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can a demultiplexer take a time varying input such as a sine or square wave? I want to generate a square wave and control which load sees the square wave using a demultiplexer and a microcontroller (like an attiny)
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Digital Demultiplexer like 74HC183 can have the square wave on one of the G inputs while the others are used to enable it out onto one of the 8 selected outputs.

http://www.ti.com/lit/ds/symlink/sn74hc138.pdf

Similarly, a sine wave can be applied  to pin 3 of Analog Demultiplexer like 74HC4051, and E input used to enable it onto  one of 8 selected outputs.
The incoming sine wave will see the equivalent of 70 ohms series resistor,  which may impact the quality and/or level of the signal being used.

http://www.ti.com/lit/ds/symlink/cd74hc4051.pdf
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the data sheet says that the demux can only output 4mA, I am trying to drive piezotransducer so this would most likely be problematic. Is a demux the wrong tool?
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I may be wrong, I dont know how much current a pzt will pull off the top of my head I would have to test the ones I currently have.
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the data sheet says that the demux can only output 4mA, I am trying to drive piezotransducer so this would most likely be problematic. Is a demux the wrong tool?
No the looks like it is the wrong demultiplexer, there are many makers, typically they can deliver 30mA but the seriese resistance varies dramatically from maker to maker.
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How many loads are you trying to drive, and what type of loads are you trying to use? You probably need some combination of a demux/switch and a power amp, but there are combined single chip solutions that may suit you.
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4-6 loads which are simply piezoelectric transducers. The input signal is a square wave with a peak-to-peak voltage of 45 V
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4-6 loads which are simply piezoelectric transducers. The input signal is a square wave with a peak-to-peak voltage of 45 V
45V!!!!
You are not going to get that through a normal multiplexer. In general you can not switch signals that are outside the rail voltage of a chip.
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