CN107157478B - Multichannel cortex electroencephalogram acquisition system based on capacitive electrodes - Google Patents

Multichannel cortex electroencephalogram acquisition system based on capacitive electrodes Download PDF

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CN107157478B
CN107157478B CN201710402635.8A CN201710402635A CN107157478B CN 107157478 B CN107157478 B CN 107157478B CN 201710402635 A CN201710402635 A CN 201710402635A CN 107157478 B CN107157478 B CN 107157478B
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capacitive
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voltage follower
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高敏
黄龙
陈昌勇
张优劲
颜卓程
林媛
潘泰松
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University of Electronic Science and Technology of China
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Abstract

The invention belongs to the field of biomedical engineering, relates to amplification and collection of cortical electroencephalogram signals, and provides a multi-channel cortical electroencephalogram collection system based on capacitive electrodes, which is used for realizing ECoG signal collection; the capacitive microelectrode array comprises a capacitive array microelectrode, a preposed impedance conversion module, an analog amplification circuit module, an AD conversion module, a main control chip, a computer upper computer and a mobile power supply; the capacitive array microelectrode is coupled with the cortical electroencephalogram signal, the signal is amplified and processed by the analog amplification circuit module after impedance conversion is carried out on the capacitive array microelectrode by the preposed impedance conversion module, the signal is transmitted to the main control chip after the signal is converted by the AD conversion module, and the data packet formed by the main control chip is transmitted to the computer upper computer for display and analysis by the serial port. The capacitive array microelectrode is used for acquiring the ECoG signal, has the characteristics of small stimulation and high spatial resolution, can realize multichannel parallel high-precision acquisition of the ECoG signal, filters high-frequency signals, and completely extracts cortical electroencephalogram.

