Patent Document

FIELD 
   The present invention relates to a circuit arrangement having an amplifier arrangement, which has an operational amplifier, and having an offset compensation arrangement connected to the amplifier arrangement and serving for the offset compensation of the operational amplifier. 
   BACKGROUND 
   Such a circuit arrangement is described for example in Finvers et al.: “A High Temperature Precision Amplifier”, IEEE Journal of Solid-State Circuits, Vol. 30, No. 2, February 1995, pages 120-128. Operational amplifiers in amplifier circuits are usually connected up in such a way that a voltage between the inputs of the operational amplifier is ideally zero. If the operational amplifier is beset with an offset, then an offset voltage not equal to zero is established between the inputs of said operational amplifier, which leads to a corruption of the measurement result. In such circuits, the offset compensation arrangement serves to detect such an offset voltage present between the inputs of the operational amplifier and to generate an offset compensation signal. Said compensation signal is fed to an offset compensation input of the operational amplifier in order to regulate the offset voltage between the inputs of the operational amplifier to zero. 
   This offset compensation by detecting the input voltage of the operational amplifier and generating the compensation signal depending on the input voltage may lead to problems when a frequently changing input signal is fed to the operational amplifier. This is because, in the event of a level change of the input signal, the input voltage difference of the operational amplifier is initially not zero until the operational amplifier has again attained a settled state. This input voltage difference not equal to zero is registered as an offset by the offset compensation arrangement and incorrectly leads to a change or adaptation of the compensation signal. 
   Offset compensation arrangements usually have an integrating behavior, which has the effect that the offset compensation signal increases with the number of level changes of the input signal or with the number of settling operations of the operational amplifier. The consequence of this is that the operational amplifier is “overcompensated”, so that precisely during the settled state of the operational amplifier there is an offset present which increases as the number of level changes of the input signal increases or as the number of settling operations of the operational amplifier increases. 
   It would therefore be advantageous to provide a circuit arrangement having an amplifier arrangement—which has an operational amplifier—and having an offset compensation arrangement which does not have these disadvantages. 
   SUMMARY 
   A circuit arrangement is disclosed comprising an amplifier arrangement having inputs for feeding in an input signal, an output for providing an output signal and having a first operational amplifier. The operational amplifier has inputs coupled to the inputs of the amplifier arrangement, an output coupled to the output of the amplifier arrangement, and a first compensation input for feeding in an offset compensation signal. The circuit arrangement additionally has a first compensation arrangement, which is coupled to the inputs of the first operational amplifier and which provides the offset compensation signal. Moreover, a deactivation circuit is present, which is designed to temporarily deactivate the first compensation arrangement. 
   In the case of the circuit arrangement according to at least some embodiments of the invention, the temporary deactivation of the compensation arrangement by the deactivation circuit prevents a situation in which, during those time durations during which the first operational amplifier settles after a level change of the input signal, a voltage difference between the inputs of the first operational amplifier is detected as an offset by the compensation arrangement. Such detection of the input voltage difference as an offset during the settling operations of the first operational amplifier would lead to an erroneous generation of the compensation signal in the manner explained above. 
   The deactivation circuit may suitably be designed to deactivate the first compensation arrangement depending on the input signal. In one embodiment, the deactivation circuit deactivates the first compensation arrangement in each case for a predetermined time duration after a level change of the input signal. 
   The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a first exemplary embodiment of a circuit arrangement according to one embodiment of the invention having an amplifier circuit, an offset compensation circuit and a deactivation circuit for the offset compensation circuit; 
       FIG. 2  shows, by way of example, temporal profiles for an input signal of the amplifier arrangement, which signal is a voltage present across a measuring resistor; 
       FIG. 3  shows an exemplary circuitry realization of an operational amplifier having one offset compensation input; 
       FIG. 4  shows a second exemplary embodiment of a circuit arrangement according to the invention; 
       FIG. 5  shows temporal profiles of selected signals occurring in the circuit according to  FIG. 4 ; 
       FIG. 6  shows a further exemplary embodiment of a circuit arrangement according to the invention; 
       FIG. 7  shows an exemplary circuitry realization of an operational amplifier having two offset compensation inputs; 
       FIG. 8  shows a further exemplary embodiment of the circuit arrangement according to the invention; and 
       FIG. 9  shows, by way of example, temporal profiles of selected signals occurring in the circuit arrangement according to  FIG. 8 . 
