Bio-impedance measuring circuit and method and electronic equipment

Abstract

A bioimpedance measurement circuit for determining a corrected body impedance is disclosed having a set of terminals, a control circuit (200), an evaluation circuit (300), and a signal processing circuit (400). The control circuit (200) is configured to control: a measurement of body impedance (Zb) in which a stimulation current (iin) is applied through the first terminal and the second terminal, and an input voltage (vin) between the third terminal and the fourth terminal is measured; measuring a first body part impedance (Zw), in which a stimulation current (iin) is applied through the first terminal and the third terminal, and an input voltage (vin) between the first terminal and the third terminal is measured; and measurement of a second body part impedance (Zf), in which the stimulation current (iin) is applied through the second terminal and the fourth terminal, and the input voltage (vin) between the second terminal and the fourth terminal is measured. The signal processing circuit (400) is configured to determine an error impedance (Zerr) based on a measured first body part impedance (Zw), a measured second body part impedance (Zf), a stored first parasitic body part impedance (Zwp), a stored second parasitic body part impedance (Zf), and a stored input impedance (Zin), and determining a corrected body impedance (Zbcorr) based on the measured body impedance (Zb) and the error impedance (Zerr).

Description

Technical Field

[0001] The present disclosure relates to a bioimpedance measurement circuit for measuring bioimpedance at both ends of a measurement electrode and an electronic device having such a bioimpedance measurement circuit. The present disclosure also relates to a corresponding bioimpedance measurement method and a computer program product thereof.

[0002] Related Applications

[0003] This patent application claims the priority of German patent applications 102022121888.0 and 102022130841.3, the disclosure content of each of which is incorporated herein by reference. Background Art

[0004] Bioimpedance measurement is a biomedical technique that determines the electrical behavior of living tissue by applying a stimulation current to a sample, such as a human body part, using an electrode arrangement and measuring the resulting voltage by a four-point measurement method. The amplitude and phase of the resulting voltage signal will depend on the BIOZ impedance. The bioimpedance of interest can be determined by dividing the measured voltage by the stimulation current injected into the tissue.

[0005] Due to the asymmetry of the current paths of the arrangement of the measuring electrodes performing the four-point measurement method, common-mode signals are generated across the body impedance. These asymmetries are caused by the mismatch of the skin / electrode contact impedance. In addition, due to the asymmetry of the voltage paths between the measuring electrodes and the differential amplifier used to evaluate the voltage across the measuring electrodes, the common-mode signals are converted into additional differential signals, which are added to the expected differential signal. Therefore, the measured amplitude of the total input differential signal applied to the differential amplifier is higher than the expected amplitude, and the phase is also affected. Summary of the invention

[0006] The object to be achieved is to provide an improved bioimpedance measurement concept which provides the bioimpedance of interest as accurately as possible and in particular reduces the influence of mismatches between skin / electrode contact impedances.

[0007] This object is achieved by the subject matter of the independent claim. Embodiments and developments result from the dependent claims.

[0008] Bioimpedance measurements are usually performed by applying a current to the sample and measuring the resulting voltage by a 4-point measurement method. The applied current is an alternating current, such as a 50Khz sine wave.

[0009] The amplitude and phase of the resulting voltage signal will depend on the bioimpedance, or generally the body impedance.

[0010] The improved bioimpedance measurement concept is based on the finding that the actual measurement of the body impedance is affected by errors caused by common mode signals due to inevitable mismatches in the contact impedances. Therefore, a correction based on making various contact impedances effective during the application of four electrodes to the body is proposed. However, since even these impedances are affected by parasitic impedances, a correction of the measured body impedance is required to determine the corrected body impedance based on the various impedances of the body parts and the corresponding stored parasitic impedances that can be determined during a calibration phase. Therefore, for each measurement of the body impedance, impedance measurements of at least two body parts have to be performed, the body parts corresponding for example to the locations where the electrodes are applied, such as the wrist and the fingers in the case of use of a wristband or a watch on the wrist for example.

