SYSTEM AND A METHOD FOR ACQUIRING AN ELECTRICAL SIGNAL AND A WEARABLE DEVICE

Abstract

A system for acquiring an electrical signal comprises: a plurality of electrodes, a plurality of signal quality detectors, each detector, being configured to detect a signal from a pair of electrodes and each detector comprising an analog-to-digital converter for providing a digital representation of a first resolution of the detected signal; a signal selection logic for determining at least one quality measure of each of the digital representations for selecting a pair of electrodes for signal acquisition; a multiplexer for selecting a pair of electrodes for signal acquisition based on a control signal from the signal selection logic; and a signal processing unit for performing analog-to-digital conversion on the selected signal and providing a digital representation having a second resolution, which is higher than the first resolution.

Claims

1. A system for acquiring an electrical signal, said system comprising: a plurality of electrodes, each being configured to acquire an electrode signal; a plurality of signal quality detectors, each signal quality detector being associated with at least a pair of electrodes among the plurality of electrodes, wherein each detector is configured to detect a signal based on input from the pair of electrodes and each detector comprises an analog-to-digital converter, which is configured to provide a digital representation of the detected signal, wherein the digital representation has a first resolution; a signal selection logic, which is configured to receive the digital representations determined by the detectors and is configured to determine at least one quality measure of each of the digital representations for selecting a pair of electrodes for signal acquisition and output a control signal; a multiplexer, which is connected to each of the plurality of electrodes and which is configured to select a pair of electrodes for signal acquisition based on the control signal from the signal selection logic; and a signal processing unit, which is configured to receive the selected signal from the multiplexer based on the selected pair of electrodes and which is configured to perform analog-to-digital conversion on the selected signal and to provide a digital representation of the selected signal, wherein the digital representation has a second resolution, which is higher than the first resolution.

2. The system according to claim 1, wherein the multiplexer is connected directly to the plurality of electrodes, such that the electrode signal by-passes the signal quality detector for being received by the multiplexer.

3. The system according to claim 1, wherein the multiplexer and the signal processing unit are a first multiplexer and a first signal processing unit, and wherein the system further comprises a second multiplexer, which is connected to each of the plurality of electrodes and which is configured to select a pair of electrodes for signal acquisition based on the control signal from the signal selection logic, and a second signal processing unit, which is configured to perform analog-to-digital conversion on the selected signal, wherein the digital representation has the second resolution, wherein the signal selection logic is configured to selectively activate one of the first or the second signal processing unit upon determining that the pair of electrodes for signal acquisition is to be changed.

4. The system according to claim 3, wherein the signal selection logic is configured to compare the at least one quality measure for pairs of electrodes to the at least one quality measure for a currently selected pair of electrodes for signal acquisition for determining that the pair of electrodes for signal acquisition is to be changed.

5. The system according to claim 1, wherein the at least one quality measure comprises a width of a frequency spectrum of the electrode signal.

6. The system according to claim 1, wherein the at least one quality measure comprises a noise floor value.

7. The system according to claim 1, wherein the at least one quality measure comprises a value representing noise in frequency domain.

8. The system according to claim 1, wherein the system comprises a first sensor unit and a second sensor unit, each of the first and the second sensor unit comprising a plurality of electrodes, a plurality of signal quality detectors, a signal selection logic and a multiplexer, wherein the first sensor unit and the second sensor unit are configured to select a pair of electrodes for signal acquisition of the sensor unit and wherein the first sensor unit and the second sensor unit are configured to acquire selected signals in different relations to a measurement subject, wherein the system further comprises an additional multiplexer, which is configured to receive the selected signals output by the multiplexers of the sensor units and to sequentially output selected signals from different sensor units to the signal processing unit.

9. The system according to claim 1, wherein the system comprises a number of detectors corresponding to a number of possible combinations of pairs of electrodes among the plurality of electrodes.

10. The system according to claim 9, wherein each detector is statically connected to a pair of electrodes among the plurality of electrodes and wherein each of the plurality of detectors is connected to a unique pair of electrodes.

11. The system according to claim 1, wherein the detectors are configured to detect a signal using a first sampling rate and wherein the signal processing unit is configured to detect the selected signal using a second sampling rate, which is higher than the first sampling rate.

12. A wearable device for acquiring an electrical signal of a subject wearing the device, the wearable device comprising: a system according to claim 1, at least one carrier, which is configured to be worn on or around a body part of the subject, wherein the at least one carrier is configured to carry the system.

