Abstract:
A transconductor converts voltage on an inductive sensor to a proportional current using two “coupling” capacitors. Responsive to movement of an electrically conductive target from the null position a resonant current is formed between the two sensor coils. A single differential transistor pair switched by periodic drive signals commutes the net alternating current at the single input to direct current.

Description:
RELATED APPLICATION 
     The present application relates to and is a divisional of Ser. No. 14/046,083 filed Oct. 4, 2013 which claims the benefit of priority to U.S. Provisional Patent Application No. 61/710,565 filed 5 Oct. 2012 which is hereby incorporated by reference in its entirety for all purposes as if fully set forth herein. 
    
    
     STATEMENT REGARDING FEDERAL SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with U.S. Government Support under HQ0147-12-C-7810 awarded by the U.S. Missile Defense Agency. The U.S. Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Embodiments of the present invention relate, in general, to inductive sensors and more particularly to a low noise inductive sensor with coupled capacitors. 
     Relevant Background 
     An inductive sensor is comprised of an induction loop that, when electric current passes through it, generates a magnetic field. In electromagnetism and electronics, inductance is the property of a conductor by which a change in current in the conductor “induces” (creates) a voltage (electromotive force) in both the conductor itself (self-inductance) and any nearby conductors (mutual inductance). To add inductance to a circuit, electronic components called inductors are used, typically consisting of coils of wire to concentrate the magnetic field and so that the magnetic field is linked into the circuit. The inductance of such a loop changes according to the material inside the magnetic field and since metals are much more effective inductors (conductors) than other materials the presence of metal in the magnetic field increases the current flowing through the loop. This change can be detected by sensing circuitry. 
     Inductive sensors are used in science, engineering, industry and medicine to measure a wide variety of physical phenomena. For example, metal detectors at airports are essentially inductive sensors as are control systems for traffic lights. Consider the pavement sensors near an intersection. These sensors can include one or more wire loops positioned within the pavement such that when a large metal mass approaches the loop, such as an automobile, the current within the loop changes. The inductive sensor described broadly above is referred to as an eddy current (EC) sensor. Eddy currents are created when a conductor experiences changes in the magnetic field. If either the conductor is moving through a steady magnetic field, or the magnetic field is changing around a stationary conductor, eddy currents will occur in the conductor. Therefore, eddy currents will be generated wherever a conducting object experiences a change in the intensity or direction of the magnetic field at any point within it, and not just at the boundaries. Eddy current sensors induce and detect eddy currents in electrically conductive bodies such as metals, liquids, industrial coatings and biological tissue. Such sensors can also measure material characteristics such as corrosion, cracking, wear and damage and are extensively used in the field of non-destructive testing (NDT). 
     Beyond basic metal detectors, EC sensors are particularly well suited to measuring the position or displacement of a metallic object because they are highly sensitive to position and motion while being highly insensitive to environmental factors such as dirt, water, temperature and radiation. EC sensors are often used in precision motion control systems such as the fine steering mirrors in satellite optics. In these systems the EC sensor provides motion feedback on the tip and tilt of a servo-controlled mirror used, for example, to image objects on the ground or to point a laser beam at another satellite. However, noise, or unwanted random signals, in the EC sensors causes the fine steering mirror to move unpredictably, generating “optical jitter” and degrading image quality or pointing accuracy. Therefore, there is a continued need for EC sensors with lower noise and higher accuracy. 
