Abstract:
A method and system for providing increased accuracy in a CMOS sensor system in one embodiment includes a plurality of sensor elements having a first terminal and a second terminal on a complementary metal oxide semiconductor substrate, a first plurality of switches configured to selectively connect the first terminal to a power source and to selectively connect the first terminal to a readout circuit, and a second plurality of switches configured to selectively connect the second terminal to the power source and to selectively connect the second terminal to the readout circuit.

Description:
FIELD  
       [0001]    This invention relates to complementary metal oxide semiconductor sensors. 
       BACKGROUND 
       [0002]    Complementary metal oxide semiconductor (CMOS) manufacturing processes are the most widely used semiconductor manufacturing processes and are recognized for their superior robustness and low cost in generating large volumes of product. Conventional CMOS processes are directed toward fabricating digital circuits, e.g., microprocessors, and the peripheral circuits, e.g., mixed-signal and radio frequency circuits. The use of CMOS sensors, however, has recently experienced rapid growth. CMOS sensors include image sensors, temperature sensors, and magnetic field sensors among many others. Additionally, many semiconductor sensors, such as pressure sensors or accelerometers, are either CMOS hybrids or CMOS systems monolithically integrated with microelectrical-mechanical systems (MEMS). Semiconductor sensors enable many new products and applications in automotive and consumer electronic market. 
         [0003]    One major challenge in using CMOS processes for fabricating sensors is the limited absolute accuracy achieved with these sensors and the sensitivity of the devices to packaging and fabrication processes. For example, the offset in Hall sensors is influenced by the piezoresistance effect. The piezoresistance effect results from the mechanical stress on the chip which are not controlled in low cost packaging technologies, e.g., plastic packages as reported in Z. Randjelovic, “Low-power High Sensitivity Integrated Hall magnetic Sensor Microsystems,” PhD Thesis, EPFL, 2000; and S. Bellekom, “Origins of Offset in Conventional and Spinning-current Hall Plates,” PhD Thesis, Delft University, 1998. Similar effects limit the performance of many other sensors. 
         [0004]    Layout techniques have been used in an attempt to mitigate various error sources. Mitigation techniques include manufacturing multiple devices on a single substrate with the theory that neighboring devices will cancel at least some of the errors introduced by the other device and/or the use of dummy devices. See, e.g., Z. Randjelovic; A. Hasting, “The Art of Analog Layout,” Prentice Hall, 2005; and J. Frounchi et al., “Integrated Hall sensor array microsystem,” ISSCC, p. 248-249, February 2001. 
         [0005]    The effectiveness of the mitigation methods identified above is limited by constancy of the mechanical stress or process gradient on the chip. Specifically, to the extent the substrate exhibits a uniform gradient in a single dimension, neighboring devices will exhibit the same errors or offsets. Accordingly, by reversing the orientation of alternate devices, the offset realized in one device is eliminated by the reversed offset in the adjacent device. 
         [0006]    In reality, however, sources of offset are not presented in a uniform gradient in a single dimension. Rather, the gradient varies within not only a single dimension, but in two dimensions, that is, both along the length of the substrate and the width of the substrate. Accordingly, neighboring devices exhibit different errors and the error of one device will not be completely cancelled by any one of the surrounding devices. Thus, sensor packages are typically costly and limited to use in applications depending on the offset tolerance of the application. 
         [0007]    The non-linear stress exhibited by some sensors may be mitigated by other techniques. For example, the spinning current technique is generally used to cancel the remainder of offset errors in the specific case of Hall sensors. See, e.g., Bellekom; Frounchi; and J. van der Meer et al., “A Fully Integrated CMOS Hall Sensor with 3.65 μT 3σ Offset for Compass Applications,” ISSCC, p. 246-247, February 2005. The increased accuracy attainable in these other techniques, however, is limited by second order effects such as Joule Heating, Seebeck, and Peltier effects. These additional techniques further require increased costs for front-end electronics. 
         [0008]    A need exists for CMOS sensors with improved accuracy without a significant increase in cost. 
       SUMMARY  
