Detection System Of Weak Current Based On ARM Micro-controller

Scanning Tunneling Microscope(STM) is a powerful tool for observing the micro-structure on the surface of an object. When the scanning tip is much closed to the surface of the conductive body, a tunneling current which is exponentially proportional to the distance between the tip and the object will be generated. Therefore the hollows and humps on the surface can be obtained by measuring the size of the current, and the cubic images of the surface will also be achieved if the tip moves over the object horizontally and vertically. But the current remains in the scale of namo ampere or even much smaller, and is difficult to be distinguished from the noise in the actual applicatiions. Relative larger error is presented even if it can be detected.A detecting and processing system with a core of ARM microprocessor is designed in this paper to improve the accuracy and sensitivity of the micro-current signal. The system is composed of a potentiostatic instrument, a signal amplifier, a programmable voltage source, an automatic gain control(AGC), a analog-to-digital convertor and a serial communication circuit. The micro current is detected by the potentiostatic instrument, and is converted to the voltage by a resistance in the feedback loop. The programmable voltage source provides the potentiostatic with a successive adjustable DC in the range of-5V to+5V. The amplified voltage is sent to the A/D. The digital signal converted is read and processed by the ARM, and transferred to the computer via a serial interface to display the data or the waveform. In order to improve the measuring precision and the signal to noise ratio, the range of the amplifier is selected by the AGC by considering the size of the current.The whole control system is based on the core of32-bits ARM microcontroller named LPC2361. The control of programmable voltage source and AGC, the read and processing of the A/D convertor, the communication with the upper monitor, the button and alarming circuits are all controlled by the control system under the PC. The neural network sutructure is designed calculate the converting output of the D/A and AGC. Program is written with the C and assembly language. Some experiments are introduced to validate the effectiveness of the proposed method. Results show that the relative errors of the output voltage of the programmable souce are reduced. Consequently the errors of the detected current are also minished dramatically, and the precision of the current is improved.Although the weak current signal detection and processing system designed in this paper is mainly used for the current in STMs, many other micro signal detection such as electrochemistry current and the concentrations of trace gases can also be implemented with different sonsors. So conclusion can be drawn that the proposed system has great universal property and wide market prospective.