       FIG. 1  shows the arrangement of a differential eddy current sensor as would be known in the prior art. In this prior art example, the driver/demodulator block  110  drives the sensor coils, Lsns+  120 , and Lsns−  130 , through the series elements Zs  140 , which can be a resistor, inductor or capacitor or combination thereof. The drive signal  150 , Vdrv, drives the two sensors  120 ,  130  with a sine wave, square wave, or similar periodic waveform generated from an oscillator  155  that is typically 500 kHz. Parallel capacitors Cp  160  resonate with the inductance of the sensors, improving linearity, temperature coefficient, and noise while reducing power consumption. The sensors are energized with an AC current at the drive signal frequency, and the magnetic field generated by the current induces eddy currents in the conductive target  170 . The eddy currents generate an opposing magnetic field that can be sensed as an impedance change at the terminals of the sensor coil. The impedance change depends on the “lift-off,” 175  or distance from the sensor coil to the target, so the voltages at RF+  135  and RF−  145  vary in amplitude and phase in response to target movement. The demodulator  110  converts the AC signal at RF+  135  and RF−  145  to DC (or baseband) at IF=V(IF+  180 −IF−  185 ). IF  180 ,  185  may be filtered, offset, scaled or digitized in a downstream signal processing stage (not shown.) 
     When a target  170  is positioned precisely equidistant from the sensors  120 ,  130  (the null point), the sensor voltages RF+  135  and RF−  145  are balanced with equal amplitude and equal phase. As the target  170  moves, it changes the inductance of the two sensor coils  120 ,  130  in a complementary way, unbalancing the voltages RF+  135  and RF−  145 . The demodulator  110  typically processes the difference (RF+−RF−) to produce a proportional DC signal at IF. 
       FIG. 2  shows a prior art demodulator circuit, which is also known as a standard Gilbert mixer. The demodulator circuit of  FIG. 2  receives the voltages on the sensors at RF+  135  and RF−  145 . The transistors QdiffP  270  and QdiffN  280  form a differential transconductor, converting the voltage difference (RF+−RF−) to a sinusoidal current which is fed into the switching quad comprised of Qsw 1   p ,  210  Qsw 1   n ,  220  Qsw 2   p ,  230  and Qsw 2   n    240 . These transistors are switched by the switch drive signals LO+ and LO− at the same frequency as the sensor drive signals, typically 500 kHz. The switching mixes the 500 kHz sensor signal (RF+−RF−)  135 ,  145  down to DC at IF+  180  and IF−  185 . Low pass filters, not shown, remove the 500 kHz signal and its harmonics producing IF=V(IF+−IF−). 
     The prior art demodulator shown in  FIG. 2  has several shortcomings when used in an eddy current sensor system such as that shown in  FIG. 1 . For example, the transistors in the transconductor generate a significant amount of noise through their base resistance, base shot noise and collector shot noise. Moreover the transconductor has a limited common mode voltage range such that the circuit must be biased with high enough voltage to accommodate the full voltage swing on the sensors that appears largely in-phase at RF+ and RF−. This causes an increase in the minimum power supply voltage and therefore an increase in power dissipation. 
     The transconductor also adds an additional level of transistors to the switch transistors, requiring an increase in power supply voltage and power consumption, and the differential pair must be degenerated with the emitter resistors Rep  250  and Ren  255  to maintain adequate linearity and to control the temperature stability. Rep  250  and Ren  255  generate significant noise and can cause further voltage drops that require an increase in supply voltage and power consumption. While Rep  250  and Ren  255  could be replaced with inductors, inductors are usually physically larger and less temperature stable than resistors. Because Rep  250  and Ren  255  have relatively low impedance the transconductor must be biased with relatively high current to maintain linearity and temperature stability, further increasing noise and power dissipation. 
     Finally the finite current gain of the differential pair, QdiffP  270  and QdiffN  280 , causes a loss of signal that is dependent on temperature and radiation exposure, degrading accuracy and stability, particularly in high radiation environments such as space. As can be seen above, there is a continued need for EC sensors with lower noise and higher accuracy. These and other deficiencies of the prior art are addressed by one or more embodiments of the present invention. 
