Patent Publication Number: US-2022233126-A1

Title: Signal processing method and apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of U.S. patent application Ser. No. 16/182,786 filed on Nov. 7, 2018 which claims the benefit under 35 USC 119(a) of Korean Patent Application Nos. 10-2014-0060542 filed on May 20, 2014, and 10-2015-0017881 filed on Feb. 5, 2015, in the Korean Intellectual Property Office, the entire disclosures of which are incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     1. Field 
     The following description relates to signal processing technology for processing an input signal and generating an output signal. 
     2. Description of Related Art 
     An instrumentation amplifier (IA) is used to measure various signals. For example, in a medical field, the IA may be used to measure and amplify a biosignal such as an electrocardiogram (ECG), an electromyogram (EMG), a photoplethysmogram (PPG), a bioimpedance, a movement signal, or any other biosignal. 
     In general, the IA may be implemented using a differential amplifier having a low offset, low noise, a high common-mode rejection, a high loop gain, and a high input resistance. The IA may include a chopper circuit to modulate a measured signal into a signal of a high-frequency band or demodulate the modulated signal into a signal of a low-frequency band. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     In one general aspect, a signal processing apparatus includes an input voltage selector configured to select an input voltage from a plurality of input voltages; an input element connected to the input voltage selector; and an input current controller configured to control an inflow of an input current in conjunction with an operation of the input voltage selector. 
     The signal processing apparatus may be configured to be operable in a voltage measuring mode and a current measuring mode. 
     The input voltage selector may be configured to apply a preset reference voltage to the input element in the current measuring mode, and apply an input voltage to be measured to the input element in the voltage measuring mode. 
     The preset reference voltage may have a fixed voltage level or a variable voltage level. 
     The input current controller may be configured to allow the inflow of the input current into the signal processing apparatus in the current measuring mode, and block the inflow of the input current in the voltage measuring mode. 
     The input element may be configured to generate a current in response to the selected input voltage in the voltage measuring mode. 
     The input current controller may include an input current compensator configured to output a compensation current to adjust a level of the input current in the current measuring mode. 
     The apparatus may further include an input current compensation controller configured to control a level of the compensation current output by the input current compensator based on an output signal of the signal processing apparatus. 
     The apparatus may further include a low-pass filter configured to extract a signal of a low-frequency band from an output signal of the signal processing apparatus and output the signal of the low-frequency band; a comparator configured to compare a level of the signal output by the low-pass filter to a level of a preset reference signal and output a signal indicating a result of the comparing; and an input current compensation controller configured to generate a control signal to control the input current compensator based on the signal output by the comparator. 
     The input current controller may include a chopper configured to modulate a frequency component of the input current based on a control signal in the current measuring mode. 
     The may further include a load element functionally connected to the input element and the input current controller. 
     An output end of the input current controller may be connected to both ends of the load element, and one end of the input element may be connected to one of the ends of the load element. 
     In another general aspect, a signal processing apparatus includes a voltage input circuit configured to receive an input voltage; a current input circuit configured to receive an input current; an amplifier configured to amplify one of the input voltage and the input current at a time; and a controller configured to control a connection between the voltage input circuit and the amplifier, and control a connection between the current input circuit and the amplifier. 
     The controller may be further configured to block the connection between the current input circuit and the amplifier and connect the voltage input circuit to the amplifier in a voltage measuring mode. 
     The controller may be further configured to block the connection between the voltage input circuit and the amplifier and connect the current input circuit to the amplifier in a current measuring mode. 
     The current input circuit may be further configured to adjust a level of the input current in response to the level of the input current being greater than a preset value and output the input current having the adjusted level in a current measuring mode. 
     The input voltage may be an electrocardiogram (ECG) signal, and the input current may be a photoplethysmogram (PPG) signal. 
     In another general aspect, a signal processing apparatus includes a light source unit configured to output a light signal to a body of a user; a light detector configured to detect a first biosignal measured based on the light signal; a bioelectrode configured to detect a second biosignal measured based on a voltage signal; a signal processor configured to amplify the first biosignal or the second biosignal based on a measurement mode; and a controller configured to control the measurement mode of the signal processor. 
     The signal processor may be further configured to be operable in a current measuring mode and a voltage measuring mode; and the controller may be further configured to control the signal processor to amplify the first biosignal in the current measuring mode, and control the signal processor to amplify the second biosignal in the voltage measuring mode. 
     The first biosignal may be a photoplethysmogram (PPG) signal, and the second biosignal may be an electrocardiogram (ECG) signal. 
     The light source unit may include a plurality of light sources; and the controller may be further configured to sequentially activate at least one light source among the light sources during each of a plurality of time intervals, and determine a light source for measuring the first biosignal from the light sources based on a level of a signal output from the light detector during each of the time intervals. 
     In another general aspect, a signal processing method includes selecting a voltage measuring mode or a current measuring mode; and controlling a type of an input voltage selected and an inflow of an input current based on a result of the selecting. 
     The controlling may include blocking the inflow of the input current and selecting a voltage to be measured as the input voltage in response to the voltage measuring mode being selected. 
     The controlling may include selecting a preset reference voltage as the input voltage and allowing the inflow of the input current in response to the current measuring mode being selected. 
     The controlling may include adjusting a level of the input current to be within a preset range. 
     In another general aspect, a non-transitory computer-readable storage medium stores instructions to cause a computer to perform the method described above. 
     In another general aspect, a signal processing apparatus includes an amplifier configured to receive a plurality of input voltages and an input current and including an amplifying circuit configured to amplify both a voltage and a current; and a controller configured to control the amplifier to amplify a selected one of the input voltages and the input current using the amplifying circuit. 
     The input voltages may include a voltage to be measured and a reference voltage; and the controller may be further configured to control the amplifier to block the input current from being applied to the amplifying circuit and apply the voltage to be measured to the amplifying circuit to measure the voltage to be measured in a voltage measuring mode, and apply the reference voltage and the input current to the amplifying circuit to measure the input current in a current measuring mode. 
     The amplifier may include a first chopper configured to selectively modulate the input current; the signal processing apparatus may further include a second chopper configured to selectively modulate the voltage to be measured, and a third chopper configured to selectively demodulate an output signal of the amplifier; and the controller may be further configured to control the first chopper to block the input current from being applied to the amplifying circuit in the voltage measuring mode, and apply the input current to the amplifying circuit in the current measuring mode. 
     The amplifying circuit may include a transconductance amplifier configured to output a current obtained by amplifying the selected one of the input voltages and the input current; and the signal processing apparatus may further include a transimpedance amplifier configured to amplify the output current of the transconductance amplifier and output a voltage obtained by amplifying the output current. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of an overall operation performed by a signal processing apparatus. 
         FIGS. 2 and 3  are diagrams illustrating an example of a circuit for implementing a signal processing apparatus. 
         FIG. 4  is a diagram illustrating another example of a circuit for implementing a signal processing apparatus. 
         FIG. 5  is a diagram illustrating an example of a signal processing apparatus operating in a voltage measuring mode. 
         FIGS. 6A and 6B  are diagrams illustrating examples of operating an input chopper based on a control signal. 
         FIGS. 7A through 7C  are diagrams illustrating examples of a control signal input for each chopper in a voltage measuring mode. 
         FIGS. 8 and 9  are diagrams illustrating examples of a signal processing apparatus operating in a current measuring mode. 
         FIGS. 10A through 11C  are diagrams illustrating examples of a control signal input for each chopper in a current measuring mode. 
