Analog circuit technology in digital technology

    Since digital circuits use short rising/falling edge pulse signals, they emit excess electromagnetic waves (noise) including high-frequency components to the outside, and respond sensitively to external electromagnetic waves (noise), causing malfunctions. In addition, there are also problems within the circuit such as intermodulation distortion between lines and sudden changes in current when digital devices are turned on and off, causing changes in the power supply voltage. In this way, it is necessary to consider the distributed constant circuit composed of wiring inductance and parasitic capacitance in digital circuits to prevent overshoot and undershoot from causing waveform confusion and signal reflection, delay, attenuation, and intermodulation distortion caused by electromagnetic interference between lines. The filters, shielding, etc. that solve this problem are all analog technologies.

    Due to the application of digital circuit technology in automobiles, trains, and radio control, high functions that were previously impossible to achieve with analog technology have been realized with high reliability. However, noise can cause system and circuit malfunctions, which is especially fatal to machines and systems. Even if there is noise in the analog circuit, it will only temporarily reduce the accuracy of the data. Once the noise disappears, it has the characteristics of self-recovery function. Therefore, the combined application of high-function digital circuits and analog circuits with self-recovery/self-confirmation capabilities will be a safe solution to prevent malfunctions caused by noise in mobile control systems and digital circuits.

Things to pay special attention to when designing circuits

    After the circuit is designed, in order to verify the work, the circuit needs to be assembled for experiments. But the result will often be that it doesn't work as designed. For example, the designed amplifier becomes an oscillator. Due to the noise from the digital circuit mixed into the analog circuit, the waveform of the analog signal is distorted, the operation is unstable, and the data cannot be obtained smoothly.

    For low-frequency circuits, no matter who assembles them, as long as the wiring is not connected incorrectly, there will be almost no difference in various installations, wiring, and circuit characteristics, and the same data can be obtained. But high frequency is different. Due to different installation methods, data with different characteristics will generally be obtained.

    In high-frequency circuits and high-speed digital circuits, if there is one line, an inductance component (parasitic) will be formed, and if there are two lines, a parasitic capacitance component and a mutual inductance component (parasitic) will be formed between the lines, which are the so-called three parasitics. The three spurious values ​​formed are very small, so it is almost not a problem at low frequencies. However, the influence of the C and L components cannot be ignored in the high frequency field (see Figures la and b).

    Recently, in order to improve the performance of machines, various circuits such as low-frequency to high-frequency analog circuits, high-speed digital circuits, micro-analog circuits, and high-current circuits are often mixed together. This can cause circuit instability and worsen frequency characteristics. The main reason is that the above three parasitics are not fully considered during design, and reliability and safety cannot be maintained.

    In addition, the circuit diagram only uses two dimensions to represent the semiconductor device and the concentrated parameters of R, C, and L, but this does not represent the performance and function of the actual circuit. The actual action is a three-dimensional space, including frequency, which is a four-dimensional space. Therefore, the microcurrent circuit formed by the combination of cross-modulation distortion, reflection, electrostatic and electromagnetic circuits will affect the characteristics and functions of high-frequency circuits. Recent ICs are designed to meet the requirements of the times. Many of them are high-speed operating devices and are very sensitive to high-frequency noise responses. Therefore, when using devices, the corresponding components should be selected according to the circuit function, and try to avoid using high-speed ICs that are higher than the requirements.

    In the circuit diagram, the impedance of the power supply, ground wire, and signal wire is usually considered as zero ohm. But in fact, there is no zero ohm, and the higher the frequency, the greater the influence of inductance and parasitic capacitance. As a result, the influence of circuit combinations and external electromagnetic fields is too great to be ignored, causing circuit instability and deterioration of frequency characteristics. In analog circuits, the problems of error, noise and time delay should be solved; in digital circuits, anti-noise should be solved, and synchronization should be used to prevent it from being affected by time delay, which is very important for improving circuit characteristics.

Attention must be paid to the influence of dynamic noise "static electricity"

    There are many noise sources that can cause malfunction of electrical equipment, such as fluorescent lamps, dust collectors, radio transceivers, transformers, converters, etc. around us. These are electromagnetic field noise sources. In addition, the noise source that causes malfunction is electrostatic discharge.

    The electrostatic discharge current and instantaneously generated high voltage will destroy the IC, causing malfunction and failure of the system or equipment. In order to prevent electrostatic discharge, necessary measures must be taken from the purchase of components to the design, production and packaging of equipment. The following measures can be taken in terms of design:

    (1) Avoid using high-speed ICs that exceed requirements, especially pay attention to the input circuit. Use differential input circuits where possible. The filter circuit should be connected close to the IC.

    (2) Input protection for semiconductors. A limiting circuit is added to the input part of the connector to control the noise below the semiconductor withstand voltage value. Since the CMOS gate has weak anti-static noise properties, it is not easy to use in the input part of the connector.

