Abstract:
A method for calibrating a current sensor which is configured to determine, in a vehicle&#39;s on-board power system, an electric operating current which flows through a measuring resistor, based on comparison of a voltage drop at the measuring resistor caused by the operating current and based on a rule which is dependent on the measuring resistor, including: determining an operating voltage drop brought about at the measuring resistor by the operating current; impressing a known electric calibration current into the measuring resistor, detecting an overall voltage drop brought about at the measuring resistor by the calibration current and the operating current, filtering the operating voltage drop from the overall voltage drop, such that a calibration voltage drop which is brought about by the calibration current remains, and calibrating the rule, dependent on the measuring resistor, based on the comparison of the calibration current and the calibration voltage drop.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase Application of PCT International Application No. PCT/EP2015/071355, filed Sep. 17, 2015, which claims priority to German Patent Application No. 10 2015 208 135.4, filed Apr. 30, 2015 and German Patent Application No. 10 2014 218 710.9 filed Sep. 17, 2014, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for measuring a current using a current sensor. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electric currents in and from a vehicle battery are measured, for example in DE 10 2009 044 992 A1, which is incorporated by reference and in DE 10 2004 062 655 A1, which is incorporated by reference, with a current sensor using a measuring resistor, also called a shunt. 
       SUMMARY OF THE INVENTION 
       [0004]    An aspect of the invention is an improvement over the known method for current measurement. 
         [0005]    According to one aspect of the invention, a method for calibrating a current sensor that is set up to determine, in an onboard power supply system of a vehicle, an electric operating current flowing through a measuring resistor based on a comparison of a voltage drop across the measuring resistor brought about by the operating current and a rule dependent on the measuring resistor, comprises the steps of determination of an operating voltage drop brought about across the measuring resistor by the operating current; impression of a known electric calibration current into the measuring resistor, detection of a total voltage drop brought about across the measuring resistor by the calibration current and the operating current, filtering of the operating voltage drop from the total voltage drop, so that a calibration voltage drop brought about by the calibration current remains, and calibration of the rule dependent on the measuring resistor based on a comparison of the calibration current and the calibration voltage drop. 
         [0006]    The specified method is based on the consideration that the measuring resistor, also called a shunt, is beset by tolerances. The measuring resistor should therefore be designed robustly with respect to these tolerances, which is accordingly expensive. Alternatively, a cheaper measuring resistor can also be used if the relevant tolerances can be detected and corrected. This could be accomplished by gauging and calibrating the measuring resistor using a known calibration current. 
         [0007]    The measuring resistor should not have the unknown operating current applied to it during calibration. This is problematic insofar as it allows only long term dynamic tolerances to be taken into consideration, as arise as a result of the aging of the measuring resistor, for example. Short term dynamic tolerances, for example caused by temperature dependencies of the measuring resistor, cannot be taken into consideration in this way because they emerge only when the measuring resistor is operated under the applied operating current or the ambient temperature. For this reason, an accordingly expensive material needs to be chosen for the measuring resistor at least with respect to its short term dynamics. 
         [0008]    This is the starting point for the specified method, with the proposal to first of all determine, during calibration, the operating voltage drop across the erroneous measuring resistor, which is consequently likewise erroneous. Subsequently, the erroneous measuring resistor subject to the operating current additionally has the calibration current applied to it and the resultant total voltage drop is tapped off. Finally, the total voltage drop has the erroneous operating voltage drop eliminated from it. This elimination also removes the error from the total voltage drop, so that the actual calibration voltage drop is then available, which can also be used to calibrate the current sensor during operation in order to suppress or cancel short term dynamic tolerances. 
         [0009]    Therefore, short term dynamic tolerances can also be ignored when choosing the measuring resistor and an accordingly cheap material can be chosen therefore. 
         [0010]    Expediently, the calibration current comprises a periodic current pulse. 
         [0011]    In a development of the specified method, the calibration current is a current pulse having a pulse width, particularly of less than 10 μs. This development is based on the consideration that the calibration current should be detected as constantly as possible with few short term dynamic alterations, as are brought about by electrically contingent temperature alterations, for example. The shorter the current pulse is chosen, therefore, the more certainly the short term dynamic alterations can be masked out. 
