Switched-Capacitor Low-Pass Filter with Pole Frequency Tuning at Constant Frequency of Switching

The circuitry of a switched-capacitor low-pass filter (SCF) is considered, in which it is possible to tune the pole frequency without changing the capacitances of the switched capacitors, for example, by selecting them of the same kind. To study the suggested SCF circuit, the analysis in the time domain was used, and the obtained characteristics of the SCF were further compared with the characteristics of a generalized second-order analog filter. It is shown that the parameters implemented by the suggested filter depend not only on the capacitance values of the switched capacitors, but also on the ratio of the resistances of two additional resistors. Moreover, the SCF pole frequency is determined by the switching frequency of the keys, the ratios of the capacitances of the switched capacitors, as well as the ratio of the resistances of additional resistors. Due to the goal-oriented selection of the resistances of these resistors, it is possible to lower the SCF pole frequency at constant and identical values of the capacitances of the switched capacitors.

I. Introduction

Despite the fact that the methods for digital processing of the analog signal are widely used in the digital transformation of industrial production they are redundant in some cases. As a result, the analog-to-discrete processing, which combines the main advantages of the analog-digital methods, is very promising. So, the switched-capacitor low-pass filters (SCF) produced by dozens of the leading microelectronic companies of the world give a significant advantage over classic digital and analog filters in terms of size, cost, current consumption, accuracy, functionality and are an effective means of constructing circuits of frequency selection and processing of analog signals in science and technology. The most promising solutions for the SC filters [1]–[70] are patented by firms in the USA, Japan, France, Taiwan, China, Germany, Great Britain, Italy, and others. At the same time, these patents solve the problem of tunning the parameters of the SC filters by changing the ratio of the switched capacitances or the switching frequency of the keys, it provides for the organization of differential inputs [31], the reduction of non-linear distortions [16], [19], the reduction of the influence of operational amplifiers (op-amps) [2], [15], the minimization of the capacitance of switched capacitors [8], the use of a special set of digitally controlled capacitances [3]. Today the SC filters are successfully used in the range of low temperatures, in the developments of NASA (the USA) in the study of planets and deep space, as well as in microwave vehicle radars.