EMI troubleshooting & design work for analogue circuits

Client Context

A high-volume manufacturer of fire alarm systems approached us with immunity-related issues in their analogue-based designs. Unlike typical EMC scenarios focused on emissions from digital or power circuits, their analogue based circuits exhibited susceptibility problems when exposed to electromagnetic disturbances.

They reached out to us because of our deep-rooted expertise in EMC and analogue system design—particularly Dr. Min Zhang's background in designing sensorless motor control systems based on analogue circuits.

Scope of Work

• Identify the weak points in the existing analogue circuitry.

• Develop cost-effective, manufacturing-friendly fixes.

• Propose design strategies for the next-generation product, balancing performance with cost.

Technical Approach

Although modern digital and power systems dominate EMC discussions, analogue circuits remain essential in many sensing and signal-processing applications. These circuits, by nature, are susceptible to electromagnetic interference, particularly demodulated RF signals, bias shifts, or stability loss due to high-frequency capacitive coupling.

Unlike digital circuits, analogue circuits can experience:

• Near-instability in amplifier feedback loops due to RF coupling.

• Demodulation effects causing DC bias shifts or non-linearities.

• Unexpected response to logic switching noise on nearby boards.

To improve immunity, the following principles were followed:

• Minimize analogue bandwidth.

• Maximize signal amplitude (SNR).

• Ensure a robust stability margin.

• Use balanced signal paths where feasible.

• Isolate sensitive inputs and outputs.

Troubleshooting & Mitigation

We applied a structured test-and-fix approach:

Using a near-field probe connected to an RF amplifier, we recreated radiated immunity failure modes observed during the far-field radiated immunity testing. For conducted immunity, we used our in-house CDN and BCI setups to accurately replicate disturbances.

Voice-frequency op-amps were found susceptible to high-frequency noise, including digital switching activity. Although these devices shouldn't respond to GHz-range signals, poor PCB layout and insufficient filtering turned them into unintentional crystal radios.

A small capacitor (CIN, typically 47 pF or less) was added across op-amp inputs to suppress RF pickup. Careful consideration was given to ensure this did not degrade the gain and phase margin. We used simulation and stability analysis tools to confirm safe operation across temperature and production tolerances.

Additional fixes included improved decoupling capacitor placement, redesigned R-C filters with optimized time constants, and careful stability loop analysis.

Results & Validation

• The system initially failed at 10 V/m field strength. After applying fixes, it passed testing up to 30 V/m without malfunction.

• Through simulation and testing, we ensured no unintended oscillation or performance degradation.

• Lessons learned were documented for the client to guide future designs, particularly under tightened immunity standards.

Conclusion

This case highlights that EMI challenges are not exclusive to high-speed digital designs. Analogue circuits—especially those used in life-critical systems—require careful attention to layout, stability, and unintended RF pickup paths. With proper design techniques and a solid understanding of immunity mechanisms, analogue systems can be made robust against real-world electromagnetic threats.