The simplified mathematics models shown in the equation (1) and (2) can be derived from the fig 1, and the print circuit board (PCB) parasitic effect is ignored.In SOC, the substrate coupling is the main form of the common impedance coupling. The substrate network in low resistive substrates behaves as a single electrical node. For high resistive substrates, the resistance between two points of the silicon surface increases with the distance, making the single node model inadequate. From analysis of the substrate conducting characteristics, a simplified circuit model can be derived, which is shown in Fig 1[1].
Fig 1 the simplified circuit model of substrate in mixed signalICs There are many methods to reduce the substrate noise in the mixed signal ICs. In this paper, the mathematics model of the substrate noise is obtained firstly, and then the main methods for reducing the substrate noise, which are the di/dt detector [2], on-chip active guard band filters [3] and the P+ guard ring/triple-well techniques [4], are analyzed using the model. Lastly an adder circuit is proposed based on the inverse relationship between the varying voltage of the digital power supply line and the one of digital ground line. The simulations prove the proposed method; the noise is reduced by 26dB, compared with triple-well techniques.Vdd