Forum Discussion
Altera_Forum
Honored Contributor
13 years agoDriving terminated controlled impedance trace single ended
Hi, I'm sure this comes up a lot but I couldn't find an exact match to my query. Consider a single ended 3.3V LVTTL Cyclone IV output, driving into a 50 ohm trace with far end parallel termin...
Altera_Forum
Honored Contributor
13 years ago --- Quote Start --- The resistor won’t damp the signal when it comes back in my opinion but you can try using parallel resistance. --- Quote End --- I don't believe that you have seriously studied transmission line termination schemes. It turns out, that for termination of I/O standards like LVTTL or CMOS, that require to achieve a full voltage swing at the receiver side, series source termination as suggested by gj_leeson is the perfect solution for most point-to-point connections. With correctly adjusted series resistor, it achieves almost reflection free waveforms. A small, usually neglectable deviation from ideal behaviour is caused by the input capacitance of receiver. It should be also noted, that the 4 mA I/O specification does not mean absolute maximum current. It's the maximum static current to keep the specified output level of the respective IO-standard. In so far it applies to parallel load R termination, but not e.g. to RC load termination or series R termination. As the main disadvantage of series source termination, it's not suited for tapped transmission lines, because the intended square waveform is only achieved at the transmission line end.