Patent Publication Number: US-2021181242-A1

Title: Current sensor for biomedical measurements

Description:
FIELD OF THE PATENT APPLICATION 
     The present patent application generally relates to electronic circuits and more specifically to a current sensor for biomedical measurements. 
     BACKGROUND 
     In biomedical or electrochemical measurements, the parameters to be measured typically vary cross orders of magnitude. Also, the biomedical or electrochemical processes to be measured are typically highly non-linear. As a result, these measurements demand the measuring circuit, which is typically a current sensing circuit or a current sensor, to have a dynamic range as wide as possible. The nature of biomedical or electrochemical measurements also demands the measuring circuit to be essentially low noise so that the measurement resolution above an acceptable level can be achieved. However, conventional current sensing circuits generally suffer low dynamic range or high noise introduced by offset or feedback mechanisms present in those current sensing circuits. 
     SUMMARY 
     The present patent application is directed to a current sensor for biomedical measurements. In one aspect, the current sensor for biomedical measurements includes: a first amplifier; a first capacitor; a second capacitor; a first switch connected in parallel with the first capacitor; a second switch connected in parallel with the second capacitor; a second amplifier; a third capacitor; a resistor; and a switched capacitor network. The first capacitor and the second capacitor are connected in series and across a first input and output of the first amplifier. The third capacitor and the resistor are respectively connected across a first input and output of the second amplifier. The switched capacitor network is connected between the output of the first amplifier and the first input of the second amplifier. 
     The current sensor for biomedical measurements may further include: a first comparator; a second comparator; an OR gate; a first flip-flop; a second flip-flop; and a third flip-flop. A first input of the first comparator and a first input of the second comparator are connected with the output of the first amplifier. Outputs of the first comparator and the second comparator are connected to inputs of the OR gate respectively, and to clock ports of the first flip-flop and the second flip-flop. Output of the OR gate is connected to clock port of the third flip-flop. D port of each of the first flip-flop, the second flip-flop, and the third flip-flop are connected with  Q  port of the flip-flop. 
     The switched capacitor network may include a fourth capacitor, a fifth capacitor, a third switch connected in parallel with the fourth capacitor, and a fourth switch connected in parallel with the fifth capacitor, the fourth capacitor and the fifth capacitor being connected in series and connected between the output of the first amplifier and the first input of the second amplifier. 
     The first switch and the fourth switch may be controlled by a first clock; and the second switch and the third switch may be controlled by a second clock that is complementary to the first clock.  Q  port of the third flip-flop may be configured to transmit the first clock; and Q port of the third flip-flop may be configured to transmit the second clock. 
     A second input of the first amplifier and a second input of the second amplifier may be biased at a first reference voltage, a second input of the first comparator may be biased at a second reference voltage, a second input of the second comparator is biased at a third reference voltage, V 2 &gt;V 1  and V 2 =−V 3 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram of a portion of a current sensor for biomedical measurements in accordance with an embodiment of the present patent application. 
         FIG. 2  is a schematic circuit diagram of another portion of the current sensor for biomedical measurements as depicted in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to a preferred embodiment of the current sensor for biomedical measurements disclosed in the present patent application, examples of which are also provided in the following description. Exemplary embodiments of the current sensor for biomedical measurements disclosed in the present patent application are described in detail, although it will be apparent to those skilled in the relevant art that some features that are not particularly important to an understanding of the current sensor for biomedical measurements may not be shown for the sake of clarity. 
     Furthermore, it should be understood that the current sensor for biomedical measurements disclosed in the present patent application is not limited to the precise embodiments described below and that various changes and modifications thereof may be effected by one skilled in the art without departing from the spirit or scope of the protection. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure. 
       FIG. 1  is a schematic circuit diagram of a portion of a current sensor for biomedical measurements in accordance with an embodiment of the present patent application. Referring to  FIG. 1 , the current sensor for biomedical measurements includes a first amplifier  101 , a first capacitor  103 , a second capacitor  105 , a first switch  107  connected in parallel with the first capacitor  103 , a second switch  109  connected in parallel with the second capacitor  105 , a second amplifier  111 , a third capacitor  113 , a resistor  115 , and a switched capacitor network  120 . 
     The first capacitor  103  and the second capacitor  105  are connected in series and across a first input (IN) and the output (Vx) of the first amplifier  101 . The third capacitor  113  and the resistor  115  are respectively connected across a first input (Vy) and the output (OUT 1 ) of the second amplifier  111 . 
     The switched capacitor network  120  is connected between the output (Vx) of the first amplifier  101  and the first input (Vy) of the second amplifier  111 . The switched capacitor network  120  includes a fourth capacitor  121 , a fifth capacitor  123 , a third switch  125  connected in parallel with the fourth capacitor  121 , and a fourth switch  127  connected in parallel with the fifth capacitor  123 . The fourth capacitor  121  and the fifth capacitor  123  are connected in series and connected between the output (Vx) of the first amplifier  101  and the first input (Vy) of the second amplifier  111 . 
