Doubly terminated coaxial cable

Question

I was going through the datasheet of ADV7125 DAC and found this sentence confusing.

For optimum performance, the analog outputs should each have a source termination resistance to ground of 75 Ω (doubly terminated 75 Ω configuration). This termination resistance should be as close as possible to the ADV7125 to minimize reflections.

Well, what I think is - since the cable impedance is 75 ohms, and the load is also 75 impedance at the end of the cable, is n't the cable already devoid of reflections ? Then why adding that 75 ohms at the beginning (double termination) ??! Also suppose the cable impedance was 50 ohms in this case, what can we do to reduce reflection ?

Answer 1

since the cable impedance is 75 ohms, and the load is also 75 impedance at the end of the cable, is n't the cable already devoid of reflections ?

Nobody's perfect.

The cable geometry won't be exactly perfect, so its characteristic impedance won't be exactly 75 ohms. It might be 73 or 78 ohms, for example.

The load termination also won't be exacctly 75 ohms and it won't be perfectly resistive.

So there will always be a slight mismatch even when the termination is designed to be matched.

By matching the source as well as the load, we provide a second location for reflections to be reduced, and produce overall lower standing wave ratio or less ringing in the cable.

Also suppose the cable impedance was 50 ohms in this case, what can we do to reduce reflection ?

The same thing, but terminate with 50 ohms instead of 75 ohms.

Since this is a current-output DAC, it will reduce the output voltage achievable.

Answer 2

Double termination ensures any reflections from the far end, and any capacitance at the far end will cause reflections back to the DAC output pin, are mostly absorbed at the DAC. Thus double-termination improves settling.

Answer 3

The DAC outputs are current sources, so the voltage levels that come out of them are dependent on the load. This allows you to choose between different standards with different signal levels specified into 75 Ohm loads by choosing the appropriate load resistor in parallel with the cable. Read further in the datasheet:

Without the resistor the signal levels would be incorrect. As you say, dealing with reflections from the far end shouldn't really be an issue as the far end will be correctly terminated by a 75 Ohm load so the local termination is a minor benefit.