# Forum: Analog Circuits Combining 2 analog signals

 Author: Ersin Oezalp (Company: BARAN) (baran-tech) Posted on: 2009-03-30 15:03

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Hello forum,

I have two analog signals. The first one is 0..+30V and the second one
is 0..-30V volt.
How can I combine them to form a -30..30V signal.

Thanks

 Author: Andreas Schwarz (andreas) (Admin) Posted on: 2009-03-30 15:41

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How do you want to combine them? Do you want to add the voltage of both
signals? You have to put your problem in mathematical terms before you
can think about implementing a solution.

 Author: Ersin Oezalp (Company: BARAN) (baran-tech) Posted on: 2009-03-30 17:13

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I actually want to add them. I tried to solve the problem with an OPAMP,
however I need a fast switching circuit. The slew rate of the high
voltage OPAMPs are very low.

 Author: lkmiller (Guest) Posted on: 2009-03-31 08:28

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> The slew rate of the high voltage OPAMPs are very low.
Whats the required bandwidth? Maximum frequency?

> The slew rate of the high voltage OPAMPs are very low.
What about a typical OPAMP (+-15V) and a discrete output stage?

 Author: Andrew Taylor (marsufant) Posted on: 2009-03-31 09:00

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Ersin Oezalp wrote:
> I actually want to add them. I tried to solve the problem with an OPAMP,
> however I need a fast switching circuit. The slew rate of the high
> voltage OPAMPs are very low.

For integrated opamps, this may be true.

If you need more than 2 MHz GBW, you will have to check for discrete
designs.
Fast video final amps will do so.

The good ol tekronix 465 (80s design) already solved this for the
vertical final amp (1 GHz GBW). Check the service manual, to get some
idea how to design this. Both discrete and ic solution.

ht,
Andrew

 Author: Ersin Oezalp (Company: BARAN) (baran-tech) Posted on: 2009-03-31 10:44

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Thanks a lot.

I found two OPAMPs from the company Cirrus Logic, PA69 and PA78. They
are expensive but I think they will solve my problem. Does anybody has
any experience with these type of OPAMPs.

 Author: 2345 (Guest) Posted on: 2009-03-31 12:00

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Yes. They work. What kind of signals do you want to add ?

 Author: Alexander Liebhold (lippi2000) Posted on: 2009-04-04 12:09

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Another way to fix this problem is to use two equivalent voltage
dividers to scale down the input voltages. Then you are able to use a
common OPAmp. After the adding stage you need a output amplifier stage
with a gain factor (reciprocal to scaling factor). I believe that's a
verry fast and precission way.

I hope you have the possibility for a spice simulation.

You can get a good spice tool from texas instruments (Tina TI).

Alexander

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