[0011] In an embodiment of a bioimpedance measurement circuit according to the improved bioimpedance measurement concept, the bioimpedance measurement circuit comprises a set of terminals, the set of terminals comprising a first terminal for connecting a first electrode, a second terminal for connecting a second electrode, a third terminal for connecting a third electrode and a fourth terminal for connecting a fourth electrode, each electrode being attached to the body. A control circuit is included to control: applying a stimulation current, in particular an AC current, with a measurement frequency through a first subset of two selected terminals in the set of terminals; and measuring an input voltage in response to the stimulation current at a second subset of two selected terminals in the set of terminals. An evaluation circuit is included for determining a measured impedance in response to the stimulation current and the measured input voltage. The signal processing circuit determines the body impedance.

[0012] The control circuit is configured to control: measurement of body impedance, wherein a stimulation current is applied through a first terminal and a second terminal, and an input voltage between a third terminal and a fourth terminal is measured; measurement of a first body part impedance, wherein a stimulation current is applied through a first terminal and a third terminal, and an input voltage between the first terminal and the third terminal is measured; and measurement of a second body part impedance, wherein a stimulation current is applied through a second terminal and a fourth terminal, and an input voltage between the second terminal and the fourth terminal is measured. The signal processing circuit is configured to determine an error impedance based on the measured first body part impedance, the measured second body part impedance, the stored first parasitic body part impedance, the stored second parasitic body part impedance, and the stored input impedance, and to determine a corrected body impedance based on the measured body impedance and the error impedance.

[0013] Thus, by only two additional measurements, namely the measurement of the body part impedance, a more accurate result for the body impedance can be achieved.

[0014] For example, the signal processing circuit is configured to determine the corrected body impedance as a difference between the measured body impedance and the error impedance.

[0015] In some implementations, the signal processing circuit is further configured to determine a first corrected body part impedance as a parallel equivalent impedance of a measured first body part impedance and a stored first parasitic body part impedance, determine a second corrected body part impedance as a parallel equivalent impedance of a measured second body part impedance and a stored second parasitic body part impedance, and determine an error impedance based on the first corrected body part impedance, the second corrected body part impedance, and the stored input impedance.

[0016] In some implementations, the control circuit is further configured to control measurement of the total impedance, wherein a stimulation current is applied through the third terminal and the fourth terminal, and an input voltage between the third terminal and the fourth terminal is measured. In this implementation, the signal processing circuit may also be configured to determine a corrected total impedance as a parallel equivalent impedance of the measured total impedance and the stored parasitic total impedance, and further determine a corrected body impedance based on the corrected total impedance.

[0017] In some implementations, the signal processing circuit is further configured to determine the stored first parasitic body site impedance, the stored second parasitic body site impedance, and the stored input impedance during the calibration phase.

[0018] For example, during the calibration phase, the first terminal, the second terminal, the third terminal, and the fourth terminal are connected to the calibration body via corresponding electrodes. The signal processing circuit is configured to determine the stored first parasitic body part impedance, the stored second parasitic body part impedance, and the stored input impedance based on the corresponding measured values ​​and the known impedance value of the calibration body corresponding to the measured values. Such calibration can be performed during the generation phase or at the end of the generation phase so that the corresponding parasitic impedance and input impedance can be stored in the memory of the bioimpedance measurement circuit.

[0019] An electrode arrangement comprising a first electrode, a second electrode, a third electrode and a fourth electrode respectively connected to corresponding terminals of the set of terminals may be part of a bioimpedance measurement circuit.

[0020] In an embodiment of a bioimpedance measurement method according to an improved bioimpedance measurement concept, a bioimpedance measurement method is performed using a set of terminals, the set of terminals including a first terminal connected to a first electrode attached to a body, a second terminal connected to a second electrode attached to the body, a third terminal connected to a third electrode attached to the body, and a fourth terminal connected to a fourth electrode attached to the body. The method comprises:

[0021] - generating a stimulation current having a measurement frequency;

[0022] - determining a body impedance in response to a stimulation current and in response to a measurement of an input voltage of the stimulation current, wherein the stimulation current is applied through a first terminal and a second terminal and the input voltage is measured between a third terminal and a fourth terminal;

[0023] - determining a first body part impedance in response to a stimulation current and a measured input voltage, wherein the stimulation current is applied through a first terminal and a third terminal and the input voltage is measured between the first terminal and the third terminal;

[0024] - determining a second body part impedance in response to a stimulation current and a measured input voltage, wherein the stimulation current is applied through the second terminal and the fourth terminal and the input voltage is measured between the second terminal and the fourth terminal;

[0025] - determining an error impedance based on the measured first body part impedance, the measured second body part impedance, the stored first parasitic body part impedance, the stored second parasitic body part impedance and the stored input impedance; and

[0026] -Determining a corrected body impedance based on the measured body impedance and the error impedance.

