Apparatus and method of measuring bio impedance

A bio impedance measurement apparatus includes a current applicator configured to provide, to terminals contacting a body, a current based on a first control signal, and a modulator configured to modulate a voltage generated as the current flows through the body, based on a second control signal. The apparatus further includes an amplifier configured to amplify the modulated voltage, and a demodulator configured to demodulate the amplified voltage based on a third control signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 USC 119(a) of Korean Patent Application No. 10-2014-0007377, filed on Jan. 21, 2014, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND

The following description relates to an apparatus and a method of measuring bio impedance.

2. Description of Related Art

Various medical equipments for diagnosing health conditions of patients are being developed. For convenience of a patient during a health examination and for a quick result of the health examination, importance of the medical equipments for measuring electrical bio signals of patients is rising. Bio impedance may be used to monitor a health condition or an emotional condition of a living body.

SUMMARY

In one general aspect, there is provided a bio impedance measurement apparatus including a current applicator configured to provide, to terminals contacting a body, a current based on a first control signal, and a modulator configured to modulate a voltage generated as the current flows through the body, based on a second control signal. The apparatus further includes an amplifier configured to amplify the modulated voltage, and a demodulator configured to demodulate the amplified voltage based on a third control signal.

The third control signal may have a third frequency that is a difference between a first frequency of the first control signal and a second frequency of the second control signal.

The third control signal may have a third frequency that is determined based on a bandwidth of the amplifier.

The second control signal may have a second frequency that is determined so that a frequency obtained by subtracting the second frequency from a first frequency of the first control signal is in a bandwidth of the amplifier.

The first control signal may be determined based on a characteristic of a bio impedance to be measured.

The amplifier may be configured to amplify a signal generated based on the second control signal determined based on the first control signal and the third control signal.

The modulator may be configured to generate a first intermediate signal of a frequency obtained by adding a second frequency of the second control signal to a first frequency of the first control signal, and a second intermediate signal of a frequency obtained by subtracting the second frequency from the first frequency. The amplifier may be configured to selectively amplify the second intermediate signal between the first intermediate signal and the second intermediate signal.

The bio impedance measurement apparatus may further include a selector configured to select the third control signal to be a fourth control signal or a fifth control signal, the fourth control signal and the fifth control signal having different phases.

The selector may be configured to select the fourth control signal to measure a real component of a bio impedance, and select the fifth control signal to measure an imaginary component of a bio impedance.

The selector may be configured to alternately select the fourth control signal and the fifth control signal at a predetermined period.

The bio impedance measurement apparatus may further include the terminals configured to contact the body so that the current flows through the body.

In another general aspect, there is provided a bio impedance measurement method including providing, to terminals contacting a body, a current based on a first control signal, and modulating a voltage generated as the current flows through the body, based on a second control signal. The method further includes amplifying the modulated voltage, and demodulating the amplified voltage based on a third control signal.

The bio impedance measurement method may further include determining the first control signal based on a characteristic of a bio impedance to be measured, determining the third control signal based on a bandwidth of an amplifier, and determining the second control signal based on the first control signal and the third control signal.

The determining of the second control signal may include determining a second frequency of the second control signal to be a difference between a first frequency of the first control signal and a third frequency of the third control signal.

The determining of the third control signal may include determining a third frequency of the third control signal so that the third frequency is in the bandwidth of the amplifier.

The modulating may include generating a first intermediate signal of a frequency obtained by adding a second frequency of the second control signal to a first frequency of the first control signal, and a second intermediate signal of a frequency obtained by subtracting the second frequency from the first frequency. The amplifying may include selectively amplifying the second intermediate signal between the first intermediate signal and the second intermediate signal.

The demodulating may include selecting the third control signal to be a fourth control signal or a fifth control signal, the fourth control signal and the fifth control signal having different phases.

The selecting may include selecting the fourth control signal to measure a real component of a bio impedance, and selecting the fifth control signal to measure an imaginary component of a bio impedance.

The selecting may include alternately selecting the fourth control signal and the fifth control signal at a predetermined period.

A non-transitory computer-readable storage medium may store a program including instructions to cause a computer to perform the method.

DETAILED DESCRIPTION

FIGS. 1A to 1Dare diagrams illustrating examples of a bio impedance measurement apparatus100. Referring toFIG. 1A, the bio impedance measurement apparatus100measures bio impedance. The bio impedance may be used to monitor a health condition or an emotional condition of a living body. The bio impedance may be in various types. For example, the bio impedance may include bio impedance indicating a skin resistance, bio impedance indicating a skin hydration, bio impedance changed depending on respiration by lungs, bio impedance changed depending on a blood current, and bio impedance present on an electrical path including skin and a measurement electrode.

The bio impedance measurement apparatus100may be applied to analysis of impedance components of a human body, such as body fat, and electrode impedance monitoring during bio signal detection such as electrocardiogram (ECG), electromyogram (EMG), electrooculogram (EOG), and brainwave. The importance of bio signal measurement is also increasing in a mobile environment, for example, ubiquitous health care. The bio impedance measurement apparatus100may be applied to various fields, for example, detecting bio signals such as body fat through measurement of the bio impedance in the mobile environment and monitoring respiration through impedance measurement of an electrode for bio signal detection.

To measure the bio impedance, a current generated at an outside of a body may be used. For example, bioelectrical impedance analysis may be performed based on a principle that a resistance of a uniform conductor having a predetermined length with a uniform cross sectional area is proportional to the length and inversely proportional to the cross sectional area. However, a living body is generally not a uniform cylinder, and conductivity of the living body is also not uniform along the living body. The living body is constituted by muscle and extracellular fluid having a relatively high conductivity, and fat tissue that is not electrically conductive. Therefore, various circuit models may be applied to explain electrical characteristics of a body.

