# Parallel Termination in a Transmission Line

Suppose we have a parallel termination like this on a high-speed trace:

If the line impedance $$\ Z_o \$$ and $$\R \$$ are matched, then there are no reflections from load to cause any ringing at the driver output or at the receiver input.

But if the source impedance $$\R_s\$$ of the driver doesn't match with the line impedance $$\ Z_o \$$, shouldn't there be reflections still at the driver output from the line? So isn't that impedance matching necessary as well?

I am asking this because, in serial termination scheme, we match the source impedance to the line impedance. But in parallel termination scheme, I just can't see this concept. So, does that mean we have to mix both the schemes to ensure signal integrity on a high-speed trace?

• What is the impedance looking into the line at the point where R is connected?
– Chu
Apr 29, 2021 at 8:00
• In series termination also, there is termination at the load and reflections will occur at load how they are hanled? Apr 29, 2021 at 8:51
• Yea, that's the drawback of series termination. The signal needs time to settle after looping back once after reflection and gets terminated by driver source impedance. Apr 29, 2021 at 8:56
• @Chu $R_s$ plays a role in selecting the value of parallel termination $R$ you mean? Apr 29, 2021 at 9:01
• Do you think that adding R at some point along the line affects the impedance seen at that point?
– Chu
Apr 30, 2021 at 11:55

But if the source impedance $$\R_s\$$ of the driver doesn't match with the line impedance $$\Z_o\$$, shouldn't there be reflections still at the driver output from the line?
You can also get mismatches caused by the receiver input capacitance/impedance shunting the load resistor $$\R\$$.