         FIG. 12  is a diagram illustrating an example of an operation performed by a signal processing apparatus in an automatic adjustment mode for adjusting an input current level. 
         FIG. 13  is a diagram illustrating an example of a configuration of a transimpedance stage. 
         FIG. 14  is a flowchart illustrating an example of a signal processing method. 
         FIGS. 15 through 16B  are diagrams illustrating examples of a wearable device including a signal processing apparatus. 
         FIG. 17  is a diagram illustrating an example of an optical sensor. 
         FIG. 18  is a diagram illustrating an example of a signal processing apparatus. 
     
    
    
     Throughout the drawings and the detailed description, the same reference numerals refer to the same elements. The drawings may not be to scale, and the relative size, proportions, and depiction of elements in the drawings may be exaggerated for clarity, illustration, and convenience. 
     DETAILED DESCRIPTION 
     The following detailed description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses, and/or systems described herein. However, various changes, modifications, and equivalents of the methods, apparatuses, and/or systems described herein will be apparent to one of ordinary skill in the art. The sequences of operations described herein are merely examples, and are not limited to those set forth herein, but may be changed as will be apparent to one of ordinary skill in the art, with the exception of operations necessarily occurring in a certain order. Also, descriptions of functions and constructions that are well known to one of ordinary skill in the art may be omitted for increased clarity and conciseness. 
     The features described herein may be embodied in different forms, and are not to be construed as being limited to the examples described herein. Rather, the examples described herein have been provided so that this disclosure will be thorough and complete, and will convey the full scope of the disclosure to one of ordinary skill in the art. 
       FIG. 1  is a diagram illustrating an example of an overall operation performed by a signal processing apparatus  100 . 
     The signal processing apparatus  100  measures a current or a voltage for each channel in a multichannel environment. The signal processing apparatus  100  may operate in a voltage measuring mode, a current measuring mode, and a combination mode. The combination mode is a measurement mode that changes between the voltage measuring mode and the current measuring mode in response to a control signal. The signal processing apparatus  100  measures an input voltage and an input current using a single integral circuit configuration. The signal processing apparatus  100  provides compatibility between a voltage measurement and a current measurement by controlling processing of the input voltage and the input current input to a measurement circuit based on a control signal. 
     In one example, the signal processing apparatus  100  is included in an instrumentation amplifier (IA), and the IA uses the signal processing apparatus  100  to selectively measure the voltage and the current. For example, the IA including the signal processing apparatus  100  may amplify a voltage measurement-based biosignal such as an electrocardiogram (ECG), or a current measurement-based biosignal such as a photoplethysmogram (PPG). The signal processing apparatus  100  measures the ECG in the voltage measuring mode, and measures the PPG in the current measuring mode. 
     Referring to  FIG. 1 , the signal processing apparatus  100  includes a current input circuit  110 , a voltage input circuit  120 , a controller  130 , and an amplifier  140 . 
     The voltage input circuit  120  receives an input voltage. The input voltage may be, for example, an ECG signal measured from a body of a user. The current input circuit  110  receives an input current. As an example, the input current may be a biosignal including a blood oxygen saturation level, for example, peripheral capillary oxygen saturation (SpO 2 ), or a PPG signal measured from the body of the user. 
     The amplifier  140  amplifies either the input voltage or the input current under a control of the controller  130 . The amplifier  140  amplifies the input voltage output from the voltage input circuit  120  or the input current output from the current input circuit  110  based on a measurement mode. The measurement mode is selected from the voltage measuring mode for measuring the input voltage and the current measuring mode for measuring the input current. 
     The controller  130  controls a connection between the voltage input circuit  120  and the amplifier  140  and a connection between the current input circuit  110  and the amplifier  140 . For example, the controller  130  performs a switching operation to select a signal to be applied to the amplifier  140  from the input current and the input voltage. In the voltage measuring mode, the controller  130  blocks the connection between the current input circuit  110  and the amplifier  140 , and connects the voltage input circuit  120  to the amplifier  140 . Accordingly, the input current received by the current input circuit  110  is not input to the amplifier  140 , and the input voltage output from the voltage input circuit  120  is input to the amplifier  140  to be amplified. 
     In the current measuring mode, the controller  130  blocks the connection between the voltage input circuit  120  and the amplifier  140 , and connects the current input circuit  110  to the amplifier  140 . Accordingly, the input voltage received by the voltage input circuit  120  is not input to the amplifier  140 , and the input current output from the current input circuit  110  is input to the amplifier  140  to be amplified. 
     In the current measuring mode, when a level of the input current is greater than a preset value, the current input circuit  110  may adjust the level of the input current so that the level of the input current is less than the present value. The current input circuit  110  may generate a compensation current to adjust the level of the input current. The compensation current may compensate the input current so that the level of the input current is adjusted to be within an operational range of the signal processing apparatus  100 . 
     Hereinafter, the operation of the signal processing apparatus  100  will be explained in detail. 
       FIGS. 2 and 3  are diagrams illustrating an example of a circuit for implementing a signal processing apparatus  200 . 
     Referring to  FIGS. 2 and 3 , the signal processing apparatus  200  measures an input voltage and an input current using a single integral circuit configuration. The signal processing apparatuses  100  and  200  control an input voltage and an input current input to a measuring circuit based on a control signal.  FIG. 2  illustrates a switching connection performed by the signal processing apparatus  200  in a voltage measuring mode.  FIG. 3  illustrates a switching connection performed by the signal processing apparatus  200  in a current measuring mode. 
     Hereinafter, the signal processing apparatus  200  operating in the voltage measuring mode and the signal processing apparatus  200  operating in the current measuring mode will be explained separately. 
     In the voltage measuring mode, the signal processing apparatus  200  operates as described below. 
     Referring to  FIG. 2 , when the signal processing apparatus  200  operates in the voltage measuring mode, the signal processing apparatus  200  controls switches SW_I ip    252 , SW_I in    254 , SW_I op    256 , and SW_I on    258  configured to control a flow of input currents I p  and I n  in the signal processing apparatus  200  to be opened. The input currents I p  and I n  actually represent a single input current that flows into the signal processing apparatus  200  as the input current I p , and flows out of the signal processing apparatus  200  as the input current I n . Alternatively, the single input current may flow into the signal processing apparatus  200  as the input current I n , and flow out of the signal processing apparatus  200  as the input current I p . 
     The signal processing apparatus  200  controls switches SW_V ip    232  and SW_V in    234  configured to control a flow of input voltages V ip  and V in  to be measured in the signal processing apparatus  200  to be closed so that an output end of a second chopper  230  is connected to an input end of an input element  240 . The signal processing apparatus  200  may control the second chopper  230  to output the input voltages V ip  and V in  directly, or to output the input voltages V ip  and V in  after converting a frequency component of the input voltages V ip  and V in . For example, the second chopper  230  may convert the frequency component of the input voltages V ip  and V in  to have a frequency higher or lower than an original frequency of the frequency component of the input voltages V ip  and V in . In one example, the input element  240  may be a voltage-current converter or a transistor. The input element  240  outputs a current based on a differential voltage between the input voltages V ip  and V in . 
     The current output I v  from the input element  240  causes a current I i  to flow through an input resistor  250 , for example, R i , constituting a load element. A current mirror  260  mirrors the current I i  flowing through the input resistor  250  to output a current I o  having a same level or an amplified level relative to the current I i . The current I o  output from the current mirror  260  generates voltages at both ends of an output resistor  280 , for example, R o . Based on the generated voltages, output voltages V op  and V on  are output from output terminals of the signal processing apparatus  200 . 
