already assembled PCB. There is no way I can disconnect them.

[DGM]

Just to be clear, when you say this is an already assembled PCB, is that to say that it's something like a development board or module that can reasonably be assumed to have a proven analog design? ... or is it simply a new untested circuit that can't be easily disassembled because it's soldered together on a protoboard?

[Giggs B.]

Hi DGM, it's a new untested circuit that can't be easily disassembled. It's not on a protoboard but a fully assembled chip (PCB).

[Walter Roberson]

Can you put through a simple sine wave?

[Giggs B.]

No I cannot, as the signal line is connected to Microcontroller. Please check the image below: the thick line (yellow arrow is pointing) is the analog signal line from microphone to microcontroller. But there is a way where I can disallow microphone to send any signal and I can probe sine wave over that signal line (I can remove the insulation, to expose copper line), but I am just worried if oscilloscope would introduce any potential voltage/current.

[Walter Roberson]

Play a sine wave for the mic to pick up?

[Giggs B.]

Yes, I can do that! But the microphone will also capture noise, I mean it has a noise floor...so it will be the noise + sine wave. But I can try!

After your earlier suggestion, I thought about it and maybe I can try sending a sine wave using a signal generator and check what is the output (since I have a few boards and I can spare one for testing). I will let you know what I get. Thank you so much for the inputs!

[Walter Roberson]

Calibration at different frequencies seems in order. If the chain seems to faithfully reproduce sine through the mic then there is probably no point in forcing the signal generator... but if through-the-mic is distorted then Yes, signal generator would be a classic debugging step.