Read-out circuitry for acquiring a multi-channel biopotential signal and a sensor for sensing a biopotential signal

A read-out circuitry for acquiring a multi-channel biopotential signal, comprises: a plurality of read-out signal channels, each receiving an input signal from a unique signal electrode; a reference channel receiving a reference signal from a reference electrode; wherein each read-out signal channel and the reference channel comprises a channel amplifier connected to receive the input signal in a first input node and with an output node connected to a second input node via a channel feedback loop; wherein each signal channel amplifier comprises a capacitor between the second input nodes of the signal channel amplifier and the reference channel amplifier, and wherein each signal channel feedback loop and the reference channel feedback loop comprise a filter.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on priority claimed on European Patent Application No. 18212714.2, filed on Dec. 14, 2018, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present inventive concept relates to a read-out circuitry for acquiring a multi-channel biopotential signal. The present inventive concept also relates to a sensor for sensing a biopotential signal.

BACKGROUND

For certain biopotential signals, recording a biopotential in multiple channels is important. For instance, for acquiring of an electroencephalogram (EEG) or an electrocardiogram (ECG), multiple channels may need to be acquired in order to enable sufficient analysis of the biopotential signals.

Like trends in many technological areas, it is of interest to reduce size of sensors for sensing a biopotential signal. Providing a small-size sensor may be facilitate general handing of the sensor. Also, it may improve experience for a subject to wearing the sensor. For instance, a sensor being integrated in a small patch may imply that the sensor is arranged on a small surface area of skin of the subject, such that wearing the sensor may not affect the subject (e.g. if the sensor is to be worn for acquiring biopotential signals over an extended period of time).

When providing a small-size sensor, it may be important to control power consumption of the sensor. For instance, if the sensor is battery-powered, a limited power consumption may ensure that battery lifetime is increased or that a smaller size battery may be included in the sensor.

In A. C. Metting van Rijn et al: “High-quality recording of bioelectric events, Part 2 Low-noise, low-power multichannel amplifier design”, Medical & Biological Engineering & Computing, 1991, vol. 29, pages 433-440, a design of a multichannel instrumentation amplifier for monopolar measurements is disclosed. The amplifier has one inverting (reference) input and a number of noninverting inputs. When compared to n independent one-channel amplifiers, the multichannel design offers a considerable reduction in the number of parts while the power consumption is reduced by approximately 40 percent.

However, with the multichannel instrumentation amplifier design, input sections influence voltage at a point Pavshared by the channels. Therefore, a badly functioning electrode may cause the amplifier not to function properly. In order to handle badly functioning electrodes, an extra common-mode sense electrode is used. This may result in a reduction of common-mode rejection ratio.

SUMMARY

An objective of the present inventive concept is to provide a read-out circuitry for acquiring a multi-channel biopotential signal with a limited power consumption. Another objective of the present inventive concept is to provide a read-out circuitry for acquiring a multi-channel biopotential signal with a high common-mode rejection ratio for the channels.

These and other objectives of the invention are at least partly met by the invention as defined in the independent claims. Preferred embodiments are set out in the dependent claims.

According to a first aspect, there is provided a read-out circuitry for acquiring a multi-channel biopotential signal, said read-out circuitry comprising: a plurality of read-out signal channels, wherein each signal channel is configured to receive an input signal from a unique signal electrode; a reference channel, which is configured to receive a reference signal from a reference electrode; wherein each of the read-out signal channels comprises a signal channel amplifier having a first input node and a second input node and an output node, wherein the first input node of the signal channel amplifier is connected to receive the input signal from the signal electrode, and wherein the output node of the signal channel amplifier is connected to the second input node of the signal channel amplifier via a signal channel feedback loop; wherein the reference channel comprises a reference channel amplifier having a first input node and a second input node and an output node, wherein the first input node of the reference channel amplifier is connected to receive the input signal from the reference electrode, and wherein the output node of the reference channel amplifier is connected to the second input node of the reference channel amplifier via a reference channel feedback loop; wherein the second input node of each signal channel amplifier is connected to the second input node of the reference channel amplifier for providing a differential input to each signal channel amplifier and for sharing amplification of the reference signal between the signal channels; wherein each signal channel amplifier comprises a capacitor between the second input node of the signal channel amplifier and the second input node of the reference channel amplifier, and wherein each signal channel feedback loop and the reference channel feedback loop comprise a filter; and wherein each read-out signal channel comprises a first output connected to the output node of the signal channel amplifier and a second output connected to the output node of the reference channel amplifier for providing a differential output by each read-out signal channel.

