Patent Publication Number: US-2022225922-A1

Title: Input circuitry for receiving electrode signals, a biopotential signal sensor system, a neural probe, and a method for amplifying electrode signals

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to the European application 21152118.2, filed on Jan. 18, 2021, incorporated herein by reference. 
     TECHNICAL FIELD 
     The present inventive concept relates to an input circuitry for receiving electrode signals. The input circuitry may be used in a biopotential signal sensor system, and in particular, in a neural probe. The present inventive concept also relates to a method for amplifying electrode signals. 
     BACKGROUND 
     In many applications, an array of electrodes is used for sensing a distribution of electrical signals. For instance, neural probes use an array of electrodes for sensing neuronal activities. In order to increase spatial resolution of measurements, a smaller pitch between electrodes in the array is needed. Further, having a readout circuitry on a common substrate with the electrodes provides high signal integrity when interconnecting the electrodes with the readout circuitry. However, the readout circuitry for an array of a large number of electrodes will occupy a large area and it is therefore desired that electrode signals are multiplexed such that a readout circuitry may support plural electrodes. 
     However, when plural electrodes are interfaced with a common instrumentation amplifier for multiplexing electrode signals in the instrumentation amplifier, the electrode signals may be degraded. In particular, in order to provide multiplexing, switches may be used for selecting which electrode signal is input to the instrumentation amplifier. When the switches are clocked in an interleaved manner, the switches together with a parasitic capacitance form a switched capacitor circuit, which results in an equivalent resistance between electrodes connected to the instrumentation amplifier. When electrode impedance is high, which in particular is true for small electrodes, signal amplitude will be degraded due to reduced equivalent input impedance of the instrumentation amplifier due to multiplexing. Also, crosstalk between electrodes will increase. 
     SUMMARY 
     An objective of the present inventive concept is to provide an input circuitry that allows multiplexing of electrode signals while maintaining high signal integrity. 
     These and other objectives of the present inventive concept 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 an input circuitry for receiving electrode signals, said input circuitry comprising: a plurality of channels for providing a multiplexed electrode signal input, wherein each channel comprises a multiplexing switch for selecting one channel at a time to provide electrode signal input, and wherein each channel comprises an input transistor configured to be connected to an electrode associated with the channel, wherein the input transistor is configured to receive an electrode signal at a gate of the input transistor; a reference input transistor, which is configured to be connected to a reference voltage at a gate of the reference input transistor; wherein an electrode signal received at a selected channel together with the reference voltage received at the reference input transistor form input signals to an instrumentation amplifier of the input circuitry; wherein the input circuitry is configured such that the input transistor of the selected channel forms part of a first flipped voltage follower of the instrumentation amplifier and the reference input transistor forms part of a second flipped voltage follower of the instrumentation amplifier. 
     The input circuitry is thus configured to receive electrode signals at different channels. Each channel comprises an input transistor, such that the electrode signals are provided at the gate of the input transistor of the respective channels. This implies that the gate of the input transistor is connected to the electrode while the channel is not selected to provide electrode signal input. Hence, the gate may follow potential variations of the electrode signal even when the channel is not selected. This implies that no input capacitance may need to be charged/discharged when an electrode signal is selected during multiplexing to avoid or at least reduce degrading of signal amplitude. Thus, the input circuitry provides high signal integrity with low crosstalk between electrodes while providing multiplexing of electrode signals. 
     According to an embodiment, each channel comprises a first multiplexing switch for selectively connecting or disconnecting a drain of the input transistor of the channel to a first shared node of the first flipped voltage follower. 
     This implies that the channels may selectively connected to a common node of the first flipped voltage follower. Hence, the first node is “shared” in that it is the node to which the drain of the input transistor of each channel is connected, when selected. However, only one channel will be selected at a time, such that multiple channels will not be simultaneously connected to the first shared node. 
     According to an embodiment, each channel further comprises a second multiplexing switch for selectively connecting or disconnecting the drain of the input transistor to a source of the input transistor. 
     The channel may be configured such that the drain of the input transistor is connected to the source of the input transistor when the channel is not selected to provide electrode signal input. Further, the channel may be configured such that the drain of the input transistor is disconnected from the source of the input transistor when the channel is selected to provide electrode signal input. 
