Signal conditioning

INTRODUCTION

Analog signal from sensor and its sampled equivalent Ways to capture sensor data One could record the signal with an analog device such as a chart recorder, which physically plots the signal on paper, or display it with an oscilloscope. Store the data using a microprocessor or computer. This process is called computer data acquisition, and it provides more compact storage of the data (magnetic, optical, or flash media vs.long rolls of paper), can result in greater data accuracy, allows use of the data in a realtime control system, and enables data processing long after the events have occurred.

Sampling To be able to input analog data to a digital circuit or microprocessor, the analog data must be transformed into coded digital values. The first step is to numerically evaluate the signal at discrete instants in time. This process is called sampling, and the result is a digitized signal composed of discrete values corresponding to each sample. The collection of sampled data points forms a data array, and although this representation is no longer continuous, it can accurately describe the original analog signal.

Sampling contd... How fast or often the signal should be sampled to obtain an accurate representation ? Ans: “as fast as you possibly can.” The problems with this conclusion Specialized, high-speed hardware is required. Large amount of computer memory is needed to store the data. A better answer is to select the minimal sampling rate required for a given application that retains all important signal information.

Sampling theorem The sampling theorem, also called Shannon’s sampling theorem, Statement: We need to sample a signal at a rate more than two times the maximum frequency component in the signal to retain all frequency components. In other words, to faithfully represent the analog signal, the digital samples must be taken at a frequency fs such that, fs > 2 fmax where fmax is the highest frequency component in the input analog signal.

Aliasing The term fs is referred to as the sampling rate, and the limit on the minimum required rate (2 fmax ) is called the Nyquist frequency.

Quantizing theory

Components used in analog to digital conversion

Various DAQ products

SAR ADC

4 bit conversion

4-bit resistor ladder D/A converter

4-bit resistor ladder D/A with digital input 0001.