| The key technique of weak signal logging instrument is NV-level minimal signal detection method, and the technical difficulty of the three-electrode logging is detect and collect the small sinusoidal signal that frequency is known. The main task of this paper is to complete weak voltage signal detection and data acquisition on the three adjacent electrodes of the metal casing.This paper firstly describes the normal resistivity logging and the three-electrode basic ideas,as well as a detailed description of the weak signal detection. Secondly, the detailed structure of the preamplifier circuit, ultra-low-noise high-precision high-speed op amp LT1028, the filter circuit and isolation of the output circuit. Thirdly, described in detail of hardware design of weak signal acquisition system, including the details of the24high-resolution signal acquisition chip ADS1271and the master-chip ADSP-2186M, the basic ideas and the corresponding simulation of the phase sensitive detection algorithm, the application of digital phase sensitive detection technology in the DSP. Finally, this paper analyzes the linearity of the preamplifier circuit and data accuracy of two voltage acquisition channels by experimental results.In this topic, low-noise preamplifier, synchronous data acquisition of the daisy chain and digital phase-sensitive detection technique are applied in detecting logging signals and100nV is achieved. The experimental result indicated that:the weak signal detection and acquisition system of three-electrode method for resistivity logging achieving high precision of detection, it is stable and meet the design requirements. |
