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Transcript
Computer Based Data Acquisition Basics Outline • Basics of data acquisition • Analog to Digital Conversion – Quantization – Aliasing Acquisition Basics Signal Physical Phenomenon Transducer Conditioning Interface Card Computer Analog to Digital Conversion • In this process, we convert an analog voltage into a number • Computers store numbers in “bits” • Typically use a 12 bit converter - converts each input voltage into some number between 0 and 4095 (212-1) • Maximum sample rate - if 100 kHz – takes 10 sec to perform the conversion Sources of Error • The fact that we are using a converter which has a finite number of bits introduces an error called “quantization error” • The fact that it takes a finite (non-zero) amount of time to perform the conversion introduces the possibility of an error called “aliasing” Quantization Error V Analog to Digital Conversion n+3 n+2 n+1 V n n-1 n-2 n-3 Quantization Error • For example, if we have a – 10 volt input range – n ranges from 0-4095 10 V V 2.44mV 4096 • Uncertainty of 1.22 mV Aliasing • Aliasing can lead to incorrect frequency information • Sampling Theorem - Maximum frequency component that can be correctly identified is that frequency which is 1/2 the sample frequency. This maximum frequency is called the Nyquist frequency. Voltage Voltage vs Time Time (sec) Voltage Voltage vs Time Time (sec) Voltage Voltage vs Time Time (sec) Voltage Voltage vs Time Time (sec) Voltage Voltage vs Time Time (sec) Aliasing • Once the observed frequency exceeds 1/2 half the sampling frequency, trouble starts 0.6 0.4 nal fmeasured /fsig Relation between Measured Frequency, Sampling Frequency and Signal Frequency 0.2 0 0 0.5 1 1.5 2 fsignal /fsample 2.5 3