The read-out circuitry is configured such that the second input node of each signal channel amplifier is connected to the second input node of the reference channel amplifier. Thus, the amplification of the reference signal is shared between signal channels, which implies that only a single amplifier for amplifying the reference channel need be provided in the read-out circuitry. This implies that the read-out circuitry may provide a low power consumption and may be area efficient.

A capacitor is provided between the input nodes of the signal channel amplifier and the reference channel amplifier. Thus, the read-out circuitry enables a channel to be configured to provide a first low pass filter characteristic with a first cut-off frequency in respect of differential mode signals of the input signal of the signal channel and the reference signal and a second low pass filter characteristic with a second cut-off frequency in respect of common mode signals of the input signal of the signal channel and the reference signal.

The low pass filter in the feedback loop may provide a high pass filter function of the amplifier. The low pass filter for differential mode signals may be configured to pass the differential DC component of the signal channel and the reference channel to enable DC offset cancellation. The low pass filter for common mode signals enables common mode aggressors to be suppressed without affecting differential mode signals at the same frequency, to enable the read-out circuitry to be provided with a high common mode rejection.

The capacitor between the input nodes of the signal channel amplifier and the reference channel amplifier may be used to define a cut-off frequency for differential signals. The capacitor may have no effect on the common mode signals. Thus, the arrangement of a filter in the signal channel feedback and the reference channel feedback loop allows for different cut-off frequencies for differential mode signals and common mode signals.

Further, the reference channel is re-used and the second input node of each signal channel amplifier is connected to the second input node of the reference channel amplifier for sharing input from the reference channel. This implies that amplification of the reference signal may be made in the reference channel and that a single amplifier may be used for amplifying the reference signal, while the reference signal is used for each of the signal channels. Hence, there is no need for amplifying the same reference signal in plural amplifiers, such that a number of components of the read-out circuitry may be limited and, also, power consumption of the read-out circuitry may be limited.

Further, each signal channel may be configured to provide a differential output such that a fully differential amplifier is provided for each channel.

According to an embodiment, the read-out circuitry further comprises a buffer associated with the reference channel.

The buffer may ensure that a low impedance node may be provided in connecting the reference channel to the signal channels. This may ensure that cross-talk between channels is prevented so as to improve isolation between different signal channels.

According to an embodiment, the buffer is implemented as an operational amplifier.

An operational amplifier may be particularly suitable for providing a buffer function.

A unity gain operational amplifier may provide a high input impedance and a low output impedance, while providing an output voltage equal to an input voltage. Thus, the buffer operational amplifier may prevent cross-talk between channels.

According to an embodiment, the buffer is arranged within the reference channel feedback loop.

This implies a low impedance is provided to the capacitors between the second input node of the signal channel amplifier and the second input node of the reference channel amplifier.

According to an alternative embodiment, the buffer is arranged between the second input node of the reference channel amplifier and the capacitors of the signal channel amplifiers.

Hence, instead of arranging the buffer within the feedback loop, the buffer may be arranged outside the feedback loop. The buffer may still provide a low impedance to the capacitors between the second input node of the signal channel amplifier and the second input node of the reference channel amplifier.

According to another alternative embodiment, the buffer is arranged between the first input node of the reference channel amplifier and the capacitors of the signal channel amplifiers.

