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So I am outputting up to 3, 0-5V signals from three separate DACs. The issue I have come into is that I would like to combine all three signals to form a single output of the same amplitude. Currently the only way I have been able to achieve combining them is with a Summing amp with 1/3 gain on each input so that when they are combined the output is the same amplitude again. As you would expect though, when only one source is provided the output is 1/3 of the needed output amplitude. (inverted)

How I am combining signals with a 1/3(ish) gain

I then also looked at the LM3900 multi-channel audio mixer IC (http://www.circuitstoday.com/multi-channel-audio-mixer-using-lm3900) But I am not sure if this accounts for the issue I am trying to fix. Does anyone know if the LM3900 outputs a constant signal amplitude irregardless of the input signal strength?

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What you need is an Automatic Gain Control (AGC) to keep the output level constant regardless of input level. A possible candidate is the Maxim MAX9814 which combines a microphone preamplifier and an AGC function with 20 dB of range. Since you have 3 inputs, which presumably have about the same level, then the maximum input level change is from 1 input active to all 3 inputs active or a factor of 3 which is about 10 dB. Thus the MAX9814 should be easily able to handle this range.

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A LM3900 Current Differencing Op Amp is similar to voltage-input op amps except that the inputs react to current rather than voltage.

When configured as a summing amplifier, it is going to behave exactly the same as a voltage-input op-amp.

In other words, if you have 3 input signals each with a gain of 1/3, the output is going to follow the sum of those inputs.

The LM3900 is an extremely neat chip. Because the inputs are current, you get current summing for free. This can simplify certain designs significantly.

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