# How can I sum two PWM signals of different frequency with a Op Amp?

I need a suggestion how to sum two PWM signals, PWM 1 + PWM 2 to an output that have them both included.

Questions:

1. Is this possible? Other suggestions?
2. What Op Amp do I need to use if I want the output handle heavy load of 12 volt? I'm going to control a large 12 DC motor and the small frequency PWM 2 is going to be the dither signal to make sure that the 12 DC motor is still in a vibrating movement. It reduces friction and makes the 12 DC motor more linear and proportional to the PWM 1 signal.
• Are the signals different magnitudes? Usually PWM is an On/Off signal at a logic level (3.3V or 5V typically), your "summed" PWM looks to have intermediate values which really isn't PWM anymore, since some of the high levels look to be less than half, and some of the upper levels not really falling to something the other side would consider "low". This may be an X-Y problem... what are you trying to solve? Sep 24, 2019 at 12:02
• I think you may have misunderstood something you've read. A sum like that would not work. You need to AND or maybe OR the two digital signals rather than adding the analog voltages.
– JRE
Sep 24, 2019 at 12:04
• Signals are the easiest to sum if you first convert them to a current. That sounds harder than it is, have a look at: electronics-tutorials.ws/opamp/opamp_4.html Sep 24, 2019 at 12:04
• Could you provide a link to whatever gave you the idea?
– JRE
Sep 24, 2019 at 12:06
• AC-coupling the signals and then using a standard "mixer" arrangement might work? Sep 24, 2019 at 12:21

You could try a non inverting summing amplifier.

https://masteringelectronicsdesign.com/the-transfer-function-of-the-summing-amplifier-with-n-input-signals/

You put your two input values on the (+) input. The link also provides the output equation as

If you make all the resistor values the same it should come out as Vout = V1+V2

• Thats a nice sugesstion. Clean and easy. Thank you. Do you think an good old ua741 will be a good tool to use here? Sep 24, 2019 at 12:33
• I think the "good old" uA741 has not been a good tool since a few decades. Use a modern opamp. Sep 24, 2019 at 12:37
• keep in mind your pwm frequency as well, most op amps are probably fast enough to deal with the average signal but checking the op amp timings is never a bad thing Sep 24, 2019 at 12:39
• Thank you all for your help. I will select another opamp. I will check the frequency. Sep 24, 2019 at 12:43
• @VladimirCravero LM358 ? Sep 24, 2019 at 12:56