Abstract:
An embodiment of a baseline drift canceling method is provided. The method removes a baseline drift signal from a bioelectric signal, and includes the steps as follows: delaying the bioelectric signal by an analog time delay circuit to generate a first bioelectric signal; according to the bioelectric signal to generate a baseline drift signal; and acquiring a second bioelectric signal according to the first bioelectric signal and the baseline drift signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application claims priority of Taiwan Patent Application No. 97151767, filed on Dec. 31, 2008, the entirety of which is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a baseline drift canceling method to remove a baseline drift signal in a bioelectric signal. 
         [0004]    2. Description of the Related Art 
         [0005]    When measuring the bioelectric signal of a human body, two sensor patches are usually applied for sensation of the human body. Take the electrocardiograph machine for example, the electrocardiograph machine senses the bioelectric signal from sensor patches on the chest or hands of the human body. However, the sensed bioelectric signal is easily affected by the breathing, movement, skin resistance or sweat on the skin of the human body. This may cause the baseline to drift or wander and when the baseline drift is too large, a resulting output signal may be saturated and the bioelectric signal would be inaccurate. 
         [0006]    One conventional method to solve the issue of a baseline drift is to directly filter out the baseline drift signal, such as that disclosed in U.S. Pat. No. 5,318,036. The method disclosed by U.S. Pat. No. 5,318,036 filters an output from a front-end filter by a digital high pass filter in a microprocessor to remove the baseline drift. Another conventional method estimates a baseline drift signal by a microprocessor and subtracts the estimated baseline drift signal from the bioelectric signal. The baseline estimation method is disclosed in U.S. Pat. No. 6,280,391 and U.S. Pub. No. 20070078354. Another baseline estimation method is disclosed in U.S. Pat. No. 6,881,191 to estimate the baseline drift signal by curve fitting. However, the described baseline drift canceling methods require complicated circuitry or increased hardware and/or software, thus, delaying results. Additionally, in certain cases, the baseline drift tolerance (the voltage range of the baseline drift/the amplitude of vibration of signal) for the described baseline drift canceling methods are limited. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    To solve the described issue, one embodiment of the invention provides a baseline drift canceling method. 
         [0008]    An embodiment of a baseline drift canceling method is provided. The method removes a baseline drift signal in a bioelectric signal, and comprises the steps as follows: delaying the bioelectric signal by an analog time delay circuit to generate a first bioelectric signal; according to the bioelectric signal to generate a baseline drift signal; and acquiring a second bioelectric signal according to the first bioelectric signal and the baseline drift signal. 
         [0009]    An embodiment of a baseline drift canceling device is provided to remove a baseline drift signal in a bioelectric signal. The baseline drift canceling device comprises a first amplifier, a baseline drift canceling unit and a digital amplifier. The first amplifier receives and amplifies a sensing signal to generate the bioelectric signal. The baseline drift canceling unit comprises a low pass filter, an analog time delay circuit and an adder. The low pass filter receives the bioelectric signal to generate the baseline drift signal. The analog time delay circuit delays the bioelectric signal to generate a first bioelectric signal. The adder receives the baseline drift signal and the first bioelectric signal to generate a second bioelectric signal. The digital amplifier receives and amplifies the second bioelectric signal to generate a digital bioelectric signal. 
         [0010]    Another embodiment of a bioelectric signal sensing apparatus is provided. The apparatus comprises a first sensor, a second sensor, a first amplifier and at least one baseline drift canceling unit. The first sensor outputs a first sensing signal. The second sensor outputs a second sensing signal. The first amplifier receives the first sensing signal and the second sensing signal to generate a bioelectric signal. The baseline drift canceling unit comprises a low pass filter, an analog time delay circuit and an adder. The low pass filter receives the bioelectric signal to generate the baseline drift signal. The analog time delay circuit delays the bioelectric signal to generate a first bioelectric signal. The adder receives the baseline drift signal and the first bioelectric signal to generate a second bioelectric signal. 
         [0011]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a schematic diagram of an embodiment of a baseline drift canceling device for a bioelectric signal according to the invention. 
           [0014]      FIG. 2  is a schematic diagram of another embodiment of a baseline drift canceling device for a bioelectric signal according to the invention. 
           [0015]      FIG. 3  is a schematic diagram of an embodiment of a baseline drift canceling unit according to the invention. 
           [0016]      FIG. 4  is a schematic diagram of an embodiment of an analog time delay circuit according to the invention 
           [0017]      FIG. 5  is a schematic diagram of another embodiment of a baseline drift canceling unit according to the invention. 