     In another example, the current I o  output from the current mirror  260  is input to a third chopper  270 , and the third chopper  270  converts a frequency component of the current I o . For example, the third chopper  270  may perform a frequency demodulation on the current I o  having a frequency component that has been converted by the second chopper  230 . 
     In one example, when a control signal (not shown) used to control a first chopper  220 , the second chopper  230 , or the third chopper  270  remains in a logic state High or a logic state Low, an input signal of a chopper controlled by the control signal is directly output without the frequency conversion being performed on the input signal. When the control signal used to control the first chopper  220 , the second chopper  230 , or the third chopper  270  is repetitively changed between the logic state High and the logic state Low as time elapses, the input signal of the chopper is output after the frequency conversion is performed on the input signal. 
     In another example, the signal processing apparatus  200  may also include an element to decrease an output impedance of the signal processing apparatus  200 . For example, the signal processing apparatus  200  may also include a buffer (not shown) connected to the output terminals of the signal processing apparatus  200  to decrease the output impedance. 
     In the current measuring mode, the signal processing apparatus  200  operates as described below. 
     Referring to  FIG. 3 , when the signal processing apparatus  200  operates in the current measuring mode, the signal processing apparatus  200  controls an input voltage input to the signal processing apparatus  100  to not be processed. The signal processing apparatus  200  controls the switches SW_V ip    232  and SW_V in    234  that are used to connect inputs of the input element  240  to the input voltages V ip  and V in  in the voltage measuring mode so that the inputs of the input element  240  are connected to reference voltages V ip_b  and V in_b  in the current measuring mode. The reference voltages V ip_b  and V in_b  may have the same voltage level as each other or different voltage levels from each another. Also, the reference voltages V ip_b  and V in _ b  may have a fixed voltage level or a variable voltage level as time elapses. 
     When the input currents I i_p  and I in  need to be amplified by a relatively high amplification level, the signal processing apparatus  200  controls the switch SW_I ip    252  controlling the flow of the input current lip and the switch SW_I In    254  controlling the flow of the input current I in  to be closed to allow the input currents I in  and lip to flow through the input resistor  250 , for example, R i . Also, the signal processing apparatus  200  controls switches SW_I op    256  and SW_I o    258  to be opened to prevent the input currents I in  and lip from flowing through the output resistor  280 , for example, R o . 
     When the input currents lip and I in  need to be amplified by a relatively low level, the signal processing apparatus  200  controls the switches SW_I ip    252  and SW_I In    254  connected to opposite ends of the input resistor  250  to be opened to prevent the input currents I in  and lip from flowing through the input resistor  250 , and controls the switches SW_I op    256  and SW_I o    258  connected to opposite ends of the output resistor  280  to be closed to allow the input currents I in  and lip to flow through the output resistor  280 . 
     In one example, when the input current to be measured exceeds an operational range of the signal processing circuit  200 , the signal processing apparatus  200  may control a level of the current flowing in the signal processing apparatus  200  to be within the operational range using an input current compensator  210 . For example, when an input current to be measured includes a direct current (DC) current of 100 microamperes (μA) and an alternating current (AC) current of 1 μA, and a maximum level of current allowed to flow through the input resistor  250  is 10 μA, the input current compensator  210  outputs a compensation current to flow in the circuit to offset the DC current of 100 μA. Due to the compensation current output by the input current compensator  210 , only the AC current of 1 μA flows through the input resistor  250 . 
     To minimize noise flowing into the signal processing apparatus  200  in a process of amplifying the input current to be measured, the signal processing apparatus  200  may modulate the frequency component of the input current using the first chopper  220 , and demodulate the frequency component using the third chopper  270 . Alternatively, in response to a modulated frequency component of the input current to be measured, the signal processing apparatus  200  may demodulate a modulated signal using the third chopper  270 . 
       FIG. 4  is a diagram illustrating another example of a circuit for implementing a signal processing apparatus  400 . 
     Referring to  FIG. 4 , the signal processing apparatus  400  includes an input voltage selector  410 , an input current controller  420 , an input element  430 , and a load element  440 . The signal processing apparatus  400  is operable in a voltage measuring mode and a current measuring mode. In one example, the signal processing apparatus  400  corresponds to a transconductance stage of an IA. 
     The input voltage selector  410  selects an input voltage from a plurality of input voltages. In the example in  FIG. 4 , the plurality of input voltages include a target voltage to be measured and a preset reference voltage. For example, a biosignal such as an ECG may be input as the target voltage to be measured. In  FIG. 4 , voltages V_TC_ip 2  and V_TC_in 2  are the target voltage to be measured, and voltages Vref_TCp and Vref_TCn are the preset reference voltage. The voltages Vref_TCp and Vref_TCn may have the same voltage level as each other or different voltage levels from each another. Also, the voltages Vref_TCp and Vref_TCn may have a fixed voltage level or a variable voltage level as time elapses. 
     The input voltage selector  410  is connected to the input element  430 . In the example in  FIG. 4 , the input element  430  is a transistor having a gate connected to the input voltage selector  410 , but is not limited thereto. The input voltage selector  410  selects an input voltage to be applied to the input element  430  using a switch. The input element  430  generates a current in response to the input voltage selected by the input voltage selector  410  in the voltage measuring mode, and thus is a transconductance element. For example, the input element  430  may be a transistor and include a plurality of transconductance elements. Thus, the input element  430  converts the input voltage to be measured into a current. The input voltage to be applied to the input element  430  is determined by a control signal. In  FIG. 4 , REG_TC_SEL_ip and REG_TC_SEL_in are control signals that control the switching operation performed by the input voltage selector  410 . A source of the input element  430  is connected to a drain of a transistor PTOP, which forms a cascode circuit with the input element  430 . A drain of the input element  430  is biased by a constant-current source providing a fixed current ib 2 . A source of the transistor PTOP is connected to a power supply voltage VDD. A gate of the transistor PTOP is biased by a voltage Voffset 2 . 
     The input current controller  420  controls an inflow of an input current in conjunction with an operation of the input voltage selector  410 . For example, the input current controller  420  blocks the inflow of the input current when an input voltage input to the signal processing apparatus  400  is the target voltage to be measured by the input voltage selector  410 . The input current to be measured is input to the signal processing apparatus  400  by the input current controller  420 . For example, a biosignal such as a PPG may be input to the signal processing apparatus  400  as the input current. The input current controller  420  controls the inflow of the input current using a switch controlled by the control signal. The input current controller  420  allows the input current to flow into the signal processing apparatus  400  or block the inflow of the input current based on a measurement mode. In  FIG. 4 , V_Current_ip and V_Current_in are the input current to be measured, and REG_TC_ch_discon is the control signal controlling the switching operation of the input current controller  420 . 
     The load element  440  is functionally connected to the input current controller  420  and the input element  430 . For example, output terminals of the input current controller  420  are connected to opposite ends of the load element  440 , and one end of the input element  430  is connected to the load element  440 . In this example, “being functionally connected” includes “being directly connected to the input current controller  420  and the input element  430 ” and “being affected by the input current controller  420  and the input element  430 ”. For example, the load element  440  may be a resistor having a resistance Ri 2 . A current flowing through the ends of the load element  440  may be changed due to the current flowing through the input element  430  or the input current flowing through the input current controller  420 . 
     In another example, the input current controller  420  may also include an input current compensator  450  to adjust a level of the input current. The input current compensator  450  generates a compensation current to adjust the level of the input current. When a level of an input current to be measured exceeds an operational range of the signal processing apparatus  400 , the input current compensator  450  a compensation current to offset the level of the input current to be within the operational range of the signal processing apparatus  400 . 
