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I am trying to feed RS-170 video signal from a single ended amp to a differential 75 ohms video amp. When I connected RS-170 signal to + of the differential and RS-170 return to - of differential, video had a lot of flickering and noise - maybe impedance matching problem? I think it is matched 75 ohm, but maybe I am wrong. The video amp is AD8023 from Analog Device. The two systems' GNDs are connected (driving side and receiving side) via other connection, but not shown in the drawing below.

What's the best way to connect TSG350 to 75 ohms differential video amp(37.5ohm into each +/- inputs of the amp)? Please advise.

enter image description here

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  • \$\begingroup\$ We have no idea what opamp you are using there, what the bandwidth requirements are, if they are met and how the signals look like. You might want to provide those along with some measurements \$\endgroup\$
    – PlasmaHH
    May 31, 2016 at 14:56
  • \$\begingroup\$ I believe the video amp is AD8023 from Analog Device. And the signal is RS-170, plain old analog video format. \$\endgroup\$
    – Humble1
    May 31, 2016 at 15:16
  • \$\begingroup\$ Add additional information in your question, not in the comments. Also, why would you not simply use a diff-amp transmitter? \$\endgroup\$
    – uint128_t
    May 31, 2016 at 15:17
  • \$\begingroup\$ The fact that you did not connect the shielding on the right side to ground tells me that you have no clue. I suggest that you search the internet for circuits that perform this function. \$\endgroup\$ May 31, 2016 at 15:19
  • \$\begingroup\$ @Humble1: there is much room for quality degradation in component video, to debug such a situation you look at the actual signal and determine its quality and try to figure out what happened to it and if it still complies. Obviously something is wrong with the signal, otherwise you would not have the noise, so it is not just a component signal, but only something similar to it. \$\endgroup\$
    – PlasmaHH
    May 31, 2016 at 15:20

1 Answer 1

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Assuming your differential video amp's input really is 75Ω...

To minimize reflections the 75Ω cable needs to be terminated with 75Ω. The differential amp's input is 37.5Ω per leg, so you need another 37.5Ω in series to pad it out to 75Ω.

Both ends must share a common ground, so if the sending and receiving ends are remote then the cable should be connected to ground at both ends.

The differential amp is expecting to see 37.5Ω on each of its inputs. A different impedance on the inverting input may change its gain. However with the signal only going to one input the amplitude is already halved, so increasing the gain may be desirable.

If the differential amp circuit is similar to what you have shown then connecting the inverting input to ground may work. If not then terminate it with a 37.5Ω resistor, and (If possible) increase the signal amplitude at the transmitting end.

schematic

simulate this circuit – Schematic created using CircuitLab

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