What Are Switched-Capacitor Circuits?

Switched-capacitor circuits are analog electronic circuits that process signals by periodically charging and discharging capacitors through MOSFET switches driven by non-overlapping clock signals. By controlling when charge is transferred between capacitors, these circuits emulate the behavior of resistors and integrators without relying on physical resistors, whose values are difficult to control precisely in integrated circuits. The technique is foundational to analog and mixed-signal design, enabling accurate filtering, data conversion, and signal generation on CMOS chips.

The approach emerged from the recognition in the 1970s that capacitor ratios in CMOS processes can be held to tolerances well below one percent, while absolute resistor and capacitor values are subject to manufacturing variation of twenty percent or more. Switched-capacitor design exploits this: because charge transfer is governed by capacitor ratios and clock frequency rather than absolute component values, circuit performance depends on quantities that can be controlled precisely.

Principles of Switched-Capacitor Operation

The fundamental building block is the switched-capacitor integrator. A capacitor connected through two MOSFET switches alternates between sampling a voltage at its input terminal and transferring the accumulated charge to an integrating capacitor at the output. The net charge transferred per clock cycle is proportional to the input voltage and to the ratio of the sampling capacitor to the integrating capacitor. This arrangement is equivalent to a resistor whose effective value equals the inverse of the product of the clock frequency and the sampling capacitance. Because clock frequency is set by a crystal oscillator, the effective resistance is determined accurately and can be tuned by adjusting the clock rate.

IEEE Solid-State Circuits publications on switched-capacitor circuit design document the systematic methods developed in the 1970s and 1980s for translating continuous-time filter specifications into discrete-time switched-capacitor networks, including bilinear and low-Q approximations that account for the sampled-data nature of the signal.

Switched-Capacitor Filters

Filters represent the most widespread application of switched-capacitor techniques. By designing networks of switched capacitors and operational amplifiers, engineers realize low-pass, high-pass, band-pass, and notch responses whose cutoff frequencies track the clock frequency precisely. This makes the response programmable: changing the clock rate shifts the entire frequency response proportionally, without modifying any component values. Anti-aliasing requirements impose limits on the ratio of clock frequency to signal bandwidth, typically requiring the clock to run at fifty or more times the highest signal frequency.

The switched-capacitor integrator, described as a circuit for all seasons in IEEE Circuits and Systems Magazine, illustrates how a single topology serves as the kernel for sigma-delta modulators, pipeline analog-to-digital converters, and sample-and-hold circuits that appear throughout mixed-signal integrated circuits.

Frequency Synthesizers

Frequency synthesizers generate stable output frequencies from a reference clock, and switched-capacitor techniques contribute to several points in the synthesis chain. Digitally controlled switched-capacitor arrays tune the resonant frequency of LC oscillators inside phase-locked loop (PLL) synthesizers, replacing variable-capacitance diodes with arrays of binary-weighted switched capacitors for finer frequency resolution and lower noise. In direct-digital synthesis, switched-capacitor interpolation filters smooth the staircase output of the digital-to-analog converter before reconstruction. Research on switched-capacitor-based frequency synthesis published in IEEE Xplore covers modern PLL architectures that integrate switched-capacitor loop filters to reduce die area compared to passive RC implementations.

Applications

Switched-capacitor circuits and synthesizers have applications in a wide range of fields, including:

• Wireless communication transceivers, where PLL synthesizers generate carrier frequencies
• Audio processing integrated circuits using switched-capacitor filters and sigma-delta converters
• Analog-to-digital and digital-to-analog conversion in instrumentation and data acquisition systems
• Biomedical sensing, including implantable devices that require precision filtering at low power
• Software-defined radio front ends with digitally reconfigurable switched-capacitor filter banks