       [0009]    In accordance with one embodiment, a method and system for providing increased accuracy in a CMOS sensor system includes a plurality of sensor elements having a first terminal and a second terminal on a complementary metal oxide semiconductor substrate, a first plurality of switches configured to selectively connect the first terminal to a power source and to selectively connect the first terminal to a readout circuit, and a second plurality of switches configured to selectively connect the second terminal to the power source and to selectively connect the second terminal to the readout circuit. 
         [0010]    In accordance with another embodiment, a method for providing increased accuracy in a complementary metal oxide semiconductor substrate (CMOS) sensor system, includes (a) establishing a first state condition by connecting a first terminal of a first of a plurality of sensor elements in an array of sensor assemblies on the CMOS substrate to an input, and connecting a second terminal of the first of the plurality of sensor elements to an output, (b) generating a first sensor element signal for the first state condition, (c) storing a first data associated with the first sensor element signal, (d) establishing a second state condition by connecting the first terminal of the first of a plurality of sensor elements to the output, and connecting the second terminal of the first of the plurality of sensor elements to the input, (e) generating a second sensor element signal for the second state condition, (f) storing a second data associated with the second sensor element signal, (g) performing (a)-(f) for each of the other of the plurality of sensor elements in the array of sensor assemblies, (h) calculating a plurality of offsets, each of the plurality of offsets associated with one of the plurality of sensor elements, using the stored first data and the stored second data, and (i) generating a sensor system output using the calculated plurality of offsets. 
         [0011]    In yet another embodiment, a complementary metal oxide semiconductor (CMOS) sensor system includes a substrate with a power bus and a readout bus, a sensor array on the substrate and including a plurality of sensor assemblies, each of the plurality of sensor assemblies including a first sensor element with a first terminal and a second terminal, a first switch, and a second switch, a memory on the substrate including command instructions for (i) generating a first sensor assembly signal with each of the first sensor elements by connecting the first terminal to the input and connecting the second terminal to the output, (ii) storing for each of the first sensor assembly signals, a first data associated with the respective first sensor assembly signal, (iii) generating a second sensor assembly signal with each of the first sensor elements by connecting the first terminal to the output and connecting the second terminal to the input, (iv) storing for each of the second sensor assembly signals, a second data associated with the respective second sensor assembly signal, and (v) calculating a respective offset for each of the plurality of sensor assemblies using the associated first data and associated second data, and a processor for executing the command instructions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  depicts a schematic of a substrate including a processor, a memory, a sensor array and a readout circuit in accordance with principles of the present invention; 
           [0013]      FIG. 2  depicts a schematic of the sensor array and readout circuit of  FIG. 1  including four sensor assemblies, each sensor assembly including four sets of switches that can be used to establish different state conditions for the sensor element in the sensor assembly; 
           [0014]      FIG. 3  depicts one of the sensor assemblies of  FIG. 2 ; 
           [0015]      FIG. 4  depicts a process that may be controlled by the processor of  FIG. 1  for obtaining offset values for each of the sensor assemblies in the sensor array; 
           [0016]      FIGS. 5-8  depict the sensor array schematic of  FIG. 2  showing different sensors configured in different states in accordance with the process of  FIG. 4 ; 
           [0017]      FIG. 9  depicts the location of a sensor array on a substrate that exhibits a stress gradient that varies in two dimensions; 
           [0018]      FIG. 10  depicts a histogram of the output of a 2×2 sensor array incorporating traditional offset cancelation techniques; 
           [0019]      FIG. 11  depicts a histogram of the output of a 16×16 sensor array incorporating principles of the invention; and 
           [0020]      FIG. 12  depicts a schematic of an alternative sensor assembly including eight switches that can be used to establish different state conditions for three different sensor elements in the sensor assembly. 
       
