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Due to the switching performance of the MOS tube, the analog switch circuit can achieve a high turn-off impedance, generally, a turn-off resistance of more than megaohm; and a very low on-resistance, generally a few ohms. So it can realize signal link switching and disconnection isolation. According to different application requirements, analog switches can be divided into audio analog switches, video analog switches, digital switches, general analog switches, etc.
Figure1. analog switch
It is an analog input signal circuit that switches and selects the desired input channel from multiple analog input signals. Field-effect transistors are widely used as analog switches. Its advantages are that the working speed can reach 10 to the 6th power/3, the on-resistance is low (5~25 ohms), and the cut-off resistance is as high as 10 to the 10th ohms.
Research has shown that: only by correctly selecting the type of multiplexers, paying attention to the reasonable matching and coordination of multiplexers and related circuits, and ensuring that each circuit unit has a proper working state, can the performance of the multiplexers be fully utilized, and even compensate for certain performance indicators. At present, the multiplexers on the market are mainly CMOS circuits.
The analog switch uses an integrated MOS tube as a switch device to realize the switching function. Due to the physical characteristics of the MOS tube itself, you need to pay attention to the following performance indicators when using it:
Switching speed:
The switching speed of the analog switch can generally reach the speed of mega Hz, which can quickly realize the link switching.
Switch withstand voltage:
Since the signal link of the analog switch is the low voltage working environment of the electronic board, the withstand voltage value is generally within 15v. Common maximum withstand voltage values are 3.3v, 5v, 12v, 15, etc. Signal link must be paid attention to when choosing the maximum voltage and the maximum withstands voltage of the device.
Maximum switch current:
The maximum current value that the analog switch can withstand when the analog switch is turned on. The maximum current of the common analog switch is generally within a few hundred milliamperes. There are very few ampere-level analog switches.
On resistance:
The on-resistance of common analog switches generally ranges from several ohms to 100 ohms. This parameter must be paid attention to when designing analog signals and weak signals.
Turn-off impedance:
The turn-off impedance represents the turn-off capability of the switch, whether it is good or bad. The turn-off impedance of a general product is sufficient to suppress the mutual interference of two adjacent signal links.
The T-switch is suitable for video and other applications with a frequency higher than 10MHz. As shown in Figure, it consists of two analog switches (S1, S3) in series. The other switch S2 is connected between the ground and the intersection of S1 and S3. The switch of this structure has higher turn-off isolation than a single switch. Because the parasitic capacitor is connected in parallel with each series switch, the capacitive crosstalk of the T-type switch in the off state increases with the increase of frequency. Therefore, the key to affecting the high-frequency characteristics of the switch is the off state of the switch rather than the on-state.
Figure 2. High-frequency T-switch
When the T-type switch is turned on, S1 and S3 are closed, and S2 is open. When the switch is open, S1 and S2 are open, and S3 is closed. At this time, those input signals that must be coupled to the output through the parasitic capacitor of the series MOSFET are bypass by S3. The off isolation of the 10MHz video T-switch (MAX4545) in the off state is up to -80dB, while the off isolation of the standard analog switch (MAX312) is only -36dB.
The advantages of CMOS switches also include 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 used 6-pin SOT23 package, the 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, MAX4610 pins are compatible with the industry-standard 4066 and can work at a lower power supply voltage (as low as 2V). With high accuracy, the maximum mismatch resistance between channels is 4Ω. These models have three different switch settings. Low on-resistance (less than 100Ω 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.
Based on Maxim's successful ESD protection interface products, ±15kV ESD protection circuits are introduced into some analog switches. The analog switches that can withstand ±15kV electrostatic shocks are fully compliant with the IEC1000-4-2 Level 4 standard. All analog input channels have passed the human body model ESD test and the air gap discharge mode test specified in IEC1000-4-2.
MAX4551-4553 pins are compatible with a variety of standard switches (such as DG201/211 and MAX391, etc.). For multiplexer series products, such as 74HC4051 and MAX4581, Maxim has also developed and produced multiplexers with ESD protection. In the new design, there is no need to use expensive TranszorbsTM devices to protect the analog input.
The power supply voltage of the analog switch limits the range of the input signal. Generally, this limitation has no effect on the use of the analog switch. However, in some applications, there is still a signal at the input end of the analog switch when the system is powered off. Because the input signal exceeds 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. The protection voltage can reach ±40V when power is off. It can handle full power swing signals and has low on-resistance. In the fault state, the input terminal is set to a high-impedance state, regardless of the switching state and load resistance, only nA leakage current flows through the signal source.
Maxim has introduced a series of new analog switches, including MAX4554-MAX4556 load-sensing switches, which are suitable for Kelvin testing in automatic testing equipment (ATE). Each model includes a low-resistance, high-current switch for loading current and a high-resistance switch for detecting a voltage or switching protection lines. When powered by ±15V, the on-resistance of the current switch is only 6Ω, and the on-resistance of the sensor switch is 60Ω. The MAX4556 has three built-in SPDT switches.