         [0012]    In one expedient development of the specified method, the operating voltage drop is determined by determining a characteristic variable for the operating voltage drop from at least two operating voltage measured values that are determined outside the current pulse. The characteristic variable is intended to be understood below to mean a value that characterizes the profile of an AC signal with respect to a particular physical property. Characteristic variables of this kind are mean values, RMS values, rectified values, and so on, for example. The development is based on the consideration that the operating voltage drop and the total voltage drop should actually be measured at common times so that the aforementioned error in the operating voltage drop can be eliminated from the total voltage drop as completely as possible. On the other hand, the operating voltage drop and the total voltage drop can, by their very nature, only be measured in succession. In order to resolve this contradiction, it is proposed as part of the present development to estimate the operating voltage drop during the applied current pulse and to estimate a suitable measured value for the operating voltage drop from the estimated operating voltage drop. This suitable measured value is described by the characteristic variable. 
         [0013]    The estimation can be effected in this case using arbitrary means, for example by means of interpolation or extrapolation. The aforementioned estimation and determination of the characteristic variable can be realized in a technically particularly simple manner if the characteristic variable detected is the mean value between the two detected operating voltage measured values. 
         [0014]    In a particular development of the specified method, the current pulse lies between the operating voltage measured values. In this way, a particularly small estimation error is achieved for estimation of the operating voltage drop, particularly for averaging. 
         [0015]    In another development of the specified method, the total voltage drop and the operating voltage measured values are each converted into a digital value using at least one analog-to-digital converter and are each stored in a separate memory. The values stored in the memories are then all available in sync, so that the calibration voltage drop that is necessary for calibrating the current sensor is determinable using a simple arithmetic and logic unit. 
         [0016]    In one development of the specified method, the total voltage drop and the operating voltage measured values are each converted using a separate analog-to-digital converter, wherein the analog-to-digital converters are interchanged with one another at intervals of time. The interchange of the individual analog-to-digital converters with one another achieves interleaving, as a result of which the error dependencies are evenly distributed over all measured values included in the calibration voltage drop. 
         [0017]    In another development of the specified method, the total voltage drop and the operating voltage measured values are buffer-stored and each converted into a digital value at staggered times using a common analog-to-digital converter. In this way, not only is it possible to use a cheap single analog-to-digital converter for determining the calibration voltage drop, the use of a standard analog-to-digital converter also introduces a standard error dependency into the measured values included in the calibration voltage drop, said measured values then being able to be canceled out again computationally. 
         [0018]    The method is developed by virtue of the total voltage drop brought about across the measuring resistor by the calibration current and the operating current being amplified by means of at least one amplifier stage, particularly before the filtering of the operating voltage drop from the total voltage drop is performed. 
         [0019]    In one aspect the amplifier stage has at least one chopper circuit connected upstream of it that converts the signal from the total voltage drop into an AC signal, wherein particularly the polarity of the signal from the total voltage drop is periodically interchanged. As a particular preference in this case, the operation and/or clocking of the chopper circuit is designed such that within an interval of time, bounded by the times at which the two operating voltage measured values that are detected outside the current pulse are determined, no polarity change and/or period change for the chopper circuit is performed. 
         [0020]    It is expedient that determination of the operating voltage drop from the total voltage drop involves a dechopper element being used, and particularly determination of the calibration voltage drop involves no dechopper element being used. 
         [0021]    According to a further aspect of the invention, a control apparatus is set up to perform a method as claimed in one of the preceding claims. 
         [0022]    In a development of the specified control apparatus, the specified apparatus has a memory and a processor. In this case, the specified method is stored in the memory in the form of a computer program and the processor is provided in order to carry out the method when the computer program is loaded from the memory into the processor. 
         [0023]    According to a further aspect of the invention, a computer program comprises program code means in order to perform all the steps of one of the specified methods when the computer program is executed on a computer or one of the specified apparatuses. 
         [0024]    According to a further aspect of the invention, a computer program product contains a program code that is stored on a computer-readable data storage medium and that, when executed on a data processing device, performs one of the specified methods. 
