Attention to the 12 questions design analog circuit no longer bothersome

- May 15, 2017-

Analog circuit design is the most troublesome engineers, but also the most fatal design part! We will be analog circuit design should pay attention to the problem were summarized, and share 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) The integral feedback circuit typically requires a small resistor (about 560 ohms) in series with each integrating capacitor greater than 10pF.

    (3) Do not use the active circuit to filter or control the EMC RF bandwidth outside the feedback loop, but only passive components (preferably RC circuits). The integral feedback method is effective only if the open loop gain of the op amp is greater than the closed loop gain. At higher frequencies, the integrating circuit can not control the frequency response.

    (4) In order to obtain a stable linear circuit, all connections must be protected using passive filters or other suppression methods such as optical isolation.

    (5) using EMC filters, and IC-related filters should be connected to the local 0V reference plane.

    (6) The input and output filters should be placed at the connection of the external cables, and any wire connections in the absence of a shielded system need to be filtered because of the antenna effect. In addition, filtering is also required at the wire connection inside the shield system of the converter having the digital signal processing or switching mode.

    (7) The analog IC's power supply and ground reference pins require high-quality RF decoupling, which is the same as for digital ICs. However, analog ICs typically require low-frequency power decoupling because the power supply noise rejection ratio (PSRR) of the analog components increases little after more than 1 kHz. RC or LC filtering should be used on the analog power supply traces for 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 achieve the desired PSRR over the entire operating frequency range.

    (8) For high-speed analog signals, transmission line technology is required, depending on their connection length and the highest frequency of communication. Even low-frequency signals, the use of transmission line technology can also improve its anti-jamming, but did not correctly match the transmission line will produce antenna effect.

    (9) Avoid the use of high-impedance inputs or outputs, which are very sensitive to the electric field.

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

    (11) The comparator must have a hysteresis (positive feedback) to prevent erroneous output transitions due to noise and interference, and to prevent oscillation at the trip point. Do not use a comparator that is faster than you need (keep dV / dt within the range that meets the requirements, as low as possible).

    (12) Some analog ICs are particularly sensitive to radio frequency fields, so it is often necessary to shield such analog components using a small metal shield that is mounted on the PCB and connected to the ground plane of the PCB. Note that to ensure that its heat sink.


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