Data Acquisition

Figure 5.20Many aliases combined with a speech signalIt could be assumed that aliasing is a phenomenon associated with high frequencies, andthat low frequencies (such as thermocouple temperature signals) are immune to this effect.Temperature changes so slowly that the input signal is almost DC; it seems therefore reasonableto sample it extremely slowly and not be concerned with frequency analyses.However, if the input signal contains a noise spike as shown in Figure 5.21(a), the resultingspectrum results in a noise floor around –60 dB, shown in Figure 5.21(b). This is because animpulse spike in the time domain spreads itself out evenly in the frequency domain. Thus,when sampled at a low frequency, the high-frequency components of the noise spike aliasdown and add to the low frequency components. The extra energy of these added frequenciescauses the temperature application to oscillate. If a low pass filter is used, the spike – with itsequivalent high-frequency components – is removed (as shown in the time domain in Figure5.21(c). The spectrum corresponding to this (Figure 5.21(d)) now has a noise floor of –80 to–90 dB, which does not affect the readings obtained by the A/D board.