| 摘要 |
PURPOSE: An apparatus for controlling the center of frequency OTA (Operational Transconductance Amplifier) filter is provided to accurately maintain the center frequency of OTA filter in regardless of a process variable by detecting the size of the process variable using PLL to compensate the center frequency. CONSTITUTION: A PLL(22) outputs comparative voltages(V_PLL1,V-PLL2) determined according to a process variable. A level comparator(20) outputs a control signal controlling the center frequency of OTA filter according to the difference between the comparative voltages(V_PLL1,V_LL2). The PLL(22) includes a voltage controlled oscillator(22d), a phase detector, and a low pass filter(22c). The voltage controlled oscillator(22d) provides an oscillation output corresponding to a control voltage applied thereto. The phase detector outputs a phase difference signal indicating difference between the oscillation output and an input frequency signal. The low pass filter(22c) gets an off set determined by a reference voltage and the process variable and provides an output signal generated base on the phase difference signal and the off set.
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