     For example, the input current compensator  450  outputs the compensation current to decrease a DC level of the input current to be within a preset range. To decrease the DC level of the input current, the input current compensator  450  outputs a compensation current having a negative DC level, for example, −3 milliamperes (mA). The level of the input current is adjusted by adding the input current flowing into the signal processing apparatus  400  and the compensation current output by the input current compensator  450 . 
     The input current compensator  450  may operate in a manual adjustment mode and an automatic adjustment mode. For example, in the manual adjustment mode, the input current compensator  450  manually adjusts the level of the input current under a control of a user. In the automatic adjustment mode, the input current compensator  450  adaptively adjusts the level of the input current based on an output signal of the signal processing apparatus  400 . For example, in the automatic adjustment mode, the input current compensator  450  may extract a signal of a low-frequency band from the output signal of the signal processing apparatus  400 , and determine whether the level of the input current is to be adjusted based on a level of the extracted signal of the low-frequency band. Based on a result of the determining, the input current compensator  450  determines a level of the compensation current to be used to adjust the level of the input current, and adjusts the level of the input current flowing in the signal processing apparatus  400  by outputting the compensation current having the determined level. 
     In the automatic adjustment mode, the signal processing apparatus  400  may also include an input current compensation controller (not shown) to generate a control signal used to control the input current compensator  450 . The input current compensation controller controls the level of the compensation current output by the input current compensator  450  based on the output signal of the signal processing apparatus  400 . In one example, the signal processing apparatus  400  includes a low-pass filter (not shown) to output the signal of the low-frequency band from the output signal of the signal processing apparatus  400 , and a comparator (not shown) to compare a level of an output signal of the low-pass filter to a level of a preset reference signal and output a result of the comparing to the input current compensation controller. The input current compensation controller generates the control signal to control the input current compensator  450  based on the result of the comparing. A detailed explanation of the input current compensation controller will be provided with reference to  FIG. 12 . 
     In another example, the input current controller  420  also includes a chopper  460  to modulate a frequency component of the input current based on a control signal. The chopper  460  changes connections between input ends and output ends of the input current controller  420  based on the control signal. Low-frequency noise, 1/f noise, or flicker noise may be reduced when the chopper  460  performs the modulation on the frequency component of the input current. For example, the low-frequency noise occurring in the signal of the low-frequency band may be reduced when the input current is modulated into a signal of a high-frequency band by the chopper  460 . When the chopper  460  does not perform the above modulation, the chopper  460  operates as a switch to allow and block the inflow of the input current. 
     Hereinafter, the signal processing apparatus  400  operating in the voltage measuring mode and the signal processing apparatus  400  operating in the current measuring mode will be described. 
     In the voltage measuring mode, the signal processing apparatus  400  operates as described below. 
     In the voltage measuring mode, the signal apparatus  400  blocks the inflow of the input current and selects a voltage to be measured as the input voltage. 
     The input voltage selector  410  applies an input voltage to be measured to the input element  430 . For example, the input current selector  410  applies an input voltage V_TC_ip 2  to be measured to a gate node V_Gp of one input element  430 , and applies an input voltage V_TC_in 2  to be measured to a gate node V_Gn of the other input element  430 . The input voltage V_TC_ip 2  and the input voltage V_TC_in 2  have a differential input relationship. When control signals REG_TC_SEL_ip and REG_TC_SEL_in have a logically high value, for example, H, the input voltage V_TC_ip 2  is applied to the gate node V_Gp and the input voltage V_TC_in 2  is applied to the gate node V_Gn so that the input voltage to be measured is input to the signal processing apparatus  400 . 
     Due to the input voltages V_TC_ip 2  and V_TC_in 2  having the differential input relationship, the current flowing through the ends of the load element  440  may be changed. 
     In response to the applied input voltage, one input element  430  generates a current flowing from a source node V_Sp to a drain node V_Dp, and another input element  430  generates a current flowing from a source node V_Sn to a drain node V_Dn. The drain node V_Dp is connected to an output node V_TC_op 2  of the signal processing apparatus  400 , and the drain node V_Dn is connected to an output node V_TC_on 2  of the signal processing apparatus  400 . 
     The input current controller  420  blocks the inflow of the input current into the signal processing apparatus  400  in the voltage measuring mode. For example, when a control signal REG_TC_ch_discon has a logically high value, switches (not shown) included in the chopper  460  of the input current controller  420  are opened so that the input current does not flow into the ends of the load element  440 . 
     In the current measuring mode, the signal processing apparatus  400  operates as described below. 
     In the current measuring mode, the signal processing apparatus  400  allows the inflow of the input current to be measured into the signal processing apparatus  400 , and selects the preset reference voltage as the input voltage in lieu of the voltage to be measured. 
     The input voltage selector  410  applies a reference voltage having a fixed voltage level to the input element  430 . For example, when the control signal REG_TC_SEL_in has a logically low value, for example, L, the input voltage selector  410  applies a reference voltage Vref_TCp having the fixed voltage level to the gate node V_Gp of one input element  430 , and applies a reference voltage Vref_TCn having the fixed voltage level to the gate node V_Gn of the other input element  430 . 
     Since a fixed current ib 2  flows through each of the input elements  430 , a constant voltage level may be maintained at each of the source nodes V_Sp and V_Sn of the input elements  430 . By adjusting a level of the reference voltage Vref_TCp, a voltage having a desired level may be provided to the source node V_Sp, and by adjusting a level of the reference voltage Vref_TCn, a voltage having a desired level may be provided to the source node V_Sn. 
     The input current controller  420  allows the inflow of the input current to be measured into the signal processing apparatus  400 . For example, the input current controller  420  allows an inflow of input currents V_Current_ip and V_Current_in to be measured. The input current V_Current_ip and the input current V_Current_in may have a differential input relationship. When the control signal REG_TC_ch_discon has a logically low value, the switches included in the chopper  460  are connected so that the input end and the output end are connected in the chopper  460 . 
     Due to the input currents V_TC_ip 2  and V_TC_in 2  having the differential input relationship, the current flowing through the ends of the load element  440  may be changed. 
     Based on the control signal REG_TC_ch_discon, the chopper  460  may operate as a switch connecting the input end of the chopper  460  with the output end of the chopper  460  and modulate the frequency component of the input current through a periodic switching operation. 
     In another example, the input current controller  420  may also include the input current compensator  450  to adjust a level of the input current. When the level of the input current to be measured is beyond an operational range of the signal processing apparatus  400 , the input current compensator  450  outputs a compensation current to offset the level of the input current to be within the operational range of the signal processing apparatus  400 . Based on the compensation current, the input current is changed into a signal having a current level that can be handled by elements of the signal processing apparatus  400 . 
       FIG. 5  is a diagram illustrating an example of a signal processing apparatus  500  operating in a voltage measuring mode. 
     Referring to  FIG. 5 , the signal processing apparatus  500  includes a transconductance stage  510  and a transimpedance stage  520 . The transconductance stage  510  corresponds to the signal processing apparatus  400  of  FIG. 4 . The transconductance stage  510  may operate in either the voltage measuring mode or a current measuring mode. The transconductance stage  510  controls an inflow of an input current and selects a type of an input voltage to be applied based on a measuring mode. 
