ne highest frequency in the analog signal. Oversampling A/D converters use sampling rates of N times this rate, where N typically ranges from 2 to 64. Both D/A and a/D converters require a voltage reference in order to achieve absolute conversion accuracy. Some conversion ICs have internal voltage references, while others accept external voltage references. For higl performance systems, an external precision reference is needed to ensure long-term stability, load regulation and control over temperature fluctuations. External precision voltage reference ICs can be found in manufac turers' data books Measurement accuracy is specified by the converters linearity Integral linearity is a measure of linearity over the entire conversion range. It is often defined as the deviation from a straight line drawn between the endpoints and through zero(or the offset value)of the conversion range. Integral linearity is also referred to as relative accuracy. The offset value is the reference level required to establish the zero or midpoint of the conversion converter. A converter is said to be monotonic if increasing input values result in increasing output values. the linearity is the linearity between code transitions. Differential linearity is a measure of the monotonicity The accuracy and linearity values of a converter are specified in the data sheet in units of the LSB of the code. The linearity can vary with temperature, so the values are often specified at +25C as well as over the entire temperature range of the device. D/A Conversion Processes Digital codes are typically converted to analog voltages by assigning a voltage weight to each bit in the digital code and then summing the voltage weights of the entire code. a general D/A converter consists of a network of precision resistors, input switches, and level shifters to activate the switches to convert a digital code to an nalog current or voltage. D/A ICs that produce an analog current output usually have a faster settling time and better linearity than those that produce a voltage output. When the output current is available, the designer can convert this to a voltage through the selection of an appropriate output amplifier to achieve the necessary given app D/A converters commonly have a fixed or variable reference level. The reference level determines the switching threshold of the precision switches that form a controlled impedance network, which in turn controls the value of the output signal. Fixed reference D/A converters produce an output signal that is proportional to the digital input. Multiplying D/A converters produce an output signal that is proportional to the product of a varying reference level times a digital code. D/A converters can produce bipolar, positive, or negative polarity signals. A four-quadrant multiplying D/A converter allows both the reference signal and the value of the binary code to have a positive or negative polarity The four-quadrant multiplying D/A converter produces bipolar output signals. D/A Converter ICs Most D/A converters are designed for general-purpose control applications. Some D/A converters, however, designed for special applications, such as video or graphic outputs, high-definition video displays, ultra high-speed signal D/A converter ICs often include special features that enable them to be interfaced easily to microprocessors or other systems. Microprocessor control inputs, input latches, buffers, input registers, and compatibility to tandard logic families are features that are readily available in D/A ICs. In addition, the ICs usually have lase trimmed precision resistors to eliminate the need for user trimming to achieve full-scale performance. A/D Conversion Processes Analog signals can be converted to digital codes by many methods, including integration, succesive approxi mation, parallel (flash)conversion, delta modulation, pulse code modulation, and sigma-delta conversion Two of the most common A/D conversion processes are successive approximation A/D conversion and parall or flash A/D conversion. Very high-resolution digital audio or video systems require specialized A/D techniques that often incorporate one of these general techniques as well as specialized A/D conversion processes. Examples e 2000 by CRC Press LLC© 2000 by CRC Press LLC the highest frequency in the analog signal. Oversampling A/D converters use sampling rates of N times this rate, where N typically ranges from 2 to 64. Both D/A and A/D converters require a voltage reference in order to achieve absolute conversion accuracy. Some conversion ICs have internal voltage references, while others accept external voltage references. For high￾performance systems, an external precision reference is needed to ensure long-term stability, load regulation, and control over temperature fluctuations. External precision voltage reference ICs can be found in manufac￾turers’ data books. Measurement accuracy is specified by the converter’s linearity. Integral linearity is a measure of linearity over the entire conversion range. It is often defined as the deviation from a straight line drawn between the endpoints and through zero (or the offset value) of the conversion range. Integral linearity is also referred to as relative accuracy. The offset value is the reference level required to establish the zero or midpoint of the conversion range. Differential linearity is the linearity between code transitions. Differential linearity is a measure of the monotonicity of the converter. A converter is said to be monotonic if increasing input values result in increasing output values. The accuracy and linearity values of a converter are specified in the data sheet in units of the LSB of the code. The linearity can vary with temperature, so the values are often specified at +25°C as well as over the entire temperature range of the device. D/A Conversion Processes Digital codes are typically converted to analog voltages by assigning a voltage weight to each bit in the digital code and then summing the voltage weights of the entire code. A general D/A converter consists of a network of precision resistors, input switches, and level shifters to activate the switches to convert a digital code to an analog current or voltage. D/A ICs that produce an analog current output usually have a faster settling time and better linearity than those that produce a voltage output. When the output current is available, the designer can convert this to a voltage through the selection of an appropriate output amplifier to achieve the necessary response speed for the given application. D/A converters commonly have a fixed or variable reference level. The reference level determines the switching threshold of the precision switches that form a controlled impedance network, which in turn controls the value of the output signal. Fixed reference D/A converters produce an output signal that is proportional to the digital input. Multiplying D/A converters produce an output signal that is proportional to the product of a varying reference level times a digital code. D/A converters can produce bipolar, positive, or negative polarity signals. A four-quadrant multiplying D/A converter allows both the reference signal and the value of the binary code to have a positive or negative polarity. The four-quadrant multiplying D/A converter produces bipolar output signals. D/A Converter ICs Most D/A converters are designed for general-purpose control applications. Some D/A converters, however, are designed for special applications, such as video or graphic outputs, high-definition video displays, ultra high-speed signal processing, digital video tape recording, digital attenuators, or high-speed function generators. D/A converter ICs often include special features that enable them to be interfaced easily to microprocessors or other systems. Microprocessor control inputs, input latches, buffers, input registers, and compatibility to standard logic families are features that are readily available in D/A ICs. In addition, the ICs usually have laser￾trimmed precision resistors to eliminate the need for user trimming to achieve full-scale performance. A/D Conversion Processes Analog signals can be converted to digital codes by many methods, including integration, succesive approxi￾mation, parallel (flash) conversion, delta modulation, pulse code modulation, and sigma-delta conversion. Two of the most common A/D conversion processes are successive approximation A/D conversion and parallel or flash A/D conversion.Very high-resolution digital audio or video systems require specialized A/D techniques that often incorporate one of these general techniques as well as specialized A/D conversion processes. Examples