How to Select an Analog Switch

The main function of analog switches and multiplexers is to switch signals. At present, integrated analog electronic switches have become the leading products in the field of small signals. Compared with the previous mechanical contact electronic switches, integrated electronic switches have many advantages, such as fast switching rate, no jitter, low power consumption, small size, reliable operation and easy control. However, there are also several disadvantages, such as large on-resistance, limited input current capacity, small dynamic range, etc. Therefore, integrated analog switches are mainly used in high-speed switching and small system size. In the lower frequency band (f<10MHz), integrated analog switches are usually made of CMOS technology: in the higher frequency band (f>10MHz), bipolar transistor technology is widely used.

The following indicators should be considered when selecting a switch:

Number of channels Integrated analog switches usually include multiple channels. The number of channels has a direct impact on the accuracy of the transmitted signal and the switching rate. The more channels there are, the greater the parasitic capacitance and leakage current. Because when one channel is selected, the other blocked channels are not completely disconnected, but are in a high-impedance state, which will generate leakage current for the conducting channel. The more channels there are, the greater the leakage current and the stronger the interference between channels.

Leakage current An ideal switch requires zero resistance when on, infinite resistance when off, and zero leakage current. However, when the actual switch is off, it is in a high-resistance state, and the leakage current is not zero. The conventional CMOS leakage current is about 1nA. If the internal resistance of the signal source is very high and the transmitted signal is a current, the leakage current of the analog switch needs to be considered. Generally, the leakage current is expected to be as small as possible.

On-resistance On-resistance flatness and on-resistance consistency On-resistance will lose signal and reduce accuracy, especially when the load in series with the switch is low impedance. In the application, a switch with sufficiently low on-resistance should be selected according to the actual situation. It must be noted that the value of on-resistance is directly related to the power supply voltage. Generally, the larger the power supply voltage, the smaller the on-resistance, and the on-resistance and leakage current are contradictory. If the on-resistance is required to be small, the channel should be enlarged, which will increase the leakage current. The on-resistance will fluctuate with the change of input voltage. The on-resistance flatness refers to the maximum fluctuation value of the on-resistance △RON=△RONMAX—△RONMIN within the limited input voltage range. It indicates the flatness of the on-resistance, and △RON should be as small as possible. On-resistance consistency represents the difference in on-resistance of each channel. The better the consistency of on-resistance, the smaller the error caused by the switch when the system collects signals from each channel.

Switching speed Refers to the speed at which a switch is turned on or off. It is usually expressed by the on time TON and the off time TOFF. For applications where fast-changing signals need to be transmitted, the analog switch is required to have a high switching speed. At the same time, it should also be considered to be compatible with the speed of the subsequent sampling and holding circuit and the A/D converter, so as to select the device with the best performance-price ratio.

In addition to the above indicators, the chip's power supply voltage range is also an important parameter, which is directly related to the switch's on-resistance and switching speed. The higher the power supply voltage, the faster the switching speed and the smaller the on-resistance. The lower the power supply voltage, the slower the switching speed and the worse the on-resistance. Therefore, for 3V or 5V voltage systems, low-voltage devices must be selected to ensure the normal operation of the system. In addition, the power supply voltage also limits the input signal range. The input signal can only reach the full power supply amplitude. If it exceeds the channel, it will be pinched off. Low-voltage devices are usually full power supply voltage amplitude, and special processes are used to ensure that the switch has a very low on-resistance at low voltage.