A switched-capacitor sample-and-hold (S&H) amplifier is an essential building block in analog circuits for data acquisition and signal processing applications. It serves the purpose of capturing and temporarily storing an analog voltage (input signal) to make it available for further processing or conversion. The main advantage of using a switched-capacitor S&H amplifier is that it provides accurate and precise sampling while using relatively simple digital control.
The operation of a switched-capacitor S&H amplifier involves two main phases: the sampling phase and the holding phase.
During the sampling phase, the switch (usually implemented using MOSFETs) connects the input analog voltage (Vin) to the input capacitor (Cin). The capacitor begins to charge up and acquires the voltage level of the input signal. The duration of the sampling phase is controlled by a clock signal (usually generated by an external clock source or an internal oscillator) that determines when the switch is closed and the capacitor starts to charge.
Once the sampling phase ends, the switch disconnects the input capacitor from the input signal. The capacitor retains the sampled voltage level from the previous phase and isolates it from the input signal. Now, the capacitor holds this voltage and acts as a temporary storage element, preventing any changes in the input signal from affecting the output.
In some cases, the sampled voltage may be too small or require further amplification. In such situations, an optional amplifier stage is connected to the output of the capacitor during the holding phase. This amplifier increases the voltage level to an appropriate range for subsequent processing or conversion.
In certain applications, it is necessary to reset the capacitor before the next sampling phase to avoid errors in the subsequent samples. To achieve this, a discharge switch can be introduced to rapidly reset the capacitor's voltage to a predefined level at the end of the holding phase.
The process then repeats for each sampling cycle, allowing the switched-capacitor S&H amplifier to capture the analog input signal accurately at specific intervals determined by the clock frequency.
Advantages of Switched-Capacitor S&H Amplifier:
High Accuracy: The sampled voltage is accurately captured and held, minimizing errors caused by variations in the input signal during the holding phase.
Simple Design: The use of digital control and MOSFET switches simplifies the implementation and reduces the need for precision analog components.
Low Input Leakage: The sampling capacitor acts as an isolation barrier between the input and the output, reducing leakage and improving the overall performance.
Compatibility: Switched-capacitor circuits are compatible with integrated circuit (IC) technologies, making them suitable for monolithic integration with other analog and digital components.
Switched-capacitor sample-and-hold amplifiers are widely used in various applications, including data converters (analog-to-digital converters and digital-to-analog converters), communication systems, signal processing, and sensor interfaces.