Sample & Hold Operation | S/H Circuit using Op-Amp | Data Acquisition System | Data Logger

 Sample & Hold Operation

In electronics, a sample and hold (also known as sample and follow) circuit is an analog device that samples (captures, takes) the voltage of a continuously varying analog signal and holds (locks, freezes) its value at a constant level for a specified minimum 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.

They are also used in electronic music, for instance to impart a random quality to successively-played notes.

 

S/H Circuit using Op-Amp

   Sample & Hold Circuit is used to sample the given input signal and to hold the sampled value.

Sample and hold circuit is used to sample an analog signal for a short interval of time in the range of 1 to 10µS and to hold on its last sampled value until the input signal is sampled again.

The holding period may be from a few milliseconds to several seconds.

 

   The following figure shows the block diagram of a typical sample and hold amplifier.

The Command terminal is in the form of a logic pulse. It controls whether to sample the input signal or hold the last sampled value of the input signal. When the pulse is high signal is sampled and when the pulse is low signal value is holded. Thus the circuit has two modes of operation depending upon the logic level of S/H command signal.

Upon receiving the input command pulse, the circuit samples the input and output follows input i.e. output tracks the input so called TRACK mode of operation. After command pulse is removed the circuit holds the output at a value which input signal had at an instant of pulse deactivation; which is called HOLD mode.

Sample and hold circuits are used in following applications.

• Analog to Digital conversion (ADCs) Out of different ADCs, successive approximation type ADC uses S/H circuit, where the signal is to be held constant while A to D conversion is taking place.
• In analog demultiplexing in data distribution and in analog delay lines.
• In general S/H circuits are used in all applications where it is necessary to freeze the analog signal for further processing.

The following figure shows a typical sample and hold circuit using op-amp

Amplifier A1 and A2 are both voltage follower circuits.

FET is operated as ON/OFF switch.

The S/H pulses controls the switching ON/OFF of FET.

Signal to be sampled is applied at Vin. Input impedance of A1 is very high so input voltage source is not loaded. While sampling output of A1 is same as Vin.

When S/H pulse is applied FET switches ON and starts conducting.

Resistance between drain and source (rdsON) is very small.

For voltage follower, A1 and A2 have 100% feedback (β=1). Therefore output impedance of A1 and A2 is very small.

Now capacitor C starts charging through rdsON and output impedance of A1.

Charging Time Constant = rdsON × rout × C

As rdsON and rout are very small, capacitor C charges through very quickly to Vin (i.e. capacitor tracks the input signal).

At the end FET is off, so almost acts as open circuit. So capacitor isolates from previous circuits and it holds the charge of last sampled value.

As input impedance of A2 is very large, capacitor discharging time is very high, so it almost holds the charge. Also Gain of A2 is unity.

Therefore

Vout = Charge on capacitor

As rout of A2 is very small, we can take Vout across any value of RL

Data Acquisition System

A typical Data Acquisition System consists of individual sensors with the necessary signal conditioning, data conversion, data processing, multiplexing, data handling and associated transmission, storage and display systems.

In order to optimise the characteristics of the system in terms of performance, handling capacity and cost, the relevant sub systems can be combined together.

Analog Data Acquisition System is generally acquired and converted into digital form for the purpose of processing, transmission, display and storage.

Processing may consist of a large variety of operations, ranging from simple comparison to complicated mathematical manipulations. It can be for such purposes as collecting information (averages, statistics), converting the data into a useful form (e.g., calculations of efficiency of motor speed, torque and power input developed), using data for controlling a process, performing repeated calculations to separate signals buried in the noise, generating information for display, and various other purposes.

Data may be transmitted over long distances (from one point to another) or short distances (from test centre to a nearby PC).

The data may be displayed on a digital panel or on a CRT. The same be stored temporarily (for immediate use) or permanently for ready reference later.

Data acquisition generally relates to the process of collecting the input data in digital form as rapidly, accurately, and economically as necessary.

To match the input requirements with the output of the sensor, some form of scaling and offsetting is necessary, and this is achieved by the use of amplifier/ attenuators.

Data Acquisition System Block Diagram

A schematic block diagram of a General Data Acquisition System (DAS) is shown in Fig.

Data Logger

The term ‘Data Logging' refers to collecting or gathering data over a period of time

A data logger is a device that can be used to store and retrieve the data.

Data loggers capture, measure, and analyze physical phenomena from the real world.

Light, temperature and pressure are examples of the different types of signals that a Data logger can measure.

A data logger is often a hand-held battery operated device which has a large amount of memory.

 

Basic parts of a Data Logger Operation

• Input scanner 
• Signal conditioner 
• A/D converter 
• Recording equipment 
• Programmer 

The input scanner is an automatic sequence switch which selects each signal in turn.

Low level signals, if any, are multiplied to bring them up to a level of 5 V.

If the signals are not linearly proportional to the measured parameter, these signals are linearised by the signal conditioner.

The analog signals are then converted to digital signals suitable for driving the recording equipment (printer or punched paper tape).

The programmer (serialiser) is used to control the sequence operation of the various items of the logger.

It tells the scanner when to step to a new channel, and receives information from the scanner, converter and recorder.

The real time clock is incorporated to automatic the system.

The clock commands the programmer to sequence one set of measurements at the intervals selected by the user.