13. The wearable device according to claim 12, wherein the system comprises a first sensor unit and a second sensor unit, each of the first and the second sensor unit comprising a plurality of electrodes, a plurality of signal quality detectors, a signal selection logic and a multiplexer, wherein the first sensor unit and the second sensor unit are configured to select a pair of electrodes for signal acquisition of the sensor unit and wherein the first sensor unit and the second sensor unit are configured to acquire selected signals in different relations to a measurement subject, wherein the system further comprises an additional multiplexer, which is configured to receive the selected signals output by the multiplexers of the sensor units and to sequentially output selected signals from different sensor units to the signal processing unit, and wherein the first sensor unit and the second sensor unit are mounted on or in different parts of the carrier, such that the first sensor unit and the second sensor unit are configured to be arranged to acquire electrode signals from different portions of the body part of the subject when the wearable device is worn.

14. A method for controlling acquisition of an electrical signal; said method comprising: acquiring electrode signals by a plurality of pairs of electrodes; for each electrode signal acquired by a pair of electrodes, determining, by a signal quality detector, a digital representation of a detected signal, wherein the digital representation has a first resolution; selecting, by a signal selection logic, a pair of electrodes for signal acquisition based on a determined at least one quality measure of each of the digital representations; controlling a multiplexer to output a selected signal based on the selected pair of electrodes to a signal processing unit; performing, by the signal processing unit, analog-to-digital conversion on the selected signal and to provide a digital representation of the selected signal, wherein the digital representation has a second resolution, which is higher than the first resolution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

[0064] FIG. 1 is a schematic view of a system for acquisition of an electrical signal according to an embodiment.

[0065] FIG. 2 is a schematic view illustrating details of a signal quality detector of the system.

[0066] FIG. 3 is a schematic view illustrating details of a signal processing unit of the system.

[0067] FIG. 4 is a flow chart illustrating a process of changing electrodes used for signal acquisition.

[0068] FIG. 5 is a schematic view of a system for acquisition of an electrical signal including a plurality of sensor units.

[0069] FIG. 6 is a schematic view of a wearable device including the system.

[0070] FIG. 7 is a flow chart of a method according to an embodiment.

DETAILED DESCRIPTION

[0071] Referring now to FIG. 1, a system 100 for acquiring of an electrical signal will be described.

[0072] The system 100 comprises a plurality of electrodes 102. Each electrode 102 may be configured to be arranged in contact with a subject for acquiring an electrode signal.

[0073] The electrodes 102 may e.g. be configured for direct galvanic contact with a skin of a person. However, it should also be realized that the electrodes 102 may be arranged within a carrier, so as to be arranged in close relation to the skin of the person for forming a capacitively coupled connection to the subject.

[0074] The electrodes 102 may be designed in many different ways as will be appreciated by a person skilled in the art. For instance, the electrodes 102 may comprise an area of conductive (e.g. metallic) material for acquiring of an electrical signal. The area of conductive material may be connected to a wire for transferring the electrical signal from the electrode 102.

[0075] The system 100 further comprises a plurality of signal quality detectors 104. Each signal quality detector 104 may be connected to a pair of electrodes 102 for receiving input from the pair of electrodes 102. The signal quality detector 104 may be statically connected to a specific pair of electrodes 102 or the connection between the pair of electrodes 102 and the signal quality detector 104 may be dynamically defined.

[0076] As further illustrated in FIG. 2, the signal quality detector 104 may comprise a differential amplifier 106, which is connected to receive input from the pair of electrodes 102 and amplify a difference in the signals from the electrodes 102.

[0077] The differential amplifier may provide a front-end interface of the pair of electrodes 102. The differential amplifier may be a relatively low power analog amplifier, which may provide a relatively high noise while requiring limited power consumption.

[0078] The signal quality detector 104 may further comprise an analog-to-digital converter 108, which may receive the signal from the differential amplifier 106 and which may convert the signal to a digital representation. The analog-to-digital converter 108 may be defined to provide a digital representation using N bits of data, providing a relatively low resolution of the signal detected by the signal quality detector 104 and, hence, requiring a relatively low power consumption.

[0079] The system 100 further comprises a signal selection logic 110. The signal selection logic 110 may be configured to receive the digital representations from each of the signal quality detectors 104.

