Signal digitisation

The accuracy with which the analogue signal is processed is influenced by two main properties of the ADC data processing interface  

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Sample Frequency. Defines the sampling frequency, in MHz, to be used in the digitisation of the A-scan signal. 

A Fourier transform infrared spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber and an infrared detector. Interference signals measured at the detector are usually amplified and then digitised. A digital computer initially records and then processes the interferogram and also allows the spectral data that result to be manipulated. Permanent records of spectral data are created using a plotter or other peripheral device. 

A spectrum in a specified ranalogue signals from eadi photodiode are digitised and transferred to a computer, where they e corrected for dark current response and transformed to absorbance. A number of digital techniques are available to increase sensitivity and to extend the use of rapid-scanning detectors to multicomponent analysis, reaction kinetics, tablet dissolution tests, process control, and detection in HPLC (A. F. Fell et al, Chrom-atographia, 1982, 16, 69-78). 

The signals are digitised in the recording cabins. The digital data, like the remote-control and telemetry data, is transmitted by radio link to the personnel base, over a... 

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,... 

The signal is then digitised for storage in the computer memory. There are two ways in which the data can be captured when the two quadrature channels are being used. A data point for each channel can be captured at the same time (called simultaneous acquisition) or, alternatively, one data point is taken at each dwell time but routed to the different channels of the phase-sensitive detector (called sequential acquisition). The relationship of the spectral width and DW is slightly different for the two approaches ... 

Figure 3.8. The NMR detection proces,s. A fixed reference frequency is subtracted from the detected NMR signal so that only the frequency differences between resonances are digitised and recorded.

Figure 3.9. Schematic illustration of NMR data collection. Pulsed rf excitation stimulates the NMR response in the probe head which is then amplified and detected in the receiver. The receiver reference frequency (that of the pulse rf) is subtracted to leave only audio frequencies (AF) which are digitised by the analogue-to-digital converter (ADC) and subsequently stored within the computer. The functions of the boxed sections are described later in the text (DSP digital signal processor).

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