The buffer may thus provide a low impedance to the capacitors between the second input node of the signal channel amplifier and the second input node of the reference channel amplifier, by the buffer being arranged between the first input node of the reference channel amplifiers and the capacitors.

In the other alternative embodiment, the second input node of the reference channel amplifier may be connected to an output node of the buffer via a capacitor.

Since the first input node of the reference channel amplifier in the other alternative embodiment is connected to the capacitors of the signal channel amplifiers, the second input node of the reference channel amplifier may not be directly coupled to the capacitors and a capacitor may need to be arranged between the second input node of the reference channel amplifier and the output node of the buffer.

According to an embodiment, the filter of each signal channel and reference channel feedback loop is a low pass filter having a first cut-off frequency in respect of a differential signal of the input signal of the read-out signal channel and the reference signal, and wherein each signal channel and reference channel feedback loop is an active low pass filter having a second cut-off frequency in respect of a common mode signal of the input signals of the read-out signal channels and the reference signal.

By providing a low pass filter function in a feedback loop, the amplifier may have a high pass filter function. The low pass filter function for differential mode signals may be configured for passing essentially only the differential DC component, and thereby enables the DC offset between inputs to be cancelled. The low pass filter function for common mode signals may be configured for passing common mode aggressors, such as mains frequencies, and thereby enables the common mode aggressors to be suppressed without affecting the desired differential mode signals at the same frequency. Thus, the second cut-off frequency may be higher than the first cut-off frequency.

According to an embodiment, the low pass filters of the signal channel feedback loop and the reference channel feedback loop each comprise a transconductance amplifier and a capacitor, wherein each transconductance amplifier comprises a first and a second input connected to the respective output node of the signal channel amplifier or the reference channel amplifier and a reference voltage.

The transconductance amplifier and capacitor may thus provide low pass gm-C filters for each feedback loop. The gm-C filters may provide filter characteristics for common mode signals and may be adapted for common mode rejection.

The filter cut-off frequency of the gm-C filters may be high. Thus, small filter capacitances may be required, which may imply that use may be made of parasitic capacitors for the low pass filters.

According to an embodiment, each transconductance amplifier is connected to receive the same reference voltage.

This may ensure that filters are configured to provide a low pass filter in respect of a common mode signal of the input signals of the read-out signal channels and the reference channel.

According to an embodiment, each read-out signal channel comprises a first transimpedance stage connected between the output node of the signal channel amplifier and the first output of the read-out signal channel, and a second transimpedance stage connected between the output node of the reference channel amplifier and the second output of the read-out signal channel.

Hence, each read-out signal channel may be configured to output a first and a second output so as to provide a fully differential amplification of the input and reference signals.

According to an embodiment, the first input node of each signal channel amplifier is a non-inverting input and the second input node of each signal channel amplifier is an inverting input, and wherein the first input node of the reference channel amplifier is a non-inverting input and the second input node of the reference channel amplifier is an inverting input.

Thus, the input signals are provided to non-inverting inputs, while feedback is coupled to inverting inputs. Hence, negative feedback is provided in the amplifiers.

According to a second aspect, there is provided a sensor for sensing a biopotential signal, said sensor comprising: a plurality of electrodes, configured to contact a skin surface of a subject and configured to provide input signals and a reference signal; and a read-out circuitry according to any one of the preceding claims for providing a differential output from each read-out signal channel of the read-out circuitry.

Effects and features of this second aspect are largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect.

Thus, a sensor is provided with a plurality of electrodes for acquiring a multi-channel biopotential signal. The re-use of amplification of the reference channel for a plurality of channels implies that power consumption of the sensor may be limited.

According to an embodiment, the sensor further comprises a patch, which is configured to carry the plurality of electrodes and the read-out circuitry.

Thus, the sensor may be implemented in a patch, which may be suited for attachment to a subject so as to arrange the plurality of electrodes in a relation to the subject for acquiring the biopotential signals.