     SUMMARY OF THE INVENTION 
     Disclosed hereafter by way of example is a current commutating demodulator (or “mixer”) circuit for use in Radio Frequency (RF) communications and inductive sensors. Also disclosed are embodiments of an inductive sensor circuit. Embodiments of the circuit present a unique topology of a differential inductive sensor, replacing the transconductor found in prior art mixers and demodulators. A transconductor is a building block employed in many analog and mixed-signal circuit applications, such as continuous-time filters, delta-sigma modulators, variable gain-amplifiers or data converters that perform voltage-to-current conversion. In one embodiment of the present invention a transconductor converts voltage on the inductive sensor to a proportional current using two “coupling” capacitors. The configuration of the present invention converts the voltage-to-current circuit from a voltage-mode of the prior art to a current-mode, removing a number of design restrictions and performance limitations. 
     By converting the critical voltage-to-current circuit to a current mode, the noise, non-linearity and temperature sensitivity of the transconductor are nearly eliminated because the coupling capacitors are essentially noiseless, highly linear, and nearly temperature independent. Moreover the coupling capacitors allow the sensors to be driven with arbitrarily large AC signals, reducing noise without compromising linearity or requiring impractically high power supply voltages. 
     The coupling capacitors can also be freely scaled to maximize the baseband signal amplitude, minimizing the required post-amplification and further reducing noise and the new circuit can eliminate at least one source of radiation-induced drift by replacing the transconductor. Finally the new circuit is more power efficient than typical RF mixers because it needs less bias current to remain linear and temperature stable. 
     According to one embodiment of the present invention an inductive sensor circuit includes a demodulator, a first and second sensor coil and an electrically conductive target. The demodulator includes a single input and is operable to convert a net alternating current (AC) signal into a direct current (DC) signal. The first sensor coil is associated with a two capacitors wherein one capacitor is in parallel with the first sensor coil and the second capacitor is interposed in series between the single input of the demodulator and the first sensor coil. The second sensor coil is also associated with two capacitors, one in parallel with the second sensor coil and the other interposed in series between the single input of the demodulator and the second sensor coil. Both sensor coils are driven by drive signals that are 180° out of phase. Lastly an electrically conductive target is interposed between the first and second sensor coils such that when the target moves from a central, null, position the inductance on the sensor coils changes resulting in a net current at the single input of the demodulator. 
     Another feature of the demodulator of the present invention is that it includes a single differential transistor pair that is switched by drive signals that commute the net alternating current at the single input to direct current. 
     According to another embodiment of the present invention an inductive sensor circuit comprises a demodulator having a single input and an electrically conductive target interposed between a first and second sensor coil wherein the first sensor coil and the second sensor coil are coupled to the demodulator at the single input and wherein movement of the electrically conductive target from a null position generates a net alternating current at the single input. 
     The features and advantages described in this disclosure and in the following detailed description are not all-inclusive. Many additional features and advantages will be apparent to one of ordinary skill in the relevant art in view of the drawings, specification, and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes and may not have been selected to delineate or circumscribe the inventive subject matter; reference to the claims is necessary to determine such inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent, and the invention itself will be best understood, by reference to the following description of one or more embodiments taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a block diagram of a differential sensor circuit as would be known to one of reasonable skill in the relevant art; 
         FIG. 2  shows a demodulator or mixer circuit as would be known to one or reasonable skill in the relevant art; 
         FIG. 3  is a circuit diagram of an improved inductive circuit with a coupled capacitor and two inductive sensors driven 180 degrees out of phase according to one embodiment of the present invention; 
         FIG. 4  is a demodulator circuit diagram for an improved inductive circuit according to one embodiment of the present invention; 
         FIG. 5  is a circuit diagram of a local oscillatory driver according to one embodiment of the present invention; 
         FIG. 6  is a high level block diagram of an improved inductive circuit according to one embodiment of the present invention that is driven differentially, 180 degrees out of phase, and combined with a differential sensor drive; 
         FIG. 7  is a demodulator for the differentially driven circuit of  FIG. 6 , according to one embodiment of the present invention; 
         FIG. 8  is a demodulator circuit with a common base input stage according to one embodiment of the present invention; 
         FIG. 9  is a demodulator for a differentially driven circuit with a common base input stage according to one embodiment of the present invention; and 
         FIG. 10  is a flowchart of one method for demodulating a current difference with a frequency mixing circuit according to one embodiment of the present invention. 
