What are the seven commonly used interface types in circuit design?
1. TTL level interface
This interface type is basically old-fashioned. Starting from studying analog circuits and digital circuits in college, for general circuit design, TTL level interfaces are basically inseparable! Its speed is generally limited to less than 30MHz. This is due to the presence of several pF of input capacitance at the input 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 few dozen mA. The signal voltage of normal operation is generally higher, and if it is close to the ECL circuit with a lower signal voltage, a more obvious crosstalk problem will occur.
We are no stranger to it, and we often deal with it. Some semiconductor characteristics about CMOS need not be wordy here. What many people know is that under normal circumstances, the power consumption and anti-interference ability of CMOS are far superior to TTL. but! It is little known that at high conversion frequencies, the CMOS series actually consumes more power than TTL. As for why this is the case, please ask the theory of semiconductor physics. Since the working voltage of CMOS can be very small at present, some FPGA cores are even close to 1.5V, which makes the noise tolerance between levels much smaller than TTL, so it is more aggravated by voltage fluctuations. Signal judgment error. As we all know, the input impedance of the CMOS circuit is very high, therefore, its coupling capacitance can be very small, without the need to use large electrolytic capacitors. Since the drive capability of CMOS circuits is usually weak, it is necessary to perform TTL conversion before driving the ECL circuit. In addition, when designing CMOS interface circuits, care should be taken to avoid overloading the capacitive load, otherwise the rise time will be slower, and the power consumption of the driving device will also increase (because the capacitive load does not consume power).
3. ECL level interface
This is an old friend inside the computer system! Because its speed "runs" fast enough, it can even run to hundreds of MHz! This is because the BJT inside the ECL is not in a saturated state when it is turned on, so that the on and off time of the BJT can be reduced, and the working speed can naturally be raised. But, this comes at a price! Its fatal injury: greater power consumption! The EMI problem caused by it is worth considering, and the anti-interference ability is not much better. If anyone 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 to say its output voltage is negative, then you need a special level shift circuit.
4, RS-232 level interface
No one knows the basics of playing electronic technology (unless he or she is just a layman in 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 a computer, a level-shifting chip MAX232 is naturally indispensable. But we have to be soberly 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. In general, this differential signal will pass through a complex noise environment during signal transmission, resulting in both lines. There is basically the same amount of noise, and the energy of the noise will be canceled at the receiving end, so it can achieve a longer distance and a higher rate of transmission. The RS-485 interface commonly used in the industry adopts the differential transmission method, which has good resistance to common mode interference.
6, optical isolation interface
Optoelectronic coupling uses optical signals as the medium to realize the coupling and transmission of electrical signals. Its "benefit" 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 photoelectric 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 described above, because the input loop and output loop of the optical isolation interface The room can withstand a high voltage of several thousand volts, which is enough for general applications. In addition, the input part and output part of the optical isolation interface must use separate power supplies, otherwise there is still electrical connection, so it is not called isolation.
It 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 booster 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 achieve impedance transformation. When matched properly, the load can obtain enough power. Therefore, the transformer coupling interface is used in the design of the power amplifier circuit. Very commonly used.
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