According to an embodiment, the patch comprises an adhesive surface, which is configured for being attached to a chest area of a subject for acquiring a multi-channel electrocardiogram, ECG, signal by the sensor.

Thus, the patch may be attached to position the electrodes in an appropriate relation to a subject. This implies that the sensor facilitates acquiring a multi-channel ECG.

Hence, a high quality multi-channel ECG may be acquired using a sensor with a small form factor for attachment to a subject. This implies that the sensor may be suitable for use of acquiring an ECG during daily life of a subject, so as to facilitate monitoring heart activity of the subject over a long period of time without affecting daily life of the subject.

DETAILED DESCRIPTION

Detailed embodiments of the present inventive concept will now be described with reference to the drawings.

Referring now toFIG. 1, a sensor100for sensing a biopotential signal will be described.

The sensor100may comprise a plurality of electrodes102. Each electrode102may be configured to be arranged in contact with a subject for acquiring a signal.

The electrodes102may e.g. be configured for direct galvanic contact with a skin of a person. However, it should also be realized that the electrodes102may 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.

The electrodes102may be designed in many different ways as will be appreciated by a person skilled in the art. For instance, the electrodes102may 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 electrode102.

The sensor100may comprise a carrier104which may be wearable by the subject. The carrier104may thus for instance comprise an adhesive patch for attachment to a skin surface of the subject. The carrier104may alternatively comprise a band element or ring-shaped element for attachment around a body part. The carrier104could for instance comprise two band parts, which may be attached to each other in an adjustable relationship for fitting the carrier104tightly around the body part, such as around a torso of the subject.

The electrodes102may be mounted in the carrier104so as to be arranged in a suitable relation to the subject, when the carrier104is attached or arranged on the subject.

The sensor100may be adapted for acquiring a biopotential signal. For instance, the sensor100may be configured for acquiring an electroencephalogram (EEG), an electrocardiogram (ECG) or an electromyogram (EMG). In the following, acquisition of an ECG will be mainly described, but it should be realized that the description of the sensor100may apply to acquisition of another biopotential signal instead.

The electrodes102may be mounted relatively close to each other in the carrier104. In particular, for miniaturizing of the sensor100, the carrier104should be small, which also implies that the electrodes102will be close to each other. Since the electrodes102are close to each other, ECG signals from a heart of the subject are not very different, which implies that a conventional ECG differential amplifier may only provide a small output signal and may also provide more or less the same signal for different channels. Hence, one of the electrodes102may be used as a common reference electrode for providing a reference signal, such that the signals for the other electrodes102are all differentially amplified in relation to the reference signal.

The sensor100may further comprise read-out circuitry110for acquiring a multi-channel biopotential signal from the electrodes102, as will be described in further detail below. Each channel may provide fully differential amplification of the input signal of an electrode102in relation to the reference electrode.

The sensor100may thus acquire biopotential signals on a plurality of channels. The biopotential signals may be further processed in further processing circuitry of the sensor100. For instance, the sensor100may comprise analog-to-digital converters (ADCs) for converting the acquired signals to digital representation.

The sensor100may further comprise a processing unit, which may be configured to process and/or analyze the acquired signals. The processing unit could for instance be configured to determine a heart rate from an acquired ECG or be configured to detect any abnormal events in heart activity.

The sensor100may further comprise a communication unit, which may be configured for wired and/or wireless communication with an external unit. Thus, the sensor100may be configured to transfer the acquired biopotential signals, possibly after further processing of the signals in the sensor100, to an external unit, which may provide extensive analysis of the signals and/or enable presenting signals on a display.

Referring now toFIG. 2, a read-out circuitry110for acquiring a multi-channel biopotential signal from the electrodes102according to a first embodiment will be described.

The read-out circuitry110comprises a plurality of read-out signal channels120a-c. Each signal channel120a-cis configured to receive an input signal inP[1]-inP[N] from a unique signal electrode102.

