Basic scheme of a S & H circuit.
In electronics, the sample and hold circuit (abbreviated S & H) is a sampler used as an interface between an analog signal that varies rapidly over time and a subsequent device, often an analog-to-digital converter (ADC, analog to digital converter ). The effect of this circuit is to maintain the constant analog value for the time needed for the converter or other subsequent circuits to perform signal operations.
In most circuits, a low capacitor C is used to conserve the analog voltage, plus a switch (usually constituted by a MOSFET, which has a small intrinsic input resistance R) which connects and disconnects the capacitor at the analog input. At the source of the transistor is a buffer that maintains the high global input impedance.
The use procedure is to inject the impulse signal into the gate of the transistor, enabling the source of the MOSFET to reach (almost instantaneously, given the size of the RC partition) the voltage of the input generator. Immediately after the impulse is entered, the switch closes, so the capacitor does not discharge, thus memorizing the input signal. Repeating this procedure several times, you get a "step-by-step" representation of the input voltage output. The frequency with which the switch is opened or closed is the sampling frequency of the system. Graph of voltages in a S & H circuit, in the example of a sinusoidal input.
The reason for the need for such a circuit is obvious: in ADC the input is compared with a series of voltages produced by an internal digital-to-analog converter until the two measures match (within a certain margin of error). If the ADC was directly connected to the signal to be converted, without a S & H circuit maintaining its value, the input may vary during the comparison process, resulting in an inaccurate or completely incorrect conversion.
Often samples and hold are used when there is a need to analyze multiple signals: each input is captured and maintained using a clock signal common to all circuits, so you can read the values comfortably.
It is essential that capacity has little leakage, so you keep the input voltage constant and that it has no load, that is, it has a large impedance of input. Voices correlateemodify wikitesto
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