     In  FIG. 5 , the signal processing apparatus  500  measures an ECG in the voltage measuring mode. ECG_ip and ECG_in are ECG signals measured by a sensor  550  such as a bioelectrode. In the voltage measuring mode, the transconductance stage  510  selects a voltage to be measured as an input voltage and blocks the inflow of the input current. For example, in the voltage measuring mode, the transconductance stage  510  selects input voltages V_TC_ip 2  and V_TC_in 2  to be measured to be applied to an internal input element, and block inflows of input currents V_Current_ip and V_Current_in. The transconductance stage  510  selects the type of the input voltage based on a control signal REG_TC_SEL_in. The transconductance stage  510  controls a switch used to determine whether the input current is allowed to flow based on a control signal REG_TC_ch_discon. 
     The transimpedance stage  520  generates output voltages V_TI_op 2  and V_TI_on 2  based on a current output from the transconductance stage  510 . The transimpedance stage  520  may include a current mirror circuit. 
     The signal processing apparatus  500  may also include an input chopper  530  to modulate a frequency component of the input voltage of the transconductance stage  510  in the voltage measuring mode. The input chopper  530  modulates the input voltage into a signal of a high-frequency band, thereby reducing low-frequency noise. A control signal REG_input_ch_discon determines whether the input chopper  530  operates. A control signal f_ch_i controls a connection of each switch included in the input chopper  530 . 
     The transimpedance stage  520  includes an output chopper  540  to demodulate into a signal of a low-frequency band the input signal that has been modulated into the signal of the high-frequency band by the input chopper  530 . A control signal f_ch_o controls a connection of each switch included in the output chopper  540 . In one example, the control signal f_ch_o applied to the output chopper  540  has the same signal waveform as the control signal f_ch_i applied to the input chopper  530 . 
       FIGS. 6A and 6B  are diagrams illustrating examples of operating an input chopper based on a control signal. 
     The control signal REG_input_ch_discon determines whether the input chopper operates. For example, when the control signal REG_input_ch_discon is in a logically high state H as shown in  FIG. 6A , all switches included in the input chopper are turned off so that a connection between an input end and an output end of the input chopper is blocked. Conversely, the input chopper operates when the control signal REG_input_ch_discon is in a logically low state L as shown in  FIG. 6B . When the input chopper operates, operations of the switches included in the input chopper are controlled based on a control signal f_input_ch. A control signal f_input_ch_bar (not shown) that is an inverse waveform of the control signal f_input_ch may be generated based on the control signal f_input_ch. Connections among the switches included in the input chopper may be changed based on the control signal f_input_ch and the control signal f_input_ch_bar. 
       FIGS. 7A through 7C  are diagrams illustrating examples of a control signal input for each chopper in a voltage measuring mode. 
     An example of a control signal f_ch_i applied to an input chopper is illustrated in  FIG. 7A . An example of a control signal f_ch_o applied to an output chopper is illustrated in  FIG. 7B . An example of a control signal f_TC_ch applied to a chopper included in an input current controller is illustrated in  FIG. 7C . In the voltage measuring mode, an input voltage is modulated into a signal of a high-frequency band by the input chopper, and the modulated input voltage is demodulated into a signal of a low-frequency band by the output chopper. In the voltage measuring mode, connections of switches included in the chopper of the input current controller are blocked, so the control signal f_TC_ch is in a logically low state. 
       FIG. 8  is a diagram illustrating an example of a signal processing apparatus  800  operating in a current measuring mode.  FIG. 9  is a diagram illustrating an example of a signal processing apparatus  900  operating in a current measuring mode. 
     In  FIG. 8 , the signal processing apparatus  800  measures a photovoltaic mode PPG in the current measuring mode. Currents V_Current_ip and V_Current_in output from a light detector  810  of  FIG. 8  are input to the transconductance stage  510 . 
     In  FIG. 9 , the signal processing apparatus  900  measures a photoconductive mode PPG in the current measuring mode. A current V_Current_ip output from a light detector  910  of  FIG. 9  is input to the transconductance stage  510 . 
     Referring to  FIGS. 8 and 9 , in the current measuring mode, the transconductance stage  510  applies reference voltages Vref_TCp and Vref_TCn, each preset as an input voltage, to internal input elements of the transconductance stage  510 . Based on a control signal REG_TC_SEL_in, the input voltage applied to an input element is determined and input voltages V_TC_in 2  and V_TC_ip 2  input to the transconductance stage  510  may be blocked. A control signal REG_TC_ch_discon is used to determine whether an input current is allowed to flow into the transconductance stage  510 . A control signal f_TC_ch is used to control a chopper included in the transconductance stage  510 . Through switching operations of switches included in the chopper, the input current may be modulated into a signal of a high-frequency band. 
     The transimpedance stage  520  includes the output chopper  540  to demodulate into a low-frequency band an input signal that has been modulated into the signal of the high-frequency band by the chopper included in the transconductance stage  510 . A control signal f_ch_o is used to control a connection of each switch included in the output chopper  540 . In one example, the control signal f_ch_o applied to the output chopper  540  has the same waveform as a control signal f_TC_ch applied to the chopper of the transconductance stage  510 . 
       FIGS. 10A through 11C  are diagrams illustrating examples of a control signal input for each chopper in a current measuring mode. 
     Examples of a control signal applied to each chopper when the chopper is operated in the current measuring mode are illustrated in  FIGS. 10A through 10C . 
     An example of control signal f_ch_i applied to an input chopper is shown in  FIG. 10A . An example of control signal f_ch_o applied to an output chopper is shown in  FIG. 10B . An example of a control signal f_TC_ch applied to a chopper included in a transconductance stage is shown in  FIG. 10C . In the current measuring mode, an input current is modulated into a signal of a high-frequency band by the chopper of the transconductance stage, and the modulated input current is demodulated into a signal of a low-frequency band by the output chopper. In a current measuring mode, the input chopper is not used, so the control signal f_ch_i is in a logically low state. 
     In one example, when the control signal f_TC_ch is in a logically high state, a first input end is connected to a first output end and a second input end is connected to a second output end in the chopper included in the transconductance stage, and when the control signal f_TC_ch is in the logically low state, the first input end is connected to the second output end and the second input end is connected to the first output end in the chopper included in the transconductance stage. 
     Examples of a control signal applied to each chopper when the chopper is not operated in the current measuring mode are illustrated in  FIGS. 11A through 11C . 
     An example of a control signal f_ch_i applied to an input chopper is shown in  FIG. 11A . An example of a control signal f_ch_o applied to an output chopper is shown in  FIG. 11B . An example of a control signal f_TC_ch applied to a chopper included in a transconductance stage is shown in  FIG. 11C . In one example, when the chopper is not operated, each of the control signal f_ch_i, the control signal f_ch_o, and the control signal f_TC_ch are in a logically low state. 
       FIG. 12  is a diagram illustrating an example of an operation performed by a signal processing apparatus  1200  in an automatic adjustment mode for adjusting an input current level. 
     Referring to  FIG. 12 , the signal processing apparatus  1200  includes the input current compensator  210  to adjust the input current level in a current measuring mode. In one example, the signal processing apparatus  1200  also includes a control circuit  1210  to control the input current compensator  210  based on an output signal of the signal processing apparatus  1200 . The control circuit  1210  includes an input current compensation controller  1250 , a low-pass filter  1220 , a first comparator  1230 , and a second comparator  1240 . 
     In one example, an output end of the signal processing apparatus  1200  is connected to an input end of the low-pass filter  1220 , and an output end of the low-pass filter  1220  is connected to an input end of each of the first comparator  1230  and the second comparator  1240 . An output end of each of the first comparator  1230  and the second comparator  1240  is connected to an input end of the input current compensation controller  1250 , and an output end of the input current compensation controller  1250  is connected to the input current compensator  210 . 