     This implies that when the channel is not selected, the drain is shorted to the source in the input transistor. This implies any leakage current may be stopped. 
     According to an embodiment, the source of the input transistor of each channel is connected to a second shared node of the first flipped voltage follower. 
     The source of the input transistor of each channel may be configured to always be connected to the second shared node, regardless whether the channel is selected or not to provide electrode signal input. This implies that the source of the input transistor may follow the potential of the other electrodes instead of being floating when the channel is not selected. Thus, settling of the instrumentation amplifier to the electrode signal will be faster when the channel is selected, since less transients are induced when an electrode is selected for providing electrode signal input. 
     According to an embodiment, the first flipped voltage follower comprises the input transistor of the selected channel forming a first transistor of the first flipped voltage follower, a second transistor having a drain connected to a source of the first transistor, a current source connected to a drain of the first transistor and a gain element connected between the drain of the first transistor and a gate of the second transistor. 
     The first flipped voltage follower provides the second shared node between the drain of the second transistor and the source of the first transistor, wherein the second shared node has a voltage following the signal of the selected electrode. 
     Thanks to feedback of the second transistor, the second shared node will have low output impedance, providing an output to an output stage of the input circuitry with low output impedance. 
     The gain element may stabilize a voltage on the first shared node. This facilitates fast settling of the instrumentation amplifier to the electrode signal when a new channel is selected. Also, the gain element may ensure that the input transistor of the selected channel is in saturation mode. 
     According to an embodiment, an output node of the first flipped voltage follower is connected via a resistor to an output node of the second flipped voltage follower. 
     The output nodes of the first flipped voltage follower and the second flipped voltage follower follow the electrode signal input and the reference voltage, respectively. The resistor may thus convert an input signal voltage into current, which may be copied to the output stage. 
     According to an embodiment, the input circuitry further comprises an electrode offset calibration block, the electrode offset calibration block connected to opposite sides of the resistor and configured to inject a compensation current into the resistor for canceling a current jump induced by different DC offsets of electrodes when switching selection of channels. 
     This may ensure that a large electrode DC offset is not allowed to saturate the instrumentation amplifier. 
     When switching from between electrode input signals, a difference of DC electrode offsets will cause a voltage step jump at the output node of the first flipped voltage follower. Thanks to the use of the electrode offset calibration block, a compensation current may be injected into the resistor such that a current jump through the resistor due to the voltage jump at the output node of the first flipped voltage follower is canceled. 
     An offset calibration may be performed before electrode measurements are started, such that the DC electrode offsets may be determined. These DC electrode offsets may be stored to allow the electrode offset calibration block to provide a corresponding compensation current. 
     According to an embodiment, the input circuitry further comprises an output stage connected to receive signals from the first flipped voltage follower and the second flipped voltage follower. 
     Thus, the output stage may form a multiplexed signal such that the input circuitry may provide a multiplexed signal output from the output stage. The output stage may provide the multiplexed signals to circuitry for further processing of the electrode signals. 
     The output stage may comprise a capacitor for integrating the received current signal copied to the output stage. The charge sampling nature of this scheme allows the amplifier to be designed with a narrower bandwidth for the same settling accuracy compared to an instantaneous voltage sampling. Thus, aliasing of electrode noise may be reduced to avoid noise affecting quality of the output electrode signal. 
     According to an embodiment, the output stage comprises a reset switch for clearing signal information between readout of electrode signals of different selected channels. 
     The reset switch may be used to reset an integrating capacitor of the output stage between readout of signals from different channels. Thus, signal information may be easily cleared between readout of different channels. 
     According to an embodiment, the input circuitry comprises at least 4 channels. 
     This implies that the input circuitry and further processing circuitry which receives the multiplexed signal may be efficiently re-used. Thus, an area required by the input circuitry and further processing circuitry may be reduced by a factor 4 or more. 
     The number of electrode signals that may be multiplexed by an input circuitry may be dependent on a context in which the electrode signals are acquired. It should be realized that, if a large number of electrodes are used for sensing signals, a plurality of input circuitries may be used associated with different sets of electrodes. 
