Figure 6 shows more examples of the advantages of isolated, independent switches. In Figure 6a , if you provide the proper closure sequence for the switches, the output is of opposite polarity to the input, of a magnitude determined by the ratio of the capacitors. In some older families of analog switches and multiplexers, such as the classic 508, you must take care to avoid damage to the FET switches under certain circumstances. If, for example, you power down a multiplexer, signal sources could provide currents that might damage the FET switches. Most modern switches and multiplexers are more robust than are older generations.
Many devices incorporate practically foolproof overvoltage and latch-up protection, for example.
Carefully scrutinize data sheets, and consult the manufacturer if a circuit's safety is in doubt. If you disconnect the inputs, the internal logic levels become undefined, and oscillation and intermittent excessive current drain can ensue. For this reason, you should connect all unused inputs of a switch or multiplexer to either the positive or negative supply. You should be aware of some of these nonideal aspects of semiconductor-based analog switches. These devices are continuously improving in terms of such parameters as on-resistance and charge injection.