     The low-pass filter  1220  extracts a signal of a low-frequency band from output signals V op  and V on  of the signal processing apparatus  1200 . For example, the signal processing apparatus  1200  may extract a signal of a frequency band less than or equal to 0.1 hertz (Hz) from the output signal of the signal processing apparatus  1200  using the low-pass filter  1220 . A signal V_LPF of the low-frequency band output from the low-pass filter  1220  is input to the first comparator  1230  and the second comparator  1240 . 
     The first comparator  1230  compares the signal V_LPF of the low-frequency band to a control signal Vlimit_H, and outputs a signal V comp_H  indicating a result of the comparing. The second comparator  1240  compares the signal V_LPF of the low-frequency band to a control signal Vlimit_L, and outputs a signal V comp_L  indicating a result of the comparing. An upper limit of a voltage range of the extracted signal of the low-frequency band is set by the control signal Vlimit_H, and a lower limit of the voltage range of the extracted signal of the low-frequency band is set by the control signal Vlimit_L. 
     In one example, when the signal V_LPF output from the low-pass filter  1220  is in a range between the control signal Vlimit_L and the control signal Vlimit_H, for example, Vlimit_L&lt;V_LPF&lt;Vlimit_H, the signal V comp_H  output from the first comparator  1230  and the signal V comp_L  output from the second comparator  1240  have a logically low value. In this example, the control signal output from the input current compensation controller  1250  to the input current compensator  210  remains the same as a previous control signal output from the input current compensation controller  1250 . 
     In another example, when a level of the signal V_LPF output from the low-pass filter  1220  is greater than a level of the control signal Vlimit_H, the first comparator  1230  outputs a logically high value, and the second comparator  1240  outputs a logically low value. In this example, the input current compensation controller  1250  outputs, to the input current compensator  210 , a control signal to decrease the level of the signal V_LPF. 
     In another example, when the level of the signal V_LPF output from the low-pass filter  1220  is less than the level of the control signal Vlimit_L, the first comparator  1230  outputs a logically low value and the second comparator  1240  outputs a logically high value. In this example, the input current compensation controller  1250  outputs, to the input current compensator  210 , a control signal to increase the level of the signal V_LPF. 
     As described above, the input current compensation controller  1250  automatically controls a level of a compensation current output by the input current compensator  210  based on an output signal of the signal processing apparatus  1200 . Other operations of the signal processing apparatus  1200  of  FIG. 12  were previously explained with reference to  FIGS. 2 and 3 . 
       FIG. 13  is a diagram illustrating an example of a configuration of a transimpedance stage  1300 . 
     The transimpedance stage  1300  generates output voltages V_TI_op 2  and V_TI_on 2  based on a current output from a transconductance stage. The transimpedance stage  1300  includes a current mirror circuit constituted by all of the elements in  FIG. 13  except for the two transistors at the top that receive V_TC_op 2  and V_TC_on 2  and the output choppers  1310 . The transimpedance stage  1300  includes the output choppers  1310  to demodulate into a signal of a low-frequency band an input signal that has been modulated into a signal of a high-frequency band by an input chopper or a chopper included in the transconductance stage. The two output choppers  1310  enable the input signal to be demodulated more accurately through two frequency demodulation processes. In  FIG. 14 , VDD denotes a power supply voltage, CASP denotes PMOS cascode transistors, Ro denotes an output resistance, and CASN denotes NMOS cascode transistors. 
       FIG. 14  is a flowchart illustrating an example of a signal processing method. 
     In operation  1410 , a signal processing apparatus selects a voltage measuring mode and a current measuring mode. The signal processing apparatus controls an inflow of an input current or selects a type of an input voltage to be applied to an input element based on the selected measuring mode. 
     In operation  1420 , the signal processing apparatus determines whether the voltage measuring mode or the current measuring mode has been selected. 
     When it is determined in operation  1420  that the voltage measuring mode has been selected, the signal processing apparatus blocks the inflow of the input current and selects an input voltage to be measured in operation  1430 . The signal processing apparatus applies the input voltage to be measured to an input element, and blocks the inflow of the input current by turning off a switch used to control the inflow of the input current. 
     When it is determined in operation  1420  that the current measuring mode has been selected, the signal processing apparatus selects a preset reference voltage and allows the inflow of the input current in operation  1440 . The signal processing apparatus applies the preset reference voltage, which may have a fixed voltage level, to the input element, and allows the inflow of the input current by turning on the switch used control the inflow of the input current. Also, the signal processing apparatus may modulate the input current into a signal of a high-frequency band using a chopper. The input current modulated into the signal of the high-frequency band may be demodulated into a signal of a low-frequency band by an output chopper. 
     In one example, the signal processing method may also include operation  1450  of adjusting a level of the input current to be within a preset range. When the level of the input current is outside an operational range of the signal processing apparatus, the signal processing apparatus may generate a compensation current and adjust the input current to a signal having an appropriate level using the compensation current. The signal processing apparatus may adjust the level of the input current in a manual adjustment mode or in an automatic adjustment mode. In the manual adjustment mode, the signal processing apparatus manually adjusts the level of the input current under the control of a user. In the automatic adjustment mode, the signal processing apparatus determines a level of the compensation current based on the output signal of the signal processing apparatus. For example, the signal processing apparatus may extract a signal of a low-frequency band from the output signal of the signal processing apparatus, and determine a sign and a level of the compensation current based on whether a level of the extracted signal is within a preset voltage range. 
       FIGS. 15 through 16B  are diagrams illustrating examples of a wearable device including a signal processing apparatus. 
     Each signal processing apparatus described with reference to  FIGS. 1 through 14  may be embedded in a wearable device  1510 . In one example, the wearable device  1510  is a device to be worn on a wrist of a user provided in the form of a watch of a bracelet. The wearable device  1510  measures various biosignals from a body of a user  1520 , and the measured biosignals are processed by a signal processing apparatus. 
     The wearable device  1510  includes various sensors for measuring a biosignal of the user  1520 . For example, the wearable device  1510  includes a bioelectrode for measuring an ECG, a sensor for measuring a PPG, a sensor for measuring a heart rate of the user  1520 , a sensor for measuring a body temperature, and a sensor for measuring a skin humidity. 
     Based on a measurement method, the biosignal may be detected as a voltage signal or a current signal, and an attribute such as a frequency bandwidth may vary in the biosignal. The signal processing apparatus processes a biosignal having a diversified attribute and converts the biosignal into a state in which the wearable device  1510  can easily analyze the biosignal. The signal processing apparatus may operate in a voltage measuring mode or in a current measuring mode. Based on a measurement mode, the signal processing apparatus controls connections between internal elements of the signal processing apparatus to analyze biosignals each having a different attribute. 
     The signal processing apparatus measures a current signal and a voltage signal by controlling the connection between the internal elements while sharing the internal elements. For example, in a process of measuring a voltage signal such as an ECG, the signal processing apparatus controls the connections between the internal elements to amplify the voltage signal in the voltage measuring mode. In contrast, in a process of measuring a current signal such as a PPG, the signal processing apparatus controls the connections between the internal elements to amplify the current signal in the current measuring mode. 
