Abstract:
The object of the procedure according to the present invention is to characterise a voltage or current converter ( 20 ) intended to be connected to a capacitive circuit ( 32 ) arranged so as to provide a capacitance difference (C1−C2) to the converter. Said converter is arranged so as to be able to receive the capacitance difference provided by the circuit, and to provide an output voltage (Vo) which is a function of the capacitance difference and a bias signal. This procedure is characterised in that it includes a sequence of steps which consist in varying the bias signal, while keeping the capacitance difference constant and measuring in response the output voltage. 
     One advantage of such a procedure lies in the fact that it allows the electric performance of the converter to be determined independently of the error link to the capacitance measuring.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns the field of integrated circuits and, more particularly, characterisation procedures for voltage converters connected to a capacitive circuit. 
     There exist a large variety of sensors as regards their shape, functions, effects and applications. 
     Within the scope of the detection of a parameter such as acceleration or pressure, a capacitive sensor such as that described hereinafter is commonly used. 
     FIG. 1 shows a conventional capacitive sensor  1 . 
     Sensor  1  is arranged to be able to measure a parameter such as the ambient pressure or the acceleration undergone by such sensor. For this purpose, sensor  1  includes a support member  5 , two plates  11  and  12  which are stationary relative to this support member, and a third plate  13  which is arranged so as to be able to move between the two plates  11  and  13 . 
     An equivalent electric diagram of sensor  1  can be shown by two capacitances C1 and C2 connected in series. In such a diagram, capacitance C1 corresponds to the capacitance of the capacitor formed by plates  11  and  13 , and capacitance  13  corresponds to the capacitance of the capacitor formed by plates  12  and  13 . 
     Capacitive sensor  1  is also arranged so as to be able to provide a capacitance difference C1−C2 which is a function of said parameter. For this purpose, sensor C1 includes a connecting terminal  15  to be able to provide such a difference. 
     The operation of sensor  1  is as follows. Via the effect of said parameter, moving plate  13  moves in sensor  1 , and this latter provides in response the capacitance difference C1−C2 which represents the arrangement of the three plates  11  to  13 , following said effect. 
     Within the scope of the detection of a parameter such as pressure or acceleration, such a capacitive sensor is connected to a voltage converter, so that the circuit formed by the sensor and the converter, provides an electric voltage which represents the change in the arameter. 
     FIG. 2 shows such a known circuit including sensor  1  of FIG. 1, which is connected to a voltage converter  20 . 
     Converter  20  includes two input terminals  201  and  202  and one output terminal  203 . Converter  20  is connected and arranged so as to be able to receive, via terminal  201 , the capacitance difference C1−C2 provided by sensor  1  and, via terminal  202 , a bias voltage vb provided by a constant voltage source  22 . Converter  20  is arranged so as to be able to provide, via terminal  203 , an output voltage Vo which depends on the capacitance difference C1−C2 and the bias voltage Vb. 
     It will be noted that bias voltage Vb can be unipolar or bipolar, and referenced relative to the earth potential of the circuit. It will also be recalled that bias voltage Vb is typically used to fix the static gain of the circuit formed by sensor  1  and converter  20  at a predetermined value. 
     Generally, conventional characterisation procedures for such a converter, rely on the determination of the characteristic feature of output voltage Vo as a function of the capacitance difference C1−C2 and, in particular, the electric performance of this converter. 
     It will be recalled that the &lt;&lt;electric performance&gt;&gt; of a converter is usually characterised by two electric parameters: the static gain As and the non-linearity coefficient L As . It will also be recal dvb that the converter static gain As is equal to          Vb     C1   +   C2       ,                          
     with reference to FIG. 2, and that non-linearity coefficient L As  represents the dispersion of output voltage Vo between the effective values of this voltage and voltage values corresponding to ideal linear properties of the converter. 
     One difficulty commonly encountered in achieving such characterisation lies in the provision of a plurality of differences in capacitance C1−C2, to measure the change in output voltage Vo as a function of capacitance difference C1−C2. 
     Indeed, in the event that the converter is connected to a capacitive sensor such as that described in relation to FIG. 1, the variation in the parameter capable of causing a capacitance difference C1−C2 is difficult to control in an industrial manufacturing environment, in particular in semi-conductor manufacturing plants, which are subject to concerns as to yield. 
     In order to answer such concerns, a circuit, whose equivalent electric diagram is close to that of a capacitive sensor, i.e., a circuit which provides an electric signal capable of representing a capacitance difference, is used as capacitive sensor. In the following description, such a circuit is called a &lt;&lt;capacitive circuit&gt;&gt;. 
