Implement monitoring and control in industrial production process need to use all kinds of automation instrument and control system and the executing agency, weak signal transmission between them both to the small signal of millivolts, ma; Have a few volts, kv, hundreds of ampere of large signal; Both dc signal, the low frequency and high frequency pulse signal, and so on, constitute the system often found transmission interference between the instrument and equipment, cause the system unstable or even wrong operation, appear this kind of circumstance in addition to the performance of each instrument, the equipment itself causes such as resistance to electromagnetic interference effects, there is also a very important reason is that all kinds of instruments and equipment according to the requirement and purpose needs to be grounded, for safety, for example, chassis need to connect the earth; In order to make the circuit work properly, the system needs to have a common reference point; In order to suppress interference and shielding cover, shield and ground, but because of the instrument and equipment between the potential difference between reference point ( Is the equipment of different location) Forming 'circulation' and 'grounding circulation' the problem is in the process of system processes the signal must solve the problem. The role of the thermocouple isolator natural lightning interference is a major source of natural disturbance, thunder and lightning interference can be transmitted to tens of thousands of kilometers. The time domain waveform of lightning interference in the background of a group of random pulse big spike. The universe is ionizing radiation noise, changes in one day. Noise and solar activity of dramatic change. Nature may be noise mainly for communication, and pointed out that bees pulse can destroy many devices, lightning energy should avoid or reduce the extent of the damage, reduce the loss. Human interference is the root cause of the change of the electromagnetic interference voltage or current in the conductor, the greater the dv/dt and di/dt and dv/dt and di/dt can make the conductor produces electromagnetic radiation. On the one hand, people can use this feature to implement specific functions, for example, wireless communication, radar, and other functions, on the other hand, the electronic devices in the work, because the wire of the dv/dt and di/dt will produce electromagnetic radiation. No matter from subjective and objective for purposes of electromagnetic environment caused by pollution. In the process of enterprise production and factories tend to have some large equipment ( Motor, frequency converter) Switch is frequent, they can also cause some capacitance, sensibility, will also affect the normal display or acquisition instrument. Voltage current mutation, electromagnetic interference will exist. Digital pulse circuit is a typical interference source, with the wide application of electronic technology, electromagnetic pollution will be more and more serious.