       FIG. 15  illustrates an example of the wearable device  1510  measuring the ECG from the body of the user  1520 . For example, when the user  1520  is wearing the wearable device  1510  on a left wrist, and touches a sensor  1530  of the wearable device  1510  with a forefinger of a right hand, the ECG is measured from the user  1520 . The sensor  1530  may be, for example, a bioelectrode for measuring the ECG. The wearable device  1510  may determine a heart rate of the user  1520 , and display heart rate information and the ECG on a display. 
       FIGS. 16A and 16B  illustrate examples of a front side and a rear side of the wearable device  1510  of  FIG. 15 . 
     Referring to  FIG. 16A , the wearable device  1510  includes fasteners  1660  and  1670  to fasten the wearable device  1510  on a wrist of a user. The wearable device  1510  includes a first electrode  1620  to measure an ECG. The first electrode  1620  corresponds to the sensor  1530  of  FIG. 15 . Also, the wearable device  1510  includes a body  1610  including a signal processing apparatus and additional elements. The wearable device  1510  includes a display  1615  to display a measured biosignal and an analysis result of the biosignal. The display  1615  is disposed on an upper portion of the body  1610 . 
     Referring to  FIG. 16B , the wearable device  1510  includes a second electrode  1630  to measure the ECG. The second electrode  1630  is electrically isolated from the first electrode  1620 , and the ECG is measured from an electrical path through the user&#39;s body between the first electrode  1620  and the second electrode  1630 . The wearable device  1510  includes a reference electrode  1650  to measure a reference voltage for measuring the ECG. 
     Also, the wearable device  1510  includes an optical sensor  1640  to measure a current measurement-based biosignal such as a PPG and a blood oxygen saturation. The optical sensor  1640  includes a light source unit to radiate a light signal of a predetermined wavelength onto the body of the user, and a light detector to detect a biosignal based on the light signal. Hereinafter, the optical sensor  1640  will be explained in detail with reference to  FIG. 17 . 
       FIG. 17  is a diagram illustrating an example of a structure of an optical sensor  1710 . 
     The optical sensor  1710  senses a current measurement-based biosignal such as a PPG and a blood oxygen saturation. The optical sensor  1710  corresponds to the optical sensor  1640  of  FIG. 16B . The optical sensor  1710  includes a light source unit to radiate a light signal onto a body of a user, and a light detector  1740  to detect a biosignal based on the light signal. The light source unit may include a plurality of light sources, and may have an array structure in which the light sources are arranged in a predetermined direction. In  FIG. 17 , as one example, the light source unit includes a first light source array  1720  and a second light source array  1730 . The first light source array  1720  includes light sources LS 11 , LS 21 , LS 31 , and LS 41 . The second light source array  1730  includes light sources LS 12 , LS 22 , LS 32 , and LS 42 . 
     In the example in  FIG. 17 , the light detector  1740  is disposed between the first light source array  1720  and the second light source array  1730 . The light detector  1740  includes a plurality of photo detectors, and has a structure in which the photo detectors are arranged in a predetermined direction. In  FIG. 17 , as one example, the light detector  1740  includes photo detectors PD 1 , PD 2 , PD 3 , and PD 4 . 
     Based on a physical characteristic of a user, a position of a radial artery  1750  in a wrist may vary. An optimal biosignal may be acquired using the array structure of the light source unit and the array structure of the light detector  1740 . For example, when the user wears the wearable device  1510 , the radial artery  1750  may be located close to the photo detector PD 2 . In this example, a biosignal measured by the photo detector PD 2  will have a higher amplitude or fluctuation when compared to a biosignal measured by the photo detectors PD 1 , PD 3 , and PD 4 . 
     The light sources included in the first light source array  1720  and the light sources included in the second light source array  1730  may radiate light having the same wavelength or light having different wavelengths. For example, when measuring a blood oxygen saturation of the user, the light sources included in the first light source array  1720  radiate red light, and the light sources included in the second light source array  1730  radiate infrared light. 
       FIG. 18  is a diagram illustrating an example of a configuration of a signal processing apparatus  1810 . 
     The signal processing apparatus  1810  measures various types of biosignals from a body of a user and processes the measured biosignals. The signal processing apparatus  1810  includes various sensors for measuring a biosignal. For example, the signal processing apparatus  1810  includes electrodes for measuring a voltage measurement-based biosignal, and an optical sensor for measuring a current measurement-based biosignal. The signal processing apparatus  1810  may operate in a voltage measuring mode or in a current measuring mode. Based on a measurement mode, the signal processing apparatus  1810  controls connections between internal elements of the signal processing apparatus  1810  to amplify biosignals having different attributes. 
     Referring to  FIG. 18 , the signal processing apparatus  1810  includes a light source unit  1840 , a light detector  1850 , a bioelectrode  1880 , a signal processor  1870 , and a controller  1820 . 
     The light source unit  1840  radiates light onto the body of the user. The light source unit  1840  includes a plurality of light sources, and has a structure in which the light sources are arranged in a predetermined direction. The light sources form a plurality of light source array structures. Light sources included in each of the light source array structures may radiate light having the same wavelength, or light having different wavelengths. In the example in  FIG. 18 , a first light source array includes light sources LS 11 , LS 21 , LS 31 , and LS 41 , and a second light source array includes light sources LS 12 , LS 22 , LS 32 , and LS 42 . For example, the light sources LS 11  and LS 12  may radiate light having the same wavelength, or light having different wavelengths. 
     In one example, the light detector  1850  detects a first biosignal measured based on a light signal output from the light source unit  1840 . For example, the first biosignal may be a PPG signal or a signal including SpO 2  information. The light detector  1850  includes a plurality of photo detectors, and has an array structure in which the photo detectors are arranged in a predetermined direction. In the example in  FIG. 18 , the light detector  1850  includes photo detectors PD 1 , PD 2 , PD 3 , and PD 4 . 
     The controller  1820  controls a light source driver  1830  to drive the light source unit  1840 , a multiplexer  1860 , and the signal processor  1870 . The controller  1820  uses the multiplexer  1860  to control connections between the signal processor  1870  and the photo detectors PD 1 , PD 2 , PD 3 , and PD 4 . The photo detectors PD 1 , PD 2 , PD 3 , and PD 4  are connected to the multiplexer  1860 . The multiplexer  1860  may apply, to the signal processor  1870 , predetermined biosignals, for example, first biosignals I In  and I i_p  among a plurality of first biosignals output from the photo detectors PD 1 , PD 2 , PD 3 , and PD 4  under a control of the controller  1820 . 
     The bioelectrode  1880  detects a second biosignal measured based on a voltage signal. For example, the second biosignal may be an ECG signal. In the example in  FIG. 18 , the bioelectrode  1880  includes a first electrode and a second electrode, and detects the second biosignal measured from an electrical path through the user&#39;s body between the first electrode and the second electrode. The first electrode and the second electrode are connected to an input end of the signal processor  1870 , and respectively apply signals V ip  and V in  to the signal processor  1870 . 
     The signal processor  1870  amplifies the first biosignal or the second biosignal depending on a measurement mode. Signals V op  and V on  are output signals of the signal processor  1870 . Detailed descriptions of the operations performed by the signal processor  1870  have been described with reference to  FIGS. 1 through 14 . The controller  1820  controls the measurement mode of the signal processor  1870 , which may be a current measuring mode or a voltage measuring mode. 
     In the current measuring mode, the controller  1820  controls the signal processor  1870  to amplify the first biosignal. The controller  1820  controls a connection between the signal processor  1870  and the bioelectrode  1880  to prevent the signal processor  1870  from amplifying the second biosignal output from the bioelectrode  1880 , and to input the first biosignal output from the light detector  1850  to the signal processor  1870  through the multiplexer  1860 . 