When an alternating current (AC) is flown through the body, the AC passes through a cell membrane, and the cell membrane may be charged with electric charges. In this case, the cell membrane may function as a capacitor, through which the electrical characteristics of the body may be modeled. In addition, electrical transmittance of a cell may be varied according to frequencies of the AC. For example, an AC of about 5 kHz does not pass through the cell membrane, and therefore may be used for measure of the extracellular fluid. An AC of about 100 kHz or higher that passes through the cell membrane may be used for measurement of a total body water (TBW).

The bio impedance measurement apparatus100includes a current applicator110. The current applicator110outputs an AC of a first frequency. The current applicator110that outputs the AC of the first frequency may be configured in various types. For example, the current applicator110may include a sine wave current generator. In this case, the sine wave current generator may output a sine wave current of the first frequency. As another example, referring toFIG. 1B, the current applicator110includes a direct current (DC) generator111and a modulator112. The DC generator111generates a DC. The modulator112outputs an AC by switching an output direction of the DC generated by the DC generator111. The modulator112may modulate the DC into the AC by switching the output direction of the DC, using the first frequency.

The current applicator110may use the first frequency corresponding to a type of bio impedance to be measured. The type of bio impedance may relate to a frequency of an AC injected into a living body for measurement of the bio impedance of the corresponding type. This is because different types of bio impedance may be measured depending on the frequency of the AC injected to the living body. The current applicator110may use various frequency bands such as 1 kHz, 5 kHz, 50 kHz, 250 kHz, 500 kHz, 1 MHz, and other frequencies as the first frequency to measure various types of the bio impedance.

For example, the bio impedance measurement apparatus100may measure various types of the bio impedance from various parts of the living body. For example, the bio impedance measurement apparatus100may measure bio impedances of various body parts such as a right arm, a left arm, a trunk, a right leg, and a left leg, using the various frequency bands. The measured bio impedances may be used for calculating a body weight, TBW, intracellular water, extracellular water, protein, mineral matter, muscle mass, lean body mass (LBM), skeletal muscle mass, fat mass, abdominal fat ratio, visceral fat area, muscle mass in each segment, muscle percentage in each segment, edema index (EI), and other parameters known to one of ordinary skill in the art.

When the modulated current is supplied to a bio impedance measured portion, a voltage drop may be caused due to a bio impedance of the bio impedance measured portion. The bio impedance measurement apparatus100may measure the bio impedance by measuring a potential difference according to the voltage drop.

Referring again toFIG. 1A, the bio impedance measurement apparatus100further includes a contactor115. The contactor115provides a plurality of terminals for contacting the living body so that the AC output by the current applicator110flows through the living body.

Referring toFIG. 1C, first and second terminals included in the contactor115are interfaced with a measurement object according to a 2-terminal measurement method. In this case, an AC is supplied to the measurement object such as the living body through the first and second terminals. As the AC flows through the living body, a potential difference is generated between the first terminal and the second terminal. The contactor115outputs the potential difference between the first terminal and the second terminal through third and fourth terminals, respectively.

As another example, referring toFIG. 1D, first through fourth terminals included in the contactor115are interfaced with a measurement object according to a 4-terminal measurement method. In this case, an AC is supplied to the measurement object such as the living body through the first and second terminals, and a potential difference between the third and fourth terminals is output. A measurement result of the 4-terminal measurement method may be more precise in comparison to a measurement result of the 2-terminal measurement method.

Hereinafter, a potential difference generated between two terminals of the contactor115may be referred to as a voltage generated between the two terminals. Since a current flowing through the living body may be an AC output by the current applicator110, having the first frequency as a central frequency, the voltage generated between the two terminals of the contactor115may be the AC having the first frequency as the central frequency.

The bio impedance measurement apparatus100further includes an amplifier130. The amplifier130amplifies the voltage generated between the two terminals of the contactor115to measure the bio impedance. The amplifier130may include an instrumentation amplifier (IA). The IA may perform amplification and filtering of micro bio signals.

A bandwidth of the amplifier130may have to include the first frequency. That is, the amplifier130may need to sufficiently amplify a signal of the first frequency used by the current applicator110to correctly measure the bio impedance to be measured.

For example, referring toFIG. 2A, a cutoff frequency fcfrom which a frequency reaction curve220of the amplifier130starts decreasing in amplitude needs to be higher than a first frequency f1210used in the current applicator110. InFIG. 2A, an x-axis denotes a frequency of a signal input to the amplifier130, and a y-axis denotes an amplitude of a signal output from the amplifier130. InFIG. 2A, a bandwidth of the amplifier130may be a frequency band less than or equal to the cutoff frequency fc.

Generally, as the bandwidth of the amplifier130includes a high frequency band, power consumption of the amplifier130increases. Therefore, when a requirement specification of the amplifier130is reduced, power consumption of the bio impedance measurement apparatus100may be reduced. The bio impedance measurement apparatus100according to an example may provide a technology of reducing power consumption needed for measuring the same bio impedance compared to other bio impedance measurement technologies.

Referring again toFIG. 1A, the bio impedance measurement apparatus100further includes an intermediate modulator120. The intermediate modulator120modulates the voltage generated between the two terminals of the contactor115, using a second frequency. The voltage generated between the two terminals may be an AC having the first frequency as the central frequency. The intermediate modulator120may be input with the AC having the first frequency as the central frequency, and modulate the input AC into an AC having a frequency lower than the first frequency as the central frequency. The intermediate modulator120may reduce the needed bandwidth of the amplifier130by reducing the frequency of the input AC, accordingly reducing the power consumption needed for operation of the amplifier130.

For example, referring toFIG. 2B, the intermediate modulator120changes the central frequency of the input signal from the first frequency f1210to a frequency f1−f2211. In this case, since the signal input to the amplifier130has the frequency f1−f2211as the central frequency, the bandwidth of the amplifier130covers up to a band corresponding to the frequency f1−f2211rather than a band corresponding to the first frequency f1210. That is, a cutoff frequency fcfrom which a frequency reaction curve221of the amplifier130starts decreasing needs to be higher than the frequency f1−f2211, but is lower than the first frequency f1210.

