Old driver summary: 11 details of the design of the simulation circuit

release time:2022-03-17Author source:SlkorBrowse:2165

Analog circuit design needs a solid foundation of analog electricity and rich experience accumulation. Here, we share the summary of an old analog electricity engineer to help you to step on the pits and take detours less. If you feel good, please give us a compliment!

system definition

System definition is the basic premise of analog circuit design. According to the design requirements, analog circuit design engineers need to make corresponding functional definitions of circuit systems and subsystems, and determine the parameter ranges of area, power consumption and other related performance.

circuit designThe selection of circuit structure is an important link in circuit design. Analog circuit design engineers need to select the appropriate circuit structure according to the functional requirements, design specifications and corresponding parameters of analog circuits, and on this basis, determine the combination of components. For the design of analog electronic circuits, there is no mature design software available at present, so engineers can only do it manually according to their own experience. To some extent, this increases the difficulty of analog circuit design and limits the development speed of analog circuit.
3.

Circuit simulation

Circuit simulation is an indispensable link in the design process of analog circuits, and it is an important basis for analog engineers to judge whether analog circuits can meet the design requirements.
According to the simulation results, engineers constantly modify and adjust the circuit until the simulation results of the analog circuit can meet the set indicators and corresponding functional requirements. Commonly used methods include parameter scanning, DC and AC analysis, Monte Carlo analysis, etc.

Layout realization

An important bridge to transform circuit design into production. After the structure and related parameters of the analog circuit are determined by the previous design and simulation results, the design engineer describes the physical geometry of the designed analog circuit and converts it into a graphic format, so as to facilitate the subsequent processing and manufacture of the analog circuit.

Physical verification

In the stage of physical verification, it is necessary to check the design rules (DRC) of the designed analog circuit. The design rule check is to check the minimum line width, hole size, minimum pattern spacing and other restricted processes based on the given design rules, so as to measure the feasibility of layout process implementation. In addition, the consistency of layout and circuit diagram should be checked (LVS). The parameters of layout can be extracted by LVS tool, and the obtained circuit diagram can be compared with the original circuit design to ensure the consistency between layout and the original circuit design.

Simulation after parasitic parameter extraction

The simulation of circuit design before layout is called "pre-simulation", which is an ideal simulation without considering parasitic parameters such as resistance and capacitance of the wiring. After parasitic parameters are added to the layout, the circuit simulation is called "post-simulation". Only when the simulation results of post-simulation meet the design index and system function requirements, the circuit design is completed.

Parasitic parameters have a great influence on analog circuits. When the simulation results of the former simulation meet the requirements, the latter simulation results cannot meet the requirements. Therefore, design engineers need to constantly modify the transistor parameters according to the post-simulation results, and sometimes even adjust the circuit structure until the post-simulation results meet the system design requirements.

At present, the design of analog circuits is difficult and complicated, the functions and system matching of EDA tools used are relatively backward, and frequent manual intervention and sensitivity to parasitic parameters are required in the design process, which to some extent limit the development of analog circuits, resulting in a relatively slow development speed of analog circuits.

At the same time, the following eleven details 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 each integral capacitor larger than 10pF.
(3) Do not use the active circuit to filter or control EMC's RF bandwidth outside the feedback loop, but only use passive components (preferably RC circuits). Only when the open-loop gain of the operational amplifier is larger than the closed-loop gain, the integral feedback method is effective. 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) Input and output filters should be placed at the connection of external cables, and any wire connection in the unshielded system needs to be filtered because of the antenna effect. In addition, filtering is also needed at the wire connection inside the shielding system of the converter with digital signal processing or switching mode.
(7) High-quality RF decoupling is required at the power supply and ground reference pins of analog IC, which is the same as that of digital IC. However, analog IC usually needs low-frequency power supply decoupling, because the PSRR of analog components increases little when it is higher than 1KHz. RC or LC filtering should be used on the analog power supply traces of every operational amplifier, comparator and data converter. The corner frequency of the power supply should compensate the PSRR corner frequency and slope of the device, so as to obtain the desired PSRR in the whole operating frequency range.
(8) For high-speed analog signals, transmission line technology is necessary according to their connection length and the highest frequency of communication. Even if it is a low-frequency signal, the anti-interference performance can be improved by using transmission line technology, but the antenna effect will be produced if there is no correct matching transmission line.
(9) Avoid using high impedance inputs or outputs, which are very sensitive to electric fields.
(10) Because most of the radiation is generated by common-mode voltage and current, and because most of the electromagnetic interference in the environment is caused by common-mode problems, using balanced transmission and reception (differential mode) technology in analog circuits will have a good EMC effect and can reduce crosstalk. The balanced circuit (difference channel) drive will not use 0V reference system as the return current loop, so it can avoid large current loop and reduce RF radiation.
(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 disconnection point. Do not use a comparator that is faster than required (keep dV/dt within the range that meets the requirements and as low as possible).

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