Abstract:
A current generator is disclosed. An example current generator includes a plurality of current cells connected in parallel, each current cell being connected to a switch. The current generator further includes a first summer configured to sum the output of each current cell of a first subset of the plurality of current cells and a second summer configured to sum the output of each current cell of a second subset of the plurality of current cells. The current generator also includes a combiner configured to combine the outputs of the first and second summers. Further, each switch is switchable according to a sequence to generate a summed output of the current cells at a plurality of quantization levels to generate positive and/or negative alternations of a pseudo-sinusoidal, alternating current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(b) to European Patent Application EP 12198406.6, filed on Dec. 20, 2012, the full disclosure of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The disclosure relates to a current generator and a method of generating a current. More particularly, but not exclusively, the disclosure relates to generating an excitation current for a biopotential measurement circuit. 
       BACKGROUND 
       [0003]    Bio-signal monitoring systems for monitoring a biological subject may involve measurement of the electrical potential differences in the tissue of the subject (biopotential). This is achieved by applying an excitation current across, in vivo, electrodes. Measurements are taken on a continuous basis. Therefore, the monitor is invariably implanted. Therefore, low power consumption whilst maintaining accuracy of the measurements is desirable. 
         [0004]    Therefore design strategies for each building block and signal monitoring methodology focus on low power consumption. In addition, measurement accuracy has to be guaranteed with minimized power consumption. There are two things which determine monitoring accuracy of portable bio-signal monitoring system. One is accuracy of the building block and the other is accuracy of monitoring method. To achieve high accuracy measurement requires high power consumption for internal building blocks and high accurate external components which increase form factor of the system. 
         [0005]    One specific bio-signal monitoring system is intra-thoracic fluid analysis. Such a measurement system is often required to resolve biopotential down to mΩ-range change (AC) superimposed on up to kΩ-range average (DC) impedance. In standard practice, this can be achieved by using a purely sinusoidal current source with very good harmonic distortion to minimize the error of impedance measurement as disclosed in, for example, Yan, L.; Bae, J.; Lee, S.; Kim, B.; Roh. T.; Song, K.; Yoo, H-J.; “A 3.9 mW 25-electrode Reconfigurable Thoracic Impedance/ECG SoC with Body-Channel Transponder,”  IEEE International Solid - State Circuits Conference , vol., no., pp.490-491, 7-11 Feb. 2010. However, this approach consumes many mWs of power which is not acceptable for an implantable device. There is research that uses multi-level quantized signal as a modulation and demodulation signal as disclosed in M. Min, and T. Parve, “Improvement of Lock-in Electrical Bio-Impedance Analyzer for Implantable Medical Devices,” IEEE Tran. On Instrumentation and Measurement, 2007. As a result, pW-range power consumption can be achieved by using a square-wave current combined with quadrature demodulation as disclosed, for example, in Yazicioglu, R. F.; Kim, S.; Torfs, T.; Merken, P.; Van Hoof, C.; , “A 30 μW Analog Signal Processor ASIC for biomedical signal monitoring,”  IEEE International Solid - State Circuits Conference , vol., no., pp.124-125, 7-11 Feb. 2010. This technique, however, requires multi-level quantized current as well as multi-level demodulation amplifier which has a complex architecture and consumes more power. In addition, this technique requires modulation and demodulation signal which have accurately generated phase shift. This limits programmability of the generated signal as well as the measurement accuracy can be degraded due to the phase variations. Further, resulting in intolerable measurement errors as high as 23% because the quadrature demodulation will fold all odd harmonics of the square wave current into the baseband. 
       Overview 
       [0006]    The present disclosure provides a current generator that would be suitable for use in an implantable biopotential monitoring system which mitigates the above-mentioned drawbacks. 
         [0007]    According to an example of the present disclosure, there is provided a current generator comprising a plurality of current cells connected in parallel, each current cell being connected to a switch; a summer configured to sum the output of each current cell; wherein each switch is switchable according to a sequence to generate a summed output of the current cells at a plurality of quantisation levels to generate positive and/or negative alternations of a pseudo-sinusoidal, alternating current. 
         [0008]    According to another example aspect of the present disclosure, there is provided a biopotential measurement system comprising a current generator such as the current generator described above. 
