Garrod. sAR"D/A and A/D Converters The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton CRC Press llc. 2000
Garrod, S.A.R. “D/A and A/D Converters” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
32 D/A and A/D Converters 32.1 D/A and A/D Circuits D/A and A/D Converter Performance Criteria. D/A Conversion Susan a.r. garrod Processes·D/ A Converter Ics·A/ D Conversion processes·A/D Converter ICs. Grounding and Bypassing on D/A and A/ Purdue University ICs. Selection Criteria for D/A and A/D Converter ICs Digital-to-analog(D/A) conversion is the process of converting digital codes into a continuous range of analog signals. Analog-to-digital (A/D)conversion is the complementary process of converting a continuous range of analog signals into digital codes. Such conversion processes are necessary to interface real-world systems, which typically monitor continuously varying analog signals, with digital systems that process, store, interpret, and manipulate the analog values. D/A and A/D applications have evolved from predominately military-driven applications to consumer- oriented applications. Up to the mid-1980s, the military applications determined the design of many D/A and A/D devices. The military applications required very high performance coupled with hermetic packaging, radiation hardening, shock and vibration testing, and military specification and record keeping. Cost was of little concern, and"low power"applications required approximately 2.8 w. The major applications up the mid 1980s included military radar warning and guidance systems, digital oscilloscopes, medical imaging, infrared systems, and professional video. O The applications requiring D/A and A/D circuits in the 1990s have different performance criteria from those earlier years. In particular, low power and high speed applications are driving the development of D/A and A/D circuits, as the devices are used extensively in battery-operated consumer products. The predominant applications include cellular telephones, hand-held camcorders, portable computers, and set-top cable Tv boxes. These applications generally have low power and long battery life requirements, and they may have high d and high resolution requirements, as is the case with the set-top cable TV boxes 32.1 D/A and A/D Circuits D/A and A/D conversion circuits are available as integrated circuits(ICs) from many manufacturers. a hug array of ICs exists, consisting of not only the D/A or A/D conversion circuits, but also closely related circuits such as sample-and-hold amplifiers, analog multiplexers, voltage-to-frequency and frequency-to-voltage con- verters,voltage references, calibrators, operation amplifiers, isolation amplifiers, instrumentation amplifiers, active filters, dc-to-dc converters, analog interfaces to digital signal processing systems, and data acquisition subsystems. Data books from the IC manufacturers contain an enormous amount of information about these devices and their applications to assist the design engineer. The ICs discussed in this chapter will be strictly the D/A and A/D conversion circuits. Table 32 1 lists a small sample of the variety of the D/A and A/d converters currently available. The ICs usually perform either D/A or A/D conversion. There are serial interface ICs, however, typically for high-performance audio and digital signal processing applications, that perform both A/D and D/a processes c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 32 D/A and A/D Converters 32.1 D/A and A/D Circuits D/A and A/D Converter Performance Criteria • D/A Conversion Processes • D/A Converter ICs • A/D Conversion Processes • A/D Converter ICs • Grounding and Bypassing on D/A and A/D ICs • Selection Criteria for D/A and A/D Converter ICs Digital-to-analog (D/A) conversion is the process of converting digital codes into a continuous range of analog signals. Analog-to-digital (A/D) conversion is the complementary process of converting a continuous range of analog signals into digital codes. Such conversion processes are necessary to interface real-world systems, which typically monitor continuously varying analog signals, with digital systems that process, store, interpret, and manipulate the analog values. D/A and A/D applications have evolved from predominately military-driven applications to consumeroriented applications. Up to the mid-1980s, the military applications determined the design of many D/A and A/D devices. The military applications required very high performance coupled with hermetic packaging, radiation hardening, shock and vibration testing, and military specification and record keeping. Cost was of little concern, and “low power” applications required approximately 2.8 W. The major applications up the mid- 1980s included military radar warning and guidance systems, digital oscilloscopes, medical imaging, infrared systems, and professional video. The applications requiring D/A and A/D circuits in the 1990s have different performance criteria from those of earlier years. In particular, low power and high speed applications are driving the development of D/A and A/D circuits, as the devices are used extensively in battery-operated consumer products. The predominant applications include cellular telephones, hand-held camcorders, portable computers, and set-top cable TV boxes. These applications generally have low power and long battery life requirements, and they may have high speed and high resolution requirements, as is the case with the set-top cable TV boxes. 32.1 D/A and A/D Circuits D/A and A/D conversion circuits are available as integrated circuits (ICs) from many manufacturers. A huge array of ICs exists, consisting of not only the D/A or A/D conversion circuits, but also closely related circuits such as sample-and-hold amplifiers, analog multiplexers, voltage-to-frequency and frequency-to-voltage converters, voltage references, calibrators, operation amplifiers, isolation amplifiers, instrumentation amplifiers, active filters, dc-to-dc converters, analog interfaces to digital signal processing systems, and data acquisition subsystems. Data books from the IC manufacturers contain an enormous amount of information about these devices and their applications to assist the design engineer. The ICs discussed in this chapter will be strictly the D/A and A/D conversion circuits. Table 32.1 lists a small sample of the variety of the D/A and A/D converters currently available. The ICs usually perform either D/A or A/D conversion. There are serial interface ICs, however, typically for high-performance audio and digital signal processing applications, that perform both A/D and D/A processes. Susan A.R. Garrod Purdue University