For example, referring toFIG. 2C, when the intermediate modulator120modulates the input signal, using the second frequency f2, a signal of a frequency f1+f2212may also be generated in addition to a signal of the frequency f1−f2211. As will be described in detail below, the bandwidth of the amplifier130may include only the band corresponding to the frequency f1−f2211instead of being required to include the band corresponding to the first frequency f1210or a band corresponding to the frequency f1+f2212. That is, the amplifier130may amplify only the band corresponding to the frequency f1−f2211, not the band corresponding to the first frequency f1210or the frequency f1+f2212.

Referring again toFIG. 1A, the bio impedance measurement apparatus100further includes a demodulator140. The demodulator140demodulates an output signal of the amplifier130, using a third frequency f3. The third frequency f3relates to the first frequency f1used in the current applicator110and the second frequency f2used in the intermediate modulator120. For example, the third frequency f3may be a frequency that is a difference between the first frequency f1and the second frequency f2. Hereinafter, the third frequency f3may be referred to as a demodulation frequency.

The bio impedance measurement apparatus100further includes a selector145. The bio impedance may include a real component and an imaginary component. The selector145selects any one of different phase signals, and provides the selected phase signal to the demodulator140so that the real component and the imaginary component of the bio impedance may be selectively measured.

For example, for detection of the bio impedance, a quadrature demodulation method may be applied in a demodulation step of a chopper stabilization method. Therefore, the real component and the imaginary component of the bio impedance may be separated. Through the real component and the imaginary component, components of a human body, such as fat and moisture, may be separately analyzed. Thus, the bio impedance measurement apparatus100may be applied to various application fields such as body fat analysis. A detailed description about the selector145will be made hereinafter.

The bio impedance measurement apparatus100further includes a filter150. The filter150may include a low pass filter (LPF) that passes signals (i.e., output voltage signals) of only a predetermined frequency or lower while interrupting signals of a frequency higher than the predetermined frequency. For example, the predetermined frequency may be the third frequency used in the demodulator140.

Hereinafter, output signals of each module illustrated inFIG. 1Awill be described in detail with reference toFIG. 2D. A graph (i) ofFIG. 2Drepresents a voltage generated as an AC output by the current applicator110flows through a living body. A central frequency of a signal231is the first frequency f1.

A graph (ii) ofFIG. 2Drepresents a voltage modulated by the intermediate modulator120. The intermediate modulator120generates a signal232and a signal233by modulating the signal231, using the second frequency f2. A central frequency of the signal232is the frequency f1−f2, and a central frequency of the signal233is the frequency f1+f2.

A graph (iii) ofFIG. 2Drepresents a voltage amplified by the amplifier130. The amplifier130amplifies only the signal232that is slower between the signal232and the signal233, but does not amplify the signal233that is faster. Alternatively, a bandwidth of the amplifier130may include only a band corresponding to the central frequency of the signal232but not a band corresponding to the central frequency of the signal233. A signal235is the signal232amplified by the amplifier130. A signal236is substantially the same as the signal233.

A signal234is a noise generated by the operation of the amplifier130. For example, a 1/f noise may be generated inside the amplifier130. The 1/f noise may be called a flicker noise, which is a unique noise generated in an active device. When the noise generated in the active device is expressed by a frequency axis, the noise may greatly increase in a low frequency band, for example, approximately 100 Hz or lower. That is, the 1/f noise may increase in inverse proportion to the frequency.

The second frequency f2is determined such that the frequency f1−f2of the voltage modulated by the intermediate modulator120is not included in a noise band generated in the amplifier130. For example, the second frequency f2is determined so that the frequency f1−f2of the signal235is located out of the band of the signal234. In this case, the measurement result of the bio impedance measurement apparatus100may not be interfered with the noise generated in the amplifier130.

A graph (iv) ofFIG. 2Drepresents a voltage demodulated by the demodulator140. The demodulator140demodulates the signal235, using the third frequency f3. The third frequency f3is the frequency f1−f2, which is the difference between the first frequency f1and the second frequency f2.

A signal237is a demodulated form of the signal235. By the operation of the demodulator140using the third frequency f3, a signal239is also generated. Also, other signals234and236than the signal235are modulated by the operation of the demodulator140using the third frequency f3. For example, a signal238is a modulated form of the signal234, and a signal240is a modulated form of the signal236.

A graph (v) ofFIG. 2Drepresents a voltage filtered by the filter150. The filter150passes signals of only a predetermined frequency or lower. The predetermined frequency is adapted to pass only the signal237but interrupt other signals238,239, and240. For example, the predetermined frequency may be the third frequency f3, that is, f1−f2. A signal241is substantially the same signal as the signal237, that is, an output signal of the bio impedance measurement apparatus100.

Thus, the bio impedance measurement apparatus100may provide a technology of measuring bio impedance while amplifying a band of the frequency f1−f2lower than the first frequency f1corresponding to the bio impedance to be measured. Accordingly, the bio impedance measurement apparatus100may provide a technology of reducing power consumption needed for measurement of the bio impedance.

FIGS. 3A and 3Bare circuit diagrams illustrating examples of a bio impedance measurement circuit300. Referring toFIG. 3A, the bio impedance measurement circuit300includes a current applicator310, a contactor320, an intermediate modulator330, an amplifier340, a demodulator350, a selector360, and a filter370.

A modulator included in the current applicator310includes a chopper. The chopper modulates a sourcing current source and a sinking current source. A frequency of the chopper that modulates the current sources may be determined according to a first frequency f1.

