A design experiment for measurement of the spectral content of substrate noise in mixed-signal integrated circuits

In mixed-signal ASICs coupling from switching digital nodes and from the digital power supply to analog circuits via the common substrate can degrade the performance of the analog circuits. This paper describes a design experiment to measure the time domain behavior and spectral content of such substrate coupling noise. To measure this noise over a wide frequency range a novel analog substrate noise sensor has been designed. Using this sensor, substrate noise has been measured in the time and frequency domain. Also the influence of supply voltage, switching activity and mounting technique on the substrate noise are experimentally investigated. Simulation results, using a SPICE substrate model, are also included. The presented measurements show that careful investigation of the spectral content of substrate noise is important in the design of mixed-signal ASICs. Differences between the peak noise levels and the noise floor can easily be 40 dB.