The use of deep-submicron CMOS processes allows for an unprecedented degree of integration in digital circuitry, but complicates the implementation of traditional RF and analog circuits. For example, frequency tuning of a low-voltage deep-submicron CMOS oscillator is a challenging task due to its highly nonlinear frequency behavior with degradation in voltage and low voltage headroom. This makes it susceptible to the variations in the power or ground supply and substrate noise. A low supply voltage can negatively impact the dynamic range of the signal and result in an increasing noise floor, thus further degrading the signal-to-noise ratio. At times, it is possible to find a specific solution, such as utilizing a voltage doubler. Unfortunately, with each CMOS feature size reduction, the supply voltage needs to be scaled down to ensure the reliability of the circuit.
The high degree of integration can lead to the generation of substantial digital switching noise that is coupled through the power supply network and substrate into noise sensitive analog circuits. Furthermore, the advanced CMOS processes typically use low resistance P-substrate which is an effective means in combating latchup problems, but exacerbates substrate noise coupling into the analog circuits. This problem only gets worse with scaling down of the supply voltage.
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