         [0009]    According to yet another example aspect of the present disclosure, there is provided a method for generating a current, the method comprising the steps of switching a plurality of current cells, each current cell being connected in parallel; summing the output of each current cell of a first subset of the plurality of current cells for generating positive alternations of the pseudo-sinusoidal, alternating current; summing the output of each current cell of a second subset of the plurality of current cells for generating negative alternations of the pseudo-sinusoidal, alternating current; combining the summations of the output of each current cell of the first subset of the plurality of current cells and the output of each current cell of the second subset of the plurality of current cells to generate a pseudo-sinusoidal alternating current; and wherein switching the plurality of current cells comprises switching each current cell according to a sequence to generate a summed output of the current cells at a plurality of quantisation levels to generate positive and/or negative alternations of a pseudo-sinusoidal, alternating current. 
         [0010]    To achieve a high accurate biopotential monitoring system with low power consumption, a multi-mode current injection circuit may be used to generate the current. A pseudo-sine wave current may be generated via internal digital control circuit which is particularly useful for monitoring biopotential as well as conventional square wave excitation current. 
         [0011]    Each current cell of the first subset of the plurality of current cells may comprise a current source. Each switch of the current cells of the first subset of the plurality of current cells may be switched according to the sequence to generate a summed output of the current cells of the first subset at a plurality of quantisation levels to generate the positive alternations of the pseudo-sinusoidal, alternating current. 
         [0012]    Each current cell of the second subset of the plurality of current cells may comprise a current sink. Each switch of the current cells of the second subset of the plurality of current cells may be switched according to the sequence to generate a summed output of the current cells of the second subset at a plurality of quantisation levels to generate the negative alternations of the pseudo-sinusoidal, alternating current. 
         [0013]    In an example embodiment, the combiner further comprises a chopper configured to chop alternating alternations of the outputs of the first and second summers to generating alternate positive and negative alternations of a pseudo-sinusoidal, alternating current. 
         [0014]    In a further example embodiment, the current generator further comprises a control module configured to provide the sequence to generate the summed output of the current cells and to adjust the number of quantization levels. 
         [0015]    The control module further may comprise a look-up table configured to store the coding for the sequence. The look-up table may comprise an integer look-up table. 
         [0016]    In yet a further example embodiment, the control module is configured to adapt the sequence in order to change the frequency and amplitude of the generated pseudo-sinusoidal, alternating current. Further, the control module is configured to change the switchable frequency of each switch and the combination frequency of the combiner. Still further, the control module is configured to increase or decrease the unit current of each current cell to increase or decrease the amplitude of the pseudo-sinusoidal alternating current. 
         [0017]    These as well as other aspects, advantages, and alternatives, will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Various exemplary embodiments are described herein, by way of example only, with reference to the following drawings, wherein like numerals denote like entities. The drawings described are schematic and are non-limiting. 
           [0019]      FIG. 1  is a simplified schematic of an example of a bio-impedance measuring circuit including the current generator of an example embodiment of the present disclosure; 
           [0020]      FIG. 2  is a simplified schematic of the current generator according to an example embodiment of the present disclosure; 
           [0021]      FIG. 3  is a simplified schematic of the current digital-to-analog converter of the current generator of  FIG. 2 ; 
           [0022]      FIG. 4  is a simplified schematic of the current generator of another example embodiment of the present disclosure; 
           [0023]      FIG. 5   a  is a graphical representation of the output of the combiner of an example embodiment of the present disclosure; 
           [0024]      FIG. 5   b  is a graphical representation of the chopper waveform of the chopper of an example embodiment of the present disclosure; 
           [0025]      FIG. 5   c  is a graphical representation of one component I 1 , of the generated current of the current generator of an example embodiment of the present disclosure; 
           [0026]      FIG. 5   d  is a graphical representation of the second component I 2  of the generated current of the current generator of an example embodiment of the present disclosure; 
           [0027]      FIG. 6  is a flowchart of a method of generating a current according to an example embodiment of the present disclosure; 
           [0028]      FIG. 7  is a flowchart of a method of generating the integer coefficient for the look-up table of the controller of the current generator according to an example embodiment of the present disclosure; and 
           [0029]      FIG. 8  is a graphical representation of a comparison of the waveforms generated by a non-integer look-up table and the integer look-up table according to an example embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    With reference to  FIG. 1 , an example biopotential circuit  100  according to an example embodiment of the present disclosure comprises a pair of receive electrodes  101 ,  103  and a pair of drive electrodes  105 ,  107 . The pair of receive electrodes  101 ,  103  is implanted into the patient&#39;s tissue. The drive electrodes  105 ,  107  are positioned either side of the receive electrodes to enclose an area  109  of the tissue. The drive electrodes may be positioned on the surface of the tissue as is known in the art. 
