27-06-2012, 02:02 PM
SAMPLE AND HOLD CIRCUIT
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INTRODUCTION
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 . 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.
CHARGE INJECTION :-
When a MOS switch is on, it operates in the triode region and its drain-to-source voltage,
VDS, is approximately zero. During the time when the transistor is on, it holds mobile
charges in its channel. Once the transistor is turned off, these mobile charges must flow
out from the channel region and into the drain and the source junctions as depicted
SWITCHED OP – AMP BASED SAMPLE – AND – HOLD
CIRCUIT :--
This S/H technique takes advantage of the fact that when a MOS transistor is in the
saturation region, the channel is pinched off and disconnected from the drain. Therefore,
if the hold capacitor is connected to the drain of the MOS transistor, charge injection will only go to the source junction, leaving the drain unaffected.
Conclusion
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. Although, this paper has described three of these alternative S/H circuits: series sampling, SOP based S/H circuit, and bottom plate S/H circuit with bootstrapped switch, 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.