How does power supply noise affect CMOS circuits?

2 Noise Effects

Power supply noise can affect CMOS circuits in several ways, depending on the frequency and magnitude of the noise. Signal degradation, timing errors, and reliability issues can all be caused by this type of noise. For instance, if the noise causes the power supply voltage to drop below the threshold voltage of a transistor, the transistor may not switch correctly and produce a wrong output. Additionally, if the noise causes the power supply voltage to rise above the nominal value, the transistor may switch faster than expected and create a short-circuit current between the power and ground rails. Furthermore, if the noise causes a voltage spike that exceeds the oxide breakdown voltage, the transistor may suffer from gate oxide degradation or rupture. As such, it is important to consider how power supply noise can affect digital circuits when designing them.

3 Noise Measurement

In order to evaluate the impact of power supply noise on CMOS circuits, it is essential to measure the noise characteristics and compare them with the circuit specifications. To do this, peak-to-peak noise, root-mean-square (RMS) noise, and power spectral density (PSD) noise should all be taken into account. Peak-to-peak noise measures the difference between the maximum and minimum values of the noise waveform over a given period of time, indicating the worst-case amplitude of the noise and its potential to cause signal degradation or reliability issues. RMS noise is calculated as the square root of the average of the squared values of the noise waveform over a given period of time, showing its contribution to power dissipation. Lastly, PSD noise measures the distribution of the noise power over different frequency bands, revealing its frequency content and potential to cause timing errors or electromagnetic interference.

4 Noise Reduction

To reduce the power supply noise in CMOS circuits, there are several techniques that can be applied at different levels of the design process. Power supply filtering, which involves adding passive or active components such as capacitors, inductors, or regulators to the power supply lines to attenuate the noise or stabilize the voltage, can be effective for reducing external noise or low-frequency noise. However, this technique may introduce parasitic effects or increase the cost and complexity of the circuit. Additionally, power distribution network design involves optimizing the layout and routing of the power supply lines to minimize parasitic resistances and inductances that cause voltage drops and noise coupling. This technique can be effective for reducing internal noise or high-frequency noise, but it may require careful trade-offs between area, performance, and reliability. Lastly, circuit design involves modifying the circuit parameters or topology to increase the noise immunity or tolerance of the CMOS devices or logic gates. This technique can be effective for reducing signal degradation or timing errors, but it may affect the power consumption, speed, or functionality of the circuit.

5 Noise Mitigation

To mitigate power supply noise in CMOS circuits, several techniques can be applied at the post-design or operational stages. Noise monitoring involves detecting the noise level or the noise-induced errors in the circuit and reporting them to a higher-level controller or system. This technique can help diagnose the noise sources or assess the circuit quality, but it may require additional hardware or software resources or overhead. Noise compensation adjusts the circuit parameters or behavior to compensate for the noise effects or errors using feedback, calibration, or correction mechanisms. This technique can improve the circuit performance or reliability, but it may require additional control or communication circuits or signals. Lastly, noise avoidance avoids the noise sources or the noise-sensitive regions of the circuit using shielding, isolation, or scheduling techniques. This technique can prevent noise interference or damage, but it may limit the circuit functionality or availability.

6 Here’s what else to consider

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