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Sample and hold circuits
Post: #1

Sample and hold circuits


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In electronics, a sample and hold circuit is an analog device that samples (captures, grabs) the voltage of a continuously varying analog signal and holds (locks, freezes) its value at a constant level for a specified minimal period of time. Sample and hold circuits and related peak detectors are the elementary analog memory devices. They are typically used in analog-to-digital converters to eliminate variations in input signal that can corrupt the conversion process.[2]
A typical sample and hold circuit stores electric charge in a capacitor and contains at least one fast FET switch and at least one operational amplifier.[1] To sample the input signal the switch connects the capacitor to the output of a buffer amplifier. The buffer amplifier charges or discharges the capacitor so that the voltage across the capacitor is practically equal, or proportional to, input voltage. In hold mode the switch disconnects the capacitor from the buffer. The capacitor is invariably discharged by its own leakage currents and useful load currents, which makes the circuit inherently volatile, but the loss of voltage (voltage drop) within a specified hold time remains within an acceptable error margin.
In the context of LCD screens, it is used to describe when a screen samples the input signal, and the frame is held there without redrawing it. This does not allow the eye to refresh and leads to blurring during motion sequences, also the transition is visible between frames because the backlight is constantly illuminated, adding to display motion blur.

Purpose

The sample and hold circuits are essentially used in linear systems. In some kinds of analog-to-digital converters, the input is often compared to a voltage generated internally from a digital-to-analog converter (D-A-C). The circuit tries a series of values and stops converting once the voltages are "the same" within some defined error margin. If the input value was permitted to change during this comparison process, the resulting conversion would be inaccurate and possibly completely unrelated to the true input value. Such successive approximation converters will often incorporate internal sample and hold circuitry. In addition, sample and hold circuits are often used when multiple samples need to be measured at the same time. Each value is sampled and held, using a common sample

Implementation

In order that the input voltage is held constant for all practical purposes, it is essential that the capacitor have very low leakage, and that it not be loaded to any significant degree which calls for a very highinput impedance.
A true sample and hold circuit is connected to the buffer for a short period of time; a track and hold circuit is designed to track input continuously.

Introduction

Connecting digital circuitry to sensor devices is simple if the sensor devices are inherently digital themselves. Switches, relays, and encoders are easily interfaced with gate circuits due to the on/off nature of their signals. However, when analog devices are involved, interfacing becomes much more complex. What is needed is a way to electronically translate analog signals into digital (binary) quantities, and vice versa. An analog-to-digital converter, or ADC, performs the former task while adigital-to-analog converter, or DAC, performs the latter.
Post: #2
In electronics a sampling and retention circuit is an analog device that samples (captures, seizes) the voltage from an analog signal that varies continuously and maintains (blocks, freezes) its value at a constant level for a specified minimum period. Sampling and retaining circuits and related peak detectors are elementary analog memory devices. They are typically used in analog-to-digital converters to eliminate variations in the input signal that may damage the conversion process.

A typical sampling and retention circuit stores the electric charge in a capacitor and contains at least one switching device such as a FET switch (field effect transistor) and usually an operational amplifier. To sample the input signal, the switch connects the capacitor to the output of a buffer amplifier. The buffer amplifier charges or discharges the capacitor so that the voltage across the capacitor is nearly equal to or proportional to the input voltage. In standby mode, the switch disconnects the capacitor from the buffer. The capacitor is invariably discharged by its own leakage currents and payload currents, which makes the circuit inherently volatile, but the voltage loss (voltage drop) within a specified timeout remains within a margin of error acceptable.

Sampling and retention circuits are used in linear systems. In some types of analog-to-digital converters, the input is compared to an internally generated voltage from a digital-to-analog converter (DAC). The circuit attempts a series of values ​​and stops converting once the voltages are equal, within a defined margin of error. If the input value were allowed to change during this comparison process, the resulting conversion would be inaccurate and possibly unrelated to the true input value. Such successive approach converters will often incorporate internal sampling and retention circuitry. In addition, sampling and retention circuits are often used when multiple samples need to be measured at the same time. Each value is sampled and retained, using a common sample clock.

For virtually all commercial liquid crystal active matrix monitors based on TN, IPS or VA electro-optical cells (excluding bi-stable phenomena), each pixel represents a small capacitor that has to be periodically charged at a level corresponding to the scale value of gray (contrast) for an image element. In order to maintain the level during a scan cycle (frame period), an additional electric capacitor is connected in parallel to each LC pixel to better retain the voltage. A thin film FET switch is directed to select a particular LC pixel and load the image information therefor. In contrast to an S / H in general electronics, there is no operational output amplifier and no electrical signal AO. Instead, the charge on the retention capacitors controls the deformation of the LC molecules and hence the optical effect as their output. The invention of this concept and its implementation in thin film technology have been honored with the IEEE Jun-ichi Nishizawa Medal in 2011.

During a scan cycle, the image does not follow the input signal. This does not allow the eye to cool and can lead to blurring during motion sequences, also the transition is visible between frames because the backlight is constantly illuminated, adding to display motion blur.
 

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