[0080] The signal selection logic 110 may be configured to determine at least one quality measure of each of the digital representations. The determination of the at least one quality measure may be used for selecting a pair of electrodes 102 to be used for signal acquisition.

[0081] According to an embodiment, the signal selection logic 110 may be configured to determine which of the digital representations from the signal quality detectors 104 that corresponds to a highest quality signal based on the at least one quality measure.

[0082] However, it should be realized that the signal selection logic 110 may alternatively be configured to select the pair of electrodes 102 for signal acquisition in another manner. For instance, a threshold of an acceptable quality may be set and the signal selection logic 110 may be configured to select a pair of electrodes 102 for signal acquisition based on a first identified digital representation that exceeds the threshold. This implies that a process of selecting a pair of electrodes 102 by the signal selection logic 110 may be quickly terminated, which may reduce power consumption by the signal selection logic 110 and may ensure that the pair of electrodes 102 for signal acquisition may be quickly selected.

[0083] According to another alternative, a maximum time may be allowed for the signal selection logic 110 to select the pair of electrodes 102. Thus, the pair of electrodes 102 providing a highest quality signal as identified within the maximum time may be selected by the signal selection logic 110.

[0084] The at least one quality measure may be any of a plurality of different measures indicating quality of a signal. For instance, the at least one quality measure may be selected from a group comprising: a width of a frequency spectrum of a signal from pair of electrodes, a noise floor value, a value representing noise in frequency domain, or a signal-to-noise ratio.

[0085] According to an embodiment, the quality measure may relate to specific signal characteristics of the electrical signal to be acquired by the electrodes 102, such as characteristics of a QRS complex in an ECG signal.

[0086] The signal selection logic 110 may be implemented in hardware, or as any combination of software and hardware. The signal selection logic 110 may be implemented as software being executed on a general-purpose computer. The system 100 may thus comprise a processing unit, such as a central processing unit (CPU), which may execute the instructions of one or more computer programs in order to implement functionality of the signal selection logic 110.

[0087] The signal selection logic 110 may alternatively be implemented as firmware arranged e.g. in an embedded system, or as a specifically designed processing unit, such as an Application-Specific Integrated Circuit (ASIC) or a Field-Programmable Gate Array (FPGA), which may be configured to implement functionality of the signal selection logic 110.

[0088] The system 100 may further comprise a multiplexer 112. The multiplexer 112 may be connected to each of the plurality of electrodes 102. The multiplexer 112 may further receive a control signal from the signal selection logic 110 for controlling selection of the pair of electrodes 102.

[0089] Based on the control signal, the multiplexer 112 may provide signals from the two selected electrodes 102 to be passed to a signal processing unit 114.

[0090] In FIG. 1, the system 100 is illustrated as having a first multiplexer 112 and a second multiplexer 122, as well as a first signal processing unit 114 and a second signal processing unit 124. However, it should be realized that the system 100 may include only a single multiplexer 112 and a single signal processing unit 114, which may be used in acquiring a signal from the selected pair of electrodes 102 and processing the acquired signal.

[0091] As further illustrated in FIG. 3, the signal processing unit 114 may comprise a differential amplifier 116, which is connected to receive input from the pair of electrodes 102 as selected by the multiplexer 112 and amplify a difference in the signals from the electrodes 102.

[0092] The differential amplifier may provide a front-end interface of the pair of electrodes 102. The differential amplifier may be a relatively high power analog amplifier, which may provide a relatively low noise while requiring limited power consumption. Thus, the differential amplifier 116 of the signal processing unit 114 may have a higher power consumption than the differential amplifier 106 of the signal quality detector 104, while providing a lower noise signal than the differential amplifier 106 of the signal quality detector 104.

[0093] The signal processing unit 114 may further comprise an analog-to-digital converter 118, which may receive the signal from the differential amplifier 116 and which may convert the signal to a digital representation. The analog-to-digital converter 118 may be defined to provide a digital representation using M bits of data. The analog-to-digital converter 118 may provide a relatively high resolution of the signal and, hence, require a relatively high power consumption. The M bits of data may be higher than the N bits of data provided by the signal quality detector 104, such that the signal processing unit 114 may require a higher power consumption for the analog-to-digital conversion than the signal quality detector 104 while providing a higher resolution of the digital representation of the signal than the signal quality detector 104.