     According to a second aspect, there is provided a biopotential signal sensor system, comprising: the input circuitry according to the first aspect; and a plurality of electrodes configured for sensing a biopotential signal, wherein each electrode is connected to the gate of an input transistor of a channel among the plurality of channels, wherein one electrode is associated with each channel. 
     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. 
     In many applications sensing biopotential signals, it may be interesting to detect signals from a plurality of electrodes. This may be useful for sensing distribution of biopotential within a body or for sensing biopotentials in several points of interest. 
     Further, when detecting biopotential signals, the electrodes are worn by a subject, and at least the input circuitry receiving the electrode signals may also need to be worn by the subject. This implies that an area required by the input circuitry affects a size of a device that is worn by the subject. Hence, in order to provide a biopotential signal sensor system that facilitates convenience to the subject from which signals are acquired, a small size of the input circuitry is desired. In this respect, the multiplexing of signals by the input circuitry, enabling the input circuitry to be shared by several electrodes facilitates a small size of a device that is to be worn by the subject. 
     The biopotential signal sensor system may for instance be configured to sense a biopotential signal relating to electrocardiography, electroencephalography, electrocorticography, or electromyography. However, as further discussed below, the biopotential signal sensor system may be of particular interest in a neural probe for sensing potentials in a brain, such as local field potential and action potential. 
     According to an embodiment, the input circuitry is arranged on a common substrate with the plurality of electrodes. 
     This implies that the biopotential signal sensor system may be compact. Further, it may ensure high signal integrity of the electrode signals received by the input circuitry. 
     According to a third aspect, the biopotential signal sensor system is a neural probe, wherein the plurality of electrodes is arranged on a carrier configured for being inserted into a brain. 
     Effects and features of this third aspect are largely analogous to those described above in connection with the first and second aspects. Embodiments mentioned in relation to the first and second aspects are largely compatible with the third aspect. 
     In neural probes, there is a desire to provide as small electrodes as possible in order to have a high spatial resolution of signals acquired from a brain. This implies that a large number of electrodes may be used, such that there may be a large need to multiplex signals in order to reduce size of an area of circuitry for processing the electrode signals. 
     Further, since electrodes are small, the electrodes have high impedance. Therefore, a high input impedance of the input circuitry is desired so as not to degrade signal quality of the electrode signals. Hence, the input circuitry is well-suited for use in a neural probe, since the input circuitry may maintain a high input impedance even though multiplexing is used. 
     According to a fourth aspect, there is provided a method for amplifying electrode signals, said method comprising: receiving electrode signals on a plurality of channels of an amplifying input circuitry for providing multiplexed amplifying of the electrode signals, wherein each channel comprises an input transistor configured to be connected to an electrode associated with the channel, wherein the input transistor is configured to receive an electrode signal at a gate of the input transistor; selecting a first electrode signal as input to multiplexed amplifying by connecting the input transistor of a first channel to form part of a first flipped voltage follower of an instrumentation amplifier and by all other channels being deselected, the first electrode signal forming an input signal pair together with a reference voltage signal to the instrumentation amplifier; and selecting a second electrode signal as input to multiplexed amplifying by connecting the input transistor of a second channel to form part of the first flipped voltage follower of the instrumentation amplifier and by all other channels being deselected, the second electrode signal forming an input signal pair together with the reference voltage signal to the instrumentation amplifier. 
     Effects and features of this fourth aspect are largely analogous to those described above in connection with the first, second, and third aspects. Embodiments mentioned in relation to the first, second, and third aspects are largely compatible with the fourth aspect. 
     Each channel comprises an input transistor, such that the electrode signals are provided at the input transistor of the respective channels, even when the channel is not selected to provide electrode signal input to the amplifying circuitry. The electrode signal may be provided at a gate of the input transistor. This implies that no input capacitance may need to be charged/discharged when an electrode signal is selected during multiplexing to avoid or at least reduce degrading of signal amplitude. Thus, the method may ensure high signal integrity while providing multiplexing of electrode signals. 
     The method may be controlled by a control unit, which may provide clocking signals for synchronizing selecting of an electrode signal of one channel and deselecting all other channels. 
     The selecting and deselecting of an electrode signal of a channel may be performed by turning on and off a first multiplexing switch. 
