Patent Publication Number: US-9833195-B2

Title: Biomedical signal sensing circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 102141664, filed on Nov. 15, 2013. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The invention relates to a sensing circuit. Particularly, the invention relates to a biomedical signal sensing circuit. 
     Related Art 
     Along with development of technology and progress of the times, people pay more and more attention to personal health, and various health-related industries are quickly developed. In order to accurately grasp various biomedical signals on human body, various manufactures have developed a wide range of instruments for measuring biomedical signals, for example, an electrocardiograph (ECG) machine, an electroencephalogram (EEG) machine, etc. 
     However, in a current biomedical signal sensing circuit, when a plurality of biomedical signals are simultaneously sensed, a plurality of independently operated sensing circuit has to be used for sensing. Since each of the sensing circuits has a little mismatch, outputs of the sensing circuits have to be summed and fed back to input terminals of the sensing circuits according to a common-mode feedback method, so as to increase a common-mode rejection ratio (CMRR). However, such method not only wastes a chip fabrication area, but also causes extra power consumption. 
     SUMMARY 
     The invention provides a biomedical signal sensing circuit including a first modulation unit, a second modulation unit, an amplifying unit, a first demodulation unit and a second demodulation unit. The first modulation unit receives a first biomedical signal, and performs a first modulation operation to the first biomedical signal according to a first signal to generate a first modulation signal. The second modulation unit receives a second biomedical signal, and performs a second modulation operation to the second biomedical signal according to a second signal to generate a second modulation signal. The second signal is orthogonal to the first signal. The amplifying unit is coupled to the first modulation unit and the second modulation unit, and amplifies the first modulation signal and the second modulation signal, and adds the amplified first and second modulation signals to generate a third modulation signal. The first demodulation unit is coupled to the amplifying unit, and performs a first demodulation operation to the third modulation signal according to the first signal to generate a first sensing signal. The second demodulation unit is coupled to the amplifying unit, and performs a second demodulation operation to the third modulation signal according to the second signal to generate a second sensing signal. 
     In an embodiment of the invention, the first modulation unit includes a first multiplier, and the first multiplier multiplies the first biomedical signal with the first signal to generate the first modulation signal. The second modulation unit includes a second multiplier, and the second multiplier multiplies the second biomedical signal with the second signal to generate the second modulation signal. 
     In an embodiment of the invention, the first signal is a sine wave, and the second signal is a cosine wave orthogonal to the sine wave. 
     In an embodiment of the invention, the amplifying unit includes a first capacitor, a second capacitor, a third capacitor and an operational amplifier. The first capacitor has a first terminal and a second terminal. The first terminal of the first capacitor receives the first modulation signal. The second capacitor has a first terminal and a second terminal. The first terminal of the second capacitor receives the second modulation signal. The third capacitor has a first terminal and a second terminal. The first terminal of the third capacitor is coupled to the second terminal of the first capacitor and the second terminal of the second capacitor. The operational amplifier has a first input terminal, a second input terminal and an output terminal. The first input terminal of the operational amplifier is coupled to the second terminal of the first capacitor. The second input terminal of the operational amplifier is coupled to a ground potential. The output terminal of the operational amplifier is coupled to the first demodulation unit and the second demodulation unit. 
     In an embodiment of the invention, the first modulation signal includes a first signal component and a second signal component. The first signal component is inverted to the second signal component. The second modulation signal includes a third signal component and a fourth signal component. The third signal component is inverted to the fourth signal component. The amplifying unit includes a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, a fifth capacitor, a sixth capacitor and an operational amplifier. The first capacitor has a first terminal and a second terminal. The first terminal of the first capacitor receives the first signal component. The second capacitor has a first terminal and a second terminal. The first terminal of the second capacitor receives the second signal component. The third capacitor has a first terminal and a second terminal. The first terminal of the third capacitor receives the third signal component. The second terminal of the third capacitor is coupled to the second terminal of the first capacitor. The fourth capacitor has a first terminal and a second terminal. The first terminal of the fourth capacitor receives the fourth signal component. The second terminal of the fourth capacitor is coupled to the second terminal of the second capacitor. The fifth capacitor has a first terminal and a second terminal. The first terminal of the fifth capacitor is coupled to the second terminal of the first capacitor. The sixth capacitor has a first terminal and a second terminal. The first terminal of the sixth capacitor is coupled to the second terminal of the second capacitor. The operational amplifier has a first input terminal, a second input terminal, a first output terminal and a second output terminal. The first input terminal of the operational amplifier is coupled to the second terminal of the first capacitor. The second input terminal of the operational amplifier is coupled to the second terminal of the second capacitor. The first output terminal of the operational amplifier is coupled to the second terminal of the fifth capacitor. The second output terminal of the operational amplifier is coupled to the second terminal of the sixth capacitor. 
