Switching devices and their selection

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1. Different types of switches 　　The commonly used devices include reed relays, dry-contact electric relays, mercury-contact relays, and CMOS analog switches. 　　Among the first three types of relay switches, the mercury contact relay offers better performance (or at least equal performance) to the first two types, aside from its higher price. Therefore, in previous data acquisition systems, mercury contact relays were an excellent switching device. However, in recent years CMOS analog switches have emerged and have been widely used. Its performance is vastly different from that of the previous three relay switches, with some parameters being much worse than those of the relay switches. For example, it has a high on-resistance and low voltage rating, while most of its parameters are much higher than those of relay switches. Therefore, people strive to mitigate the impact of high on-resistance by adopting measures such as overvoltage protection, in order to overcome these disadvantages and make use of its advantages to create data acquisition systems that are highly reliable, compact, fast, and inexpensive. 　　2. CMOS analog switch Its key feature is its ability to handle analog input voltages that are much higher than the supply voltage (V+ = +20V, V- = -20V). It does not suffer from self-locking, has a high allowable supply voltage (±20V), low leakage current, but a relatively high on-resistance. It is suitable for harsh field environments. The AD7501/AD7502 feature low leakage current, low on-resistance, and low voltage difference, making them suitable for applications with high precision requirements, such as scientific research instruments. Meanwhile, CD4051/CD4052 and CD4067/CD4097 represent the cheapest standard types. Using them can reduce costs, but they have a relatively high on-resistance, a limited power supply voltage range, and especially high leakage current. This demands even more reasonable design.