     By way of illustration, FIG. 3 shows a conventional capacitive circuit  25 , capable of providing five capacitance difference values C1 i −C2 i  i=1, 2, 3, 4, 5. For this purpose, capacitive circuit  25  comprises connection means  251  and five first capacitors C1 i  (i=1, 2, 3, 4, 5) respectively connected in series to five second capacitors C2 i  (i=1, 2, 3, 4, 5). Capacitive circuit  25  is arranged so as to be able to establish a connection between one of capacitors C1 i , connection means  251  and the associated capacitor C2, so that this circuit provides, via connection means  251 , one of the five capacitance difference values C1 i −C2 i . 
     One problem which is encountered in the determination of the electric performance of a voltage converter lies in the fact that such determination is unreliable, since the capacitance values are tainted by an intrinsic inaccuracy linked to the tolerance on the components and an extrinsic inaccuracy linked to the connection means of the capacitive circuit, these inaccuracies being all the more inconvenient if one desires to characterise a converter capable of processing low capacitance values. 
     FIG. 4 a  shows a curve  40  illustrating the voltage characteristic of converter  20  of FIG. 2, which is connected to capacitive circuit  25  of FIG. 3, and a curve  42  illustrating the linear regression of this characteristic. 
     It will be recalled that capacitive circuit  25  can provide successively five capacitance difference values C1 i −C2 i  (i=1, 2, 3, 4 , 5). In the event that capacitive circuit  25  is used to simulate capacitive sensor  1  in the circuit of FIG. 2, five values for output voltage Vo can be obtained for the five respective capacitance difference values Vo i  (i=1, 2, 3, 4, 5). In other words, five pairs of data items (C1 i −C2 i , Vo i ) are thus obtained. 
     In order to determine the electric performance of converter  20 , as described hereinbefore, these five pairs of data items are then extrapolated by a linear regression which is show in FIG. 4 a  by curve  42 . This regression allows gain As, and coefficient L As  to be determined. 
     It will be noted in FIG. 4 a  that the measurement error in output voltage Vo is essentially due to the error ε in the effective value of the differences in capacitance C1 i −C2 i . 
     For a sensor such as described in relation to FIG. 1, this sensor can provide capacitance differences C1−C2 which are less than several tens of femtofarads (1 fF=10 −15 F), over a range of variation in capacitance difference C1−C2 which is typically comprised between several picofarads and several tens of picofarads (1 pF−10 −12 F). For example, a converter having a 12 bit resolution allows a variation of 2.4 fF to be measured for a variation range of 10 pF. 
     FIG. 4 b  shows a theoretical curve  43  illustrating a relative accuracy designated ΔVo/Vo, which is linked to output voltage Vo obtained from FIG. 4 a , as a function of capacitance difference C1−C2. It will be noted in FIG. 4 b  that the measuring accuracy of the electric performance of converter  20 , determined by such a characterisation procedure, is tainted by error ε which is typically of the order of 1%, this value having been established by the Applicant of the present invention, by considering capacitances whose tolerances are of the order of 1%. It will be noted that this error is higher the lower the measured capacitance difference. 
     In other words, such a characterisation procedure does not answer current requirements as to accuracy and yield, which are common industrial concerns, in particular in semi-conductor manufacturing plants. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a characterisation procedure for a voltage or current converter connected to a capacitive circuit, this procedure overcoming the aforementioned drawbacks of the prior art, in particular by allowing the converter voltage characteristic to be calculated accurately, in particular the electric performance thereof. 
     Another object of the present invention is to provide a characterisation procedure which answers current industrial concerns as to accuracy and yields, in particular in semi-conductor manufacturing plants. 
     These objects, in addition to others, are achieved by the characterisation procedure according to claim  1 . 
