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TLV320AIC33 CODEC DAC OUTPUTS, OUT-OF-BAND NOİSE

mustafa su
Prodigy 20 points
Other Parts Discussed in Thread: TLV320AIC33, TPA2010D1

Hi,

We are using TLV320AIC33 Codec with line inputs and line outputs.

We use operational amplifiers in the line outputs in order to increase the signal levels.

My question is that must we use external low pass filter in our line outputs for the out-of-band issues?

While first using this codec we thought this codec has internal audio filters.

Codec has some digital filters and analog reconstruction filter but i think an external filter is needed for out-of-band noise. Is that true?  

I read some articles about out-of-band noise of audio DACs and my understanding, if it is true, is that the external low pass filters are necessary

in order to make true dynamic performance measurements such as THD+N, SNR or SINAD.

But  i am suspicious about using a low pass filter in the circuit itself, not only for measurements.

In EVM kit  of TLV320AIC33 there is no external filters on outputs.

At the moment we have a design without an external filter and our line outputs are noisy when looking with oscilloscope.(100mVpp noise on a 800mVpp sinus signal).

In the FFT plot this noise is seen between 100KHz and 500 KHz.

My main question is how big this out-of-band noise problem is?

Can i think this is not a problem because the noise is not in auido band?

or

Must  use a sufficient external low pass filter in order not to have any problem?

Thanks.

Mustafa Sü

Digital Design Engineer 

 

 

  • 0
    TI__Guru**** 189399 points

    Hi, Mustafa,

    Welcome to e2e, and thanks for your interest in our products.

    It sounds like you've already built boards with the AIC33 on them. This is a pretty old part, we probably have a better, cheaper, newer part, but if you've already started using this one, that is fine.

    Out of Band Noise (OOBN) is tricky. It depends on what you are driving. if you are driving passive head phones, for example, then the noise it above 20 kHz, and no one can hear it, so it doesn't present any problems.

    if you connect the output of the DAC to a TPA2010D1 class-D amplifier, for example, the OOBN will essentially alias back down into the audio band due to the sampling nature of the class-D amplifier. So, in this case, you would definitely need an external RF filter to reduce the amplitude of the OOBN.

    Some of our newer amplifiers have built-in low-pass filters to avoid this problem.

    What are you driving with your AIC33?

    -d2 

  • 0 in reply to Don Dapkus
    Prodigy 20 points

     

    Hi,

    Thanks for your reply. This ouput is a 150 ohm line output of a radio device. Actually, this output will be used for driving passive headphones but any other device can be connected to this radio, as our output is a line output. We can not be sure that only passive headphones will be connected to our device. So i think it will be better to put an external low pass filter after AIC33. I tried a simple RC low pass filter with 20 KHz corner frequency by making a handmade modification to our board and it gave good results of suppressing the OOBN. 

    Have you got a newer CODEC which has on-chip LPF similar to AIC33?

    Thanks.

    Mustafa Sü

    Digital Design Engineer 

     

        

  • 0 in reply to mustafa su
    TI__Mastermind 28585 points

    Hi Mustafa,

    All of the AIC codecs will require an external filter if OOBN is a concern.