# Common-Mode Voltage

I have an application which is using a differential transmitter to transmit a video signal(PAL).Transmitter is placed in a remote device which is connected through a long cable.The device is powered up through two conductors(one for 24V and one for GND) which each conductor has 1.8ohms.My doubt is if the device consumes 5 amps I will have a ground difference of about 9V, so from start the common mode voltage on the twisted pair of differential transmitter will be 9V.Is it true ?

– user76844
Dec 10, 2015 at 21:16
• Yes, cable has about 200m.It s normal his resistance. Dec 10, 2015 at 21:18
• In that case it's a crime to use 24V. Use 96, even 48. Just think, how much power you waste. As for the video, you must use isolated driver, so the gnd voltage will not matter.
– user76844
Dec 10, 2015 at 21:19
• I know I will change to 220V AC an decrease the voltage with a transformer.I just want to know if my assumption is correct about common mode voltage. Dec 10, 2015 at 21:20
• What about the receiver? Where is it, how is it powered?
– user76844
Dec 10, 2015 at 21:21

Assuming that the differential signals are generated relative to ground, then the answer would be yes, as seen from the other end.

The ground would have floated up by 9V relative to the supply's ground, and the 24V would float down by the same amount - leaving only 6V at the transmitter end.

Not actually relevant to the question asked, but do you really need 5 amps to power what sounds like a camera? If so, you're going to need a higher voltage.

• Well, this is a crawler for pipe inspection, so it is equipped with some type of motors for movement.I know that it is a waste of power but this was only a prototype.Right now I am changing the whole design, the power will be transmitted AC at 220V and in the crawler I will have a power transformer to step down the voltage. Dec 11, 2015 at 6:47

Well the receiver is powered from the start of wires, so the remote device is powered through that resistance of cable.The ground difference at 5 amps is 9V(1.8 * 5).The receiver is powered up from the same supply just that the resistance to ground is almost 0.

Sorry I wanted to be a comment.

• Just use isolated supply to disconnect the gnd.
– user76844
Dec 10, 2015 at 21:37
• Not necessarily. Depends on specific output stage.
– user76844
Dec 10, 2015 at 21:40
• The point of differential output is 4hat common mode component may change and it doesn't matter. So it may be 0 or 9v or 24v, the signal is in diff mode, s8 it doesn't matter. But i hardly believe that your output buffer has such wide range, and i also don't think that your receiver will live well with signal created 9V above.
– user76844
Dec 10, 2015 at 21:43
• Trust me, i understand you completely. Which is why i suggest following my advices: use higher voltage, isolate your circuit from input voltage.
– user76844
Dec 10, 2015 at 21:50
• Look, having isolated power supply will allow you to ignore the voltage drop across the wires.
– user76844
Dec 10, 2015 at 21:53