aaron_do
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Hi,
if you have distortion only when hooking all the outputs together, then probably your other switches are turning ON somehow. Check the waveforms (1.3 V is not very small). Your calculation of input and output impedance is not really correct since these transmission gates are operating passively. Therefore when turned ON, they should just act like a small resistance in the signal path. Make sure they really are operating passively (VDS = 0) otherwise you might have distortion.
Your idea about how to implement an analog MUX seems ok, but could be improved. Look up literature or datasheets on switches since they are essentially the same thing (SPDT, SP4T, SP8T etc). For instance, the way you are blocking the signal is to turn off the device to make it high impedance. However, you could implement each branch as a T-network, with one shunt branch to ground in the center to short the unwanted signal, and two branches in series which are high-impedance for the unwanted signal. This should result in much better distortion performance.
regards, Aaron
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