       FIG. 2  is a schematic circuit diagram of another portion of the current sensor for biomedical measurements as depicted in  FIG. 1 . Referring to  FIG. 2 , this portion of the current sensor circuit includes a first comparator  201 , a second comparator  203 , an OR gate  205 , a first flip-flop  207 , a second flip-flop  209 , and a third flip-flop  211 . A first input of the first comparator  201  and a first input of the second comparator  203  are connected with the output (Vx) of the first amplifier  101 . The outputs of the first comparator  201  and the second comparator  203  are connected to inputs of the OR gate  205  respectively, and to clock ports of the first flip-flop  207  and the second flip-flop  209 . The output of the OR gate  205  is connected to the clock port of the third flip-flop  211 . For each of the first flip-flop  207 , the second flip-flop  209 , and the third flip-flop  211 , D port of flip-flop is connected with port of the flip-flop. 
     In this embodiment, a second input of the first amplifier  101  and a second input of the second amplifier  111  are biased at a first reference voltage V 1 . A second input of the first comparator  201  is biased at a second reference voltage V 2 . A second input of the second comparator  203  is biased at a third reference voltage V 3 . In this embodiment, V 2 &gt;V 1  and V 2 =−V 3 . 
     The first switch  107  and the fourth switch  127  are controlled by a first clock A. The second switch  109  and the third switch  125  are controlled by a second clock B. The second clock B is complementary to the first clock A. In this embodiment,  Q  port (CLOCK A) of the third flip-flop  211  is configured to transmit the first clock A. Q port (CLOCK B) of the third flip-flop  211  is configured to transmit the second clock B. 
     When the first clock A is high (“1”), and the second clock B is low (“0”), the first switch  107  is closed while the second switch  109  is open. Therefore, the first capacitor  103  is reset while the second capacitor  105  is charging. In the same period, the fourth switch  127  is closed while the third switch  125  is open. Therefore, the fifth capacitor  123  is reset while the fourth capacitor  121  is charging. 
     When the first clock A is low (“0”), and the second clock B is high (“1”), the first switch  107  is open while the second switch  109  is closed. Therefore, the first capacitor  103  is charging while the second capacitor  105  is reset. In the same period, the fourth switch  127  is open while the third switch  125  is closed. Therefore, the fifth capacitor  123  is charging while the fourth capacitor  121  is reset. 
     In the aforementioned charge conserving configuration, electrical charges for charging the capacitors  103 ,  105 ,  121 ,  123  are locally provided instead of being provided by the amplifiers  101  and  111 . The operations of the capacitors are much faster than the settling time of the amplifiers. Therefore, reset transients and recovery time of the circuit are minimized. 
     The output (OUT 1 ) of the second amplifier  111  is a first output port of the current sensor for biomedical measurements, and is configured to output a voltage that is linearly related to the current I IN  at the first input (IN). More specifically, V OUT1 =V 1 +C 1 ·I IN , where C 1  is a constant determined by the first, second, fourth, fifth capacitors  103 ,  105 ,  121 ,  123  and the resistor  115 . 
     The Q port (OUT 2 ) of the first flip-flop  207  or the Q port (OUT 3 ) of the second flip-flop  209  is configured to output a digital signal with a frequency being proportional to the current I IN  at the first input (IN), depending on the direction of the current I IN . More specifically, the output (Vx) of the first amplifier  101  periodically increases linearly with time until it reaches V 2  or V 3 . When Vx reaches V 2  or V 3 , the first comparator  201  or the second comparator  203  is configured to output a digital “1”, which inverts the output at the ports CLOCK A, CLOCK B, and OUT 2  (or OUT 3 ) and resets Vx to zero. Within each period, the rate at which the output (Vx) of the first amplifier  101  increases with time is proportional to I IN , therefore, the frequency of the signal output by OUT 2  (or OUT 3 ) is proportional to I IN . The Q port (OUT 2 ) of the first flip-flop  207  and the Q port (OUT 3 ) of the second flip-flop  209  thus serve as a second and a third output ports of the current sensor for biomedical measurements. 
     In this embodiment, for the current I IN  that is relatively small and of higher frequency, the output (OUT 1 ) of the second amplifier  111 , as the first output port of the current sensor, provides a measurement of the current with relatively low noise. For a relatively large current I IN , the second or the third output port of the current sensor for biomedical measurements provides a frequency output that is proportional to the current I IN . Therefore, the dynamic range of the current sensor for biomedical measurements is greatly widened. In addition, the current sensor for biomedical measurements provided by the embodiment does not require any external reset clock or sample clock, and therefore bandwidth of the current sensor is not limited by any sample rate. 
     While the present patent application has been shown and described with particular references to a number of embodiments thereof, it should be noted that various other changes or modifications may be made without departing from the scope of the present invention.