Electronic equipment such as telecommunications systems utilizing, for example, pulse-code modulation (PCM) and other voice-band systems require precision high-order filters. Conventional filtering approaches implementing audio and other low-frequency filters have utilized resistor-capacitor differential integrators. With the development of metal-oxide-semiconductor (MOS) technology such conventional filters have been fabricated monolithically as resistor-capacitor products. Such fabrication has required large amounts of semiconductor substrate area with the absolute values of both resistive and capacitive circuit elements being tightly controlled. This control is extremely difficult for typical temperature and processing variations.
Monolithic implementation of low-frequency filters requires filter operation similar to passive low-frequency filters having long time constants but is required to be fabricated in small semiconductor substrate areas and utilize transfer functions that are insensitive to parameter variations. Additionally, in such monolithic implementation it is desirable to obtain precise responses without external trimming operations. Previously developed conventional active filters implemented with thin-film or hybrid circuits have not provided a precision filter insensitive to temperature and processing variations.
As a result of the need for trimming techniques to ensure absolute values of components utilized in high-order filters and the need for maintaining temperature and processing variations independence, switched-capacitor integrator filters have been developed. Such switched-capacitor circuits closely approximate conventional differential integrators. The differential switched-capacitor integrator is operated with two-phase nonoverlapping clocks. A discussion of MOS switched-capacitor filters is found in a paper by Allstot et al, "MOS Switched Capacitor Ladder Filters"; IEEE Journal of Solid-State Circuits, Vol. SC-13, No. 6, December 1978.
In order to simulate capacitor and inductor elements used in passive filters, a switched-capacitor integrator must not introduce any phase shift in the signal being processed. Previously developed switched-capacitor filters simulating inductor-capacitor filters have introduced phase shifts such that these filter elements appeared as lossy inductors and lossy capacitors. With the introduction of a phase shift in the input signal, a degregation in the output response is produced such that existing switched-capacitor filters do not precisely simulate passive filter elements.
A need has thus arisen for a switched-capacitor filter for simulating capacitor and inductor elements in passive ladder filters in which the simulated capacitor and inductor elements have no loss due to an effective zero phase shift in the circuit simulating these elements in voltage and current domain. Such a switched-capacitor filter must be insensitive to parameter variations introduced by temperature and processing controls. Additionally, a need has arisen for a switched-capacitor filter in which circuit component values can be realized without a need for trimming. A need has further arisen for switched-capacitor filter circuit to implement doubly terminated ladder filters such as, high-pass, low-pass and bandpass filters being insensitive to component variations. Additionally, a need has further arisen for a switched-capacitor filter in which filter elements can be fabricated in small areas of a semiconductor substrate to permit for densely packed chips while simultaneously eliminating the effect of parasitic capacitances.