Learn the basics of analog SWITCHES before designing analog circuits.

For example, analog SWITCHES are primarily used when you want to insert SWITCHES into a circuit, such as for switching LEDs, POWER SUPPLIES supplies, audio signals on/off, or analog signals. Analog SWITCHES are connected in series with electrical signals or POWER SUPPLIES lines and can switch between ON and OFF. While SWITCHES have mechanical contacts, this explanation will focus on analog SWITCHES composed of semiconductor circuits.

Products are available with a wide range of compatible POWER SUPPLIES voltages, from a few volts to over 100 volts. Furthermore, for the negative POWER SUPPLIES, there are types that support not only GND but also other negative voltages.
The inside of an analog SWITCHES is basically composed of semiconductor SWITCHES, specifically a ch channel MOS FETS and a p channel MOS FETS, as shown in Figure 1. The ON/OFF state of MOS FETS is controlled by applying a voltage to the gate of MOS FETS.

Figure 1: Symbol and internal configuration of an analog SWITCHES

When the positive POWER SUPPLIES is VDD and the negative POWER SUPPLIES is GND, the voltage range of the input signal is basically limited to GND to VDD. For example, as shown in Figure 2, if VDD = 5V, a voltage of 0 to 5V can be input to IN. When IN is around 0V, the VGS of the nch MOS FETS becomes above the threshold voltage and turns ON (pch is OFF). When IN is around 5V, the VGS of the pch MOS FETS becomes above the threshold voltage and turns ON (nch is OFF). When IN is around 2.5V, both nch and pch are above the threshold voltage and both are ON.

Figure 2 Path due to input voltage

When an analog SWITCHES is OFF, it has high impedance, but when it is ON, the nch/pch MOS FETS that make up SWITCHES can be considered as resistive elements, and thus act as the ON resistance of the nch/pch MOS FETS. In applications where little current flows, you don't need to worry too much about the ON resistance, but in applications where a certain amount of current flows, you need to be careful about the power loss in this resistance. Also, when dealing with AC signals or when the signal is amplified in a subsequent stage, this ON resistance can affect the signal, so caution is necessary.
As explained in the "Operation" section, the ON/OFF state of nch and pch changes depending on the input voltage, as shown in Figure 3, and the ON resistance also changes accordingly. Therefore, this point must also be taken into consideration.

Figure 3: Change in ON resistance due to input voltage