   

   In the figures, unless specified otherwise, identical reference symbols designate identical circuit components and signals with the same meaning. 
   DESCRIPTION 
   An exemplary circuit arrangement according to at least one embodiment of the invention as illustrated in FIG.  1  has an amplifier arrangement  10  having inputs  101 ,  102  for feeding in an input signal Vin and having outputs  103 ,  104  for providing an output signal Vout. In the example, the input signal Vin and the output signal Vout are voltages which are referred in each case to a reference-ground potential GND. The input  102  and the output  104  are at said reference-ground potential GND. The amplifier arrangement  10  has a first operational amplifier  11  having inputs  111 ,  112  and an output  113 . In the example, the noninverting input  111  of the operational amplifier  11  is connected to the input  101  of the amplifier arrangement  10  via a resistor  14 , and the inverting input  112  of the operational amplifier  11  is connected to the input  102  of the amplifier arrangement  10  via a further input resistor  15 . The output  113  of the first operational amplifier  11  is connected to the output  103  of the amplifier arrangement  10 . The output  113  of the operational amplifier  11  is furthermore feedback-connected to the inverting input  112  of said operational amplifier via a feedback resistor  13 . After application of an input voltage Vin, a differential voltage Vdiff=0 is established between the inputs  111 ,  112  of the operational amplifier  11  after a settling operation has proceeded. In the case of the circuitry illustrated, the following holds true for the output voltage Vout of the operational amplifier  11 :
 
 V out= R 2 /R 1  (1)
 
   In this case, R 1  designates the resistances of the input resistors  14 ,  15  and R 2  designates the resistance of the feedback resistor  13 . 
   For example due to temperature influences or else due to production-dictated variations in the parameters of the components present in the operational amplifier  11  (not illustrated in greater detail), an offset, that is to say an input difference Vdiff not equal to zero, may be present in the settled state of the operational amplifier  11 . In order to compensate for such an input offset, the first operational amplifier  11  has a compensation input  114  for feeding in a compensation signal. In the example, said compensation signal is a voltage V 12  present across a first capacitive storage element, which in the example is connected between the compensation input  114  and reference-ground potential GND. 
   Said compensation signal V 12  is generated by a compensation circuit  20  connected to the inputs  111 ,  112  of the first operational amplifier  11  in order to detect the input voltage difference Vdiff thereof and to generate the compensation signal V 12  depending on said input voltage difference Vdiff in conjunction with the capacitive storage element  12 . The compensation circuit  20  has a second operational amplifier  21 , which is designed as a transconductance amplifier in the example and which thus generates an output current I 20  dependent on the input voltage difference Vdiff in order to charge the capacitive storage element  12 . In principle, the compensation arrangement  20  generates an output current I 20  as long as the input difference Vdiff of the first operational amplifier  11  is not equal to zero, in order thereby to change the compensation signal V 12  until it has been adjusted to a value at which the offset or the input voltage difference Vdiff is zero. 
   Since the transconductance amplifier  21  of the compensation arrangement  20  may also be beset with an offset the compensation arrangement  20  has a further compensation arrangement serving for the offset compensation of the transconductance amplifier  21 . In accordance with the first operational amplifier  11  of the amplifier arrangement  10 , the transconductance amplifier  21  has a compensation input  214  for feeding in an offset compensation signal V 22 . A second capacitive storage element  22  is connected between said compensation input  214  and reference-ground potential GND, the offset compensation signal V 22  of said transconductance amplifier  21  being present across said second capacitive storage element. Said second capacitive storage element  22  is part of the compensation arrangement of said transconductance amplifier  21 . 