     In the voltage measuring mode, the controller  1820  controls the signal processor  1870  to amplify the second biosignal. The controller  1820  uses the multiplexer  1860  to block the first biosignal output from the light detector  1850  from being input to the signal processor  1870 . Also, the controller  1820  controls the signal processor to amplify the second biosignal output from the bioelectrode  1880 . 
     Additionally, the controller  1820  may select at least one light source for measuring the biosignal from the light sources LS 11  to LS 42  included in the light source unit  1840 . The controller  1820  may sequentially activate at least one light source among the light sources during each of a plurality of time intervals, and select a light source for measuring the first biosignal from the light sources LS 11  to LS 42  based on a level of a signal output from the light detector  1850  during each time interval. 
     In one example, the controller  1820  controls the light source driver  1830  to activate only the light sources LS 11  and LS 12  during a first time interval, and controls the multiplexer  1860  to apply an output signal of the photo detector PD 1  to an input terminal of the signal processor  1870 . During a second time interval following the first time interval, the controller  1820  activates only the light sources LS 21  and LS 22 , and controls the multiplexer  1860  to apply an output signal of the photo detector PD 2  to the input terminal of the signal processor  1870 . During a third time interval following the second time interval, the controller  1820  activates only the light sources LS 31  and LS 32 , and controls the multiplexer  1860  to apply an output signal of the photo detector PD 3  to the input terminal of the signal processor  1870 . During a fourth time interval provided following the third time interval, the controller  1820  activates only the light sources LS 41  and LS 42 , and controls the multiplexer  1860  to apply an output signal of the photo detector PD 4  to the input terminal of the signal processor  1870 . 
     The controller  1820  may repetitively perform the aforementioned procedure and analyze the level of the signal output by the light detector during each time interval, thereby determining a pair of a light source and a photo detector to be used for measuring the first biosignal. The controller  1820  may operate only the determined light source and the determined photo detector, and control the multiplexer  1860  so that an output signal of the determined photo detector is applied to the input terminal of the signal processor  1870 . For example, when the level of the signal output by the photo detector PD 2  during the second time interval is higher than levels of the signals output by the photo detectors PD 1 , PD 3 , and PD 4  during the first, third, and fourth time intervals, the controller  1820  may determine the light sources LS 21  and LS 22  as light sources for measuring the second biosignal, and control the multiplexer  1860  to apply an output signal of the photo detector PD 2  to the signal processor  1870 . 
     In another example, the controller  1820  may periodically perform the aforementioned procedure of determining the pair of light source and photo detector, thereby determining an optimal sensing position for measuring the first biosignal. Based on light emitting timing information of the light sources LS 11  to LS 42  received from the controller  1820 , the signal processor  1870  may determine a time interval during which the determined light source emits light, and amplify the signal of the photo detector input to the signal processor  1870  only during the determined time interval, thereby amplifying only the signal of the determined photo detector. 
     The controllers  130  and  1820  in  FIGS. 1 and 18 , the input current compensators  210  and  450  in  FIGS. 2-4 and 12 , the input current compensation controller  1250  in  FIG. 12 , and the light source driver  1830  and the multiplexer  1860  in  FIG. 18  that perform the operations described herein with respect to  FIGS. 1-18  are implemented by hardware components. Examples of hardware components include controllers, generators, drivers, and any other electronic components known to one of ordinary skill in the art. In one example, the hardware components are implemented by one or more processors or computers. A processor or computer is implemented by one or more processing elements, such as an array of logic gates, a controller and an arithmetic logic unit, a digital signal processor, a microcomputer, a programmable logic controller, a field-programmable gate array, a programmable logic array, a microprocessor, or any other device or combination of devices known to one of ordinary skill in the art that is capable of responding to and executing instructions in a defined manner to achieve a desired result. In one example, a processor or computer includes, or is connected to, one or more memories storing instructions or software that are executed by the processor or computer. Hardware components implemented by a processor or computer execute instructions or software, such as an operating system (OS) and one or more software applications that run on the OS, to perform the operations described herein with respect to  FIGS. 1-18 . The hardware components also access, manipulate, process, create, and store data in response to execution of the instructions or software. For simplicity, the singular term “processor” or “computer” may be used in the description of the examples described herein, but in other examples multiple processors or computers are used, or a processor or computer includes multiple processing elements, or multiple types of processing elements, or both. In one example, a hardware component includes multiple processors, and in another example, a hardware component includes a processor and a controller. A hardware component has any one or more of different processing configurations, examples of which include a single processor, independent processors, parallel processors, single-instruction single-data (SISD) multiprocessing, single-instruction multiple-data (SIMD) multiprocessing, multiple-instruction single-data (MISD) multiprocessing, and multiple-instruction multiple-data (MIMD) multiprocessing. 
     The method illustrated in  FIG. 14  that performs the operations described herein with respect to  FIGS. 1-18  is performed by a processor or a computer as described above executing instructions or software to perform the operations described herein. 
     Instructions or software to control a processor or computer to implement the hardware components and perform the methods as described above are written as computer programs, code segments, instructions or any combination thereof, for individually or collectively instructing or configuring the processor or computer to operate as a machine or special-purpose computer to perform the operations performed by the hardware components and the methods as described above. In one example, the instructions or software include machine code that is directly executed by the processor or computer, such as machine code produced by a compiler. In another example, the instructions or software include higher-level code that is executed by the processor or computer using an interpreter. Programmers of ordinary skill in the art can readily write the instructions or software based on the block diagrams and the flow charts illustrated in the drawings and the corresponding descriptions in the specification, which disclose algorithms for performing the operations performed by the hardware components and the methods as described above. 
     The instructions or software to control a processor or computer to implement the hardware components and perform the methods as described above, and any associated data, data files, and data structures, are recorded, stored, or fixed in or on one or more non-transitory computer-readable storage media. Examples of a non-transitory computer-readable storage medium include read-only memory (ROM), random-access memory (RAM), flash memory, CD-ROMs, CD-Rs, CD+Rs, CD-RWs, CD+RWs, DVD-ROMs, DVD-Rs, DVD+Rs, DVD-RWs, DVD+RWs, DVD-RAMs, BD-ROMs, BD-Rs, BD-R LTHs, BD-REs, magnetic tapes, floppy disks, magneto-optical data storage devices, optical data storage devices, hard disks, solid-state disks, and any device known to one of ordinary skill in the art that is capable of storing the instructions or software and any associated data, data files, and data structures in a non-transitory manner and providing the instructions or software and any associated data, data files, and data structures to a processor or computer so that the processor or computer can execute the instructions. In one example, the instructions or software and any associated data, data files, and data structures are distributed over network-coupled computer systems so that the instructions and software and any associated data, data files, and data structures are stored, accessed, and executed in a distributed fashion by the processor or computer. 
     While this disclosure includes specific examples, it will be apparent to one of ordinary skill in the art that various changes in form and details may be made in these examples without departing from the spirit and scope of the claims and their equivalents. The examples described herein are to be considered in a descriptive sense only, and not for purposes of limitation. Descriptions of features or aspects in each example are to be considered as being applicable to similar features or aspects in other examples. Suitable results may be achieved if the described techniques are performed in a different order, and/or if components in a described system, architecture, device, or circuit are combined in a different manner, and/or replaced or supplemented by other components or their equivalents. Therefore, the scope of the disclosure is defined not by the detailed description, but by the claims and their equivalents, and all variations within the scope of the claims and their equivalents are to be construed as being included in the disclosure.