Analog circuit design is the most painful, but deadly, part of design for engineers! We have summarized the issues that you should be aware of in analog circuit design and shared them with you.
1, To obtain a feedback circuit with good stability, it is often 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 integral capacitor larger than 10pF.
3, do not use active circuits outside the feedback loop for filtering or controlling the RF bandwidth of the EMC, but use only passive components (preferably RC circuits). 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 integral circuit does not control the frequency response.
4, To obtain a stable linear circuit, all connections must be protected with passive filters or other rejection methods such as opto-isolation.
5, use EMC filters, and any filters associated with the IC should be connected to the local 0V reference plane.
6, input and output filters should be placed at external cable connections, and any wire connections inside an unshielded system need to be filtered because of the antenna effect. Also, wire connections inside shielded systems with digital signal processing or switch mode converters need to be filtered.
7, high quality RF decoupling is required at the power and ground reference pins of analog ICs, just as it is for digital ICs. But analog ICs typically 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 supply alignment of each op-amp, comparator, and data converter. The corner frequency of the power supply 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 depending on the length of the connection and the maximum frequency of the communication. Even for low frequency signals, the use of transmission line technology can improve their immunity to interference, but without properly matched transmission lines there will be antenna effects.
9, avoid using high impedance inputs or outputs, which are very sensitive to electric fields.
10, Since most of the radiation is generated by common mode voltages and currents, and because most of the ambient EMI is generated by common mode issues, using balanced transmit and receive (differential mode) techniques in analog circuits will have good EMC results and will reduce crosstalk. Balanced circuit (differential circuit) drivers do not use a 0V reference system as a return current loop, so large current loops can be avoided, thus reducing RF emissions.
11, the comparator must have hysteresis (positive feedback) to prevent incorrect output transformations due to noise and interference, and also to prevent oscillations at break points. Do not use a comparator that is faster than needed (keep dV/dt as low as possible to meet the requirements).
12, Some analog ICs are inherently particularly sensitive to RF fields, so it is often necessary to shield such analog components using a small metal shield box mounted on the PCB and connected to the ground plane of the PCB. Note that it is important to ensure its heat sink strip.