         [0025]    According to another aspect of the invention, a current sensor for measuring an electric current comprises an electrical measuring resistor via which the electric current to be measured is routable to one of the specified control apparatuses. 
         [0026]    According to another aspect of the invention, a vehicle comprises one of the specified control apparatuses and/or the specified current sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The properties, features and advantages of this invention that are described above and also the way in which they are achieved will become clearer and more distinctly comprehensible in connection with the following description of the exemplary embodiments, which are explained in more detail in connection with the drawings, in which: 
           [0028]      FIG. 1  shows a basic depiction of a current sensor connected to an onboard power supply system of a vehicle, 
           [0029]      FIG. 2  shows the basic depiction from  FIG. 1  with an alternative line choice, 
           [0030]      FIG. 3  shows a timing diagram with voltages dropped in the current sensor of  FIG. 1 , 
           [0031]      FIG. 4  shows a timing diagram with control signals for the current sensor from  FIG. 1 , 
           [0032]      FIG. 5  shows a basic depiction of an alternative current sensor connected to an onboard power supply system of a vehicle, and 
           [0033]      FIG. 6  shows a timing diagram with control signals for the current sensor from  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    In the figures, like technical elements are provided with like reference symbols and described only once. 
         [0035]    Reference is made to  FIG. 1 , which shows a basic depiction of a current sensor  4  connected to an onboard power supply system  2  of a vehicle, not depicted further. The lines depicted in finely dashed form are initially intended to be ignored in  FIG. 1  in order to explain the operating principle of the current sensor  4  in more detail. The elements depicted in finely dashed form in  FIG. 1  are therefore also not provided with a reference symbol. 
         [0036]    The onboard power supply system  2  comprises a vehicle battery  6  having a positive pole  8  and a negative pole  10 , which is connected to a reference-ground potential  11 , such as ground. The vehicle battery  6  delivers, via the positive pole  8 , an electric operating current  12  that supplies electric power to and thus operates various electrical loads  14  in the onboard power supply system  2 . The operating current  12  is then returned to the negative pole  10 . For practical reasons, this normally takes place via the chassis  16  of the vehicle. 
         [0037]    The current sensor  4  is intended to detect the operating current  12  and to make it available to a processing computation device, such as a battery management system indicated in  FIG. 1 . The battery management system  18  is fundamentally a circuit that monitors the vehicle battery  6  and/or regulates the state of charge thereof. As such, various protection functions are implemented, one of which is known from DE 20 2010 015 132 U1, which is incorporated by reference and protects the vehicle battery  6  against deep discharge, for example. 
         [0038]    For the purpose of detecting the operating current  12 , the onboard power supply system  2  has a measuring resistor  20 , also called a shunt, arranged in it. The operating current  16  flows through this measuring resistor  20  and thus ensures an operating voltage drop  22  across the measuring resistor  20 . Fundamentally, the current sensor  2  detects the operating voltage drop  22  and takes the electrical properties of the measuring resistor  20  as the basis for determining the operating current  12  in a manner known per se. To this end, the measuring resistor  20  is expediently connected up directly to the reference-ground potential  11  at the negative pole  10 , since in this way only a further potential tap  24  is necessary in order to detect the operating voltage drop  22 . 
         [0039]    To determine the operating current  12  based on the operating voltage drop  22 , the current sensor  2  taps off the operating voltage drop  22  from the potential tap  24 . This operating voltage drop  22  is normally so small that it is unusable for direct signal processing in order to determine the operating current  12 . Therefore, the operating voltage drop  22  is initially amplified before the operating current  12  is determined. 
