Design Of Multi-functional Transcranial Direct Current Stimulator Based On FPGA

Transcranial Direct Current Stimulation(tDCS),as a kind of non-invasive nerve stimulation technology,can be used to regulate the neural plasticity by stimulating the cerebral cortex with weak direct currents,and it can also improve the learning-memory ability.This technique is one of the research hotspots in neuroscience,neurological rehabilitation medicine and neurology fields.There are significant differences in the Lemple-Zie Complexity(LZC)between patients with cognitive disorder and the normal,so it can be an index for judge tDCS to record the EEG signals produced by the spontaneous electrical activity of brain cells.A multi-functional transcranial direct current stimulator based on FPGA is developed in our paper,it can not only give direct current stimulation,but also work to monitor and analysis EEG signals.The instrument employ FPGA chip EP2C8Q208C8 N as the control center,and program with VHDL language.Firstly,the wireless Bluetooth system receive related parameters of stimulus waveform and control command transmitting from the software which based on the labview platform on the PC.Secondly,these signals drive FPGA chip to generate digital waveforms with adjustable parameters,such as intensity,waveform type,frequency and duty cycle.Then the digital signals are converted into analog signals by A/D circuit.And finally,constant current is obtained to stimulate the target human body through the voltage controlled current source.Besides,Current value flowing through the workloads can be acquired according to a reference resistance,and transmit to PC by the wireless Bluetooth system.Thus,the user can observe the output current,and regulate the output current to achieve closed-loop control,as well.Moreover,the EEG's module in our system can be used to collect EEG signals in real-time,and send the EEG signals to PC through wireless transmission,and do some complex analysis and data storage in the PC.This system can stably output the waveform with specific amplitude,frequency and duty cycle according to the parameters and instructions that is advanced set,and analysis the collected EEG signals.For the stimulator module,the amplitude of stimulating current is precisely less than 2%,and sine wave at the frequency of 0.01~0.1Hz,0.1~1Hz and 1~250Hz can reach a step value of 0.01 Hz,0.1Hz and 1Hz respectively.A stimulate current with an output of 2 mA can gain a maximum load of 12 K?.The stimulating current is controlled by instructions like start,suspend and cancel.For the EEG signal acquisition module,8 Channel EEG acquisition and analysis is available.Common mode rejection ratio is up to 106 dB,and the system noise is less than 3 ?V.the amplitude changes of ?,?,? and ? wave can be detected.The result shows a different LZC in the advantages of semi brain regions of every subject.Different channels of raw and processed EEG data can be saves in TXT files by the option setting,and the document data playback and analysis are also realized.