A switched-capacitor filter (SCF) is an analog signal processing circuit that performs filtering using the principle of capacitor charging and discharging. It is a popular technique for implementing various types of analog filters, such as low-pass, high-pass, band-pass, and notch filters. The primary advantage of switched-capacitor filters is their compatibility with integrated circuit (IC) fabrication processes, making them well-suited for use in modern mixed-signal and digital systems.
Here's a general overview of how a switched-capacitor filter performs analog filtering:
Basic Principle: Charge Redistribution
At the heart of a switched-capacitor filter is the concept of charge redistribution. It involves alternately charging and discharging capacitors by connecting them to input and output voltages through switches. These switches operate in a cyclic manner, enabling the capacitor to transfer charge back and forth between different nodes within the filter circuit.
Sampling the Input Signal
The first step in the filtering process is to sample the input analog signal. This is achieved by closing the appropriate switches to connect the input signal to the capacitor. The capacitor then charges or discharges to the voltage level of the input signal.
Transfer of Charge
Once the capacitor is charged to the input voltage, the switches are reconfigured to connect the charged capacitor to different parts of the circuit. This is where the filtering action takes place. The capacitor's charge is effectively transferred to other capacitors or nodes within the filter circuit, thereby achieving the desired filtering effect.
Time Domain to Frequency Domain Conversion
By carefully controlling the timing of the switches, the switched-capacitor filter can approximate continuous-time filtering characteristics, such as low-pass or high-pass responses. The charge transfer process effectively converts the input signal from the time domain to the frequency domain, where filtering occurs.
Output Signal
The filtered output signal is obtained by measuring the voltage across one or more capacitors after the charge transfer process is complete. Depending on the design and configuration of the filter, the output voltage represents the filtered version of the original input signal.
Clocking and Control
The operation of the switched-capacitor filter relies on an external clock signal that controls the timing of the switches' opening and closing. Properly synchronizing the switches is essential for achieving accurate filtering characteristics.
Advantages of Switched-Capacitor Filters:
Flexibility: The filter's cutoff frequency and response shape can be adjusted by changing the clock frequency and switch timings.
Accuracy: Switched-capacitor filters can achieve high precision and linearity.
Compatibility: They are well-suited for implementation in integrated circuits.
Low component variation: The performance of SCFs is less sensitive to variations in the capacitor values compared to other analog filter implementations.
However, it's worth noting that switched-capacitor filters are subject to certain limitations, such as clock-induced noise and limited bandwidth due to the discrete-time nature of their operation. Nonetheless, their versatility and integration-friendly characteristics have made them valuable components in a wide range of analog signal processing applications.