Abstract:
A mechanism to provider a bootstrapped power source for a differential operational amplifier includes a three-winding transformer having a first winding disposed between a positive voltage and a plus power input to the amplifier, yielding an initial plus voltage, a second winding disposed between a negative voltage and a minus power input to the amplifier yielding an initial minus voltage, and a third winding disposed between a ground and a plus input to the amplifier providing a feedback path. The differential operational amplifier output is connected to ground. The said amplifier minus input is connect to a signal and when the signal is displaced a first amount from a first voltage, the plus and minus power inputs are displaced approximately the same amount from the initial positive and negative voltages.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
     Capacitive displacement-sensing gauges are known in the art. These gauges use the change in capacitance between a capacitive displacement probe (probe) and the structure over which it is positioned (target) to measure the change in distance between the probe and the target. 
     These gauges are characterized by the equation that describes the capacitance of a parallel-plate capacitor.              C   =       k                   ɛ   0        A     d             (   1   )                                
     where 
     C=capacitance 
     ∈hd 0=permittivity of free space 
     k=dielectric constant 
     A=plate area 
     d=distance between plates 
     When using a capacitive-displacement probe, the plate area, which is the area of the probe&#39;s sensing element, is held constant, and the dielectric constant, typically air, is also constant. With this configuration, distance (d) is inversely proportional to the capacitance (C). 
     The voltage (v) and current (i) in a capacitor are related by equation 2.              v   =     i     s                 C               (   2   )                                
     where s is the complex frequency variable 
     When Equation 1 is substituted into Equation 2, v is expressed by equation 3.              v   =       i                 d       s                 k                   ɛ   0        A               (   3   )                                
     Equation 3 implies that if frequency (s) and current (i) are held constant, voltage v is proportional to the distance d between the probe and target. This relationship leads to an operational mode where capacitive displacement probes are commonly driven with a constant current, and the voltage across the probe provides an output that varies linearly as the distance varies. Because the voltage is most likely not directly usable, it is usually processed, for instance, by a unity-gain buffer before being used. 
     To ensure that a linear relationship between the circuit&#39;s transfer function and distance holds in practice, the probe must be guarded to prevent parasitic capacitances from appearing in parallel with the probe capacitance. Total probe capacitances are often tenths or even hundredths of a picofarad. The linear relationship between the circuit&#39;s transfer function and distance is further ensured by utilizing a high impedance input stage in the circuit receiving the input. When a unity-gain buffer is connected to the probe, the buffer amplifier&#39;s power-supply connections must be bootstrapped to ensure that the input impedance is high enough to maintain the transfer function of the gauge. 
     The frequency response of the unity-gain buffer should be flat and wideband to track the capacitance. Since the power-supply characteristics affect the buffer&#39;s frequency response, it is necessary to utilize a more costly and complex power-supply in this configuration. 
     Another way to connect to the probe and maintain the desired transfer function is to use a differential amplifier, such as an operational amplifier, with a grounded output in place of the unity-gain buffer. In prior art, in order to be able to ground the output, the amplifier is powered by a floating power supply. The floating power supply is an additional component that increases the size and complexity of the gauge system. The required transformer may need to be large, expensive and may be susceptible to stray electric and magnetic fields. To achieve the desired function, it may need to be custom made. 
     BRIEF SUMMARY OF THE INVENTION 
     A capacitive displacement-sensing gauge in which the transducer interface stage generates its own bootstrapping voltages provides a smaller more cost-effective gauge. A differential amplifier with a grounded output is used as a transducer interface stage and power and common connections to the stage are made via a 3-winding transformer in which the three windings are equal in turns and are closely coupled. The current that flows to ground through the grounded output of this stage by necessity flows through a winding of the transformer, causing the common voltage and both supply voltages to be perfectly bootstrapped. 
     No active circuitry, with its concomitant limitations in phase and frequency response, is required. Cost and size are reduced by elimination of active circuitry and/or the means necessary to create a floating supply. Reliability is improved by reducing the number of circuit elements that are required. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be understood from the following detailed description in conjunction with the drawings, of which: 
     FIG. 1 is a conceptual schematic of a idealized capacitive displacement probe with the output processed by a unity-gain buffer as is known in the art; 
     FIG. 2 is a conceptual schematic of the capacitive displacement probe of FIG. 1 with the probe capacitance guarded; 
     FIG. 3 is a conceptual schematic of a capacitive displacement probe circuit incorporating bootstrapped power inputs to a unitary buffer amp; 
     FIG. 4 is a conceptual schematic of a idealized capacitive displacement probe with the output processed by a differential amplifier and a floating supply as is known in the art; 
     FIG. 5 is a conceptual schematic of a idealized capacitive displacement probe with the output processed by a differential amplifier and a driven supply as is known in the art; and 
     FIG. 6 is a conceptual schematic of a idealized capacitive displacement probe with a self-bootstrapping transducer interface stage. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Capacitive displacement-sensing gauges use the change in capacitance between a capacitive displacement probe (probe) and the structure over which it is positioned (target) to measure the change in distance between the probe and the target. 