TABLE 32.1 D/A and A/D Integrated Circuits Multiplying D/A Converter ICs Resolution, b vs Fixed Reference Settling Time, us Input Data Format Analog Devices AD558 8 Fixed reference parallel Analog Devices AD7524 Multiplying 0.400 Parallel Analog Devices AD390 ad 12 Fixed reference parallel Analog Devices AD1856 16 Fixed reference Serial Burr-Brown DAC729 18 Fixed reference 8 Parallel DATEL DAC-HFS 8 multiplying 0.025 National DACo800 Multiplying parallel A/D Converter ICs Resolution, b Signal Inputs Conversion Speed, Ls Output Data Forma Analog Devices AD572 Serial parallel Burr-Brown ADC803 12 Parallel Burr-Brown adc701 16 National ADC1005B TI. National ADC0808 8 8 100 TI. National ADC0834 TI TLCO820 Parallel TI TLC1540 A/D and D/A Interface ICs Resolution, b On-Board Filters Sampling Rate, kHz Data Format TI TLC32040 19.2(programmable) Serial TI 2914 PCM codec filter 8 Serial D/A and A/D Converter Performance Criteria The major factors that determine the quality of performance of D/A and A/D converters are resolution, sampling The resolution of a D/A circuit is the smallest change in the output analog signal. In an A/D system, the resolution is the smallest change in voltage that can be detected by the system and that can produce a change in the digital code. The resolution determines the total number of digital codes, or quantization levels, that will be recognized or produced by the circuit. The resolution of a D/A or A/D IC is usually specified in terms of the bits in the digital code or in terms of the least significant bit(LSB)of the system. An n-bit code allows for 2" quantization levels, or 2"-1 steps tween quantization levels. As the number of bits increases, the step size between quantization levels decreases, herefore increasing the accuracy of the system when a conversion is made between an analog and digital signal. The system resolution can be specified also as the voltage step size between quantization levels. For A/D circuits, the resolution is the smallest input voltage that is detected by the system The speed of a D/A or A/D converter is determined by the time it takes to perform the conversion process or D/A converters, the speed is specified as the settling time. For A/D converters, the speed is specified as the code; thus, it is specified in the data sheet with the appropriate conditions stated and transition in the digital A/D converters have a maximum sampling rate that limits the speed at which they can perform continuou conversions. The sampling rate is the number of times per second that the analog signal can be sampled and converted into a digital code. For proper A/D conversion, the minimum sampling rate must be at least two times the highest frequency of the analog signal being sampled to satisfy the Nyquist sampling criterion. The nversion speed and other timing factors must be taken into consideration to determine the maximum sampling rate of an A/D converter Nyquist A/D converters use a sampling rate that is slightly more than twice c2000 by CRC Press LLC
© 2000 by CRC Press LLC D/A and A/D Converter Performance Criteria The major factors that determine the quality of performance of D/A and A/D converters are resolution, sampling rate, speed, and linearity. The resolution of a D/A circuit is the smallest change in the output analog signal. In an A/D system, the resolution is the smallest change in voltage that can be detected by the system and that can produce a change in the digital code. The resolution determines the total number of digital codes, or quantization levels, that will be recognized or produced by the circuit. The resolution of a D/A or A/D IC is usually specified in terms of the bits in the digital code or in terms of the least significant bit (LSB) of the system. An n-bit code allows for 2n quantization levels, or 2n – 1 steps between quantization levels. As the number of bits increases, the step size between quantization levels decreases, therefore increasing the accuracy of the system when a conversion is made between an analog and digital signal. The system resolution can be specified also as the voltage step size between quantization levels. For A/D circuits, the resolution is the smallest input voltage that is detected by the system. The speed of a D/A or A/D converter is determined by the time it takes to perform the conversion process. For D/A converters, the speed is specified as the settling time. For A/D converters, the speed is specified as the conversion time. The settling time for D/A converters will vary with supply voltage and transition in the digital code; thus, it is specified in the data sheet with the appropriate conditions stated. A/D converters have a maximum sampling rate that limits the speed at which they can perform continuous conversions. The sampling rate is the number of times per second that the analog signal can be sampled and converted into a digital code. For proper A/D conversion, the minimum sampling rate must be at least two times the highest frequency of the analog signal being sampled to satisfy the Nyquist sampling criterion. The conversion speed and other timing factors must be taken into consideration to determine the