[0027] The bioimpedance measurement method may be a computer-implemented method, for example executed by a processor or programmable circuit.

[0028] According to the various implementations described above in conjunction with the bioimpedance measurement circuit, further implementations of the method will become obvious to those skilled in the art.

[0029] According to one embodiment of the improved bioimpedance measurement concept, a computer program product is disclosed, which includes instructions that, when executed on one or more processors associated with, for example, the first terminal, the second terminal, the third terminal, and the fourth terminal as described above, cause the one or more processors to perform a bioimpedance measurement method according to one of the disclosed implementations.

[0030] Various implementations of bioimpedance measurement circuits and methods can be used in various applications and products, such as electronic devices, vital sign monitoring in wearable devices (smart watches) or healthcare applications such as diagnostics. Body impedance can, for example, be the basis for determining body cell mass composition (BCM), hydration level detection, calorie consumption, fat, muscle ratio, stress level, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The improved bioimpedance measurement concept will be described in more detail below with the aid of the accompanying drawings. Elements and functional blocks having the same or similar functions have the same reference numerals throughout the accompanying drawings. Therefore, their description need not be repeated in the following drawings.

[0032] In the attached picture:

[0033] Figure 1 An example implementation of a bioimpedance measurement circuit with current / voltage path asymmetry due to a mismatch between skin / electrode contact impedances is shown;

[0034] Figure 2 Another example implementation of a bioimpedance measurement circuit is shown;

[0035] Figure 3 Shows Figure 2 Details of the bioimpedance measurement circuit;

[0036] Figure 4 Various measurement configurations of the calibration phase of a bioimpedance measurement circuit are shown;

[0037] Figure 5 Various measurement configurations of the measurement phase of the bioimpedance measurement circuit are shown;

[0038] Figure 6 Shows Figure 5 and Figure 6 Details of the bioimpedance measurement circuit used in;

[0039] Figure 7 An example block diagram showing stages of operation in a bioimpedance measurement circuit;

[0040] Figure 8 Another example block diagram showing stages of operation in a bioimpedance measurement circuit; and

[0041] Fig. 9 Another example implementation of a bioimpedance measurement circuit is shown. DETAILED DESCRIPTION

[0042] Figure 1 A bioimpedance measurement circuit based on a four-point measurement method is shown. The bioimpedance measurement circuit includes an electrode arrangement 100 for attachment to a human body. The electrode arrangement 100 includes a first pair of electrodes 110, 120 for applying a stimulation current iin. The electrodes 110, 120 are attached to nodes A and B of the human body. It is assumed that the electrode 110 has an electrode contact impedance Z3, and the electrode 120 has an electrode contact impedance Z4. The electrode arrangement also includes a second pair of electrodes 130, 140, which are used to measure the voltage vin between the second pair of electrodes 130, 140. The electrodes 130, 140 are attached to nodes C and D of the human body. It is assumed that the electrode 130 has an electrode contact impedance Z1, and the electrode 140 has an electrode contact impedance Z2.

[0043] The electrodes 110, 120, 130 and 140 are placed to measure the bioimpedance of interest BIOZ between nodes C and D of the human body or between electrode 130 and electrode 140. The resulting voltage vin is applied to and evaluated by an evaluation circuit 300, which is shown in simplified form as Figure 1 The differential amplifier 310 in FIG. The amplitude and phase of the resulting voltage signal vin depend on the bioimpedance BIOZ of interest, and the bioimpedance BIOZ is the quotient vin / iin.

