AD / DA conversion technology trends and the course of development

（AD/DA转换技术的发展历程及其趋势）

INTRODUCTION

With the extent of the electronics industry figures and the continuing development of gradually formed a digital system as the main pattern. A / D converter circuit as analog and digital interfaces are now being increasingly widespread concern. With the rapid development of digital technology, people on the A / D converters, the higher and higher demands, the new analog / digital conversion technologies continue to emerge. This paper introduces the current Several of the analog / digital conversion technology, and through the recent development of digital technology analysis of the analog / digital conversion technology in the future.

A / D converter history of the development

Computers, digital communications system is the number of digital signal processing circuit system. However, in the practical application of, any big changes are continuous analog, therefore, need an interface circuit will be converted to analog signal digital signal. A / D converter is based on such a request the right moment. In the early 1970s, due to the accuracy of MOS Technology also is not high enough, so some of the general simulation using bipolar process, and the number of adopted MOS technology, and digital and analog part is not done in the same part of a chip. Therefore, the A / D converter can only be achieved with multi-chip, high cost. 1975, a process of using NMOS 10 successive approximation A / D converter to become the first single-chip A / D converters.

In 1976, there has been a resolution of 11 single-chip CMOS integral type A / D converter. At this time the monolithic A / D converter, the digital part of the main, analog part only played a secondary role and, when compared to the MOS process bipolar process still many deficiencies. The 1980s, there has been produced using BiCMOS process monolithic A / D converter, but complex process, the high cost. With the continuous development of CMOS technology, produced using CMOS single-chip A / D converter has become mainstream. This A / D converters, low cost, small power consumption. During the 1990s, portable electronics applications require the universal A / D converters, the power consumption as low as possible. At that time, the A / D converter for mW power level, and now has to be μ W-class. A / D converters, the conversion speed and accuracy is improving now, A / D converters, the conversion rate has reached hundreds MSPS, the resolution has already reached 24.

Analog / digital converter technology development status

Normally, A / D converter with three basic functions: sampling, quantization and coding. How to achieve these three functions, the decision A / D converter circuit structure and performance. A / D conversion of the type many Here are some commonly used analog / digital conversion technology.

All parallel analog / digital converter

All parallel A / D converters, the principle is very simple, analog input signal at the same time with two N-a reference voltage for comparison, just one conversion at the same time can produce n-bit digital output. It is by far the fastest A / D converter, the maximum sampling rate can reach 500 MSPS. However, there are also a lot of inadequacies it. First, the hardware cost, power consumption and size and resolution exponentially; Secondly, the parallel structure of duplication between the comparator must be precise matching, any mismatch will cause static error. Furthermore, the A / D converters also easily lead to uncertainty and discrete output, the so-called "flashing yards." Therefore, the entire parallel A / D converter is only applicable to low-resolution situation.

Reduce all parallel A / D converters, the input capacitance and series resistance network is the key to improving its performance. In order to achieve this objective, using a variety of new technologies, such as all parallel structures and interpolation technology, can reduce power consumption and size, which would enable the entire parallel A / D converter for more high-precision analog / digital conversion. Lane C. designed a 10 60 MSPS conversion rate of the entire parallel A / D converter, through the use of interpolation techniques, the number of comparator reduced from 1,023 to 512, and significant power savings area.

Two-step type analog / digital converter

Two-step type A / D converter structure. First of all, from a coarse whole-parallel A / D converter input to the high conversion, a high-N1 digital output, and this output through digital / analog converter, analog resume then, and the original input voltage Analog subtraction, the remaining quantity of amplification, a more sophisticated again to the entire parallel analog / digital converter to convert, produce low-N2 digital output; Finally, the two A / D converters, the output parallel, As a general digital output.

