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If we strip away the DIGITAL from Digital Signal Processing, we are left with something that weve been doing in electronics since it was first invented, Signal Processing! Signal processing is all about taking a signal, applying some change to it, and then getting a new signal out. That change might be amplification or filtration or something else, but nearly all electronic circuits can be considered to be signal processors. Looked on in this way, the signal processor as a black box might be composed of discrete components like capacitors and resistors, or it could be a complex integrated circuit with many circuits to accomplish a more complex task, or it could be a digital system which accepts a signal on its input and outputs the changed signal. So long as it accomplishes its defined task, it doesn't matter how the box works internally.
Digital signal processors require several things to work properly:
A processor fast enough and with enough precision to support the mathematics it needs to implement.
Supporting memory to store programming, samples, intermediate results, and final results.
Analog-to-Digital (A/D) and Digital-to-Analog (D/A) Converters to bring real signals into and out of the digital domain.
Programming to do the job.
Digital signal processors, even single chip DSP systems, are built from these elements. Twenty years ago, anyone using DSP had to be quite a mathematician to be able to implement and use the algorithms. Today, DSP can be incorporated into devices so simple that they can be mass-produced and operated with as little as the press of a button.
All communications circuits contain some noise. This is true whether the signals are analogÂ or digital, and regardless of the type of information conveyed. Noise is the eternal bane of communications engineers, who are always striving to find new ways to improve the signal-to-noise ratio in communications systems. Traditional methods of optimizing S/N ratio include increasing the transmitted signal power and increasing the receiver sensitivity. (In wirelessÂ systems, specialized antenna systems can also help.) Digital signal processing dramatically improves the sensitivity of a receiving unit. The effect is most noticeable when noise competes with a desired signal. A good DSP circuit can sometimes seem like an electronic miracle worker. But there are limits to what it can do. If the noise is so strong that all traces of the signal are obliterated, a DSP circuit cannot find any order in the chaos, and no signal will be received.
If an incoming signal is analog, for example a standard television broadcast station, the signal is first converted to digital form by anÂ Analog to Digital Converter (ADC), as depicted in Figure 1.1. The resulting digital signal has two or more levels. Ideally, these levels are always predictable, exact voltages or currents. However, because the incoming signal contains noise, the levels are not always at the standard values. The DSP circuit adjusts the levels so they are at the correct values. This practically eliminates the noise. The digital signal is then converted back to analog from via aÂ Digital to Analog Converter (DAC) [1-3].
Figure 1.1: Digital Communication System
Need of Digital to Analog Converter
A digital-to-analog converter (DAC or D-to-A) is a device for converting a digital (usually binary) code to an analog signal (current, voltage or charges). Digital-to-Analog Converters are the interface between the abstract digital world and the analog real life. Simple switches, a network of resistors, current sources or capacitors may implement this conversion.
In DSP System if an incoming signal is analog, for example a standard television broadcast station; the signal is first converted to digital form by anÂ ADC. The resulting digital signal has two or more levels. Ideally, these levels are always predictable, exact voltages or currents. However, because the incoming signal contains noise, the levels are not always at the standard values. The DSP circuit adjusts the levels so they are at the correct values. This practically eliminates the noise. The digital signal is then converted back to analog from via aÂ DAC .
Types of DAC System
The most commonly used electronic DAC's are:
The Pulse Width Modulator the simplest DAC type. A stable current (electricity) or voltage is switched into a low pass analog filter with a duration determined by the digital input code. This technique is often used for electric motor speed control, and is now becoming common in high-fidelity audio.
The Binary Weighted DAC, which contains one resistor or current source for each bit of the DAC connected to a summing point. These precise voltages or currents sum to the correct output value. This is one of the fastest conversion methods but suffers from poor accuracy because of the high precision required for each individual voltage or current. Such high-precision resistors and current-sources are expensive, so this type of converter is usually limited to 8-bit resolution or less.
The R2R Ladder DAC, which is a binary weighted DAC that creates each value with a repeating structure of 2 resistor values, R and R times two. This improves DAC precision due to the ease of producing many equal matched values of resistors or current sources, but lowers conversion speed due to parasitic capacitance.
The Segmented DAC, which contains an equal resistor or current source segment for each possible value of DAC output. An 8-bit binary-segmented DAC would have 255 segments, and a 16-bit binary-segmented DAC would have 65,535 segments. This is perhaps the fastest and highest precision DAC architecture but at the expense of high cost. Conversion speeds of >1 billion samples per second have been reached with this type of DAC.
