Common Interface Type Description Of Circuit Design

We know that there may be some problems when exchanging data among various sub-modules of the circuit system, resulting in signals not being able to “circulate” normally and with high quality. For example, sometimes there is a deviation in the working timing of each circuit sub-module (such as CPU and peripherals) or The respective signal types are inconsistent (such as sensors detecting light signals), etc. At this time, we should consider handling this problem well through the corresponding interface methods.Today qihe smt pick and place machine sharing key points of 7 common interface types in circuits .

The following is an explanation of the key points of 7 commonly used interface types in circuit design:

TTL level interface: This interface type is basically a cliché. Starting from studying analog circuits and digital circuits in college, for general circuit design, TTL level interface is basically inseparable! Its speed is generally limited to 30MHz. This is because there are several pF input capacitance at the input end of the BJT (constituting an LPF). If the input signal exceeds a certain frequency, the signal will be “lost”. Its driving capability is generally a maximum of dozens of milliamperes. The signal voltage of normal operation is generally higher. If it is placed close to the ECL circuit with lower signal voltage, obvious crosstalk problems will occur.

CMOS level interface: We are not unfamiliar with it and have often dealt with it. There is no need to go into details about some semiconductor characteristics of CMOS here. What many people know is that under normal circumstances, the power consumption and anti-interference ability of CMOS are much better than TTL. but! What is little 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 working voltage of CMOS can now be very small, and some FPGA core working voltages are even close to 1.5V, this makes the noise margin between levels much smaller than TTL, which further aggravates the noise caused by voltage fluctuations. Signal judgment error. As we all know, the input impedance of a CMOS circuit is very high, so its coupling capacitance can be very small without the need to use a large electrolytic capacitor. Since CMOS circuits usually have weak driving capabilities, TTL conversion must be performed first before driving the ECL circuit. In addition, when designing a CMOS interface circuit, 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).

ECL level interface: This is an old friend within the computer system! Because it “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 the turn-on and cut-off time of the BJT can be reduced, and the working speed can naturally be increased. But, there is a price to pay! Its fatal flaw: higher power consumption! The EMI problem it causes is worth considering, and the anti-interference ability is not much better. If anyone can compromise these two factors, then he or she will make a fortune. It should also be noted that generally ECL integrated circuits require a negative power supply, which means that their output voltage is negative. In this case, a special level shifting circuit is required.

RS-232 level interface: Basically no one who plays electronic technology does not know it (unless he or she is just a “layman” majoring 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 the computer, a level conversion chip MAX232 is naturally indispensable. But we have to be clearly aware of some of its shortcomings, such as relatively slow data transmission speed and short transmission distance.

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 during signal transmission. The environment causes basically the same amount of noise to be generated on both lines, and the energy of the noise will be offset at the receiving end, so it can achieve longer distance and higher speed transmission. The RS-485 interface commonly used in industry uses differential transmission, which has good anti-common mode interference capabilities.

Optical isolation interface: Optoelectric 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 that the circuit operating frequency is very high, basically only high-speed optical isolation interface circuits 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 loop and output loop of the optical isolation interface It can withstand high voltages of several thousand volts, which is enough for general applications. In addition, the input and output parts of the optical isolation interface must use independent power supplies. Otherwise, there is still an electrical connection, and it is not called isolation.

Coil coupling interface: It has good electrical isolation characteristics, but the allowed signal bandwidth is limited. For example, transformer coupling has a very high power transmission efficiency and its 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 voltage. 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 enough power. Therefore, the transformer coupling interface plays an important role in the design of power amplifier circuits. Very “popular”.

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