     The method may further comprise connecting a drain of the input transistor to a source of the input transistor when a channel is deselected. This implies that when the channel is not selected, the drain is shorted to the source in the input transistor. This implies any leakage current may be stopped. 
     The connecting of the drain of the input transistor to the source of the input transistor may be performed by turning on a second multiplexing switch. Further, when the channel is selected, the second multiplexing switch may be turned off to disconnect the drain of the input transistor from the source of the input transistor. 
     According to an embodiment, the method further comprises, when selecting the second electrode signal, providing a reset signal to an output stage of the instrumentation amplifier for clearing signal information between readout of electrode signals of different selected channels. 
     The reset switch may be used to reset an integrating capacitor of the output stage between readout of signals from different channels. Thus, signal information may be easily cleared between readout of different channels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a schematic view of an input circuitry according to an embodiment. 
         FIGS. 2 a - c    are schematic views of electrodes being connected to the input circuitry at different points in time. 
         FIG. 3  is a detailed schematic view of a first flipped voltage follower of the input circuitry according to an embodiment. 
         FIG. 4  is a flow chart of a method according to an embodiment. 
         FIG. 5  is a clocking diagram of control signals for controlling multiplexing switches of the input circuitry according to an embodiment. 
         FIG. 6  is a schematic view of a biopotential signal sensor system according to an embodiment. 
         FIG. 7  is a schematic view of a neural probe according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , an input circuitry  100  for receiving electrode signals according to an embodiment will be described. The input circuitry  100  is configured to receive signals from a plurality of electrodes  102  and the input circuitry  100  is configured to form multiplexed signal from the signals of the plurality of electrodes  102 . 
     The electrodes  102  may be any type of electrode  102  which is configured to sense an electrical potential at a location of the electrode  102 . The electrode  102  may therefore comprise a conducting part which is configured to sense the electrical potential. The electrode  102  could for instance be used for sensing biopotential, i.e. an electrical potential in a body of a human being or an animal. However, the electrode  102  could be used in numerous other applications. 
     The input circuitry  100  comprises a plurality of channels  110 , wherein each channel is associated with one electrode  102 . Each channel  110  comprises an input transistor  112 . The electrode  102  associated with the channel  110  is connected to a gate of the input transistor  112 , such that an electrode signal from the electrode  102  is received at the gate. 
     The input circuitry  100  is configured to select one channel  110  at a time to provide electrode signal input to the input circuitry  100 . Thus, at each time instant, one channel may be selected while all other channels may be deselected. 
     Each channel  110  may comprise a first multiplexing switch  114 , which is configured to select the electrode signal of the channel  110  to provide the electrode signal input of the input circuitry  100 . 
     The gate of the input transistor  112  is connected to the electrode  102 , even when the channel  110  is not selected. This implies that the gate of the input transistor  112  may follow the potential variations of the electrode signal from the electrode  102  associated with the channel  110  even when the channel  110  is not selected. Hence, no charge/discharge from the electrode  102  is required when the channel  110  is selected, such that signal amplitude is not degraded. 
     The electrode signal provided at the gate of the input transistor  112  of the selected channel  110  together with a reference voltage forms a pair of input signals to an instrumentation amplifier of the input circuitry  100 . The input circuitry  100  may further comprise a reference input transistor  142  which is configured to receive the reference voltage at a gate of the reference input transistor  142 . The reference voltage may be received by the reference input transistor  142  being connected to a reference electrode or to a known potential, such as ground. 
     The input circuitry  100  forms a first flipped voltage follower  120  with the input transistor  112  of the selected channel forming a first transistor of the first flipped voltage follower  120 . The input circuitry  100  further comprises a second flipped voltage follower  140  with the reference input transistor  142  forming a first transistor of the second flipped voltage follower. 
     The first flipped voltage follower  120  is configured to receive the electrode signal input (at the selected channel  110 ) and is configured to provide a potential following the potential of the electrode signal input at a first output node  124 . The second flipped voltage follower  140  is configured to provide a potential following the potential of the reference voltage at a second output node  144 . 
     The first and second output nodes  124  and  144  are arranged on opposite sides of a resistor  160 , such that the voltage across the resistor  160  is converted to a current. The current is copied to an output stage  170  of the input circuitry  100 . 
