# Digital signal into the video cable

The input signal is simply composite sync signal, without any additional video information. It is CMOS - 0 to 5V levels. I need the same type of signal to appear in the cable along with the RGB lines.

The difficulties I face relate to the best way to tranform the signal - in terms of level and impedance.

First, the 5V CMOS signal can be easily transformed into 300 mV-pp or 600 mV-pp signal using simple resistor network, not introducing much currents and affecting logic output of the previous chip:

Vref will simply be a positive supply rail, and I can, and most probably will, DC bias the resulting signal somwhere within 1-2 V (relatively far away from the power rails).

As for the operational amplifier, I would be very happy using ADA4891 device for the purpose.

And then I have difficulties to decide how to proceed, as I need to match output impedance of the operational amplifier with the cables' impedance (75 Ohms).

1. Use G=+2 configuration. I will make input signal 300 mV-pp, and it will be converted to 600 mV-pp, and then divided back to 300 mV-pp by the 75-Ohm resistor network.

However my environment does assume single supply power. I read somewhere (e.g. here) that putting gain resistor onto the ground in this configuration is bad idea as it creates unnecessary currents from the output to ground (=to negative power rail of opamp), and capacitor must be used. Then it is stated that gain and capacitor will form a filter which will change the frequency response of the circuit. That sounds somehow complicated. I reviewed a number of sources and some of them signify these problems, but most just connect Rg onto the ground without further exploring the issues.

1. Use G=+1. It will be as simple as voltage follower, with no relation of the feedback to the power rails. I will make input signal as 600 mV-pp, and just translate it to 600 mV-pp at the output with low impedance - and then divided to 300 mV-pp by the 75 Ohm resistor network. That would be so simple if datasheet would not constantly recommend using this operational amplifier in G=+2 configuration for the video, even in its headline, e.g. Video specifications (G = +2, RL = 150 Ω).

Can I use the op-amp in G=+1 configuration as explained in point 2 and what risks will it present? Probaly datasheet has technical information on it, but I am unable to derive it out of there. Analysing plots does not give me any warning signs of possible problems...