Abstract:
An improved biometric data sensing circuit, for example adapted for fingerprint sensing, uses a charge subtraction technique at the input of the circuit integrator to cancel the so called “common mode” signal from the circuit output. The result is an output signal that is (a) linear, (b) free from any amplification effect due to the presence of the detected object (e.g. a finger), and (c) indicative of the detected object&#39;s fine surface geometry (i.e., indicative of the fingerprint&#39;s ridges and valleys).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present disclosure is related to and claims priority from copending U.S. Provisional Patent Application, Ser. No. 60/987,739, filed on Nov. 13, 2007, and further which, in its entirety, is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to biometric data (e.g., fingerprint) sensing circuitry, and more particularly to a circuit and method for improving dynamic range in such circuitry. 
     2. Description of the Prior Art 
     Capacitive biometric data sensing circuits (also referred to as pixels) are well known. Such a circuit is shown and described in U.S. Pat. No. 6,512,381, incorporated herein by reference. More specifically, with reference to  FIG. 1  hereof, there is shown a schematic illustration of a circuit for  10  biometric data sensing.  FIG. 2  is an illustration of a device  20  implementing the circuit  10  of  FIG. 1 . 
     A common application of such circuits is imaging of a user&#39;s fingerprint pattern found on the tip of one of the user&#39;s fingers, for example for user identity verification. Such circuits sense field changes due in part to proximity of the finger of the user. These circuits are very sensitive, and are in fact able to detect, for example, differences in field strength in the presence of a peak of a fingerprint and field strength in the presence of a valley of a fingerprint on a pixel-by-pixel basis. For the circuit  10  of  FIG. 1 , in terms of the sensing capacitance, C sense , at capacitor  12 , 
               C   sense     =     {             C     S   ⁢           ⁢   0       ⁢           ⁢   in   ⁢           ⁢   absence   ⁢           ⁢   of   ⁢           ⁢   a   ⁢           ⁢   fingerprint   ⁢           ⁢   ridge                   C     S   ⁢           ⁢   0       ⁡     (     1   -   α     )       ⁢           ⁢   in   ⁢           ⁢   presence   ⁢           ⁢   of   ⁢           ⁢   a   ⁢           ⁢   fingerprint   ⁢           ⁢   ridge   ⁢           ⁢     (     α   ⁢           ⁢     typ   .           ⁢   approx   .           ⁢   0.1       )                     
In terms of the voltages at the input node  14  and output node  16  of the circuit,
 
                          Δ   ⁢           ⁢     V   out            =            Δ   ⁢           ⁢     V   in            ·       C   in       C   sense                 (   1   )               
Thus, the difference in output voltages between pixels can be used to generate a local image of the user&#39;s fingerprint.
 