Figure 3. 4-wire measurement
Load sensing switches are mainly used in high-precision systems and long-distance measurement systems. In the 4-wire measurement, the 2-wire is the load loading voltage or current, and the remaining 2-wires are directly connected to the load-sensing line. In a 2-wire system, the load voltage sensing line and the loading line are connected to both ends of the load. Because the load voltage or current will produce a voltage drop along the line, the load voltage is slightly lower than the signal source voltage, and the distance between the load and the signal source, load current, on-resistance, etc. will cause the load voltage to attenuate. The 4-wire method can reduce signal fading, and the current flowing through the two additional voltage sensing wires in the 4-wire method can be ignored. The new load-sensing switch simplifies many applications, such as nV-level voltmeters, Fei-ohm-level resistance meters, etc.
Calibrated 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 a precise voltage ratio from the input reference voltage and a high-precision resistor Voltage divider. Among these devices, MAX4539 and MAX4540 can be used to correct two main errors in A/D conversion: offset error and gain error. Using the internal precision voltage divider, the gain and offset voltage is measured under the control of the serial interface of the microprocessor. The reference ratio is 15/4096 and 4081/4096, accurate to 15 bits. To measure the offset error, the controller records the difference between the binary number 000000001111 and the actual output of the ADC and uses the error value to correct the offset voltage. In order to measure the gain error, the calibration multiplexer is replaced with 4081/4096 (VREFHI-VREFLO). The microcontroller records the difference between the binary number 111111110000 and the actual output of the ADC. After the offset error and gain error of the ADC are known, the system software can establish the correction coefficient, adjust the subsequent output to the correction value.
When switching between audio and video signals, the difficulty lies in how to avoid introducing noise and signal loss due to device resistance or incidental capacitance. Although CMOS analog switches are both effective and efficient, designers need to understand the key parameter trade-offs to use them properly. Switching between audio or video sources can be tricky. Most mechanical switches or relays are not designed for switching multimedia signals and may cause interference, such as loud pops or visual interference. The switch circuit can be designed from scratch, but this will increase design complexity and time.
To solve this problem, a simple CMOS analog switch can be used. Their working principle is similar to that of small semiconductor relays, allowing current to flow in both directions with low loss. With features such as opening-before-closing and low on-resistance, audio or visual noise during switching can be eliminated while reducing signal loss. But in practice, before using analog switches, designers also need to consider the trade-offs of various specifications.
The analog switch uses parallel P-channel MOSFET and N-channel MOSFET to create a bidirectional switch. ON Semiconductor's NS5B1G384 SPST normally closed analog switch is a simple CMOS analog switch example (Figure below). The control input sends the appropriate inverting and non-inverting signals to the MOSFET gate according to whether the device configuration is normally open (NO) or normally closed (NC).
Figure 4: High-level representation of a simple SPST analog switch. A single contact is turned on and off according to the state of the control input signal IN
Ideally, analog switches should have the lowest possible switch resistance (RON). The way to achieve this is to design a CMOS switch to increase the drain/source area of the MOSFET to create more surface area for the flow of electrons and reduce the on-resistance. However, increasing the surface area has the disadvantage of increasing parasitic capacitance. At higher frequencies, this parasitic capacitance may become a problem, that is, forming a low-pass filter and causing distortion. Capacitors also cause propagation delays due to charging and discharging time.
When choosing a CMOS switch for a given application, the trade-off between RON and parasitic capacitance is the key. Not every application requires low RON, and in some cases, analog switches are connected in series with resistive loads, making RON negligible. But for video signals, it becomes important to weigh RON and parasitic capacitance. As RON decreases, the parasitic capacitance will increase. This will cut off high-frequency signals, resulting in bandwidth reduction or distortion.
For the NS5B1G384 case shown in Figure, the device has a low RON of 4.0 Ω (typical). The parasitic capacitance is very low, 12 picofarads (pF), so this switch can be applied to signals up to 330 MHz.
To switch the audio input signal between the two audio signal outputs, the audio input must be connected to the COM pins of the two NS5B1G384 switches. Connect the NC pin of each switch to its respective transducers, such as earphones and speakers. Please note that only one IN pin can be selected at a time.
In this configuration, the on-time and off-time of the analog switch become important. For NS5B1G384, the on-time is 6.0 nanoseconds (ns) and the off time is 2.0 ns. When multiple switches are used, a faster turn-off time can achieve the open-before-close function. This ensures that one switch is disconnected before the other switch is connected, thereby preventing two loads from being connected at the same time. This also reduces the popping noise that is heard on audio equipment from time to time when switching audio signals.
Another alternative solution for switching between two audio signal outputs is to use two SPDT analog switches. For example, ADG884BCPZ-REEL from Analog Devices contains two SPDT analog switches in one package. When using a 5 V power supply, the RON of both switches is very low, between 0.28 Ω (typical value) and 0.41 Ω (maximum), so it is suitable for low-loss audio signal switching. But such a low RON also has a price. When the switch is open, the parasitic capacitance between the analog switch contacts is 295 pF.
The ADG884 can handle 400 mA of current through a switch, making it suitable for driving speakers directly from an audio amplifier (Figure 5).
Figure 5: This basic circuit uses a single Analog Devices ADG884 to switch between two audio output devices
In order to minimize the possibility of EMI injecting noise into the audio output, the location of the audio amplifier on the printed circuit board should be as close as possible to the ADG884. The headphone jack should also be as close as possible to the ADG884. If the speaker does not use a jack, a shielded audio cable should be used between the ADG884 and the speaker.
If the audio input signal is a differential pair, the signal pairs S1A/S1B, S2A/S2B, and D1/D2 should be placed adjacent to each other on the printed circuit board to cancel any common interference and eliminate speaker or earphone noise.
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