   Said compensation arrangement additionally has a first switch  23  for interrupting the connection between the first input  211  of the transconductance amplifier  21  and the first input  111  of the operational amplifier  11 , a second switch  24  for short-circuiting the inputs  211 ,  212  of the transconductance amplifier  21 , a third switch  25  for connecting the output  213  of the transconductance amplifier  21  to the first capacitive storage element  12 , and also a fourth switch  26  for connecting the output  213  of the transconductance amplifier  21  to the second capacitive storage element  22 . Said switches  23 ,  24 ,  25 ,  26  of the compensation arrangement of the transconductance amplifier  21  are driven by switching signals p 1 , p 2  generated by a control circuit  200 , which is merely illustrated schematically. Said control signals p 1 , p 2  are complementary to one another and chosen such that the first and fourth switches  23 ,  25  are always opened and closed together and that the second and fourth switches  24 ,  26  are always opened and closed together. In this case, the first and third switches  23 ,  25 , on the one hand, and the second and fourth switches,  24 ,  26 , on the other hand, are always driven complementarily to one another. 
   The control circuit  200  controls the offset compensation of the transconductance amplifier  21  by the second compensation arrangement. During a compensation operation in which the compensation signal V 22  is generated, the first and third switches  23 ,  25  are opened in order to decouple the transconductance amplifier  21  from the first operational amplifier  11 . The second and fourth switches  24 ,  26  are closed in order to short-circuit the inputs  211 ,  212  of the transconductance amplifier  21  and in order to connect the output  213  of the transconductance amplifier  21  to the second capacitive storage element  22 . If the transconductance amplifier  21  has an offset, then there is available at its output  213  despite short-circuited inputs  211 ,  212 , an output current which charges the capacitor  22  via the fourth switch  26  in order to increase the second compensation signal V 22 . Said compensation signal V 22  serves for offset compensation internally in the transconductance amplifier  21 . 
   The offset of the transconductance amplifier  21  is completely compensated for when the output current of said transconductance amplifier becomes zero and, as a result, the compensation signal V 22  does not rise any further. After the conclusion of the compensation operation, the second and fourth switches  24 ,  26  are opened and the first and third switches  23 ,  25  of the compensation arrangement of the transconductance amplifier  21  are closed. 
   The compensation operation explained above for the transconductance amplifier  21  may suitably be repeated at regular time intervals, in which case, during the compensation operation, in the manner explained, the second and fourth switches  24 ,  26  are for example closed for a fixedly predetermined time duration and the other two switches  23 ,  25  are opened for this time duration. In a manner that is not illustrated in more specific detail, there is in this connection also the possibility of providing a discharge circuit for the capacitive storage element  22  which completely discharges the capacitive storage element  22  in each case before the beginning of a compensation operation, in order subsequently to generate a second compensation signal V 22  again with the second and fourth switches  24 ,  26  closed. It should be pointed out in this connection that the compensation signal V 22  is maintained after the opening of the fourth switch  26 , so that only the first compensation signal V 12  is changed during the compensation operation. 
   An operating state of the transconductance amplifier in which the second and fourth switches  24 ,  26  are closed is referred to below as “compensation operating state”, while an operating state in which said switches  24 ,  26  are open and the other two switches  23 ,  25  are closed is referred to as “normal operating state”. 
   The task of the first compensation arrangement  20  is, in conjunction with the first capacitive storage element  12  connected to the offset compensation input  114  of the operational amplifier  11 , to generate an offset compensation signal V 12  for the operational amplifier  11 . 
   In order to generate the first offset compensation signal V 12 , the transconductance amplifier  21  is operated in the normal operating mode. The transconductance amplifier  21  then detects the voltage Vdiff present between the inputs  111 ,  112  of the operational amplifier  11  and generates an output current I 20  at its output  213 , said output current being dependent on said voltage difference Vdiff. In the ideal situation, if the operational amplifier  11  is not beset with an offset, this input voltage difference Vdiff is zero in the settled state of the operational amplifier  11 . In this case, the output current I 20  of the transconductance amplifier  21  is likewise zero provided that the transconductance amplifier  21  is not itself beset with an offset, which is assumed below. 
