Sample-and-hold (S/H) circuits are important analog building blocks that are used in applications like analog-to-digital converters and switched-capacitor filters. The simplest S/H circuit uses an MOS transistor as a sampling switch and a hold capacitor. Switched op-amp based S/H circuits reduce errors from charge injection and clock feedthrough. S/H circuits are used to hold input signals constant for comparisons in circuits like successive approximation ADCs and to simultaneously sample multiple signals.

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EIE 7TH SEM

2.  Sample-and-hold (S/H) is an important analog
building block with many applications, including
analog-to-digital converters (ADCs) and switched-
capacitor filters.
 The function of the S/H circuit is to sample an
analog input signal and hold this value over a
certain length of time for subsequent processing.
 Taking advantages of the excellent properties of
MOS capacitors and switches, traditional switched
capacitor techniques can be used to realize
different S/H circuits .

3.  The simplest S/H circuit in MOS technology is
shown in Figure 1, where Vin is the input
signal, M1 is an MOS transistor operating as
the sampling switch, Ch is the hold capacitor,ck
is the clock signal, and Vout is the resulting
sample-and-hold output signal.

4. Switched op-amp based sample and hold circuitSwitched op-
amp based sample and hold circuit

5.  During sample mode, the SOP behaves just like
a regular op-amp, in which the value of the
output follows the value of the input.
 During hold mode, the MOS transistors at the
output node of the SOP are turned off while
they are still operating in saturation, thus
preventing any channel charge from flowing
into the output of the SOP.
 In addition, the SOP is shut off and its output is
held at high impedance, allowing the charge on
Ch to be preserved throughout the hold mode.

6.  On the other hand, the output buffer of this
S/H circuit is always operational during
sample and hold mode and is always
providing the voltage on Ch to the output of
the S/H circuit.

7.  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.

8.  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 clock.

9.  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 high input 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.

10.  - Obtain a low droop rate during holding mode
 - Stability is determined by the stabilities of OP
Amps.
 - OP Amps offset can constrain the accuracy of
SHA.
 Needs a special circuitry to stabilize the input
amplifier during the holding mode

11.  Sample-and-hold (S/H) is an important analog
building block that has many applications.
 The simplest S/H circuit can be constructed using
only one MOS transistor and one hold capacitor.
 However, due to the limitations of the MOS
transistor switches, errors due to charge injection
and clock feed through restrict the performance of
S/H circuits.
 As a result, different S/H techniques and
architectures are developed with the intention to
reduce or eliminate these errors.

12.  More new S/H techniques and architectures
need to be proposed in order to meet the
increasing demand for high-speed, low-
power, and low voltage S/H circuits for data
acquisition systems.