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I have three light sensors which outputs appropriate AC currents with respect to light intensities. I only have one pin left for my ADC IC that I have on my board so I can't sample three sensors using my ADC. What I want is that I want to average those three light sensor's current by sampling their sum feeding that number into one of my ADC and dividing it by 3 using micro controller and so on. I need help with adding three AC signals. I can't find out appropriate circuit for it.

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Use an op-amp mixer circuit. This will add three signals into one: -

Alternatively, if you did the math you would find that three independent voltage sources connected together thru 3 resistors of equal value would produce a node voltage that is the average of the three voltages. This might be easier: -

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An alternate to summing all three together would be to add a 4-channel analog mux, such as the ADG804 to your single ADC input, and retain the ability to read each sensor individually. The part is available at Digi-Key for under $3.

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