[0094] In addition, the signal quality detector 106 may possibly use a lower sampling rate than the signal processing unit 114, such that the signal quality detector 106 may further limit its power consumption.

[0095] As illustrated in FIG. 1, the signal processing unit 114 may output a digital representation of the selected signal to the signal selection logic 110. The signal selection logic 110 may then further output the signal to an external unit or another part of the system 100, wherein the signal may be e.g. further processed or may be output for display to a user. However, it should be realized that the signal processing unit 114 may directly output the digital representation of the selected signal to the external unit or another part of the system 100.

[0096] As mentioned above, the system 100 may comprise a first multiplexer 112 and a first signal processing unit 114 and a second multiplexer 122 and a second signal processing unit 124.

[0097] The signal selection logic 110 may selectively activate one of the first multiplexer 112 and first signal processing unit 114 or the second multiplexer 122 and the second signal processing unit 124. This may be used by the system 100 such that, when a change in electrodes 102 to be used for signal acquisition is to be made, an inactive multiplexer and signal processing unit may be allowed to settle before the currently active multiplexer and signal processing unit is inactivated.

[0098] Hence, since the electrodes 102 are connected to both the first multiplexer 112 and the second multiplexer 122, it is possible to freely choose the pair of electrodes 102 selected by each of the multiplexers 112, 122 to provide a signal to the first and the second signal processing unit 114, 124, respectively. Thus, when it is identified that a change in the pair of electrodes 102 being used for signal acquisition is needed, the inactive multiplexer and signal processing unit may be activated to acquire a signal using a new pair of electrodes 102 providing a signal of high quality. Then, the newly activated signal processing unit has been allowed to settle while the previously active signal processing unit continues to provide an acquired signal. Once the newly activated signal processing unit has been allowed to settle, the signal selection logic 110 may switch the signal being output by the signal selection logic 110 to use the signal received from the newly activated signal processing unit. Thus, a signal may be continuously output by the signal selection logic 110 to an external unit or another part of the system 100, without any interruptions in the signal due to a settling time of the signal processing unit.

[0099] Once the signal selection logic 110 has switched the signal being output, the previously active signal processing unit may be turned off and may be ready to be activated using a new pair of electrodes 102 if it is decided that another change of the pair of electrodes 102 being selected for signal acquisition is needed for maintaining quality of the output signal.

[0100] Referring now to FIG. 4, a process of changing electrodes 102 being used for signal acquisition is further described.

[0101] The process starts with the first multiplexer 112 and the first signal processing unit 114 being active, step 402. The first multiplexer 112 is configured to select a pair of electrodes 102 for signal acquisition. The selected electrodes by the first multiplexer 112 are herein denoted i and j. The signal processing unit 114 receives the signal from the electrodes i and j and processes the received signal to provide a digital representation of the signal.

[0102] While the first multiplexer 112 and the first signal processing unit 114 are active, the signal quality detectors 104 detect a signal from each pair of electrodes 102 to which the signal quality detectors 104 are connected and form digital representations of the detected signals. The signal selection logic 110 receives the digital representations from the signal quality detectors 104 and determines at least one quality measure for each of the digital representations. The signal selection logic 110 determines the pair of electrodes, herein denoted electrodes I and m, providing a best data quality based on the at least one quality measure, step 404.

[0103] Then, the quality of data Q(l,m) of the electrodes I and m is compared to the quality of data Q(i,j) of presently used electrodes i and j, step 406. If the quality of data Q(i,j) of electrodes i and j is better, the signal acquisition continues to be based on the electrodes i and j already selected by the first multiplexer 112 and the process returns to step 402. However, if the quality of data Q(l,m) of electrodes I and m is better, the pair of electrodes 102 used for signal acquisition needs to be changed and the process initiates changing of electrodes.

[0104] It should be realized that the determining of which pair of electrodes provides a best quality may be performed in many different ways. The quality of data obtained by the presently used electrodes i and j, as determined based on the digital representation from the signal quality detector 104 connected to electrodes i and j, may be compared in any order to the quality of data of the other electrode pairs. For instance, as indicated above, the best quality measure of non-selected pair of electrodes 102 may first be determined and then compared to the quality measure of the active electrodes i and j, as indicated above. According to one alternative, the quality measures may be pairwise compared, storing the best quality measure, until all quality measures have been considered, whereby the electrodes corresponding to the best quality measure is identified as the pair of electrodes providing best quality of data. If the presently used electrodes i and j provide best quality of data, the process returns to step 402; otherwise the process initiates changing of electrodes.

