What Is an Analog Switch?

The analog switch mainly completes the signal switching function in the signal chain. The switching mode of the MOS tube is used to turn off or on the signal link. Because its function is similar to a switch, it is implemented with the characteristics of an analog device and becomes an analog switch.

Analog switches in electronic equipment mainly play the role of on or off signals. Because analog switches have the characteristics of low power consumption, fast speed, no mechanical contacts, small size and long service life, they have been widely used in automatic control systems and computers. ^[1]

Chinese name: Analog switch

Foreign name: Analog switches

Analog switch

Analog switch, English name Analog switches; mainly completes the signal switching function in the signal link. The switching mode of the MOS tube is used to turn off or on the signal link. Because its function is similar to a switch, it is implemented with the characteristics of an analog device and becomes an analog switch.

Classification of analog switches

Due to the breaking performance of the MOS tube, the analog switch circuit can achieve a high turn-off impedance, which is generally a turn-off impedance above megaohms; and a very low turn-on impedance, which is generally a few ohms, so it can Good implementation of signal link switching and disconnection. According to different application requirements; analog switches can be divided into audio analog switches, video analog switches, digital switches, general analog switches, and so on.

Multi-channel analog switch

It is a circuit that selects the required input channel analog input signal from multiple analog input signals. Field effect transistors are widely used as analog switches. The advantage is that the working speed can reach 10 to the third power of / 3, the on-resistance is low (5 to 25 ohms), and the off-resistance is up to 10 to the tenth power.

Studies have shown that: only by properly selecting the type of multiplexer, paying attention to the reasonable matching and coordination of the multiplexer and related circuits, and ensuring that each circuit unit has a proper working state, can the performance of the multiplexer be fully utilized, and even make up for the performance indicators Lack of results and expected results. The current multi-channel switches on the market are mainly CMOS circuits.

Performance indicators of analog switches

Because the analog switch uses the integrated MOS tube as the switch device to achieve the switching function; due to the physical characteristics of the MOS tube, you need to pay attention to several performance indicators when using it:

Analog switch switching speed:

The switching speed of analog switches can generally reach the speed of terahertz, which can quickly realize link switching.

Withstand voltage of analog switch :

Because the signal link of its application is the low-voltage working environment of the electronic board, the withstand voltage value of the switch is generally within 15v; there are common maximum withstand voltage values such as 3.3v, 5v, 12v, 15, and so on; you must pay attention to the signal link The maximum voltage and the maximum withstand voltage of the device.

Analog switch maximum current:

The maximum current value that the analog switch can withstand. The current maximum current of the common analog switch is generally within a few hundred milliamps; there are very few amp-level analog switches.

Analog switch on resistance:

The on-resistance of common analog switches generally ranges from a few ohms to 100 ohms; when designing analog signals and weak signals, you must pay attention to this parameter.

Analog switch off impedance:

The turn-off impedance represents the turn-off ability of the switch, and the turn-off is good or bad. The turn-off impedance of general products is sufficient to achieve the ability to suppress mutual interference between two adjacent signal links.

Several special analog switches:

1.High-frequency T-switch

T switch is suitable for video and other applications with frequency higher than 10MHz. As shown in Figure 4, it consists of two analog switches (S1, S3) connected in series, and the other switch S2 is connected between ground and the intersection of S1 and S3. The switch of this structure has higher off-isolation than a single switch. As the parasitic capacitance is connected in parallel with each series switch, the capacitive crosstalk of the T-switch in the open state increases with the frequency. Therefore, the key to affecting the high-frequency characteristics of the switch lies in the off state rather than the on state of the switch ^[2] .

When the T-type switch is on, S1 and S3 are closed and S2 is open; when the switch is off, S1 and S2 are open and S3 is closed. At this time, those input signals that are to be coupled to the output terminal through the parasitic capacitance of the series MOSFET are S3 bypass, the off-state isolation of the 10MHz video T-switch (MAX4545) in the open state is -80dB, while the off-isolation of the standard analog switch (MAX312) is only -36dB.