     In an embodiment of the invention, the third modulation signal includes a first component and a second component. The first component is inverted to the second component. The first sensing signal includes a third component and a fourth component. The third component is inverted to the fourth component. The second sensing signal includes a fifth component and a sixth component. The fifth component is inverted to the sixth component. The first demodulation unit has a first input terminal, a second input terminal, a third input terminal, a first output terminal and a second output terminal. The first input terminal of the first demodulation unit is coupled to the first output terminal of the operational amplifier to receive the first component. The second input terminal of the first demodulation unit is coupled to the second output terminal of the operational amplifier to receive the second component. The third input terminal of the first demodulation unit receives the first signal. The first output terminal of the first demodulation unit outputs the third component. The second output terminal of the first demodulation unit outputs the fourth component. The second demodulation unit has a first input terminal, a second input terminal, a third input terminal, a first output terminal and a second output terminal. The first input terminal of the second demodulation unit is coupled to the first output terminal of the operational amplifier to receive the first component. The second input terminal of the second demodulation unit is coupled to the second output terminal of the operational amplifier to receive the second component. The third input terminal of the second demodulation unit receives the second signal. The first output terminal of the second demodulation unit outputs the fifth component. The second output terminal of the second demodulation unit outputs the sixth component. 
     In an embodiment of the invention, the first biomedical signal includes a first differential signal component and a second differential signal component. The first differential signal component is inverted to the second differential signal component. The second biomedical signal includes a third differential signal component and a fourth differential signal component. The third differential signal component is inverted to the fourth differential signal component. The first signal includes a first portion and a second portion. The first portion is inverted to the second portion. The second signal includes a third portion and a fourth portion. The third portion is inverted to the fourth portion. The first modulation unit has a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first output terminal and a second output terminal. The first input terminal of the first modulation unit receives the first differential signal component. The second input terminal of the first modulation unit receives the second differential signal component. The third input terminal of the first modulation unit receives the first portion. The fourth input terminal of the first modulation unit receives the second portion. The second modulation unit has a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first output terminal and a second output terminal. The first input terminal of the second modulation unit receives the third differential signal component. The second input terminal of the second modulation unit receives the fourth differential signal component. The third input terminal of the second modulation unit receives the third portion. The fourth input terminal of the second modulation unit receives the fourth portion. The first output terminal of the second modulation unit is coupled to the second output terminal of the first modulation unit. 
     In an embodiment of the invention, the amplifying unit includes a first capacitor, a second capacitor, a third capacitor, a fourth capacitor and an operational amplifier. The first capacitor has a first terminal and a second terminal. The first terminal of the first capacitor is coupled to the first output terminal of the first modulation unit for receiving the first modulation signal. The second capacitor has a first terminal and a second terminal. The first terminal of the second capacitor is coupled to the second output terminal of the second modulation unit for receiving the second modulation signal. The third capacitor has a first terminal and a second terminal. The first terminal of the third capacitor is coupled to the second terminal of the first capacitor. The fourth capacitor has a first terminal and a second terminal. The first terminal of the fourth capacitor is coupled to the second terminal of the second capacitor. The operational amplifier has a first input terminal, a second input terminal, a first output terminal and a second output terminal. The first input terminal of the operational amplifier is coupled to the second terminal of the first capacitor. The second input terminal of the operational amplifier is coupled to the second terminal of the second capacitor. The first output terminal of the operational amplifier is coupled to the second terminal of the third capacitor. The second output terminal of the operational amplifier is coupled to the second terminal of the fourth capacitor. 
     In an embodiment of the invention, the first modulation unit includes a first transistor, a second transistor, a third transistor and a fourth transistor. The first transistor has a first terminal, a second terminal and a control terminal. The first terminal of the first transistor receives the first differential signal component. The control terminal of the first transistor receives the second portion. The second transistor has a first terminal, a second terminal and a control terminal. The first terminal of the second transistor receives the first differential signal component. The control terminal of the second transistor receives the first portion. The third transistor has a first terminal, a second terminal and a control terminal. The first terminal of the third transistor receives the second differential signal component. The control terminal of the third transistor receives the first portion. The second terminal of the third transistor is coupled to the second terminal of the first transistor. The fourth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the fourth transistor receives the second differential signal component, and the control terminal of the fourth transistor receives the second portion. The second terminal of the fourth transistor is coupled to the second terminal of the second transistor. The second modulation unit includes a fifth transistor, a sixth transistor, a seventh transistor and an eighth transistor. The fifth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the fifth transistor receives the third differential signal component. The control terminal of the fifth transistor receives the fourth portion. The sixth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the sixth transistor receives the third differential signal component. The control terminal of the sixth transistor receives the third portion. The seventh transistor has a first terminal, a second terminal and a control terminal. The first terminal of the seventh transistor receives the fourth differential signal component. The control terminal of the seventh transistor receives the third portion. The second terminal of the seventh transistor is coupled to the second terminal of the fifth transistor. The eighth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the eighth transistor receives the fourth differential signal component. The control terminal of the eighth transistor receives the fourth portion. The second terminal of the eighth transistor is coupled to the second terminal of the sixth transistor. 
