After uncovering these insights, many engineers have found themselves stuck in the frustrating stage of debugging and fixing circuit issues. In fact, a significant portion of project time is often spent on this phase, with many projects getting stuck and not moving forward. If you're looking to finish your project faster, eliminate the headaches of experimental debugging, and quickly identify the root cause of problems, it's essential to understand the key details behind the circuit design.
Here are some critical design considerations for achieving stable and reliable analog circuits:
1. To achieve a feedback circuit with good stability, it’s common practice to use a small resistor or choke outside the feedback loop. This helps buffer the capacitive load and improves overall performance.
2. For integral feedback circuits, a small resistor (around 560 ohms) should be placed in series with each integrating capacitor that exceeds 10pF. This helps prevent unwanted oscillations and ensures proper operation.
3. Avoid using active circuits outside the feedback loop for filtering or controlling EMC RF bandwidth. Instead, rely on passive components, preferably RC circuits. The integration method works best when the op-amp’s open-loop gain is higher than its closed-loop gain. At higher frequencies, integration may fail to control the frequency response effectively.
4. To ensure a stable linear circuit, all connections must be protected using passive filters or other suppression techniques like opto-isolation.
5. Implement an EMC filter, and connect IC-related filters to the local 0V reference plane to minimize noise and interference.
6. Place input and output filters at the point where external cables connect. Any internal wire connection within a shielded system should also be filtered due to the antenna effect. Filtering is especially important in systems with digital signal processing or switching mode converters.
7. High-quality RF decoupling is essential for the power and ground pins of analog ICs, just as it is for digital ICs. However, analog components typically require low-frequency decoupling since their PSRR (Power Supply Rejection Ratio) doesn’t improve much above 1 kHz. Use RC or LC filters on the analog power traces of each op-amp, comparator, and data converter. The corner frequency of the line filter should be chosen to match the device’s PSRR characteristics, ensuring optimal performance across the operating range.
8. For high-speed analog signals, transmission line technology becomes necessary depending on the signal length and frequency. Even for low-frequency signals, using transmission lines can enhance immunity to interference. However, improper matching can lead to antenna-like effects, increasing EMI.
9. Avoid using high-impedance inputs or outputs that are highly sensitive to electric fields, as they can easily pick up noise and degrade performance.
10. Balanced transmit and receive (differential mode) techniques are widely used in analog circuits because most radiation comes from common-mode voltages and currents. Since most environmental EMI is a common-mode issue, differential signaling provides better EMC performance and reduces crosstalk. Differential circuits do not rely on a 0V reference for return current, which minimizes large current loops and lowers RF emissions.
11. Comparators must include hysteresis (positive feedback) to prevent false triggering from noise and interference, and to avoid oscillation at the threshold voltage. It’s also advisable to avoid using comparators that are faster than necessary—keep the dV/dt as low as possible within the required range.
12. Some analog ICs are inherently sensitive to RF fields, so it’s often necessary to shield them using a small metal enclosure mounted on the PCB and connected to the ground plane to reduce interference.
By applying these design principles, you can significantly improve the reliability, stability, and electromagnetic compatibility of your analog circuits, saving time and reducing frustration during the debugging process.
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