Propeller Backpack Video Problem

sonny

I have a small surplus of these small TFT displays. I can drive the display with the composite video output from a DVD player with no problem. I can drive a TV monitor with the backpack with no problem as well. However, if I try to drive the display with the backpack, it cooks the composite video input of the display and its no longer usable. The first one I thought was a fluke. The second one though I confirmed worked perfectly before hooking it up to the backpack, now it just gives a white screen.

So, as far as power and ground go, the display takes 8 volts. The ground shares the same ground as the composite video shield wire. The backpack is on that same ground as well, but is powered from a separate 5 volt regulator.Does anybody know of a way to prevent this damage from happening? I feel like I need to isolate the grounds from each other, but cannot figure out how. Maybe im totally off base....

Thanks

Sonny

Phil Pilgrim (PhiPi)

Sonny,

The backpack expects the displays it drives to have a 75-ohm input impedance. Check to make sure the resistance across the display's input leads is 75 ohms. If it's not, add a 75-ohm resistor across the Backpack's video output. This will limit the output voltage to 1V P-P from 2V P-P. Still, though, it would be highly unusual for a 2V signal to destoy a video input, since all non-loaded video sources output the same 2V P-P signal.

You could also have a common-mode voltage difference between your two grounds. Disconnect the Backpack from the monitor, and connect the grounds together through a 100K resistor and measure the voltage across it. It should be zero.

-Phil

sonny

The input to the display does measure 75 ohms. I didn't physically check for common mode voltage differences on the grounds, but all grounds to tie back to 1 single point, so I assume that would eliminate that (if I understand common mode correctly).

Also, I did check the datasheet for the display and it lists the Vp-p to be 1 volt typical and 2 volts max.

I did also verify that the two grounds going in to the display are connected together by a fairly wide trace on the board.

Sonny

Phil Pilgrim (PhiPi)

If there is a voltage difference between the grounds when they're connected by a 100K resistor, there could be destructive ground currents flowing when they're connected wire-to-wire. That's why I wanted you to check the voltage difference.

-Phil