Adding +/-5V offset to video signal

Dana Petran

Other Parts Discussed in Thread: THS3001

Hello:

We are using THS3001 op amp in our new video multicoupler 100Hz-30MHz. The input is AC coupled and the driver fans out into three output amplifiers. Each output is AC coupled into 75ohm load. This part works quite satisfactory so far, but now we need to add DC offset to each output. All three offsets must be variable between +/-5V (simultaneously, using one control pot). Does anyone have good solution?

Thank you,

DanaP

over 12 years ago

0 Randy S over 12 years ago

TI__Expert 3030 points

Dana,

There are several potential methods to achieve this. I am attaching one method using TINA Spice. Let me know if you have any questions about this.

Cheers,

Randy

7713.THS3001 Video Distribution with Offset.TSC

0 Dana Petran over 12 years ago in reply to Randy S

Prodigy 40 points

Randy,

thank you much for your input. I may have not explained our circuit clearly, so I am attaching schematic in PDF. The DC offset must be added after ac coupling, at very output. Everything that we tried so far affects gain and flatness on top of it.

Dana

RF0117-SC01 no format.pdf

0 Dana Petran over 12 years ago in reply to Dana Petran

Prodigy 40 points

I forgot-the resistor 10K on the offset voltage must go away, so we are loosing isolation...

0 Randy S over 12 years ago in reply to Dana Petran

TI__Expert 3030 points

Dana,

If you need to remove the 10k resistor, then I do not see an "easy" way to add a +/-5V offset to the signal. As you saw, there will be interaction between the pull-up/resistor and the video signal. One way around this is to add anotehr amplifier at the outputs, but this makes no sense.

I would argue to simply DC couple the output of the THS3001 amplfiers in a similiar manner to what I porposed. You reach the same end-goal with this approach.

0 Dana Petran over 12 years ago in reply to Randy S

Prodigy 40 points

Randy,

you are absolutely correct, thank you. Very helpful!

Dana

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Adding +/-5V offset to video signal