   If the first operational amplifier  11  is beset with an offset, then the input voltage difference Vdiff is not equal to zero and the transconductance amplifier  21  supplies an output current I 20  not equal to zero, which charges the capacitive storage element  12  in order thereby to increase the offset compensation voltage V 12 . In this case, the compensation voltage V 12  is increased until the input voltage Vdiff of the first operational amplifier is zero and the offset of the first operational amplifier  11  has thus been compensated for. The first compensation signal V 12  is maintained if the transconductance amplifier  21  undergoes transition from the normal operating state to the compensation operating state and the third switch  25  is opened. 
   As already explained, the input voltage difference Vdiff of the operational amplifier  11  is normally zero. In particular during a settling phase after a change in the input signal Vin, however, said input voltage difference Vdiff may assume a value not equal to zero. Unless additional measures are taken, said input voltage difference Vdiff, during the settling phase, would be interpreted as an offset by the compensation arrangement  20 , which would lead to an increase in the offset compensation signal V 12  of the operational amplifier  11 . 
   The compensation arrangement  20  with the capacitive storage element  12  has an integrating behavior, which equivalently means that those input voltage differences Vdiff which are not equal to zero would be integrated during the settling operations explained and would lead to a continuous increase in the offset compensation signal V 12  unless additional measures are implemented. 
   Input voltage differences Vdiff not equal to zero can furthermore also be generated by the input voltage source that generates the input voltage Vin, as is explained below with reference to  FIG. 2 . 
     FIG. 2  shows, in the left-hand part, a device for generating an input voltage Vin of the amplifier arrangement  20 . This arrangement  50  comprises a measuring resistor or shunt resistor, through which a measurement current I 50  flows. It shall be assumed that said measurement current I 50  is a pulsed current that is switched on and off. The amplifier arrangement  10  generates an output signal Vout dependent on said measurement current I 50  by means of the measuring resistor  50 . The measuring resistor  50  comprises a nonreactive resistance component R 50  and a parasitic inductance component L 50 . As is illustrated in the right-hand part of  FIG. 2 , said parasitic inductance component leads to voltage spikes in the input voltage Vin both when the measurement current I 50  is switched on and when the measurement current I 50  is switched off. Said voltage spikes likewise lead to an input voltage difference Vdiff not equal to zero and would be integrated by the compensation arrangement  20  unless further measures are implemented. 
   In order to avoid a situation in which input voltage differences Vdiff not equal to zero which are caused by the parasitic effects explained or by settling operations of the operational amplifier  11  lead to a corruption of the offset compensation signal V 12 , the circuit arrangement has, according to at least one embodiment of the invention, a deactivation circuit  30 , which is designed to temporarily deactivate the first compensation circuit  20 . In the example, said deactivation circuit  30  has a switch  31 , which is connected downstream of an output of the compensation arrangement  20  and which prevents, in the open state, a changing of the offset compensation signal V 12  by the compensation arrangement  20 . A drive circuit  32  is present for driving said switch  31 , which drive circuit is designed to open the switch  31  temporarily, such as during settling operations of the operational amplifier  11  or during predetermined time durations after changes in the input signal Vin. Input voltage differences Vdiff not equal to zero which occur during these time durations thus cannot affect the offset compensation signal V 12  of the operational amplifier  11 . 
   In the example, the compensation arrangement  20  is deactivated when it does not supply an output signal which can change the compensation signal V 21  generated up to that point. 
   In order to afford a better understanding of the function of the offset compensation signal V 12 ,  FIG. 3  shows a simple exemplary circuitry realization of an operational amplifier having an offset compensation input  114 . This operational amplifier has a differential amplifier stage having first and second input transistors  121 ,  122 , the control terminals of which form the inputs  111 ,  112  of the operational amplifier. In the example, said transistors  121 ,  122  are formed as n-channel MOSFETs whose source terminals are connected to one another and are connected to reference-ground potential GND via a current source  126  serving as a load. The drain terminals of said MOSFETs  121 ,  122  are connected to a supply potential Vbb via a current mirror having two further transistors  124 ,  125 . Said current mirror  124 ,  125  comprises two p-channel transistors, a first current mirror transistor  124  of which is connected up as a diode. The current mirror  124 ,  125  maps a current flowing through the first input transistor  121  onto a current flowing through the second current mirror transistor  125 . An output stage of this operational amplifier is formed by a series circuit comprising a further n-channel MOSFET  123  and a further current source  127 . In this case, the output  113  of the operational amplifier  11  is formed by a node common to the further transistor  123  and the current source  127 . A control terminal of said further n-channel transistor  123  is connected to a node common to the second current mirror transistor  125  and the second input transistor  122 . 
