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Flash converters

The ultra fast analog to digital converters are called flash converters. They are also called simultaneous converters. Flash converters have two digital output lines. An analog circuit called comparator is used here. Comparator is always represented by a triangle symbol with two inputs and one output. When the voltage at the upper input is relatively positive with respect to the lower input, comparator output will be ‘1’. When the upper input is negative with respect to the lower input, comparator output will be ‘0’. Flash converters of many varieties exist. Flash converters of various sizes and different speeds are available. The latest ultra fast single-chip converters perform 8-bit conversions at a 100 million conversions per second rate. The major problem here is the requirement of very large number of comparators.

Flash memory

The flash memory cell consists of one transistor with a floating gate. Threshold voltage of this gate can be programmed by applying an electric field. Transistor is the memory cell with two states corresponding to the existence of charges (electrons) at the floating gate. In NOR flash memory cell, F-N tunneling mechanism for the erase operation and the hot electron injection mechanism for the programming operation. In NAND flash memory cell, F-N tunneling mechanism is used for the erase operation. In flash memory circuit, charge-pump mechanism is implemented with n-MOS or p-MOS transistors for the generation of a high positive or a negative voltage. The charge-pump circuit consists of a chain of the diode and capacitors which are discharged successfully after each half-clock cycle.

Computer story

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