7 commonly used interface types including TTL level interface/CMOS level interface/coil coupling interface

fish001

7 commonly used interface types including TTL level interface/CMOS level interface/coil coupling interface [Copy link]

We know that when the submodules of the circuit system exchange data, there may be some problems that cause the signal to not "circulate" normally and with high quality. For example, sometimes the working timing of the submodules of the circuit is deviated (such as the CPU and the peripherals) or the signal types of each submodule are inconsistent (such as the sensor detecting the light signal). At this time, we should consider using the corresponding interface method to deal with this problem well. The following is an explanation of the key points of the 7 commonly used interface types in circuit design: 1 Level interface This interface type is basically a commonplace. Since I started studying analog circuits and digital circuits in college, for general circuit design, the level interface is basically inseparable! Its speed is generally limited to 30MHz. This is because there are several pF input capacitors at the input end of the BJT (forming an LPF). If the input signal exceeds a certain frequency, the signal will be "lost". Its driving capacity is generally a maximum of tens of milliamperes. The signal voltage of normal operation is generally high. If it is close to the ECL circuit with a lower signal voltage, a more obvious crosstalk problem will occur. 2 Level interface We are not unfamiliar with it, and we often deal with it. Some semiconductor characteristics do not need to be elaborated here. Many people know that the power consumption and anti-interference ability under normal circumstances are much better. However! What is less known is that at high conversion frequencies, the CMOS series actually consumes more power than TTL. As for why this is the case, please ask the semiconductor physics theory. Since the operating voltage of CMOS can be very small at present, the operating voltage of some FPGA cores is even close to 1.5V, which makes the noise margin between levels much smaller than TTL, thus further aggravating the signal judgment errors caused by voltage fluctuations. As we all know, the input impedance of CMOS circuits is very high, so its coupling capacitor capacity can be very small, and there is no need to use large electrolytic capacitors. Since CMOS circuits usually have weak driving capabilities, they must be converted to TTL before driving ECL circuits. In addition, when designing CMOS interface circuits, care should be taken to avoid excessive capacitive loads, otherwise the rise time will be slowed down, and the power consumption of the driving device will also increase (because capacitive loads do not consume power). 3ECL level interface This is an old friend inside the computer system! Because its speed "runs" fast enough, it can even run up to hundreds of MHz! This is because the BJT inside the ECL is not in a saturated state when it is turned on, so the turn-on and turn-off time of the BJT can be reduced, and the working speed can naturally be increased. But, this comes at a price! Its fatal flaw: high power consumption! The EMI problem it causes is also worth considering, and the anti-interference ability is not much better. If someone can compromise these two factors, then he (she) should make a fortune. It should also be noted that the general ECL integrated circuit requires a negative power supply, that is, its output voltage is negative, and a special level shifting circuit is required at this time. 4RS-232 level interface There is basically no one who doesn't know it when playing with electronic technology (unless he or she is just a "layman" in the field of electronic technology). It is a low-speed serial communication interface standard. It should be noted that its level standard is a bit "abnormal": the high level is -12V, and the low level is +12V. So, when we try to communicate with peripherals through computers, a level conversion chip MAX232 is naturally indispensable. But we have to be aware of some of its shortcomings, such as the relatively slow data transmission speed and short transmission distance. 5 Differential balanced level interface It uses the relative output voltage (uA-uB) of a pair of terminals A and B to represent the signal. Under normal circumstances, this differential signal will pass through a complex noise environment during signal transmission, resulting in basically the same amount of noise on both lines, and the noise energy will be offset at the receiving end, so it can achieve long-distance and high-speed transmission. The RS-485 interface commonly used in industry adopts differential transmission, which has good anti-common mode interference ability. 6 Optical isolation interface Optical coupling uses optical signals as the medium to realize the coupling and transmission of electrical signals. Its "advantage" is that it can achieve electrical isolation, so it has excellent anti-interference ability. Under the condition of high circuit operating frequency, basically only high-speed optical isolation interface circuit can meet the needs of data transmission. Sometimes, in order to achieve high voltage and high current control, we must design and use optical isolation interface circuits to connect these low-level, low-current TTL or CMOS circuits as mentioned above, because the input and output circuits of the optical isolation interface can withstand a high voltage of several thousand volts, which is enough to meet general applications. In addition, the input and output parts of the optical isolation interface must use independent power supplies, otherwise there is still electrical connection, and it is not called isolation. 7. The coil coupling interface has good electrical isolation characteristics, but the allowed signal bandwidth is limited. For example, transformer coupling, its power transmission efficiency is very high, and the output power is basically close to its input power. Therefore, for a step-up transformer, it can have a higher output voltage, but it can only give a lower current. In addition, the high-frequency and low-frequency characteristics of the transformer are not optimistic, but its biggest feature is that it can realize impedance transformation. When matched properly, the load can obtain a large enough power. Therefore, the transformer coupling interface is very popular in the design of power amplifier circuits.