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.
Best Answer
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.
simulate this circuit – Schematic created using CircuitLab