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DS90UB913A-Q1: DS90UB913A-Q1:

Beomkyu Kim
Prodigy 70 points
Part Number: DS90UB913A-Q1

Hi Team, I have some questions about PoC.

The figure below is a picture of Application note, "Sending Power Over Coax in DS90UB913A Design".

1. Usually, I know that power source is used in Rx and power regulator is used in Tx. Depending on the situation, is it possible to change the position of power source and power regulator?

   And, is it common to use this?

2. AC coupling capacitor, 100nF, is used for data transfer without noise. Also, I have already checked why TI selected and recommended 100nF in the application note.

   But, I cannot see why TI selected and recommended "47nF". I want to know the purpose of 47nF and why this value(47nF) is used.

Thank you

  • 0
    TI__Guru* 98485 points
    Hello Beomkyu,

    1. The position of the power source on TX or RX side does not matter. Most commonly the power is being provided to the remote imager, but from a circuit standpoint it doesn't matter if they are reversed.

    2. 47nF is the closest value to 50nF available. In coax mode, the device pins must still be balanced to allow for echo cancellation. With the two connected devices in series the D+ 100nF and 100nF makes an effective capacitance of 50nF. Using 47nF on the negative side balances the driver/receiver pins to the positive side.

    Best Regards,
    Casey
  • 0 in reply to Casey McCrea
    Prodigy 70 points

    Hello Mr. Casey McCrea.

    Thank you for your comment.

    It was great helpful for me to understand PoC concept.

    Based on your comment, I would like to check that I understood your comment exactly.

    Regarding No.2 question, in coax mode, two series capacitors(100nF) in D+ should be used for AC coupling of data&control(Forward&Back) channel and DC block. These 100nF capacitors can be selected by calculating the value that get low impedance at the used bandwidth of forward and back channel. These two series capacitors makes an effective capacitance of 50nF. Meanwhile, if there is no capacitor on the negative side(GND), echo(reflection by mismatching) can be occured due to unbalance. This can make the degradation of signal quality. So, By using 47nF on the negative side(GND), which is simliar to effective capacitance of 50nF in D+, we can have balance between positive and negative sides. Normally, the used termination resistance in coax mode is 50 ohm it's because the characteristic impedance of coax cable is 50 ohm.

    Did I understancd it correctly?

    Thanks in advance.

    Beomkyu Kim

  • 0 in reply to Beomkyu Kim
    TI__Guru* 98485 points

    Hello Beomkyu,

    Yes you are completely correct! :)

    Best Regards,

    Casey 

  • 0 in reply to Casey McCrea
    Prodigy 70 points

    Hello Casey McCrea.

    Thank you for your commet!!

    It was great helpful for me.

    Best Regards,

    Beomkyu Kim