Analog switches (sometimes referred to as "switches") are switching devices that switch or route analog signals (which can be at any level within a specified range) based on the level of a digital control signal. Usually consisting of a "transmission gate circuit", analog switches function like relays.
The analog switch is a tri-stable circuit that determines the state of the input and output according to the level of the selector. When the selector is in the select state, the state of the output depends on the state of the input; when the selector is in the cutoff state, the output is in the high resistance state regardless of the level of the input.
Analog switches are mainly used to turn on or turn off signals in electronic devices. Because of the low power consumption, high speed, no mechanical contact, small size and long service life, analog switches are widely used in automatic control systems and computers.
Analog switch due to the use of integrated MOS tube as a switching device to achieve the switching function; due to the physical characteristics of the MOS tube itself, the following performance indicators need to be noted when using.
#1. Switching speed
The switching speed of analog switches can generally reach megahertz speed, which can quickly achieve link switching.
#2. Switching withstand voltage
Analog open due to its application of the signal link for the electronic board low-voltage operating environment, off the withstand voltage value is generally within 15v; common 3.3v, 5v, 12v, 15, and other maximum withstand voltage value; selection must pay attention to the maximum voltage of the signal link and the maximum withstand voltage value of the device.
#3. Switch maximum current
The maximum current value that the conduction of the analog switch can withstand. Nowadays, the maximum current of the common analog switch is generally within a few hundred milliamps; ampere-level analog switches are rare.
The on-resistance of common analog switches generally ranges from a few ohms to 100 ohms; attention must be paid to this parameter when using analog switches in analog signal and weak signal designs.
#5. Off Impedance
The turn-off impedance represents the switch's ability to turn off, turn off good or bad, and the general product's turn-off impedance is sufficient to achieve the ability to suppress mutual interference between two adjacent signal links.
Although analog switches have switching properties, they differ from mechanical switches in that they also have semiconductor characteristics of their own.
1. The on-resistance (Ron) varies with the input signal (VIN)
Figure a is a simple schematic diagram of the analog switch, from the figure can be seen in the analog switch normally open and normally closed channel is actually composed of two pairs of N-channel MOSFETs and P-channel MOSFETs, can make the signal bi-directional transmission, if the different VIN values corresponding to the P-channel MOSFETs and N-channel MOSFET on-resistance in parallel, you can get Figure b parallel structure of Ron with the input.
The variation relationship of Ron with the input voltage (VIN), if the influence of temperature and supply voltage is not considered, Ron is linear with Vin, which will lead to the variation of insertion loss and make the analog switch produce total harmonic distortion (THD). In addition, Ron is also affected by the supply voltage and usually decreases as the supply voltage rises.
2. Analog switch input has a strict input signal range
As the analog switch is a semiconductor device, when the input signal is too low (below zero potential) or too high (above the supply voltage), the MOSFET is in reverse bias, and when the voltage reaches a certain value (beyond the limit of 0.3V), the switch cannot work properly at this time, or even damaged in serious cases. Therefore, analog switches in the application, be sure to pay attention to the input signal does not exceed the specified range.
3. Charge injection
Application of mechanical switches we certainly hope that the lower the Ron the better, because low resistance can reduce the loss of signal. However, for analog switches, a low Ron is not suitable for all applications. A lower Ron requires a larger chip area, which results in a larger input capacitance and consumes more current during its charging and discharging process in each switching cycle. The time constant t = RC and the charging time depends on the load resistance (R) and capacitance (C) and typically lasts for a few tens of nanoseconds. This indicates that low Ron has longer on and off times. For this reason, the selection of an analog switch should be a combined trade-off between Ron and injected charge.
4. Induced signal still leaks out when the switch is disconnected
This characteristic refers to the fact that when an analog switch transmits an AC signal, a portion of the signal is still transmitted by induction from the input to the output, or from one channel to the other, when it is disconnected. Usually the higher the frequency of the signal, the more severe the signal leakage.
5. Transmitted current is relatively small
Unlike mechanical switches, analog switches can usually only transmit small currents. Most of the current CMOS process analog switches allow continuous current transmission of less than 500mA.