A first frequency signal F1of the chopper that corresponds to the first frequency f1may be implemented by a square wave or a sine wave. When only a fundamental term excluding harmonics is considered among frequency components of the first frequency signal F1, the first frequency signal F1may be expressed by Equation 1. For a concise expression, an amplitude of the first frequency signal F1may be presumed to be 1.
F1=sin(2π·f1t)  [Equation 1]

In Equation 1, F1denotes the first frequency signal, f1denotes the first frequency, and t denotes a time.

When a current output from the current applicator310is injected to a living body325through the contactor320, a first voltage V1according to a bio impedance is generated. As aforementioned with reference toFIGS. 1C and 1D, a plurality of terminals included in the contactor320ofFIG. 3Aare connected to the living body325according to the 2-terminal measurement method, and a plurality of terminals included in the contactor320ofFIG. 3Bare connected to the living body325according to the 4-terminal measurement method.

The intermediate modulator330includes a chopper. The chopper outputs a second voltage V2by modulating the first voltage V1. A frequency of the chopper that modulates the first voltage V1may be determined by a second frequency f2.

A second frequency signal F2of the chopper that corresponds to the second frequency f2may be implemented by a square wave or a sine wave. Considering only a fundamental term excluding harmonics among frequency components of the second frequency signal F2, the second frequency signal F2may be expressed by Equation 2. For a concise expression, an amplitude of the second frequency signal F2may be presumed to be 1.
F2=sin(2π·f2t)  [Equation 2]

In Equation 2, F2denotes the second frequency signal, f2denotes the second frequency, and t denotes the time.

The amplifier340includes an IA. The IA outputs a third voltage V3by amplifying the second voltage V2.

The demodulator350includes a chopper. The chopper outputs a fourth voltage V4by demodulating the third voltage V3. A frequency of the chopper that demodulates the third voltage V3may be determined by a third frequency f3.

The selector360includes a multiplexer (MUX). The MUX selects any one of a plurality of signals depending on a select signal SEL. For example, the MUX may select any one of two signals having a phase difference of about 90 degrees, depending on the select signal SEL. However, the phase difference is not limited to 90 degrees. For example, the phase difference may range from 0 degrees to 180 degrees, depending on purposes. Hereinafter, an example in which two signals having a phase difference of about 90 degrees are used will be described.

The MUX selects any one of a third sine signal F3,Sand a third cosine signal F3,Cdepending on the select signal SEL. In this example, the MUX selects the third sine signal F3,Swhen the select signal SEL is a logical value 0, and selects the third cosine signal F3,Cwhen the select signal SEL is a logical value 1.

The third sine signal F3,Sand the third cosine signal F3,Cmay both correspond to the third frequency f3. However, the phase difference between the third sine signal F3,Sand the third cosine signal F3,Cmay be approximately 90 degrees. That is, although each of a frequency of the third sine signal F3,Sand a frequency of the third cosine signal F3,Cis the third frequency f3, the phase difference between the third sine signal F3,Sand the third cosine signal F3,Cmay be approximately 90 degrees.

Each of the third sine signal F3,Sand a frequency of the third cosine signal F3,Cmay be implemented by a square wave or a sine wave. Considering only a fundamental term excluding harmonics among frequency components of the third sine signal F3,S, the third sine signal F3,Smay be expressed by Equation 3. For a concise expression, an amplitude of the third sine signal F3,Smay be presumed to be 1.
F3,S=sin(2π·f3t)  [Equation 3]

In Equation 3, F3,Sdenotes the third sine signal, f3denotes the third frequency, and t denotes the time. The third frequency f3may be the frequency f1−f2that is a difference between the first frequency f1and the second frequency f2.

Considering only a fundamental term excluding harmonics among frequency components of the third cosine signal F3,C, the third cosine signal F3,cmay be expressed by Equation 4. For a concise expression, an amplitude of the third cosine signal F3,Cmay be presumed to be 1.
F3,C=cos(2π·f3t)  [Equation 4]

In Equation 4, F3,Cdenotes the third cosine signal, f3denotes the third frequency, and t denotes the time. The third frequency f3may be the frequency f1−f2that is the difference between the first frequency f1and the second frequency f2.

The filter370includes an LPF. The LPF cancels high frequency components included in the fourth voltage V4, and finally outputs an output voltage V0via terminals OUTP and OUTN.

Hereinafter, an operation principle of the bio impedance measurement circuit300will be described in detail. For example, the bio impedance measurement circuit300may measure a real component ZREof the bio impedance.

The chopper of the current applicator310is driven by the first frequency signal F1. The first voltage V1may be generated by the current output by the current applicator310and the real component ZREof the bio impedance. The first voltage V1may be expressed by Equation 5.
V1=ZRE·I·sin(2π·f1t)  [Equation 5]

In Equation 5, V1denotes the first voltage, ZREdenotes the real component of the bio impedance, I denotes a magnitude of a current supplied by a current source, f1denotes the first frequency, and t denotes the time. The first frequency f1may be a fundamental frequency of the first frequency signal F1.

The chopper of the intermediate modulator330is driven by the second frequency signal F2. The first voltage V1is modulated into the second voltage V2through the chopper of the intermediate modulator330. The second voltage V2may be expressed by Equation 6.

In Equation 6, V1denotes the first voltage, V2denotes the second voltage, F2denotes the second frequency signal, ZREdenotes the real component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, t denotes the time, and f2denotes the second frequency. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

The IA of the amplifier340amplifies the second voltage V2, and outputs the third voltage V3. The third voltage V3may be expressed by Equation 7.

In Equation 7, V2denotes the second voltage, V3denotes the third voltage, A denotes a voltage gain of the IA, ZREdenotes the real component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, f2denotes the second frequency, and t denotes the time. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

The chopper of the demodulator350is driven by the third cosine signal F3,Cor the third sine signal F3,S. The chopper of the demodulator350outputs the fourth voltage V4by demodulating the third voltage V3. The MUX of the selector360selects the third cosine signal F3,Cor the third sine signal F3,Sto be provided to the chopper of the demodulator350. To detect the real component ZREof the bio impedance, the MUX of the selector360selects the third cosine signal F3,Cbetween the third sine signal F3,Sand the third cosine signal F3,C. In this example, the fourth voltage V4may be expressed by Equation 8.

