From Mixed Signal VLSI Wireless Design: Circuits and Systems

7.6 TYPES OF ANALOG-TO-DIGITAL CONVERTERS

Analog-to-digital converters are classified into one-step architectures, such as: flash, folding and interpolative topologies, and multi-step architectures, such as: Successive approximation and pipeline topologies.

7.6.1 Flash Analog-to-Digital Converter

The flash architecture is potentially the fastest analog-to-digital converter, because of the parallelism it employs. However, this parallelism puts a practical limit on the resolution of the flash analog-to-digital converter. For an n-bit analog-to-digital converter, the flash architecture employs (2 n ?1) comparators and decoding logic. A resistive ladder consisting of 2 n equal resistors divides the reference voltage into 2 n equally spaced voltages as shown in Figure 7.15


Figure 7.15: Block diagram of an n-bit flash analog-to-digital converter.

The output of the comparator constitutes a thermometer code. Assume that the analog input signal ? in is such that:

(7.33)

Therefore, the output of comparators C 1, C 2 , C i is logic 1, while the output of comparator C i +1 , C i +2 , is logic 0. The decoding logic transfers the thermometer code which has (2 n ?1) bits into a binary signal which has n bits. Table 7.1 shows the mapping between the thermometer code and the binary coded signal when n=3.

Table 7.1: The thermometer code of a binary coded signal for n=3.

Thermometer code

Binary code

0000000

000

0000001

001

0000011

010

0000111

011

0001111

100

0011111

101

0111111

110

1111111

111

Copyright Kluwer Academic Publishers 2000 under license agreement with Books24x7

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Analog-to-Digital Converters
Analog-to-digital converters (ADC) sample an analog signal and convert it to a series of digital values to represent the signal to a computer processor.
Digital-to-Analog Converters
Digital-to-analog converters (DAC) transform a digital number into a corresponding analog voltage or current.
Analog Comparators
Analog comparators are amplifiers that compare the magnitude of voltages at two inputs. An analog comparator is an operational amplifier with negative feedback removed, and with no feedback and very high gain, the output voltage goes to one extreme to the other.
Logic Comparators
Digital comparators are circuits used to compare the magnitude of two binary quantities and to determine the relationship of those quantities.

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