[0044] Ideally, the voltage vin at the input of the evaluation circuit 300 is equal to the desired differential signal vdm, which represents the voltage drop across the bioimpedance BIOZ. However, in practice, due to the asymmetry of the current path, a common-mode signal is generated across the body impedance BIOZ. Assuming that in some implementations, the common mode at nodes A and B is adjusted to a certain internally defined reference voltage vref, the common-mode signal between nodes C and D is:

[0045]

[0046] Wherein, VA is the voltage potential at node A, VB is the voltage potential at node B, VC is the voltage potential at node C, and VD is the voltage potential at node D.

[0047] Therefore, a common mode signal vcm is generated on the bioimpedance BIOZ between the node C and the node D, wherein the common mode signal vcm is proportional to the stimulation current:

[0048]

[0049] Due to the asymmetries in the voltage paths, the common-mode signal Vcm is converted into an unwanted additional differential signal Vdmx. These asymmetries arise from the mismatch between the skin / electrode contact impedances.

[0050] Figure 2 Another example implementation of a bioimpedance measurement circuit is shown, which is similar to Figure 1 , and includes an input impedance Zin at the input of the amplifier 310. Furthermore, it can be seen that the impedance at the electrodes 110, 120, 130, 140 has both a resistive component and a capacitive component.

[0051] Figure 3 Shows Figure 2 The details of the bioimpedance measurement circuit are shown in Figure 3. Let Zin be the single-ended input impedance of amplifier 310. This impedance is expected to be primarily capacitive, but can generally be modeled as a parallel combination of a resistor and a capacitor:

[0052]

[0053] Due to the mismatch of contact impedance, the common mode signal in the body will be converted into a differential signal vdmx at the input of amplifier 310:

[0054]

[0055] Using the expression of vcm obtained above, we get vdmx:

[0056]

[0057] Return to reference Figure 2 , let Zb be the body impedance, and then assuming that there is no significant attenuation in the differential path, the expected measured differential voltage vdm is:

[0058] vdm=iin·Zb

[0059] However, due to the error in the common-mode signal, the total input differential voltage will be:

[0060] vin=vdm+vdmx, or

[0061]

[0062] The measured (body) impedance will then be:

[0063]

[0064] So the final expression of error impedance Zerr is:

[0065]

[0066] Assuming that the contact impedances at the two input terminals of the amplifier 310 are equal, that is, for a first body part impedance such as wrist impedance Zw and a second body part impedance such as finger impedance Zf,

[0067] Z3≈Z1=Zw and

[0068] Z4≈Z2=Zf,

[0069] The error impedance expression for Zerr becomes:

[0070]

[0071] Therefore, the corrected body impedance results in the difference between the measured body impedance Zmeas and the error impedance Zerr.

[0072] For example, a correction method according to the improved bioimpedance measurement concept includes estimating the values ​​of Zf, Zw and Zin through measurement and calibration, calculating the error impedance Zerr and subtracting the error impedance Zerr from the measured body impedance.

[0073] Figure 4 Various measurement configurations a) to f) of the calibration phase of the bioimpedance measurement circuit are shown. As described, the error impedance Zerr depends on the first body part impedance or wrist impedance Zw, the second body part impedance or finger impedance Zf and the input impedance Zin, which values ​​must be known for each measurement of the body impedance. Although it can be assumed that the input impedance Zin of the amplifier remains constant, the effective body part impedance may change with, for example, changing the application of electrodes.

[0074] For example, in configuration a) for measuring body impedance, a stimulation current iin is applied through the first terminal and the second terminal, and an input voltage vin is measured between the third terminal and the fourth terminal or between corresponding electrodes, respectively. Similarly, in configuration b) for measuring the impedance of a first body part or wrist impedance Zw, a stimulation current iin is applied through the first terminal and the third terminal, and an input voltage vin is measured between the first terminal and the third terminal. In configuration c), the total impedance Zt can be measured, wherein a stimulation current iin is applied through the third terminal and the fourth terminal, and an input voltage vin is measured between the third terminal and the fourth terminal. Similarly, in configuration d) for measuring the impedance of a second body part or finger impedance Zf, a stimulation current iin is applied through the second terminal and the fourth terminal, and an input voltage vin is measured between the second terminal and the fourth terminal or between corresponding electrodes, respectively.

