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B-mode instrumentation
Published in Peter R Hoskins, Kevin Martin, Abigail Thrush, Diagnostic Ultrasound, 2019
At regular, frequent intervals in time, the amplitude of the analogue echo signal is measured, or sampled, producing a sequence of numbers corresponding to the amplitude values of the samples (Figure 4.5a). The digital output from the ADC for each sample is in the form of a binary number (Figure 4.5b). A stream of such binary numbers is produced corresponding to the sequence of signal samples. A binary number is formed from a set of digits, which can each have the value one or zero. Each digit of the binary number (1 or 0) is referred to as a bit (binary digit). The output from the ADC in Figure 4.5b is made up of 8 bits. There are 256 possible combinations of the 8 ones or zeros, so the digital output from this ADC can express 256 possible values of signal amplitude. Unlike the analogue signal, which was continuously variable, the digital signal can have only a finite number of values, which is determined by the number of bits. Each additional bit doubles the number of possible signal levels. The ADCs used in commercial B-mode systems are typically 12-bit, giving 4,096 possible values of signal level, so that the signal is a much more faithful recording of the analogue original. Also, since the smallest value that can be digitized is smaller, the range of values, i.e. the ratio of the largest to smallest value, is greater.
Acquiring and Processing Turbulent Flow Data
Published in Richard J. Goldstein, Fluid Mechanics Measurements, 2017
In digital processing the first step is to convert the anemometer output into digital form compatible with the computer and interface bus. Digital processing systems generally have 8-, 12-, 16-, or 32-bit character lengths. Computers generally use 16- and 32-bit words (“word” reflects the natural width of the computer’s CPU registers). In computer jargon, a 4-bit array is a “nibble” and an 8-bit array is a “byte.” A bit is a binary integer, and a binary word is a sequence of bits representing a number. For instance, 101 is a 3-bit word representing the decimal number 5. The right-hand bit, which is called the least significant bit, is the units column, the next bit is the 21 column, and the next bit, the most significant bit in this case, is the 22 column. An 8-bit array can represent any decimal integer between 0 and 256; a 12-bit array, 0-4095; a 16-bit array, 0-65535; etc. An 8-bit array allows maximum resolution of 1 part in 256 (1/2%); a 12-bit, 0.025%; a 16-bit, 0.0015%; etc. Eight-bit resolution may require offsetting an unlinearized hot-wire signal before digitizing, whereas 12-bit resolution would allow digitizing the signal directly.
Applied engineering systems
Published in Mike Tooley, Engineering A Level, 2006
Analogue to digital conversion is necessary when analogue signals (such as audio or video) are required to be processed or transmitted in digital form. The process of conversion (see Figure 4.141a) involves sampling the incoming analogue signal at a fast rate and then converting each sample to a corresponding digital code. The more binary digits (bits) used in the conversion process and the faster the sampling rate, the more accurate will be the digital representation of the analogue signal. In practice, it is necessary to sample an analogue signal at a rate (in terms of samples per seconds) that is at least twice the highest frequency present in the analogue signal. Resolutions of 8-bits can be satisfactory for simple applications (corresponding to 256 different signal voltage levels—see page 323), as many as 10 and 12 bits are used in many practical analogue to digital converters.
A new self-calibration method for electronic current transformers
Published in Journal of the Chinese Institute of Engineers, 2018
Kun-Long Chen, Guan-Jie Huang, Nanming Chen, Wei-Jen Lee
The thermal drift situation can be detected based on the difference between Vdc and Vc. In the case that this difference is smaller than the second threshold (), this indicates that thermal drift occurs. The reason for setting this threshold is that the ambient temperature varies slowly (less than 1 °C/min) and the resolution error of digital sampling should be considered. The resolution of a 12-bit analog-to-digital converter with the input voltage range of ±5-V is 2.44 mV/bit. The value of 2.44 mV is approximately (). Hence, the value of , based on the ambient temperature change rate (less than 1 °C/min), is selected to be the second threshold. Using Equation (1), the actual current value for this threshold is only approximately 1.215 A. Later, the Vc is updated to be Vdc, and then is used to adjust the quiescent output voltage. In the situation that the difference is larger than the second threshold, the thermal drift determination has failed, and then the proposed method will recalculate the Vdc.