           [0018]      FIG. 6  is a schematic diagram of another embodiment of two cascaded baseline drift canceling units according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0020]      FIG. 1  is a schematic diagram of an embodiment of a baseline drift canceling device for a bioelectric signal according to the invention. The baseline drift canceling device  10  comprises a first amplifier  11 , baseline drift canceling unit and digital amplifier  17 . The baseline drift canceling unit comprises a low pass filter (LPF)  13 , an adder  14  and an analog time delay circuit  15 . The digital amplifier  17  further comprises a second amplifier  16  and an analog to digital converter (ADC)  18 . The first amplifier  11  receives and amplifies a sensing signal to generate a bioelectric signal, and the bioelectric signal is transmitted to the baseline drift canceling unit, i.e., the bioelectric signal is transmitted to both the low pass filter  13  and analog time delay circuit  15 . The low pass filter  13  low-pass filters the received bioelectric signal to estimate a baseline drift (BLD) signal. 
         [0021]    The analog time delay circuit  15  delays the received bioelectric signal for a predetermined time to generate a first bioelectric signal, and the predetermined time varies according to the processing time of the low pass filter  13 . In other words, the output of the analog time delay circuit  15  synchronizes with the output of the low pass filter  13 . The adder  14  comprises a non-inverting input terminal (+) and an inverting input terminal (−) which respectively receives the first bioelectric signal from the analog time delay circuit  15  and the estimated baseline drift signal from low pass filter  13 . The adder  14  subtracts the estimated baseline drift signal from the first bioelectric signal to generate a second bioelectric signal. The digital amplifier  17  receives and amplifies the second bioelectric signal output by the adder  14  to generate a digital bioelectric signal, wherein the second amplifier  16  amplifies the second bioelectric signal and the analog to digital converter  18  converts the second bioelectric signal into the digital bioelectric signal. 
         [0022]    In this embodiment, the bioelectric signal is illustrated as an electrocardiography (ECG) signal, but the invention is not limited thereto. In another embodiment, the bioelectric signal may be an electroencephalogram (EEG) signal, an electromyogram (EMG) signal or an electric ocular graph (EOG) signal. 
         [0023]      FIG. 2  is a schematic diagram of another embodiment of a baseline drift canceling device for a bioelectric signal according to the invention. The embodiment is illustrated as an electrocardiography (ECG) signal, but the invention is not limited thereto. The first amplifier  21  receives and amplifies two sensing signals from the bioelectricity sensors E+ and E− to generate an ECG signal. The bioelectricity sensors E+ and E− directly contact with the body of a patient. Since a voltage difference between the body of the patient and a ground voltage level exists, an inverting amplifier  29  is used to eliminate the voltage difference. In other words, the inverting amplifier  29  is used to reduce the common mode voltage. In another embodiment, the inverting amplifier  29  is further coupled to a filter. 
         [0024]    The first filter  22  filters the ECG signal from the first amplifier  21 . Then, the filtered ECG signal is processed by the first baseline drift canceling unit  23 , the second baseline drift canceling unit  24   a  and the Nth baseline drift canceling unit  24   b  to remove the baseline drift from the ECG signal. In this embodiment, the number of baseline drift canceling units is not limit to the number shown in  FIG. 2 . Those skilled in the art can use more baseline drift canceling units for a smooth ECG signal according to the baseline drift in the ECG signal. The second amplifier  25  receives and amplifies the processed ECG signal from the Nth baseline drift canceling unit  24   b . The second filter  26  filters the ECG signal from the second amplifier  25 . In this embodiment, the first filter  22  is a high pass filter, such as a high pass filter with high common mode rejection ration. The second filter  26  is a low pass filter. In this embodiment, the gain of the first amplifier  21  is about 5-10, and the gain of the second amplifier  25  is about 100. 
         [0025]    The notch filter  27  receives a signal from the second filter  26  and eliminates the frequency interference therein, wherein the frequency interference is caused by 50 Hz or 60 Hz alternating current (AC) power. The smooth ECG signal output by the notch filter  27  is transmitted to the R wave detector  28  to generate a heartbeat pulse signal S R . In this embodiment, it is illustrated as only one notch filter, but it can use more notch filters to eliminate the frequency interference caused by the alternating current frequency. It is noted that the frequency interference which can be eliminated by the notch filter is not limit to the frequency interference of 50 Hz or 60 Hz. In this embodiment, a gain compensation circuit is coupled to the notch filter  27  to adjust the gain of the output of the second filter  26 . 
