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PCM5242: The RC value of LPF questions

Alex Lee
Prodigy 160 points
Part Number: PCM5242
Other Parts Discussed in Thread: PCM5102A

Hello

I found that there are 4 RC values in datasheet and EVM

in datasheet 8.7.1. Many applications require an external low-pass RC filter (470Ω + 1.2nF) to provide sufficient out-of-band noise rejection.
470R+1.2nF

in datasheet 8.7.1. The PCM5242 can also support single ended outputs, using OUTLP and OUTRP respectively. A single 470Ω and 2.2nF capacitor can be used on each pin in single ended mode.
470R+2.2nF

in datasheet 8.7.1. Figure 66. Optional Low Pass Filters
470R+4.3nF

in EVM 6.1 Schematic
499R+1nF

There are some question confusing me..

1. Are those values respresenting the filter values of following?
470R+1.2nF ~ 282k
470R+2.2nF ~ 154k
470R+4.3nF ~ 78k
499R+1nF ~ 318k
If so, how to determine the optimal value? why does single end need 154k, but differential can choose between 78k-318k?

2. What is the range of R should be use?
the datasheet and EVM are using 470-499R,how to determine this value as well?
I just tried 1k in PCM5102A(not sure if it's same as PCM5242), the sound was horrible compare to 500R.

Thank you very much.

  • 0
    TI__Mastermind 26125 points

    Hi Alex,

    Our DAC expert is out of office today and will return to answer your question Monday.

    Thanks for your patience,
    Jeff McPherson

  • 0
    TI__Mastermind 20830 points

    Hello Alex,

    Thanks for your patience while I was out of office.

    Datasheet indicates that many application need  470R+1.2nF or about 282kHZ BW so this is just setting the stage for requirements of certain applications.

    In most applications, however  we use 470R+2.2n for single ended outputs , that is a 2.2nF cap to GND from each output. Note that for differential output you can connect  one 4.3nF between the two outputs ( not to the GND) . Also  PCM514x, PCM512x and PCM510x use 470+2.2nF on their EVMs and datasheet for single ended output as suggested above. At the end of the day, the application determines the bandwidth of the filter. and for some reason the  PCM5242 EVM is using a wider BW filter even though the  audio range is  <20kHZ.

    The resistor choice can effect the distortion and harmonics  though , the value  of about 470 ohm is a good starting point.

    Regards,

    Arash

  • 0 in reply to Arash Loloee
    Prodigy 160 points

    Hello Arash

    Got it! Thank you so much for your help.
    I'll try different combination to test it out.