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With 4 timers of a mcu I am generating 4 square signals , each of them have an amplitud of 3.3v and are generated at diferent frequencies (8KHz 9KHz 10KHz and 11KHz), to generate a multitone signal I am adding them with an op amp adder circuit, everything is OK, but I would like to try with more of 4 signals if it is possible, What would be the maximum number of input signals for an adder circuit?, the op amp is working with 3.3 volts (ad822).

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3 Answers 3

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The more inputs you add, the more noise the op-amp produces - this is because the noise (regard it as in series with the grounded non-inverting input), gets amplified each time a new input is added. Consider the non-inverting op-amp amplifier: -

enter image description here

Now, regard Vin as the op-amp input noise with respect to 0 volts but it gets amplified as shown in the formula. Regard R2 is one limb of your mixer. Every time you add a limb, R2 gets smaller and the noise on the output gets higher. For instance, if you have one input, the output noise might be X but, with 10 inputs the output noise will be 10X.

It's not normally a show-stopper but it's something to take into account and anyway you did ask!

Clearly, the sum total of all the input voltages x gain has to produce an output voltage that avoids clipping or slew limiting as well.

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  • \$\begingroup\$ Hmm that's a subtle point. In general with a summing amp I'd expect that the input R's (R2 in your example) would be made bigger as more inputs were added.. such that the sum of all the inputs won't overload the output. \$\endgroup\$ Feb 22, 2017 at 17:14
  • \$\begingroup\$ @george yes it sounds that simple but you in effect make a bigger capacitor to ground with more inputs and that also increases noise gain. For audio you can probably get away with many inputs but I'm not sure this question or answer should be limited in that respect. \$\endgroup\$
    – Andy aka
    Feb 22, 2017 at 18:06
  • \$\begingroup\$ ah a random downvote. \$\endgroup\$
    – Andy aka
    Feb 26, 2017 at 14:30
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There is no maximum number of inputs you can use, so long as the output does not exceed the op amp capabilities.

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You could compress the signal, using diodes. Keep the resistances low, so the node-capacitance (the summing node) does not cause excessive phaseshift and turn your summer into an oscillator.

schematic

simulate this circuit – Schematic created using CircuitLab

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