| ??? 01/26/05 23:44 Read: times |
#85876 - Series termination Responding to: ???'s previous message |
Payam said:
Of course there's been a couple of precautions done in carrying the paralell signals from the mainboard to the drivers, things like having connected every other wire of the flat to GND on both sides. Ok, this and that you transmit the LED supply voltage from the same point reduces the danger of susceptibility against EMI somewhat. I guess the LED display is totally isolated from any earth connected frames? Otherwise interference would run across cable, demanding symmetrical signal transmission technique, providing some common mode rejection. Ribbon flat cable sometimes show a bit unprecise characteristic impedances. So, I would heavily recommend, that you check the correct impedance of series termination resistors. This can be done by connecting the probe of scope to the recieving end of cable. Series termination is choosen properly if clean edges can be seen, without ringing and overshots. Signal must not look like a staircase! If you have a look at the drivers side, here a staircase can be seen, when using series termination technique. That's normal, nothing to worry. Take care, all measurements have to be done with a very low capacitance probe! For the case that your application violates the CE radiation tests, feed the ribbon cable through a soft ferrite choke, best at the point, where the cable leaves the board. Good luck, Kai |