         [0031]    An excitation current is applied to the drive electrodes  105 ,  107  and the potential of the enclosed area  109  of the tissue is measured by the receive electrode pair  101 ,  103 . The biopotential circuit  100  further comprises a current generator  111  for generating the excitation current described in more detail below. The bio-potential circuit  100  further comprises an oscillator  113  and a demodulation clock  115 . The oscillator  113  provides the reference clocks for the current generator  111  and the demodulation clock  115 . 
         [0032]    The receive electrode outputs are fed to first and second analyzers  117 ,  119 . Each analyzer  117 ,  119  is clocked by the demodulation clock  115 . The second analyzer  119  compares the in phase component of the outputs of the pair of receive electrodes  101 ,  103  and the first analyzer  117  compares the quadrature phase components of the outputs of the pair of receive electrodes  101 ,  103  to determine the biopotential of the area  109  of the tissue. The respective components of the biopotential pass to respective first and second programmable gain amplifiers  121 ,  123 , respective filters  125 ,  127  and buffers  129 ,  131  to output the respective in phase component of the biopotential and the quadrature phase component of the bio-potential. 
         [0033]    The current generator  111  comprises a current digital-to-analog converter (DAC)  133  and a control module  135 . As shown in more detail in  FIG. 2 . The control module  135  comprises a code generation module  201 , a clock divider module  203  and a DAC selection module  205 . 
         [0034]    The current DAC  133  comprises a first subset  207  of a plurality of parallel current cells  209 _ 1  to  209 _n. Each current cell  209 _ 1  to  209 _n of the first subset  207  comprises a pMOSFET  211 _ 1  to  211 _n. The pMOSFETs  211 _ 1  to  211 _n may be substantially similar, each generating the same amount of current. The source of each pMOSFET  211 _ 1  to  211 _n of the first subset  207  is connected in parallel to a common supply voltage V s . The gate of each pMOSFET  211 _ 1  to  211 _n of the first subset  207  is connected in common to a first bias voltage V biasp . The drain of each pMOSFET  211 _ 1  to  211 _n of the first subset  207  is connected to the respective source of a cascode pMOSFET  218 _ 1  to  218 _n of a first subset  217 . The drain of each cascode pMOSFET  218 _ 1  to  218 _n of the first subset  217  is connected to a first terminal of a respective switch  213 _ 1  to  213 _n of a first subset  215 . The second terminals of each switch  213 _ 1  to  213 _n of the first subset  215  are connected together to a first input of a combiner  219 . The gate of each cascade pMOSFET  218 _ 1  to  218 _n of the first subset  217  is connected to a third bias V casp . The current cells  209 _ 1  to  209 _n of the first subset  207  provide a current source to generate positive alternations of a pseudo-sinusoidal alternating current. 
         [0035]    The current DAC  133  further comprises a second subset  221  of a plurality of parallel current cells  223 _ 1  to  223 _n. Each current cell  223 _ 1  to  223 _n of the second subset  221  comprises an nMOSFET  225 _ 1  to  225 _n. The nMOSFET  225 _ 1  to  225 _n may be substantially similar, each sinking the same amount of current. The source of each nMOSFET  225 _ 1  to  225 _n of the second subset  221  is connected in parallel to ground. The gate of each nMOSFET  225 _ 1  to  225 _n of the second subset  221  is connected in common to a second bias voltage V biasn . The drain of each nMOSFET  225 _ 1  to  225 _n of the second subset  221  is connected to the source of a respective cascode nMOSFET  232 _ 1  to  232 _n of a second subset  231 . The drain of each cascode nMOSFET  232 _ 1  to  232 _n of the second subset  231  is connected to the first terminal of a respective switch  227 _ 1  to  227 _n of a second subset  221 . The second terminal each switch  213 _ 1  to  213 _n of the first subset  215  are connected together to a second input of the combiner  219 . The gate of each nMOSFET  232 _ 1  to  232 _n of the second subset  231  is connected to a fourth bias V casn . The current cells  223 _ 1  to  223 _n of the second subset  221  provide a current sink to generate the negative alternations of a pseudo-sinusoidal alternating current. 