    
    
     DESCRIPTION 
       [0021]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
         [0022]      FIG. 1  depicts a CMOS sensor  100  which includes a processor or digital signal processing block  102 , a controller  104 , a sensor array  106 , a readout circuit  108  and a memory  110 . The processor  102 , controller  104 , sensor array  106 , readout circuit  108  and memory  110  in this embodiment are all located on a substrate  112 . In alternative embodiments, various combinations of the components are located remote from the sensor array  106 . A positive power terminal  114  and a negative power terminal  116  are also provided on the substrate  112  along with other terminals  118 ,  120 ,  122 ,  124 ,  126 , and  128  which may be used for power, communications, control, and other connections. More or fewer pins may be provided. 
         [0023]    With reference to  FIG. 2 , the sensor array  106  includes four sensor assemblies  130   1-4 . While only four sensor assemblies  130   x  are shown in the embodiment of  FIG. 1 , more sensor assemblies  130   x  may be included on a substrate if desired. Each of the sensor assemblies  130   1-4  in  FIG. 2  are identical and are described with reference to sensor assembly  130   1  which is shown more clearly in  FIG. 3 . 
         [0024]    The sensor assembly  130   1  includes a sensor element  132   1 , eight power switches  134   1NM ,  134   1NP ,  134   1SM ,  134   1SP ,  134   1EM ,  134   1EP ,  134   1WM , and  134   1WP  and eight readout switches  136   1NW ,  136   1NP ,  136   1SM ,  136   1SP ,  136   1EM ,  136   1EP ,  136   1WM , and  136   1WP . Each of the switches  134   1XM  can be individually controlled by the controller  104  to operably connect the sensor element  132   1  to a negative bias bus  138   1  through any of four sensor element terminals  140   N ,  140   E ,  140   S , or  140   W  and each of the switches  134   1XP  can be individually controlled by the controller  104  to operably connect the sensor element  132   1  to a positive bias bus  142   1  through any of the four sensor element terminals  140   1N ,  140   1F ,  140   1S , or  140   1W . The negative bias bus  138   1  is operably connected to the negative bias terminal  116  and the positive bias bus  142   1  is operably connected to the positive bias terminal  114 . 
         [0025]    Additionally, each of the switches  136   1XM  can be individually controlled by the controller  104  to operably connect the sensor element  132   1  to a negative readout bus  144   1  through any of the four sensor element terminals  140   1N ,  140   1E ,  140   1S , or  140   1W  and each of the switches  136   1XP  can be individually controlled by the controller  104  to operably connect the sensor element  132   1  to a positive readout bus  146   1 . The negative readout bus  144   1  is operably connected to a negative input  150  of the readout circuit  108  and the positive readout bus  146   1  is operably connected to a positive input  152  of the readout circuit  108  as shown in  FIG. 2 . 
         [0026]    Specifically, the processor  102  executes command instructions which are stored in the memory  110  to command the controller  104  to sequentially connect a sensor element terminal  140   XX  in each of the sensor assemblies  130   1-4  to the positive power terminal  114  through a positive power supply bus  142   X  and another sensor element terminal  140   XX  to the negative power terminal  116  through a negative power supply bus  138   X . In conjunction, the processor  102  executes command instructions which are stored in the memory  110  to command the controller  104  to sequentially connect another of the sensor element terminals  140   XX  in each of the sensor assemblies  130   1-4  to the positive input  152  of the readout circuit  108  through a positive readout bus  146   X  to the positive input  152  of the readout circuit  108  through a positive readout bus  146   X  and another sensor element terminal  140   XX  to the negative input  150  of the readout circuit  108  through a negative readout bus  144   X . 
         [0027]    By controlling the particular combination of power switches  134   1SM ,  134   1NP ,  134   1SM ,  134   1SP ,  134   1EM ,  134   1EP ,  134   1WM , and  134   1WP , and readout switches  136   1NM ,  136   1NP ,  136   1SM ,  136   1SP ,  136   1EM ,  136   1EP ,  136   1WM , and  136   1WP  which are used to connect the sensor element terminals  140   XX  to the negative power supply bus  138   X , the positive power supply bus  142   X , the negative readout bus  144   X , and the positive readout bus  146   X , the state of the respective sensor element  122   X  can be varied and sensed as described more fully below. The state may simply be different polarities, or different directions of current flow. Alternatively, different states may be effected by control of other onchip (or offchip) sources. By way of example, coils or resistors may be used to generate a particular magnetic field or heat. 
         [0028]    The output of the sensor array  106  is provided to the readout circuit  108  which may include biasing components, a programmable amplifier, and an analog-to-digital converter. The output of the readout circuit  108  is in turn provided to the processor  102 . The output provided to the processor  102  is used to estimate the offset for each device and to perform an adaptive calibration of the output of the sensor array  106 . 
         [0029]    The offset estimation process  160  of  FIG. 4  begins at  162  with the selection of a sensor assembly  130   x . For this example, the sensor assembly  130   x  that is initially selected is sensor assembly  130   1 . At block  164 , a state condition is established. In this embodiment, the sensor elements  132   X  are Hall Effect sensors. Accordingly, a first state condition may be established by controlling the power switches  134   1NP  and  134   1SM  to connect to the terminals  140   1N  and  140   1S , respectively. Additionally, the readout switches  136   1EP  and  136   1WM  are controlled to connect to the terminals  140   1E  and  140   1W , respectively. This configuration is shown in  FIG. 5 . 
         [0030]    At the block  166 , the processor  102  controls the readout circuit  108  to obtain the output of the sensor assembly  130   1 . The data corresponding to the output obtained by the readout circuit  108  is then stored in the memory  110  at the block  168 . 
         [0031]    At the block  170  the power switches  134   1NP  and  134   1SM  are controlled to disconnect from the terminals  140   1N  and  140   1S , respectively. In one embodiment, each sensor assembly  130   X  is selected and the first condition established prior to establishing a second state condition. In the embodiment of  FIG. 4 , however, a second state condition for the sensor assembly  130   1  is established by controlling the power switches  134   1EP  and  134   1WM  to connect to the terminals  140   1E  and  140   1W , respectively. Additionally, the readout switches  136   1SP  and  136   1NM  are controlled to connect to the terminals  140   1S  and  140   1N , respectively. This configuration for the sensor assembly  130   1  is shown in  FIG. 6 . Accordingly, the state of the sensor assembly  130   1  is modified from the state associated with  FIG. 5 . 
         [0032]    At the block  174 , the processor  102  controls the readout circuit  108  to obtain the output of the sensor assembly  130   1 . The data corresponding to the output obtained by the readout circuit  108  is then stored in the memory  110  at the block  176 . 
         [0033]    With two values from the sensor assembly  130   1  at different states, the offset of the sensor assembly  130   1  may be determined at the block  178 . Offset may be calculated using the following equation: 
         [0000]    
       
