A unique EEG amplifier detection circuit design

1 Introduction

Electroencephalogram (EEG] by brain activities and is always present in the central nervous system of spontaneous potential, rich in brain activity information, brain research, Physiology research, clinical brain disease diagnosis of an important means.

By EEG recording to provide clinical data and diagnostic purposes. Therefore the extraction of the EEG has very important clinical significance.

2 design often encounter technical difficulties

(1) is very weak, the EEG is generally only around 50 μ V, amplitude range 5 μ μ V-100 V.

Therefore it requires amplification than ordinary instrument much higher;

(2) of the EEG frequency is low, its scope is generally 0.5 Hz ~ 35Hz that make amplification of low-frequency cut-off selection very difficult, when are the peak pulse interference or lead switch, prone to clogging the amplifier;

(3) the existence of power frequency 50 Hz and polarization voltage, and other powerful background interference.

Which frequency common mode interference mainly 50Hz, amplitude is large, so the EEG amplifier must have a high common-mode rejection ratio. While the existence of polarization voltage interference makes EEG amplifier gain not before level too high;

(4) because of the human body is a high resistance signal source, the internal resistance of up to tens thousand euro and even a few hundred thousand euro, but it's not only easy to change in impedance, and possible imbalance in each branch, EEG amplifier input impedance must be more than a few megabytes of Europe.

Visible, to design a high-quality EEG amplifier, preamplifier must have input impedance is high, and high common-mode rejection ratio (CMBR), low noise, nonlinearity of small, strong anti-interference ability, as well as the appropriate band and dynamic range, which makes the performance such as RF amplifiers in the design of the major difficulties, but this is the entire EEG signal acquisition system designed for the success of the most important key aspect.

3 signal amplification detection circuit design

EEG amplifier detection circuit shown in Figure 1.

From this figure, the main leading from the buffer level, differential amplifier, 50 Hz frequency, wave circuit, voltage amplifier, low-pass filter, level control circuitry, linear optical coupling circuits, etc.

　　

In the human body and EEG pre-amplifier between buffering level mainly in order to achieve higher input impedance, level adjustment circuit was designed to meet A/D converter input range of needs.

3.1 front differential amplification circuit

3.1.1 circuit composition and characteristics

Preamplifier is the key link of EEG detector.

This design in "three amplifier", through the use of the new circuit structure, the ingenious use of the instrumentation amplifier gain and common-mode rejection ratio (see table 1), and combined with RC coupling circuit, common mode driving technology, floating ground track circuits, etc., can inhibit DC interference under high common-mode rejection ratio, have external passive components parameters not sensitive features. Specific circuit design is shown in Figure 2.

　　

　　

UlA, UlB constitute parallel dual operational amplifier instrument amplifier, the operational amplifier is the ideal case, parallel differential amplifier input impedance is infinite, cmrr also is infinite, and the cmrr and peripheral resistance matches.

The part of the circuit have increased input impedance and provide voltage buffer.

RC-coupled circuit by C1, C2, R6, R7, etc., mainly from the isolation effect of polarization DC signals, as after the class instrument amplifier gain, which in turn increase for circuit cmrr.

LT1167 constitute after the class amplifier, dual-and differential output signal is converted to a common single-ended output signal.

This tier can achieve very high gain (by changing the size of the Rg), resulting in a high common-mode rejection ratio.

Common-mode signal sampling driver circuit consists of two equal resistor R4, R5, and voltage by UlC constitute follow, etc.

Due to the composition of the U1A and U1B parallel dual operational amplifier instrumentation amplifier with very low output impedance, through the use of common-mode driver technology, you can avoid resistor coupling circuit resistance, capacitance component parameters asymmetry leads to common mode interference into differential mode interference.

Human and common-mode feedback circuit (right leg drive circuit).

By common-mode sampling driver circuit of two electrodes after U2A common-mode voltage (voltage follower) and UlD (inverting amplifier) feedback to the head, with the original common-mode voltage together, forming a common-mode voltage negative feedback circuit, reduces the value of the input common-mode voltage, thereby increasing the circuit resistance capacity of power-frequency interference.

