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8.2. ASIC pad selection, placement, and package type

The position and type of ASIC pad that you select depends on the following factors:

  • the ability to minimize the noise and coupling between trace and other signals

  • the ability to drive the external load.

The quality of the TRACECLK signal, as observed by the TPA, has the greatest effect on the reliability of the system. It is vital that TRACECLK transitions move cleanly through the threshold region of the input circuitry of the TPA, without glitches or ringing.

With certain types of package and pin placement (for example, pads on the corner or the edge of a package), the signal coupling between the trace data signals and the trace clock can be significant. If this problem is encountered during simulations, place GND or static I/O signals on both sides of the TRACECLK signal.

The quality of the package, and specifically the presence or absence of a ground plane in the package, can significantly affect the quality of the output signal. In general, ASIC pads are specified in terms of current drive and signal slew rate. For calculating the PCB signal quality you are likely to also have to determine:

  • the signal rise and fall times

  • the pad output impedance.

    Note

    Matched impedance output pads give a significantly improved performance.

You must also consider the pad placement, to ensure that the PCB tracking to the trace port connector is possible. You are recommended to place the pads so that they are:

  • on the outside of the package

  • grouped together

  • in the same order as the connector.

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