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AFE5801: is there any length matching requirements for the Analog input signals of AFE5801

Tarang Jindal68
Intellectual 820 points
Part Number: AFE5801

Hello,

The AFE5801 has written about length matching of digital output signals which is 150 mils.

but there is no statement regarding length matching of Analog inputs so, please let us know that "is there any length matching requirements for the Analog input signals of AFE5801"? 

I have also seen in previous queries on forum that, these analog signal should be routed with 50E impedance, as these are differential analog signals, so we should route them as per diff 100E impedance.  Please confirm. 

Thanks and Regards

Tarang Jindal

  • 0
    TI__Genius 12025 points
    Hi Tarang,
    How are you?
    Thanks for using AFE5801 device.
    We will look into the question of AFE5801 you mentioned very soon.

    Thank you very much!
    Have a nice day!

    Best regards,
    Chen
  • 0 in reply to Chen Kung
    TI__Genius 12025 points

    Hi Tarang,

    How are you?

    Thank you for using AFE5801 device.

    For your question #1:

    Ideally the length matching for Analog Input Signals need to be totally matched

    from one channel to the other channel. This is necessary so that is why the datasheet did not

    mention any statement. However in the real application, it would need to depend on

    your requirement and application purpose.

    Please look at the following plot:

    When the Input Clock (CLKIN) starts rising-up

    and the first Input Signal can be Sampled at (for example) "Sample N" position.

    Then at the same time, the Second Input Signal can be Sampled at (not shown on this plot) "Sample M" position, for example.

    and so on.

     Therefore if all the Analog Input Signal trace length are not matched,

    then the sampled signals will cause different time-delay from one channel to the other channel

    as you can see.

    So please think in advance that

    how much % tolerance could be allowed by your system design.


    For your question #2:

    The queries you saw probably could be shown from the AFE5801 User's Guide EVM schematic plot

    (shown as below):

    The left hand side (plot in red) means the Analog Input Signal comes from a signal source

    with internal serial 50ohm impedance.

    Also because the signal source signal is single-ended,

    then the AFE5801 EVM uses a transformer (ADTT1) to convert input single-ended

    into a pair of differential signals such as CH1P and CH1N (for example). 

    Since ADTT1 imp ratio is 1:1, then it uses 50ohm (25ohm+25ohm) for impedance matching at the end.

    Of course, if your input signal source can generate differential output signals (one for CH1P and the other for CH1N),

    then you need to route a 100ohm (50ohm+50ohm) impedance matching at the end.

    (In this case, assume you are not using ADTT1-1)

    Thank you and best regards,

    Chen