Abstract:
Amplifying device for a sensor ( 10 ) delivering a response in voltage or impedance comprising:  
     voltage controlling means ( 22  connected to output terminals ( 16, 20 ) of the sensor to maintain a noticeably constant voltage between the said terminals,  
     at least an additional impedance ( 12 ) connected in series with the sensor within a polarisation circuit, between the output terminals ( 18 - 16; 18 - 20 ) of the said device.  
     Application to temperatures, irradiation levels and pH measurement.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an amplifying device for sensors and a measuring system for physical quantities using this device.  
           [0002]    The device of the invention is intended, in particular, for voltage and/or impedance response sensors. Such a voltage and/or impedance response sensor is a sensor at which terminals one obtains a voltage variation and/or an impedance variation related to the modification of the physical quantity to which the sensor is sensitive.  
           [0003]    The invention has applications in the manufacturing of measuring systems, and mainly measuring systems which comprise miniaturised or embedded (integrated) sensors, known as micro-sensors. As an example, in no way restrictive for the use of the device, the invention may apply to the realisation of thermal probes, dosimeters or photometers.  
         DESCRIPTION OF THE PRIOR ART  
         [0004]    The above mentioned voltage and/or impedance response sensors may be modelled by assuming that they are equivalent to a source of a fixed voltage in series with a variable impedance, or equivalent to a source of a variable voltage in series with a fixed impedance, or equivalent to a source of a variable voltage in series with a variable impedance.  
           [0005]    By letting E Th  and Z Th  be the voltage and impedance values featured by the sensor, and X a physical quantity to which the sensor is sensitive, one may define the voltage sensitivity e, or the impedance sensitivity z of the sensor as: 
             e=dE   Th   /dX    
           and 
             z=dZ   Th   /dX    
           [0006]    A sensor whose sensitivity to a physical quantity only affects its impedance may be characterised by dE Th =0 and dZ Th =zdX. A sensor whose sensitivity to a physical quantity only affects its voltage source may be characterised by dE Th =edX and dZ TH =0.  
           [0007]    The sensitivity (e and z) of sensors to physical quantities are usually very low. It is especially the case when theses sensors are involved in the detection of secondary physical quantities which are generated by the phenomenon to be observed (temperature measurement by variable resistors, for example).  
           [0008]    This low sensitivity of sensors induces a limitation of measurement precision and a strong influence of the measurement noise.  
         DESCRIPTION OF THE INVENTION  
         [0009]    The purpose of the invention is to present an amplifying device for the sensors and a measurement system which discards the above mentioned limitations.  
           [0010]    One particular purpose of the invention is to propose an amplifying device allowing to yield a measurement signal at terminals of an additional impedance distinct from the sensor.  
           [0011]    One object of the invention is also to propose a measuring system whose sensitivity to the physical quantities to be measured is widely increased.  
           [0012]    One object of the invention is also to propose a system which features a good measuring precision, which is immune to noise and thus permits a reduction of detection thresholds.  
           [0013]    To fulfil these objectives, the object of the invention is an amplifier for a voltage and/or impedance response sensor. The device includes:  
           [0014]    voltage control means, connected to the sensor output terminals to maintain a noticeably constant voltage between these terminals,  
           [0015]    at least one additional impedance series connected with the sensor in a polarisation circuit, between the terminals of the said device.  
           [0016]    Through the voltage control means, any modification of the equivalent impedance of the sensor or any modification of the voltage delivered by the equivalent voltage source of the sensor yields a modification of the current flow through the sensor. This results from the fact that the voltage at its terminals is forced to be fixed.  
           [0017]    As the additional impedance is series connected with the sensor in the polarisation circuit, the sensor current also flows into this impedance. The current modification then appears as a voltage modification at the additional impedance ends.  
           [0018]    The additional impedance is connected between output circuit terminals so that the voltage variation at the ends of the additional impedance are included in the output signal.  
           [0019]    The output terminals of the device may be, for example, the additional impedance ends or the ends of the assembly including the additional impedance in series with the sensor (the voltage being constant at ends thereof).  
