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Aic33 codec inband noise

kobi elbaz
Prodigy 150 points

hello,

I'm using an array of 4 AIC33 codecs connected in TDM mode to the McBSP of c6424, at atartup each D/A receives zeros from the TDM bus, once a slight change in the data occurs (even one lsb is changed to 1), the codec starts to create an inband noise(several milivolts) If i return to transmit zeros, the noise remains.

The configuration basiclally works, I can sample signals and generate signals, only once I generate even a very week signal, noise is added to whatever i generate from now on.

any ideas?

thanks

Kobi

  • 0
    TI__Mastermind 28585 points

    Hi Kobi,

    by default, the modulators are powered down. Even once the converter is powered up, the modulators do not start until data is sent. The SNR numbers in the datasheet are measured with the modulators running. Once started, the modulators will continue to run until the part is powered down or reset.

    If you are looking at the outputs with a scope, the measurement will be dominated by out of band noise. How are you measuring the outputs?

    I would recommend measuring the band limited SNR with an Audio Precision or something similar. If you don't think the device is meeting spec, please attach some measurements and we can take a look.

  • 0 in reply to Converter Apps
    Prodigy 150 points

    Thanks for the reply.

    Did some more tests, and compared to the EVM6424.

    On the EVM the noise looks similar on the scope but it can hardly be heard , EVM is with the same codec configuration (except inputs that are single ended).

    On our custom board the noise is heard.

    Looking with a signal analyzer confirms this measurement, there is a strong low-frequency noise (0-100kHz, the range of the instrument ) in our board, but not on the EVM.

    However this noise starts only when the modulator starts, so they are somehow linked.

    Can you give us some ideas how to detect the source of this phenomena? How to protect our board against it?

     

    Many Thanks

     

    Kobi

     

  • 0
    Prodigy 120 points

    I am facing exactly the same problem

    i am also using aic33 connected to c6424 using McBSP

    there is no output from the codec until the first data sent to it (using the edma) , then the codec outputs inband noise (more than 100 mv)
    after that the noise will be added to any output signal until powering the evm off and on again. even when output is zeros the noise never disapear .

    another thing is when changing the channel output gain (adjusting values of resgister 86 and 93 for the left and right channels) the noise level changes accordingly , for example when the channel gain set to 9db  the noise level reach 1V.

    what is the source of this noise ? and how it could be reduced?