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GND issues related to Mic input and LO output on TPS65950A3.

Oscar Gomez Fuente
Expert 2066 points
Other Parts Discussed in Thread: DM3730, LM4675, TPS65950

Hi, I am working in a new design based on TPS65950A3 with DM3730 OMAP3 platform to develop a SIP telephone. I've got some questions related to audio module.

I need one michrophone input (mono configuration and differential mode) and one line out (mono configuration and single-ended) to feed an amplifier of 2.65 Watts [LM4675 of Analog Device (TI)].

I'm thinking in use the MIC.MAIN input and the Predrive.LEFT output to feed the amplifier LM4675. Can I use this configuration? Are this configuration compatible?

I attach the schematics in pdf format: tps65950a3_power.pdf.

I've got some doubts related to MICBIAS.GND (Pin D3). I've read that this GND can be isolated to the others GNDS, but too pin MICBIAS.GND has to be shared with AGND (Pin N15)...

Page 87 of TPS65950 Integrated Power Management/Audio Codec Silicon Revision 1.2 "SWCS032E.pdf":

NOTE
To improve the rejection, it is highly recommended to ensure that MICBIAS_GND is as clean as possible. This ground must be shared with AGND of TPS65950 and must not share with AVSS4,
which is the ground used by RX class-AB output stages. In differential mode, adding a low-pass filter (made by RSB and CB) is highly recommended if coupling between RX output stages
and the microphone is too high (and there is not enough attenuation by the echo cancellation algorithm). The coupling can come from:
• The internal TPS65950 coupling between MICBIAS.OUT voltage and RX output stages
• Coupling noise between MICBIAS.GND and AVSS4

I don't understand this issue. How do I have to connect MICBIAS.GND, AGND and AVSS4 pins?

Thank you so much. Best regards.

  • 0
    TI__Intellectual 2045 points

    Oscar,

    I don't see any issue using the Main microphone and the predriver output as in your schematic.

    One comments, AC coupling capatiors are missing in serie with MIC.MAIN.P and MIC.MAIN.M. 100nF is ok.

     

    Regarding the ground connection. it is recomended to have a dedicated shape connecting MICBIAS.GND to the microphone connector. External capacitors/resistors linked to the microphone must be connected to this shape only. This shape should be connected in one point(under the ball) to the board maing ground layer. We must avoid any connection of this ground on top with any otehr ground.

     

    regrads,

    Alexis