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TLV320AIC3254: ADC noise reduction using AGC

David Trotter
Intellectual 740 points

Part Number: TLV320AIC3254

Hi there, 

We are using the 3254 with Minidsp for a portable speaker. The aux 3.5mm input goes pretty much directly to the IN1 single-ended inputs and ADC with just a 10k series resistor to half the voltage and avoid clipping the input. There is a lot more noise using the ADC with a line level signal source than there is using the I2S digital input. I have narrowed this down to digital noise in the ADC. 

As a workaround, I am trying to use the AGC to boost the analog signal level using the PGA and then reduce the digital audio level. For example, setting the PGA to +10db and the ADC digital level to -10db has a much lower noise floor than setting both to 0db. 

The issue I am having, is that as soon as I enable the AGC, I lose control of the ADC digital volume level. I guess this is now being controlled by the AGC? So I am unable to turn down the digital gain and turn up the analog gain. I normally set my AGC like this:

//###### AGC config ###############
0,0, // set page 0
0x56,0x82, // set noise to -70db and target level to -5.5db
0x5E,0x82, // right
0x57,0x6A, // turn on and set rest of settings
0x5F,0x6A, // right
0x58,0x10, // set max ADC gain = 8db
0x60,0x10, // right
0x59,0x18, // set attack to 5ms
0x61,0x18, // right
0x5A,0xF9, // set decay to 1.5 seconds
0x62,0xF9, // right
0x5B,0x0F, // set noise debounce 1.5s
0x63,0x0F, // right
0x5C,0x0F, // set signal debounce 1.5s
0x64,0x0F, // right

How would you recommend I go about setting it up to get the lowest noise level and best SNR? I need to acheive a certain signal chain gain to max out the speaker given a typical line input of 0.75Vrms. 

I do not have enough memory space in the MiniDSP left to use any noise reduction algorithms. 

Thanks

David

  • 0
    TI__Guru 57800 points

    HI, David,

    As you mention, the AGC is actually controlling the Digital gain as well, so it is not possible to do fine tuning with the digital volume controller while AGC is active.

    The AGC is mostly used in mic recording applications to keep the record volume constant in different situations, like where a person is speaking and moves closer or away from the mic.  For line input applications,  it is complicated to tune the AGC as the level changes on the line signals are actually expected sometimes. The noise issue you are pointing out may be related to the external circuit used at the ADC input. Sometimes, out-of-band noise can couple back to the ADC if the Audio source DAC does not have a proper filter in the output. Have you verified if the noise you are getting in the system is the same when using different audio sources?.

    Best Regards,

      -Diego Meléndez López
       Audio Applications Engineer

  • 0 in reply to Diego Melendez
    Intellectual 740 points

    Yes I understand all of this. Some devices are worse than others, some Macbooks, for example, have a lot of out of band noise which couples back in. Most phones are pretty good and fairly consistent. Together with the 10k input resistor, I have a 330pF capacitor forming a RC filter to LPF some of this. 

    I've done everything I can with the layout. This may be a more complex issue which cannot be discussed here. Perhaps I can get in touch directly to share some more detail schematics etc? 

    Thanks

    David

  • 0 in reply to David Trotter
    Intellectual 740 points

    It seems like this really is a problem with other devices out of band noise coupling back into the audio band.

    The iphone 4s for example has a huge amount of noise around 500kHz when the output is on. iphone 6s is fine.

    It seems a single RC filter on the inputs is not enough to filter this noise. 

    Is there any way to resolve this?