[0075] During the calibration phase, electrodes are placed on the body or other object with known impedances, such as known body impedance Zbk, known wrist impedances Zwlk, Zw2k, and known finger impedances Zf1k, Zf2k.

[0076] In configurations e) and f), zero impedance and a known resistor impedance, such as 2 kΩ, are measured.

[0077] For example, ideally, using Figure 4 In configuration b), 2*Zw can be measured directly. However, when the magnitude of 2*Zw is high enough, the current leakage through the parasitic parallel path can lead to significant errors. In order to correct this error, it is proposed to have a calibration phase in which the impedance Zwp of the parallel parasitic path is estimated.

[0078] During the calibration phase, a known reference impedance is used, so the value of the impedance is known in advance.

[0079] Let Zw1k, Zw2k be the known wrist impedances of the selected reference impedance used for calibration. Therefore, due to the parasitic parallel path, instead of measuring Zw1k+Zw2k, the system will represent the measurement as a different value of Zwm.

[0080] The measured contact impedance will be

[0081] Zwm=(Zw1k+Zw2k)||Zwp

[0082] By using the known impedances Zw1k, Zw2k, the parasitic impedance can be calculated by solving the following equation:

[0083]

[0084] Similar methods for other configurations will be apparent to readers skilled in the art.

[0085] Typically, during calibration, a known reference impedance is connected and four values ​​are measured:

[0086] -Body impedance Zb

[0087] -Wrist impedance Zw (divide the result by 2 to get Zw)

[0088] - Finger impedance Zf (divide the result by 2 to get Zf)

[0089] -Total impedance Zt (optional)

[0090] The known reference impedance value will be expressed as:

[0091] -Known body impedance: Zbk

[0092] -Known wrist impedance: Zw1k, Zwk2

[0093] -Known finger impedance: Zf1k, Zf2k

[0094] - Known total impedance: Ztk = Zbk + Zw2k + Zf2k

[0095] Based on the previous information, 4 values ​​are calculated

[0096] -Equivalent impedance in parallel with the wrist: Zwp

[0097] -Equivalent impedance in parallel with the finger: Zfp

[0098] -Equivalent impedance in parallel with the total impedance: Ztp (optional)

[0099] -Equivalent single-ended input impedance: Zin

[0100] For example, during calibration the formula is:

[0101] Intermediate parameter fac (current shunt coefficient of total impedance):

[0102]

[0103] Intermediate parameter Zberr (body impedance error):

[0104]

[0105] To determine the input impedance Zin, solve the quadratic equation for the error impedance Zerr:

[0106] a=2·Zberr

[0107] b=2·Zberr·(Zfk+Zwk)-(Zfk-Zwk) 2

[0108] c=2·Zberr·Zfk·Zwk

[0109] delta=b 2 -4·a·c

[0110]

[0111] If the imaginary part of Zin is positive, then

[0112]

[0113] Now refer to Figure 5 , shows various measurement configurations of the measurement phase of the bioimpedance measurement circuit. For example, Figure 6 The multiplexer 250 shown implements the corresponding configurations a) to d) corresponding to the Figure 4 The only difference is that the respective impedances are unknown. However, at this stage, the parasitic impedances can be used to perform corrections.

[0114] Figure 7 An example block diagram of the operation phases in the bioimpedance measurement circuit is shown. For example, as described above, the system model and correction estimate correspond to the calibration phase with 2+4 measurements and known impedance as its input. During normal operation or measurement operation, only Figure 5 The four measurements which form the basis for the actual determination of body impedance are also described above.

[0115] like Figure 8As shown, the calibration routine receives the measured values ​​Zw, Zf, Zt, Zb and the known impedances Zwk, Zfk, Zbk, Ztk, where Zt may be optional for a known body. As described, the result is the parasitic impedances Zwp, Zfp, Ztp and the input impedance Zin. During normal operation or measurement operation, the correction routine (also) receives the measured values ​​Zw, Zf, Zt, Zb (where Zt may be optional) and performs a corresponding correction based on the parasitic impedances Zwp, Zfp, Ztp and the input impedance Zin to determine the corrected body impedance Zb.

[0116] The formula used during measurement and calibration is:

[0117] The measured values ​​are expressed as Zwm, Zfm, Zbm, Ztm.

