12 issues you should pay attention to in analog circuit design

- Dec 27, 2019-

12 issues you should pay attention to in analog circuit design

The design of analog circuits is the most troublesome, but also the deadliest, part of design for engineers! We have summarized the issues that should be noticed in the design of analog circuits and share them with you.

  (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) Integral feedback circuits usually require a small resistor (approximately 560 ohms) in series with each integrating capacitor greater than 10pF.

  (3) Do not use active circuits outside the feedback loop to filter or control the RF bandwidth of EMC, but only use passive components (preferably RC circuits). The integral feedback method is effective only at frequencies where the open-loop gain of the op amp is greater than the closed-loop gain. At higher frequencies, the integrating 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) An input / output filter should be placed at the connection of the external cable. Any connection at the 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 a converter with digital signal processing or switch mode.

  (7) The power and ground reference pins of analog ICs need high-quality RF decoupling, which is the same as digital ICs. However, analog ICs usually require low-frequency power supply decoupling because the power supply noise rejection ratio (PSRR) of analog components increases very little above 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 for the PSRR corner frequency and slope of the device to obtain the desired PSRR over the entire operating frequency range.

   (8) For high-speed analog signals, transmission line technology is necessary based on its connection length and the highest frequency of communication. Even for low-frequency signals, the use of transmission line technology can improve its anti-jamming, but transmission lines that are not properly matched will produce antenna effects.

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

   (10) Since most of the radiation is generated by common-mode voltage and current, and because most of the environment's electromagnetic interference is caused by common-mode problems, balanced transmission and reception (differential mode) technology is used in analog circuits. Will have a good EMC effect, and can reduce crosstalk. The balanced circuit (differential circuit) driver does not use a 0V reference system as the return current loop, so it can avoid large current loops and reduce RF emissions.

   (11) The comparator must have hysteresis (positive feedback) to prevent erroneous output conversion due to noise and interference, and also to prevent oscillation at the trip point. Don't use a comparator that is faster than needed (keep dV / dt within the required range, as low as possible).

   (12) Some analog ICs are particularly sensitive to the RF field, so a small metal shielding box installed on the PCB and connected to the ground plane of the PCB is often used to shield such analog components. Take care to ensure its heat sink.

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