In Equation 7, V3denotes the third voltage, V4denotes the fourth voltage, F3,Cdenotes the third cosine signal, A denotes the voltage gain of the IA, ZREdenotes the real component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, f2denotes the second frequency, and t denotes the time. The third frequency f3may be a fundamental frequency of the third cosine signal F3,C, that is, the frequency f1−f2that is the difference between the first frequency f1and the second frequency f2. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

The LPF of the filter370passes signals of only a band of a cutoff frequency or lower. When the cutoff frequency is the third frequency f3, the output voltage V0of the filter370may be expressed by Equation 9.

In Equation 9, V0denotes the output voltage, A denotes the voltage gain of the IA, ZREdenotes the real component of the bio impedance, and I denotes the magnitude of the current supplied by the current source. Referring to Equation 9, the output voltage V0may be determined by the input current I, the voltage gain A of the IA, and the real component ZREof the bio impedance.

As aforementioned, the IA of the amplifier340amplifies only frequency components included in a bandwidth of the IA, instead of amplifying all frequency components of the second voltage V2. For example, the bandwidth of the amplifier340may not include a band corresponding to the frequency f1+f2, but include a band corresponding to the frequency f1−f2. In this example, Equation 7 may be approximated to Equation 10.

In Equation 10, V2denotes the second voltage, V3denotes the third voltage, A denotes the voltage gain of the IA, ZREdenotes the real component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, f2denotes the second frequency, and t denotes the time. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

In this example, Equation 8 may be approximated to Equation 11.

In Equation 11, V3denotes the third voltage, V4denotes the fourth voltage, F3,Cdenotes the third cosine signal, A denotes the voltage gain of the IA, ZREdenotes the real component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, f2denotes the second frequency, and t denotes the time. The third frequency f3may be a fundamental frequency of the third cosine signal F3,C, that is, the frequency f1−f2that is the difference between the first frequency f1and the second frequency f2. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

Even when Equation 7 and Equation 8 are approximated to Equation 10 and Equation 11, respectively, the output voltage V0may be expressed in the same way as in Equation 9. Thus, although the bandwidth of the IA of the amplifier340is reduced, the bio impedance may be accurately measured.

As another example, the bio impedance measurement circuit300may measure an imaginary component ZIMof the bio impedance.

The chopper of the current applicator310is driven by the first frequency signal F1. The first voltage V1may be generated by the current output by the current applicator310and the imaginary component ZIMof the bio impedance. The first voltage V1may be expressed by Equation 12.
V1=ZIM·I·cos(2π·f1t)  [Equation 12]

In Equation 12, V1denotes the first voltage, ZIMdenotes the imaginary component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, and t denotes the time. The first frequency f1may be the fundamental frequency of the first frequency signal F1.

The chopper of the intermediate modulator330is driven by the second frequency signal F2. The first voltage V1is modulated into the second voltage V2through the chopper of the intermediate modulator330. The second voltage V2may be expressed by Equation 13.

In Equation 12, V1denotes the first voltage, V2denotes the second voltage, F2denotes the second frequency signal, ZIMdenotes the imaginary component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, t denotes the time, and f2denotes the second frequency. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

The IA of the amplifier340outputs the third voltage V3by amplifying the second voltage V2. The third voltage V3may be expressed by Equation 14.

In Equation 14, V2denotes the second voltage, V3denotes the third voltage, A denotes the voltage gain of the IA, ZIMdenotes the imaginary component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, f2denotes the second frequency, and t denotes the time. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

The chopper of the demodulator350is driven by the third cosine signal F3,Cor the third sine signal F3,S. The chopper of the demodulator350outputs the fourth voltage V4by demodulating the third voltage V3. The MUX of the selector360selects the third cosine signal F3,Cor the third sine signal F3,Sto be provided to the chopper. To detect the imaginary component ZIMof the bio impedance, the MUX of the selector360selects the third sine signal F3,Sbetween the third sine signal F3,Sand the third cosine signal F3,C. In this example, the fourth voltage V4may be expressed by Equation 15.

In Equation 15, V3denotes the third voltage, V4denotes the fourth voltage, F3,Sdenotes the third sine signal, A denotes the voltage gain of the IA, ZIMdenotes the imaginary component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, f2denotes the second frequency, and t denotes the time. The third frequency f3may be the fundamental frequency of the third sine signal F3,S, that is, the frequency f1−f2that is the difference between the first frequency f1and the second frequency f2. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

The LPF of the filter370passes only signals of a band of a cutoff frequency or lower. When the cutoff frequency is the third frequency f3, the output voltage V0of the filter370may be expressed by Equation 16.

In Equation 16, V0denotes the output voltage, A denotes the voltage gain of the IA, ZIMdenotes the imaginary component of the bio impedance, and I denotes the magnitude of the current supplied by the current source. Referring to Equation 16, the output voltage V0may be determined by the input current I, the voltage gain A of the IA, and the imaginary component ZIMof the bio impedance.

As aforementioned, the IA of the amplifier340amplifies only frequency components included in the bandwidth of the IA, instead of amplifying all frequency components of the second voltage V2. For example, the bandwidth of the IA may not include a band corresponding to the frequency f1+f2, but include a band corresponding to the frequency f1−f2. Therefore, Equation 14 may be approximated to Equation 17.

In Equation 17, V2denotes the second voltage, V3denotes the third voltage, A denotes the voltage gain of the IA, ZIMdenotes the imaginary component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, f2denotes the second frequency, and t denotes the time. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

In this example, Equation 15 may be approximated to Equation 18.