     In an embodiment of the invention, the third modulation signal includes a first component and a second component. The first component is inverted to the second component. The first sensing signal includes a third component and a fourth component. The third component is inverted to the fourth component. The second sensing signal includes a fifth component and a sixth component. The fifth component is inverted to the sixth component. The first demodulation unit has a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first output terminal and a second output terminal. The first input terminal of the first demodulation unit is coupled to the first output terminal of the operational amplifier to receive the first component. The second input terminal of the first demodulation unit is coupled to the second output terminal of the operational amplifier to receive the second component. The third input terminal of the first demodulation unit receives the first portion. The fourth input terminal of the first demodulation unit receives the second portion. The first output terminal of the first demodulation unit outputs the third component. The second output terminal of the first demodulation unit outputs the fourth component. The second demodulation unit has a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first output terminal and a second output terminal. The first input terminal of the second demodulation unit is coupled to the first output terminal of the operational amplifier to receive the first component. The second input terminal of the second demodulation unit is coupled to the second output terminal of the operational amplifier to receive the second component. The third input terminal of the second demodulation unit receives the third portion. The fourth input terminal of the second demodulation unit receives the fourth portion. The first output terminal of the second demodulation unit outputs the fifth component. The second output terminal of the second demodulation unit outputs the sixth component. 
     In an embodiment of the invention, the first demodulation unit includes a ninth transistor, a tenth transistor, an eleventh transistor and a twelfth transistor. The ninth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the ninth transistor receives the first component. The control terminal of the ninth transistor receives the second portion. The tenth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the tenth transistor receives the first component. The control terminal of the tenth transistor receives the first portion. The eleventh transistor has a first terminal, a second terminal and a control terminal. The first terminal of the eleventh transistor receives the second component. The control terminal of the eleventh transistor receives the first portion. The second terminal of the eleventh transistor is coupled to the second terminal of the ninth transistor. The twelfth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the twelfth transistor receives the second component, and the control terminal of the twelfth transistor receives the second portion. The second terminal of the twelfth transistor is coupled to the second terminal of the tenth transistor. The second demodulation unit includes a thirteenth transistor, a fourteenth transistor, a fifteenth transistor and a sixteenth transistor. The thirteenth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the thirteenth transistor receives the first component. The control terminal of the thirteenth transistor receives the fourth portion. The fourteenth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the fourteenth transistor receives the first component. The control terminal of the fourteenth transistor receives the third portion. The fifteenth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the fifteenth transistor receives the second component. The control terminal of the fifteenth transistor receives the third portion. The second terminal of the fifteenth transistor is coupled to the second terminal of the thirteenth transistor. The sixteenth transistor has a first terminal, a second terminal and a control terminal. The first terminal of the sixteenth transistor receives the second component. The control terminal of the sixteenth transistor receives the fourth portion. The second terminal of the sixteenth transistor is coupled to the second terminal of the fourteenth transistor. 
     According to the above descriptions, the biomedical signal sensing circuit of the invention is capable of simultaneously processing a plurality of biomedical signals through a plurality of modulation units and demodulation units, so as to correspondingly generate a plurality of sensing signals that do not interfere with each other according to the biomedical signals. 
     In order to make the aforementioned and other features and advantages of the invention comprehensible, several exemplary embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a functional block diagram of a biomedical signal sensing circuit according to an embodiment of the invention. 
         FIG. 2  is a schematic diagram of a biomedical signal sensing circuit according to the embodiment of  FIG. 1 . 
         FIG. 3  is a schematic diagram of a biomedical signal sensing circuit according to the embodiment of  FIG. 1 . 
         FIG. 4  is a schematic diagram of a biomedical signal sensing circuit according to the embodiment of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS 
       FIG. 1  is a functional block diagram of a biomedical signal sensing circuit according to an embodiment of the invention. In the present embodiment, the biomedical signal sensing circuit  100  includes a first modulation unit  110 , a second modulation unit  120 , an amplifying unit  130 , a first demodulation unit  140  and a second demodulation unit  150 . 
     The first modulation unit  110  receives a first biomedical signal BS 1 , and performs a first modulation operation to the first biomedical signal BS 1  according to a first signal S 1  to generate a first modulation signal MS 1 . In an embodiment, the first modulation unit  110  is, for example, a first multiplier, which multiplies the first biomedical signal BS 1  by the first signal S 1  to generate the first modulation signal MS 1 . 
     The second modulation unit  120  receives a second biomedical signal BS 2 , and performs a second modulation operation to the second biomedical signal BS 2  according to a second signal S 2  to generate a second modulation signal MS 2 . In an embodiment, the second modulation unit  120  is, for example, a second multiplier, which multiplies the second biomedical signal BS 2  by the second signal S 2  to generate the second modulation signal MS 2 . 
     In an embodiment, the first biomedical signal BS 1  and the second biomedical signal BS 2  are, for example, electrocardiogram (ECG) signals, electroencephalogram (EEG) signals or other physiological measurement signals. 
     In an embodiment, the second signal S 2  is orthogonal to the first signal S 1 . For example, the first signal S 1  is, for example, a sine wave, and the second signal S 2  is, for example, a cosine wave orthogonal to the sine wave. Alternatively, in other embodiments, the first signal S 1  and the second signal S 2  can be respectively implemented by pseudo random codes orthogonal to each other, or other signals orthogonal to each other. 
