Switched-capacitor circuits are highly suited to on-chip analog signal processing. Their main component—the switched-capacitor (SC) resistor—is easy to build into an integrated circuit for any application requiring better resistance, high-precision frequency responses, and small chip area. Filters, comparators, and power conversion for computing, telecommunication, and the Internet of Things (IoT) are some of the common switched-capacitor circuit applications. Here’s a look at how switched-capacitor circuits work and their benefits.
What Is a Switched-Capacitor Circuit?
This circuit uses a switched-capacitor to aid in discrete time signal processing. It includes switches that are opened and closed to allow for the transfer of charge in and out of a capacitor. Typically, non-overlapping clocks control the switches to ascertain predictable/controllable charge transfer.
The Role of the Switched-Capacitor Resistor
A typical switched-capacitor circuit comprises a capacitor and two switches. The switches connect the capacitor alternately to the input and output voltage. During each switching period, a charge is transferred from the input to the output at the clock frequency. Therefore, the switched-capacitor circuit functions as a resistor controlled by the capacitor value and the switching frequency.
Switched-Capacitor Circuit Advantages
Switched-capacitor circuits allow for the design of tightly controlled resistance on-chip, based on the clock frequency and capacitor value. This circuit design approach has multiple advantages, including:
- Saves space while achieving high resistance values. Using actual resistors would require a larger amount of silicon space.
- Achieve a frequency response accuracy of the order of 0.1% with switched-capacitor filters. Matching similar IC components(such as capacitor to capacitor) instead of two different components(such as capacitor to resistor) allows for this level of precision. In contrast, a continuous-time filter that uses RC time constants to control the frequency can cause frequency variations of as much as 20% because of the mismatch between resistors and capacitors.
- Easier to reliably implement with a broader range of values, making it an ideal replacement for traditional resistors in any integrated circuit application. Similarly, it’s possible to adjust the equivalent resistor value by changing the clock speed.
Switched-capacitor circuits have the right properties for the design of ICs for electronic filters, comparators, power converters, telecommunication systems, and other applications. They offer precise frequency responses that are difficult to achieve with resistor-to-capacitor circuits. Their dynamic range is also an important attribute.
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