     
    
    
     The Figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention are hereafter described in detail with reference to the accompanying Figures. Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention. 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the present invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Like numbers refer to like elements throughout. In the figures, the sizes of certain lines, layers, components, elements or features may be exaggerated for clarity. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     As used herein any reference to “one embodiment” or “an embodiment” means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
     By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and/or clarity. 
     It will be also understood that when an element is referred to as being “on”, “attached” to, “connected” to, “coupled” with, “contacting”, “mounted” etc., another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, “directly on”, “directly attached” to, “directly connected” to, “directly coupled” with or “directly contacting” another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed “adjacent” another feature may have portions that overlap or underlie the adjacent feature. 
     Spatially relative terms, such as “under”, “below”, “lower”, “over”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of a device in use or operation in addition to the orientation depicted in the figures. For example, if a device in the figures is inverted, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. Thus, the exemplary term “under” can encompass both an orientation of “over” and “under”. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Similarly, the terms “upwardly”, “downwardly”, “vertical”, “horizontal” and the like are used herein for the purpose of explanation only unless specifically indicated otherwise. 
     The inductive sensor design of the present invention reduces noise and provides an improvement in accuracy over eddy current sensors and other inductive sensors. Since these sensors can be used to measure precise displacement and material properties in a wide variety of systems, the invention is potentially of use in a wide variety of applications, including precision position and displacement sensing in optics, robotics, semiconductor equipment, and industrial manufacturing applications. 
     Non-destructive inspection and testing of materials for cracks, corrosion, and other flaws and material characterization and quality control, such as measurement of electrical conductivity, magnetic permeability, thickness and the thickness of conductive and non-conductive coating are other applications for the present invention as is Magnetic bearing servo systems. 
     The lower noise in the sensor leads to higher accuracy and higher sensitivity. Various embodiments of the present invention reduce noise, provide better linearity and increased temperature stability. The design also uses lower voltage and power as it is generally a more simplified circuit than those of the prior art. 
       FIG. 3  shows a block diagram an improved inductor sensor circuit according to one embodiment of the present invention. A signal generated by an Oscillator  155  at typically 500 kHz provides a periodic signal  350  that is split into two drive signals Vdrv+  352  and Vdrv−  354  that are 180° out of phase. These two signals, Vdrv+  352  and Vdrv− drive a demodulator/mixer  375 . The signal passes through a series of elements Zs which can be resistors, capacitors, inductors or any combination thereof to arrive at a signal Vp and Vn that drivers two inductive sensors Lsns+  320  and Lsns−  330 . As with the prior art the drive signal is periodic in nature however in this embodiment the drive signal is 180° out of phase. Each inductive sensor Lsns+  320  and Lsns−  330  is associated with a capacitor Cp 1   360  and Cp 3   365 , respectively, configured in parallel, that shunts a portion of the current from a resonant current produced by the sensor. 
     Interposed between the two inductive sensors Lsns+  320 , Lsns−  330  is an electrically conductive target  370 . As the target moves from a middle (null) position the inductance in each of the two sensors is changed resulting in a resonant current or resonant frequency RF. For illustration purposes two counterclockwise currents are shown in sensor Lsns+  320  and Lsns−  330 . One skilled in the art will appreciate that no current exists when then target  370  is at a null position. Also, a clockwise current exists in both sensors Lsns+  320 . Lsns−  330  when the target  370  moves in the opposite direction. 