     The amplifying unit  130  is coupled to the first modulation unit  110  and the second modulation unit  120 . The amplifying unit  130  amplifies the first modulation signal MS 1  and the second modulation signal MS 2 , and adds the amplified first and second modulation signals MS 1  and MS 2  to generate a third modulation signal MS 3 . The first demodulation unit  140  is coupled to the amplifying unit  130 , and performs a first demodulation operation to the third modulation signal MS 3  according to the first signal S 1  to generate a first sensing signal SS 1 . The second demodulation unit  150  is coupled to the amplifying unit  130 , and performs a second demodulation operation to the third modulation signal MS 3  according to the second signal S 2  to generate a second sensing signal SS 2 . 
     Since the first signal S 1  and the second signal S 2  are orthogonal to each other, even if the third modulation signal MS 3  includes the added and amplified first and second modulation signals MS 1  and MS 2 , the first demodulation unit  140  and the second demodulation unit  150  can still eliminate an influence of the other modulation signal when performing the first and second demodulation operations, so as to obtain a correct measuring result. For example, when the first demodulation unit  140  performs the first demodulation operation to the third modulation signal MS 3  according to the first signal S 1 , since the first signal S 1  is orthogonal to a component of the second signal S 2  in the second modulation signal MS 2 , the first demodulation unit  140  can eliminate the influence of the second modulation signal MS 2  from the third modulation signal MS 3 , so as to generate the first sensing signal SS 1  not including the component related to the second modulation signal MS 2 . On the other hand, when the second demodulation unit  150  performs the second demodulation operation to the third modulation signal MS 3  according to the second signal S 2 , since the second signal S 2  is orthogonal to a component of the first signal S 1  in the first modulation signal MS 1 , the second demodulation unit  150  can eliminate the influence of the first modulation signal MS 1  from the third modulation signal MS 3 , so as to generate the second sensing signal SS 2  not including the component related to the first modulation signal MS 1 . 
     According to another aspect, since the first signal S 1  and the second signal S 2  are orthogonal to each other, the biomedical signal sensing circuit  100  can process a plurality of biomedical signals simultaneously by only including the single amplifying unit  130 , such that the biomedical signal sensing circuit  100  may achieve a smaller volume and lower implementation cost. 
     Moreover, those skilled in the art should understand that the biomedical signal sensing circuit  100  may also include other modulation units and corresponding demodulation units. In this case, as long as the signals (for example, the first signal S 1  and the second signal S 2 ) according which each of the modulation units and the corresponding demodulation unit perform the modulation operation and the demodulation operation are orthogonal to each other, the biomedical signal sensing circuit  100  can simultaneously receive a plurality of biomedical signals, and correspondingly generate a plurality of sensing signals that do not influence each other. 
       FIG. 2  is a schematic diagram of a biomedical signal sensing circuit according to the embodiment of  FIG. 1 . In the present embodiment, the biomedical signal sensing circuit  200  includes a first modulation unit  210 , a second modulation unit  220 , an amplifying unit  230 , a first demodulation unit  240  and a second demodulation unit  250 . The first modulation unit  210 , the second modulation unit  220 , the first demodulation unit  240  and the second demodulation unit  250  can be respectively implemented by multipliers, though the invention is not limited thereto. 
     The first modulation unit  210  receives the first biomedical signal BS 1 , and multiplies the first biomedical signal BS 1  by the first signal S 1  to generate the first modulation signal MS 1 , and the second modulation unit  220  receives the second biomedical signal BS 2 , and multiplies the second biomedical signal BS 2  by the second signal S 2  to generate the second modulation signal MS 2 . 
     The amplifying unit  230  includes a first capacitor C 21 , a second capacitor C 22 , a third capacitor C 23  and an operational amplifier  235 . The first capacitor C 21  has a first terminal and a second terminal. The first terminal of the first capacitor C 21  receives the first modulation signal MS 1 . The second capacitor C 22  has a first terminal and a second terminal. The first terminal of the second capacitor C 22  receives the second modulation signal MS 2 . The third capacitor C 23  has a first terminal and a second terminal. The first terminal of the third capacitor C 23  is coupled to the second terminal of the first capacitor C 21  and the second terminal of the second capacitor C 22 . The operational amplifier  235  has a first input terminal, a second input terminal and an output terminal. The first input terminal of the operational amplifier  235  is coupled to the second terminal of the first capacitor C 21 . The second input terminal of the operational amplifier  235  is coupled to a ground potential GND. The output terminal of the operational amplifier  235  is coupled to the first demodulation unit  240  and the second demodulation unit  250 . 
     In detail, the first modulation signal MS 1  and the second modulation signal MS 2  can be respectively a first voltage signal and a second voltage signal, and the two voltage signals can be respectively converted into a first current signal and a second current signal by the first capacitor C 21  and the second capacitor C 22 . Then, the two current signals can be added to produce a third current signal at the second terminal of the first capacitor C 21  (i.e. the second terminal of the second capacitor C 22 ). Thereafter, the operational amplifier  235  amplifies the third current signal to generate the third modulation signal MS 3 . 