     The output of a typical sensing circuit is comprised of a “differential mode” (that portion of the output signal of interest, commonly referred to as data), a “common mode” (base data present in each pixel regardless of the presence of a finger), and noise, attributable to many source such as the design of the circuit itself, the environment in which the circuit is operated, etc.  FIG. 3  illustrates a graph of the output voltage from a pixel as a ridge of a fingerprint passes over the pixel, showing differential and common modes (noise is omitted for clarity), for two respective pixels. A description of the timing of the various signals is beyond the scope of the present disclosure. If at the pixel location p 1  a valley of a fingerprint is present over the sensing circuit, the output voltage from the pixel is V 1 . If at the pixel location p 2  a ridge of a fingerprint is present, the output voltage is V 2 . It is the difference between V 1  and V 2  (ΔV) that is of interest. Thus, at the limit V 1  can be considered the “common mode” (although in certain embodiments, a voltage measured in the absence of a finger may be considered the “common mode.”) 
     At present, the common mode may account for as much as 90% of the pixel output signal, with a mere 10% of that output signal representing data (the information needed to construct a biometric image such as a fingerprint). As the ratio of data (signal) to noise decreases in an output, the difficulty in accurately determining data in that output increases. It is a goal of circuit design to minimize the common mode so that the data is more easily and accurately recognized. 
     Commonly in the prior art, gain and offset adjustments are made to compensate in part for the common mode signal. However, adjusting the gain and offset affect not only the undesired “common mode” signal, but also the desired data (signal). Thus in the prior art, the output dynamics of the sensing circuit are not used efficiently to reflect the presence of surface modulations, i.e., the fingerprint features. 
     Compounding this poor signal-to-noise problem is a decrease in typical operating voltage for fingerprint sensors (driven, for example, by the desire to increase battery life in portable devices using such fingerprint sensors). Circuits of the type illustrated in  FIG. 1  have typically been operated at 5 volts. This has provided acceptable dynamic range for sensing. Dynamic range is the greatest possible range of output signals, taking away the noise or common mode. Basically, dynamic range is the difference between an output signal in the presence of a ridge and in the presence of a valley. However, there is a trend to design biometric sensing circuits to operate at 3 volts or lower. In doing so, the dynamic range is compressed. That is, the actual voltage representing the data decreases to a point that it is difficult to detect, especially as compared to the voltage representing the common mode. Thus, as dynamic range is compressed, the noise portion of the pixel output dominates the data, and generation of a biometric image (e.g., fingerprint) becomes more difficult and less accurate (if possible at all). 
     SUMMARY OF THE INVENTION 
     Accordingly, there is a need in the art to provide a method of boosting dynamic range within the constraints of pixel design. We have developed a technique, disclosed herein, for accomplished by removing the common mode from pixels in an array, leaving as output from those pixels the data of interest. 
     The output of the present invention is an electronic signal that indicates the fine surface features of the detected object (e.g., an image of a fingerprint pattern). Using a charge subtraction technique at the input of the circuit integrator, the so called “common mode” signal is subtracted from the circuit output. The result is an output signal that is (a) linear, (b) free from any amplification effect due to the presence of the detected object (e.g., a finger), and (c) indicative of the detected object&#39;s fine surface geometry (e.g., indicative of the fingerprint&#39;s ridges and valleys). With the common mode signal effectively removed from the output, the circuit makes better use of the available output dynamics of the sensing pixel circuit compared to the prior art. This can improve the signal-to-noise ratio (SNR) of the pixel circuit, as well as enable low power pixel circuit implementations. 
