SN65HVD3086E - AC Termination

Hello Kawai-san,

You are correct that AC Termination can be a solution for decreasing driver power consumption and still allows for open input receiver failsafe during idle or open line conditions.

The value of RT is typically chosen to match the characteristic impedance of the line (usually between 100-150ohms depending on the cable).

The value of CT is selected to equal the round trip delay of the cable divided by the cable's characteristic impedance.  So in order to calculate this value, a couple of parameters from the actual application or system will need to be identified in advance. 

1. First, determine the type of cable and identify the propagation velocity of the cable. 
2. Then determine the length of the cable that will be used in the system application.
3. Finally determine the actual cable characteristic impedance.

As an example:

Cable length = 100 feet

Propagation Velocity = 1.7 ns/foot

Characteristic Impedance = 100 ohm

CT = (cable length * velocity) / characteristic impedance

Yields CT = (100 ft * 1.7 ns/ft) / 100 ohm = 3400 pF

This means that the value of CT should be chosen to be 3400pF or less.

There is a tradeoff to using AC termination which is that it introduces a Low Pass Filtering effect on the signals and therefore the maximum data rate used with AC termination will be less than the maximum data rate achieved with a parallel termination technique. The resulting RC time constant should be less than or equal to 10% of the unit interval (TUI).  The maximum switching rate for the above example would be 300 kHz.

Additional information, and the information I provided you today can be found in this Application Note.  I hope this is the information you seek.

Regards,

Jonathan