A feedforward-based active shielding technique for digital noise suppression is more preferred for its capability of reducing the noise on the entire area inside the guard ring. In order to compensate for the variation of substrate parameters, an automatic control scheme to tune the gain of the active shield circuit is proposed. Simulation results show the effectiveness of the proposed system in reducing the digital noise regardless of circuit layout. Simulation results also show that noise suppression improvement from passive guard ring to active shield with tuning is 20 dB or one tenth while that from active shield without tuning to active shield with tuning is 12 dB.

An active shield circuit which effectively reduces the substrate noise on the entire area inside the guard ring regardless of the noise source position is proposed. Simulation result shows that the proposed circuit can reduce the noise level to -85 dB while a conventional guard ring gives -52 dB.

A design optimization of active shield circuit using noise averaging method is proposed. The relation between the averaged noise and the design parameters of the active shield circuit such as circuit gain and on-chip layout is examined. A simple design guideline is also provided. Simulation results show that the active shield circuit designed by the proposed optimization method gives a better noise suppression performance of about 28% than the conventional one.

A fully on-chip active guard band circuit is proposed. The proposed circuit is mainly composed of current mirrors and based on a DC bias technique. HSPICE simulations and experiment results confirm the validity of the proposed active guard band circuit.

A method called "active guard band filtering" is proposed for reducing substrate noise in analog and digital mixed-signal integrated circuits. A noise cancellation signal having an inverse value to the substrate noise is actively input into a guard band to suppress the substrate noise. An operational amplifier produces the noise cancellation signal based upon the substrate noise detected by one guard band and feeds this signal through another quard band into the substrate. This is done within the amplifier feedback loop, which includes the guard bands and the substrate. The noise suppression effect was measured by using 0.8µm CMOS test chip. Using active guard band filtering suppressed substrate noise to -40 dB of the original non-canceled noise level at 8 MHz. The noise suppression effect was also observed at frequencies up to 20MHz, with an external operational amplifier. The influence of parasitic impedance was found to be a key factor in noise suppression. An active guard band filter with an on-chip noise cancellation circuit will be even more effective for high frequencies, because it eliminates parasitic impedance due to external components.