     According to one aspect of the present invention, a difference circuit is provided permitting mathematical cancelling of the common mode. In a specific embodiment of the present invention, a feedback capacitor takes the place of the sensing capacitor of the prior art, and the sensing capacitor is moved to the input node of the op amp. Dual inputs drive the circuit, with the input waveforms being of opposite phase with respect to one another. Capacitive values, which are controlled through the physical device embodying the circuit, are selected such that terms contributing to the output voltage attributable to common mode may cancel each other. The circuit output voltage then represents measured capacitance changes, such as when a fingerprint transitions from a valley to a ridge as it slides over the sensor circuit in operation. 
     While term cancellation is one embodiment of the present invention, other embodiments are contemplated by the present invention. For example, by knowing the relative capacitive values of C in  and C f , it is possible to take into account the values of terms attributable to common mode without actually cancelling terms from the voltage calculation. Many different methods of data analysis and circuit design are therefore provided by the present invention. 
     The above is a summary of a number of the unique aspects, features, and advantages of the present invention. However, this summary is not exhaustive. Thus, these and other aspects, features, and advantages of the present invention will become more apparent from the following detailed description and the appended drawings, when considered in light of the claims provided herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings appended hereto like reference numerals denote like elements between the various drawings. While illustrative, the drawings are not drawn to scale. In the drawings: 
         FIG. 1  is a schematic illustration of a circuit for biometric data acquisition according to the prior art. 
         FIG. 2  is a device embodying the circuit of  FIG. 1  for biometric data acquisition according to the prior art. 
         FIG. 3  is a graph of output voltage versus time for a biometric sensing circuit sensing a user&#39;s fingerprint, and in particular showing the difference between the sensing of a ridge and a valley of a fingerprint, the common mode, and data from such sensing. 
         FIG. 4  is a schematic illustration of a circuit for biometric data acquisition according to one embodiment of the present invention. 
         FIG. 5  is a combined cross section and schematic view of a biometric data acquisition device, such as a fingerprint sensor, showing placement of a user&#39;s finger thereover, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to remove the common mode, we have developed an improved pixel circuit  20 , as illustrated in  FIG. 4 . Pixel circuit  20  comprises an operational amplifier  22  having an input connected to a first capacitor  24 , a second capacitor  26  which serves as the sensing capacitor, and a third capacitor  26 , which is the feedback capacitance (C f ). A charge P is applied to capacitor  24  as V inP , and a charge N is applied to capacitor  26  as V inN . The magnitudes of charges P and N may be different, but they are essentially of opposite phase with respect to one another. 
     Again, in terms of C S0 , the sensing capacitance C sense , is given by 
               C   sense     =     {             C     S   ⁢           ⁢   0       ⁢           ⁢   in   ⁢             ⁢             ⁢   the   ⁢           ⁢   absence   ⁢           ⁢   of   ⁢           ⁢   a   ⁢           ⁢   fingerprint   ⁢           ⁢   ridge   ⁢           ⁢     (       C     S   ⁢           ⁢   0       =     C   sense       )                   C     S   ⁢           ⁢   0   ⁢           ⁢     (     1   -   α     )         ⁢           ⁢   in   ⁢           ⁢   the   ⁢           ⁢   presence   ⁢           ⁢   of   ⁢           ⁢   a   ⁢           ⁢   fingerprint   ⁢           ⁢   ridge   ⁢           ⁢     (     the   ⁢           ⁢   pixel   ⁢           ⁢   location   ⁢           ⁢   p   ⁢           ⁢     typ   .           ⁢   approx   .           ⁢   0.1       )                     
In the absence of a fingerprint we obtain
 