         [0036]    The example current DAC  133  is shown in more detail in  FIG. 3 . The first and second bias voltages V biasp , V biasn  are provided via a current mirror pMOSFET  301 . A reference current I REF  is provided to the current mirror by a current reference circuit  305 . The combiner  219  comprises a combination module  307  and a chopper  309 . The chopper  309  outputs the generated components of the excitation current and the negative I 2  for drive electrodes  105 ,  107 . 
         [0037]    In an example embodiment as shown in  FIG. 4 , the DAC selection module  205  comprises a first decoder  405 . The output of the first decoder  405  provides the value to control the current generated by each current cell  209 _ 1  to  209 _n of the first subset  207  or sunk by each current cell  223 _ 1  to  223 _n of the second subset  221 . In this embodiment, each current cell comprises a plurality of MOSFETs connected in parallel (illustrated as a single variable MOSFET  411 _ 1  and  423 _ 1  in  FIG. 4  for simplicity), each of the parallel connected MOSFETs are turned on and off based on the control signal I AMP  output by the first decoder  405 . The input of the decoder  405  is connected to a third control signal AMPLITUDE. The code generation module  201  comprises a second decoder  439 . The output of the second decoder  439  is connected to a multiplexer  441 . The output of the multiplexer  441  is connected to a pseudo-sine synthesizer  443 . The pseudo-sine synthesizer  443  is connected to a look-up table  445 . The output of the pseudo-sine synthesizer  443  is connected to a wave selector  447 . The output of the wave selector  447  is provided to control switching of the switches  213 _ 1  to  213 _n of the first subset  215  and the switches  227 _ 1  to  227 _n of the second subset  229 . The pseudo-sine synthesizer  443  receives a first control signal SIGNAL_TYPE and the wave selector  447  receives a second control signal FREQUENCY. The clock divider module  203  also receives the second control signal FREQUENCY. The outputs of the clock divider  203  are connected to the second decoder  439  of the code generation module  201 , the multiplexer  441  of the code generation module  201 , the wave selector  447  of the code generation module  201  and the combiner  219 . The clock divider  203  has a reference clock  437  connected to the clocking terminal of the clock divider  203 . 
         [0038]    Operation of the example current generator of  FIGS. 2 to 5   d  will be described with reference to  FIGS. 6 to 8 . 
         [0039]    The actuation of the switches is controlled by the control module  135 . The switches are actuated according to a sequence. The coding for the switching sequence is stored in the look-up table  445  of the code generation module  201 . The sequence is selected by the pseudo-sine synthesizer  443  from the coding stored in the look-up table  445  in response to the first control signal, SIGNAL_TYPE. The clock divider module  203  outputs the required modulation frequency selected by the second control signal, FREQUENCY. The wave selector  447  controls the switching frequency of each switch of the first and second subsets  215 ,  229  in accordance with the sequence provided by the pseudo-sine synthesizer  443 . The DAC selection module  205  controls the amplitude of the modulation current output by the current generator in response to a third control signal AMPLITUDE by controlling the unit current for each current cell  209 _ 1  to  209 _n and  225 _ 1  to  225 _n. The unit current of the first subset and the second subset is substantially the same. In an example, to generate pseudo-sine-wave, an integer look-up table (LUT) with  64  values is designed which enables 16-level quantization. The third control signal AMPLITUDE is decoded by the first decoder  405  (in this specific example a 2 to 4 decoder) to generate a 4-bit I AMP &lt;0:3&gt; signal. In this example, each current cell comprises four MOSFETs  411 _ 1 ,  423 _ 1  (for simplicity, the four MOSFETs are illustrated in the Figure as a single MOSFET) connected in parallel, each of the four MOSFETs are turned on and off based on the control signal I AMP &lt;0:3&gt; output by the first decoder  405  to control the amplitude of the pseudo-sine waveform. The clock divider  203  generates a 4-bit IT&lt;0:3&gt; signal. This is used to generate a 14 bit signal output by the multiplexer  441  which controls the pseudo-sine synthesizer  443  for accessing the LUT  445  with a SEL signal. SIGNAL_TYPE control signal selects the type of waveform for the output current, for example, a pseudo-sine-wave or square-wave. The 14 bit signals after Wave Selector are used as a switch control signals inside the unit current DACs. 