         [0040]    To amplify the operating voltage drop  12 , the current sensor  2  comprises an amplifier  25 , which is of two stage design in the present embodiment. Fundamentally, a single-stage amplifier would suffice for determining the operating current  12 . The amplifier  25  comprises a low pass filter  26  that taps off the operating voltage drop  22  from the potential tap  24  and filters undesirable signal components, such as switching transients, for example, therefrom. The low pass filtered operating voltage drop  22  tapped off is then converted into an AC signal using a chopper circuit  28 . The chopper circuit takes a chopper signal  30  prescribed by a chopper controller  29  as a basis for separating the operating voltage drop  22  into signal periods and interchanges the polarity of single instances of these separated signal periods. In this way, an AC voltage  31  is produced from the operating voltage drop  22 . Chopper circuits are known from DE 10 2012 105 162 A1, which is incorporated by reference, for example, and are not intended to be explained in more detail below. The task of the chopper circuit  28  is to eliminate an offset from a subsequent first amplifier stage  32  and second amplifier stage  32  routed via a further low pass filter  26 . An intermediate tap  33  between the two amplifier stages  32  is discussed at a later juncture. 
         [0041]    The amplified AC voltage  31 ′ at the output of the second amplifier stage  32  is then finally digitized in a signal conditioning circuit  33  and converted into a digital operating DC voltage value  22 ′. To this end, the signal conditioning circuit  33  has an analog-to-digital converter, called an A/D converter  34 , that converts the amplified AC voltage  31 ′ into a digital signal, not shown. Subsequently, the resolution of the digitized amplified AC voltage  31 ′ can be optionally increased in an input filter  35  if the resolution previously provided by the A/D converter  34  is inadequate. In any case, the digitized amplified AC voltage  31 ′ is then converted back into an offset-containing AC voltage, not shown further, in a dechopper element  36 . To this end, the digitized amplified AC voltage  31 ′ is multiplied by −1 and +1 in the dechopper element  36  in the periods from the chopper controller  29  based on an inverted chopper signal  30 ′. As a result, the negative periods of the digitized amplified AC voltage  31 ′ become positive or, to put it crudely, are “folded upward”. As the two amplifier stages  32  amplify the AC voltage  31  with an offset, for technical reasons, the originally positive periods and the periods “folded upward” have a different amplitude, which is why this is an offset-containing AC voltage. This offset-containing AC voltage is then smoothed in a terminating filter  37  by averaging, as a result of which the offset introduced by the two amplifier stages  32  is canceled. From the signal conditioning circuit  33 , the operating voltage drop  22  is thus output in digitized form, i.e. as a digital operating voltage drop  22 ′. 
         [0042]    From the digital operating voltage drop  22 ′, it is then possible, in a conversion device  38 , for example using a characteristic curve  21  describing the physical properties of the measuring resistor  20  from a memory  23 , to determine and output the operating current  12  to be measured. The conversion in the conversion device  38  can in this case take place based on known physical laws, such as Ohm&#39;s law, on the basis of the aforementioned physical properties  21  of the measuring resistor  20 . A value for the operating voltage  22  is then assigned an explicit value for the operating current  12 . However, this is the case only when the aforementioned electrical properties of the measuring resistor  20  and hence the characteristic curve  21  thereof are stable in the long and short terms. Normally, however, they change contingent on age, state and/or environment, for example. As such, ambient temperature is known to have an influence on electrical resistance. The material for the measuring resistor  20  must be chosen to be accordingly robust for all the aforementioned influences, this normally being possible only with very cost intensive materials. 
         [0043]    This is intended to be the starting point for the present exemplary embodiment, as described with reference to  FIG. 2 , in which the elements from  FIG. 1  that are depicted in finely dashed form are now depicted as normal elements and provided with a reference symbol. 
         [0044]    The concept behind the present embodiment is to impress, during operation of the onboard power supply system  2  and hence the current sensor  4 , a known reference current  40 , also called calibration current, into the measuring resistor  20  from a reference current source  39 , also called calibration current source, and to determine the actual physical properties of the measuring resistor  20  from the known reference current  40  and the voltage drop that can be tapped off from the output of the signal conditioning circuit  33 , and in this way to correct the measurement result from the current sensor  4 . 
         [0045]    A fundamental obstacle to this idea, however, is that, during operation of the onboard power supply system  2 , the operating current  12  also flows and thus has the reference current  40  superimposed on it. Hence, a total voltage drop  42  across the measuring resistor  20  that can be tapped off from the output of the signal conditioning circuit  33  is confronted not with a known reference current  40  but rather with a total current  41  from the operating current  12  and the reference current  40 , so that the actual physical properties of the measuring resistor  20  are not ascertainable in this manner during operation. 
