[Design of amplifier circuit for thermal conductivity detector in micro gas chromatography]

Agilent 3000 + is a typical micro gas chromatograph (micro GC) which is widely used for its fast analysis, high resolution, wide dynamic range and energy-efficient. However its amplifier circuit and analog-to-digital convertor (ADC) are of high power consumption and high working temperature. Based on the results of theoretical calculation, ADS1255, a 24-bit delta-sigma ADC from TI, was selected as the core component for its low noise and energy-efficient. Furthermore, a low noise, high common-mode voltage durable full differential amplifier circuit was designed to accomplish the functions of impedance matching, filtering, and level shifting in front of ADC. The full differential amplifier was optimized with the analysis of noise model and theoretical calculation. In addition, a testing platform was developed to test the full differential amplifier and ADC. The testing results showed that the American Society for Testing and Materials (ASTM) noise value of new full different amplifier and ADC was as low as 1.25 microV and the power dissipation was 3.7 W lower than that of the old circuit. The new circuit is low noise, energy-efficient, compact and cheap and can cater for the requirement of the micro GC of next generation.