     The input circuitry  100  further comprises an electrode offset calibration block  162 , which is connected to opposite sides of the resistor  160 . The electrode offset calibration block  162  is configured to inject a compensation current into the resistor  160  for canceling a current jump induced by different DC offsets of the electrodes  102  when switching selection of channels  110 . 
     An offset calibration may be performed before receipt of electrode signals from the electrodes  102  are started, such that the DC electrode offsets may be determined for each of the electrodes  102 . These DC electrode offsets may be stored to allow the electrode offset calibration block  162  to provide a corresponding compensation current. 
     The electrode offset calibration block  162  may be configured to receive a digital signal corresponding to the stored DC electrode offset for a selected electrode  102  and may convert the received digital signal to an analog compensation current to be injected into the resistor  160 . 
     The current through the resistor  160  flows through a second transistor  126  of the first flipped voltage follower  120  and through a second transistor  146  of the second flipped voltage follower  140 . The signal on a gate of the second transistor  126  of the first flipped voltage follower  120  is also provided to a gate of a first transistor  172  of the output stage  170 . The signal on a gate of the second transistor  146  of the second flipped voltage follower  140  is also provided to a gate of a second transistor  174  of the output stage  170 . Thus, the current flowing through the resistor  160 , the second transistor  126  of the first flipped voltage follower and the second transistor  146  of the second flipped voltage follower is copied to the output stage  170 . 
     The output stage  170  comprises a capacitor  176 , which is configured to receive the current copied from the resistor  160 . The capacitor  176  is configured to integrate the received current signal. The charge sampling nature of this scheme allows the input circuitry  100  to be designed with a narrower bandwidth for the same settling accuracy compared to an instantaneous voltage sampling. Thus, aliasing of electrode noise may be reduced to avoid noise affecting quality of the output electrode signal. 
     Output nodes  178 ,  180  are connected on opposite sides of the capacitor  176  for providing a differential output signal from the output stage  170  of the input circuitry  100 . The output stage  170  further comprises a reset switch  182 . The reset switch  182  is configured to be enabled when a new electrode  102  is selected to provide the electrode signal input. The reset switch  182  clears the signal information between readout of different channels  110 . Also, the reset switch  182  may isolate the output nodes  178 ,  180  from any glitched due to the switching between channels  110 . 
     Referring now to  FIGS. 2 a - c   , an embodiment for selection of channels  110  among N channels, where N is an integer number, will be further described. In each of  FIGS. 2 a - c   , a first channel  11  associated with a first electrode  102   a , a second channel  110   b  associated with a second electrode  102   b , and an N′th channel  110   n  associated with an N′th electrode  102   n  are shown. 
     Each channel  110  comprises the first multiplexing switch  114 . The first multiplexing switch is configured to selectively connect or disconnect a drain of the input transistor  112  to a first shared node  118  of the first flipped voltage follower  120 . The first node  118  is “shared” in that it is the node to which the drain of the input transistor  112  of each channel  110  is connected, when selected. However, only one channel  110  will be selected at a time, such that multiple channels will not be simultaneously connected to the first shared node  118 . 
     When the first multiplexing switch  114  of a channel  110  is turned on to select the channel  110 , the input transistor  112  of the channel  110  forms part of the first flipped voltage follower  120 , by the drain of the input transistor  112  being connected to the first shared node  118 , which is part of the first flipped voltage follower  120 . 
     Each channel  110  may further be connected to a second shared node  124 , which corresponds to the first output node  124  of the first flipped voltage follower  120 . The source of the input transistor  112  may be connected to the second shared node  124 , even when the channel  110  is not selected. This implies that the source of the input transistor  112  follows potentials of the other electrodes  102  instead of being floating when the channel  110  is not selected. This implies that settling to the electrode signal will be fast when the channel  110  is selected, since less transients are induced when an electrode  102  is selected for providing electrode signal input. 
     Each channel  110  may further comprise a second multiplexing switch  116 . The second multiplexing switch  116  may selectively connect or disconnect the drain of the input transistor  112  to the source of the input transistor  112 . When a channel  110  is not selected, the drain is shorted to the source of the input transistor  112  by the second multiplexing switch  116  being turned on. This implies that any leakage current from the input transistor  112  affecting readout of an electrode signal of another channel  110  may be stopped. 