Oversampling DACs such as the Delta-Sigma DAC, a pulse density conversion technique. The oversampling technique allows for the use of a lower resolution DAC internally. A simple 1-bit DAC is often chosen, as it is inherently linear. The DAC is driven with a pulse density modulated signal, created through negative feedback. The negative feedback will act as a high-pass filter for the quantization (signal processing) noise, thus pushing this noise out of the pass-band. Most very high resolution DACs (greater than 16 bits) are of this type due to its high linearity and low cost. Speeds of greater than 100 thousand samples per second and resolutions of 24 bits are attainable with Delta-Sigma DACs. Simple first order Delta-Sigma modulators or higher order topologies such as MASH - 'Multi stage' noise Shaping can be used to generate the pulse density signal. Higher oversampling rates relax the specifications of the output Low-pass filter and enable further suppression of quantization noise.
Hybrid DACs, which use a combination of the above techniques in a single converter. Most DAC integrated circuits are of this type due to the difficulty of getting low cost, high speed and high precision in one device.
The concept of oversampling and interpolation can be used in a similar manner with a reconstruction DAC. For instance, oversampling is common in digital audio CD players, where the basic update rate of the data from the CD is 44.1 kSPS (Kilo Samples per Second). Early CD players used traditional binary DACs and inserted "zeros" into the parallel data, thereby increasing the effective update rate to 4-times, 8-times, or 16-times the fundamental throughput rate. The 4Ã-, 8Ã-, or 16Ã- data stream is passed through a digital interpolation filter which generates the extra data points. The high oversampling rate moves the image frequencies higher, thereby allowing a less complex lower cost filter with a wider transition band. In addition, there is an increase in the SNR within the signal bandwidth because of the process gain. The sigma-delta DAC architecture uses a much higher oversampling rate and represents the ultimate extension of this concept and has become popular in modern CD players [4, 5].
Introduction to Oversampling Sigma Delta DAC
Sigma-delta DACs operate very similarly to sigma-delta ADCs, however in a sigma-delta DAC, the noise shaping function is accomplished with a digital modulator rather than an analog one.
An Î£-Î” DAC, unlike the Î£-Î” ADC, is mostly digital (see Figure 1.2). It consists of an "interpolation filter" (a digital circuit which accepts data at a low rate, inserts zeros at a high rate, and then applies a digital filter algorithm and outputs data at a high rate), a Î£-Î” modulator (which effectively acts as a low pass filter to the signal but as a high pass filter to the quantization noise, and converts the resulting data to a high speed bit stream), and a 1-bit DAC whose output switches between equal positive and negative reference voltages. The output is filtered in an external analog LPF. Because of the high oversampling frequency, the complexity of the LPF is much less than the case of traditional Nyquist operation. It is possible to use more than one bit in the Î£-Î” DAC, and this leads to the multibit architecture. The Sigma Delta architecture is ideal for converters for measurement, voiceband and audio application (CD/ SACD/ DVD) .
Figure 1.2: Single bit output Sigma Delta DAC.
The growing demands for digital audio and voice band codec necessitate high-resolution A/D and D/A converters that can be integrated with digital processors in a standard CMOS technology. Oversampling D/A converters based on sigma-delta (S-D) modulation easily fulfills the need for high resolution by sacrificing circuit speed for the sake of amplitude resolution. Also, these D/A converters are suitable for the integration with digital circuit because of their relaxed analog filtering requirements and their improved tolerance for component mismatch. Oversampling used in conjunction with digital filtering is a powerful tool in modern sampled data systems. A primary advantage is the relaxation of the requirements on the antialiasing/anti-imaging filter. Another advantage is the increase in SNR which occurs because of the process gain.
This thesis contains five chapters, below is the brief detail of them. Present chapter discusses the basics of digital communication systems requiring the ADC & DAC. Further the types of the DACs are discussed & Oversampled DAC is introduced.
Chapter 2 deals with the findings of related work till date. It has also attempted to bring out some of the limitations of existing work. Chapter 3 describes the problem identified during the literature review.
Chapter 4 deeply explains the methodology which has been proposed in this thesis. The architecture of 128x oversampled sigma delta DAC is presented, which contains the interpolator block and the delta sigma modulator.
Chapter 5 shows the outcome of the intermediate result at each stage of the work. Chapter 6 deals with the conclusion of the implemented work with relevant application and the possible future work.