In Equation 18, V3denotes the third voltage, V4denotes the fourth voltage, F3,Sdenotes the third sine signal, A denotes the voltage gain of the IA, ZIMdenotes the imaginary component of the bio impedance, I denotes the magnitude of the current supplied by the current source, f1denotes the first frequency, f2denotes the second frequency, and t denotes the time. The third frequency f3may be the fundamental frequency of the third sine signal F3,S, that is, the frequency f1−f2that is the difference between the first frequency f1and the second frequency f2. The first frequency f1may be the fundamental frequency of the first frequency signal F1. The second frequency f2may be the fundamental frequency of the second frequency signal F2.

Even when Equation 14 and Equation 15 are approximated to Equation 17 and Equation 18, respectively, the output voltage V0may be expressed in the same way as in Equation 16. Thus, although the bandwidth of the IA of the amplifier340is reduced, the bio impedance may be accurately measured.

In an example, when the first frequency f1corresponding to a type of the bio impedance to be measured is 2fo, the second frequency f2may be determined to be fo. The third frequency f3may also be determined to be fo. As aforementioned, the bio impedance measurement apparatus may accurately measure the bio impedance to be measured even when a frequency fccorresponding to the bandwidth of the IA is higher than fobut lower than 2fo. For example, when the first frequency f1is 2fo, and the second frequency f2and the third frequency f3are 2fo, requirements of the bandwidth of the IA may be reduced by half in comparison to a conventional impedance measurement method.

That is, when the first frequency is 2fo, and the second frequency and the third frequency are 2fo, the first frequency signal F1, the second frequency signal F2, the third sine signal F3,S, and the third cosine signal F3,Cmay be expressed by Equation 19 to Equation 22, respectively.
F1=sin(2π·2fot)  [Equation 19]
F2=sin(2π·fot)  [Equation 20]
F3,S=sin(2π·fot)  [Equation 21]
F3,C=cos(2π·fot)  [Equation 22]

In Equation 19 to Equation 22, F1denotes the first frequency signal, F2denotes the second frequency signal, F3,Sdenotes the third sine signal, F3,Cdenotes the third cosine signal, and t denotes the time.

When the real component ZREof the bio impedance is measured, the first voltage V1, the second voltage V2, the third voltage V3, and the fourth voltage V4may be expressed by Equation 23 to Equation 26, respectively.

In Equation 23 to Equation 26, V1denotes the first voltage, V2denotes the second voltage, V3denotes the third voltage, and V4denotes the fourth voltage. ZREdenotes the real component of the bio impedance, I denotes the magnitude of the current supplied by the current source, and t denotes the time. F2denotes the second frequency signal, F3,Cdenotes the third cosine signal, and A denotes the voltage gain of the IA.

Since the frequency fccorresponding to the bandwidth of the IA is higher than fobut lower than 2fo, an amplification level obtained in a frequency band higher than 2fomay not be sufficient. In this example, Equation 25 and Equation 26 may be approximated to Equation 27 and Equation 28, respectively.

In Equation 27 and Equation 28, V3denotes the third voltage, V4denotes the fourth voltage, A denotes the voltage gain of the IA, ZREdenotes the real component of the bio impedance, I denotes the magnitude of the current supplied by the current source, and t denotes the time.

Since the bandwidth of the IA is lower than the frequency 2fo, even when the third voltage V3and the fourth voltage V4are approximated to Equation 27 and Equation 28, the output voltage Vomay be expressed by Equation 29 in the same manner as when the bandwidth of the IA is greater than or equal to 2fo.

In Equation 29, Vodenotes the output voltage, A denotes the voltage gain of the IA, ZREdenotes the real component of the bio impedance, and I denotes the magnitude of the current supplied by the current source.

Therefore, according to the examples, power consumption reduction of the bio impedance measurement circuit may be achieved by using the IA having a relatively low bandwidth when measuring impedance of a frequency. In addition, bio impedance of a high frequency may be measured even with the IA having the low bandwidth.

When the imaginary component ZIMof the bio impedance is measured, the first voltage V1, the second voltage V2, the third voltage V3, and the fourth voltage V4may be expressed by Equation 30 to Equation 33, respectively.

In Equation 30 to Equation 33, V1denotes the first voltage, V2denotes the second voltage, V3denotes the third voltage, and V4denotes the fourth voltage. ZIMdenotes the imaginary component of the bio impedance, I denotes the magnitude of the current supplied by the current source, and t denotes the time. F2denotes the second frequency signal, F3,Sdenotes the third sine signal, and A denotes the voltage gain of the IA.

Since the frequency fccorresponding to the bandwidth of the IA is higher than the frequency fobut lower than 2fo, an amplification level obtained in a frequency band higher than 2fomay not be sufficient. In this example, Equation 32 and Equation 33 may be approximated to Equation 34 and Equation 35, respectively.

In Equation 34 and Equation 35, V3denotes the third voltage, V4denotes the fourth voltage, A denotes the voltage gain of the IA, ZIMdenotes the imaginary component of the bio impedance, I denotes the magnitude of the current supplied by the current source, and t denotes the time.

Since the bandwidth of the IA is lower than the frequency 2fo, even when the third voltage V3and the fourth voltage V4are approximated to Equation 34 and Equation 35, the output voltage Vomay be expressed by Equation 36 in the same manner as when the bandwidth of the IA is greater than or equal to 2fo.

In Equation 36, Vodenotes the output voltage, A denotes the voltage gain of the IA, ZIMdenotes the imaginary component of the bio impedance, and I denotes the magnitude of the current supplied by the current source.

Therefore, according to the examples, power consumption reduction of the bio impedance measurement circuit may be achieved by using the IA having a relatively low bandwidth when measuring impedance of a frequency. In addition, bio impedance of a high frequency may be measured even with the IA having the low bandwidth.

In the above description, the choppers illustrated inFIGS. 3A and 3Bmay be implemented by a multiplier.