[0105] When electrodes selected for acquisition are to be changed, the signal selection logic 110 transmits an enable signal to the second multiplexer 122 and the second signal processing unit 124, configuring the second multiplexer 122 to select electrodes I and m for signal acquisition, step 408. The first multiplexer 112 and the first signal processing unit 122 may be turned off after the second signal processing unit 124 has been allowed to settle.

[0106] Similar to the step 404, the signal selection logic 110 receives the digital representations from the signal quality detectors 104 and determines at least one quality measure for each of the digital representations. The signal selection logic 110 determines the pair of electrodes, herein denoted electrodes i and j, providing a best data quality based on the at least one quality measure, step 410.

[0107] Then, the quality of data Q(i,j) of the electrodes i and j is compared to the quality of data Q(l,m) of presently used electrodes I and m, step 412. If the quality of data Q(l,m) of electrodes I and m is better, the signal acquisition continues to be based on the electrodes I and m already selected by the second multiplexer 122 and the process returns to step 408. However, if the quality of data Q(i,j) of electrodes i and j is better, the pair of electrodes 102 used for signal acquisition needs to be changed and the process initiates changing of electrodes, returning to step 402.

[0108] It should be realized that, although the process of changing electrodes 102 being used for signal acquisition has been described above in relation to using first and second multiplexers 112, 122 and first and second signal processing units 122, 124, a single multiplexer and a single signal processing unit may be used instead. In such case, when it is identified that the pair of electrodes 102 being used for signal acquisition is to be changed, the multiplexer may receive a control signal to control a change in the electrodes being selected by the multiplexer so as to provide a selected signal from a different pair of electrodes 102 to the signal processing unit. However, in such a set-up, interruptions in the acquired signal may occur while the signal processing unit settles when a signal from a newly selected pair of electrodes is being provided. If the pair of electrodes 102 used for signal acquisition is rarely changed, such interruptions may however be acceptable.

[0109] Referring now to FIG. 5, a system 500 according to another embodiment will be described.

[0110] The system 500 comprises a plurality of sensor units 530. The system 500 may comprise at least two sensor units 530, but is illustrated here as comprising four different sensor units 530a, 530b, 530c and 530d.

[0111] Each sensor unit 530a-d may comprise a plurality of electrodes 502, a plurality of signal quality detectors 504, a signal selection logic 510 and a multiplexer 512. The signal selection logic 510 and the signal quality detectors 504 of each sensor unit 530a-d may function, as described above, to determine the pair of electrodes 502 of the respective sensor unit 530a-d providing a best quality signal. The signal selection logic 510 may further control the multiplexer 512 of each sensor unit 530a-d to select the signal from the pair of electrodes 502 providing best quality for output by the multiplexer 512.

[0112] The multiplexers 512 of the sensor units 530a-d are connected to an additional multiplexer 532 of the system 500. The additional multiplexer 532 may be further configured to selectively pass the signal from a multiplexer 512 of one of the sensor units 530a-d. The additional multiplexer 532 may be connected to a signal processing unit 514 of the system 500 for providing a selected signal to the signal processing unit 514. The signal processing unit 514 may then perform analog-to-digital conversion on the selected signal and provide a digital representation of the selected signal, which may be output to an external unit or another part of the system 500, e.g. for further processing of the signal.

[0113] The plurality of sensor units 530a-d may be configured to acquire selected signals in different relations to a measurement subject. This implies that, e.g. when the measurement subject moves, the sensor unit 530a-d providing a best quality of data may change. Thanks to the signal selection logic 510 and the signal quality detectors 504 of each of the sensor units 530a-d, each sensor unit 530a-d may output the signal from the pair of electrodes 502 providing a highest quality data among the electrodes 502 of the respective sensor unit 530a-d.

[0114] With the set-up of the system 500, multiplexing may be handled in a hierarchical way. The signal processing unit 514 may only receive a few different signals from different sensor units 530a-d. The signal processing unit 514 may determine a best quality of the signals from the different sensor units 530a-d for using a selected sensor unit 530a-d in further signal acquisition. When quality of signal deteriorates, a new pair of electrodes 502 from any of the sensor units 530a-d may be selected.

