Engineering Application - Essay Example

Connecting digital circuitry to sensors is simple if the sensor devices are inherently digital. Switches, electromagnetic relays, en rs and de rs are easily interfaced with gate circuits due to the on/off nature of their signals. However, when analogue devices are involved, interfacing is very complicated and we will need a way to translate analog signals into digital quantities. With this growing demand for and capability of digital signal processing, many analog signal conditioning are performed in the digital domain today. Because of this, there is also a high demand for the most accurate digital representation of the analog input before performing any type of signal processing. Analog to Digital So how do we make a digital representation of analog signals? We use an Analog to Digital Converter (ADC). The ADC is an electronic device that “accepts an analog input-a voltage or a current-and converts it to a digital value that can be read by a microprocessor” (Embedded.com). The ADC lies at the heart of the encoding side of digital systems, and is “perhaps the most critical component in the entire signal chain” (Pohlmann). ADCs discretely sample analog signals, measure its amplitude, and finally represent the measurement as a binary word. Pohlmann describes the importance of highly accurate ADCs in his paper: Whereas conversions made with the [ADC] counterpart—the digital-to-analog converter [DAC]—can subsequently be improved for higher-fidelity playback, errors introduced by the [ADC] will accompany the audio signal throughout digital processing and storage and, ultimately, back into its analog state. Thus, the choice of the [ADC] irrevocably affects the fidelity of the resulting signal. For critical applications such as the long-term preservation of historic audio signals, to the greatest extent possible, the [ADC] must exhibit audio transparency—that is, it should neither add to nor subtract from the sound. To assess the degree of transparency, the converter’s electrical measurements and subjective aural performance, as well as the converter’s operating parameters such as sampling frequency and word length, must be considered. Finally, the signal-level input to the converter, converter-component design, and external conditions such as grounding and shielding can greatly affect the fidelity of the resulting file. (Pohlmann) That said, we therefore need to know the specifications to describe ADC performance. But before going to its specifications, let us first describe some types or structures of ADCs. Types The type or structure of an ADC describes the way it is implemented. Microlink Engineering Solutions classifies ADC types into five – (1) Successive approximation converter, (2) Dual slope integrating converter, (3) Charge balancing converter, (4) Flash converter and (5) Sigma-Delta converter. These types described in the following paragraph. Successive approximation converters provide a fast conversion of an instantaneous value of the input signal. This type of ADC works by comparing the input with a voltage which is half of the input range, then compares it with three-quarters of the range if it was over the level, and so on. Repeating these twelve times results to a 12-bit resolution. “While [the] comparisons are taking place the signal is frozen in a sample and hold circuit. After [ADC] the resulting bytes are placed into either a pipeline or buffer store. A pipeline store enables the [ADC] to do another conversion while the previous data is transferred to the computer. Buffered [ADC] place the data into a queue held in buffer memory. The computer can read the converted value immediately, or can allow values to accumulate in the buffer and read them when it is convenient. This frees the computer from having to deal with the samples in real time, allowing them to be processed in convenient batches without losing any data” (“Technical Notes: Analogue-to-Digital”). Dual slope integrating converters, which also go by the names ramp-compare ADC and multi-slope ADC, introduces less noise but is slower than the successive approximation type. Charge balancing converters create pulse trains. The pulses going out of the capacitor can be translated to input voltage. The capacitor is charged for a fixed time but is simultaneously discharged in units of charge packets. Integrating the input signal over the capacitor charging time results to a decrease in noise. Flash converters perform “by a highly parallel comparison of an input analog signal to each of a set of reference voltages” (“Analog to digital converter”). They are the fastest type due to their use of numerous comparators. Sigma-Delta converters are “useful for high resolution conversion of low-frequency signals as well as low-distortion conversion of signals containing audio frequencies” (“Technical Notes: Analogue-to-Digital”) in that they have good linearity and high accuracy. Another type of ADC that is widely documented is the Pipeline ADC. Although slower than flash ADCs, this type provides conversion that is faster than most other ADC architectures. Its only difference form flash ADCs is 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” (“Analog to digital converter”). Pipeline ADCs include stage amplifiers which translate to the number of output bits. Its advantage over other types is lies in the separation of each stage. Specifications There are many types of specifications for ADCs quoted by hardware manufacturers. This report will briefly discuss resolution, response, accuracy, sampling rate, aliasing, throughput and dither. The resolution of an ADC can be described in terms of bits as the number of discrete values it can produce over the range of analog values. It can also be electrically defined as the smallest voltage change that can be measured. It is “the same as the smallest step size, and can be calculated by dividing the reference voltage by the number of possible conversion values” (Embedded.com). ADC response can be categorized into linear and nonlinear. Linear means that the input values have a linear relationship with the output values. On the other hand nonlinearity can be subdivided into integral and differential non-linearity. Differential non-linearity describes the error in each voltage step from its previous step, while integral non-linearity represents the nonlinearity over the entire range of the ADC. The accuracy of ADC is vastly affected by a number of errors including quantization error, aperture error, offset error, gain error, and the nonlinearities discussed earlier. Quantization error results from an infinite level analog signal being mapped to a finite number of voltages. Aperture error is a transient effect that introduces errors between the sample and hold modes. Offset error occurs results from a difference between the value of the first code transition and the ideal value of 0.5 LSB. Gain error occurs when the slope of the transfer characteristic differs from that of an ideal ADC. Since analog signals are continuous, it is necessary to define the rate at which new digital values are sampled from the analog signal, or sampling rate. If the input signal is changing much faster than the sample rate, spurious signals called aliases will be produced at the output of the DAC. It should be noted that aliasing works both ways. “An out-of-band signal in the analog domain can interfere in the digital spectrum if it is above Nyquist range and becomes ‘aliased" in-band of the digital version. Thus, ‘anti-alias’ filters are typically used before [ADCs]” (Embedded.com). The throughput is the maximum rate at which the A-D converter can output data values. “Dither is a low-amplitude noise deliberately added to the audio signal” (Pohlmann) to improve linearity, reduce distortion, and greatly extend converter’s dynamic range. Applications ADCs are used in many applications including communication applications to provide an effective way of converting analog signals into digital signals, wireless communication devices which typically process and generate both digital and analog and “in high-speed applications such as CCD imaging, ultrasonic medical imaging, digital videos, cable modems, and fast Ethernets” (“Analog to digital converter”). DACs A digital-to-analog converter, also referred to as a DAC, is an electronic decoding device that receives a digitally coded signal and outputs corresponding analog output current or voltage signal. Digital-to-analog conversion is the process opposite that done by ADC. Engineering applications include “applications involving digital signal processing of real-world signals such as those found in communication systems, instrumentation, and audio and video processing systems” (“Analog to digital converter”). Both commercial ADCs and DACs are available in the market today in the form of integrated circuits. We just need to study their data sheets to find the ADC or DAC perfect for out needed application. Works Cited “Analog to digital converter." Electronics Information Online. 06 December 2006. 24 March 2009 . “Digital to analog converter." Electronics Information Online. 06 December 2006. 24 March 2009 . Embedded.com. TechInsights. 24 March 2009 . Pohlmann, Ken. Measurement and Evaluation of Analog-to-Digital Converters Used in the Long-Term Preservation of Audio Recordings. Washington: National Recording Preservation Board., 2006. “Technical Notes: Analogue-to-Digital Converters." MicrolinkEngineering Solutions. 24 March 2009 . Read More