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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 ? enter image description here

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3 Answers 3

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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.

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  • \$\begingroup\$ So if that resistance wasn't there at the source, there will be some reflection from the load side due to this little mismatching, which then causes standing waves? \$\endgroup\$
    – Mitu Raj
    Commented Nov 14, 2017 at 17:12
  • \$\begingroup\$ Standing waves are caused by reflecting waves back and forth between source and load? \$\endgroup\$
    – Mitu Raj
    Commented Nov 14, 2017 at 17:13
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    \$\begingroup\$ There will be some reflection from the load no matter what. Termination at the source means that reflection only bounces once before it gets (mostly) absorbed. \$\endgroup\$
    – The Photon
    Commented Nov 14, 2017 at 17:14
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    \$\begingroup\$ @ThePhoton "Nobody's perfect.", You're perfect. \$\endgroup\$ Commented Nov 14, 2017 at 17:14
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    \$\begingroup\$ Standing waves is what you'll see if your signal is a narrowband rf signal. Ring is what you'll see if your signal is digital. \$\endgroup\$
    – The Photon
    Commented Nov 14, 2017 at 17:15
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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.

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    \$\begingroup\$ what would have happened if it wasn't doubly terminated ? \$\endgroup\$
    – Mitu Raj
    Commented Nov 14, 2017 at 16:56
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    \$\begingroup\$ The settling is delayed. \$\endgroup\$ Commented Nov 16, 2017 at 6:34
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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:

enter image description here

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.

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