         [0026]      FIG. 3  is a schematic diagram of an embodiment of a baseline drift canceling unit according to the invention. The ECG signal is transmitted to a low pass filter  32  and an analog time delay circuit  31 . The low pass filter  32  filters the ECG signal to estimate a baseline drift (BLD) signal. 
         [0027]    The analog time delay circuit  31  delays the received ECG signal for a predetermined time, wherein the predetermined time varies according to the processing time of the low pass filter  32 . In other words, the output of the analog time delay circuit  31  synchronizes with the output of the low pass filter  32 . The adder  33  comprises a non-inverting input terminal (+) and an inverting input terminal (−) which respectively receives the output signal from the analog time delay circuit  31  and the output signal from low pass filter  32 . The adder  33  subtracts the estimated baseline drift signal from the output signal of the analog time delay circuit  31  to generate a bioelectric signal ECG′. In another embodiment, the adder  33  can be replaced by a subtractor. 
         [0028]      FIG. 4  is a schematic diagram of an embodiment of an analog time delay circuit according to the invention. The analog time delay circuit is made up of an operational amplifier  41 , resistors R, R g , R f  and a capacitor C. Resistors R g  and R f  determine the gain. Resistor R and capacitor C determines the delay time. The mathematical equation of the delay time can be expressed as follows: 
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         [0029]    When τ S is much smaller than 1, the equation can be approximated as the following: 
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         [0030]    Therefore, the delay time or phase lag derived from the inverting amplifier and the delay time of the low pass filter can be estimated and the resistance of resistor R and capacitance of capacitor C according to the described equation can be determined. Then, the corresponding analog time delay circuit can be implemented. 
         [0031]      FIG. 5  is a schematic diagram of another embodiment of a baseline drift canceling unit according to the invention. The detailed circuitry of a low pass filter  51 , an analog time delay circuit  52  and an analog adder  53  is shown in  FIG. 5 . In this embodiment, the 3 dB frequency of the low pass filter  51  is 1 Hz. In another embodiment, the 3 dB frequency of the low pass filter  51  can be between 0.5 Hz to 1 Hz by adjusting the capacitance of the capacitor C 3  and the resistance of resistor R 35 . In  FIG. 5 , a time constant corresponding to the 3 dB frequency of the low pass filter  51  is between 2 seconds and 1 second, a time constant of the analog time delay circuit  52  corresponding to the 3 dB frequency of the low pass filter  51  is between 90 milliseconds and 50 millisecond, and a compensation gain of the low pass filter  51  and the adder  53  corresponding to the 3 dB frequency of the low pass filter  51  is between 1.25 and 1.05. 
         [0032]      FIG. 6  is a schematic diagram of another embodiment of two cascaded baseline drift canceling units according to the invention. In this embodiment, the baseline drift canceling units  61   a  and  61   b  eliminate the baseline drift in the ECG signal. In this embodiment, the 3 dB frequency of the low pass filters in both baseline drift canceling units  61   a  and  61   b  is 0.67 Hz. 
         [0033]    According to the above description, the disclosed baseline drift canceling method removes the baseline drift signal during the front-end filtering process and accordingly the baseline drift tolerance can be also increased. In a conventional baseline drift canceling method, the bioelectric signal is amplified by a first amplifier and a digital amplifier during the front-end filtering operation, and the baseline drift signal is removed from the amplified bioelectric signal during the back-end operation. This may cause the baseline drift signal to be too large and exceed the output range, thus causing error. According to the disclosed baseline drift canceling method, the baseline drift canceling procedure is applied to the bioelectric signal after the bioelectric signal is amplified by a first amplifier, thus baseline drift tolerance is efficiently increased. 
         [0034]    Please refer to  FIG. 1 . Assuming the gain of the first amplifier  11  is 5, the gain of second amplifier  16  is 100, the circuit outputting the ECG signal is powered by a single one power source (5V), the operative voltage of the circuit is from 0.25V to 4.75V and the magnitude of the ECG signal is 0.5V, after the ECG signal is filtered in the front-end filter, the maximum variation of the baseline drift signal is 4V. If the ECG signal input to the second amplifier  16  is 5 mV (5V/100), and the maximum variation of the baseline drift signal is 4.5V. In other word, the ECG signal input to the first amplifier  11  is 1 mV (5 mV/5), and the maximum variation of the baseline drift signal is 0.9V (4.5V/5). Thus, according to the described baseline drift canceling method, the baseline drift tolerance is 900 (0.9V/1 mV). Compared with the convention baseline drift tolerance (4V/0.5V=8), the baseline drift tolerance is significantly increased. 
         [0035]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.