     When using a capacitive displacement probe, the plate area, which is the area of the probe&#39;s sensing element, is held constant, and the dielectric constant, typically air, is also constant, so that the distance (d) is inversely proportional to capacitance (C). 
     A common technique for interfacing to the probe and providing a constant-current drive while allowing for sensing the voltage across the probe is shown schematically in FIG.  1 . Note that FIG. 1, and all figures used herein are conceptual schematics, omitting details of the circuit that relate to fine points of a particular implementation. This simplification improves the clarity of the presentation and prevents implementation details from obscuring the broader concepts of each particular approach. In FIG. 1, the sum of an input voltage V i , and an output voltage V o  are equal to the voltage V s  presented to the capacitive divider (C ref +C probe ). The voltage on the probe capacitor C probe  is the input to a unitary buffer 2 having the output voltage V o . An idealized unitary buffer has infinite input impedance and exhibits the same voltage at the input and output. 
     Circuit analysis applied to FIG. 1 shows that the relationship between the output voltage and the input voltage is expressed by equation 4.                    v   i     +     v   o       =     v   s            
              v   s          (       C   ref         C   ref     +     C   probe         )       =     v   0               (4a)                     v   o       v   i       =       C   ref       C   probe         ,           (   4   )                                
     where V o  and V i  represent the output voltage and input excitation source respectively, and C ref  and C probe  represent a reference capacitor and the probe capacitor respectively. The combination of the reference capacitor and the excitation source set the current in the probe to a constant value that will yield an output voltage that is within a desired range. Substituting Equation 1 into Equation 4 yields a relationship between V o  and V i  expressed in Equation 5.                  v   o       v   i       =         C   ref        d       k                   ɛ   0        A               (   5   )                                
     Equation 5 confirms that for the circuit of FIG. 1, there is a linear relationship between V o  and d. 
     The circuit of FIG. 1 does not protect the probe capacitance from parasitic effects. To realize the linear relationship between the circuit&#39;s transfer function and the distance, the probe must be guarded to prevent parasitic capacitances from appearing in parallel with the probe capacitance. The output voltage, which tracks the probe voltage, (Eq. 4a) is typically used to guard the probe capacitor, as shown in FIG.  2 . 
     In FIG. 2, further details of C p  probe are shown. The target  10  is typically grounded. The plate  12  of capacitor C p  probe is a distance d away from the target  10 . Output voltage V o  is connected the shields  14  protecting plate  12  and the plate&#39;s connection to the input of the unity-gain buffer  2 . 
     Although a connection mechanism such as illustrated in FIG. 2 would seem to be fully functional, a problem exists. Typical probe capacitances C p  probe are often tenths of even hundredths of a picofarad. The input impedance of the unity-gain buffer  2 , connected to the circuit at point  16 , must be high enough to not substantially alter the transfer function of the capacitive divider circuit. For buffer amp  2 , the primary means to assure the high input impedance, is to bootstrap the buffer amp&#39;s power-supply connections (not shown). When an amplifier&#39;s power-supply is boot strapped, the − and − power inputs are slaved to the output voltage of the amplifier. As the output rises, both the + and − rise, maintaining the voltage differential, but moving the center point in concert with the output. 
     An illustrative schematic of a circuit incorporating bootstrapped power inputs to a unitary buffer amp  2 ′ is shown in FIG.  3 . The power inputs to unitary buffer amplifier  2 ′ are explicitly shown as +V D  and −V D . In FIG. 3, +V and −V represent the external power connections to a transformer and output V o  connects to a differential amplifier  20  whose output V d  drives the transformer that provides bootstrapped voltages +V D  and −V D . The two driven voltages, +V D  and −V D  are used to power the unity-gain buffer  2 ′. The ratio of R 1  and R 2  is typically set so the gain through them is slightly less than one. Although FIG. 3 shows +V D  and −V D  as the only power-supply connections to the unity-gain buffer  2 ′, implementations in which +V D  and −V D , as well as +V and −V connect to the buffer are also possible. A transformer implements the driven supply in the illustrative example of FIG. 3, but other means are known and can be used for this purpose. 