[0118] The correction values ​​are expressed as Zwc, Zfc, Zbc, and Ztc.

[0119] Corrected impedance:

[0120] Zwc=Zwm||(-Zwp)

[0121] Zfc=Zfm||(-Zfp)

[0122] Ztc=Ztm||(-Ztp)

[0123] For each measurement, the error impedance Zerr disclosed above needs to be calculated using the correction impedances Zwc and Zfc as well as Zin.

[0124] Fig. 9 A bioimpedance measurement circuit 10 is shown that allows the determination of the real part I and the imaginary part Q of the measured bioimpedance. The measurement circuit 10 comprises an electrode arrangement 100 for attachment to a body. The electrode arrangement 100 comprises four electrodes 110, 120, 130, 140 to apply a stimulation current iin and measure an input voltage vin according to a configuration selected via a multiplexer 250. The electrodes 110, 120 are attached to nodes A and B of the human body, wherein Z3 represents the electrode contact impedance between the skin of the body and the electrode 110, and Z4 represents the electrode contact impedance between the skin of the body and the electrode 120.

[0125] The bioimpedance measurement circuit 10 further comprises a control circuit 200 controlling the respective impedance measurement, and an evaluation circuit 300. The evaluation circuit 300 is configured to determine the real part I and the imaginary part Q of the measured impedance in response to the stimulation current iin and the measured input voltage vin.

[0126] The evaluation circuit 300 comprises a differential amplifier 310 to apply the measured input voltage vin. The output side of the differential amplifier 310 is coupled to a first path comprising a modulator 320 and a low-pass filter 350 and to a second path comprising a modulator 330 and a low-pass filter 360. The modulators 320 and 330 are coupled to an oscillator 340. An arrangement in which the modulators 320, 330 are coupled to the oscillator 340 and the low-pass filters 350, 360 are arranged after the modulators 320, 330 is provided to achieve quadrature demodulation, which allows the real part I and the imaginary part Q of the measured impedance to be measured.

[0127] Reference Fig. 9 The bioimpedance measurement circuit comprises a signal processing circuit 400, such as a processor for determining a correction value of the bioimpedance (BIOZ), i.e. for performing the corresponding calculations as described above. To this end, the signal processing circuit 400 may comprise or be connected to a memory for storing the values ​​required for the correction.

[0128] The control circuit 200 is configured to control the measurement of the impedance by applying a stimulation current iin having a measurement frequency F to the electrode arrangement 100, in particular by controlling the multiplexer 250 according to a desired impedance.

[0129] The control circuit 200 can also control different operation modes, ie calibration operation and normal operation or measurement operation, respectively. The control circuit 200 itself can be controlled by the signal processing circuit 400 .

[0130] In order to familiarize the reader with the novel aspects of the implementation of the improved bioimpedance measurement concept, the embodiments of the improved bioimpedance measurement concept disclosed herein have been discussed. Although preferred embodiments have been shown and described, many changes, modifications, equivalents and alternatives may be made to the disclosed concept by those skilled in the art without departing from the scope of the claims.

[0131] In particular, the implementation of the improved bioimpedance measurement concept is not limited to the disclosed embodiments, and examples of many possible alternatives of the features included in the discussed embodiments are given. However, it is intended that any modifications, equivalents and alternatives of the disclosed concept are included within the scope of the claims attached hereto.

[0132] Features recited in separate dependent claims may advantageously be combined. Furthermore, reference signs used in the claims are not to be construed as limiting the scope of the claims.

[0133] Furthermore, as used herein, the term "comprising" does not exclude other elements. In addition, as used herein, the articles "a" and "an" are intended to include one or more than one components or elements, and are not limited to being interpreted as meaning only one.

