Tuesday, July 06, 2010

High-Speed, Current-Feedback Amplifier Drives and Equalizes up to 100-m VGA Cables

In classrooms, lecture halls, and conference rooms, PCs transmit RGB video signals to projectors through VGA cables. The cable length depends on the room size and ceiling height, but most cables are shorter than 100 m. This article shows how the ADA4858-3 triple high-speed current-feedback op amp with integrated charge pump can drive and equalize up to 100 m of VGA cable.


At 9:05 AM, Blogger Joseph Hogan Wilks said...

I never knew that there was such a loss of signal for how long the cable was. Is it the same way for both analog cable and digital cable? I suppose it does not matter if the cable is optimized like you have shown. I guess the expensive cables really do make a difference.

At 9:54 AM, Anonymous Anonymous said...

Hi Joseph,
Digital cable are looking for the eye pattern, cable fedelity is not as important. All you need is to recover the info 1 vs 0. However, for long cables and fast bitrate the data might not be recoverable. This is when it would be good to use equalization like the one we show in this article. It all comes down to maintaining the frequency and loss through the length of the cable.

At 10:40 AM, Anonymous Charly El-Khoury said...

Do expensive cables make a difference?
The answer is: it all depends.
If 2 cables have the same loss vs length and frequency but one cost more then the other, than you should not buy the more expensive one.
However if a cable manufacturing comapany is charging more for a cable, this mean that the performance of that cable should be better. i.e. less cable loss vs length, or frequency, Less crosstalk... This could be achieved using different type of material, like copper vs. aluminum. Or they could be well insolated for less crosstalk and so forth.
So always look at the cable specs.

At 4:46 AM, Blogger auckaradej said...

I need To Know to value of R1-R7,C1-C6 and ZD1

At 11:52 AM, Anonymous Anonymous said...

R1, R2, R3 are the input terminations so if you have a 75 Ohm system then you make them equal to that.
R4, R5 and R6 are the output series impedance, in video world you make them 75 ohms.
ZD1 is a zener you can leave open if you are using USB power.
C2 = 10 uF, C3=0.1uF, C5 = C4 = C6 = 1uF.


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