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PCM5242: Noise on DSP-Board Output

Emanuel Lemnitz
Prodigy 50 points
Part Number: PCM5242
Other Parts Discussed in Thread: OPA1656


Hello TI-Team,
We have a problem with the noise at the output of our DSP board. We want to ensure that the noise is not coming from our output circuitry by evaluating the performance of used ICs. The output is converted with the DAC PCM5242. The analog audio signal is then amplified with the OPA1656ID operational amplifier. Our question is whether the noise problem is due to incorrect configuration of the DAC or OPA.


Is the low-pass filter at the output of the DAC correctly dimensioned?
Can the resistor R612 and the DC coupling capacitor C615 be removed, e.g. with OUTRP?
Another question is whether an impedance adjustment must be made between the two ICs in order to keep the noise floor as low as possible?


I have attached a schematic page with the DAC and the OPA used.

Thank you for your support!
Best regards
Emanuel Lemnitz

crispAudio DSP DAC OPA.pdf

  • 0
    TI__Mastermind 20830 points

    Hello Emanuel  

    I took a look at the schematics and here are few suggestions regarding DAC that I am familiar with:

    The filter component value is ok 

    I would use dedicated decupling capacitors for each supply pin-  right at the pin or as close as possible.

    I usually add a 22 to 100ohm series resistors for I2S signals

    I also use pull up resistors for I2C signals. You can read more at   SLVA689–February 2015

    If XSMT is not required, it can be directly connected to AVDD.

    The PCM5242 provides  ground-centered allowing  to eliminate DC blocking capacitors on the output, so you can remove  DC coupling. In fact if you check the EVM schematic, we do not have any DC blocking or resistor except for the filters  ( Figure 7. PCM5242EVM Schematic Evaluation Module  in SLAU592A user's guide). 

    While it is always a good practice to have impedance matched, I have never used OPA1656 so if you want  please post a new post for  OPA question.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 50 points

    Hello Arash,

    Thank you for your support!

    In SLAU592A user's guide (Figure 7. ) there are no resistors after the Low-Pass Filter at the Output of the DAC. But in the datasheet of the PCM5242 (Figure 80.) there are 402Ohm resistors used. What is the reason for these? Do I need them in my device?

    Best regards

    Emanuel Lemnitz

  • 0 in reply to Emanuel Lemnitz
    TI__Mastermind 20830 points

    Hello Emanuel,

    In the schematic of the user's guide the output voltage of the DAC  is filtered and being sent to a XLR  connector for external connection.

    I believe the amp used in datasheet diagram is a current amp,  so it uses a series resistor to convert the DAC voltage to current to send it to amp.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 50 points

    Hello Arash,

    the Amp used in datasheet diagram is a Headphone Amplifier. Since we are operating an OPA at the output of the DAC, are the series resistors also required for the circuit I have realized? 

  • +1 in reply to Emanuel Lemnitz
    TI__Mastermind 20830 points

    No need for series resistors.

    Regards,

    Arash