     One advantage of the characterisation procedure according to the present invention lies in the fact that it relies on the measurement of the converter output voltage, as a function of different bias voltage values, which allows the electric performance of the converter to be determined independently of the error due to the measurement of a capacitance. There results a procedure which is accurate and simple to implement in an industrial environment, in particular in the semi-conductor field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These objects, features and advantages, in addition to others, will appear more clearly upon reading the detailed description of a preferred embodiment of the invention, given solely by way of example, in relation to the annexed Figures, in which: 
     FIG. 1 which has already been cited, shows a conventional capacitive sensor, 
     FIG. 2 which has already been cited, shows a circuit diagram including the sensor of FIG. 1, connected to a voltage converter, 
     FIG. 3 which has already been cited shows a conventional capacitive circuit, 
     FIG. 4 a  which has already been cited, shows two curves respectively illustrating the characteristic of a voltage converter, and the linear regression of such characteristic, according to a conventional characterisation procedure, 
     FIG. 4 b  which has already been cited, shows a theoretical curve illustrating a relative accuracy linked to a voltage converter as a function of a capacitance difference, capable of being obtained from a conventional characterisation procedure, 
     FIG. 5 shows a circuit diagram including a capacitive circuit, and a voltage converter to be characterised by the procedure according to the present invention, 
     FIG. 6 shows a flow diagram of the characterisation procedure according to the present invention, and 
     FIG. 7 a  shows two curves illustrating respectively the voltage characteristic of a converter, and the linear regression of such characteristic according to the characterisation procedure of FIG. 6, and 
     FIG. 7 b  shows a theoretical curve illustrating a relative accuracy linked to a voltage converter, as a function of a voltage, capable of being obtained from the characterisation procedure according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The characterisation procedure according to the resent invention is intended to determine the characteristic of a voltage converter connected to a capacitive circuit. 
     By way of illustration only, FIG. 5 shows a circuit  30  including voltage converter  20  of FIG. 2, which is connected to a capacitive circuit  32 , and intended to be characterised by the characterisation procedure according to the present invention. 
     It will be noted that FIG. 5 shows elements designated by references which are used to designate the elements described in relation to FIG.  2 . The references used to designate elements in FIGS. 2 and 5, designate the same elements in the two Figures, in particular in relation to converter  20 . 
     Moreover, as FIG. 5 shows, capacitive circuit  32  includes a connecting terminal  320  connected to the input terminal  201  of converter  20 . Capacitive circuit  32  has a structure as described hereinbefore. In other words, this circuit is formed of a circuit whose equivalent electric diagram is close to that of the sensor of FIG. 1, i.e. of two capacitances C1 and C2 connected in series. Thus, capacitive circuit  32  is arranged so as to provide, via terminal  320 , a capacitance difference C1−C2 to converter  20 . 
     It will be recalled that output voltage Vo of converter  20  is a function of capacitance difference C1−C2 and bias voltage Vb. 
     It will be noted that converter  20  receives, via terminal  202 , bias voltage Vb which is provided by conventional supply means (not shown) arranged so as to be able to provide a variable value of bias voltage Vb. 
     The procedure according to the present invention will now be described, used to characterise converter  20  in circuit  30  of FIG.  5 . 
     FIG. 6 shows a flow diagram of this procedure. 
     The procedure according to the present invention includes a sequence of steps designated &lt;&lt;a&gt;&gt; to &lt;&lt;d&gt;&gt;. 
     Step &lt;&lt;a&gt;&gt; consists in measuring at least three experimental values of output voltage Vo from at least three respective values of bias voltage Vb. It will be noted that capacitance difference C1−C2 is kept constant. 
     The references Vo mes   1 , Vo mes   2  and Vo mes   3  designate respectively the three experimental values of output voltage Vo, and the references Vb 1 , Vb 2  and Vb 3  designate respectively the three values of bias voltage Vb. It will be noted that values Vb 1 , Vb 2  and Vb 3  are comprised within the operating range of converter  31 . 
     At the end of step &lt;&lt;a&gt;&gt;, three pairs of values are thus obtained (Vb 1,  Vo mes   1 ), (Vb 2,  Vo mes   2 ), (Vb 3,  Vo mes   3 ). 
     Step &lt;&lt;a&gt;&gt; is followed by step &lt;&lt;b&gt;&gt;, which includes two sub-steps designated &lt;&lt;b 1 &gt;&gt; and&lt;&lt;b 2 &gt;&gt;. 
     Sub-step &lt;&lt;b 1 &gt;&gt; consists in determining by regression an analytical curve from the three pairs of values (Vb 1,  Vo mes   1 ), (Vb 2,  Vo mes   2 ), (Vb 3,  Vo mes   3 ). The curve obtained is an analytical representation of the voltage characteristic of converter  20 , based on an analytical model which depends on the nature of the capacitive circuit, as will be described in more detail hereinafter. 
     It will be noted that the procedure according to the present invention is particularly advantageous, since it allows the characteristic of converter  20  to be determined from values Vb 1 , Vb 2  and Vb 3  of bias voltage Vb, unlike the conventional procedure. Indeed, during the conventional procedure, the characteristic of converter  20  is determined from capacitance difference values C1 i −C2 i , for i=1, 2, 3, 4, 5, the value of bias voltage vb being kept constant. Consequently, the accuracy of measurement in the characterisation procedure according to the present invention is typically of the order of 1 mV, which allows a capacitance difference of 10 pF to be measured with an accuracy of 2 fF (1 fF=10 −15  F), for a bias voltage Vb equal to 5 V for example. 