       FIG. 2 a    illustrates a point in time when the first channel  110   a  is selected for providing electrode signal input from the electrode  102   a . At the point in time illustrated in  FIG. 2 a   , a clocking signal controlling the first multiplexing switch  114  of the first channel  110   a  is high and a clocking signal controlling the second multiplexing switch  116  of the first channel  110   a  is low. This implies that the drain is disconnected from the source of the input transistor  112  in the first channel  110   a  and the drain of the input transistor  112  of the first channel  110   a  is connected to the first shared node  118 . 
     Further, at the point in time illustrated in  FIG. 2 a   , clocking signals controlling the first multiplexing switches  114  of all the remaining channels, illustrated by channels  110   b ,  110   n , are low to disconnect the drain of the input transistors  112  of these channels  110   b ,  110   n  from the first shared node  110 . Also, clocking signals controlling the second multiplexing switches  116  of all the remaining channels, illustrated by channels  110   b ,  110   n , are high to short the drain to the source of the input transistors  112  of these channels  110 . 
       FIG. 2 b    illustrates a point in time when the second channel  110   b  is selected for providing electrode signal input from the electrode  102   a . At the point in time illustrated in  FIG. 2 b   , the clocking signal controlling the first multiplexing switch  114  of the second channel  110   b  is high and the clocking signal controlling the second multiplexing switch  116  of the second channel  110   b  is low. This implies that the drain is disconnected from the source of the input transistor  112  in the second channel  110   b  and the drain of the input transistor  112  of the second channel  110   b  is connected to the first shared node  118 . 
     Further, at the point in time illustrated in  FIG. 2 b   , clocking signals controlling the first multiplexing switches  114  of all the remaining channels, illustrated by channels  110   a ,  110   n , are low to disconnect the drain of the input transistors  112  of these channels  110   a ,  110   n  from the first shared node  110 . Also, clocking signals controlling the second multiplexing switches  116  of all the remaining channels, illustrated by channels  110   a ,  110   n , are high to short the drain to the source of the input transistors  112  of these channels  110 . 
     The channels  110  may be sequentially selected one by one to provide the electrode signal input in a multiplexed manner.  FIG. 2 c    illustrates a point in time when the N′th channel  110   n  is selected for providing electrode signal input from the electrode  102   n . At the point in time illustrated in  FIG. 2 c   , the clocking signal controlling the first multiplexing switch  114  of the N′th channel  110   n  is high and the clocking signal controlling the second multiplexing switch  116  of the N′th channel  110   n  is low. This implies that the drain is disconnected from the source of the input transistor  112  in the N′th channel  110   n  and the drain of the input transistor  112  of the N′th channel  110   n  is connected to the first shared node  118 . 
     Further, at the point in time illustrated in  FIG. 2 c   , clocking signals controlling the first multiplexing switches  114  of all the remaining channels, illustrated by channels  110   a ,  110   b , are low to disconnect the drain of the input transistors  112  of these channels  110   a ,  110   b  from the first shared node  110 . Also, clocking signals controlling the second multiplexing switches  116  of all the remaining channels, illustrated by channels  110   a ,  110   b , are high to short the drain to the source of the input transistors  112  of these channels  110 . 
     Once all channels  110   a ,  110   b ,  110   n  have been selected to provide electrode signal input, the first channel  110   a  may again be selected, such that a new sequence of sequentially selecting each channel  110   a ,  110   b ,  110   n  one at a time may be initiated. 
     Referring now to  FIG. 3 , the first flipped voltage follower  120  according to an embodiment will be described in further detail. 
     The flipped voltage follower  120  is formed by the input transistor  112  of the selected channel  110  (forming a first transistor  112 ), the second transistor  126 , a current source  128 , and a gain element  130 . The first transistor  112  and the second transistor  126  may be p-type metal-oxide-semiconductor (PMOS) transistors. 
     The current source  128  enables a current through the first transistor  112  to be held stable, independent of an output current from the flipped voltage follower. 
     The output node  124  is arranged between a drain of the second transistor  126  and a source of the first transistor  112 . The output node  124  will have a potential following the electrode signal received at the gate of the first transistor  112 . 
