D.2.2. Termination options

There are four termination options:

Matched impedance

Where available, the best termination scheme is to have the ASIC manufacturer match the output impedance of the driver to the impedance of the PCB track on your board. This produces the best possible signal.

Series (source) termination

This method requires a resistor fitted in series with signal. The resistor value plus the output impedance of the driver must be equal to the PCB track impedance.

DC parallel termination

This requires either a single resistor to ground or a pull-up/pull-down combination of resistors (Thevenin termination), fitted at the end of each signal and as close as possible to the Mictor connector. If a single resistor is used, its value must be set equal to the PCB track impedance. If the pull-up/pull-down combination is used, their resistance values must be selected so that their parallel combination equals the PCB track impedance.

Caution

At lower frequencies, parallel termination requires considerably more drive capability from the ASIC than series termination and so, in practice, DC parallel termination is rarely used.

AC parallel termination

This typically uses a resistor and capacitor in series to ground.

Caution

AC termination can only be used with signals with a 1:1 mark/space ratio (DC balanced) and is not suitable for asymmetric signals. For this reason, AC termination is not recommended.

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