   The operational amplifier has a compensation stage having a first compensation transistor  126 , which is formed as a p-channel transistor  126  in the example, and a second compensation transistor  127 , which is formed as an n-channel transistor in the example. The two compensation transistors  126 ,  127  are jointly driven by the compensation signal V 12  present at the compensation input  114 . For this purpose, the gate terminals of these two transistors  126 ,  127  are connected to the compensation input  114 . The task of the compensation transistors  126 ,  127  is to reduce or increase the current I 122  through the input transistor  122  according to the compensation signal V 12 . For this purpose, a node common to the two compensation transistors  126 ,  127  is connected to a node common to the current mirror transistor  125  and the input transistor  122 . 
   The functioning of the illustrated operational amplifier having the compensation stage is explained below: 
   This operational amplifier is not beset with an offset when the currents I 121 , I 122  through the input transistors  121 ,  122  are of identical magnitude given identical input voltages at the inputs  111 ,  112 . Identical input voltages are present when the voltage between the two inputs  111 ,  112  is zero. 
   The operational amplifier is beset with an offset if these two currents I 121 , I 122  are not identical given identical input voltages. In the case of an input voltage difference equal to zero, an output voltage not equal to zero is available. If such an operational amplifier beset with an offset is connected up in the manner illustrated in  FIG. 1  such that the output is feedback-connected to one of the inputs, then such an offset has an effect such that an input voltage difference not equal to zero is established. Such an offset is compensated for by the compensation stage  126 ,  127  in that, according to the compensation signal V 12 , the current through the second input transistor  122  is increased or decreased in order to adapt the current I 122  through said input transistor  122  to the current I 121  through the other input transistor  121 . 
   If the compensation signal V 12  in the case of this arrangement assumes a first value, at which the two compensation transistors  126 ,  127  are driven identically, then a current flowing from the compensation stage  126 ,  127  is equal to zero. The compensation current Ik is positive in order to increase the current through the input transistor  122  if the compensation signal V 12  falls below the first value. In this case, the first compensation transistor  126  is driven to a greater extent than the second compensation transistor  127 . The compensation current Ik is negative in order to reduce the current through the input transistor  122  if the compensation signal V 12  rises above the first value. In this case, the second compensation transistor  127  is driven to a greater extent than the first compensation transistor  126 . 
     FIG. 4  illustrates a use of the circuit arrangement according to at least one embodiment of the invention for determining a load current I 50  flowing through a semiconductor switch  52  of a half-bridge circuit. In the example, the half-bridge circuit has two first and second semiconductor switches  51 ,  52  connected in series between a supply potential Vcc and reference-ground potential GND. In the example, said half-bridge circuit serves for driving a load  56  connected between an output  58  of the half-bridge circuit and reference-ground potential GND. A current measuring resistor  50  is connected in series with the second semiconductor switch  52 . Said measuring resistor  50  supplies the input voltage Vin for the amplifier arrangement  10 . 
   The driving of the two semiconductor switches  51 ,  52  is effected according to control signals S 53 , S 54  provided by a control circuit  55 . Driver circuits  53 ,  54  serve for amplifying said control signals S 53 , S 54  or for converting the levels of said control signals S 53 , S 54  to levels suitable for driving the semiconductor switches  51 ,  52 . 
     FIG. 5  shows, by way of example, the temporal profile of the drive signal S 54  of the semiconductor switch  52 . In this case, the temporal profile of said control signal S 54  corresponds qualitatively to the temporal profile of the current I 50  flowing through the second semiconductor switch  52  if the load  56  is an inductive load, such as a motor for example.  FIG. 5  additionally shows the temporal profile of the input voltage Vin present across the measuring resistor  50  assuming that the measuring resistor  50  has a parasitic inductance. 