     The second transistor  126  provides feedback of the flipped voltage follower  120 . The output node  124  will have a low output impedance because of the feedback of the second transistor  126 . 
     The signal on the first shared node  118  is passed through a gain element  130  to be provided to a gate of the second input transistor  126 . The signal on the first shared node  118  will have a small ac component due to attenuation of the gain element  130 . Thanks to use of the gain element  130 , it is easy to maintain the first transistor  112  in saturation mode and also to provide a fast settling time during multiplexing. 
     As shown in enlargement in  FIG. 3 , the gain element  130  may comprise an n-type metal-oxide-semiconductor (NMOS) transistor  132 , with the source connected to the first shared node  118  and the drain connected to the gate of the second transistor  126 . The NMOS transistor  132  may further receive a bias voltage on a gate of the NMOS transistor  132 . 
     The second flipped voltage follower  140  may be configured in a corresponding manner, receiving the reference voltage at the reference input transistor  142 . As shown in  FIG. 1 , the second flipped voltage follower  140  may thus comprise the reference input transistor  142 , the second transistor  146 , a current source  148 , and a gain element  150 . 
     Referring now to  FIGS. 4 and 5 , a method according to an embodiment will be described in relation to a flow chart in  FIG. 4  and a clocking diagram illustrating signals of switches in  FIG. 5 . 
     The method comprises receiving  202  electrode signals on a plurality of channels  110  of the amplifying input circuitry  100  for providing multiplexed amplifying of the electrode signals. 
     The method further comprises selecting  204  a first electrode signal as input to multiplexed amplifying by connecting the input transistor  112  of the first channel  110   a  to form part of the first flipped voltage follower  120  of the instrumentation amplifier and by all other channels  110   b ,  110   n  being deselected. Thus, the first electrode signal forms an input signal pair together with the reference voltage signal to the instrumentation amplifier. 
     As shown in  FIG. 5 , a first clocking signal  302  for controlling the first multiplexing switch  114  of the first channel  110   a  is thus set to be high, while the clocking signals  304 ,  306  for controlling the first multiplexing switch  114  of remaining channels  110   b ,  110   n  are low, during a first period of time  310 . Thus, the first electrode  102   a  is selected to provide the electrode signal input during the first period of time  310 . 
     The method further comprises selecting  206  a second electrode signal as input to multiplexed amplifying by connecting the input transistor  112  of the second channel  110   b  to form part of the first flipped voltage follower  120  of the instrumentation amplifier and by all other channels  110   a ,  110   n  being deselected. Thus, the second electrode signal forms an input signal pair together with the reference voltage signal to the instrumentation amplifier. 
     As shown in  FIG. 5 , the second clocking signal  304  for controlling the first multiplexing switch  114  of the second channel  110   b  is thus set to be high, while the clocking signals  302 ,  306  for controlling the first multiplexing switch  114  of remaining channels  110   a ,  110   n  are low, during a second period of time  312 . Thus, the second electrode  102   b  is selected to provide the electrode signal input during the second period of time  312 . 
     Further, the selection of electrodes to provide the electrode signal input proceeds until all electrodes have been selected. Thus, in a N′th period of time  314 , the N′th clocking signal  306  for controlling the first multiplexing switch  114  of the N′th channel  110   n  is thus set to be high, while the clocking signals  302 ,  304  for controlling the first multiplexing switch  114  of remaining channels  110   a ,  110   b  are low. 
     As can be seen in  FIG. 5 , the electrodes  102  and their associated input transistors  112  are connected into the instrumentation amplifier in a time interleaved manner controlled by the clocking signals  302 ,  304 ,  306 . The clocking signal  302 ,  304 ,  306  controlling a first multiplexing switch  114  will be high during a single period of time, when the electrode associated with the channel controlled by the clocking signal is selected, and will then be low during multiple periods of time when electrodes associated with other channels are selected. 
     Although not shown in  FIG. 5 , clocking signals controlling the second multiplexing switch  116  of each channel may also be synchronized with the clocking signals  302 ,  304 ,  306 . The clocking signal controlling the second multiplexing switch  116  will be low during a single period of time, when the electrode associated with the channel controlled by the clocking signal is selected, and will then be high during multiple periods of time when electrodes associated with other channels are selected. 
