Computer Based Data Acquisition Basics Outline Basics of

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 k. Hz – 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 • Uncertainty of 1. 22 m. V

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.

Aliasing • Once the observed frequency exceeds 1/2 half the sampling frequency, trouble starts