[0134] Reference numerals

[0135] 10 Bioimpedance Measurement Circuit

[0136] 100 Electrode Arrangement

[0137] 110, 120, 130, 140 electrodes

[0138] 200 Control circuit

[0139] 250 Multiplexer

[0140] 300 Evaluation Circuit

[0141] 310 Differential Amplifier

[0142] 320 Modulator

[0143] 330 Modulator

[0144] 340 Oscillator

[0145] 350 Low pass filter

[0146] 360 Low Pass Filter

[0147] 400 Signal Processing Circuit

[0148] Z1, Z2, Z3, Z4 electrode contact impedance

[0149] BIOZ, Zb Bioimpedance

[0150] I Real part

[0151] Q imaginary part

Claims

1. A bioimpedance measurement circuit, comprising: - a set of terminals, the set of terminals comprising: - a first terminal for connecting a first electrode (110) to be attached to the body; - a second terminal for connecting a second electrode (120) to be attached to the body; - a third terminal for connecting a third electrode (130) to be attached to the body; - a fourth terminal for connecting a fourth electrode (140) to be attached to the body; - a control circuit (200) for controlling: applying a stimulation current (iin) having a measurement frequency (F) through a first subset of two selected terminals of the set of terminals, and measuring an input voltage (vin) in response to the stimulation current (iin) at a second subset of two selected terminals of the set of terminals; - an evaluation circuit (300) for determining a measured impedance in response to a stimulation current (iin) and a measured input voltage (vin); and - a signal processing circuit (400) for determining the body impedance (Zb), Wherein, the control circuit (200) is configured to control the measurement of the following items: - body impedance (Zb), wherein a stimulation current (iin) is applied through the first terminal and the second terminal and an input voltage (vin) is measured between the third terminal and the fourth terminal; - a first body part impedance (Zw), wherein a stimulation current (iin) is applied through the first terminal and the third terminal and an input voltage (vin) between the first terminal and the third terminal is measured; and - a second body part impedance (Zf), wherein a stimulation current (iin) is applied through the second terminal and the fourth terminal and an input voltage (vin) between the second terminal and the fourth terminal is measured; and Wherein, the signal processing circuit (400) is configured to: - determining an error impedance (Zerr) based on the measured first body part impedance (Zw), the measured second body part impedance (Zf), the stored first parasitic body part impedance (Zwp), the stored second parasitic body part impedance (Zf) and the stored input impedance (Zin); and - determining a corrected body impedance (Zbcorr) based on the measured body impedance (Zb) and said error impedance (Zerr).

2. The bioimpedance measurement circuit according to claim 1, wherein: The signal processing circuit (400) is configured to determine the corrected body impedance (Zbcorr) as a difference between the measured body impedance (Zb) and the error impedance (Zerr).

3. The bioimpedance measurement circuit according to claim 1 or 2, wherein: The signal processing circuit (400) is further configured to: - determining a first corrected body part impedance (Zwc) as a parallel equivalent impedance of the measured first body part impedance (Zw) and the stored first parasitic body part impedance (Zwp); - determining a second corrected body part impedance (Zfc) as a parallel equivalent impedance of the measured second body part impedance (Zf) and the stored second parasitic body part impedance (Zfp); and - determining the error impedance (Zerr) based on the first corrected body part impedance (Zwc), the second corrected body part impedance (Zfc) and the stored input impedance (Zin).

4. The bioimpedance measurement circuit according to one of claims 1 to 3, wherein: The control circuit (200) is also configured to control the measurement of the total impedance (Zt), wherein a stimulation current (iin) is applied through the third terminal and the fourth terminal, and an input voltage (vin) between the third terminal and the fourth terminal is measured, and wherein the signal processing circuit (400) is also configured to: determine the corrected total impedance (Ztc) as the parallel equivalent impedance of the measured total impedance (Zt) and the stored parasitic total impedance (Ztp), and also determine the corrected body impedance (Zbcorr) based on the corrected total impedance (Ztc).

5. The bioimpedance measurement circuit according to one of claims 1 to 4, wherein: The signal processing circuit (400) is also configured to determine a stored first parasitic body part impedance (Zwp), a stored second parasitic body part impedance (Zf), and a stored input impedance (Zin) during a calibration phase.

6. The bioimpedance measurement circuit according to claim 5, wherein: During the calibration phase, the first terminal, the second terminal, the third terminal and the fourth terminal are connected to a calibration body via corresponding electrodes, and wherein the signal processing circuit (400) is configured to determine a stored first parasitic body part impedance (Zwp), a stored second parasitic body part impedance (Zf) and a stored input impedance (Zin) based on corresponding measurement values ​​and known impedance values ​​of the calibration body corresponding to the measurement values.