     During each transition from a current electrode being selected to a next electrode being selected, a reset signal  308  is enabled to turn on the reset switch  182  and reset the capacitor  176 , which clears the signal information of the current electrode. Also, at each transition, digital signal input to the electrode offset calibration block  162  is updated to a signal associated with the next electrode. 
     Referring now to  FIG. 6 , a biopotential signal sensor system  400  according to an embodiment will be described. 
     The biopotential signal sensor system  400  comprises a plurality of electrodes  402  configured for sensing a biopotential signal, which may correspond to the plurality of electrodes  102  described above. The biopotential signal sensor system  400  further comprises input circuitry for receiving the biopotential signals from the electrodes  402  and for providing multiplexed amplifying of the biopotential signals. The biopotential signal sensor system  400  may thus comprise the input circuitry  100  according to the embodiment described above. 
     The biopotential signals from the electrodes  402  may each be connected to the gate of the input transistor  112  of a respective channel  110  of the input circuitry  100 . Thus, the input circuitry  100  may allow multiplexing of the biopotential signals. 
     The biopotential signal sensor system  400  may be configured to be worn by a subject. The biopotential signal sensor system  400  may thus comprise a carrier configured for attachment to the subject or for arrangement around a body part of the subject. For instance, the biopotential signal sensor system  400  may comprise a patch  404  for attaching the biopotential signal sensor system  400  to the subject. 
     The biopotential signal sensor system  400  may comprise further processing circuitry for further processing of the multiplexed electrode signals output by the input circuitry  100 . Thus, the biopotential signal sensor system  400  may provide analysis of the electrode signals. 
     The biopotential signal sensor system  400  may also or alternatively comprise a communication unit for wired or wireless communication to a remote unit for further processing of the electrode signals. The biopotential signal sensor system  400  may communicate the multiplexed electrode signals output by the input circuitry  100  to the remote unit or may further process the multiplexed electrode signals before communicating the further processed signals to the remote unit. 
     The input circuitry  100  may be arranged on a common substrate  406  with the plurality of electrodes  402 . This implies that the biopotential signal sensor system  400  may be compact. Further, it may ensure high signal integrity of the electrode signals received by the input circuitry  100 . 
     The biopotential signal sensor system  400  may for instance be configured to sense a biopotential signal relating to electrocardiography, electroencephalography, electrocorticography, or electromyography. 
     Referring now to  FIG. 7 , a neural probe  500  according to an embodiment will be described. The neural probe  500  may incorporate electrodes and input circuitry of the biopotential signal sensor system  400  as described above. 
     In the neural probe  500 , small electrodes  502  are used in order to have a high spatial resolution of signals acquired from a brain. This implies that a large number of electrodes  502  may be used, such that there may be a large need to multiplex signals in order to reduce size of an area of circuitry for processing the electrode signals. 
     Further, since electrodes  502  are small, the electrodes  502  have high impedance. Therefore, a high input impedance of the input circuitry  100  is desired so as not to degrade signal quality of the electrode signals. Hence, the input circuitry  100  as described above is well-suited for use in the neural probe  500 , since the input circuitry  100  may maintain a high input impedance even though multiplexing is used. 
     The electrodes  502  may be arranged on a carrier  504  configured for being inserted into the brain to allow the electrodes  502  to acquire signals from the brain. The carrier  504  may have a pointed tip  506  for facilitating insertion into the brain and may have a base portion  508  which is intended not to be inserted into the brain. 
     The electrodes  502  are arranged in the portion of the carrier  504  intended to be inserted into the brain. The input circuitry  100  may be arranged in the portion of the carrier  504  intended to be inserted into the brain, such that the input circuitry  100  may be arranged in a layer below the electrodes  502 , illustrated as dashed lines for an input circuitry  100  supporting the electrodes  502  above the input circuitry  100 . According to an alternative, the input circuitry  100  may be arranged in the base portion  508  so as to enable minimizing a size of a part the neural probe  500  to be inserted into the brain. 
     The electrodes  502  and the input circuitry  100  may still be arranged on a common substrate to ensure high signal integrity of the electrode signals received by the input circuitry  100 . 
     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.