FIG. 4is a block diagram illustrating an example of a bio impedance measurement apparatus400further including a controller160. Referring toFIG. 4, the bio impedance measurement apparatus400includes the current applicator110, the contractor115, the intermediate modulator120, the amplifier130, the demodulator140, the selector145, and the filter150, ofFIG. 1, further includes the controller160.

The controller160determines a first frequency used in the current applicator110, a second frequency used in the intermediate modulator120, and a third frequency used in the demodulator140.

For example, the controller160may determine the first frequency based on a type of a bio impedance to be measured. The controller160may use a mapping table in determining the first frequency. When the type of the bio impedance to be measured is determined, the controller160may determine a frequency corresponding to the type of the bio impedance, using the mapping table. The controller160may determine the frequency corresponding to the type of the bio impedance to be the first frequency.

The controller160may determine the third frequency based on a specification of the amplifier130. For example, the controller160may determine the third frequency so that a signal of the third frequency to be demodulated by the demodulator140is fully amplified by the amplifier130. In this example, the controller160may determine the second frequency based on the first frequency and the third frequency. For example, the controller160may determine the second frequency to be a frequency that is a difference between the first frequency and the third frequency.

Alternatively, the controller160may determine the second frequency so that an output signal of the intermediate modulator120is included in a bandwidth of the IA of the amplifier130. That is, the controller160may determine the second frequency so that a frequency having a value obtained by subtracting the second frequency from the first frequency is lower than or equal to a frequency fccorresponding to the bandwidth of the amplifier. In this example, the controller160may determine the third frequency based on the first frequency and the second frequency. For example, the controller160may determine the third frequency to be the frequency that is a difference between the first frequency and the second frequency.

Although not shown in the drawings, the controller160may further include a frequency generator. The frequency generator may generate a first frequency signal corresponding to the first frequency, a second frequency signal corresponding to the second frequency, a third sine signal corresponding to the third frequency, and a third cosine signal corresponding to the third frequency. The controller160may provide the first frequency signal to the current applicator110, the second frequency signal to the intermediate modulator120, and the third sine signal and the third cosine signal to the selector145.

The controller160may provide a select signal SEL for controlling the MUX of the selector145. The controller160may provide a select signal SEL having a first logical value to the selector145to measure a real component of the bio impedance. In addition, the controller160may provide a select signal SEL having a second logical value to the selector145to measure an imaginary component of the bio impedance.

The controller160may alternately provide the select signal SEL having the first logical value and the select signal SEL having the second logical signal to the selector145at a predetermined period. In this example, the selector145may alternately select the third sine signal and the third cosine signal at the predetermined period. The demodulator140may alternately demodulate the real component and the imaginary component of the bio impedance at the predetermined period.

FIGS. 5A and 5Bare circuit diagrams illustrating examples of a bio impedance measurement circuit500further including a controller510. Referring toFIGS. 5A and 5B, the bio impedance measurement circuit500includes the current applicator310, the contactor320, the intermediate modulator330, the amplifier340, the demodulator350, the selector360, and the filter370, ofFIGS. 3A and 3B, further includes the controller510.

The controller510provides the first frequency signal F1to the chopper included in the current applicator310. The controller510provides the second frequency signal F2to the chopper included in the intermediate modulator330. The controller510provides the third sine signal F3,Sand the third cosine signal F3,Cto the MUX included in the selector360. The controller510provides the select signal SEL to the MUX of the selector360. Since technical features illustrated with reference toFIGS. 3A, 3B, and 4may be directly applied, a detailed description will be omitted.

Two terminals included in the contactor320ofFIG. 5Aare connected to the living body325according to the 2-terminal measurement method. Four terminals included in the contactor320ofFIG. 5Bare connected to the living body325according to the 4-terminal measurement method.

FIG. 6is a block diagram illustrating an example of a bio impedance measurement apparatus600including a plurality of demodulators. Referring toFIG. 6, the bio impedance measurement apparatus600includes the current applicator110, the contactor115, the intermediate modulator120, and the amplifier130, ofFIG. 1, and includes a real component demodulator610and an imaginary component demodulator630, separately.

The real component demodulator610demodulates an output signal of the amplifier130to measure a real component of a bio impedance. The imaginary component demodulator630demodulates the output signal of the amplifier130to measure an imaginary component of a bio impedance.

A filter620cancels high frequency components of an output signal of the real component demodulator610to generate a first output voltage. A filter640cancels high frequency components of an output signal of the imaginary component demodulator630to generate a second output voltage.

Since the real component demodulator610and the imaginary component demodulator630may apply the technical features illustrated with reference toFIGS. 1A to 3B, a detailed description will be omitted.

FIGS. 7A and 7Bare circuit diagrams illustrating examples of a bio impedance measurement circuit700including a plurality of demodulators. Referring toFIGS. 7A and 7B, the bio impedance measurement circuit700includes the current applicator310, the contactor320, and the intermediate modulator330, ofFIGS. 3A and 3B, and further includes a primary amplifier750, a first demodulator710, a first filter720, a second demodulator730, and a second filter740. The first demodulator710and the second demodulator730each include a secondary amplifier and a chopper. The first filter720and the second filter740each include an LPF. The chopper of the first demodulator710and the chopper of the second demodulator730are input with an output of an IA included in the primary amplifier750.

The secondary amplifier of the first demodulator710outputs the fourth voltage V4by amplifying the third voltage V3. The chopper of the first demodulator710generates a fifth voltage V5by demodulating the fourth voltage V4, using the third cosine signal F3,C, to measure the real component of the bio impedance. The secondary amplifier of the second demodulator730outputs a sixth voltage V6by amplifying the third voltage V3, and the chopper of the second demodulator730generates a seventh voltage V7by demodulating the sixth voltage V6, using the third sine signal F3,S, to measure the imaginary component of the bio impedance. Since the technical features illustrated with reference toFIGS. 1A to 3B and 6may be directly applied to the chopper of the first demodulator710and the chopper of the second demodulator730, a detailed description will be omitted.

