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A new method for modeling the electromagnetically coupled substrate noise from digital gates in large mixed-signal integrated circuits was developed. The unique feature of the method is that it employs a modular approach to model noise coupling via current injection from the digital gates which alleviates the problems associated with the current methodology in so far as the circuit size, instability the effects of parasitics and the accuracy issues are concerned. The method was demonstrated on the characterization of the substrate noise issues in FLASH analog-to-digital (A/D) converters. The model was then used to study the addition of the p+ guards around the input devices of comparators in order to asses their effectiveness in reducing the substrate noise. The effect of variations in the package bond-wire inductances in the net injected current noise was also modeled.