     As shown the demodulator  310  includes a single input or low impedance node N 1   375 . The node N 1   375  is maintained essentially at AC ground. The single input N 1   375  is coupled to each inductive sensor Lsns+  320 , Lsns−  330  through an additional capacitor Cp 2   335 , Cp 4   325 , respectively. According to one embodiment of the present invention capacitors Cp 2   335 , Cp 4   325  possesses the same capacitance. 
     When the target is at the null point, RF+ and RF− are equal in amplitude and 180° out of phase, so the AC voltage on node N 1   375  is zero and no net AC current flows into the demodulator  310  at node N 1   375 . When the target is displaced from the null point the sensor inductances are unequal, RF+ differs from RF− in amplitude and phase, and a net AC current flows into node N 1   375 . 
       FIG. 4  is a circuit diagram of a demodulator suitable for receiving a single AC input according to one embodiment. The demodulator  400  of the present invention includes a single differential NPN transistor pair, QswP  410  and QswN  420 , that are switched by LO+  425  and LO−  430  at the frequency of signal  350 , which is typically 500 kHz. Net current from the sensors, RF, enters the low impedance emitters of the transistors, so N 1   375  is maintained near AC ground. The transistors  410 ,  420  commutate this current, mixing the sensor frequency from 500 kHz down to DC at the IF+  440  and IF−  445  outputs. Resistor Rns  435  isolates the sensors Lsns+  320 , Lsns−  330  and the capacitors Cp 1   360 , Cp 3   365  and Cp 2   335 , Cp 4   325  respectively, from the emitters of the transistors, reducing noise and improving the high frequency stability of the transistors. According to one embodiment of the present invention, capacitors Cp 1   360  and Cp 3   365  can possess the same capacitance. Also, one of reasonable skill in the art will recognize that resistor Rns  435  may be replaced with an arbitrary impedance  470 ,  460  such as an inductor for the same purpose. 
     The inductive sensor of  FIG. 3  and demodulator of  FIG. 4  provide significant advantages over the prior art. The noise, non-linearity, temperature sensitivity and radiation sensitivity of the prior art transconductor (illustrated by example in  FIGS. 1 and 3 ) are nearly eliminated. The coupling capacitors, CP 2   335  and CP 4   325 , substantially reduce noise to the point of being essentially noiseless, demonstrating highly linear characteristics, and are nearly immune to temperature drift and radiation degradation. 
     In contrast to the input of the prior art demodulator circuit (for example voltage input RF+  135 , RF−  145 ), embodiments of the current invention convert the input to a current input at node N 1   375 , so the demodulator  310 ,  400  is oblivious to the full voltage on the sensors. The capacitors, CP 2   335  and CP 4   325 , are also highly linear further aiding to the improved characteristics of the circuit. These factors allow the sensors Lsns+  320 , Lsns−  330 , to be driven with a relatively large signal  350  to minimize noise. These factors also permit a reduction in the power supply voltage, reducing total power consumption. 
     As illustrated in  FIG. 3  the coupling capacitors Cp 2   335 , Cp 4   325  are connected to a low impedance node N 1   375 , so they are essentially held at AC ground. Thus the capacitors experience very little additional series resistance, preserving and enhancing their high quality factor, reducing noise, and increasing sensitivity to target movement. These capacitors also see zero additional series resistance at the null point, as there is no net current that flows into N 1  when the target is in its null position. Therefore there is zero degradation in quality factor at the null point, which is significant in mechanical servo systems such as magnetic bearing control and alignment of segmented mirrors in astronomical telescopes, aiming devices and the like. In those applications, and many others, the servo system drives and maintains the target position at the null point where accuracy and low noise is critical. 
