Research Of Preamplifier And Data Acquisition In Quadrupole Mass Spectrometer Detection System

Quadrupole mass spectrometer and quadrupole ion trap mass spectrometer which can detect the chemical compositions of the material are high sensitivity, high resolution instruments. The transimpedance preamplifier and data acquisition system are the key equipment in QMS, and they directly affect the system sensitivity and resolution. For the research of low-current amplification, it has not only research the high-current and low-frequency current signal amplification, but also research suppressing noise and signal bandwidth expansion of current amplification. Particularly, in QMS and QIT-MS, there are needs for the radiofrequency and power frequency noise is serious, low-abundance ion signal detection and fast scanning, so low-current amplification is required to reduce internal noise, amplifier ultra-low current signal and reduce the single scanning time of acquisition data. There are a lot of researches on current amplification circuit at home and abroad, and most researches focus on feedback network parameters and circuit structure of transimpedance preamplifier. In the detection system designing for commercial application instruments and laboratory studies, problems like higher power, larger volume, larger noise and lower signal to noise ratio are existed. The purpose of this PHD dissertation is to research low noise and high bandwidth transimpedance preamplifier, and fast data acquisition control system in QMS and QIT-MS, The transimpedance preamplifier is designed to suppress noise, extend bandwidth, improve the signal gain and improve stability. Single scanning time is reduced by improving the speed of data acquisition control logic.In the dissertation, the current-to-voltage amplifier is designed to suppress noise, extend bandwidth, increase gain and system stability on the QMS and QIT-MS. Real-time acquisition and high-speed large-capacity FIFO data buffer are implemented with field-programmable gate array (FPGA). For MS low current detection, non-inverting input composite preamplifier (NIICP) with internal feedback network, and composite transimpedance JFET preamplifier are researched, and the corresponding improvement solution are given for the problems. For ultra-low MS current detection, Bandwidth sharing program is designed for low current signal testing, two-stage ac boosting amplifier is designed to extend bandwidth. Low current-to-voltage amplifier solution in different conditions is tested and verified by theoretical analysis, physical modeling and experimental validation. In addition, external noise suppression method is analyzed and designed in detail, synchronous acquisition control and high-speed data storage buffer is also designed. The main researches are as follows:First, concerning on the MS peak spectral range and main noise source in the QMS, the ultra low current measurement and data acquisition system is designed using composite transimpedance preamplifier, fourth-order active low-pass filter and successive approximation analog-to-digital converter (SAR ADC). By analyzing impact of gain and bandwidth on signal-to-noise radio (SNR),full width at half maximum (FWHM) and signal intensity, suitable gain and bandwidth are selected. In high-rate scanning, the amplifier requires high bandwidth. In the situation of bandwidth meets the requirement of peak bandwidth, the impacts of different gains to SNR, peak intensity and FWHM were analyzed. In slow-rate scanning, as bandwidth need is low, an ultra high signal gain preamplifier that obtains higher SNR can increase the sensitivity of small quadrupole mass spectrometer.Secondly, FPGA control logic of MS data acquisition and data buffer in the QMS was proposed. FPGA was designed to control SRAM memory to realize the high-speed high-capacity FIFO (first in first out, FIFO) memory, real-time synchronous data acquisition and automatic simultaneous read/write. Time multiplexing was designed to simultaneous read and write. Pre-reading resolution can void the problem that system read pulse time is not enough to complete time multiplexing. In the end, 1M*16 bits FIFO storage and 20Mbps access speed were realized. The system was used on the QMS and QIT-MS to reduce the time of single MS scanning analysis.Additionally, a non-inverting input composite preamplifier (NIICP) with internal feedback network was designed. Composite open-loop gain is changed by the internal feedback parameters, and transfer function of signal gain and noise gain change too, so reducing noise at lower gain and expanding bandwidth in higher gain can be realized respectively. In the lower gain, voltage noise bandwidth which has a significant effect on overall output noise can be reduced by suppressing composite open-loop gain. While in the higher gain, combined with a lower open-loop gain, the error function of the signal gain can occur peak gain. The peak gain is used for recovering the suppressed signal gain to expand bandwidth. Compared with inverting input composite preamplifier (IICP), experimental results indicate that NIICP reduces 5-30% in the 106-107V/A gain, reduces 20% in the 107V/A gain, expands bandwidth 1.8 times and increases 10-15% MS peak amplitude in the 108 V/A gain.Pre-JFET composite transimpedance amplifier (PJCTA) with lower noise is designed. JFET which is used as the input of a cascade amplifier can reduce the noise. Cascode amplifier is used as the input stage of composite amplifier, as cascode amplifier can eliminate the Miller effect to increase the signal bandwidth. FET operational amplifier is used as the cascode promotion stage to drive external load. Small signal models of pre-JFET composite transimpedance amplifier are analyzed to realize low-noise design.Finally, in the 109V/A gain, two-stage ac boosting amplifier (TSABA) with high bandwidth was designed. The first stage is the stable, extremely high gain, low bandwidth and low noise negative-feedback transimpedance preamplifier. The second stage cancels out circuit pole to recover signal gain damped down by -3dB frequency turning point of the first stage. Two stage ac boosting amplifier parameters with high gain and low noise were selected by signal gain of transfer characteristics which was derived by analyzing small model of two stage amplifiers.In this dissertation, MS low current detection and data acquisition control system were improved. The low noise and high bandwidth NIICP, a lower noise PJCTA and a wider bandwidth TSABA were analyzed and designed. FPGA control logic with synchronous acquisition and high-speed data buffer was designed. All the above are the innovations of this dissertation. Compared with former QMS and QIT-MS detectors, these optimized transimpedance amplifier can achieve higher sensitivity and resolution, and can detect very low abundance ions, and the single scanning time meet for high speed scanning demanding.