Electronic Design - Dry goods｜Old engineers share some experience in analog circuit design

The design of analog PCB circuits is the most troublesome for engineers, but it is also the most deadly part of the design. 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 be replaced, such as the design of RF radio frequency circuits. The problems that should be paid attention to in the design of analog circuits are summarized as follows:

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.

The integral feedback circuit usually requires a small resistor (about 560 ohms) in series with an integral capacitor greater than 10pF.

Do not use active circuits to filter or control the RF bandwidth of EMC outside the feedback loop, but only use passive components (RC circuits are best). 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.

In order to obtain a stable linear circuit, all connections must be protected by passive filters or other suppression methods (such as photoelectric isolation).
Use EMC filters, and all IC-related filters should be connected to the local 0V reference plane.

Input and output filters should be placed at the connection of the external cable; filtering is required at any wire connection inside the unshielded system 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.

The power and ground reference pins in analog ICs require high-quality RF decoupling, just like digital ICs. But analog ICs usually require low-frequency power supply decoupling, because the power supply noise rejection ratio (PSRR) of analog components increases 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 to obtain the desired PSRR within the entire operating frequency range.

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 if the transmission line is not properly matched, an antenna effect will occur.

Avoid using high impedance inputs or outputs, they are very sensitive to electric fields.

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, the use of balanced transmission and reception (differential mode) technology in analog circuits can be very good EMC effect, and can reduce crosstalk. The balanced circuit (differential circuit) drive does not use the 0V reference system as the return current loop, so a large current loop can be avoided, thereby reducing RF radiation.
The comparator must have hysteresis (positive feedback) to prevent erroneous output conversion due to noise and interference, and to prevent oscillation at the break point. Do not use a comparator that is faster than the required speed (keep dV/dt within the required range, as low as possible).
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 the heat dissipation conditions must be ensured.