    (3) Avoid using edge-emitting ICs and use gated or latch circuits.

    (4) In order to suppress the occurrence rate of malfunction, the control terminal and output terminal should be made into low effective logic.

    (5) Filter the high-sensitivity signal input. It is important to filter out the high frequencies outside the frequency band so that the operational amplifier does not input too large signals. Also pay attention to the lead inductance of the capacitor used.

    (6) Some measures must also be taken in terms of software. Since electrostatic discharge is a one-time transition pulse, erroneous data can be detected through multiple verifications. A watchdog circuit (monitoring circuit) is installed in the microcomputer to prevent unexpected stop.

    (7) Electronic circuits and wiring should be kept away from metal chassis that discharge static electricity.

    (8) The metal and metal connecting parts of the chassis should be tightly connected with the paint removed and fixed with screws as much as possible.

    In order to reduce the influence of the electromagnetic field generated by the discharge current, the following measures should be taken on the printed circuit board:

    (1) Reduce the ring area (see Figure 2).

    Due to the cross-linking of magnetic flux in the formed ring, a current will be induced in the ring. The larger the area of ​​the ring, the more magnetic flux will be cross-linked, and the greater the induced current will be. Therefore, in order to minimize the loop area formed by the power supply and ground wires, the power supply and ground wires should be routed as close as possible. Install high-frequency bypass capacitors between the power supply and ground wires to reduce the loop area (see Figure 3). In order to reduce the loop area formed between the signal line and the ground line, route the signal close to the ground line.

    (2) Keep the wiring as short as possible. It is necessary to consider the distribution of signal line lengths. When designing, lengthen the low-effective signal line and make the high-effective signal line the shortest. (See Figure 4). Make the wiring between each device as short as possible, and install the devices connected to the input and output lines near the terminals.

    (3) Use multi-layer circuit boards, which are seen in analog circuits and high-speed digital circuits.

    In high-speed digital circuits, the spectrum of pulse signals has a very wide range of high-order harmonic components. The higher the operating frequency used, the greater the influence of parasitic capacitance and inductance. Assuming that a high-frequency current I flows through the pattern with an inductor L, the voltage drop generated by the inductor L is:

    V=L·di/dt

    The pattern acts like an antenna, sending radiated noise out. Making the ground wire into a flat surface can reduce the impedance of the ground wire and reduce the voltage drop caused by the discharge current.

    Anti-static measures should be taken for interface cables:

    The shielded wires of the cable are connected to the chassis at both ends. Add bypass capacitors to short-circuit high frequencies where ground loops may occur. It should not be connected to logic ground when there is no chassis ground. For flat cables, a ground wire can be added between the signal wires.

Issues that should be paid attention to when switching power supply is used as analog signal power supply

    The so-called switching power supply is a power supply circuit form that stabilizes the output voltage through pulse modulation. In this method, only the switching part consumes power, so the faster the switching speed, the higher the efficiency of the power supply, so high-speed switching devices are generally used. Due to its high efficiency, this power supply is widely used in everything from high-power machines to small and lightweight machines. However, there is a disadvantage of leakage of switching noise due to high-speed switching operation. This kind of power supply used for analog circuits will cause many problems.

    When a switching power supply is used as a power supply for analog circuits, the high-frequency noise shown in Figure 5 will enter the frequency band of the analog signal, worsening the signal/noise ratio of the analog signal. Although switching noise is generally only 50 to 100mVpp, which is quite small, such noise often causes problems due to the large dynamic range of analog signals. Especially when used in equipment such as A/D converters, if noise is superimposed on the signal at the time of level conversion, conversion errors will occur and the expected accuracy will not be achieved.

    In order to solve the problems of using switching power supplies in analog circuits, you can pay attention to the following two aspects when selecting switching power supplies:

    (1) The noise level of the switching power supply should be as small as possible;

    (2) The switching noise component does not enter the frequency band of the signal.

    Since the analog signal level is high, switching noise will have no impact on the signal-to-noise ratio. In order to keep switching noise out of the signal band, the simplest way is to choose a power supply with a higher switching frequency than the highest frequency band of the analog signal.

    When the above method cannot be selected, it is necessary to find ways to reduce the switching noise generated by the power supply. These methods include:

    (1) Add an external capacitor.

    (2) Switching noise generated by external power supply.

    (3) Use series regulator together.

    Figure 6 is a circuit example of a low-noise switching power supply. The transformer of this power supply uses three windings to eliminate noise between the windings. This power supply is a high-efficiency power supply that can be used in communication equipment that supplies power through transmission lines. The receiving part of the communication machine uses an analog circuit with very low signal inductance. When using this low-noise switching power supply, the efficiency problem and the noise problem can be solved at the same time.