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AFE7900: Regarding the requirement of polarity inversion in RXB , RXC channels with respect to RXA and RXD channels of AFE7900 front end

Part Number: AFE7900

 Hello support team,

We are planning to design a custom AFE7900 board having same identical RF front end chain with the TCM2-33WX+ RF Balun in all 4 Rx channels ( RXA, RXB, RXC and RXD). For all other sections, we have taken a reference from AFE7900EVM board. 

As per reference schematic of AFE7900EVM, what I have observed that RXB and RXC RF front end channels are having polarity inversion with respect to RXA and RXD channels. 

Based on that only, we have put the TCM2-33WX+ in all 4 Rx channels ( RXA, RXB, RXC, RXD) in such a way that RXB and RXC channels are having polarity inversion with respect to RXA and RXD channels. 

Please check above screenshot of the front end of RXA, RXB, RXC and RXD. (RXA : P, N, RXB: N, P , RXC: P, N , RXD: N, P)

But due to usage of same RF balun in all 4 channels , we are facing issue in layout for signals crossing if we use like this. 

So, Is it possible to not use this kind of polarity  inversion in RXB, RXC w.r.t RXA and RXD same as eval board, and what will be the effect if we do not use this kind of polarity inversion? Is there any performance degradation if we do not use polarity inversion in terms of isolation or any other parameter?

Could you please suggest a way to resolve this issue. Awaited your feedback.

( FYI: please check below screenshots for the layout issue. )

  • Hi Maitry,

    Polarity inversion of RXB, RXC w.r.t RXA and RXD is done on the EVM for easier PCB routing. You can follow the same for your custom board design. The digital output data from RXB and RXC can be multiplied by -1 in your FPGA to correct this inversion. 

    There would be no impact on ADC performance due to this inversion. 

    Regards,

    Vijay

  • Hello Vijay,

    Thank you for your quick response. 

    Actually by following the same as evaluation board, we are having this layout cross signal issues( routing issue as mentioned in the above screenshots). So, we are planning not to use polarity inversion in all 4 Rx channels. Is that ok? 

    As per our PCB routing , below are the possible way

    RXA = P, N

    RXB = N, P

    RXC = N, P

    RXD = P, N

    Kindly confirm.

  • Hi Maitry,

    Yes. This is okay. You can choose to invert channels as needed for your layout. As I mentioned, these inversions can be corrected by multiplying ADC output data by -1 in FPGA. 

    Regards,

    Vijay