How can I add three AC signals?

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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Other than with an op amp? – Ignacio Vazquez-Abrams Jun 12 '14 at 6:04
What kind of light sensor are you using? – Bitrex Jun 12 '14 at 6:32
Summing currents is trivial: just connect the current outputs, and Kirchhoff will do the work. Ar you sure your sensors have current outputs? And what about your ADC? – Wouter van Ooijen Jun 12 '14 at 9:31
Can you provide a link to the datasheet for your sensor? A light sensor with an AC output sounds like a light-to-frequency converter (such as this one). If this is what you have, you connect it to a digital (not analog) input of your microprocessor and write firmware to analyze the frequency (or period) of the pulse stream that it produces. – Dave Tweed Jun 12 '14 at 11:44

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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wait, op says he wants to sum currents... – Vladimir Cravero Jun 12 '14 at 8:40
The first circuit will sum currents to produce a voltage output. Ditto the 2nd with the addition of one resistor. – Andy aka Jun 12 '14 at 10:01
Yep you can omit R2, 3 and 4 then, they can't be used to tune the gain as suggested by the blue formula if the inputs are currents, ditto the second circuit. What I bet is that the outputs are voltages... But that's another story. – Vladimir Cravero Jun 12 '14 at 10:04
Yes, I was looking for something like a mixer circuit. I think I got my answer. Since I have three current ouputs from each sensors, I could just add tham using the kirchoff's law and connect that node to transimpedance amplifier's inverting input to convert it into approprite voltage range for ADC. Let me know if this idea seems sloppy? – dr3patel Jun 12 '14 at 15:14
@dr3patel that's exactly my first diagram with low input value resistors. Use resistors if voltage sources and no resistance needed for current signals. – Andy aka Jun 12 '14 at 16:29

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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That is a good approach but then i will have to read the values sequentially for each sensor. I want sample the sum of all three sensor output at single sample. The current of othert sensor could change while i am sampling another one. I am looking for something like a mixer of three singnals. – dr3patel Jun 12 '14 at 15:08
@dr3patel ok, I went by your statement "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" thinking that if you had three ADC channels available you would have used them. Depending on your ADC, you can probably read all three in several hundred µS to a ms; if things are changing faster than that and the extra information obtained from having individual results isn't of any value, then the summer would be better. – tcrosley Jun 12 '14 at 16:24
yeap, this make sense! – dr3patel Jun 14 '14 at 12:15