           [0020]    Following a particular realisation of the device according to the invention, the voltage control means may include a field effect transistor (FET). The sensor is then connected between the gate and the source of the transistor, whereas the additional impedance is connected between the gate and the drain of the transistor.  
           [0021]    The transistor, for example, is a MOS (Metal-Oxide Semiconductor) mounted in common source and preferably polarised in saturation. The application of an large polarisation current maintains a steady state of the gate-source voltage. The polarisation means may for example include a current source connected between the source and the drain of the transistor.  
           [0022]    The invention relates also to a measuring system for physical quantities involving a voltage response or impedance response sensor operating with an amplifying device as described above.  
           [0023]    Being liable to be realised with any type of voltage and/or impedance sensor, the measuring system may include in particular any one of the following sensors: a thermocouple, a dosimeter, a chemical sensor or a bio-sensor. A bio-sensor is understood as any sensor sensitive to organic species such as ADN, various proteins, bacteria, etc.  
           [0024]    Other features and advantages of the invention will be made clearer in the following description, referring to the figures in annex. This description is in no way restrictive with respect to the scope of the invention and gives only an example of the various realisation of the device according to the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a simplified schematic block diagram of a measuring device according to the invention;  
         [0026]    [0026]FIG. 2 is an electric schematics showing a possible design of the measuring system which involves a field effect transistor;  
         [0027]    FIGS.  3  to  5  are electric schematics featuring three particular applications of the measuring system following FIG. 2;  
         [0028]    [0028]FIG. 6 is a plot of the evolution in time of the output voltage of a measuring system following the invention, including a sensor sensitive to hydrogen;  
         [0029]    [0029]FIG. 7 is a plot showing the maximum of the output voltage of the said measuring system in terms of hydrogen potentials measured in a solution. 
     
    
     DETAILED DESCRIPTIONS OF EMBODIMENTS OF THE INVENTION  
       [0030]    In the following description, the elements which are either identical or similar have a same numeric reference.  
         [0031]    [0031]FIG. 1 illustrates the general operating principle of the invention.  
         [0032]    A sensor  10  and an additional impedance  12  are series connected in a polarisation circuit connected to polarisation means  14 . The polarisation means  14  provide sensor  10  and additional impedance  12  with a current noted I on the figure.  
         [0033]    The additional impedance  12  is connected between a node  16 , which connects it to sensor  10  and an output terminal  18 . The sensor  10  is connected between node  16  and a ground terminal  20 .  
         [0034]    Voltage control means  22  are also connected to node  16 . They maintain a constant voltage between the load ends, that is, in the example described on the figure, between node  16  and ground terminal  20 .  
         [0035]    The input of the device corresponds to the sensor terminals and the output voltage of the device can be picked up at terminals of the additional impedance, that is between node  16  and output terminal  18 .  
         [0036]    As the voltage at sensor terminals is constant, the output voltage of the device can also be picked up between output terminal  18  and ground terminal  20 . This alternative solution is retained in the following.  
         [0037]    [0037]FIG. 2 shows a particular embodiment of the measurement system wherein current control means  22  involve an insulated gate field effect transistor (IGFET), for example an N channel MOSFET, and wherein the polarisation means  14  feature a current source. The drain, the source and the gate of transistor  22  are respectively connected to output terminal  18 , ground terminal  20  and node  16 .  
         [0038]    The current source delivers a current, I 0 , which is split into a first current, I DS  which flows through the channel of the transistor, and a second current I which flows through the additional impedance  12 . The strong polarisation current involved (I 0 &gt;&gt;I) permits to operate the transistor  22  in saturation and then to fix its gate voltage (here the gate-source voltage) at an equilibrium value which avers practically constant. This value, applied to terminals of the sensor  10 , is noted V GSo  in the following.  
         [0039]    The current I which flows through the additional impedance  12  is noticeably the same as the current which flows into the load  10  in series with the additional impedance. Indeed, the value I G  of a gate current of the transistor which leaves node  16  is extremely low (I&gt;&gt;I G ).  