And the entire parallel A / D converter compared to this type of A / D converters while lowering the conversion rate, but save power and area to resolve the entire parallel A / D converter resolution increase in the number of components and upsurge in the number of contradictions. Therefore, the two-step type A / D converters can be used in more than 10 analog / digital converter, but its remaining amplifier in high demand, and the remaining amount must be enlarged to a second A / D converter input simulation volume, or else will have nonlinear and lost yards. In addition, the first-class A / D converters and D / A converters, the set-up time and accuracy to restrict two-step type A / D converter speed work an important factor, if time is not fully established, it is bound to lead to a result of the conversion error, Therefore, the majority of two-step type A / D converters use a digital correction techniques to improve this problem. Razavi, and B. Wooley, BA correction technology developed by the two-step type A / D converters, the first-class set-up time compared with only 10 ns, offset voltage can be up to 5 mV, conversion rate as high as 5 MSPS, resolution 12.

Interpolation folded analog / digital converter
         
Folding structure, and its basic principle is simulated through a special pretreatment produced more than poor voltage, and the subsequent figures, a minimum effective bits (LSB), the maximum effective bits (MSB) is parallel with the work of the circuit folding rough parallel at all A / D converters are, in almost the same time the signal sampling, and sampling of the more than poor.

In this circuit described by folding the transfer function is the ideal situation, the actual circuit difficult to achieve. Therefore, the general folding with nonlinear structures, but the Department of nonlinear 0.1 0. If only consider these zero, the difference between Vin and Vrj the correct polarity can be determined by interpolation to generate additional ways to solve a 0.1 low. This is the basic idea of folding interpolation, it used folding characteristics, without the additional nonlinear.

  

The use of new technologies, interpolation folding type A / D converter performance continues to increase. Here introduced two new technologies: a current-type interpolation system and the cascade structure. Resistance to achieve voltage-interpolation, matching its accuracy by the resistance of the restrictions in the current-type interpolation of the signal is indicated by the current rate, the higher accuracy, but also suitable for low voltage power supply work . Li, YC, and others through the use of detailed quantitative pathway on the current mode signal processing technology to reduce the voltage swing, with access to 300 MSPS conversion rate, the input signal bandwidth of 60 MHz, 7-bit resolution A / D converters. Another way is to use an improved cascade structure. No parallel increase in the input stage and sub A / D converter in comparison with the number of conditions, the stacked structure conversion accuracy can be increased to more than eight.
       Vorenkamp, P., who designed the 12-fold interpolation A / D converter using three-step cascade-type structure, in which three components of the rough, three concerning quantified, six sub-quantifiable. The A / D converter only 50 comparison, conversion rate of 60 MSPS.



Line-analog / digital converter
         Pipelined A / D converters for two-step type A / D converter for further expansion. It's an n-bit high-resolution analog / digital converter divided into low-level conversion, and then converting the results at all levels combined, a total output. Each circuit from a sample / hold circuit (S / H), low-resolution A / D converters, D / A converters, and can provide subtraction for the gain of amplifier stages.
         This type of A / D converter has the following advantages: redundancy at every level to optimize the overlap of error correction, with good linearity and low offset each have a separate sample-and-hold amplifier, thus allowing Pipeline levels at the same time on a number of sample processing, thereby improving the conversion speed resolution of the same circumstances, the circuit size and power consumption greatly reduced. But it also, there are some drawbacks: the complex structure of benchmark circuits and bias input signal must pass through several circuit, causing water delay the need for strict synchronization of all output latch timing; on the printed circuit board process defects and more sensitive, Nonlinear gain will be affected, disorders and other parameters.
         At present, two commonly used new technology to improve pipeline A / D converter performance. One is the time-cutting technology, a number of lines parallel to the work. By adopting this technology will greatly improve the conversion rate, but the number of parallel channels can not be too much, otherwise, will be greatly increased chip area and power consumption, but also between the various access requires a high degree of match, it is difficult to achieve in the process. Sumanen, L., who designed a four-channel parallel pipelined A / D converters, a 0.5 μ mCMOS process to achieve. The A / D converter sample rates up to 200 MSPS, a resolution of 10. Another new technology is the digital calibration technology, and its main idea is to calibration cycle to the measurement error stored in memory, and then in the normal operation cycle, addressable through the source code, be proofread yards, and through the source code and proofreading Code calculation, the number of final output. This technology can be an imbalance of analog circuits do not match, as well as the introduction of nonlinear effective error correction, so that pipelined A / D converters, the accuracy of more than 10. Hakarainen, V., who developed cutting-pipelined A / D converters, use this correction technology to correction of D / A converters of error, and the parallel between the channel gain and offset voltage mismatch compensation, in the 10 to match the accuracy of the device under a 14 conversion accuracy.