         [0046]    So as still to make the actual physical properties of the measuring resistor  20  ascertainable based on the reference current  40 , it is proposed as part of the present embodiment to eliminate an estimated operating voltage drop  22  from the total voltage drop  42 . The operating voltage drop  22  during superimposition of the operating current  12  and the reference current  40  can be estimated with sufficient accuracy, which will be explained briefly with reference to  FIG. 3  prior to further explanation of  FIG. 2 . 
         [0047]    In  FIG. 3 , voltage values  43  from the operating voltage drop  22  (without the action of the reference current  40 ), from a reference voltage drop  44  (without the action of the operating current  12 ), also called calibration voltage drop, and from the total voltage drop  42  are plotted over time  45 . The profile of the operating voltage drop  22  and of the reference voltage drop  44 , which profiles differ from the total voltage drop  42 , are depicted in dotted form in  FIG. 3 . Further,  FIG. 3  is intended to be understood on a purely qualitative basis, the actual ratios between the voltages not being reproduced. 
         [0048]    To estimate the operating voltage drop  22 , it is proposed that the reference current  40  be embodied in the form of a pulse, so that the reference voltage drop  44  also takes on the shape of a pulse  46  indicated in  FIG. 3 . As a continuous operating voltage drop  22  can be assumed, this should be estimated in temporal proximity to the pulse  46 . To estimate the operating voltage drop  22 , it is possible to use any desired method of estimation, including a Kalmann filter. 
         [0049]    It is technically particularly simple and efficient to implement detecting a measured value  43 , accordingly a first measured value  49  and a second measured value  50 , for the operating voltage drop  22  at a first measurement time  47  in direct temporal proximity before the pulse  46  and at a second measurement time  48  in direct temporal proximity after the pulse. From the two measured values  49 ,  50 , it is then possible to determine a mean value  51  that describes the operating voltage drop  22  in the centre  52  of the pulse  46  with sufficient tolerance. 
         [0050]    The mean value  51  describes the actual voltage value  43  more accurately the more linear the profile of the operating voltage drop  22  in the region between the two measurement times  47 ,  48 . Therefore, the pulse width  53  of the pulse  46  should be chosen to be as small as possible and restricted only by the boundaries of the signal processing. A useful pulse width  53  has been found to be less than 10 μs. 
         [0051]    The mean value  51  at the middle time  52  can be used to detect the total measured value  54  for the total voltage drop  42  at the middle time  52  and to eliminate the operating voltage drop  22  from said total measured value, so that the reference voltage drop  44  brought about by the reference current  40  at the middle time  52  is known and can be used for calibrating the current sensor  4 . In other words, in the present embodiment, all the necessary information for calibrating the current sensor  4  is available at the middle time  52 . 
         [0052]    The technical explanation of the calibration will now be explained further with reference to  FIG. 2 . 
         [0053]    In the current sensor  4 , the averaging between the measured values  49 ,  50  takes place in an averager  55  and the subtraction between the total measured value  54  and the mean value  51  to determine the reference voltage drop  44  takes place in a subtraction stage  56 . Finally, in a division stage  57 , the reference current  40  is compared with the reference voltage drop  44  by division, which results in a correction factor  58  as a basis for the correction, of the uncorrected operating current results. The correction factor  58  can, finally, optionally also be filtered using a filter element in the form of a demodulator  59 , in order to smooth it. The demodulator  59  is based on the consideration that, considered over a comparatively long period of time, the operating current  12  is periodic because it is made up of a charging current flowing into the vehicle battery  6  and a discharge current flowing out of the vehicle battery  6 . Since the capacity of the vehicle battery  6  is limited, the operating current  12  must therefore have a periodic profile and be equal to 0 on average, when considered over a long period of time. The demodulator  59  therefore considers any remaining component of operating current  12  in the correction factor  58  to be a carrier signal and filters it therefrom by means of averaging, analogously to demodulation (for example amplitude demodulation). 
