Integrated interface circuit for processing the signal supplied by a capacitive sensor
BACKGROUND OF THE DISCLOSURE 
     The present invention relates to an interface circuit for use between a sensor with capacitive impedance, particularly a piezoelectric accelerometric sensor, and a circuit for processing the signal provided by the sensor, particularly a filtering circuit with switched capacitors. 
     Piezoelectric accelerometric sensors are used, for example, in systems for detecting pinking (knocking) in Otto-cycle internal combustion engines for motor vehicles. In this type of application, the signal provided by a piezoelectric accelerometric sensor is processed by processing circuits which include, in particular, integrated filtering circuits of the type with switched capacitors. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an interface circuit of the aforesaid type which can optimise the characteristics of the signal coming from the sensor in view of the subsequent processing to which the signal will be subject. 
     According to the invention, this object is achieved by means of an interface circuit, characterised in that it comprises, integrated on a single substrate by CMOS technology, 
     an amplitude-limiting circuit, including a first CMOS transistor whose source is intended to be connected to the output of the sensor and whose gate and drain are intended to be kept at a reference potential, and a second CMOS transistor whose gate and drain are connected to the source of the first transistor and whose source is intended to be kept at the reference potential; and 
     a circuit for filtering the signal and changing its level, connected downstream of the amplitude-limiting circuit and comprising an inverting amplifier including 
     an operational amplifier formed by CMOS technology and having its non-inverting input connected to a source of a direct-current reference voltage of lower value than the supply voltage, 
     a first capacitive impedance circuit connected between the inverting input of the operational amplifier and the amplitude limiter, and 
     a second capacitive impedance circuit connected between the inverting input and the output of the operational amplifier; 
     the capacitive impedance. circuits being such that, together, they have a band-pass filtering effect adapted to attentuate the low-frequency components of the sensor signal and to limit its band width. 
     A capacitive sensor of the aforesaid type typically has a reference terminal which, in use, is connected to earth or to a voltage supply, and a signal terminal at which the useful signal, which varies relative to the voltage applied to the reference terminal, is available in operation. 
     According to the invention, the transistors of the amplitude-limiting circuit are p-MOS devices produced by p-well CMOS technology or n-MOS devices produced by n-well CMOS technology, according to whether the reference terminal of the sensor is to be connected to earth or to a voltage supply, in use. This solution prevents any problems due to the leakage of stray currents associated with the known &#34;latch-up&#34; phenomenon into the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and advantages of the invention will become clear from the detailed description which follows, with reference to the appended drawings, provided purely by way of non-limiting example, in which: 
     FIG. 1 shows an integrated device (shown in the form of a block diagram) for interfacing between a capacitive sensor and a switched-capacitance filtering circuit, 
     FIG. 2 is a more detailed circuit diagram of one embodiment of the interface circuit, and 
     FIG. 3 shows a further embodiment of the circuit according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, a capacitive-impedance sensor, for example a piezoelectric accelerometric sensor, is indicated S. The sensor has a reference terminal r and a signal terminal s. In the embodiment illustrated, the terminal r is connected to earth while the terminal s is connected to the input 1 of an interface circuit, generally indicated IC, interposed between the sensor and a circuit 2, such as a filtering circuit with switched capacitors, for the subsequent processing of the sensor signal. Since the terminal r of the sensor S is connected to earth, the useful signal available at the terminal s in use is a signal which varies relative to zero. 
     As already stated, the reference terminal r of the sensor S could, however, be connected to a potential other than earth, for example to a direct-current voltage supply, and in this case the useful signal would be a signal whose amplitude would vary relative to the level of that direct-current voltage. 
     The interface circuit IC, which is connected as a whole to a direct-current voltage supply V DD  and to earth, as shown in the conceptual diagram of FIG. 1, comprises essentially an amplitude limiter 3 connected to a circuit, generally indicated 4, for filtering the signal and changing its level. 
     The amplitude limiter 3 limits the input signal and protects the interface circuit from excess voltage. 
     The circuit for filtering the signal and changing its level comprises an inverting amplifier including 
     an operational amplifier A whose non-inverting input is connected to a direct-current reference voltage V R  which is lower than the supply voltage V DD  and equal, for example, to V DD  /2, 
     a capacitive impedance circuit Z 1  connected between the inverting input of the operational amplifier A and the amplitude limiter 3, and 
     a capacitive impedance circuit Z 2  connected between the inverting input and the output of the operational amplifier A. 
     Assuming that the amplitude limiter 3 operates in the linear region of its characteristic, the transfer function of the interface circuit IC, between its input and its output, is given by 
     
         V.sub.o /V.sub.in =-Y1/Y2=-H.sub.lp ·H.sub.hp 
    
     in which 
     V in  is the voltage input to the interface circuit, 
     V o  is the voltage output by the interface circuit, 
     Y1 is the admittance of the circuit Z 1 . 
     Y2 is the admittance of the circuit Z 2 , 
     H lp  is a term representing a low-pass filtering function, and 
     H hp  is a term representing a high-pass filtering function. 
