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PCM1803A: HPF by external RC(R1,C1,R2,C2) shown in figure24 and BYPAS=low

Yoshikazu Kawasaki
Intellectual 515 points
Part Number: PCM1803A

Hello,

My question is related to the followings, but the 2nd one isn't available now, so please let me ask you a few questions.

https://e2e.ti.com/support/audio-group/audio/f/audio-forum/532658/pcm1803a-adc-input-range-without-dc-blocking-cap?tisearch=e2e-sitesearch&keymatch=PCM1803A%2525252526%2525252526input#

https://e2e.ti.com/support/data_converters/audio_converters/f/64/p/403678/1432310#1432310 <= The link isn't available now.

I understand the RC on the input pins(VINL/VINR) shown in figure24 on page 17 in the datasheet is for HPF as the note.A says.  I also do that there is an internal HPF which can be bypassed(BYPAS=high) or used(BYPAS=low).  The cutoff frequency is different between those, but both work as HPF.

1. Would you please tell me if the input filters for VINL/VINR pins are really mandatory since 8.2.2.3 on page 18 says "AC coupling capacitor muse be placed in series with the input"?  If yes, would you please tell me why it is because it has internal HPF instead which can be configured by BYPAS=low?

2. Would you please tell me what would happen if the external RC filter isn't used, but only the internal HPF by BYPAS=low is done?

3. If my customer can use BYPAS=low without doing the external RC, is the following all they have to check for the VINL/VINR pins?

 

4. Would you please tell me the differences between using the external R1/C1/R2/C2 and using internal HPF by BYPAS pin(#8)=low?  I know the Fc is different, but I'm not sure other than that.

Best Regards,

Yoshikazu Kawasaki

  • 0
    TI__Genius 11230 points

    HI Kawasaki-san,

    1. The RC low pass filter is optional as this is mostly to help reduce out of band noise in the input signal path. The AC-coupling in series is to remove any external DC component from the analog signal since the device has is its own internal biasing to reach optimal input common mode.

    2.  Again, the external RC filter is optional to help remove noise. The device will operate normally.

    3. Yes, that is correct. 

    4. C1 and C2 are recommended. C1 and C2 will create a "internal" HPF when paired with the impedance of 40kOhm.  Bypassing the HPF adds the DC bias from the input signal when it is in the digital signal chain and is represented on the DOUT pin. 

    R1,R2,C7,C8 are optional and only for additional filtering.

    Regards,

  • 0 in reply to Daveon Douglas
    Intellectual 515 points

    Hello Daveon-san,

    Thank you very much for your quick reply.

    I should have separated HPF and LPF.  I'd like to ask you here is HPF and understand as shown below.  Please correct me if I'm wrong.

    - Adding C1/C2 on VINL(VINR) is recommendation, but can be removed even though the datasheet on page 18 says "AC coupling capacitor must be placed in series with the input".

    - Input signal shown below and BYPAS=low without C1/C2 will also work well for PCM1803A.

    Best Regards,

    Yoshikazu Kawasaki

  • +1 in reply to Yoshikazu Kawasaki
    TI__Genius 11230 points

    Hi Kawasaki-san,

    Yes, the device should be okay if C1 and C2 are removed.

    Yoshikazu Kawasaki said:
    - Input signal shown below and BYPAS=low without C1/C2 will also work well for PCM1803A.

    Yes, the device should work okay.

    Regards,