Control noise from SSM2167

Dear Respected,

 

We are planning to use SSM2167 in one of our project.

Where output of MIC is connected to input of SSM2167 and output of SSM2167 is connected to Audio codec, we have tested with external setup by soldering this IC in general purpose board and it is working fine.

But, there is small amount of background noise which is always present irrespective of audio. This noise is not present, when we connect MIC output directly to audio codec by bypassing SSM2167.

 

Please suggest to resolve said issue at the earliest.

 

Thanks,

Amar

91 80 6709 2249

  • 0
    •  Analog Employees 
    on Sep 24, 2016 12:12 AM over 4 years ago

    Hello Amar,

    We will have to see a schematic of what you are doing and what the other parts are. Connecting in a part using a prototype board is not the cleanest way to test the performance of a part. Ground leads are not always good and the board does not have power and ground planes. The wires are long and unshielded which will pick up external RF signals easily. This is good to determine if a part will function correctly and to learn what it requires for support circuitry but it is not a good way to evaluate the performance.

    What is the input impedance of the codec input?

    Dave T

  • Dear Dave,

    Thanks for your response.

    Please find attached schematic which includes MIC, SSM2167 circuit and Auido CODEC for your reference.

    Now, we are planning to make separate board for attached schematic, so that board will have proper ground and power connection, we will interface this small board to the original board. Hoping that, this will eliminate background noise and provide clear audio output.

    Thanks,

    Amar

  • 0
    •  Analog Employees 
    on Sep 28, 2016 10:57 PM over 4 years ago

    Hello Amar,

    I took a look at your schematic. The first thing you should do is get rid of L11 and tie the two grounds together. Putting a ferrite bead in series with a ground connection will turn the ground plane into a radiating plane antenna for high frequency noise. I often use a 0-ohm jumper to keep digital ground nets separate from analog ground nets when I want to be careful of how they are run and where they go but ultimately, I tie them together on the board. That said, most of our codec boards we only use one ground for both analog and digital grounds.

    C17 I would remove. The output of the SSM2167 has a max capacitive load of 2nf. L6 is OK.  If you need to have C17 as the antialiasing filter for the input of the codec then make it into an RC filter so the 2167 does not "see" the 100nf load directly with no series resistance. So place it on the codec side of L6 and still add a series resistor.

    For the codec I do not see any series dampening resistors on the I2S data and clock lines and also for the MCLK line. You must properly terminate the transmission lines to prevent overshoot and undershoot. This can cause noise to be injected into the power and ground planes and that can bleed into the audio.

    So that is all I see. So if you still have a noise problem then I would like to know more about the noise. What it sounds like? What the frequency spectrum of the noise is? This way we could try to determine the source of the noise which will help to figure out how to reduce it. The power supplies are also something to look into. They were not on your schematic so I could not comment.

    Thanks!

    Dave T

  • Dear Dave,

    Thanks for all your suggestions.

    Let me work on all points you suggested and revert back for any further issues.

    Thanks once again.

    --Amar