      
       o 
       i,j 
       =o 
       i,j 
       1 
       −o 
       i,j 
       2  
      
     
         [0000]    wherein 
         [0034]    “i” is the column of the array that the sensor is in, 
         [0035]    “j” is the row of the array that the sensor is in, 
         [0036]    “1” identifies a first state, and 
         [0037]    “2” identifies a second state. 
         [0038]    The value of the offset determined at the block  178  is stored in the memory  110  at the block  180 . The next sensor assembly is then selected at the block  182 , and a first state condition is established for the selected sensor assembly at the block  184 . By way of example,  FIG. 7  shows a state condition established for the sensor assembly  1302  by controlling the switches  134   2NP  and  134   2SM  to connect to the terminals  140   2N  and  140   2S , respectively. Additionally, the readout switches  136   2EP  and  136   2WM  are controlled to connect to the terminals  140   2E  and  140   2W , respectively. The second state for the sensor assembly  130   2  is shown in  FIG. 8  with the power switches  134   2EP  and  134   2WM  connected to the terminals  140   1E  and  140   1W , respectively. Additionally, the readout switches  136   2SP  and  136   2NM  are controlled to connect to the terminals  140   2S  and  140   2N , respectively. 
         [0039]    The offset calculation process  160  continues until data is available for all of the sensor assemblies  130   x . The offset calculation process  160  then continues with the first sensor assembly  130   X  and continues to provide updated offset values for each of the sensor assemblies  130   X . Continued updating of the offset values for each of the sensor assemblies  130   x  provides increased accuracy. The offset values, however, may include a significant noise element. Thus, the offset values obtained in the first and second state are described by the following equations: 
         [0000]        o   i,j   1   =v   h   +v   i,j   offset +noise 
         [0000]        o   i,j   2   =v   h   −v   i,j   offset +noise 
         [0000]    wherein 
         [0040]    v h  corresponds to Hall voltage, and 
         [0041]    v offset  corresponds to offset voltage. 
         [0042]    Accordingly, performing o i,j   1 −o i,j   2  as described at block  178  results in offset with a high noise component. The effect of noise and other errors on the accuracy of the output of the sensor  100  is mitigated by applying a robust polynomial 2-D least-squares fit of the offset data for the sensor array  106 . The command instructions for the polynomial 2-D least-squares fit may be stored in the memory  110  for execution by the processor  102 . Execution of the command instructions for the polynomial 2-D least-squares fit provides a smoothed offset value (ô i,j ), for each sensor element. The smoothed offset value exhibits reduced inaccuracies caused by noise, including the 1/f component, as compared to the offset values. 
         [0043]    In one embodiment, the smoothed offset value is used to identify sensor assemblies  130   X  which exhibit very high offsets. The least squares fit may be improved by either ignoring the abnormally high value or substituting a normalized value for the particular sensor assembly  130   X  to generate a corrected smoothed offset value. A correlation detector may be used to evaluate packaging properties by calculating the uniformity of the mechanical stress. The corrected smoothed offset value thus reduces the effect of non-stress related inaccuracies of a single sensor assembly on the calculated smoothed offset value of adjacent sensors. Additionally or alternatively, the output from the sensor assemblies  130   X  exhibiting abnormally high offset may be excluded from the sensor array output. 
         [0044]    The smoothed offset value, corrected or non-corrected, is applied by the processor  102  in generating an output signal on a selected one of the terminals  118 ,  120 ,  122 ,  124 ,  126 , or  128 . Specifically, there will generally be at least two output values stored in the memory  110  for each of the sensor assemblies  130   X , one value for each of the state conditions stored at the blocks  168  and  176  of  FIG. 4 . At a predetermined interval, the processor  102  obtains the two most recently stored outputs for each of the sensor assemblies  130   X . The smoothed offset value associated with the respective sensor assembly  130   X  is then applied to the most recent sensor assembly  130   X  outputs to produce a corrected sensor assembly output for the respective sensor assembly  130   X . 
         [0045]    The corrected sensor assembly outputs for each of the sensor assemblies  130   X  are then added together and divided by the number of sensor assemblies  130   X  to produce an offset canceled output for the sensor  100 . In this embodiment, the offset canceled output is calculated using the following formula: 
         [0000]    
       
