Tips for Analog Circuit Design (Part 2)
Analog circuit design is one of the most complicated design parts, also becomes a headache problem for most engineers. Although currently digital circuits and large-scale integrated circuit develop quickly, the analog circuit design is still inevitable, and sometimes digital circuits cannot be replaced, such as RF circuit design. The following tips are practical for your analog circuit design.
7. The power supply and ground reference pins in the analog IC 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 and slope frequency to achieve the desired PSRR over the entire operating frequency range.
8. For high-speed analog signals, transmission line technology is required according to connection length and the highest communication frequency. Even for low-frequency signals, the use of transmission line technology can also improve its anti-jamming, but mismatch transmission line will cause 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 the common-mode voltage and current, and most of the environment's electromagnetic interference is a common-mode problem, so the use of balanced transmission and reception (differential mode) techniques in analog circuits will have 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 which can avoid large current loops, thereby RF emissions can be largely reduced.
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 as low as possible but still within the required range).
12. Some analog ICs are particularly sensitive to radio frequency fields, so a small metal shield is often used which is mounted on a PCB and connected to the ground plane of the PCB to shield such analog components.