Twelve Experiences in Analog Circuit Design

The design of analog circuits is the most troublesome but also the most deadly part of the design for engineers. Although the current development of digital circuits and large-scale integrated circuits is very rapid, the design of analog circuits is still inevitable, and sometimes digital circuits cannot replace them. , Such as the design of RF radio frequency circuit! Here is a summary of the issues that should be paid attention to in analog circuit design. (1) In order to obtain a feedback circuit with good stability, it is usually required to use a small resistor or choke outside the feedback loop to provide a buffer for the capacitive load. (2) The integral feedback circuit usually requires a small resistor (about 560 ohms) in series with an integral capacitor larger than 10pF. (3) Do not use active circuits to filter or control the RF bandwidth of EMC outside the feedback loop, but only use passive components (preferably RC circuits). The integral feedback method is only effective at frequencies where the open-loop gain of the op amp is greater than the closed-loop gain. At higher frequencies, the integrator circuit cannot control the frequency response. (4) In order to obtain a stable linear circuit, all connections must be protected by passive filters or other suppression methods (such as photoelectric isolation). (5) Use EMC filters, and all IC-related filters should be connected to the local 0V reference plane. (6) Input and output filters should be placed at the connection of the external cable, and any wire connection inside the unshielded system needs to be filtered because of the antenna effect. In addition, filtering is also required at the wire connections inside the shielding system of the converter with digital signal processing or switch mode. (7) The power and ground reference pins of analog ICs require high-quality RF decoupling, which is the same as digital ICs. But analog ICs usually require low-frequency power supply decoupling, because the power supply noise rejection ratio (PSRR) of analog components increases very little after being higher than 1KHz. RC or LC filtering should be used on the analog power traces of each op amp, comparator and data converter. The corner frequency of the power filter should compensate the PSRR corner frequency and slope of the device, so as to obtain the desired PSRR within the entire operating frequency range. (8) For high-speed analog signals, transmission line technology is necessary according to its connection length and the highest frequency of communication. Even if it is a low-frequency signal, the use of transmission line technology can improve its anti-interference performance, but a transmission line that is not properly matched will produce an antenna effect. (9) Avoid using high impedance input or output, they are very sensitive to electric field. (10) Since most of the radiation is generated by common-mode voltage and current, and because most environmental electromagnetic interference is caused by common-mode problems, balanced transmission and reception (differential mode) technology is used in analog circuits It will have a good EMC effect and can reduce crosstalk. The balanced circuit (differential circuit) drive will not use the 0V reference system as the return current loop, so a large current loop can be avoided, thereby reducing RF radiation. (11) The comparator must have hysteresis (positive feedback) to prevent erroneous output conversion due to noise and interference, and to prevent oscillation at the disconnection point. Do not use a comparator that is faster than required (keep dV/dt within the required range, as low as possible). (12) Some analog ICs themselves are particularly sensitive to radio frequency fields, so it is often necessary to use a small metal shielding box mounted on the PCB and connected to the ground plane of the PCB to shield such analog components. Note that it is necessary to ensure that its heat sink enters the pricing page https://www./