     The new demodulator  400  illustrated in  FIG. 4  taps a portion of the current circulating in the sensors at Lo+  425  and Lo−  445 . In practice the circulating current can be relatively large (e.g., tens of milliamps in a typical system). By changing the ratio of Cp 1   360 , Cp 3   365  and Cp 2   335 , Cp 4   325  any portion of the circulating current can be routed into the demodulator. Thus the demodulator can work with a large current-mode input signal rather than the relatively weak voltage mode signal in the prior art. The demodulator  400  can therefore develop higher gain (also known as conversion gain) than the circuits of the prior art. Such a higher gain minimizes or even eliminates the need for post amplification of the demodulator output, IF+  440 , IF−  445 . The effects of noise in the demodulator  400  and the downstream circuits (filter, amplifiers, ADC and the like) are therefore reduced. 
     Embodiments of the current circuit are also more power efficient than the circuits of the prior art because they need less bias current to remain linear and temperature stable. The impedance of the capacitors Cp 2   335 , Cp 4   325  is, according to one embodiment of the present invention, higher than the impedance seen at the emitters of the transistors QswP  410  and QswN  420 . In addition and according to another embodiment of the present invention, the impedance of capacitors Cp 2   335 , Cp 4   325  is higher that the resistance Rep  250  and Ren  255  the prior art circuit shown in  FIG. 2 . The lower bias current reduces power but also reduces noise because it reduces transistor base and collector shot noise. 
       FIG. 5  is a circuit diagram of a local oscillatory driver according to one embodiment of the present invention. The local oscillatory driver of  FIG. 5  reduces the effects of variations in the emitter voltage of QswP  410  and QswN  420 . Diodes D+  510  and D−  520  clamp the LO+  425  and LO−  430  signals to the voltage VD  530  above ground, where VD  530  is the forward biased diode voltage. The emitter voltage rises to VD  530 −VBE, where VBE is the base-emitter voltage of the transistors. Even if the oscillator output voltages, Q+ and Q−, fluctuate or ring, the diodes D+  510 , D−  520  will attenuate the variation at the LO+  425  and LO−  430  nodes. 
     The diodes D+  510 , D−  520  can, in one embodiment of the present invention, be made with diode connected NPN transistors that match QswP  410  and QswN  420 . In that case the emitter voltage will be temperature and radiation compensated to a great degree, stabilizing the voltage on the bias current source Ibias  550 . This in turn helps stabilize the bias current, particularly if Ibias  550  is made merely with a resistor. 
       FIG. 6  is a high level block diagram of an improved inductive circuit according to one embodiment of the present invention that is driven differentially, 180 degrees out of phase, and combined with a differential sensor drive. In the embodiment of the present invention shown in  FIG. 6 , the two sensors, Lsns+  320 , Lsns−  330 , are driven differentially and are 180 degrees out of phase. 
     Vdrv+  652  and Vdrv−  654  are periodic (sine wave or square wave) signals that are applied to both sensor terminals, Lsns+  320 , Lsns−  330 , through the four series impedances Zs  610 . Four capacitors Cp 5   625  couple the sensor voltages into two low-impedance inputs RF+  635  and RF−  645  to the demodulator. The voltages on the sensor terminals are labeled Vp+, Vp−, Vn+ and Vn−. At the null point, or the point at which the target  370  is equidistant from the two sensors Lsns+  320 , Lsns−  330 , the two sensors have equal impedances and the circuit operates in a symmetrical mode. Vdrv+  652  and Vdrv−  654  are 180° out of phase, so Vdrv+  652 =−Vdrv−  654 . By symmetry we see that Vp+=−Vp− and Vn+=−Vn−. Also by symmetry Vp+=−Vn+ and Vp−=−Vn− and there is no net current flowing into nodes RF+  635  and RF−  645 . As the target  370  moves away from the null point, the sensor impedances become unequal and net current flows into RF+  635  and RF−  645 . However, symmetry also reveals that the current into RF+  635  is equal and opposite that in RF−  645 . The demodulator  700  processes these currents, mixing them down to DC. 
