Connection of NTSC analog video signal input to ADV7280

Question

The standard circuit of ADV7280 data sheet is as follows,

If the input NTSC analog video signal has a long distance from the housing connector to Ain 
 and there is also a relay on the FPC connector (see below),

is it better to receive it with a differential operational amplifier somewhere? 
 Or is it okay if I don't care too much?

<Route from input NTSC analog video signal to Ain>
Housing connector-(PWB pattern 50mm*)-FPC connector-(FPC pattern 70mm*)-FPC connector-(PWB pattern 50mm*)-Ain
* Both PWB pattern and FPC pattern have impedance matching at 75Ω.

Then, for the GND that is the return side of the NTSC analog video signal, 
 which is better, a normal pattern or a solid pattern, in the above PWB/FPC path? 
 
 Thank you, 
 
 o-taka

GuenterL · Accepted Answer

As Poornima has said video is based on 75 Ohm impedance. If the cable from the housing to the board is kept at 75 Ohms you should be OK without differential op-amps. Differential op-amps are good for noisy environments. I am not sure what you mean by normal or solid pattern. If the cable from the housing to the PCB is 75 Ohms it's good 
 Often PCBs are designed for 50 Ohm traces so if the incoming video connector is 75 Ohms you can place the 24/51 Ohm resistors right next to the connector and convert the video path to 50 Ohms and run it to Ain. Also the 24/51 Ohm network is required to reduce the video amplitude to the working range of the ADV7280 input. 
 I you have mis-matches, you will see ghosting in the video depending on the length of the mismatch. Small segments of mismatch won't be a problem. Ghosting is caused by the distance a reflection has to travel and with a large pixel time width and signals speed of PCBs about 20 cm/ns, CVBS can handle some impedance mismatch lengths. 
 Hope this helps

Electronic Design Services - PartnerZone

Connection of NTSC analog video signal input to ADV7280

Top Replies