         [0054]    The filtered correction factor  58  is then supplied to the conversion device  38 , which can correct the characteristic curve  20  for determining the operating current  20  based on the correction factor  58 , for example. 
         [0055]    The measured values  40 ,  50 ,  54 ,  59  needed for determining the correction factor  58  are stored in appropriate memories  60 ,  61 ,  62 ,  63 . These memories  60 ,  61 ,  62 ,  63  are depicted twice in  FIGS. 1 and 2  merely for the sake of clarity so that the signal paths do not cross. In fact, each memory is present only once. 
         [0056]    To fill the memories  60 ,  61 ,  62 ,  63  and hence to initiate calibration, there are various control signals in the present exemplary embodiment. A reference current control signal  64  is used to close a switch  65 , which conducts the reference current  40  via a reference resistor  66  to the measuring resistor  20 . In principle, the reference resistor  66  is not necessary, because the method could be performed with the reference current source  39  alone. However, the demands on the reference current source  39  with respect to robustness and so on are then very high. Using the reference resistor  66 , these demands can then be lowered, because the reference current  40  can be determined based on the electrical properties  67  of the reference resistor  66 , which properties may be stored in an appropriate memory  68 , and a corresponding voltage drop  69  across the reference resistor  66 . The voltage drop  69  across the reference resistor  66  is, in this case, not the reference voltage drop  44  that is dropped across the measuring resistor  20  and is contained in the total voltage drop  42 . In order to determine the voltage drop  69  across the reference resistor  66  as simply as possible, the reference resistor  66  should be chosen to be a multiple larger than the measuring resistor  20 . Further, the reference current  40  should be chosen to be a multiple higher than the operating current  12 . During normal operation, when the operating current  12  supplies electric power only to display elements as electrical loads  14 , for example, said operating current is in the region of a few mA. It is then possible for the reference current  40  to be chosen to be in the region of one amp. In this way, the voltage drop  69  across the reference resistor  66  can be tapped off via a single tap point  70  with reference to the reference-ground potential  11 , the total voltage drop  42  being able to be tapped off from the measuring resistor  20 . 
         [0057]    The voltage drop  69  across the reference resistor  66  is stored in a first buffer store  72  with the reference current control signal  64  via a further switch  65 , a high pass filter  71  and a low pass filter  26  and can be tapped off again from said buffer store via a further switch  65 . Further, the total measured value  54  for the total voltage drop  42  is also tapped off from the intermediate tap  33  with the reference current control signal  64  via a high pass filter  71  and stored in a second buffer store  73 . The total voltage drop  42  is very high from the perspective of the high reference current  40 . This is where the tap off from the intermediate tap  33  comes to fruition. In actual fact, the two amplifier stages  32  have to amplify a comparatively small operating voltage  22 . The contrastingly high total voltage drop  42  would drive at least the second amplifier stage  32  to saturation and hence corrupt the measurement. Therefore, the total voltage drop  42  should be tapped off from the intermediate tap  33 . 
         [0058]    However, the intermediate tap  33  has the disadvantage that the relevant signals tapped off therefrom do not pass through the dechopper element  36 . In order to overcome this disadvantage, it should be ensured that all measured values  49 ,  50 ,  54  tapped off from the intermediate tap  33  are tapped off in a pulse period of the chopper signal  30 . The offset of the first amplifier stage  32  is then automatically canceled out when the correction factor  58  is determined. If the first measured value  49  is denoted by U 0 , the middle  54  of the pulse  46  is denoted by U 1 , the second measured value  50  is denoted by U 2 , the reference current  40  is denoted by I ref , the correction factor  58  is denoted by K and the offset of the first amplifier stage  32  is denoted by x, then the determination of the correction factor  58  according to  FIG. 2  can be depicted by the following formula: 
         [0000]        K=I   ref /[( U   1   +x )−½{( U   D   +x )+( U   2   +x )}]
 
         [0059]    The offset of the first amplifier stage  32  is canceled out from this formula. 
