Noise digitisation

Figure 11 FTIR spectra of a 10% solution of sucrose in water A) and of water B) and the difference spectrum obtained by spectral subtraction (A-B). The features in the difference spectrum are the absorption bands of sucrose. The region between 3500 and 3150 cm-i is obscured by digitisation noise due to the intense water absorption band in this region.

Figure 3.32. Enhanced sensitivity can be realised in favourable cases by oversampling the data and hence reducing digitisation noise. Spectrum (a) shows part of a conventional proton spectrum sampled according to the Nyquist criterion. Oversampling the data by a factor of 24 as in (b) provides a sensitivity gain (all other conditions as for (a)).

The corrosion process is observed as a series of events which all contribute to the overall corrosion rate. Measurement of rest potential fluctuations between two identical electrodes of potential fluctuations with respect to a fixed reference can be carried out. The electrochemical noise output spectrum is analysed using digitised data. The interpretation requires electrochemical expertise, and the method is therefore usually provided as a specialised service. 

The microvolt NMR signal generated in the coil requires amplification prior to detection and digitisation. The first stage is typically 20-30 dB amplification, using a preamplifier whose most important characteristic is the noise figure (NF). This is essentially a measure of the noise added to the signal by the amplifier, and is defined by,... 

Figure 3.27. Dynamic range and the detection of small signals in the presence of large ones. As the digitiser resolution and hence its dynamic range are reduced, the carbon-13 satellites of the parent proton resonance become masked by noise until they are barely discernible with only 6-bit resolution (all other acquisition parameters were identical for each spectrum). The increased noise is digitisation or quantisation noise (see text below).

Figure 3JO. Digitisation errors. Discrete digital sampling of an analogue waveform introduces errors in amplitude measurements, indicated by the vertical bars. These errors ultimately contribute to additional noise in the spectrum.

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