Research On Detection Of Weak Direct Current

This paper's purpose is researching on the methods of detecting weak direct current (level of pA) and realizing detection with high precision. Weak current belongs to the "Weak Signals" category. Weak signals normally mean their amplitude are very weak or faint, they more exactly mean the signals which are submerged in various noises. So the weak signals cannot be detected just by using amplification. It's necessary to reject noise and improve SNR with the purpose of realizing the pick-up of useful signals. Accordingly, the detection theoretics, methods and equipments as well as the noises' sources, characteristics, causation and transmitting laws have to be researched. On the basis of this, the noises can be rejected effectively by using some pertinent measures. It can be used extensively in military scout, physics, chemistry, electrochemistry, biomedicine, astronomy, physiography, magnetics, etc. The detection technology of weak signals is important and even necessary to develop new and high technologies, to explore and discover the orders of nature, etc, so it will be used in more areas with the development of science and technology.On the basis of consulting lots of relative reference books and papers, this paper researched the noise sorts, characteristics, transmitting mechanism, and the noise models of some electronic parts firstly, and then suggested a list of design principles such as low noise, high accuracy, long life, low cost and simple circuit. According to these principles, this paper showed the detection model and the circuit for detecting weak direct current (level of pA) after computing parameters and choosing parts. Also, the work of PCB EMC design, depurating, torrefying, assembling parts, reducing leakage current, dustproof and dampproof design, shielding and grounding designed, etc are described detailedly in this paper.The experimental results indicate that the measurement precision is±0.005 pA(±5 fA), the peak excursion in one hour is not more than 0.0065 pA(6.5 fA) when the input current is 1 pA or 2 pA, when the input current is 3 pA, the peak excursion is only 0.002 pA(2 fA).