sample and hold circuit

[Aman bharti]

Sample and hold circuit

Definition and Introduction 

Sample and hold circuit is an electronic circuit which makes the samples of input voltage signals and holds these samples signal for some very small time duration.

 

Sample and hold circuit is basically is an analog to digital converter circuit. Input voltage signals to be sampled and hold for some duration (in microseconds) using the capacitor and give the output in the form of digital pulse. Because this works by the holds the sampled analog input signal this is called the sample and hold circuit. Holding of the signal is done using a capacitor called holding capacitor, capacitor has the ability to store the voltage by charging until it will discharge. This charging and storing property of the capacitor is used here for the holding of the signal.

Sample

Some Parts consider or Take the values of a given continuous input signal at each different time is called sample.

Hold

catch the sampled signals and take hold for a time t is called hold.

Sampling time

The time during which generates the samples of input signals called sampling time. This is usually 1us to 14 u sec.

Holding time

The time during which the circuit holds the sampled value is called the holding time. Holding time to be set according to the required for application.

Basic sample and holding circuit

A basic sample and hold circuit consists a switch and a capacitor. A combination of a control switch and a capacitor is called a simple sample and hold circuit. Input voltage applied at the point Vin and then a switch connected after that a capacitor connected between the switch and output. One side of the capacitor is connected with the ground. A JFET or a MOSFET is used as a Control switch.

Working

When the switch is closed the connected capacitor charged and when the switch is opened the capacitor will not get discharged because there is no path for discharging and then the capacitor takes to hold the signal.

Sample  hold circuit using op-amp

In this circuit two voltage follower circuit, a capacitor and A MOSFET connected. Input voltage given at The control voltage is given on the gate terminal of MOSFET, here MOSFET is used as a switch. The reason of using the Voltage follower circuit is this circuit gives output same as the input signal and there is a very high input impedance so no load is considered at the input and the capacitator will also not get discharged because of very high impedance at the input of the op-amp.

The MOSFET is used here as the control switch for applying the voltage pulse. The Gate terminal of MOSFET is connected with a controlled pulsed signal generator.  Astable multivibrator can be used to produce frequency. This astable frequency switch ON-OFF Very fast the MOSFET. By the ON-OFF, ON-OFF of the MOSFET a pulsed form of the input voltage signal is applied to the NON-Inverting terminal of Op-amp. The capacitor will charge as a pulse signal gets for the 1^st pulse to its max level and this level is HOLD for a definite time by the capacitor because this capacitor will not discharge. And now the next pulse falls and the capacitor will again charge as this pulse voltage level and this level hold and this process is continuously proceeding as per pulse signal and the amplitude level of the pulse signal.

So, in this whole process, the output is sampled and also hold for a time duration, these are done by the main capacitor used in this circuit. We know that the sampled signals are the digital signals, So we can say that the sample and hold circuit converts analog or continuous-time signal into digital signals.

Uses or applications of Sample and Hold  circuit

• ADCs (Analog-to-Digital Conversion)
• DACs (Digital-to-Analog Conversion)
• In Analog Demultiplexing
• In Linear Systems
• In Data Distribution System
• In Digital Voltmeters
• In Signal Constructional Filters

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