A Low Noise Amplifier And∑-△Modulator For Implantable Device For Neural Signal Acquisition

With the development of Biomedical Science, exploration of biological neural signals appears to be urgent and in-depth. Bio-signal interface emerges as the time requires. They are inserted into bodies through electrodes, which contacts with neurons, extracting neural signals. Neural signals are divided into two parts, one part is local field potentials (LFP), forming electrocorticogram(ECoG); the other is part of the action potentials(spike), forming electroencephalogram (EEG) signals on the scalp. When biological action occurs, potentials of neurons will change, generating action potentials, which is the main object of study. Neural signal transmission to the electrodes will be extracted, amplified and processed into digital code, delivering through wireless transmitter and transceiver to the computer terminals or DSPs and so on. The microchip modules include:filter, ADC, digital module, RF transmitter and RF receiver. This paper mainly focuses on the front end analog design, namely the filter and sigma-delta modulator, which is a veryimportant part to determine the performance of the Σ-△ADC.This Filter is two-order active filter, containing low noise amplifier inside. Creative point of the work is using MOSFETs working in the sub-threshold region instead of high resistance in the filter network. This, on one hand, turns impossible to achieve ultra low cut-off frequency to possible, saving the chip area, on the other hand, avoids generating the thermal noise, which is the most appropriate application of this work.Sigma-delta modulator part requires high precision, so as to restore the biological neural signal which is very weak (in general, a few tens of microvolts to two hundred microvolts). Design index of the ADC is16bit, which can better accomplish this task.The chip implanted in SMIC0.18μm1P6M CMOS process, with1.8V power supply. According to the test, the sigma delta modulator’s SNDR reached76.04dB, and the effective number of bits is12.34bit. The whole front end of analog part’s SNDR achieved74.72dB, and the effective number of bits is12.12bit, proving the pre amplifier to do is good, which has a strong drive capability, closed to the ideal signal source.