Hi Guenter,
That is what we see, but for some reason the chip seems unable to float our signal at this level, so we have to help it.
It is as if it's biasing circuit is not strong enough. The AC coupling cap seems to be able to hold the signal higher, but it eventually it will drift down to 0 to 1.0V which takes it outside of the range the ADV7401 wants it to be in.
We are able to correct the issue by voltage dividing the signal to the level the ADV7401 wants it to be. In this state the ADV7401 operates correctly, however, it is more ideal to tune the ADV7401 correctly to do it on its own.
Our application places the VGA driver 0.5" from the ADV7401 and on shared ground plane, no cabling, no real potential difference between transmitter and receiver. We believe this situation, which is not typical for the device is linked to the reason why it cannot float the signal to the correct level.
Questions:
1) Can we strengthen the BIAS circuit by tuning the values connected to BIAS, CML or CAP network of the device?
2) Does the recommended bead and input filtering have an impact on the chips ability to float the signal? If so, can you explain how it works?
3) Our current filtering model includes inductor/capacitor combo, pull down resistors for both sides of cable and AC coupling (DC blocking). We find that a higher coupling capacitor allows the ADV7401 to float the signal longer, but the result is always that it slowly pulls down over time.