   In the example, the deactivation circuit  30  has an edge detection circuit  33 , to which the control signal S 54  according to which the load current I 50  is generated is fed. The edge detection circuit  33  is designed to detect rising and falling edges of said control signal S 54  and to open the switch  31  after a rising and a falling edge of the control signal S 54  in each case for a predetermined time duration, in order thereby to deactivate the compensation arrangement  20 . The output signal S 33  of said edge detection circuit  33  is likewise illustrated in  FIG. 5 . In the example, it is assumed that the switch  31  is closed when said output signal S 33  has a high level and is open when said output signal S 33  has a low level. The time durations for which the output signal S 33  of the edge detector  33  in each case assumes a low level after a rising or falling edge of the control signal S 54 , in order to deactivate the compensation arrangement, may suitably be adapted to the time durations during which the input voltage Vin has voltage spikes on account of the parasitic inductance of the measuring resistor  50 , or these time durations are adapted to settling durations of the operational amplifier  11  after a level change of the input signal Vin. 
   Interference signals may arise during the opening and closing of the switch  31  that deactivates the second compensation arrangement  20 , said interference signals being referred to as so-called “switching noise”. In order to prevent said switching noise from adversely affecting the generation of the offset compensation signal V 12 , the operational amplifier  11  may be formed as an operational amplifier with differential offset compensation. 
     FIG. 6  shows a modification of the circuit arrangement illustrated in  FIG. 1 , in which the operational amplifier  11  is formed as an operational amplifier with differential offset compensation. The operational amplifier  11  has two offset compensation inputs  114 _ 1 ,  114 _ 2 . In this case, the capacitive storage element  12  already explained is connected to the first compensation input  114 _ 1 , said storage element being connected to the output of the first compensation arrangement  20 . A further capacitive storage element  17  is connected between the second compensation input  114 _ 2  and reference-ground potential GND. Said further capacitive storage element  17  is connected to a terminal for a reference potential Vref 2  via a second switch  34  of the deactivation circuit. This further switch  34  is opened and closed jointly with the switch  31  connected between the compensation arrangement  20  and the first capacitive storage element  12 . 
   An exemplary circuitry realization of an operational amplifier with differential offset compensation is illustrated in  FIG. 7 . The basic construction of this operational amplifier corresponds to the construction of the operational amplifier illustrated in  FIG. 3  with the difference that the compensation stage is formed as a differential compensation stage. 
   In the case of this operational amplifier, the compensation stage comprises a first and second compensation transistor  128 ,  129 . The first compensation transistor  128  is driven by the first compensation signal V 12  and its load path is connected between a node common to the current mirror transistor  125  and the second input transistor  122  and reference-ground potential. The second compensation transistor  129  is driven by a compensation signal V 17  present across the second capacitive storage element ( 17  in  FIG. 6 ) and its load path is connected between a node common to the current mirror transistor  125  and the second input transistor  122  and reference-ground potential. The compensation signal which is present at the input of the second compensation transistor  129  and corresponds to the second reference potential with switches  28 ,  34  closed is referred to below as constant compensation signal. 
   In the case of the operational amplifier illustrated, a change in the current I 122  through the second input transistor  122  with respect to the current through the first input transistor  121  is effected by means of a change in the compensation signal V 12  in comparison with the fixed compensation signal V 17 . Said fixed compensation signal has the effect that part of the current I 124  flowing through the current mirror transistor  124  flows away to reference-ground potential via the second compensation transistor  129 . If the compensation signal V 12  corresponds to the fixed compensation signal V 17 , then the current I 128  flowing through the first compensation transistor  128  corresponds to the current I 129  flowing through the second compensation transistor  129 . When an offset is not present, the currents I 121 , I 122  through the input transistors are then identical. 
   The operational amplifier is beset with an offset if the currents I 121 , I 122  through the input transistors  121 ,  122  are not identical. Depending on the type of offset, compensation of said offset necessitates increasing or reducing the current I 121  through the second input transistor  122  in comparison with the current I 121  through the first input transistor  121 . 
   In order to increase the current I 122  through the second input transistor  122  with respect to the current I 121  through the first input transistor  121 , the compensation signal V 12  is increased in comparison with the fixed compensation signal V 17 . In this case, the first compensation transistor  128  is regulated down. In order to reduce the current I 122  through the second input transistor  122  with respect to the current I 121  through the first input transistor  121 , the compensation signal V 12  is reduced in comparison with the fixed compensation signal V 17 . In this case, the first compensation transistor  128  is driven up. 
