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The inter-poly-oxide (IPO) capacitor is one of the main elements in present mixed-mode CMOS process technologies. However, the phenomenon of poly depletion causes a significant change in the measured capacitance with applied voltage. This effect, expressed in terms of the voltage coefficient of capacitance (VCC), can seriously deteriorate analog precision. In this letter, a novel cross-coupled scheme is proposed for the IPO capacitors. The resulting VCC has a very low measured value of 2 ppm/V in a 0.35-/spl mu/m standard mixed-mode CMOS process, achieved without any unconventional approaches.
Date of Publication: July 1999