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Cryogenic Detection Circuit for a Fourier Transform Mass Spectrometer
Keywords: Cryogenic , Spectrometer, GaAs MESFET
Fourier transform mass spectrometry (FTMS) has emerged as one of the best analytical instruments for the analysis of biological samples due to its high mass accuracy, high resolution and wide dynamic range. Two additional important performance measures of an FTMS are sensitivity and limits-of-detection. This works aims to improve both by developing a new, higher performance preamplifier. The first stage in the detection circuit of a FTMS is a differential transimpedance preamplifier. Here we report ongoing progress in the development of such a low noise preamplifier which can be cooled to 4oK to reduce the intrinsic thermal noise. The bandwidth of interest in the unamplified output signal from a FTMS is about 1 MHz and the signal has typical differential amplitude of a few pA. Thus, for the best use of the standard digitizers, the complete amplifier is required to have a transimpedance gain of approximately 1e8 Ω. We have used GaAs MESFET as the active devices for the preamplifier. The power dissipation in the current design is approximately 50 mW, which is below the allotted 100 mW to the detection circuit. The preamplifier has a transimpedance gain of 95 kΩ with a 1 MHz bandwidth. The output signal from preamplifier is amplified further using a room temperature JFET amplifier. The equivalent input voltage noise of the preamplifier was measured to be 11 nV/rtHz at 100 kHz at 77oK, and further characterization of the preamplifier and the MESFET leakage current at 4oK is undergoing.
Mr. Raman Mathur, Boston University
Boston University
Boston, MA

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