     The circuit 4 as a whole thus changes the signal to a d.c. signal and has a low-pass filtering effect. This circuit is thus able both to limit the band width of the signal supplied by the sensor (which prevents aliasing effects in the subsequent sampling of the signal by switched-capacitance filters) and to have a low-pass filtering effect which attenuates the very low-frequency components of the signal which are of no interest. 
     The whole interface circuit IC can be produced in integrated form on a single substrate by CMOS technology, for example, in the manner which will now be described with reference to FIG. 2. In this drawing, parts and components already described above have again been given the same alphabetical and numerical symbols. 
     In the embodiment of FIG. 2, the amplitude limiter 3 comprises two transistors MP1 and MP2 produced by CMOS technology. The transistor MP1 has its source connected to the input terminal 1 through a resistor R&#39; 1  and its gate and its drain connected to earth. The transistor MP2 has its gate and its drain connected to the source of MP1 and its source connected to earth. 
     The transistors MP1 and MP2, which are normally in the cutoff condition, limit the level of the input signal to ±V t  (the threshold voltage of the transistors). The level is limited when one of the two transistors starts to conduct, short-circuiting the input signal to earth. More particularly, the transistor MP1 limits positive amplitudes and protects the interface circuit from excessive voltages in the input signal whilst the transistor MP2 limits the negative amplitude of the signal. 
     The resistor R&#39; 1  limits the current through the transistors MP1 and MP2 when the signal reaches levels close to the supply voltage. 
     If, as in the embodiment shown in the drawings, the sensor S has its reference terminal connected to earth, the transistors MP1 and MP2 are p-MOS transistors produced by p-well CMOS technology. If, however, the reference terminal r of the sensor S is connected to a direct-current supply, the transistors are n-MOS devices produced by n-well CMOS technology. In this case, the gate and the drain of MP1 and the source of MP2 are connected to V DD  instead of to earth. 
     As stated above, these solutions prevent the problems which may result from the leakage of stray currents into the substrate. 
     The use of p-MOS or n-MOS transistors is more advantageous than the use of ordinary diodes with MOS-technology junctions since they permit a more dynamic input signal and prevent any current leakage caused by the stray bipolar transistors. 
     The filtering and level-changing circuit in the diagram of FIG. 2 has a transfer function which corresponds to a band-pass filtering effect between its input and its output. For this purpose, a resistor R&#34; 1  and a capacitor C 1  are interposed in series between the amplitude-limiting circuit 3 and the inverting input of the operational amplifier A. Moreover, a resistor R 2  and a capacitor C 2  are connected in parallel between the inverting input and the output of A. 
     The transfer function achieved thus assumes the following expression: ##EQU1## in which R 1  =R&#39; 1  +R&#34; 1 . 
     In particular, this transfer function has two poles which determine its low-pass and high-pass behaviour respectively at the frequencies ##EQU2## The pole at frequency f lp  is intended to limit the band width of the signal and may, for example, be at a frequency of approximately 60 kHz. The formation of R 1  and C 1  by CMOS technology in order to ensure such a value of f lp  does not pose particular problems. In fact a pole at 60 kHz can easily be achieved, for example, with a 20 pf capacitor and a 132 kohm resistor, which values can be integrated on a silicon substrate of quite large, but nevertheless acceptable area. 
     Problems may arise, however, in the formation of the pole at the frequency f hp , particularly when this frequency has to be quite low. If, for example, this frequency must assume a value of about 600 Hz and if C 2  has a value of 20 pF, a resistor with a resistance R 2  of about 13 Mohms is required and this is an unacceptable value for production in an integrated circuit. 
     In such cases, the solution shown in FIG. 3 may be adopted. In the arrangement shown in this drawing, a switched-capacitance circuit, generally indicated 10, is provided in parallel with the capacitor C 2 . This circuit comprises a capacitor C R  whose plates are connected to those of C 2  through two switching circuits comprising respective first switches 11 and 21, connected between the plates of C R  and the homologous plates of C 2 , and respective second switches 12 and 22, connected between the plates of C R  and earth. 
     In use, the switches 11 and 21 are operatively connected so as to switch on and off in phase with each other by means of a clock signal φ of frequency f ck . 
     The switches 12 and 22, however, are also operatively connected to switch on and off in phase with each other, but in counterphase with the switches 11 and 21 (φ). 
     The switched capacitor circuit 10 as a whole simulates a resistor with a resistance equivalent to ##EQU3## where f ck  is the clock frequency used. 
     Thus if f ck  is 150 kHz, C 2  is 20 pF and C R  is 0.5 pF, a pole is obtained at the frequency ##EQU4## It is important to stress that the solution in FIG. 3 is compatible with the need to limit the band width of the signal in order to prevent aliasing of the sampled signal. In fact, the operational amplifier A, which operates by sampling in discrete time domains, &#34;sees&#34;, as its input, the signal current which in effect is filtered continuously by the resistance R 1  =R&#39; 1  +R&#34; 1  and by the capacitor C 1 . 
     Finally, the resistor R&#34; 1  can be formed in a distributed manner so as to increase the equivalent attenuation at high frequencies. 
     The interface circuit described above can be integrated by CMOS technology on a single substrate on which circuits for the subsequent processing of the signal can also be integrated conveniently.