[0115] Alternatively, the signal processing unit 514 may receive time-multiplexed signals from the sensor units 530a-d, such that the signal processing unit 514 may acquire signals from all sensor units 530a-d in a time-multiplexed manner. The acquired signals by the signal processing unit 514 may then not need to be compared from a quality perspective and quality of data of the signal from each of the sensor units 530a-d is ensured by the selection of the best pair of electrodes 502, as determined within each sensor unit 530a-d by the respective signal selection logic 510 and signal quality detectors 504.

[0116] Referring now to FIG. 6, a wearable device 600 is shown. The wearable device 600 may comprise at least one carrier 602, which may be worn by a subject, herein illustrated as a bracelet. The carrier 602 may further be configured to carry the system 100; 500 of any of the above-described embodiments. Thus, by the system 100; 500 being mounted on or in the carrier 602, the electrodes 102, 502 may be arranged in close proximity to the subject for acquiring an electrical signal of the subject wearing the wearable device 600.

[0117] The carrier 602 may have any shape or form facilitating the carrier 602 being worn by the subject. The carrier 602 may thus e.g. be designed as a bracelet which may be worn around a wrist or an ankle of the subject or as a strap, which may be arranged around a chest of the subject. According to further alternatives, the carrier 602 may be designed as a patch, which may comprise or be associated with an adhesive surface for attaching the patch to the subject, or as a headset for arranging the system 100, 500 on a head of the subject. The electrodes 102, 502 may be mounted on or in the carrier 602 in such manner that the electrodes 102, 502 are arranged in a suitable relation to the subject, when the carrier 602 is worn by the subject, for facilitating acquisition of the electrical signal. The electrodes 102, 502 may then be further associated with signal quality detectors, signal selection logic, multiplexer(s), and signal processing unit(s) as schematically illustrated by connections to the electrodes 102, 502 in FIG. 6.

[0118] Thanks to the system 100; 500 allowing a large number of electrodes to be selectively used for acquisition of a signal while allowing a low power consumption of the device, the wearable device 600 may not need to be arranged in a very accurate relation to the subject in order to ensure that a high quality signal may be acquired. Thus, the electrodes 102, 502 may be distributed over a large area of the carrier 602 in order to be arranged in different relations to the subject, such that at least one pair of electrodes 102, 502 may be arranged in such relation to the subject to enable acquisition of a high quality signal.

[0119] This implies that the wearable device 600 may be easy to use and to properly arrange on the subject, as it may not be necessary to arrange the wearable device 600 in a specific relation to the subject to enable acquiring of the electrical signal with high quality. For instance, it may not be necessary to arrange a bracelet in a tight relationship around a wrist of the subject, which implies that the bracelet may be comfortable to wear.

[0120] Referring now to FIG. 7, a method for controlling acquisition of an electrical signal according to an embodiment will be briefly summarized.

[0121] The method comprises acquiring, step 702, electrode signals by a plurality of pairs of electrodes. The method further comprises, for each electrode signal acquired by a pair of electrodes, determining, step 704, by a signal quality detector, a digital representation of a detected signal, wherein the digital representation has a first resolution. Thus, for each pair of electrodes, a signal is detected in order to allow a quality measure of the signal to be determined. The digital representation is determined with a first, low resolution, such that the method detects the signals of each pair of electrodes with a low power consumption.

[0122] The method further comprises selecting, step 706, by a signal selection logic, a pair of electrodes for signal acquisition. The signal selection logic may use at least one quality measure of each of the digital representations determined by the signal quality detectors and may determine a pair of electrodes that provides a highest signal quality for selecting that pair of electrodes for signal acquisition.

[0123] The method further comprises controlling, step 708, a multiplexer to output a selected signal based on the selected pair of electrodes to a signal processing unit. Thus, the signal from the selected pair of electrodes, which provides a highest signal quality, may be output by the multiplexer.

[0124] The method further comprises performing, step 710, by the signal processing unit, analog-to-digital conversion on the selected signal being output by the multiplexer. The signal processing unit may thus provide a digital representation of the selected signal, wherein the digital representation has a second resolution, which is higher than the first resolution. Thus, the relatively large power consumption associated with the analog-to-digital conversion to a high resolution digital representation is restricted to analog-to-digital conversion on the selected signal from one pair of electrodes, whereas a low power consumption is required for the analog-to-digital conversions of the plurality of signals from different pairs of electrodes used for selecting the signal to be output to the signal processing unit.

[0125] In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.