     A demodulator circuit  700 , according to another embodiment of the present invention, for the differential driven system in  FIG. 6  is shown in  FIG. 7 . This circuit differs from the double-balanced mixer shown in  FIG. 2  in that the differencing function of the transconductor is performed by coupling capacitors, Cp 5   625  (shown in  FIG. 6 ). As can be seen in  FIG. 7 , and as compared to the demodulator of  FIG. 2 , the two switching pairs (Qsw 1   p    710 /Qsw 1   n    770  and Qsw 2   p    730 /Qsw 2   n    740 ) are coupled directly  710 ,  720  to the emitters RF+  635  and RF−  645 . LO+ and LO− are periodic (sine wave or square wave) signals that fully switch their current in these pairs. The signals from the sensors Lsns+  320 , Lsns−  330  in  FIG. 6  are injected into the emitters  710 ,  720  of the switching pairs (Qsw 1   p    710 /Qsw 1   n    770  and Qsw 2   p    730 /Qsw 2   n    740 ) at the low-impedance nodes RF+  635  and RF−  645 . The switching pairs commutate the current from the sensors, mixing the AC sensor signal down to DC at the nodes IF+  780  and IF−  785 . 
     According to one embodiment of the present invention, driving the sensors with complementary signals at Vdrv+  652  and Vdrv−  654  doubles the voltages across the sensors Lsns+  320 , Lsns−  330 . This increase in voltage decreases noise and increases immunity to radio-frequency interference (RFI). Moreover, the noise power contributed by the demodulator and downstream electronics is nominally halved. The balanced connections to both terminals of each sensor Lsns+  320 , Lsns−  330  helps reject RFI because RFI now appears as a common mode signal that can be rejected by the differential symmetry of the demodulator  700 . 
     Compared to demodulator circuit shown in  FIG. 4 , which is a version of a single-balanced mixer, the double-balanced configuration of the demodulator depicted in  FIG. 7  suppresses local oscillator (LO) feed through, making the demodulator output significantly easier to filter. 
       FIG. 8  is a demodulator circuit with a common base input stage according to one embodiment of the present invention. As shown a common base transistor Qcb  810  is coupled to the emitter of the switching pair QswP  410 , QswN  420 . 
       FIG. 9  shows a demodulator for a differentially driven circuit with a common base input stage according to one embodiment of the present invention. In the demodulator of  FIG. 9  common base transistors QcbP  910  and QcbN  920  are coupled to the emitters of the switching pairs (Qsw 1   p    710 /Qsw 1   n    770  and Qsw 2   p    730 /Qsw 2   n    740 ). 
     According to another embodiment of the present invention a demodulator or mixer circuit for use in Radio Frequency (RF) communications and inductive sensors uses current commuting to lower noise, increase reliability and provide improved versatility.  FIG. 10  is a flowchart depicting one method embodiment of signal demodulation using current commutation. In the following description, it will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by on or more equivalent circuit designs. These circuits create a means for implementing the functions specified in the flowchart block or blocks shown in  FIG. 10 . Accordingly, blocks of the flowchart illustrations support combinations of means for performing the specified functions and combinations of steps for performing the specified functions. It will also be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by special purpose hardware systems that perform the specified functions or steps, or combinations of special purpose hardware. 
     In this example the demodulation process begins  1005  with sensing  1010  a physical parameter with two magnetic sensors. As one of reasonable skill in the relevant art can appreciate the present invention can accept signals from a variety of type of sensors with equal effectiveness. The sensors are thereafter driven  1020  with an AC signal that is exactly 180 degrees out of phase. The sensors are moreover coupled to a common reference point  1030  with, in one embodiment, a passive circuit thereby automatically converting the two voltages to current  1040 . 
     A difference between the two currents is determined  1050  that is then demodulated  1060  with a frequency mixing circuit. A desired frequency range can thereafter be selected  1070  with a filter or similar circuit configuration to extract a signal suitable for the physical parameter of interest. 
     While there have been described above the principles of the present invention in conjunction with an inductive sensor circuit, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features that are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The Applicant hereby reserves the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.