Substrate Noise Reduction Using Active Guard Band Filters in Mixed-Signal Integrated Circuits

摘要

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 guard 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 0.8 μm 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 20 MHz, 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.