     Then, the first demodulation unit  240  receives the third modulation signal MS 3  and multiplies the same by the first signal S 1  to generate the first sensing signal SS 1 . Moreover, the second demodulation unit  250  receives the third modulation signal MS 3  and multiplies the same by the second signal S 2  to generate the second sensing signal SS 2 . 
     Similar to the instructions of the embodiment of  FIG. 1 , since the first signal S 1  is orthogonal to the second signal S 2 , when the first demodulation unit  240  multiplies the third modulation signal MS 3  by the first signal S 1 , the first demodulation unit  240  can correspondingly eliminate the component related to the second modulation signal MS 2  from the third modulation signal MS 3 , so as to generate the first sensing signal SS 1  not including the component related to the second modulation signal MS 2 . Similarly, the second demodulation unit  250  can also generate the second sensing signal SS 2  not including the component related to the first modulation signal MS 1 , and details thereof are not repeated. 
     In other embodiments, the biomedical signal sensing circuit  200  may further include a first low-pass filter  260  and a second low-pass filter  270 . The first low-pass filter  260  is coupled to the first demodulation unit  240 , and performs a first low-pass filtering operation to the first sensing signal SS 1  to generate a first filtering signal SS 1 ′. The second low-pass filter  270  is coupled to the second demodulation unit  250 , and performs a second low-pass filtering operation to the second sensing signal SS 2  to generate a second filtering signal SS 2 ′. 
     In other embodiments, various devices in the biomedical signal sensing circuit can also be implemented by differential circuits, so as to further decrease a noise, for example, a common-mode noise, etc. 
       FIG. 3  is a schematic diagram of a biomedical signal sensing circuit according to the embodiment of  FIG. 1 . In the present embodiment, the biomedical signal sensing circuit  300  includes a first modulation unit  310 , a second modulation unit  320 , an amplifying unit  330 , a first demodulation unit  340  and a second demodulation unit  350 . 
     In the present embodiment, the first biomedical signal BS 1  includes a first differential signal component BS 1 _ 1  and a second differential signal component BS 1 _ 2 . The first differential signal component BS 1 _ 1  is inverted to the second differential signal component BS 1 _ 2 . The second biomedical signal BS 2  includes a third differential signal component BS 2 _ 1  and a fourth differential signal component BS 2 _ 2 . The third differential signal component BS 2 _ 1  is inverted to the fourth differential signal component BS 2 _ 2 . 
     The first modulation unit  310  has a first input terminal, a second input terminal, a third input terminal, a first output terminal and a second output terminal. The first input terminal of the first modulation unit  310  receives the first differential signal BS 1 _ 1 . The second input terminal of the first modulation unit  310  receives the second differential signal BS 1 _ 2 . The third input terminal of the first modulation unit  310  receives the first signal. The second modulation unit  320  has a first input terminal, a second input terminal, a third input terminal, a first output terminal and a second output terminal. The first input terminal of the second modulation unit  320  receives the third differential signal BS 2 _ 1 . The second input terminal of the second modulation unit  320  receives the fourth differential signal BS 2 _ 2 . The third input terminal of the second modulation unit  320  receives the second signal. 
     In the present embodiment, the first modulation signal MS 1  includes a first signal component MS 1 _ 1  and a second signal component MS 1 _ 2 . The first signal component MS 1 _ 1  is inverted to the second signal component MS 1 _ 2 . The second modulation signal MS 2  includes a third signal component MS 2 _ 1  and a fourth signal component MS 2 _ 2 . The third signal component MS 2 _ 1  is inverted to the fourth signal component MS 2 _ 2 . 
     The amplifying unit  330  includes a first capacitor C 31 , a second capacitor C 32 , a third capacitor C 33 , a fourth capacitor C 34 , a fifth capacitor C 35 , a sixth capacitor C 36  and an operational amplifier  335 . The first capacitor C 31  has a first terminal and a second terminal. The first terminal of the first capacitor C 31  receives the first signal component MS 1 _ 1 . The second capacitor C 32  has a first terminal and a second terminal. The first terminal of the second capacitor C 32  receives the second signal component MS 1 _ 2 . The third capacitor C 33  has a first terminal and a second terminal. The first terminal of the third capacitor C 33  receives the third signal component MS 2 _ 1 . The second terminal of the third capacitor C 33  is coupled to the second terminal of the first capacitor C 31 . The fourth capacitor C 34  has a first terminal and a second terminal. The first terminal of the fourth capacitor C 34  receives the fourth signal component MS 2 _ 2 . The second terminal of the fourth capacitor C 34  is coupled to the second terminal of the second capacitor C 32 . The fifth capacitor C 35  has a first terminal and a second terminal. The first terminal of the fifth capacitor C 35  is coupled to the second terminal of the first capacitor C 31 . The sixth capacitor C 36  has a first terminal and a second terminal. The first terminal of the sixth capacitor C 36  is coupled to the second terminal of the second capacitor C 32 . The operational amplifier  335  has a first input terminal, a second input terminal, a first output terminal and a second output terminal. The first input terminal of the operational amplifier  335  is coupled to the second terminal of the first capacitor C 31 . The second input terminal of the operational amplifier  335  is coupled to the second terminal of the second capacitor C 32 . The first output terminal of the operational amplifier  335  is coupled to the second terminal of the fifth capacitor C 35 . The second output terminal of the operational amplifier  335  is coupled to the second terminal of the sixth capacitor C 36 . 
