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December 27 2019


Basic Procedure and Cautions of Analog Circuit Design

An analog signal is an electrical signal that changes continuously. And analog circuit refers to a circuit used to transmit, transform, process, amplify, measure, and display analog signals, which are the basis of electronic circuits. They mainly include amplifier circuits, signal operation and processing circuits, oscillator circuits, modulation and demodulation circuits, and power supplies.

I Basic Procedure of Analog Circuit Design

1. System Definition

System definition is the basic premise of analog circuit design. According to the design requirements, the analog circuit design engineer needs to make corresponding function definitions for the circuit systems and subsystems, and determine the parameter range of related performances such as area and power consumption.

2. Circuit Design

The choice of circuit structure is an important part of circuit design. Analog circuit design engineers need to select the appropriate circuit structure according to the functional requirements, design specifications and corresponding parameter indicators that the analog circuit needs to implement, and then determine the combination of components.

For the design of analog electronic circuits, there is currently no mature design software available. Therefore, it can only be done manually by engineers based on their own experience. This increases the difficulty of analog circuit design to a certain extent and limits the development of analog circuits.

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.

The engineer continues to modify and adjust the circuit according to the simulation results until it can meet the given indicators and corresponding functional requirements. Commonly used methods are parameter scanning method, DC and AC analysis method, Monte Carlo analysis, etc.

4. Layout Implementation

The layout implementation is an important bridge for transforming circuit design into production. After the structure and related parameters of the analog circuit were determined from the previous design and simulation results, the design engineer described the physical and geometrical features of the designed analog circuit and converted it into graphic format to facilitate subsequent processing and production.

5. Physical Verification

During the physical verification phase, a design rule check (DRC) of the designed analog circuit is required. Design rule check is based on the given design rules to check the minimum line width, hole size, minimum pattern spacing and other limiting processes to measure the feasibility of layout process. In addition, the consistency of layout and circuit diagrams is checked. You can use the layout versus schematic (LVS) tool to extract the layout parameters and compare the obtained circuit diagram with the original circuit design diagram to ensure the consistency of the layout and the original circuit design.

6. Simulation after Extraction of Parasitic Parameters

The simulation of the circuit design before the layout is called "pre-simulation", and the "pre-simulation" is an ideal simulation without considering the parasitic parameters such as the resistance and capacitance of the wires. The circuit simulation performed after adding parasitic parameters to the layout is called "post-simulation". Only when the simulation results of the post-simulation meet the design specifications and system function requirements can the circuit design work be completed. The parasitic parameters have a great influence on the analog circuits. If the simulation results of the previous simulation are satisfied, the results of the post simulation may not meet the requirements. Therefore, design engineers need to continuously modify the transistor parameters based on 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, and the functions and system compatibility of the EDA tools are relatively backward. Besides, frequent manual intervention is required in the design process. These factors, to a certain extent, restrict the development of analog circuits.


II Cautions of Analog Circuit Design

1. In order to obtain a feedback circuit with good stability, it is usually required to use a small resistor or choking coil outside the feedback loop to provide a buffer for the capacitive load.

2. The integral feedback circuit usually needs a small resistor (approximately 560 ohms) in series with each integral capacitor larger than 10pF.

3. Do not use active circuits outside the feedback loop to filter or control the RF bandwidth of the EMC, instead, use only 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 withpassive filters or other suppression methods such as optical isolation.

5. Use EMC filters, and all 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. Any connection ofthe wires 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 powersupply of analog IC and ground reference pins require high-quality RF decoupling just like digital IC. However, analog IC usually requireslow-frequency power supply decoupling because the power supply noise rejection ratio (PSRR) of analog components increases very little when it’s 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 in the entire operating frequency range.

8. For high-speed analog signals, transmission line technology is very necessarybased 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-jammingperformance, but transmission lines that are not properly matched will produce antenna effects.

9. Avoid using high impedance inputs or outputsbecause theyare very sensitive to electric fields.

10. Since most of the radiation is generated by common-mode voltage and current, and most of the electromagnetic interference is caused by common-mode problems, using balanced transmit and receive (differential mode) technology in analog circuits will havea 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 radiations.

11. The comparator must have positive feedback to preventincorrectoutput transitions due to noise and interference, and to prevent oscillation at the open-circuit contact. Don't use a comparator that is faster than needed speed(keep dV / dt within the required range, which is as low as possible).


Related Source:

Introduction of Simple Analog Circuits

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