This paper describes a method of estimating the internal impedance and the internal source voltage of digital ICs at electromagnetic interference frequencies. Estimates of the internal impedance of various types of CMOS digital ICs in operation-clock frequencies are performed. The result shows that there is a difference between the values of the internal impedance in the operation-clock frequencies and the values of the internal impedance measured by direct current. It is concluded that the output current from digital ICs when the load is changed can be estimated in accordance with the obtained values of the internal impedance of the digital ICs in the operation-clock frequencies.
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