Each signal channel120a-ccomprises a signal channel amplifier122a-c. The signal channel120a-cmay be configured to receive the input signal inP[1]-inP[N] on a first input node of the signal channel amplifier122a-c. The first input node of the signal channel amplifier122a-cmay be a non-inverting input such that the signal channel120a-cmay be configured to receive the input signal inP[1]-inP[N] on a non-inverting input of the signal channel amplifier122a-c.

The signal channel120a-cmay further comprise a signal channel feedback loop connecting an output node of the signal channel amplifier122a-cto a second input node of the signal channel amplifier122a-c. The feedback loop may be connected to an inverting input of the signal channel amplifier122a-cto provide a negative feedback.

The read-out circuitry110may further comprise a reference channel130, which is configured to receive a reference signal inN from a reference electrode, which acts as a common reference for each of the input signals inP[1]-inP[N] of the signal electrodes102.

The reference channel may comprise a reference channel amplifier132. The reference channel130may be configured to receive the reference signal inN on a first input node of the reference channel amplifier132. The first input node of the reference channel amplifier132may be a non-inverting input such that the reference channel132may be configured to receive the reference signal inN on a non-inverting input of the reference channel amplifier132.

The reference channel amplifier132may further comprise a reference channel feedback loop connecting an output node of the reference channel amplifier132to a second input node of the reference channel amplifier132. The feedback loop may be connected to an inverting input of the reference channel amplifier132to provide a negative feedback.

The second input node of each signal channel amplifier122a-cis connected to the second input node of the reference channel amplifier132via a capacitor124a-cof each signal channel120a-c.

The signal channel feedback loop and the reference channel feedback loop together form a feedback network. The signal channel feedback loop may comprise a low pass filter. The low pass filter may for instance be based on a transconductance amplifier126a-cand a capacitor128a-c. Also, the reference channel feedback loop may comprise a low pass filter. The low pass filter may for instance be based on a transconductance amplifier134and a capacitor136. The low pass filters may provide a common mode filter characteristic of the feedback network.

In this regard, the transconductance amplifiers126a-c,134may each be configured to receive a differential input based on the output from the signal channel amplifier122a-c,132and a reference voltage.

Since the low pass filters are implemented in negative feedback loops, a high pass filter characteristic of the signal channel amplifiers122a-cmay be provided, providing DC offset cancellation.

The low pass filters may comprise a transconductor or transconductance amplifier126a-c,134and a shunt capacitor128a-c,136. For common mode signals, the filters should pass a full frequency band of interest, thus including e.g. mains aggressors. This implies that a high cut-off frequency in respect of the common mode signals may be provided. Hence, the shunt capacitor128a-c,136may be small and may, for example, be provided by parasitic capacitances of the transconductance amplifier126a-c,134.

The capacitor124a-cmay provide different filter responses for common mode and differential mode signals, as the capacitor124a-cdoes not affect the common mode signals. Thus, the feedback network may be configured to provide a cut-off frequency in respect of differential signals, which is different from the cut-off frequency in respect of common mode signals. Thus, the feedback network may further be configured to provide DC offset cancellation.

Each read-out signal channel120a-cmay further comprise a first output connected to the output node of the signal channel amplifier122a-cand a second output connected to the output node of the reference channel amplifier for providing a differential output of a first output signal outP[1]-outP[N] and a second output signal outN[1]-outN[N] by each read-out signal channel120a-c.

The signal channel amplifiers122a-cand the reference channel amplifiers132may be transconductance amplifiers. Each signal channel120a-cmay further comprise a first transimpedance amplifier140a-cconnected to the output node of the signal channel transconductance amplifier122a-cand a second transimpedance amplifier142a-cconnected to the output of the reference channel transconductance amplifier132. The signal channel120a-cmay hence act as an overall voltage amplifier.