     Sub-step &lt;&lt;b 1 &gt;&gt; is followed by sub-step &lt;&lt;b 2 &gt;&gt; which consists in providing, from the analytical representation obtained in sub-step &lt;&lt;b 1 &gt;&gt;, and values Vb 1 , Vb 2  and Vb 3 , three values calculated from output voltage Vo, which are respectively designated Vo cal   1 , Vo cal   2  and Vo cal   3 , and which correspond respectively to the three experimental values Vo mes   1 , Vo mes   2  and Vo mes   3 . 
     Within the framework of determining the electric performance of converter  20 , as described hereinbefore, sub-step &lt;&lt;b 2 &gt;&gt; is followed by step &lt;&lt;c&gt;&gt; which is intended to determine gain As of converter  20 , then step &lt;&lt;d&gt;&gt; which is intended to determine coefficient L As  of the converter. 
     Thus step &lt;&lt;c&gt;&gt; consists in determining, from the previously measured experimental values Vo mes   1 , Vo mes   2  and Vo mes   3  and the respective calculated values Vo cal   1 , Vo cal   2  and Vo cal   3 , the converter gain As., by performing the following calculation, for i, j=1, 2, 3 and i≠j:          A                 s     =         V                   o   cal   i       -     V                   o   cal   j             V                   b   i       -     V                   b   j                                  
     It will be noted that this gain is calculated with the accuracy of measurement relating to bias voltage Vb. 
     Step &lt;&lt;c&gt;&gt; is followed by step &lt;&lt;d&gt;&gt; which consists in determining, from the previously measured experimental values (Vo mes   1 , Vo mes   2 , Vo mes   3 ), and the respective calculated values (Vo cal   1 , Vo cal   2 , Vo cal   3 ), the converter non-linearity coefficient L As  by performing the following calculation:          L   As     =       Max     i   ,   j            (         V                   o   mes   i       -     V                   o   mes   j             V                   o   max       -     V                   o   min           )                              
     where Vo max -Vo min  corresponds to the length of the range of variations in the output signal. 
     It will be noted that coefficient L As  is calculated with the accuracy of measurement relative to bias voltage Vb. 
     As was already mentioned hereinbefore, the curve obtained at the end of step &lt;&lt;b 1 &gt;&gt; is an analytical representation of the voltage characteristic of converter  20 , based on an analytical model which depends on the nature of capacitive circuit  32 . The following two types of capacitive circuits will be considered. 
     Let us consider the first type of capacitive circuit which has a structure whose equivalent electric diagram includes a capacitance C1 which is fixed, and a capacitance C2 which is variable. In this case, the regression used in step &lt;&lt;b 1 &gt;&gt; is polynomial. 
     Let us now consider the second type of capacitive circuit which has a structure whose equivalent electric diagram includes two capacitances C1 and C2 the sum of which is fixed. In this case, the regression used in step &lt;&lt;b 1 &gt;&gt; is linear. 
     By way of illustration only, FIG. 7 a  shows a curve  70  illustrating the linear regression of the voltage characteristic of a voltage converter, in the event that the latter is connected to a capacitive circuit of the second type. 
     Five pairs of data are thus shown in FIG. 7 a , and designated by the references (Vb i , Vo mes   1 ) for i=1, 2, 3, 4, 5. These pairs of data have been extrapolated by the linear regression illustrated by curve  70 , to determined gain As and coefficient L as . 
     It will be noted in FIG. 7 b  that the measurement error in output voltage Vo is essentially due to the error in the effective value of bias voltage vb. 
     FIG. 7 b  shows a theoretical curve  72  illustrating a relative accuracy designated ΔVo/Vo, which is linked to output voltage Vo described in relation to FIG. 7 a , as a function of bias voltage Vb. It will be noted in FIG. 7 b  that the accuracy of measurement of the electric performance of the converter, determined by such a characterisation procedure, is tainted by an error which is typically of the order of 0.02%, this value having been established by the Applicant of the present invention. 
     It goes without saying for those skilled in the art that the detailed description hereinbefore can undergo various modifications without departing from the scope of the present invention. By way of example, the characterisation procedure according to the present invention can be used to determine the characteristic of a current converter. Also by way of alternative, the bias voltage can be replaced by a bias current.