         
           
             
               O 
               offset_canceled 
             
             ∝ 
             
               
                 1 
                 
                   n 
                   2 
                 
               
                
               
                 
                   ∑ 
                   
                     i 
                     = 
                     1 
                   
                   n 
                 
                  
                 
                   
                     ∑ 
                     
                       j 
                       = 
                       1 
                     
                     n 
                   
                    
                   
                     ( 
                     
                       
                         o 
                         
                           i 
                           , 
                           j 
                         
                         1 
                       
                       - 
                       
                         
                           o 
                           ^ 
                         
                         
                           i 
                           , 
                           j 
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0046]    In an another embodiment, the foregoing procedure is modified by using only the most recently stored output for each sensor assembly  130   X  for the most recent state conditions. In this embodiment, the value for ô i,j  is modified to reflect the use of a single measurement through each of the sensor elements. 
         [0047]    Some sensors are subjected to environments which produce varying gain factors, e.g., differnt temperatures. In such unstable temperature environments, an offset canceled output may be generated by using a weighted average of the sensor assembly outputs. A weighted average offset canceled output may be calculated using the formula set forth below: 
         [0000]    
       
         
           
             
               O 
               
                 offset_normalize 
                  
                 d 
               
             
             ∝ 
             
               
                 1 
                 
                   n 
                   2 
                 
               
                
               
                 
                   ∑ 
                   
                     i 
                     = 
                     1 
                   
                   n 
                 
                  
                 
                   
                     ∑ 
                     
                       i 
                       = 
                       1 
                     
                     n 
                   
                    
                   
                     
                       ( 
                       
                         
                           o 
                           
                             i 
                             , 
                             j 
                           
                           1 
                         
                         + 
                         
                           o 
                           
                             i 
                             , 
                             j 
                           
                           2 
                         
                       
                       ) 
                     
                     
                       
                         o 
                         ^ 
                       
                       
                         i 
                         , 
                         j 
                       
                     
                   
                 
               
             
           
         
       