6. Very small drive current at the logic control side
The drive current of the logic control side of mechanical switches is often in the milliamp level, and sometimes it is difficult to drive by digital I/O alone. In contrast, the drive current of the logic control side of analog switches is extremely small, generally below the nanoampere level. Therefore, it can be driven directly by the digital I/O, thus reducing power consumption and simplifying the circuit
Since it is called an analog switch, naturally it also has switching characteristics, which are as follows.
1. The signal can be transmitted in both directions
Some people are accustomed to calling the two normally open and normally closed terminals of an analog switch the inputs and the common terminal the output, but this is only a temporary definition based on the specific application of the analog switch. Most analog switches can transmit signals in both directions, and if this is ignored, it is easy for the circuit to generate problems, such as reverse biasing the voltage and backing up the current.
2. Very low leakage current after switch disconnection
The leakage current between the two transmission ends is extremely small, generally below the nanoamp level, such as SGM3001, SGM3002 and SGM3005 series analog switches, whose leakage current after disconnection is 1nA. Such a weak current is negligible in the application, and the analog switch can be considered as ideally disconnected at this time.
In conclusion, analog switch is a semiconductor device with switching function. In the process of application, it is necessary to make full use of its switching function and consider its semiconductor characteristics, otherwise unexpected troubles may occur.
The analog switch circuit consists of two non-gates, two field-effect tubes and one non-gate, as shown in Figure 1. The true value table of the analog switch is shown in Table I.
The operating principle of the analog switch is as follows.
When the selector E and the input A are both 1, the S2 terminal is 0 and the S1 terminal is 1. At this time, VT1 is on, VT2 is off, and the output B is 1. A=B, which is equivalent to the input and output being connected.
When the selector E is 0, and the input A is 0, then S2 is 1 and S1 is 0, then VT1 is cut off, VT2 is on, and output B is 0, A = B. This is also equivalent to the input and output being connected.
When the selected terminal E is 0, VT1 and VT2 are cutoff and the circuit output is high resistance.
From the above analysis, it can be seen that only when the elective terminal E is high, the analog switch will be turned on and information can be transmitted from A to B. When the input terminal A is low, the analog switch is turned off and information transmission is stopped.
1. High Frequency T-Switch
T-switch is suitable for video and other applications with frequencies higher than 10MHz, as shown in the figure above, it consists of two analog switches (S1, S3) connected in series, and another switch S2 is connected between ground and the intersection of S1 and S3, the switching isolation of this structure is higher than that of a single switch, because the parasitic capacitance is connected in parallel with each series switch, the capacitive crosstalk of the T-switch in the disconnected state increases with the frequency . Therefore, the key to the high frequency characteristics of the switch is the off state of the switch rather than the on state.
When the T-switch is on, S1 and S3 are closed and S2 is disconnected; when the switch is off, S1 and S2 are disconnected and S3 is closed, at which point those input signals to be coupled to the output through the parasitic capacitance of the series MOSFET are bypassed by S3. The off isolation of the 10 MHz video T-switch (MAX4545) in the off state reaches -80 dB, while the off isolation of the standard analog switch ( MAX312) has a turn-off isolation of only -36dB.
2. ESD protection switch
Based on Maxim's successful ESD protection interface products, ±15kV ESD protection circuitry has been introduced into certain analog switches. The newly introduced analog switches that can withstand ±15kV electrostatic shock are fully compliant with IEC1000-4-2Level4 standard, and all analog input paths are tested for human model ESD and the air gap discharge mode specified in IEC1000-4-2. model test.
The MAX4551-4553 pins are compatible with a variety of standard switches such as the DG201/211 and MAX391, and for multiplexer families such as the 74HC4051 and MAX4581, Maxim has also developed and produced multiplexers with ESD protection.
3. Fail-safe Switches
The supply voltage of an analog switch limits the input signal. Normally, this limitation has no effect on the use of the analog switch, but in some applications, the signal is still present at the input of the analog switch when the system is powered down, and in this case, permanent damage to the switch will be possible because the input signal is out of the range of the supply voltage.
Maxim's new analog switches and multiplexers with fault protection are capable of providing ±25V overvoltage protection and up to ±40V protection in the event of a power failure, while handling full supply swing signals and having low on-resistance. In the fault state, the input is set to a high resistance state, independent of the switching state and load resistance, and only nA-level leakage current flows through the signal source.