Successive Approximation of analog / digital converter
         Successive approximation A / D converters, the principle is as follows: a sample of the input signal and D / A converters, the output value of the initial cut, I was poor compared with quantitative, quantifiable value and then guiding control logic is increasing or decreasing D / A converter output; then, the new D / A converter output value again from the input sample values were less, and constantly repeat this process until the accuracy requirements are met. Clearly, this A / D converter in only one clock cycle to complete a conversion, N-bit conversion will require N clock cycles, it is the sampling rate is not high, and lower input bandwidth, but a simple circuit structure , area and power consumption of small, but there is no question of delay.
         Successive approximation A / D converter is a key part of D / A converters, it restricts the A / D converters, precision and speed. D / A converter is the traditional production methods with precision resistor network, but its accuracy is not high. Based on the capacitor array, a charge redistribution technology D / A converter can be higher accuracy, mainly due to the MOS circuit easier to produce small volume precision capacitance and capacitive loss minimal. Gan, JH, who use non-binary capacitor array structure to achieve D / A converters, and a self-calibration technology to raise the matching capacitance, the D / A converters, high precision 22, to produce 50 mW power dissipation the high-performance 16-bit 150 MSPS successive approximation A / D converters.

Δ Σ-analog / digital converter
         Σ-Δ A / D converters, the structure shown in Figure 6, from Δ Σ-modulator and digital filter components. Modulator integrator and include a comparator, as well as containing a 1-bit D / A converters, the feedback loop with noise shaping function will be quantization noise from base-band move to a higher frequency band, thereby enhancing the SNR. Moreover, in Δ Σ-modulation, beyond Nyquist sampling rate higher than the frequency of the analog signal is sampled, can be reduced within the scope of base-band noise power, to further improve the accuracy of conversion. The modulator output is a high-speed digital Σ2Δ flow, contain a large number of high-frequency noise, it was necessary to digital filter, filtering high-frequency noise, reduced sampling frequency.
         Σ-Δ A / D converter is the highest accuracy of the A / D converter. In addition, it is extremely excellent linearity, without fine-tuning, as well as lower-confusion advantage. However, the technical requirements oversampling sampling frequency much higher than the input signal frequency, which limits the input signal bandwidth; Moreover, with a sampling rate of increase, power will be greatly increased. Therefore, in ensuring the accuracy of the premise, as far as possible to reduce the sampling rate has become very crucial. It is now generally used There are two main methods: multi-level noise shaping technology (MASH), which uses a number of cascade, and stability of the first order or second-order loop; the other is the structure of a number of Σ-Δ A / D converter, the structure contains an n-bit parallel A / D converter and an n-bit D / A converters. In order to obtain better results, usually a combination of these two methods. 2001, delRio, R., and other artificial ADSL application design of the four-Δ Σ order modulator using 2-1-1 three-tier structure, with the last one containing four quantizer. The A / D converters, the sampling rate is only 16, Resolution 12, 4 MSPS sampling rate, power of 77 mW.
         In addition, there are several new technologies are applied to Σ-Δ A / D converter in order to improve its performance. Bandpass Σ-Δ A / D converter using band-pass filter replacement integrator, quantization noise was useful both upward and downward out of band, and then from band-pass digital filter bandwidth will be useful in other signals and quantization noise filter In addition to direct high-precision intermediate frequency signal conversion. Schreier, R., and others use 0.35 μ m BiCMOS technology produced by band-pass Σ-Δ A / D converter, the bandwidth of 333 kHz, 90 dB dynamic range, power consumption is 50 mW, the clock frequency of up to 32 MHz. Heterojunction produced by the continuous-time process Σ-Δ A / D converters, its bandwidth than switched-capacitor type Σ-Δ A / D converter much so that Σ-Δ A / D converters can be used in the field of RF . InPHBT a process used to achieve the second-Δ Σ modulator, with a resolution of 12, the signal bandwidth of 50 MHz, 3.2 GHz sampling rate. Multiple Σ-Δ A / D converter parallel, simulation pretreatment on the importation of digital post-processing output, and increase the available sampling than the same effect, the realization of Nyquist sampling rate of Σ-Δ A / D converters (oversampling ratio of 1), so as to further enhance the input signal bandwidth. Nyquist sampling rate Σ-Δ A / D converters, the number of parallel channels 8, the input signal bandwidth of 160 kHz.