By common-mode sampling driver circuit of common-mode voltage between two electrodes are connected to two input R13 electrode shielding layer.

It can reduce lead distribution effect of shunt capacitance to amplifier input impedance to minimize the impact, making the CMRR is not reduced.

Floating ground track circuits (also called floating along) by U2B, R17, R17 termination front-end positive and negative power supply on the public side, so that power floating.

If you have the ideal characteristics U2B, make positive and negative power supply voltage fluctuation range and size of common-mode input voltage. Although the common mode input voltage remains at the U1A, U1B phase ends, but because the amplifier itself the power to the common mode input signal trace function to influence greatly weakened. Even if the parameters of the UlB U1A, not completely symmetrical, but effective common-mode voltage reduced into differential of error voltage seldom, equivalent to improve lead-common mode rejection.

In Figure 2, the input signal is taken from the UlC

U1A (input to output as Vin1, V01) and U1B (input to output as Vin2, V02) outputs two series resistor R4 and R5 's midpoint voltage Vc, Vc = l/2x (V01 + V02), when only differential-mode signal (V01 =-V02) output, Vc = 0, the amplifier output voltage equal to UlC O, equivalent to the Earth; and when there is a common-mode voltage and differential-mode signal input, the total output U1C contains only common mode input signal part Vc = l/2x (Vin1 + Vin2). Making common mode signal without RC coupling circuit pressure directly on the integrated instrument amplifier input of U3, avoids due to capacitance coupled circuit does not match the overall reduction circuit cmrr.

Figure 2 shows the circuit of differential output can be determined by the calculation of (1):

　　

Which is integrated amp Adu3 LT1167 magnification of differential mode and Adu3 = (49.9 k Ω/R g) + 1

The circuit of high pass cutoff frequency fc can be expressed as:

　　

Where: F1, F2, ...

As the noise amplifiers, K1, K2 ... For all levels of amplifier power gain. Visible for multistage Amplifier noise coefficient will mainly by the decision of the first level of noise. Preamplifier total equivalent input noise voltage and closed-loop gain inversely, appropriate to increase the level of magnification benefit chamo noise reduction. So in the design selection with very low noise of the integrated amplifier LMH6626, after the class amplifier selection LT1167.

3.1.2 devices introduction

LMH6626 is a United States national launched a dual very low noise broadband amplifier.

Its main features are as follows:

Extremely low noise, low input voltage noise to 0.92 nV/(Hz), 1/2 (typical values), enter the current noise typical 1.8 pA/(Hz), 1/2;

Gain bandwidth 1.3 GHz;

Power supply voltage ± 6 V, input offset voltage of soil o.1mV, temperature drift than 0 μ ± V/° c, input offset current typical value is 0.05 A μ, temperature drift 0.7 nA/℃;

Open-loop gain is 81 dB (typical), common-mode rejection ratio up to 95 dB CMRR, and slew rate (SR) 340 V/μ s;

LTll67 is Linear company new instrumentation amplifier that combines FET amplifier and bipolar op amp advantages, its high input impedance, low bias current near FET devices, and the noise level is the same as with bipolar op amp.

Specific performance indicators are as follows:

Laser-trimmed resistors guarantees the common mode rejection ratio CMRR > 110dB (G = 10);

High input impedance l 000 G Ω, parallel 1 pF; noise voltage V 0.28 μ p-10 p, noise current pAp-p (0.1 Hz to 10 Hz);

Input bias current quiescent current 50pA; < 13 mA.

LTll67 comparison applies to medical devices.

The CMRR and closed-loop gain in differential mode are shown in table 1.

3.2 linear optical coupling circuits

Isolation amplifier with excellent resistance to common mode interference ability, effectively blocking the preamplifier and back-level data collection circuit between the electrical contact, but does not block the signal passed between them.

Therefore in EEG acquisition system uses the isolation is necessary, are also possible. The function of General from transformers and optical coupling components is complete, the transformer used for coupling AC signal and optical Coupler is used for coupling DC signal.