Description

Multichannel cortex electroencephalogram acquisition system based on capacitive electrodes
Technical Field
The invention belongs to the field of biomedical engineering, relates to amplification and collection of cortical electroencephalogram signals, and particularly relates to a multi-channel cortical electroencephalogram collection system based on capacitive electrodes, which is applied to cortical electroencephalogram signal collection in the aspect of neuroelectrophysiology.
Background
With the promotion of brain plans in China, the research footsteps of people on the brain become faster and faster; with the research of brain science, modern medicine for treating brain diseases is rapidly developed.
In recent years, people start to detect and research bioelectricity activity generated by thinking activity of human brain, and an ECoG acquisition mode belongs to 'implantation' and needs to carry out craniotomy on a tested object and implant electrodes into cerebral cortex in the skull. ECoG has higher spatial resolution and amplitude, and wider bandwidth than EEG; in the research of brain-computer interfaces, ECoG is also increasingly used. At present, many research groups have published the research results of the ECoG brain-computer interface based on the motor function, the language function and the visual system at home and abroad; the cortical electroencephalogram signal is widely applied to the research and application fields of epilepsy, brain-computer interfaces and the like due to the unique advantages. In order to ensure the accuracy of the research, an acquisition system which can be used for the brain function research needs to be designed according to the characteristics of the animal ECoG signals. In the study of cortical electroencephalogram signals, animals such as rats are often selected for experiments because craniotomy is performed on the subject.
Disclosure of Invention
The invention aims to provide a multi-channel cortical electroencephalogram acquisition system based on capacitive electrodes, which is used for realizing ECoG signal acquisition; the technical scheme adopted by the invention is as follows:
a multi-channel cortical electroencephalogram acquisition system based on capacitive electrodes comprises capacitive array microelectrodes, a preposed impedance conversion module, an analog amplification circuit module, an AD conversion module, a main control chip, a computer upper computer and a mobile power supply; the capacitive array microelectrode is coupled with the cortical electroencephalogram signal, the signal is amplified and processed by the analog amplification circuit module after impedance conversion is carried out on the capacitive array microelectrode by the preposed impedance conversion module, and then the signal is transmitted to the main control chip after being converted by the AD conversion module, and the data packet formed by the main control chip is transmitted to the computer upper computer through the serial port for display and analysis; the mobile power supply is used for supplying power to the system.
Furthermore, the capacitive array microelectrode is composed of an insulating medium layer, a metal electrode layer and an insulating protection layer which are sequentially overlapped from bottom to top, the metal electrode layer comprises a plurality of horizontally arranged electrode points, and the insulating medium layer and each electrode point form a parallel plate capacitor.
Furthermore, the preposed impedance conversion module is matched with the capacitive array microelectrode and comprises a voltage follower and a bias resistor (R)b) The positive phase end of the voltage follower is connected with the capacitive array microelectrode, one end of the bias resistor is connected with the positive phase end of the voltage follower, the other end of the bias resistor is grounded, the amplifier and the feedback capacitor form a feedback loop, and the output of the voltage followerThe voltage follower is connected with the analog amplifying circuit module through the amplifier and the feedback capacitor to the positive phase end of the voltage follower in sequence.
Further, the amplification factor of the amplifier is: 1+ alpha, and the capacitance value of the feedback capacitor is as follows: and Cc is Ci/alpha and Ci is the input capacitance of the voltage follower.
In the invention, as shown in FIG. 3, the pre-impedance conversion module adapted to the capacitive array microelectrode is mainly composed of a voltage follower with unit gain; however, because the capacitive impedance of the capacitive array microelectrode is very large, in order to realize matching, it is particularly important to increase the input impedance of the back-end circuit to avoid attenuation of the acquired signals; the invention connects a large-resistance grounding bias resistor (R) to the capacitive array microelectrodeb) Providing a loop through which a bias current flows; meanwhile, the equivalent capacitance (Cs) of the capacitive array microelectrode, the input capacitance (Ci) of an operational amplifier in the voltage follower and a grounding bias resistor (R)b) Together forming a passive high-pass filter; thereby realizing the adaptation of the preposed impedance conversion module and the capacitive array microelectrode.
The invention has the beneficial effects that: the multichannel cortical electroencephalogram acquisition system based on the capacitive electrodes has the advantages that the capacitive array microelectrode is adopted to acquire ECoG signals, the system has the characteristics of small stimulation and high spatial resolution, and meanwhile, multichannel parallel high-precision acquisition of the ECoG signals can be realized through the matched pre-impedance conversion module, the analog amplification circuit module, the AD conversion module, the main control chip, the computer upper computer and the mobile power supply, high-frequency signals are filtered, and the cortical electroencephalogram is extracted completely.
Drawings
FIG. 1 is a schematic diagram of the principle of a multi-channel cortical electroencephalogram acquisition system based on capacitive electrodes provided by the invention.
FIG. 2 is a schematic diagram showing a capacitive array microelectrode according to an embodiment of the present invention.
Fig. 3 is an equivalent model diagram of the pre-impedance transformation module in the embodiment of the present invention.
Detailed Description
The embodiments of the present invention will be described in detail below with reference to the accompanying drawings: the present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.
The embodiment provides a multichannel cortex electroencephalogram acquisition system based on capacitive electrodes, the principle of which is shown in fig. 1, and the system comprises: the capacitive array microelectrode comprises a capacitive array microelectrode, a preposed impedance conversion module which is adapted to the capacitive electrode, an analog amplification circuit, an analog-to-digital conversion module, a microprocessor, an upper computer and a mobile power supply.
The capacitive microelectrode array is used for implanting electrodes into the cerebral cortex of a rat through craniotomy, and comprises an insulating dielectric layer 3, a metal electrode layer 2 and an insulating protective layer 1 which are sequentially overlapped, wherein the insulating protective layer is used for preventing brain tissues from directly contacting the metal electrodes, 9 electrode points in the metal electrode layer are connected to a rear-end circuit interface through wiring, 7 of the electrode points are used as movable electrodes, 1 electrode point is used as a reference electrode and 1 right leg driving electrode, and each electrode point in the insulating dielectric layer and the metal electrode layer forms a parallel plate capacitor; the cortical electroencephalogram signal is coupled to the metal electrode layer through the insulating medium layer, the error of the equivalent capacitance value of each electrode is small, and the common-mode rejection ratio of the system is further improved. Because the voltage signal is coupled through the capacitor, the signal is transmitted without direct contact in a strict sense, the leakage current is avoided, the rat is safe enough, and the anaphylactic reaction caused by the implantation of the electrode is avoided; and transmitting the voltage signal to a post-stage pre-impedance conversion module through the capacitive coupling cortical potential.
The pre-impedance conversion module is mainly used for impedance matching through the unit gain voltage follower, so that the input impedance is increased, and signal attenuation is avoided; the equivalent circuit diagram is shown in FIG. 3, where Rb is the bias resistor and CSRi and Ci are the input impedance and input capacitance of the voltage follower amplifier, which is the electrode equivalent capacitance; when only ac signals are considered:
Figure BDA0001310089870000031
when the gains are comprehensively considered:
Figure BDA0001310089870000032
therefore, the fact that the coupled electroencephalogram signals are attenuated by the overhigh input capacitor of the voltage follower can be obtained, and therefore when an amplifier of the voltage follower is selected, low noise, low input capacitance and high input impedance become more important;
in addition, the output Vout of the voltage follower is fed back to the non-inverting input end of the voltage follower through a feedback loop; the feedback loop passes through an amplifier with the amplification factor of 1+ alpha, then a feedback capacitor with the capacitance value of Cc ═ Ci/alpha is accessed, the relational expression is satisfied, so that the current flowing through the feedback capacitor Cc is exactly equal to the current flowing through the equivalent input capacitor Ci, and the attenuation of the input capacitor Ci to the coupling electroencephalogram signal is reduced by the method;
at the same time, the electrode equivalent capacitance CSAmplifier input capacitance and bias resistor RbForming a passive high-pass filter; the high-pass cut-off frequency is:
Figure BDA0001310089870000033
the analog amplification circuit corresponds to the capacitive array microelectrode, and comprises 7 independent analog amplification modules for corresponding to 7 channels; the digital-to-analog conversion module correspondingly adopts a 24-bit high-precision AD; the analog amplification module is divided into two stages, an instrument operational amplifier AD8221 is adopted at the front stage, high-pass filtering is simulated by an integral feedback circuit, a second-order voltage-controlled active low-pass filtering circuit is adopted at the rear stage, amplified single-ended signals are converted into differential signals through THS4521 and input to the analog-digital conversion module, and an ADS1278 cycle is adopted by the analog-digital conversion chip to process analog signals for each channel and output 24-bit high-precision digital signals.
The microprocessor adopts an STM32F103 series 32-bit ARM microcontroller, controls data to be transmitted to a host computer for digital filtering processing, and filters power frequency interference and harmonic thereof; and then the data is transmitted to an upper computer through Bluetooth or USB for displaying and analyzing.
While the invention has been described with reference to specific embodiments, any feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise; all of the disclosed features, or all of the method or process steps, may be combined in any combination, except mutually exclusive features and/or steps.