   Common-mode interference signals which are superposed on the two compensation signals V 12 , V 17  do not affect the offset compensation in the case of this compensation arrangement  128 ,  129 . If an identical interference signal is superposed on the two compensation signals V 12 , V 17 , then the two compensation transistors  128 ,  129  are regulated down or driven up in the same way with the result that the current I 122  through the second input transistor  122  does not change in comparison with the current I 121  through the first input transistor  121 ; a difference between these two currents I 122  and I 121  remains the same and is equal to zero in the case of complete offset compensation. An interference signal which is superposed on the two compensation signals V 12 , V 17  may be for example a switching noise that arises as a result of simultaneous switching of the switches  31 ,  34 . 
   Referring to  FIG. 6 , a fifth switch  28  of the compensation arrangement  20  is connected between the further capacitive storage element  117  and the terminal for reference potential Vref 2 . Said switch  28  is driven by means of the control signal p 2  jointly with the fourth switch  25  connected between the output of the transconductance amplifier  21  and the first capacitive storage element  12 . In the case where the switches  31 ,  34  of the deactivation circuit are closed and the compensation arrangement  20  is changed over from the compensation state to the normal state, and vice versa, the fourth switch  25  is opened or closed in the manner explained. In order to prevent switching noise that occurs during the opening and closing of said fourth switch  25  from adversely affecting the offset compensation of the operational amplifier  11 , the fifth switch  28  is opened and closed in a manner corresponding to said fourth switch  25 . 
   In accordance with the first operational amplifier  11  of the amplifier arrangement  10 , the transconductance amplifier  21  of the compensation arrangement  20  may also be embodied as an operational amplifier with differential offset compensation. In this case, the capacitive storage element  22  that has already been explained previously is connected to a first offset compensation input  214 _ 2  of the transconductance amplifier  21 , and a second capacitive storage element  27  is connected to a second offset compensation input  214 _ 2  of said transconductance amplifier  21 . The second capacitive storage element  27  is connected to a reference potential Vref 1  via a sixth switch  29 . Said sixth switch  29  is opened and closed synchronously with the fourth switch  26  by means of the control signal p 1 . Switching noise that arises as a result of the opening and closing of the fourth switch  26  and could adversely affect the generation of the second compensation signal V 22  is compensated for by synchronous opening and closing of the sixth switch  29 , which is connected to the second capacitive storage element  27 , in such a way that the switching noise does not affect the offset compensation of the transconductance amplifier  21 . 
   The noninverting input of the operational amplifier  11  is connected to a further reference potential Vref 1  via a further resistor  16 . In the example, said further resistor  16  has the same resistance R 2  as the feedback resistor  13  of the operational amplifier  11 . The quiescent value output voltage Vout, that is to say the value at which the output voltage Vout is established in the case of an-input voltage Vin=0, is set by way of the ratio of said two resistors  13 ,  16 . Given identical resistors  13 ,  16 , said quiescent value corresponds to the further reference potential. Upon application of an input voltage Vin not equal to zero, the output voltage Vout then changes proceeding from said quiescent value. 
   The further reference potential Vref 2  may correspond to the reference potential Vref 1  present for compensation purposes. 
   In the case of the exemplary embodiments explained above with reference to  FIGS. 4 and 6 , the compensation arrangement  20  is deactivated depending on a signal S 54  which triggers a level change of the input voltage Vin of the amplifier arrangement  10 . 
