Computer Based Data Acquisition Basics

Outline Basics of data acquisition Analog to Digital Conversion –Quantization –Aliasing

Acquisition Basics Physical Phenomenon Transducer Signal 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 ( ) 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 n+3 n+2 n+1 n n-1 n-2 n-3 VV Analog to Digital Conversion

Quantization Error For example, if we have a –10 volt input range –n ranges from 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.

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