maximum sampling rate of an A/D converter. Nyquist A/D converters use a sampling rate that is slightly more than twice TABLE 32.1 D/A and A/D Integrated Circuits Multiplying D/A Converter ICs Resolution, b vs. Fixed Reference Settling Time, ms Input Data Format Analog Devices AD558 8 Fixed reference 3 Parallel Analog Devices AD7524 8 Multiplying 0.400 Parallel Analog Devices AD390 Quad, 12 Fixed reference 8 Parallel Analog Devices AD1856 16 Fixed reference 1.5 Serial Burr-Brown DAC729 18 Fixed reference 8 Parallel DATEL DAC-HF8 8 Multiplying 0.025 Parallel National DAC0800 8 Multiplying 0.1 Parallel A/D Converter ICs Resolution, b Signal Inputs Conversion Speed, ms Output Data Format Analog Devices AD572 12 1 25 Serial & parallel Burr-Brown ADC803 12 1 1.5 Parallel Burr-Brown ADC701 16 1 1.5 Parallel National ADC1005B 10 1 50 Parallel TI, National ADC0808 8 8 100 Parallel TI, National ADC0834 8 4 32 Serial TI TLC0820 8 1 1 Parallel TI TLC1540 10 11 21 Serial A/D and D/A Interface ICs Resolution, b On-Board Filters Sampling Rate, kHz Data Format TI TLC32040 14 Yes 19.2 (programmable) Serial TI 2914 PCM codec & filter 8 Yes 8 Serial
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 highperformance 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 manufacturers’ 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 lasertrimmed 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 approximation, 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
Analog Comparator Shift Register Clock FIGURE 32 1 Successive approximation A/D converter block diagram. of specialized A/D conversion techniques are pulse code modulation(PCM), and sigma-delta conversion. PCM is a common voice encoding scheme used not only by the audio industry in digital audio recordings but also by the telecommunications industry for voice encoding and multiplexing. Sigma-delta conversion is an over- sampling A/D conversion where signals are sampled at very high frequencies. It has very high resolution and low distortion and is being used in the digital audio recording industry Successive approximation A/D conversion is a technique that is commonly used in medium-to high-speed data acquisition applications. It is one of the fastest A/D conversion techniques that requires a minimum amount of circuitry. The conversion times for successive approximation A/D conversion typically range from 10 to 300 for 8-bit systems The successive approximation A/D converter can approximate the analog signal to form an n-bit digital code in n steps. The successive approximation register (SAR) individually compares an analog input voltage to the midpoint of one of n ranges to determine the value of one bit. This process is repeated a total of n times, using n ranges, to determine the n bits in the code. The comparison is accomplished as follows: The Sar determines if the analog input is above or below the midpoint and sets the bit of the digital code accordingly. The SAr assigns the bits beginning with the most significant bit. The bit is set to a l if the analog input is greater than the midpoint voltage, or it is set to a 0 if it is less than the midpoint voltage. The SAR then moves to the next bit and sets it to a I or a 0 based on the results of comparing the analog input with the midpoint of the next allowed range. Because the SaR must perform one approximation for each bit in the digital code, an n-bit code requires n approximations. A successive approximation A/D converter consists of four functional blocks, as shown in Fig. 32. 1: the SAR, the analog comparator, a D/A converter, and a clock. c2000 by CRC Press LLC
© 2000 by CRC Press LLC of specialized A/D conversion techniques are pulse code modulation (PCM), and sigma-delta conversion. PCM is a common voice encoding scheme used not only by the audio industry in digital audio recordings but also by the telecommunications industry for voice encoding and multiplexing. Sigma-delta conversion is an oversampling A/D conversion where signals are sampled at very high frequencies. It has very high resolution and low distortion and is being used in the digital audio recording industry. Successive approximation A/D conversion is a technique that is commonly used in medium- to high-speed data acquisition applications.It is one of the fastest A/D conversion techniques that requires a minimum amount of circuitry. The conversion times for successive approximation A/D conversion typically range from 10 to 300 ms for 8-bit systems. The successive approximation A/D converter can approximate the analog signal to form an n-bit digital code in n steps. The successive approximation register (SAR) individually compares an analog input voltage to the midpoint of one of n ranges to determine the value of one bit. This process is repeated a total of n times, using n ranges, to determine the n bits in the code. The comparison is accomplished as follows: The SAR determines if the analog input is above or below the midpoint and sets the bit of the digital code accordingly. The SAR assigns the bits beginning with the most significant bit. The bit is set to a 1 if the analog input is greater than the midpoint voltage, or it is set to a 0 if it is less than the midpoint voltage. The SAR then moves to the next bit and sets it to a 1 or a 0 based on the results of comparing the analog input with the midpoint of the next allowed range. Because the SAR must perform one approximation for each bit in the digital code, an n-bit code requires n approximations. A successive approximation A/D converter consists of four functional blocks, as shown in Fig. 32.1: the SAR, the analog comparator, a D/A converter, and a clock. FIGURE 32.1 Successive approximation A/D converter block diagram. Control Clock Control Shift Register Successive Approximation Register (SAR) D/A Resistor Ladder Network Analog Comparator Analog Input Voltage Output Latch Digital Output Code Control Clocking Signals + –