Solid state photodiode system matched to high-gain low-noise d.c. and lock-in amplifiers for use in multichannel emission spectrochemical analysis

Abstract

Various configurations of planar silicon photodiode arrays were tested as detection devices for multichannel emission spectrometry. The electronics comprised a laboratory-built preamplifier (open loop gain 25,000) that allowed measurements with commercial high-gain low-noise d.c. and lock-in amplifiers. An a.c. noise level of 2.5 × 10 −15 A (modulating frequency = 400 Hz, frequency band pass ≈ 1 Hz) and a low frequency noise level of 6 × 10 −15 A (3-db high-frequency roll-off = 3 Hz) at the output of the detector was reached at room temperature. This noise can be ascribed completely to the preamplifier. Cooling the photodiodes has no effect on the noise level and can be omitted. A perfectly linear response over at least three orders of magnitude above the noise level and a spectral resolution equivalent to that attained with photomultiplier and exit slit were established. Comparison of the signal-to-noise (S/N) ratio of the photodiodes with that of various types of photomultiplier showed that photomultipliers were superior by a factor of 100–500 in the S/N ratio at low light levels in the wavelength region between 2500 and 5500 Å. Prospects for further research on photodiode devices for dual-channel intensity measurements (simultaneous measurement of line-plus-background and background) are discussed.