1.3 Signal Filtering

The sensor output is prone to get affected by the surrounding operating environment, resulting in additional noise and fluctuations. Analog or digital signal filters are required to remove noise and fluctuations. The frequency-cutoff characteristics of passive filters can change with a change in load, whereas active filters using op amp are used to maintain stable filter characteristics. Active filters also help to attenuate/amplify signals to obtain the desired voltage range without losing signal characteristics. For example, PIR sensors, photodiodes, current measurement, flow sensors and capacitive touch sensors need filtering. Figure 1-3 shows the low-pass, high-pass and band-pass active filters, respectively.