Referring now toFIG. 3, a read-out circuitry210for acquiring a multi-channel biopotential signal from the electrodes102according to a second embodiment will be described.

The read-out circuitry210according to the second embodiment is in many aspects very similar to the read-out circuitry110according to the first embodiment. Thus, features of the read-out circuitry210of the second embodiment which are similar to the read-out circuitry110of the first embodiment will not be described in detail here.

The read-out circuitry210according to the second embodiment comprises a plurality of read-out signal channels220a-chaving signal channel amplifiers222a-cwhich are configured to receive an input signal inP[1]-inP[N] on a first input node. The read-out circuitry210further comprises a reference channel230having a reference channel amplifier232, which is configured to receive a reference signal inN. The signal channel amplifiers222a-cand the reference channel amplifier232are provided with feedback loops to provide input on a second input node of the signal channel amplifiers222a-cand the reference channel amplifier232, respectively.

The second input node of each signal channel amplifier222a-cis connected to the second input node of the reference channel amplifier232for providing a differential mode filter characteristic. Each signal channel and reference channel feedback loop further provides an active low pass filter (illustrated as a transconductance amplifier226a-c,234and a shunt capacitor228a-c,236) having a cut-off frequency in respect of a common mode signal of the input signals inP[1]-inP[N] of the read-out signal channels220a-cand the reference signal inN.

Each read-out signal channel220a-cmay further comprise a first transimpedance amplifier240a-cconnected to the output node of the signal channel amplifier222a-cand a second transimpedance amplifier242a-cconnected to the output node of the reference channel amplifier232for providing a differential output of a first output signal outP[1]-outP[N] and a second output signal outN[1]-outN[N] by each read-out signal channel220a-c.

In difference to the first embodiment, the read-out circuitry210of the second embodiment comprises a buffer238associated with the reference channel232.

The buffer238may ensure that a low impedance node is provided in connecting the reference channel230to the signal channels220a-c. Thanks to the buffer238, a low impedance node may be provided to the capacitors234. This may ensure that cross-talk between channels220a-cis prevented so as to improve isolation between different signal channels220a-c.

As illustrated inFIG. 3, the buffer238may be arranged within the reference channel feedback loop. The buffer238may thus be arranged between the transconductance amplifier234and the second input node of the reference channel amplifier232.

The buffer238may be implemented as an operational amplifier, e.g. as a unity gain operational amplifier. The unity gain operational amplifier may provide a high input impedance and a low output impedance, while providing an output voltage equal to an input voltage.

Referring now toFIG. 4, a read-out circuitry310for acquiring a multi-channel biopotential signal from the electrodes102according to a third embodiment will be described.

The read-out circuitry310according to the third embodiment is in many aspects very similar to the read-out circuitries110,210according to the first and second embodiments. Thus, features of the read-out circuitry310of the third embodiment which are similar to the read-out circuitries110,210of the first and second embodiments will not be described in detail here.

The read-out circuitry310according to the third embodiment comprises a plurality of read-out signal channels320a-chaving signal channel amplifiers322a-cwhich are configured to receive an input signal inP[1]-inP[N] on a first input node. The read-out circuitry310further comprises a reference channel330having a reference channel amplifier332, which is configured to receive a reference signal inN. The signal channel amplifiers322a-cand the reference channel amplifier332are provided with feedback loops to provide input on a second input node of the signal channel amplifiers322a-cand the reference channel amplifier332, respectively.

The second input node of each signal channel amplifier322a-cis connected to the second input node of the reference channel amplifier332for providing a differential mode filter characteristic. Each signal channel and reference channel feedback loop further provides an active low pass filter (illustrated as a transconductance amplifier326a-c,334and a shunt capacitor328a-c,336) having a cut-off frequency in respect of a common mode signal of the input signals inP[1]-inP[N] of the read-out signal channels320a-cand the reference signal inN.

