On the Susceptibility to Switching Noise of Operational Amplifiers

This paper investigates the susceptibility of operational amplifiers (OpAmps) to switching noise generated by the commutation of output transistors in power ASICs. Such a switching noise couples to the analog control circuitry through the shared substrate and package parasitic elements. While primarily present at high frequencies, the switching noise is down converted within the analog channel bandwidth by the demodulation carried out by the OpAmp input stage. To better analyze this problem, a reference system comprising of a buck converter and a feedback OpAmp integrated in the same silicon die is considered. The disturbance at the OpAmp input is first evaluated through time-domain simulations and then analyzed in the frequency domain to gain further insight. The resulting noise is then applied to the inputs of three commercial OpAmps with different technologies, architectures, and electrical parameters through a Direct Power Injection (DPI) setup to compare their immunity.