         [0060]    A first measured value control signal  74  can be used analogously to the total measured value  54  for the total voltage drop  42  to detect the first measured value  49  and to store it in a third buffer store  75 , while a second measured value control signal  76  can be used to detect the second measured value  50  and to store it in a fourth buffer store  77 . 
         [0061]    The first buffer store  72  can then be read using a first read signal  78  via a switch  65 , while the second buffer store  73  can be read using a second read signal  79  via a switch  65 , the third buffer store  75  can be read using a third read signal  80  via a switch  65  and the fourth buffer store  77  can be read using a fourth read signal  81  via a switch  65 . 
         [0062]    The control signals  30 ,  64 ,  74 ,  76 ,  78 ,  79 ,  80 ,  81  in  FIGS. 1 and 2  are plotted over time  45  in  FIG. 4 . 
         [0063]    As can be seen in  FIG. 4 , at the first measurement time  47 , the first measured value  49  is first of all sampled with the first measured value signal  74  from the total voltage drop  42  using the relevant switch  65  and stored in the relevant buffer store  75 . As soon as the first measured value  49  is present in the relevant buffer store  75 , it is read using the first read signal  78  and stored in its memory  62  via an A/D converter  34 . 
         [0064]    During the actual reading of the buffer store  75  for the first measured value  49 , the total measured value  54  itself for the total voltage drop  42  and also the voltage drop  69  across the reference resistor  66  are sampled in the middle  52  of the pulse  46  using the relevant switches  65  and are stored in the relevant buffer stores  72 ,  73 . Subsequently, the buffer store  73  and the buffer store  72  are read in succession using the second read signal  79  and the third read signal  80 , respectively. The sampled total measured value  54  for the total voltage drop  42  is stored in the relevant memory  61 , while the voltage drop  69  across the reference resistor  66  is first of all converted into the reference current  40  based on the physical properties  67  of the reference resistor  66  before the reference current  40  determined in this manner is then stored in the relevant memory  60 . 
         [0065]    Following the sampling of the total measured value  54  for the total voltage drop  42  and of the voltage drop  69  across the reference resistor  66 , the second measured value  50  is finally sampled with the second measured value signal  76  using the switch  65  at the second measurement time  48  and stored in the relevant buffer store  77 . Reading of the buffer store  77  takes place analogously to the reading of the buffer store  73 , but with the fourth read signal  81 , the second measured value  50  being stored in the relevant memory  63 . 
         [0066]    In this way, all the measured values  40 ,  49 ,  50 ,  54  are present in the memories  60  to  63 , so that the correction factor  58  can be determined according to  FIG. 2 . 
         [0067]    The method and the embodiment of the current sensor  4  according to  FIG. 2  have the advantage that digitization of the measured values  40 ,  49 ,  50 ,  54  requires only a single A/D converter  34 , because the digitization of the measured values  40 ,  49 ,  50 ,  54  can be equalized by the temporal arrangement of the control signals according to  FIG. 4 . In this way, the digitization errors that are included at least in the measured values  49 ,  50 ,  54  by the A/D converter  34  involved in the digitization are also canceled out from the correction factor  58  analogously to the aforementioned offset of the first amplifier stage  32 . 
         [0068]    The comparatively high complexity of control signals can be reduced, however, using an alternative design of the current sensor  4  according to  FIG. 5 . 
         [0069]    In this case, instead of the read signals  78  to  81 , a multiplexer  82  is used. This is used to connect the relevant measured values  69 ,  54 ,  49 ,  50  directly to A/D converters  34  connected in parallel. This requires the A/D converters  34  to be designed to operate accordingly quickly, however. 
         [0070]    The multiplexer  82  itself is not necessary. It is also possible to use switches  65 , as shown in  FIGS. 1 and 2 . The multiplexer  82  can be used to rotate or interchange the individual A/D converters  34  during conversion of the measured values  69 ,  54 ,  49 ,  50 , however, as a result of which different digitization errors from the individual A/D converters  34  are evenly distributed over the individual measured values  69 ,  54 ,  49 ,  50 . In this way, the digitization errors are likewise canceled out during determination of the correction factor  58  analogously to the aforementioned offset of the first amplifier stage  32 .