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Analog to digital converter
Saturday, 07 October 2006

Analog to digital (A/D, ADC) converters are electrical circuit devices that convert continuous signals, such as voltages or currents, from the analog domain to the digital domain where the signals are represented by numbers. Most processing equipment today are digital in nature, and they work with signals which are binary valued. In a digital or binary representation, a signal is represented by a word, which is composed of a finite number of bits. The processing of signals is preferably carried out in the digital domain because digital processing is fast, accurate and reliable. Analog to digital converters are widely used for converting analog signals to corresponding digital signals for many electronic circuits. Analog to digital converters allow the use of sophisticated digital signal processing systems to process analog signals, which are common in the real world. Many modern electronic systems require conversion of signals from analog to digital or from digital to analog form. Circuits for performing these functions are now required in numerous common consumer devices such as digital cameras, cellular telephones, wireless data network equipment, audio devices such as MP3 players, and video equipment such as digital video disk (DVD) players, high definition digital television (HDTV), and numerous other products. Analog to digital converters (ADC's) form an essential link in the signal processing pathway at the interface between the analog and digital domains. Advances in ADC technology have increased the speed, lowered the cost, and reduced the power requirements of analog to digital converters, and resulted in a proliferation of ADC applications.

Analog to digital converters perform a common and basic function that is necessary in many different types of applications. The primary function of an ADC is to convert an analog input signal to a digital value or binary code for use by various circuits and electronic devices. A simple ADC generally provides a low resolution digital representation for each sample, such as an eight-bit value, for example. More complex ADCs provide higher accuracy's, such a sixteen-bit values, or higher. The A/D converter may be part of a large analog system, and is frequently the component that limits the performance of the system. A process of converting an analog signal into a digital signal comprises measuring the amplitude of the analog signal at consistent time intervals and producing a set of signals representing the measured digital value. The information in the digital signals and the known time interval enables one to convert the digital signal back to the analog signal. Analog to digital conversion of a continuous input signal normally occurs in two steps: sampling and quantization. The sampler takes a time-varying analog input signal and converts it to a fixed voltage, current, electrical charge, or other output level. The quantizer takes the constant sampled level and compares it to the closest level from a discrete range of values called quantization levels. The performance of analog and digital converters is typically quantified by two primary parameters, speed (in samples per second) and resolution (in bits). Higher resolution A/D converters typically require a large signal to noise ratio and good linearity. A/D converters with high sampling rates are frequently desired, but generally have lower resolution. There are two basic techniques for performing analog to digital conversion: an open-loop technique and a feedback technique. An analog to digital converter is typically tested to assure proper function during operation by identifying repeatable discrepancies between the sampled analog signal and the corresponding value of the converted digital instance.

Analog to digital converters provide the link between analog and digital domains. The ADC is required to be capable of converting analog data to digital data in an accurate manner, appropriate to the bandwidth and resolution requirements of particular application. Analog ICs often require the use of, and constantly consume, a DC bias current. Digital integrated circuits are ICs which process digital signals. A/D converters are designed to process analog signals over a specified range of analog signal values. When the input signal exceeds the specified peak input signal level, the output registers of the A/D converter overflow. A/D converters are often used with microprocessors to convert an analog signal to a corresponding digital signal which is processed by the microprocessor. Microprocessors are general purpose processors which provide high instruction throughputs in order to execute software running thereon, and can have a wide range of processing requirements depending on the particular software applications involved. Due to the increased speed of microprocessors, there is a need for faster A/D converters. A/D converters of the parallel type and the serial and parallel type are advantageous for high speed operation. Such analog-to-digital converters generally comprise a plurality of comparators for comparing an analog input signal with analog reference voltages and an encoder for converting output signals of the comparators to a multibit digital signal. Multiple level or multilevel analog to digital data converters are useful in high speed, high power applications, for example for converting an input analog signal into a digital signal prior to digital signal processing. An analog to digital converter may be implemented using a variety of topologies. For example, an analog to digital converter may be implemented utilizing sigma-delta technology, mash converter technology, successive approximation technology, flash converter technology, or variations thereof. Therefore, a variety of A/D converter types exist, including flash ADC, sub-ranging ADC, successive approximation ADC, pipelined ADC, integrating ADC, and sigma delta or delta sigma A/D converters.

Flash ADC is performed by a highly parallel comparison of an input analog signal to each of a set of reference voltages. A flash converter uses a resistive divider to obtain the quantization. A flash converter has multiple reference levels and comparators. Each comparator compares the analog input signal to one of the reference levels and produces an output that indicates whether the input is above or below the reference level. Flash ADC can provide very high speed and accuracy at the cost of high component count and high power consumption. Flash ADCs are limited by higher input capacitance, power consumption, and device yield constraints associated with the high number of comparators in the circuitry. Successive approximation ADC uses one or a few comparators, operated iteratively, to yield high accuracy conversion with far fewer components than flash conversion. A/D converter using successive approximation technique effectively performs a binary search in a digital analog look up table and using a digital to analog converter (DAC) and comparator circuit. Successive approximation converters also allow higher resolutions but tend to be slower since they usually require N cycles to produce the answer. Successive approximation ADC operates at much slower conversion rates than flash ADC. Subranging analog to digital converters provide an intermediate compromise between flash ADCs and successive approximation ADCs. Subranging analog to digital converters typically use a low resolution flash quantizer during a first or coarse pass to convert the analog input signal into the most significant bits (MSB) of its digital value. A digital to analog converter (DAC) then generates an analog version of the MSB word. The residue signal is sent through one or more fine passes to produce the lower significant bits of the input signal. The lower significant bits and the MSB word are then combined by digital error correcting circuitry to produce the desired digital output word. A switched capacitor analog to digital converter (ADC) operated according to successive approximation register technique comprises a plurality of weighted capacitors with associated switches and a local DAC. The capacitors are charged by a voltage sample of an analog signal to be converted. The voltage sample is compared with an analog signal generated by the local DAC.

