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 m 41 m, respectively.
An active guarding technique is demonstrated for substrate immunity improvement on LC-tank oscillators. Active components in the oscillator, which are sensitive to substrate noise and contribute to the output most, are guarded from the substrate perturbation by applying the active guarding circuit to decouple and also compensate the noise current. To evaluate the effectiveness of the proposed technique, noise signals of both single and multiple tones of various frequencies are injected to the substrate close to the guarded transistors and measure the sideband spurs by switching the active guarding circuit on/off respectively. Experimental results showed more than 11 dB noise suppression performances can be achieved in a wide frequency range from DC to 10 GHz. By applying the active guarding circuit to a LC-tank oscillator, more than 17 dB spur suppression performance is obtained in the frequency range of interest.
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