ADV7842 analog input resistances

Hello,

I saw ADV7842 evaluation board schematic and it uses a combination of 51.1R and 24R resistors. My question is, can I just replace them with a single 75R termination resistor to ground from each analog video signal (composite, component, s-video, rgb, vga...etc)?

thanks

  • 0
    •  Analog Employees 
    •  Super User 
    on Sep 14, 2021 6:00 PM

    No,  The analog inputs expect to see the input reduced by 50/(25+50) and the input cables expect to see 75 Ohms.  These resistors accomplish this.  Place the resistors as close to the connect as possible, then use a 50 Ohm trace to the 7842.  Also note the 50 Ohm trace impedance is a common PCB trace impedance which matches the normal digital trace impedances.  No messing around with 75 Ohm trace impedances.

  • this is interesting to read, I don't think I found this info in the datasheet but I could be mistaken. I guess this wasn't the case for ADV7800 right? if it is a must, then I will follow it.

    to my understanding, I should put those 2 resistors as close as possible to video connector itself not the adv7842 pins right? then use 50R routing for each individual trace from the 2 resistors to adv7842 pins right?

    notice that the input video will come from 75-ohm terminated cables, how will that affect the operation? and maybe it will come from non-terminated cables... please advise if there is a difference.

    thanks

  • +1
    •  Analog Employees 
    •  Super User 
    on Sep 14, 2021 9:16 PM in reply to VEGETA

    Yes, place 2 resistors close to the connector.  Use 50 Ohm traces from there.  Don't forget the DC blocking caps.

    The 25 + 50 to ground will match the 75 Ohm cable.  The 7842 input is high impedance so it really doesn't affect the cable.  The eval. board works fine up to UXGA60 which is ~154MHz data rate.

    In general you want to place the 7842 close to the connector too.  Don't run it across the board.  Although your formats are so slow the whole layout is rather tolerant.  Running long distances with poor impedance matching at high pixel rates can cause reflections causing image ghosting.  Been there, done that.  Copy the eval board.

    Older designs where based on 75 Ohm PCBs.  You'd really need to check the layout to see that it's designed that way.