| 摘要 |
The electronic filter has three integrating operational amplifiers (12, 14, 16) connected in series. An input voltage (Vin) to be filtered is supplied during alternating clock pulses (phi, phi') to the first integrator (12) from a capacitor (30) and to the capacitor for storage, respectively, by way of MOS switching devices (18-24). Capacitances (40, 44) are connected in a feedback loop of the first amplifier, of which the output is connected via capacitor (68) and MOS switches (58, 62, 70, 72) to the inverting input of the second integrator (14). The stray capacitances are eliminated effectively by the additional pair of switches for each switched capacitor, by the alternate charging and discharging of the stray capacitors on the alternate clock pulses. A third order elliptic filter characteristic is achieved having two complex conjugate poles, a real pole and two complex conjugate zeroes. Magnitudes of the zeroes are not affected by the capacitor valves alpha i, where i equals 1, 2... . In the transfer function, the magnitude of the transmission zero is the constant term of the quadratic factor in the numerator and is always unity no matter what the capacitor values are. |
