Key points of seven commonly used interface types in circuit design

TTL level interface


This interface type is basically a cliché. Starting from college to study analog circuits and digital circuits, for general circuit design, the TTL level interface basically is basically inseparable. Its speed is generally limited to 30MHz, which is due to there are several pF input capacitors at the input of the BJT (constituting an LPF). If the input signal exceeds a certain frequency, the signal will be “lost”.


It's driving capacity is generally up to tens of milliamps. The signal voltage for normal operation is generally high, and if it is close to the ECL circuit with a lower signal voltage, a more significant crosstalk problem will occur.


CMOS level interface


We are no stranger to it, and we often deal with it. What many people know is that CMOS's power consumption and anti-interference ability are much better than TTL under normal conditions. However, it is little known that at high switching frequencies, the CMOS series actually consumes more power than TTL. As for why it is, the semiconductor physics theory will explain it.


Since the operating voltage of CMOS can be very small at present, some FPGA cores work at even close to 1.5V, which makes the noise margin between levels much smaller than TTL, thus increasing the voltage fluctuation. The signal is judged incorrectly.


It is well known that the input impedance of a CMOS circuit is very high, so its coupling capacitance can be small without using a large electrolytic capacitor. Since the CMOS circuit usually has a weak driving capability, it is necessary to perform TTL conversion before driving the ECL circuit. In addition, when designing a CMOS interface circuit, care should be taken to avoid excessive capacitive loading, otherwise, the rise time will be slower and the power consumption of the driver device will increase (because the capacitive load does not consume power).


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 saturated when it is turned on, so it can reduce BJT's turn-on and turn-off time, the speed of work can naturally be raised.


But, this is a price to pay! Its fatal injury: power consumption is large! The EMI problem caused by it is worth considering, and the anti-interference ability is not much better, if anyone can compromise these two points, then he or she should make a fortune. It should also be noted that a general ECL integrated circuit requires a negative power supply, that is, its output voltage is negative, and a special level shift circuit is required.


RS-232 level interface


No one who knows the basics of playing electronic technology does not know it (unless he or she is just an "outsider" in electronics technology). It is a low-speed serial communication interface standard. It should be noted that it's level standard is a bit "abnormal": high level is -12V, and 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 aware of some of its shortcomings, such as slower data transmission speeds and shorter transmission distances.


Differential balanced level interface


It uses a pair of terminals A and B relative output voltage (uA-uB) to represent the signal. In general, this differential signal will pass through a complex noise environment during signal transmission, resulting in both lines being generated. Basically the same amount of noise and the noise of the noise will be offset at the receiving end, so it can achieve longer distance, higher rate transmission. The RS-485 interface commonly used in the industry uses a differential transmission method, which has good resistance to common-mode interference.


Optical isolation interface


Photoelectric coupling is the coupling and transmission of electrical signals based on optical signals. Its benefit is to achieve electrical isolation, so it has an excellent anti-interference ability. Under the condition that the circuit operates at a high frequency, basically only the 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 optically isolation interface circuits to connect these low-level, low-current TTL or CMOS circuits as described above. Because the input and output circuits of the optical isolation interface can withstand a high voltage of several thousand volts, which is enough for general applications.


In addition, the input and output sections of the optical isolation interface must use separate power supplies, otherwise, there is electrical contact, which is not called isolation.


Coil coupling interface


Its electrical isolation characteristics are good, but the allowed signal bandwidth is limited. For example, transformer coupling, its power transmission efficiency is very high, the output power is basically close to its input power. Therefore, for a step-up transformer, it can have a higher output voltage, but 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 impedance transformation can be realized. When matched reasonably, the load can obtain enough power. Therefore, the transformer coupling interface is frequently used in the power amplifier circuit design.