2.Miniature package

The advantages of CMOS switches also include the small package size. For example, the 6-pin SOT23 switch does not contain any mechanical parts (unlike reed relays). The small video switch (MAX4529) provided by Maxim and the standard low-voltage SPDT switch (MAX4544) are all 6-pin SOT23 package, power supply range is 2.7V to 12V. In addition, Maxim has a variety of general-purpose analog switches like CD4066, such as the newly released MAX4610-MAX4612 low-cost four analog switches. Among them, the MAX4610 is pin-compatible with the industry-standard 4066 and can work at lower supply voltages (down to 2V), with higher accuracy, the maximum mismatch resistance between channels is 4; the flatness is within 8. These models have three different switch settings. Low on-resistance (less than 100 at 5V) is suitable for low voltage applications. The compact 14-pin TSSOP package (6.5 x 5.1 x 1.1 mm3) solves the problem of tight circuit board size. ^[2] .

3.ESD protection switch

Based on Maxim's successful ESD protection interface products, ± 15kV ESD protection circuits have been introduced into some analog switches. The newly introduced analog switches that can withstand ± 15kV electrostatic shock fully comply with the IEC1000-4-2 Level 4 standard, and all analog input channels All have passed the human body model ESD test and the air gap discharge mode test specified in IEC1000-4-2.

The MAX4551-4553 is compatible with a variety of standard switches (such as DG201 / 211 and MAX391, etc.). For multiplexer series products, such as 74HC4051 and MAX4581, Maxim also develops and produces multiplexers with ESD protection. . The new design eliminates the need to protect the analog inputs with expensive TranszorbsTM devices.

4.Failure protection switch

The power supply voltage of the analog switch limits the range of the input signal. In general, this limitation has no effect on the use of the analog switch, but in some applications, the signal still exists on the input end of the analog switch when the system is powered off. Because the input signal is out of the range of the power supply voltage, it will cause permanent damage to the switch.

Maxim's new analog switches and multiplexers with fault protection can provide ± 25V overvoltage protection, protection voltage up to ± 40V at power down, and can handle full power swing signals with low on resistance . In the fault state, the input terminal is set to a high-impedance state, which has nothing to do with the switching state and load resistance. Only nA level leakage current flows through the signal source ^[2] .

5.Load-induction switch

Over the past few years, Maxim has introduced a series of new analog switches, the MAX4554-MAX4556 load-sensing switches for Kelvin detection in automatic inspection equipment (ATE). Each model includes a low resistance for loading current, a high current switch, and a high resistance switch for detecting voltage or switching protection lines. When ± 15V power is supplied, the on-resistance of the current switch is only 6, and the on-resistance of the inductive switch is 60. The MAX4556 has three built-in SPDT switches.

The load-sensing switch is mainly used for high-precision systems and long-distance measurement systems (Figure 5). In 4-wire measurement, 2 wires load the voltage or current of the load, and the remaining 2 wires are directly connected to the load sense wire. In a 2-wire system, the load voltage sensing line and the load line are connected to both ends of the load. Because the load voltage or current will cause a voltage drop along the line, the load voltage is slightly lower than the signal source voltage. The distance between the load and the signal source, as well as the load current and on-resistance, will cause the load voltage to decay. The 4-wire method can reduce signal fading, and the current flowing through the 2 additional voltage sensing lines in the 4-wire method can be ignored. The new load-sensing switch simplifies many applications, such as nV-level voltmeters, femto-ohm resistors, etc. ^[2] .

6.Calibration Multiplexer

Calibration multiplexers (Cal-muxes) are mainly used in high-precision A / D converters and self-monitoring systems. The combined structure in the chip mainly includes: analog switches that generate precise voltage ratios from the input reference voltage, and high-precision resistors Voltage divider for selecting a multiplexer with different inputs. In this class of devices, the MAX4539 and MAX4540 can be used to correct two main errors in A / D conversion: offset error and gain error. Utilize the internal precision voltage divider to measure the gain and offset voltage under the control of the serial interface of the microprocessor. The reference ratios are 15/4096 and 4081/4096, accurate to 15 bits. For A / D conversion with a reference of 4.096V The reference voltage of 15/4096 times is 15mV and the binary digital output is 000000001111. In order to measure the offset error, the controller records the difference between the binary number 000000001111 and the actual output of the ADC and uses this error value to correct the offset voltage. To measure the gain error, the calibration multiplexer is replaced by 4081/4096 (VREFHI-VREFLO). The microcontroller records the difference between the binary number 111111110000 and the actual ADC output. After the ADC's offset error and gain error are known, the system software can establish The correction coefficient adjusts the subsequent output to the correction value. ^[2]