+5 volts 4.375 Volts Analog Comparators 3. 125 Volts Encoder Latch 2.5 Volts MSB 1.875 Volts Code 1.25 Volts 625 Volts Analog Ground Resistor Ladder Network FIGURE 32.2 Flash A/D converter block diagram. Parallel or flash A/D conversion is used in high-speed applications such as video signal processing, medical imaging, and radar detection systems. A flash A/D converter simultaneously compares the input analog voltage to 2"-1 threshold voltages to produce an n-bit digital code representing the analog voltage. Typical flash a/D converters with 8-bit resolution operate at 20 to 100 MHz The functional blocks of a flash A/D converter are shown in Fig. 32. 2. The circuitry consists of a precision resistor ladder network, 2m-1 analog comparators, and a digital priority encoder. The resistor network estab- lishes threshold voltages for each allowed quantization level. The analog comparators indicate whether or not the input analog voltage is above or below the threshold at each level. The output of the analog comparators is input to the digital priority encoder. The priority encoder produces the final digital output code that is stored in an output latch. An 8-bit flash A/D converter requires 255 comparators. The cost of high-resolution A/D comparators escalates as the circuit complexity increases and as the number of analog converters rises by 2" -1. As a low-cost alternative, some manufacturers produce modified flash A/D converters that perform the A/D conversion in two steps to reduce the amount of circuitry required. These modified flash A/d converters are also referred to as half-flash A/D converters, since they perform only half of the conversion simultaneously A/D Converter ICs A/D converter ICs can be classified as general-purpose, high-speed, flash, and sampling A/D converters. The general-purpose A/D converters are typically low speed and low cost, with conversion times ranging from 2 us to 33 ms A/D conversion techniques used by these devices typically include successive approximation, tracking, and integrating. The general-purpose A/D converters often have control signals for simplified microprocessor interfacing. These ICs are appropriate for many process control, industrial, and instrumentation applications, as well as for environmental monitoring such as seismology, oceanography, meteorology, and pollution monitoring. e 2000 by CRC Press LLC© 2000 by CRC Press LLC Parallel or flash A/D conversion is used in high-speed applications such as video signal processing, medical imaging, and radar detection systems. A flash A/D converter simultaneously compares the input analog voltage to 2n – 1 threshold voltages to produce an n-bit digital code representing the analog voltage. Typical flash A/D converters with 8-bit resolution operate at 20 to 100 MHz. The functional blocks of a flash A/D converter are shown in Fig. 32.2. The circuitry consists of a precision resistor ladder network, 2n – 1 analog comparators, and a digital priority encoder. The resistor network estab￾lishes threshold voltages for each allowed quantization level. The analog comparators indicate whether or not the input analog voltage is above or below the threshold at each level. The output of the analog comparators is input to the digital priority encoder. The priority encoder produces the final digital output code that is stored in an output latch. An 8-bit flash A/D converter requires 255 comparators. The cost of high-resolution A/D comparators escalates as the circuit complexity increases and as the number of analog converters rises by 2n – 1. As a low-cost alternative, some manufacturers produce modified flash A/D converters that perform the A/D conversion in two steps to reduce the amount of circuitry required. These modified flash A/D converters are also referred to as half-flash A/D converters, since they perform only half of the conversion simultaneously. A/D Converter ICs A/D converter ICs can be classified as general-purpose, high-speed, flash, and sampling A/D converters. The general-purpose A/D converters are typically low speed and low cost, with conversion times ranging from 2 ms to 33 ms. A/D conversion techniques used by these devices typically include successive approximation, tracking, and integrating. The general-purpose A/D converters often have control signals for simplified microprocessor interfacing. These ICs are appropriate for many process control, industrial, and instrumentation applications, as well as for environmental monitoring such as seismology, oceanography, meteorology, and pollution monitoring. FIGURE 32.2 Flash A/D converter block diagram. + – + – + – + – + – + – + – +5 Volts Analog Input Voltage Analog Comparators Digital Output Code Analog Ground Resistor Ladder Network 4.375 Volts 1K ½ 1K ½ 1K ½ 1K ½ 1K ½ 1K ½ 1K ½ 1K ½ 3.75 Volts 3.125 Volts 2.5 Volts 1.875 Volts 1.25 Volts .625 Volts C MSB Octal Priority Encoder Output Latch LSB 1 2 3 4 5 6 7 B A