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In this paper, an active guarding circuit is presented for wideband substrate noise suppression. A feed-forward compensation mechanism is proposed to extend the noise suppression bandwidth and to adjust the amplitude of phase-inversed noise cancellation current by introducing a zero and an amplitude controller. A noise decoupling mechanism is developed to provide a decoupling path and to sense the noise level for generating noise cancellation current. With substrate characterization, parameters of substrate network impedance, decoupling factor, and amplitude of noise cancellation current can be either obtained or determined. The active guarding circuit is implemented in a 90-nm CMOS process based on substrate characterization. The measured substrate noise suppression is better than -9 dB from dc to 250 MHz. The -3-dB suppression bandwidth is effectively extended to 1.2 GHz. The power consumption and circuit area are 2.5 mW and 20 [micro]m x 41 [micro]m, respectively. Index Terms--Analog circuits, CMOS integrated circuits (ICs), substrate coupling noise, substrate crosstalk, substrate isolation.