     The third modulation signal MS 3  includes a first component MS 3 _ 1  and a second component MS 3 _ 2 . The first component MS 3 _ 1  is inverted to the second component MS 3 _ 2 . The first sensing signal SS 1  includes a third component SS 1 _ 1  and a fourth component SS 1 _ 2 . The third component SS 1 _ 1  is inverted to the fourth component SS 1 _ 2 . The second sensing signal SS 2  includes a fifth component SS 2 _ 1  and a sixth component SS 2 _ 2 . The fifth component SS 2 _ 1  is inverted to the sixth component SS 2 _ 2 . 
     The first demodulation unit  340  has a first input terminal, a second input terminal, a third input terminal, a first output terminal and a second output terminal. The first input terminal of the first demodulation unit  340  is coupled to the first output terminal of the operational amplifier  335  to receive the first component MS 3 _ 1 . The second input terminal of the first demodulation unit  340  is coupled to the second output terminal of the operational amplifier  335  to receive the second component MS 3 _ 2 . The third input terminal of the first demodulation unit  340  receives the first signal S 1 . The first output terminal of the first demodulation unit  340  outputs the third component SS 1 _ 1 . The second output terminal of the first demodulation unit  340  outputs the fourth component SS 1 _ 2 . The second demodulation unit  350  has a first input terminal, a second input terminal, a third input terminal, a first output terminal and a second output terminal. The first input terminal of the second demodulation unit  350  is coupled to the first output terminal of the operational amplifier  335  to receive the first component MS 3 _ 1 . The second input terminal of the second demodulation unit  350  is coupled to the second output terminal of the operational amplifier  335  to receive the second component MS 3 _ 2 . The third input terminal of the second demodulation unit  350  receives the second signal S 2 . The first output terminal of the second demodulation unit  350  outputs the fifth component SS 2 _ 1 . The second output terminal of the second demodulation unit  350  outputs the sixth component SS 2 _ 2 . 
     Compared to the biomedical signal sensing circuit  200  in the embodiment of  FIG. 2 , besides that the biomedical signal sensing circuit  300  of the present embodiment can correctly generate the first sensing signal SS 1  and the second sensing signal SS 2  that do not interfere with each other, a whole common-mode noise can be further decreased. 
     In other embodiments, the designer can decrease the required number of the capacitors by adjusting a circuit connection method of the biomedical signal sensing circuit, so as to decrease a whole circuit area and the manufacturing cost. 
     Referring to  FIG. 4 ,  FIG. 4  is a schematic diagram of a biomedical signal sensing circuit according to an embodiment of the invention. As that shown in  FIG. 4 , the biomedical signal sensing circuit  400  includes a first modulation unit  410 , a second modulation unit  420 , an amplifying unit  430 , a first demodulation unit  440  and a second demodulation unit  450 . 
     In the present embodiment, the first biomedical signal BS 1  includes a first differential signal component BS 1 _ 1  and a second differential signal component BS 1 _ 2 . The first differential signal component BS 1 _ 1  is inverted to the second differential signal component BS 1 _ 2 . The second biomedical signal BS 2  includes a third differential signal component BS 2 _ 1  and a fourth differential signal component BS 2 _ 2 . The third differential signal component BS 2 _ 1  is inverted to the fourth differential signal component BS 2 _ 2 . The first signal S 1  includes a first portion S 1 _ 1  and a second portion S 1 _ 2 . The first portion S 1 _ 1  is inverted to the second portion S 1 _ 2 . The second signal S 2  includes a third portion S 2 _ 1  and a fourth portion S 2 _ 2 . The third portion S 2 _ 1  is inverted to the fourth portion S 2 _ 2 . 
     The first modulation unit  410  has a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first output terminal and a second output terminal. The first input terminal of the first modulation unit  410  receives the first differential signal BS 1 _ 1 . The second input terminal of the first modulation unit  410  receives the second differential signal BS 1 _ 2 . The third input terminal of the first modulation unit  410  receives the first portion S 1 _ 1 . The fourth input terminal of the first modulation unit  410  receives the second portion S 1 _ 2 . 
     The second modulation unit  420  has a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first output terminal and a second output terminal. The first input terminal of the second modulation unit  420  receives the third differential signal BS 2 _ 1 . The second input terminal of the second modulation unit  420  receives the fourth differential signal BS 2 _ 2 . The third input terminal of the second modulation unit  420  receives the third portion S 2 _ 1 . The fourth input terminal of the second modulation unit  420  receives the fourth portion S 2 _ 2 . The first output terminal of the second modulation unit  420  is coupled to the second output terminal of the first modulation unit  410 . 