         [0040]    The assembly composed of sensor  10  and additional impedance  12  form a voltage dividing bridge between the drain-source voltage of the transistor, noted V DS , and the gate-source voltage.  
         [0041]    Besides, the additional impedance  12 , whose value is noted Z, connected between the drain and the gate of the transistor  22 , acts as a counter-reaction loop, which servo-controls and stabilises the gate-source voltage, that is the voltage at sensor terminals.  
         [0042]    One may see, on the figure, that sensor  10  is modelled as an equivalent voltage source  24 , whose value is E Th , in series with an equivalent impedance  26  whose value is Z Th . The figure details these Thevenin&#39;s equivalence.  
         [0043]    As the voltage V GSo  is constant, a variation dE Th  or dZ Th  of the characteristics E Th , Z Th  of sensor  10 , in response to a variation to be detected of a physical quantity, are transformed into a variation of the current I. These variations, amplified through the additional impedance  12 , whose value is Z, are measured as variations dV DS  of the drain-source voltage V DS  of the transistor.  
         [0044]    The variation dV DS  can be derived from the variations dZ Th  and dE Th  via the following relation: 
           dV   DS =−( V   GSo   −E   Th )·( Z/Z   Th    2 )· dZ   Th −( Z/Z   Th ) dE   Th    
         [0045]    For sensors sensitive to a value X only through their equivalent voltage source E Th , with a sensitivity value e such as dE Th =edX and dZ Th =0, the circuit is advantageous, in particular when the following condition is satisfied: |Z/Z Th |&gt;1.  
         [0046]    This condition permits to obtain an output signal proving a higher amplitude than the signal which should be delivered by the sensor alone.  
         [0047]    In other words, the value Z of the additional impedance is preferably selected larger than the value Z Th  of the sensor internal impedance  26 .  
         [0048]    For sensors sensitive to a physical quantity only through the impedance Z Th  with a sensitivity z such as dZ Th =zdX and dE Th =0, the circuit is advantageous, in particular when the following condition is satisfied: |(V so −E Th )·(Z/Z Th   2 )|&gt;1.  
         [0049]    [0049]FIG. 3 illustrates the measuring system of FIG. 2 adapted to a thermal sensor of the kind of thermocouples.  
         [0050]    The thermocouples involve the Seebeck effect. This effect relates the variations of temperature T to thermoelectric voltage variations. The impedance of a thermocouple is noticeably constant.  
         [0051]    So, the thermocouple may be modelled as shown in FIG. 3, by a variable voltage source  24  whose voltage value is E Th , in series with a merely resistive impedance  26 , whose value is R Th .  
         [0052]    In this example, it is advisable to select an additional impedance  12  purely resistive, whose value is R.  
         [0053]    In the case when transistor  22  is a N channel MOS type featuring a threshold voltage Vt, the optimal operation is obtain when the following relations are satisfied:  
         [0054]    I G &lt;&lt;I&lt;&lt;I 0    
         [0055]    0&lt;V T &lt;V GSo &lt;V DS    
         [0056]    E Th &lt;V GSo    
         [0057]    R/R Th &gt;1  
         [0058]    By assuming that V DS  is the output voltage, the sensitivity in temperature measurement dV DS /dT is amplified and becomes |dV DS /dT|=(R/R Th )a where a is the intrinsic thermocouple sensitivity.  
         [0059]    [0059]FIG. 4 shows another example of application of the invention wherein the sensor  10  is a dosimeter, i.e., a sensor which is sensitive to radiation. Such a component for example features a MOS type (Metal-Oxide-Semiconductor) capacitor. The radiation level is measured through the variation of an electrical potential V Fb  said flat band potential. Sensor  10  may be modelled as a variable voltage source  24 , value E Th , whose constant sensitivity is e, in series with a purely capacitor impedance, value C Th .  
