What Are Sampled Data Circuits?

Sampled data circuits are electronic circuits that process analog or mixed signals by operating on discrete-time samples rather than on the continuous waveform. Instead of responding instantaneously to every variation in the input, a sampled data circuit captures the signal at regular intervals defined by a clock and performs its intended function, such as filtering, amplification, or comparison, on those captured values. This discrete-time mode of operation bridges purely analog circuit design and digital signal processing, enabling implementations that achieve high accuracy in standard CMOS technology without requiring precision resistors.

The conceptual foundation of sampled data circuits rests on the Nyquist-Shannon sampling theorem, which establishes that a bandlimited signal can be reconstructed without error if sampled at a rate at least twice its highest frequency component. In practice, anti-aliasing filters precede the sampling stage to ensure out-of-band content does not fold into the signal band during the sampling process. The field evolved rapidly in the 1970s when researchers recognized that a MOSFET switch combined with a capacitor could functionally replace a resistor in an active filter, enabling filters whose characteristics depend on capacitor ratios and clock frequencies rather than on absolute resistor values.

Switched-Capacitor Circuits

The switched-capacitor technique is the dominant implementation paradigm for sampled data circuits in integrated form. In a switched-capacitor resistor equivalent, two non-overlapping clocks alternately connect a capacitor to an input node and then to an output node, transferring a packet of charge proportional to the capacitor value and the voltage difference per clock cycle. The effective resistance seen by the circuit equals the inverse of the product of clock frequency and capacitance, meaning the filtering characteristics scale precisely with the clock rate. Switched-capacitor filters, integrators, and amplifiers built on this principle are standard building blocks in CMOS processes and are described in depth in UCLA's course material on analog integrated circuits. The accuracy of signal-processing functions in switched-capacitor designs is set primarily by the matching of on-chip capacitor ratios, which CMOS processes can control to within 0.1 percent, enabling filter poles and zeros to be placed with precision impractical for continuous-time RC networks.

Clocking and Noise Considerations

Two-phase non-overlapping clocks govern the operation of most sampled data circuits, with each phase dedicated to either sampling or processing to prevent charge sharing errors. Clock feedthrough and charge injection from MOSFET switches introduce small voltage errors at each transition, and designers use fully differential topologies and complementary switch pairs to cancel these effects by symmetry. Thermal noise sampled onto hold capacitors, called kT/C noise, sets a fundamental lower bound on the noise floor: the rms noise voltage across a capacitor C at temperature T equals the square root of kT/C, so larger capacitors are used to meet demanding dynamic range specifications. A tutorial on switched-capacitor noise analysis published in Analog Integrated Circuits and Signal Processing provides a systematic hand-calculation approach to predicting noise in multi-stage sampled data topologies.

Filters, Converters, and Data Paths

Sampled data circuits serve as the core building blocks in pipeline and successive-approximation ADCs, delta-sigma modulators, switched-capacitor filters for anti-aliasing and reconstruction, and correlated double sampling circuits used to suppress offset and 1/f noise in image sensors. The history and taxonomy of analog sampled-data signal processing, from bucket-brigade devices through modern switched-capacitor and continuous-time delta-sigma converters, is surveyed in a retrospective article in Analog Integrated Circuits and Signal Processing. Modern system-on-chip designs routinely integrate sampled data circuits alongside digital logic in standard deep-submicron CMOS, making them indispensable to communications, sensing, and control applications.

Applications

Sampled data circuits have applications in a range of fields, including:

• Analog-to-digital and digital-to-analog conversion in communications systems
• Switched-capacitor filters in audio codecs and baseband processors
• Correlated double sampling in CMOS image sensors and scientific detectors
• Delta-sigma modulators for high-resolution measurement instrumentation
• Sample-and-hold front-ends for multiplexed sensor interfaces
• Charge redistribution networks in successive-approximation ADCs