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PCM1791A: VCOM Capacitors

Sadanori Kato
Guru 36530 points
Part Number: PCM1791A
Other Parts Discussed in Thread: DSD1791

Hi,

According to PCM791A data sheet, I found that it is recommended to add a 1uF chemical capacitor between VCOM and GND.
For a 1uF capacitor, is it possible to use a 1uF ceramic instead of a 1uF chemical capacitor?
Additionally, according to the following evm user's guide, a 0.1uF ceramic capacitor is added. So, should a 0.1uF ceramic capacitor be added between VCOM and GND as well?

DEM-PCM/DSD1791/1793 EVM User's Guide
https://www.ti.com/lit/pdf/sleu038

Best regards,
Kato

  • 0
    TI__Guru 53453 points

    Hi Kato-san,

    The VCOM pin is a reference voltage for the DAC outputs that is created by a simple resistor divider.  The larger the capacitor, the lower frequency the fC of this filter is, which will help limit the noise in the audible band.  I believe the resistor divider is created by two 15kΩ resistors.  From a noise perspective, these would be in parallel, so you have R=7.5kΩ for the RC filter.  Changing the cap value will determine the cutoff frequency. 10µF will have an fC of about 2.1Hz, while C=1µF will have an fC =21Hz.  This might be well below the audible range for their application, so it might not be an issue.  larger capacitor values also slow the ramp time of VCOM, which will limit the time from power-on before the device can play audio.

    In the EVM, they are likely using two values to better attenuate higher frequency noise as well.  This is probably not necessary in application, as the output LPF will help with this as well.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Guru 36530 points

    Hi Paul-san,

    Thank you for the information.

    I understand.

    Best regards,
    Kato