Switched-Capacitor
A switched-capacitor is an electronic circuit technique that uses capacitors and switches to simulate resistors and create discrete-time analog signal processing functions.
Switched-Capacitor Circuits
Switched-capacitor (SC) circuits represent a fundamental approach in analog integrated circuits that enables the implementation of complex signal processing functions using only capacitors, switches, and operational amplifiers. This technique emerged in the late 1970s as a revolutionary solution for integrating analog filters in CMOS technology.
Operating Principle
The core concept relies on the periodic switching of capacitors to emulate resistive behavior:
- During phase φ1, a capacitor charges to an input voltage
- During phase φ2, the charge transfers to another node
- This charge transfer, occurring at a fixed frequency, creates an equivalent resistance
The equivalent resistance R_eq is given by:
R_eq = 1/(f_s * C)
where f_s is the switching frequency and C is the capacitor value.
Advantages
- High precision due to excellent capacitor matching in CMOS processes
- Temperature compensation against temperature variations
- Reduced chip area compared to traditional resistor implementations
- Programmability through clock frequency adjustment
- Common-mode rejection noise immunity
Common Applications
1. Filters
- Active filter
- Anti-aliasing filters
- Delta-sigma modulator
2. Data Conversion
- Sample and hold circuits
- Analog-to-digital converter front-ends
- Digital-to-analog converter output stages
3. Signal Processing
- Discrete-time signal processing
- Gain control circuits
- Integration and differentiators
Design Considerations
Several factors require careful attention when designing SC circuits:
- Clock Feedthrough
- Clock feedthrough
- Bottom-plate switching techniques
- Compensation techniques methods
- Noise
- kT/C noise from switching
- Thermal noise considerations
- Sampling theory effects
- Component Selection
- MOSFET resistance
- Capacitor matching requirements
- Op-amp specifications bandwidth
Modern Developments
Contemporary applications have extended SC techniques into new domains:
The flexibility and precision of switched-capacitor circuits continue to make them essential in modern integrated circuit design, particularly in applications requiring precise analog signal processing in digital CMOS processes.