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Control of on-chip power supply noise has become a major challenge for continuous scaling of CMOS technology. Conventional passive decoupling capacitors (decaps) exhibit significant area and leakage penalties. To improve the efficiency of power supply regulation, this paper proposes a distributed active decap circuit for use in digital integrated circuits (ICs). The proposed design uses an operational amplifier to boost the performance of conventional decaps. Simulations proved its enhanced decoupling effect in comparison with passive decaps. The proposed active decap also shows advantages in providing additional damping to the on-chip resonant noise. To verify the performance from the proposed circuit, a 0.18-mu m test chip with on-chip noise generators and sensors has been fabricated. Measurements show a 4-11times boost in decap value over conventional passive decaps for frequencies up to 1 GHz with a total area saving of 40%. Local supply noise distribution and decap gating capability were also examined from the test chip.