         [0060]    In the example of FIG. 4, the additional impedance  12  is also selected as a pure capacitor, of value C. Transistor  22  is a P channel MOS type transistor with a threshold voltage V T . It must be noted that, in this case, the flow I 0  is inverted, with respect to the previous figures, which corresponds to the use of N channel transistors.  
         [0061]    In this example, the circuit operates optimally with components selected for satisfying the following relations.  
         [0062]    I G &lt;&lt;I&lt;&lt;I 0    
         [0063]    V DS &lt;V GSo &lt;V T &lt;0  
         [0064]    V GSo &lt;E Th    
         [0065]    C Th /C &gt;1  
         [0066]    By forcing the linearity of the MOS capacitor and thus defining its intrinsic sensitivity as s, the sensitivity of radiation dose measurements D, noted dV DS /dD expresses as: 
           dV   DS   /dD =( C   Th   /C )· s    
         [0067]    [0067]FIG. 5 shows a third example wherein sensor  10  is a field effect ion-sensitive capacitor (ISFEC). Such sensors are used for pH-measurement. Measuring the ph of a solution is performed by the variation of an electrical potential onto an isulating/electrolytic interface.  
         [0068]    The ISFEC-type sensors may be modelled as a variable voltage source  24 , value E Th , whose sensitivity is s, in series with an impedance which is a MOS-type capacitor  26   a , of constant value C Th , in series with a resistor  26   b , whose value is R s . This resistive value comes from the series resistance of the electrolyte and a sensor reference electrode.  
         [0069]    One may see on the figure that the additional impedance  12  includes a capacitive value C, a capacitor  12   a  in parallel with a resistive value R, a resistor  12   b.    
         [0070]    It is also shown that a resistor  28 , whose value is R p , is connected in parallel to the load between node  16  and ground terminal  20 .  
         [0071]    High values leak resistances result from the current leaks of the MOS capacitor  26   a  and from the gate-source capacitor of transistor  22 . The resistors  12   b  and  28 , which prove lower values, create a dividing bridge and allow to neglect these leak resistances, whose value is difficult to evaluate. They then make it possible to control leak currents.  
         [0072]    The amplification of pH detection capabilities obtained in the bridge including sensor  10  and capacitor  12   a  is however altered by the value of the series resistor, value R s , and the effects of the bridge including the resistors  12   b - 28 .  
         [0073]    Transistor  22  is a P channel type transistor with a threshold voltage V T.    
         [0074]    For optimal operation of the circuit, the components are selected so as to satisfy the following relations:  
         [0075]    I G &lt;&lt;I&lt;&lt;I 0    
         [0076]    V DS &lt;V GSo &gt;V T &lt;0  
         [0077]    V GSo &lt;E Th    
         [0078]    C Th /C&lt;1  
         [0079]    C Th /C&lt;R/R p    
         [0080]    The voltage V DS  produced by the circuit in FIG. 5 evolves in time. It shows first a transient phase A due to the effect of the bridge  10 ,  12   a  sensor/capacitor then a steady state B due to the effect of the resistive bridge  28 ,  12   b . These phases appear in FIG. 6. This figure plots the voltage V DS  in ordinate, in volts, versus time in abscissa, in milliseconds. The maximum value of V DS , namely V max , is thus representative of the amplification of detection properties. The hydrogen potential measurement sensitivity is brought up to the value: 
         • dV   max   /dpH •=α( C   Th   /C )·s   
         [0081]    The ratio C Th /C represents the amplification of the capacitive bridge whereas coefficient α represents the amplification attenuation due to the resistive bridge (R, R p ) and series resistor R s  (α&lt;1).  
         [0082]    On the whole, and over a wide scale of pH values, the invention brings up an increase of the sensitivity to pH measurements up to values much larger than the theoretical maximum (Nernst law: s=59 mv/pH at ambient temperature) currently obtained when using standard techniques. This appears in FIG. 7 which plots in ordinate, in volts, the V max  voltages measured with the system of FIG. 5, versus the pH values in abscissa.