         [0040]    Since the control signals, SIGNAL_TYPE, FREQUENCY and AMPLITUDE are independently provided to the current generator, a high programmable modulation current is generated. 
         [0041]    The switching  601 ,  605  of each switch of the first and second subset is therefore controlled in accordance with a sequence determined by the output of the DAC selection module  205 . The output of each current cell  209 _ 1  to  209 _n of the first subset are summed,  603 , by a first summer, formed in this embodiment, by the common connection of the switches. The output of the first summer is provided on the first input terminal of the combiner  219 . 
         [0042]    The output of each current cell  223 _ 1  to  223 _n of the second subset are summed,  607  by the second summer, formed in this embodiment, by the common connection of the switches  227 _ 1  to  227 _n of the second subset  229 . The output of the second summer is provided on the second input terminal of the combiner  219 . 
         [0043]    The summation M*I REF +ΔI p  (wherein M is the number of current cells of the first subset connected at any one time, ΔI p  is the error of the summed output of the current cells of the first subset  207  due to mismatch) and the summation—M*I REF −ΔI n  (wherein M is the number of current cells of the second subset connected at any one time, ΔI n  is the error of the summed output of the current cells of the second subset  221  due to mismatch) are combined,  609 , to form the combined waveform as shown in  FIG. 5   a . The combined waveform is input to the chopper  309  which combines the waveform of  FIG. 5   a  with the square waveform of  FIG. 5   b  to output and I 2  as shown in  FIGS. 5   c  and  5   d . The alternate alternations are then chopped,  611  to form a pseudo-sinusoidal alternating current I 1 =2M*I REF +Δ(I p +I n ) and the negative I 2 =−{2M*I REF +Δ(I p +I n )}. 
         [0044]    In an example, to limit the Total Harmonic Distortion (THD) due to folded odd harmonics to less than 1%, a 16-level quantized-sinusoidal current source is implemented which will suppress all harmonics power below the 63 rd  harmonic. In order to generate such a quantized current source with ideal sinewave characteristics, the switches of the DAC are switched in accordance with a code retrieved from the look-up table. An adaptively quantized integer LUT is used to reduce conversion error and reduce the requirements of a very high frequency clock as well as a high resolution DAC and this increasing power consumption. By assigning different number of currents cells based on the amplitude of the resulting modulation signal, conversion errors decease by 1.71-fold. Using current cells minimizes the mismatch of the current generator and prevents glitches due to simultaneous switching of the DAC as well as enables easy configuration of the injection current amplitude. The integer LUT is achieved by the method shown in  FIG. 7 . Each non-integer coefficient for generating the pseudo-sinewave (shown in  FIGS. 5   c  and  5   d ) is received,  701 , and converted into an integer,  703 , for example, by rounding to the nearest integer. Adaptive quantisation,  705  is carried out on the converted integer so that the level of the quantisation is varied depending on the slop of the generated sine wave. As a result, when the slope is gentle, the quantisation is coarse with greater interval between changes in the quantisation level, for steeper slopes, the change in quantisation is lower at smaller change intervals. The adapted integer is the converted,  707  to Thermometer code and stored in the look-up for later retrieval. As illustrated in  FIG. 8 , the ideal non-integer LUT and the integer LUT of an example embodiment of the present disclosure lead to similar performance. As a result, demodulation can still be executed by a square wave using chopper demodulation while assuring 1% THD. 
         [0045]    By adopting a pseudo-sine wave current generator with low power current DAC architecture, high resolution bio-impedance monitoring is enabled as well as low power monitoring by adopting a square wave current generator. 
         [0046]    The use of the PMOS and NMOS of the current cells of the DAC to generate the positive and negative alternations enables the NMOS sink matrix to reuse the current from the PMOS source matrix to maximize the power efficiency. Further, the output chopper switch can mitigate the AC current mismatch between the current source and the current sink by alternating outputs at f CHOP  frequency. Furthermore, scaling the size of each PMOS and NMOS can simply adjust the current magnitude (10-40 μA p-p ) to enable the high dynamic biopotential measurement range. 
         [0047]    While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims, along with the full scope of equivalents to which such claims are entitled. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. 
         [0048]    Although embodiments of the present disclosure have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed, but capable of numerous modifications without departing from the scope of the invention as set out in the following claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.