7. The bioimpedance measurement circuit according to one of claims 1 to 6, further comprising an electrode arrangement (100) for attachment to a body, the electrode arrangement (100) comprising a first electrode (110), a second electrode (120), a third electrode (130) and a fourth electrode (140) respectively connected to corresponding terminals of the set of terminals.

8. An electronic device comprising the bioimpedance measurement circuit according to one of claims 1 to 7.

9. A bioimpedance measurement method performed using a set of terminals, the set of terminals comprising: - a first terminal connected to a first electrode (110) attached to the body; - a second terminal connected to a second electrode (120) attached to the body; - a third terminal connected to a third electrode (130) attached to the body; as well as - a fourth terminal connected to a fourth electrode (140) attached to the body; The method comprises: - generating a stimulation current (iin) having a measurement frequency (F); - determining a body impedance (Zb) in response to a stimulation current (iin) and in response to a measured input voltage (vin) of the stimulation current (iin), wherein the stimulation current (iin) is applied through the first terminal and the second terminal and the input voltage (vin) is measured between the third terminal and the fourth terminal; - determining a first body part impedance (Zw) in response to a stimulation current (iin) and a measured input voltage (vin), wherein the stimulation current (iin) is applied through the first terminal and the third terminal and the input voltage (vin) is measured between the first terminal and the third terminal; - determining a second body part impedance (Zf) in response to a stimulation current (iin) and a measured input voltage (vin), wherein the stimulation current (iin) is applied through the second terminal and the fourth terminal and the input voltage (vin) is measured between the second terminal and the fourth terminal; - determining an error impedance (Zerr) based on the measured first body part impedance (Zw), the measured second body part impedance (Zf), the stored first parasitic body part impedance (Zwp), the stored second parasitic body part impedance (Zf) and the stored input impedance (Zin); and - determining a corrected body impedance (Zbcorr) based on the measured body impedance (Zb) and said error impedance (Zerr).

10. The method according to claim 9, wherein: The corrected body impedance (Zbcorr) is determined as the difference between the measured body impedance (Zb) and the error impedance (Zerr).

11. The method according to claim 9 or 10, wherein: Determining the error impedance (Zerr) includes: - determining a first corrected body part impedance (Zwc) as a parallel equivalent impedance of the measured first body part impedance (Zw) and the stored first parasitic body part impedance (Zwp); - determining a second corrected body part impedance (Zfc) as a parallel equivalent impedance of the measured second body part impedance (Zf) and the stored second parasitic body part impedance (Zfp); and - determining the error impedance (Zerr) based on the first corrected body part impedance (Zwc), the second corrected body part impedance (Zfc) and the stored input impedance (Zin).

12. The method according to one of claims 9 to 11, further comprising: - determining a total impedance (Zt) in response to a stimulation current (iin) and a measured input voltage (vin), wherein the stimulation current (iin) is applied through the third terminal and the fourth terminal and the input voltage (vin) is measured between the third terminal and the fourth terminal; - determining a corrected total impedance (Ztc) as the parallel equivalent impedance of the measured total impedance (Zt) and the stored parasitic total impedance (Ztp); and - determining said corrected body impedance (Zbcorr) also based on said corrected total impedance (Ztc).

13. The method according to one of claims 9 to 12, wherein: Determining the stored first parasitic body part impedance (Zwp), the stored second parasitic body part impedance (Zf), and the stored input impedance (Zin) is performed during a calibration phase.

14. The method according to claim 13, wherein: During the calibration phase, the first terminal, the second terminal, the third terminal and the fourth terminal are connected to a calibration body via corresponding electrodes, and wherein a stored first parasitic body part impedance (Zwp), a stored second parasitic body part impedance (Zf) and a stored input impedance (Zin) are determined based on corresponding measured values ​​and known impedance values ​​of the calibration body corresponding to the measured values.

15. A computer program product comprising instructions, which, when executed on one or more processors associated with a first terminal, a second terminal, a third terminal and a fourth terminal, cause the one or more processors to perform the bioimpedance measurement method according to one of claims 9 to 14.

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