Impact of Spread Spectrum Modulated Switching Noise on Feedback Operational Amplifiers

This paper investigates the susceptibility of operational amplifiers (OpAmps) to spread-spectrum–modulated switching noise in power ASICs. The fast commutation of large switching devices injects electric charges into the silicon substrate they share with other building blocks like the analog ones, that results in switching noise superimposed onto the nominal signals. While primarily present at high frequencies, the switching noise is also demodulated by the nonlinear devices in the analog section, generating in-band components. Since such disturbances are inherently synchronous with the PWM clock, any spread-spectrum modulation applied to the switching frequency directly shapes the substrate-coupled interference. Therefore, input-referred switching-interference waveforms are first derived using a simplified substrate-coupling model that captures the interaction between the power and analog/ control sections. The disturbances are then shaped through spread-spectrum modulation, whose parameters are optimized for EMI reduction, and injected into a commercial operational amplifier using the Direct Power Injection (DPI) approach. Experimental results show that the spread-spectrum–modulated substrate noise is demodulated by the device under test, generating an in-band component at the modulation frequency.