• Resolved


Guru 11375 points

Our application is to record the two audio inputs VinR and VinL to the PC (via USB). We are not playing any sound to the audio outputs.

Do we need to add low pass filter before VinR and VinL (PCM2904  audio inputs)?

in The picture  from page 27 of the datasheet. The application is using a regulator IC1. There is another schematics in page 28 without the regulator but according to the datasheet the A/D performance will be degraded.

Please explain this in more details. Please let me know if we need IC1 for our application and if there is a lower cost alternative.

  • Hi Eli,

    As with any audio ADC, there is no need to low pass filter the inputs. The LPF is typically required at the output side (i.e. DAC output stage). You should take care to ensure that your input source has sufficient level to match the ADC's 0dBFS level and that its output impedance can drive the ADC's input impedance (30K) properly.

    The high performance application example shows an external REG103 regulator driving VCCCI (therefore presumably bypassing the internal 3.3V regulator). This is recommended because a USB host's 5V supply is typically "dirty" and the internal regulator can only do so much. A good external regulator helps ensure the codec's internal PLL doesn't create too much jitter.

    A much lower cost alternative to the REG103 is the TPS78833. It's rated at up to 150mA (and a plus - it's designed for USB power) but it has a noise figure of 56uV versus 33uV for the REG103.

    Disclaimer: I do not work for TI.

  • Guru 11375 points

    In reply to John Yap:

    Thank you for the detailed reply

    It seems that the PCM2900 and PCM2902 have the same performance as the PCM2904. Is there a reason why not to use the PCM2900 or PCM2902 in our design? They have EVB and we can test exactly the configuration of our product. What is the advantage of using the PCM2904

    The PCM2900 EVB has external filters at both inputs and outputs. From what you wrote, input filtering is not necessary but some amplification may be required. Is this correct?

    Thanks again


  • In reply to eli:


    For this application the PCM2900 can be used equally to the PCM2904.  The PCM2902 would be used if S/PDIF functionality is needed.  Your statement is correct. 

    Patrick Galvin