Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 155-158).This thesis presents a study of the effects of substrate noise on analog circuits in mixed-signal chips and techniques for minimizing these harmful effects on sensitive analog circuits. A microchip built in a 0.25um CMOS epitaxial process was designed, fabricated, and tested for this research. Through the use of an on-chip sampling scope, the effect of substrate noise generated by digital inverters with coupling capacitors to the substrate on analog circuits was characterized. Substrate noise coupled into a representative analog circuit, a switched capacitor delta-sigma modulator primarily through the asymmetrical parasitics of the input sampling circuit. Furthermore, since some of the parasitics are nonlinear with input voltage, substrate noise couples into the analog circuits producing an input signal dependent component and an input signal independent component. The substrate noise, with decay time constants of a few nanoseconds and ringing frequencies of few hundred megahertz, can decrease analog circuit performance. In the case of a delta-sigma modulator, substrate noise caused the signal to noise power ratio to decrease by more than 18dB, 3 bits in terms of analog-to-digital converter metrics. In addition, two techniques of minimizing the substrate noise and its effects were explored. The first used a replica delta-sigma modulator on the same chip to subtract the effects of substrate noise from the original delta-sigma modulator. This method proved useful for removing input signal independent substrate noise, but not input signal dependent substrate noise which dominates in-band noise for large input signal magnitudes. The second technique involved an active substrate noise cancellation system.(cont.) A discrete time feedback loop senses the substrate noise, processes it through a filter, and uses an array of digital inverters to cancel the substrate noise. The principal advantages of this technique are the shaping of substrate noise through a designed filter without a significant power penalty and design independence from the analog and digital components. Measured data shows that this technique is capable of over 20dB reduction in substrate noise on the substrate voltage itself. Measured data also shows over 10dB improvement in SNDR of the delta-sigma modulator in certain cases.by Mark Shane Peng.Ph.D