Two terminals of the contactor320ofFIG. 7Aare connected to the living body325according to the 2-terminal measurement method. Four terminals of the contactor320ofFIG. 7Bare connected to the living body325according to the 4-terminal measurement method.

FIG. 8is an operation flowchart illustrating an example of a bio impedance measurement method. Referring toFIG. 8, in operation810, the bio impedance measurement method includes providing a current of a first frequency to electrodes contacting a living body. In operation820, the bio impedance measurement method includes modulating a voltage generated as current flows through the living body, using a second frequency. In operation830, the bio impedance measurement method includes amplifying the modulated voltage by an amplifier. In operation840, the bio impedance measurement method includes demodulating the amplified voltage using a third frequency. In operation850, the bio impedance measurement includes filtering the demodulated voltage, namely, canceling a signal of a higher frequency than the third frequency from a frequency band of the demodulated voltage. Since the technical features illustrated with reference toFIGS. 1A to 7Bmay be directly applied to the operations ofFIG. 8, a detailed description will be omitted.

FIG. 9is an operation flowchart illustrating an example of a method of determining frequencies. Referring toFIG. 9, in operation910, the method includes determining a first frequency based on a type of bio impedance to be measured. In operation920, the method includes determining a third frequency based on a bandwidth of an amplifier. In operation930, the method includes determining a second frequency based on the first frequency and the third frequency.

The method of determining the frequencies may be performed by the controller160ofFIG. 4, the controller510ofFIG. 5A, or the controller510ofFIG. 5B. For example, the bandwidth of the amplifier may be about 10 kHz, and a frequency of an AC output from a current applicator may be about 100 kHz. The controller may determine the third frequency to be about 2 kHz so that the third frequency is fully included in the bandwidth of the amplifier. Furthermore, the controller may determine the second frequency by subtracting the third frequency from the first frequency. The second frequency may be determined as 100 kHz−2 kHz=98 kHz.

The various modules, elements, and methods described above may be implemented using one or more hardware components, one or more software components, or a combination of one or more hardware components and one or more software components.

A hardware component may be, for example, a physical device that physically performs one or more operations, but is not limited thereto. Examples of hardware components include microphones, amplifiers, low-pass filters, high-pass filters, band-pass filters, analog-to-digital converters, digital-to-analog converters, and processing devices.

A software component may be implemented, for example, by a processing device controlled by software or instructions to perform one or more operations, but is not limited thereto. A computer, controller, or other control device may cause the processing device to run the software or execute the instructions. One software component may be implemented by one processing device, or two or more software components may be implemented by one processing device, or one software component may be implemented by two or more processing devices, or two or more software components may be implemented by two or more processing devices.

A processing device may be implemented using one or more general-purpose or special-purpose computers, such as, for example, a processor, a controller and an arithmetic logic unit, a digital signal processor, a microcomputer, a field-programmable array, a programmable logic unit, a microprocessor, or any other device capable of running software or executing instructions. The processing device may run an operating system (OS), and may run one or more software applications that operate under the OS. The processing device may access, store, manipulate, process, and create data when running the software or executing the instructions. For simplicity, the singular term “processing device” may be used in the description, but one of ordinary skill in the art will appreciate that a processing device may include multiple processing elements and multiple types of processing elements. For example, a processing device may include one or more processors, or one or more processors and one or more controllers. In addition, different processing configurations are possible, such as parallel processors or multi-core processors.

A processing device configured to implement a software component to perform an operation A may include a processor programmed to run software or execute instructions to control the processor to perform operation A. In addition, a processing device configured to implement a software component to perform an operation A, an operation B, and an operation C may have various configurations, such as, for example, a processor configured to implement a software component to perform operations A, B, and C; a first processor configured to implement a software component to perform operation A, and a second processor configured to implement a software component to perform operations B and C; a first processor configured to implement a software component to perform operations A and B, and a second processor configured to implement a software component to perform operation C; a first processor configured to implement a software component to perform operation A, a second processor configured to implement a software component to perform operation B, and a third processor configured to implement a software component to perform operation C; a first processor configured to implement a software component to perform operations A, B, and C, and a second processor configured to implement a software component to perform operations A, B, and C, or any other configuration of one or more processors each implementing one or more of operations A, B, and C. Although these examples refer to three operations A, B, C, the number of operations that may implemented is not limited to three, but may be any number of operations required to achieve a desired result or perform a desired task.

Software or instructions for controlling a processing device to implement a software component may include a computer program, a piece of code, an instruction, or some combination thereof, for independently or collectively instructing or configuring the processing device to perform one or more desired operations. The software or instructions may include machine code that may be directly executed by the processing device, such as machine code produced by a compiler, and/or higher-level code that may be executed by the processing device using an interpreter. The software or instructions and any associated data, data files, and data structures may be embodied permanently or temporarily in any type of machine, component, physical or virtual equipment, computer storage medium or device, or a propagated signal wave capable of providing instructions or data to or being interpreted by the processing device. The software or instructions and any associated data, data files, and data structures also may be distributed over network-coupled computer systems so that the software or instructions and any associated data, data files, and data structures are stored and executed in a distributed fashion.

For example, the software or instructions and any associated data, data files, and data structures may be recorded, stored, or fixed in one or more non-transitory computer-readable storage media. A non-transitory computer-readable storage medium may be any data storage device that is capable of storing the software or instructions and any associated data, data files, and data structures so that they can be read by a computer system or processing device. 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, or any other non-transitory computer-readable storage medium known to one of ordinary skill in the art.

Functional programs, codes, and code segments for implementing the examples disclosed herein can be easily constructed by a programmer skilled in the art to which the examples pertain based on the drawings and their corresponding descriptions as provided herein.