     FIG. 8  shows an exemplary embodiment of a circuit arrangement according to at least one embodiment of the invention having a deactivation circuit  31 ,  60 , in the case of which no “advance information” about an imminent level change of the input voltage Vin is required for the deactivation of the first compensation arrangement  20 . The deactivation circuit has the switch  31  explained previously, said switch being connected between the output  20  of the compensation arrangement and the first capacitive storage element  12 . A drive signal S 31  is generated by a detector circuit  60 , which detects level changes of the input voltage Vin. Said detector circuit  60  has an amplifier arrangement constructed in accordance with the amplifier arrangement  10 . This amplifier arrangement of the detector circuit  60  comprises an auxiliary operational amplifier  61  having inputs  611 ,  612  and an output  613 . The inputs  611 ,  612  of said auxiliary operational amplifier  61  are connected to the inputs  101 ,  102  of the amplifier arrangement  10  via input resistors  64 ,  65 . The input resistors  64 ,  65  may suitably be dimensioned in accordance with the input resistors  14 ,  15  of the amplifier arrangement  10  and have a resistance R 1 . The auxiliary operational amplifier  61  is connected up in accordance with the operational amplifier  11  of the amplifier arrangement. For this purpose, the output  613  of said auxiliary operational amplifier is feedback-connected to the inverting input  612  of the auxiliary operational amplifier  61  via a feedback resistor  63 , which is dimensioned in accordance with the feedback resistor  13  of the operational amplifier  11 . 
   The first operational amplifier  11  and the auxiliary operational amplifier  61  are dimensioned such that they have different time constants, and that is to say that they react at different speeds to changes in the input voltage Vin. This is explained on the basis of temporal profiles of the output voltages Vout of the operational amplifier  11  and Vout′ of the auxiliary operational amplifier  61  with reference to  FIG. 9 . 
     FIG. 9  shows, by way of example, a temporal profile of the input voltage Vin, which has a rising edge at an instant t 1  and a falling edge at an instant t 2  and which has a constant signal level between these two instants t 1 , t 2 . With the rising edge of the input signal Vin, the output voltages Vout, Vout′ of the two amplifiers  11 ,  61  start to rise, but at different rates on account of the different time constants. In the example, the time constant of the auxiliary operational amplifier  61  is shorter than the time constant of the first operational amplifier  11 , so that the output voltage Vout′ of the auxiliary operational amplifier  61  rises more rapidly than the output voltage Vout of the operational amplifier  11  after a rising edge of the input voltage Vin and falls more rapidly than the output voltage Vout of the operational amplifier  11  after a falling edge of the input voltage Vin. The operational amplifiers  11 ,  61  are dimensioned and connected up in such a way that the output voltage Vout of the operational amplifier  11  corresponds to the output voltage Vout′ of the auxiliary operational amplifier  61  in the settled state. After a rising edge of the input voltage Vin and after a falling edge of the input voltage Vin, the output voltages Vout, Vout′ differ on account of the different time constants in each case for time durations Δt 1 , Δt 2 . 
   An evaluation circuit having two comparators  66 ,  67  and a logic gate  68  evaluates the output voltages Vout, Vout′ of the two amplifiers  11 ,  61  in order to generate the drive signal S 31  of the switch  31  therefrom. This evaluation circuit  66 ,  67 ,  68  has the task of opening the switch  31  during the time durations Δt 1 , Δt 2  after rising and falling edges of the input voltage Vin. For this purpose, the output voltage Vout of the operational amplifier  11  and the output voltage Vout′ of the auxiliary operational amplifier  61  are in each case fed to the comparators  66 ,  67 . During the time durations Δt 1 , Δt 2  during which these two voltages Vout, Vout′ deviate from one another, the output signal of a respective one of these two comparators  66 ,  67  has a high level. In this example, the logic gate  68  is formed as an XOR gate, which supplies a high level at its output in each case when the two comparator output signals deviate from one another. This output signal of the XOR gate is inverted by means of an inverter  69 . The control signal S 31  for the switch  31  is present at the output of said inverter  69 . Said control signal S 31  assumes a low level in each case during the time durations Δt 1 , Δt 2  in order to open the switch  31  and thereby to deactivate the compensation arrangement  20 . 
   The deactivation circuit  60 ,  31  explained ensures that the compensation arrangement  20  is deactivated in each case after rising and falling edges of the input voltage Vin, as a result of which settling operations of the operational amplifier  11  that follow such a level change of the input voltage Vin, by means of the compensation arrangement  20 , do not affect the generation of the offset compensation signal V 21 . 
   While the invention disclosed herein has been described in terms of several preferred embodiments, there are numerous alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.

Technology Category: h