     The amplifying unit  430  includes a first capacitor C 41 , a second capacitor C 42 , a third capacitor C 43 , a fourth capacitor C 44  and an operational amplifier  435 . The first capacitor C 41  has a first terminal and a second terminal. The first terminal of the first capacitor C 41  is coupled to the first output terminal of the first modulation unit  410  for receiving the first modulation signal MS 1 . The second capacitor C 42  has a first terminal and a second terminal. The first terminal of the second capacitor C 42  is coupled to the second output terminal of the second modulation unit  420  for receiving the second modulation signal MS 2 . The third capacitor C 43  has a first terminal and a second terminal. The first terminal of the third capacitor C 43  is coupled to the second terminal of the first capacitor C 41 . The fourth capacitor C 44  has a first terminal and a second terminal. The first terminal of the fourth capacitor C 44  is coupled to the second terminal of the second capacitor C 42 . The operational amplifier  435  has a first input terminal, a second input terminal, a first output terminal and a second output terminal. The first input terminal of the operational amplifier  435  is coupled to the second terminal of the first capacitor C 41 . The second input terminal of the operational amplifier  435  is coupled to the second terminal of the second capacitor C 42 . The first output terminal of the operational amplifier  435  is coupled to the second terminal of the third capacitor C 43 . The second output terminal of the operational amplifier  435  is coupled to the second terminal of the fourth capacitor C 44 . 
     In the present embodiment, the first modulation unit  410  includes a first transistor T 1 , a second transistor T 2 , a third transistor T 3  and a fourth transistor T 4 . The first transistor T 1  has a first terminal, a second terminal and a control terminal. The first terminal of the first transistor T 1  receives the first differential signal component BS 1 _ 1 . The control terminal of the first transistor T 1  receives the second portion S 1 _ 2 . The second transistor T 2  has a first terminal, a second terminal and a control terminal. The first terminal of the second transistor T 2  receives the first differential signal component BS 1 _ 1 . The control terminal of the second transistor T 2  receives the first portion S 1 _ 1 . The third transistor T 3  has a first terminal, a second terminal and a control terminal. The first terminal of the third transistor T 3  receives the second differential signal component BS 1 _ 2 . The control terminal of the third transistor T 3  receives the first portion S 1 _ 1 . The second terminal of the third transistor T 3  is coupled to the second terminal of the first transistor T 1 . The fourth transistor T 4  has a first terminal, a second terminal and a control terminal. The first terminal of the fourth transistor T 4  receives the second differential signal component BS 1 _ 2 , and the control terminal of the fourth transistor T 4  receives the second portion S 1 _ 2 . The second terminal of the fourth transistor T 4  is coupled to the second terminal of the second transistor T 2 . 
     The second modulation unit  420  includes a fifth transistor T 5 , a sixth transistor T 6 , a seventh transistor T 7  and an eighth transistor T 8 . The fifth transistor T 5  has a first terminal, a second terminal and a control terminal. The first terminal of the fifth transistor T 5  receives the third differential signal component BS 2 _ 1 . The control terminal of the fifth transistor T 5  receives the fourth portion S 2 _ 2 . The sixth transistor T 6  has a first terminal, a second terminal and a control terminal. The first terminal of the sixth transistor T 6  receives the third differential signal component BS 2 _ 1 . The control terminal of the sixth transistor T 6  receives the third portion S 2 _ 1 . The seventh transistor T 7  has a first terminal, a second terminal and a control terminal. The first terminal of the seventh transistor T 7  receives the fourth differential signal component BS 2 _ 2 . The control terminal of the seventh transistor T 7  receives the third portion S 2 _ 1 . The second terminal of the seventh transistor T 7  is coupled to the second terminal of the fifth transistor T 5 . The eighth transistor T 8  has a first terminal, a second terminal and a control terminal. The first terminal of the eighth transistor T 8  receives the fourth differential signal component BS 2 _ 2 . The control terminal of the eighth transistor T 8  receives the fourth portion S 2 _ 2 . The second terminal of the eighth transistor T 8  is coupled to the second terminal of the sixth transistor T 6 . 
     The third modulation signal MS 3  includes a first component MS 3 _ 1  and a second component MS 3 _ 2 . The first component MS 3 _ 1  is inverted to the second component MS 3 _ 2 . The first sensing signal SS 1  includes a third component SS 1 _ 1  and a fourth component SS 1 _ 2 . The third component SS 1 _ 1  is inverted to the fourth component SS 1 _ 2 . The second sensing signal SS 2  includes a fifth component SS 2 _ 1  and a sixth component SS 2 _ 2 . The fifth component SS 2 _ 1  is inverted to the sixth component SS 2 _ 2 . 
     The first demodulation unit  440  has a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first output terminal and a second output terminal. The first input terminal of the first demodulation unit  440  is coupled to the first output terminal of the operational amplifier  435  to receive the first component MS 3 _ 1 . The second input terminal of the first demodulation unit  440  is coupled to the second output terminal of the operational amplifier  435  to receive the second component MS 3 _ 2 . The third input terminal of the first demodulation unit  440  receives the first portion S 1 _ 1 . The fourth input terminal of the first demodulation unit  440  receives the second portion S 1 _ 2 . The first output terminal of the first demodulation unit  440  outputs the third component SS 1 _ 1 . The second output terminal of the first demodulation unit  440  outputs the fourth component SS 1 _ 2 . 