Claims (1)

1. A multi-channel cortical electroencephalogram acquisition system based on capacitive electrodes comprises capacitive array microelectrodes, a preposed impedance conversion module, an analog amplification circuit module, an AD conversion module, a main control chip, a computer upper computer and a mobile power supply; the capacitive array microelectrode is coupled with the cortical electroencephalogram signal, the signal is amplified and processed by the analog amplification circuit module after impedance conversion is carried out on the capacitive array microelectrode by the preposed impedance conversion module, and then the signal is transmitted to the main control chip after being converted by the AD conversion module, and the data packet formed by the main control chip is transmitted to the computer upper computer through the serial port for display and analysis; the mobile power supply is used for supplying power to the system; the front impedance conversion module is matched with the capacitive array microelectrode and comprises a voltage follower and a bias resistor (R)b) The positive phase end of the voltage follower is connected with the capacitive array microelectrode, one end of the bias resistor is connected with the positive phase end of the voltage follower, the other end of the bias resistor is grounded, the amplifier and the feedback capacitor form a feedback loop, the output of the voltage follower sequentially passes through the amplifier and the feedback capacitor to the positive phase end of the voltage follower, and the output end of the voltage follower is connected with the analog amplification circuit module; the capacitive array microelectrode is composed of an insulating medium layer, a metal electrode layer and an insulating protection layer which are sequentially overlapped from bottom to top, wherein the metal electrode layer comprises a plurality of horizontally arranged electrode points, and the insulating medium layer and each electrode point form a parallel plate capacitor; the amplification factor of the amplifier is as follows: 1+ alpha, and the capacitance value of the feedback capacitor is as follows: and Cc is Ci/alpha and Ci is the input capacitance of the voltage follower.
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