How to match impedance when driving 75ohm differential video amp with single ended RS-170 video output?
Assuming your differential video amp's input really is 75&ohm;...

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

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 37.5&ohm; 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 37.5&ohm; resistor, and (If possible) increase the signal amplitude at the transmitting end. 







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