Data acquisition and interferogram data treatment in FT-IR spectrometers

Abstract

Traditional FT-IR spectrometers use the zero crossings of an interferogram from a control HeNe laser as a trigger signal for digitizing the analog signal of the main interferometer. Usually, successive approximation analog to digital converters (ADCs) with 16 to 20 bit resolution with or without gain ranging functions are employed. Advances in digital signal processing and progress in Audio-CD and DVD recording have brought a new breed of ADCs with higher resolutions, but with the apparent disadvantage of a fixed digitization rate. Initially, this kind of ADC (“Delta Sigma” ADC) could not be used in FT-IR spectrometers because FT-IR data acquisition requires that the infrared signal be digitized at equal increments of optical path difference. However, in a paper published by J. Brault it was shown that it is possible to use these “Delta Sigma” ADCs very efficiently in FT-IR instrumentation by simultaneously sampling both the analog signal of the main interferometer and the rate of change of the optical path difference. Digital signal processing then allows recalculation of the interferogram at the traditional zero crossings of the interferogram of the HeNe laser, and subsequent calculation of the spectrum. Differences between the traditional approach using successive approximation ADCs and state of the art “Delta Sigma” ADCs are shown, along with additional data processing functions that enhance the usability of the spectrometer.