A / D converters, the comparison and classification
         According to A / D converters, the speed and accuracy can be broadly divided into three categories.
         1) High-speed low (or medium) precision A / D converter, the specific structure of the entire parallel, two-step type, type and line interpolation folding type. Such A / D converter speed, but accuracy is not high, and the power consumption, the use of silicon area is also very strong, mainly for video processing, communications, and high-speed digital measuring instruments and radar, and other fields.
         2) Medium-speed precision A / D converters. This type of A / D converter is speed for accuracy, such as successive approximation A / D converters. This type A / D converters, the output data is usually a serial, and their conversion rate in the tens of kHz to a few hundred kHz between accuracy than high-speed A / D converter high (10 ~ 16), the main for the sensor, automatic control, audio processing, and other fields.
         3) low-speed or high-precision A / D converters. Such A / D converter speed unpleasant, but high precision (16 ~ 24), such as Σ-Δ A / D converters. The type A / D converters are primarily for audio, communications, geophysical measurements, test instrument, automatic control, and other fields.
                                                     

Analog / digital converter technology development trends
         At present, the digital processing system is rapid development in the areas of video, high-definition digital television (HDTV), the emergence of radio and television have been pushed to a higher level, the resolution of HDTV compared with ordinary television at least one times. In the communications field, in the past the design of wireless communication systems are static, only in the specific provisions of the band on the use of specialized modulator, and channel encoder agreement. And software radio technology (SDR) can be more flexible and efficient use of spectrum, and can easily escalate and tracking technology, has greatly promoted the development of wireless communication system. In the field of high-precision measurement, the High Resolution Instrument has been improving, the current reached μ A magnitude, or even lower voltage mV arrived in the area of audio, high performance professional audio processing equipment are constantly emerging, such as DVD-Audio and Super Audio CD (SACD), which can handle higher-quality audio signal.

         In order to meet the requirements for the development of digital systems, A / D converters, the performance must continue to improve, it will mainly develop in the direction of the following: high-speed: the modern digital data processing system faster, the speed of data access requests also rising. For example, in software radio system, the A / D converters, the location is very critical, it requires A / D converters, the maximum input signal frequency in the 1 GHz and 5 GHz between, with the present technology level, it is still difficult to achieve. Thus, the super-high-speed A / D converters direction of the trend is clearly visible.
     High accuracy: a resolution of modern digital system has been improving, for example, the smallest senior instrument can be measured continuously decreased, therefore, A / D converters, the resolution must also improve in the professional audio processing system , in order to obtain a more realistic sound effects, the need for high-precision A / D converters. At present, the most accurate up to 24 A / D converter can not meet the requirements. Now, people are more committed to the development of high-precision A / D converters.
     Low power consumption: system-on-chip (SOC) integrated circuits has become the trend of development, with a chip on both analog circuits are digital circuitry. In order to complete the complicated system, a system in each sub-module power consumption should be as low as possible, therefore, low-power A / D converter is essential. In the past in the design of 5 MSPS8 ~ 12-bit resolution A / D converters, the typical power consumption of 100 to 150 mW. This far from being able to meet the development of system-on-demand, therefore, will be a low-power A / D converters an inevitable trend of development.
         In short, a variety of technical and process of mutual infiltration, the non-suitable for the development of various applications, to meet the different needs of the A / D converter, analog / digital converter technology trends for the future development of high-speed, high-precision, low - power A / D converters will be the future development of the data converters focus.

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