Each read-out signal channel320a-cmay further comprise a first transimpedance amplifier340a-cconnected to the output node of the signal channel amplifier322a-cand a second transimpedance amplifier342a-cconnected to the output node of the reference channel amplifier332for providing a differential output of a first output signal outP[1]-outP[N] and a second output signal outN[1]-outN[N] by each read-out signal channel320a-c.

Similar to the second embodiment, the read-out circuitry310of the third embodiment comprises a buffer338associated with the reference channel332.

Again, the buffer338may ensure that a low impedance node is provided in connecting the reference channel330to the signal channels320a-c. Thanks to the buffer338, a low impedance node may be provided to the capacitors334. This may ensure that cross-talk between channels320a-cis prevented so as to improve isolation between different signal channels320a-c.

In contrast to the second embodiment, the buffer338of the third embodiment of the read-out circuitry310is arranged outside the reference channel feedback loop. The buffer338may thus be arranged between the second input node of the reference channel amplifier232and the capacitors224a-c.

The buffer338may be implemented as an operational amplifier, e.g. as a unity gain operational amplifier. The unity gain operational amplifier may provide a high input impedance and a low output impedance, while providing an output voltage equal to an input voltage.

Referring now toFIG. 5, a read-out circuitry410for acquiring a multi-channel biopotential signal from the electrodes102according to a fourth embodiment will be described.

The read-out circuitry410according to the fourth embodiment is in many aspects very similar to the read-out circuitries110,210,310according to the first, second, and third embodiments. Thus, features of the read-out circuitry310of the fourth embodiment which are similar to the read-out circuitries110,210,310of the first, second and third embodiments will not be described in detail here.

The read-out circuitry410according to the fourth embodiment comprises a plurality of read-out signal channels420a-chaving signal channel amplifiers422a-cwhich are configured to receive an input signal inP[1]-inP[N] on a first input node. The read-out circuitry410further comprises a reference channel430having a reference channel amplifier432, which is configured to receive a reference signal inN. The signal channel amplifiers422a-cand the reference channel amplifier432are provided with feedback loops to provide input on a second input node of the signal channel amplifiers422a-cand the reference channel amplifier432, respectively.

The second input node of each signal channel amplifier422a-cis connected to the second input node of the reference channel amplifier432for providing a differential mode filter characteristic. Each signal channel and reference channel feedback loop further provides an active low pass filter (illustrated as a transconductance amplifier426a-c,434and a shunt capacitor428a-c,436) having a cut-off frequency in respect of a common mode signal of the input signals inP[1]-inP[N] of the read-out signal channels420a-cand the reference signal inN.

Each read-out signal channel420a-cmay further comprise a first transimpedance amplifier440a-cconnected to the output node of the signal channel amplifier422a-cand a second transimpedance amplifier442a-cconnected to the output node of the reference channel amplifier432for providing a differential output of a first output signal outP[1]-outP[N] and a second output signal outN[1]-outN[N] by each read-out signal channel420a-c.

Similar to the second and third embodiments, the read-out circuitry410of the fourth embodiment comprises a buffer438associated with the reference channel432.

Again, the buffer438may ensure that a low impedance node is provided in connecting the reference channel430to the signal channels420a-c. Thanks to the buffer438, a low impedance node may be provided to the capacitors434. This may ensure that cross-talk between channels420a-cis prevented so as to improve isolation between different signal channels420a-c.

In the fourth embodiment, the buffer438is arranged between the first input node of the reference channel amplifier432and the capacitors424a-c.

Here, the first input node of the reference channel amplifier432is connected to the capacitors424a-cvia the buffer438. Thus, the second input node of the reference channel amplifier432may not be directly coupled to the capacitors424a-cand an additional capacitor439may need to be arranged between the second input node of the reference channel amplifier432and the output node of the buffer438.

The buffer338may be implemented as an operational amplifier, e.g. as a unity gain operational amplifier. The unity gain operational amplifier may provide a high input impedance and a low output impedance, while providing an output voltage equal to an input voltage.