     The second demodulation unit  450  has a first input terminal, a second input terminal, a third input terminal, a fourth input terminal, a first output terminal and a second output terminal. The first input terminal of the second demodulation unit  450  is coupled to the first output terminal of the operational amplifier  435  to receive the first component MS 3 _ 1 . The second input terminal of the second demodulation unit  450  is coupled to the second output terminal of the operational amplifier  435  to receive the second component MS 3 _ 2 . The third input terminal of the second demodulation unit  450  receives the third portion S 2 _ 1 . The fourth input terminal of the second demodulation unit  450  receives the fourth portion S 2 _ 2 . The first output terminal of the second demodulation unit  450  outputs the fifth component SS 2 _ 1 . The second output terminal of the second demodulation unit  450  outputs the sixth component SS 2 _ 2 . 
     The first demodulation unit  440  includes a ninth transistor T 9 , a tenth transistor T 10 , an eleventh transistor T 11  and a twelfth transistor T 12 . The ninth transistor T 9  has a first terminal, a second terminal and a control terminal. The first terminal of the ninth transistor T 9  receives the first component MS 3 _ 1 . The control terminal of the ninth transistor T 9  receives the second portion S 1 _ 2 . The tenth transistor T 10  has a first terminal, a second terminal and a control terminal. The first terminal of the tenth transistor T 10  receives the first component MS 3 _ 1 . The control terminal of the tenth transistor T 10  receives the first portion S 1 _ 1 . The eleventh transistor T 11  has a first terminal, a second terminal and a control terminal. The first terminal of the eleventh transistor T 11  receives the second component MS 3 _ 2 . The control terminal of the eleventh transistor T 11  receives the first portion S 1 _ 1 . The second terminal of the eleventh transistor T 11  is coupled to the second terminal of the ninth transistor T 9 . The twelfth transistor T 12  has a first terminal, a second terminal and a control terminal. The first terminal of the twelfth transistor T 12  receives the second component MS 3 _ 2 . The control terminal of the twelfth transistor T 12  receives the second portion S 1 _ 2 . The second terminal of the twelfth transistor T 12  is coupled to the second terminal of the tenth transistor T 10 . 
     The second demodulation unit  450  includes a thirteenth transistor T 13 , a fourteenth transistor T 14 , a fifteenth transistor T 15  and a sixteenth transistor T 16 . The thirteenth transistor T 13  has a first terminal, a second terminal and a control terminal. The first terminal of the thirteenth transistor T 13  receives the first component MS 3 _ 1 . The control terminal of the thirteenth transistor T 13  receives the fourth portion S 2 _ 2 . The fourteenth transistor T 14  has a first terminal, a second terminal and a control terminal. The first terminal of the fourteenth transistor T 14  receives the first component MS 3 _ 1 . The control terminal of the fourteenth transistor T 14  receives the third portion S 2 _ 1 . The fifteenth transistor T 15  has a first terminal, a second terminal and a control terminal. The first terminal of the fifteenth transistor T 15  receives the second component MS 3 _ 2 . The control terminal of the fifteenth transistor T 15  receives the third portion S 2 _ 1 . The second terminal of the fifteenth transistor T 15  is coupled to the second terminal of the thirteenth transistor T 13 . The sixteenth transistor T 16  has a first terminal, a second terminal and a control terminal. The first terminal of the sixteenth transistor T 16  receives the second component MS 32 . The control terminal of the sixteenth transistor T 16  receives the fourth portion S 2 _ 2 . The second terminal of the sixteenth transistor T 16  is coupled to the second terminal of the fourteenth transistor T 14 . 
     Compared to the biomedical signal sensing circuit  300  of  FIG. 3  that includes six capacitors, the biomedical signal sensing circuit  400  of  FIG. 4  can achieve the effect of generating a plurality of sensing signals that do not interfere with each other by only using four capacitors, such that the whole circuit volume and the manufacturing cost are further decreased. 
     In summary, the biomedical signal sensing circuit of the invention is capable of simultaneously processing a plurality of biomedical signals through a plurality of modulation units and demodulation units, so as to correspondingly generate a plurality of sensing signals that do not interfere with each other according to the biomedical signals. It should be noticed that the biomedical signal sensing circuit of the invention may simultaneously process a plurality of biomedical signals in case that only a single amplifying unit is included, which is different to the conventional biomedical signal sensing circuit that can only process a single biomedical signal at one time. Therefore, compared to the conventional biomedical signal sensing circuit, the biomedical signal sensing circuit of the invention may have a smaller volume and lower implementation cost. 
     Moreover, since the biomedical signal sensing circuit of the invention is unnecessary to use a plurality of sensing circuits to sense the biomedical signals, mismatch of the sensing circuits is avoided, such that common-mode feedback is avoided, and the power consumption is reduced. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.