Evaluation of using active circuitry for substrate noise suppression

Abstract

The performance of system-on-chips can be severely degraded if noisy circuits interfere with sensitive circuits through the common silicon substrate. Many methods have been proposed to suppress such substrate noise, ranging from designing circuits that generate less noise to using guard bands to prevent noise from reaching the sensitive parts. In this paper we have investigated a proposed method for substrate noise suppression using active circuitry, and compared it with two passive methods; resistive grounding and capacitive decoupling. Published results indicate that active noise suppression outperforms passive noise suppression. However, these results are based on simulations using oversimplified substrate models, like single node or resistor T-models. In this paper we confirm these previous results, but we also show that when using a more realistic substrate model these advantages disappear! We claim that there is no noticeable difference between active noise suppression and DC grounding; these two methods are comparable and better than capacitive decoupling. The substrate resistivity in system-on-chip solutions typically result in large substrate resistances that decrease the efficiency of active decoupling compared to simple DC grounding.