The Study Of Measurement System For Visual Evoked Potentials

Visual Evoked Potentials (VEPs) are produced by electrical activity of the visual cortex in response to light or pattern stimulation of the retina. It can detect functional loss in the visual pathway from retina to the visual cortex. VEP examination is a nondestructive, objective, qualitative, quantitative and repeatable measurement; it is the influential role to the visual disease diagnosis and distinction. For study of VEP, it is vital significance to research and develop VEP measurement system.Vision evoked potentials are low frequency and weak signal, measurement for VEP are subject to strong background noise and interference. Amplification is the most critical step in measuring and analyzing VEP. This paper has presented the principle and characteristic of VEP, analyzed the equivalent circuit model of VEP with picked up by bioelectrode, researched the internal noise and external interference to influence on the outcome of VEP, discussed the influencing factor for high impedance weak signal measurement, analyzed the device's value deviation from normal to influence the circuit performance, designed a low noise and high CMRR amplification circuit with guarding, driven right led circuit, isolation etc, simulated the circuit system using ORCAD/PSpice simulation software, designed the data acquisition and control system with microcontroller LPC2148, and designed the printed circuit board of measurement system. Finally, performed the experiment by pattern reversal stimulation, and recorded the results of pattern reversal VEP.The experimental results show that the VEP measurement system performance is that gain is 10 000~200 000, bandwidth is 1~300Hz, short-circuits equivalent input noise voltage peak-peak value is 2~4μV, CMRR is 106.7dB. It is undergoes 64 averaging be able to succeed recording VEP.