                     Δ   ⁢           ⁢     v   out       =         -   Δ     ⁢           ⁢       v     in   P       ·       C   in       C   f           +     Δ   ⁢           ⁢       V     in   N       ·       C     S   ⁢           ⁢   0         C   f                     (   2   )               
And in the presence of a fingerprint we obtain
 
     
       
         
           
             
               
                 
                   
                     
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     Now, we choose the fixed capacitance in the circuit, C f , C SO  and C in  all to be equal, as well as setting V inP =V inN . It will be noted that upon doing so, 
     
       
         
           
             
               
                 
                   
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     Therefore, due to the negative sign in equation (2), the first two terms cancel one another, leaving 
     
       
         
           
             
               
                 
                   
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     The right side of equation (6) is simply the contribution to the output signal due to the presence of the ridge—the data we were originally interested in viewing from the sensing circuit. Accordingly, we have cancelled the “common mode” (the part of the output signal due to noise) and left as the output signal only the data of interest. 
       FIG. 5  is an illustration of one embodiment  30  of a fingerprint sensor device according to the present invention.  FIG. 5  shows two sensor cells  32 ,  34  which are spaced from each other as might typically be implemented in an array of such sensor cells. For ease of illustration, the first cell  32  is shown with a ridge region of a finger  36  directly thereover, whereas the second cell  34  is shown with a valley region of a finger  36  directly thereover so that the differences in operation can be easily explained and understood. 
     Concerning first the situation in which a ridge portion  38  of a fingerprint pattern borne by the finger  36  is directly over a cell  32 , the distance between the fingerprint surface (i.e., user&#39;s skin) and upper surfaces of first and second capacitor plates  38 ,  40  is D 1 , since the skin is in direct contact with the upper surface  42  of the sensor body structure  44 . In this instance, the effect of the ridge portion  38  of the fingerprint pattern borne by the finger  36  is to reduce the value of the fringe capacitance between plates  42  and  44 . In particular, the capacitance between plates  42  and  44  is shown by the fringe field lines with fewer field lines illustrating a reduction in the capacitance value. Namely, since the ridge  38  is close to the plates  42  and  44 , some of the field lines are interfered with by the ridge portion  38  so that the overall capacitance value between plates  42  and  44  is reduced due to the fringing effect of an adjacent conductor. This is represented by some of the field lines going into the finger  36  rather than extending between capacitor plate  42  and  44 . 
     Concerning the second situation in which a valley portion  40  of a fingerprint pattern borne by the finger  36  is directly over a cell  34 , the distance between the fingerprint surface (i.e., user&#39;s skin) and upper surfaces of first and second capacitor plates  52 ,  54  is D 2 . Taking the case in which D 2  is relatively large, there will be little or no interference between the fringe capacitive field lines that extend between the plates  52  and  54  of the feedback capacitor to the amplifier  56  of cell  34 . In particular, most or all of the fringe field lines which existed between plates  52  and  54  still exist when the valley is adjacent the cell such that the finger does not interfere with the fringe capacitance value in any significant way. 
     As detailed in the aforementioned U.S. Pat. No. 6,512,381, the actual local capacitance of each individual cell  32 ,  34  will not be precisely known, and will vary over a wide range. However, with C f  and C in  both fixed, the sense capacitance C sense  will be at a minimum value while at a ridge portion with the skin in direct contact with the upper surface  46  of sensor structure  48 . The sense capacitance C sense  will increase as the skin moves away from plates  42 ,  44  (or similarly  52 ,  54 ), and be at a maximum value in the presence of a valley portion of a fingerprint. Since the fingerprint pattern of an individual varies between ridge and valleys, it will be appreciated that the actual sense capacitance will also vary from the two extremes of the minimum and maximum value. 
     The present invention provides many advantages over prior art original single-ended capacitance sensing pixel circuit architectures, including:
         a) The output signal represents (ideally) only the real gain modulation due to a user&#39;s finger&#39;s ridges and valleys (rather than the presence of the finger);   b) The output dynamics of the circuit are utilized more efficiently (improving the signal-to-noise ratio) and enables the use of this circuit with low power supplies;   c) The circuit is very simple and its timing requirements are minimal (therefore, the transistor matching tolerance can be increased, and the statistical offset can be reduced);   d) Only the input capacitances need be matched; and   e) The components of this design are well known in the art and are readily implemented using existing integrated circuit fabrication techniques.       

     The physics of modern electrical devices and the methods of their production are not absolutes, but rather statistical efforts to produce a desired device and/or result. Even with the utmost of attention being paid to repeatability of processes, the cleanliness of manufacturing facilities, the purity of starting and processing materials, and so forth, variations and imperfections result. Accordingly, no limitation in the description of the present disclosure or its claims can or should be read as absolute. The limitations of the claims are intended to define the boundaries of the present disclosure, up to and including those limitations. To further highlight this, the term “substantially” may occasionally be used herein in association with a claim limitation (although consideration for variations and imperfections is not restricted to only those limitations used with that term). While as difficult to precisely define as the limitations of the present disclosure themselves, we intend that this term be interpreted as “to a large extent”, “as nearly as practicable”, “within technical limitations”, and the like. 
     Furthermore, while a plurality of preferred exemplary embodiments have been presented in the foregoing detailed description, it should be understood that a vast number of variations exist, and these preferred exemplary embodiments are merely representative examples, and are not intended to limit the scope, applicability or configuration of the disclosure in any way. Various of the above-disclosed and other features and functions, or alternative thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications variations, or improvements therein or thereon may be subsequently made by those skilled in the art which are also intended to be encompassed by the claims, below. 
     Therefore, the foregoing description provides those of ordinary skill in the art with a convenient guide for implementation of the disclosure, and contemplates that various changes in the functions and arrangements of the described embodiments may be made without departing from the spirit and scope of the disclosure defined by the claims thereto.