Previous optical isolation products generally do not have online sex circuit, but rather will be LED and photodiode of optocoupler application in digital signal isolation, due mainly to the period of optocoupler certain nonlinear and instability so that such devices do not quite fit this very weak EEG physiological signal of high precision measurement.

And the transformer isolation is a large, complex wiring, etc.

LOCllO linear optical Coupler is a United States company's new CLARE optical Coupler, and General optical coupling components, the design mode of the servo LOC110, to compensate for the light-emitting diodes for nonlinear time and temperature, both AC and DC signal coupling.

LOCllO to replace the EEG signal acquisition application mass transformer and nonlinear optical coupler provides another better choices.

LOCll0 light voltage and photoconductivity of the two working modes.

Optical voltage mode using LOCllO optical Coupler can achieve the best linearity, a minimum of interference and drift performance. In this mode, you can reach the circuit linearity 12-bit, although this is a smaller 40 kHz bandwidth costs, but does not affect the application of the design. This design, work in optical LOC110 voltage mode, its principle is shown in Figure 3.

　　

Figure 3 definition: servo gain IF K1 = I1/I2/, K2 = IF, K3 = K1 K2/, IF the light-emitting diode current of clockwise.

The Vin = I1.R1 = IF.K1.R1, Vout = R2 = I2 · IF · K2 · R2, Vout = Vin · (K2.R2)/(K1.R1) = Vin · K3 · R2/Rl, generally take Kl = K2, Vout = Vin.R2/R1, R2/R and Vout1.

3.3 5O Hz frequency notch circuit

Frequency interference is the main interference EEG, although preamplifier for common mode interference with strong inhibitory effect, but some frequency interference is the differential mode signal access circuits and frequency in the EEG frequency bands, coupled with the electrode and input circuit instability and other factors, the front-stage circuit output of EEG signal is exist strong power frequency interference, it must be designed to filter out.

Specific design shown in Figure 4, simulation curve and the experimental data curve is shown in Figure 5 (a), (b). Figure 4 LT1112 is Linear in the company's production of dual low power, high precision, Appian input operational amplifier.

　　

　　

3.4 voltage amplifier

Because of the low frequency EEG signals, so the circuit adopts AC bootstrap techniques, also in low frequency high input impedance, and thus have strong ability of AC-coupling.

Bootstrapping to fully play its role, must be in Figure 4 R1 bottom provides special voltage near its top. In this way, the resistance to flow of current on the R1 is very small and very large. impedance Otherwise, they play not from lifting effect. The input impedance of ZIN = Xcl + Rl + R2 + R2/R1 · KC2 ≈ R1.R2/XC2. According to figure 4 circuit parameters can be found: ZIN (1 Hz) = 188.46 k Ω, ZIN (10 Hz) = 2 M Ω, to further improve the input impedance, you must increase the Rl, R2, C2. Specific design shown in Figure 6.

　　

3.5 low-pass filter design

Low-pass filter with broad-spectrum noise suppression and ADC before anti-aliasing of the dual mandate, both low-pass filter passband as flat, rolled down rate the sooner the better, in order to obtain a high signal to noise ratio.

According to the signal characteristics, selection of Butterworth-order filter, 100 Hz cutoff frequency selected. Specific design shown in Figure 7, simulation curve and the experimental data curve is shown in Figure 8 (a), (b).

　　

4 closing

This topic is part of a research project of Guangxi Zhuang Autonomous Region Department of education: the high-performance digital EEG Diagnostics and monitoring machine (Guangxi branch research [2004], no. 20).

Because of the weak and EEG signal is exist strong interference, making it is difficult to measure, passed on the circuit structure of careful design and selection of new devices, you can overcome the EEG signal extraction often encountered some difficulties, the preamplifier with high common-mode rejection ratio, enabling better zoom the EEG detected. In the design process, through PspICe simulation software helps circuit design and debug. If combined with data acquisition and USB interfaces, you can realize the EEG in the PC's display, analysis and other functions to enable real-time monitoring of EEG, with traditional EEG devices for better performance.

EEG is a relatively weak human physiological signals, through appropriate improvement, the circuit can also be used to measure other human physiological signals or other weak signal.