     The supply-driver circuit must have a flat, wideband frequency response, because the supply inputs to the buffer stage substantially affect the buffer&#39;s frequency response. The required flat, wideband frequency response of the power supply adds cost and complexity to the design of FIG.  3 . 
     An alternate implementation, shown in FIG. 4, obtains the desired transfer function using a differential amplifier  30 , such as an operational amplifier, in place of the unity-gain buffer  2 ,  2 ′. This implementation connects the differential amplifier&#39;s output  32  to ground. As is known in the art, one alternative when the differential amplifier&#39;s output  32  is grounded, is to power the amplifier  30  by a floating power supply  34 . A floating power supply  34  is one that is isolated at DC from ground and has sufficient AC isolation to not unduly load the differential amplifier  30 . Although the circuit topology of FIG. 4 looks quite different from that of FIG. 3, circuit analysis shows that they exhibit the same transfer function, thereby performing the same function. Because the differential amplifier&#39;s power supply connections +V F , −V F  move with the amplifier&#39;s summing junction  16 , the topology of FIG. 4 is inherently bootstrapped. The disadvantage of this implementation is the cost, size, and complexity associated with the floating supply. 
     An alternate implementation known in the art, shown in FIG. 5, uses a differential amplifier  30 ′ with a driven supply  40  rather than the floating supply  34  of FIG.  4 . The supply-driver circuit  44  keeps the output  42  of the transducer-interface amplifier  30 ′ at ground potential. The component labeled C 1  represents a compensation network, which in any particular implementation, may consist of other components as is known in the art. Alternate implementations of the driven supply circuit  44  function are known in the art. The circuits of FIGS. 3 and 5 share the characteristic that the phase and frequency response of the supply driver circuit  44  affects the response of the differential amplifier  30 ,  30 ′ that may have connections to both the driven and the external power supply. Each of the prior art circuits of FIGS. 3-5 have disadvantages that are associated with the need for the driven or floating supplies as listed above. 
     FIG. 6 illustrates a circuit in which the transducer interface stage generates its own bootstrapping voltages. A differential amplifier  30 ″ with an output  50  grounded is used as a transducer interface stage as described with reference to the circuit of FIG.  4 . The power  54 ,  56  and common  50  connections to the transducer interface stage are made via a 3-winding transformer  52  in which the three windings are equal in turns and are closely coupled. In this circuit, current that flows to ground through the grounded output  50  by necessity flows through one of the windings of the illustrated transformer, causing the common voltage and both supply voltages to be perfectly bootstrapped. 
     As an illustration, if the input voltage on line  16  tries to fall, the output  50  tries to move up at the same time. This causes current to flow in the +V arm of the transformer  52 . The transformer presents a high impedance load to power input  54 causing the voltage at  54  to fall. Since transformer  52  is closely coupled, the voltage drop in the +V/ 54  line causes drops in the −V/ 56  line and the GND/ 58  line. 
     The transformer windings, being identical, function symmetrically depending on the change in input  16 . When the transistor, internal to differential amplifier  30 ″, connected between +V input  54  and the output  30  is turned on, the upper winding of the transformer  52  acts as the primary, and the two lower windings act as secondaries. Analogously, when the transistor, internal to differential amplifier  30 ″, connected between −V input  56  and output  50  is turned on, the lower winding of the transformer  52  acts as the primary, and the two upper windings act as secondaries. 
     The circuit of FIG. 6 has several advantages over the prior-art methods. Because the circuit utilizes no active components, the limitations in phase and frequency response imposed by active components are not seen, also the cost associated with active circuitry is eliminated. Cost is further reduced by the elimination of the driven or floating power supply. The size of a package containing the circuit is reduced by elimination of active circuitry and the means necessary to create a floating supply. The reduction in the number of parts, active and passive, improves the reliability of the product. 
     The transformer  52  used in the new circuit is simpler to manufacture than transformer  40  used in the prior art. All windings are identical and there are no concerns about inter-winding capacitance. The transformer  52  can be a tri-filer, simple to manufacture using a common core, providing a very accurate wideband transfer function. The invention requires no separate power supply. 
     Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should only be limited by the spirit and scope of the appended claims.