Integrating A/D converters integrate unknown and fixed charges over fixed and measured time periods respectively. Dual ramp integrating A/D converters integrate the signal being measured for a fixed period of time onto a capacitor and then de-integrate the capacitor charge back to the starting voltage using a reference voltage as the input. The converter measures the time it takes to return to the starting voltage. The resulting digital conversion in the time counter or digital accumulator is then proportional to the ratio of the unknown and reference signals. Delta sigma or sigma delta modulators are often used in mixed signal integrated A/D converters, because of their insensitivity to CMOS process linearity and matching problems when compared to other A/D converter types. These features make delta sigma based mixed signal solutions very attractive for a number of applications, such as audio, receiver channels of communication devices, sensor interface circuits, and measurement systems. A sigma-delta converter generally includes an analog modulator portion and digital filtering and decimation portion. The analog modulator portion essentially digitizes an analog input signal at very high sampling rates greater than the Nyquist rate, in order to perform a noise shaping function. A noise shaping or loop filter, typically a lowpass filter is commonly provided in the forward signal path of the delta sigma modulator to push some of the quantization noise into the higher frequency spectrum beyond the band of interest. Digital filtering is performed on the oversampled digital output to achieve a high resolution. The digital filtering portion allows the ADC to achieve a high resolution. Decimation is thereafter used to reduce the effective sampling rate back to the Nyquist rate. Sigma delta techniques allow much higher resolutions, but are relatively slow since the requisite level resolution is achieved by oversampling the input signal and noise shaping. Folding is a high speed technique in which the signal is folded by using several folding amplifiers to replicate the input signal and by detecting zero crossings of the folding amplifiers to produce the digital output.

Pipeline ADC provides analog to digital conversion that, while slower than flash conversion, is faster than most other ADC architectures. Pipeline converters behave similarly to flash converters except that there is a finite latency between the analog sample and the digital representation of the sample which is dependent on the number of stages in the pipeline. A pipeline analog to digital converter may include stage amplifiers. The number of stage amplifiers may be substantially equal to the number of output bits. The advantage of pipelined analog-to-digital conversion is that each stage of resolution is separated. A typical pipelined ADC includes a series of stages, wherein each stage provides one or more output digital bits. Each stage accepts an analog input and produces digital bits representing the band in which the input signal falls. The stage also creates an analog output representing the difference between the digital representation of the signal and the actual analog signal. The output digital bits from each stage taken together represent the digital value of an input signal provided to the ADC. Once the analog signal is resolved at the first stage and the result passed to the second stage, a new signal can be processed by the first stage. Multi-stage pipelined analog to digital converters (ADC) provide efficient high speed conversion of analog signals to digital equivalents. Pipelined ADCs have many applications. They are particularly useful when low voltage, high speed, high resolution quantization is required. This feature makes it ideal for high volume telecommunications application such as various digital subscriber lines, digital signal processing at video rates, and for stand alone high speed analog-to-digital converters. Unlike flash converters, for which component counts increase exponentially with converter resolution, the component counts of pipeline ADC converters increase linearly with resolution. Therefore, pipeline ADC converters are relatively compact, inexpensive, and power efficient. Accordingly, pipeline ADC's are widely used in portable signal processing apparatus.

Analog to digital converters (ADC's) are used in many applications. ADC's are used to convert an analog signal to a digital bit representation for analog video signal, audio signals and any other suitable analog signals. Analog to digital converters have been used in communication applications to provide an effective way of converting analog signals into digital signals. Wireless communication devices are widely used to communicate data, voice, and other information between physical locations without the use of wires. Wireless communications products and other modern electronic devices typically process and generate both digital and analog signals. To perform their intended functions, these systems often convert analog signals into digital signals. ADCs enable many systems to implement real-time processing of analog data. Such data capture and processing systems usually include sensors for collection of analog information and digital signal processors or microprocessors for processing of the data. Analog to digital converters range in size, complexity, and accuracy or resolution, where each of these factors depends upon the particular needs of the underlying application. Different architectures are suited to different needs. Serial analog-to-digital architecture offers the widest range of performance in analog-to-digital conversion, from low power and low resolution to quantizations with very high resolutions. Parallel analog-to-digital architecture provides the fastest quantization rate per analog signal. The pipelined analog-to-digital converter has become a popular ADC architecture for use in high-speed applications such as CCD imaging, ultrasonic medical imaging, digital videos, cable modems, and fast Ethernets.