     
         [0000]    Thus, because the offset is determined and corrected digitally, as opposed to depending upon the offset signal of one sensor to cancel the offset signal of an adjacent sensor with a reverse polarity, the offset correction may be tailored to the particular sensor environment. 
         [0048]    Principles of the embodiments were validated as discussed with reference to  FIGS. 9-11 . With initial reference to  FIG. 9 , a substrate  190  is subjected to a stress with an origin  192  that is not centered on the substrate  190 . The stress at the origin  192  was simulated at 80 MPa. Each isobar  194  indicates a 5 MPa decrease in the stress, with the stress at the lower corners (as viewed in  FIG. 9 ) being 30 MPa. A location  196  was selected for placement of a 2×2 array (not shown). The 2×2 array was oriented to fit within the location  196 . The four sensors in the 2×2 array were connected in parallel pairs, each of the sensors in a pair having a polarity opposite to the polarity of the other sensor in the pair in order to cancel the offset of the paired sensor. 
         [0049]    The 2×2 array was then sampled 1000 times with the offset correction effected by the reversed polarities and the results plotted on the histogram  2000  of  FIG. 10 . The X-axis of the histogram  200  identifies the sensed magnetic field in micro-Tesla (μT) for the 2×2 array. The Y-axis identifies the number of samples, each sample being an average of the 4 sensors in the 2×2 array, which were obtained at the associated level.  FIG. 10  reveals a standard deviation of 1.2 μT centered on about −8184 μT, with about 60% of the samples between about −8175 μT and −8192 μT. 
         [0050]    A 16×16 array was then oriented to fit within the location  196 . The 256 sensors in the 16×16 array were operated using the procedure discussed with reference to  FIG. 4 , and 1000 smoothed offset values were obtained. Each of the 1000 smoothed offset values represented the average smoothed offset value of the 256 sensors in the 16×16 array. 
         [0051]    The results of the 1000 samples of the 16×16 array are plotted on the histogram  210 , which has the same axes as the histogram  200 .  FIG. 10  reveals a standard deviation of 96 ηT centered upon 4 ηT, with more than 60% of the samples within ±0.1 μT of 0.0 μT. 
         [0052]    Thus, by generating a smoothed offset value, the offset in the sensor output resulting from a non-linear stress is reduced by four orders of magnitude and the spread is likewise significantly reduced. 
         [0053]    Of course, the foregoing examples discussed the use of only two possible states for the sensor assemblies  130   x . Additional accuracy may be obtained by incorporating additional switch configurations to provide additional state conditions. Moreover, while the state conditions for the sensor array  106  were varied by switching the polarity of the individual sensor elements  132   X  using the switches  134   XXX , state conditions for other types of sensors, including pressure and optical sensors, may be modified using other devices which may or may not be located on the same substrate as the sensor array. 
         [0054]    Additional increases in accuracy may be obtained in a variety of ways. By way of example, a smoothed offset value may be combined with an extra reference or a factory side calibration. The accuracy of the smoothed offset value may also be enhanced by applying a strong source (e.g., high magnetic field) either by an external source or by on-chip actuators (e.g., coils) during offset determination. In magnetic sensor embodiments incorporating a coil, the coil may further be used to perform a gain calibration. 
         [0055]    Moreover, the process of  FIG. 4  may be modified to obtain readings of the sensors at different currents. Analysis of the readings obtained at different current levels may be used to isolate the offset which results from the “Seebeck effect” since the Seebeck effect is a third order term in the offset expression which is a function of bias current in a Hall sensor. 
         [0056]    The smoothed offset value may be further refined in sensor arrays incorporating sensor elements of different types. By way of example, the sensor array  230  of  FIG. 12  includes four sensor assemblies  232   1-4 . The sensor assemblies  232   1-4  may be powered by positive and negative power buses  234  and  236 , respectively, and readouts from the sensor assemblies  232   1-4  are obtained by positive and negative readout buses  238  and  236 , respectively. 
         [0057]    The sensor assemblies  232   1-4  include sensor elements  242   1-4 . The sensor element  242   1  and  242   2  are both Hall Effect sensors while the sensor element  242   3  is a diode sensor element and the sensor element  242   4  is a strain sensor element. The power and readout switches for the sensor assemblies  232   1 ,  232   2 , and  232   4  are substantially the same as the power and readout switches for the sensor assemblies  130   x . The main difference between the sensor assemblies  232   1  and  232   2  is that the sensor assembly  232   2  is angularly rotated on the sensor array  230 . The sensor assembly  232   3  differs from the other sensor assemblies in that the sensor element  242   3  has only two terminals,  244   3N  and  244   3S . Accordingly, only four power switches  246   3NM ,  246   3NP ,  246   3SM  and  246   3SP  are incorporated for establishing different state conditions. Similarly, only four readout switches  248   3NM ,  248   3NP ,  248   3SM  and  248   3SP  are incorporated for obtaining sensor signals from the sensor element  242   3 . 
         [0058]    The sensor array  230  may be controlled to act as a dedicated optical sensor, a dedicated hall sensor, a temperature sensor, or a dedicated strain sensor. Alternatively, the sensor array  230  may be controlled to act as a multi-sensor. The sensor array  230  may further be controlled to distinguish between different types of errors across an array of the sensor assemblies  232   X . For example, distinctions may be obtained between offsets caused by temperature gradients on the chip and offsets caused by stress gradients on the chip. This information may be used in calibrating the sensor array  230  or the data may be stored for later analysis of the performance of the sensor array  230 . 
         [0059]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.