Patent Application
20060284627
1. Field of the Invention
This disclosure generally relates to apparatus and methods for measuring and conditioning electronic circuits, and specifically to apparatus and methods for correcting electrical signals in electric circuits.
2. Related Art
Piezoresistors are typically used in a Wheatstone bridge circuit. Typically, four piezoresistors are coupled, and use either constant-voltage excitation or a constant-current excitation. As is well known, piezoresistors are resistors that vary their resistance value in response to an applied strain.
A full four wire Wheatstone bridge exhibits immunity to noise. Common-mode noise coupled into the first sense wire 106 and the second sense wire 104 can be cancelled using well-known noise cancellation techniques. In the embodiments wherein all piezoresistors 110a, 110b, 110c, and 110d have identical resistance, (i.e., 110a=110b=110c=110d=Ro), a variation in a sensed parameter, which causes a change in resistance (±ΔRo), in the piezoresistors 110a, 110b, 110c, and 110d, causes a differential voltage to appear between the first sense wire 106 and the second sense wire 104. This differential voltage is proportional to the variation in the sensed parameter. For a constant voltage excitation, the differential voltage has a value of VcΔ. For a constant current excitation, the differential voltage has a value of IcΔRo.
A differential voltage which exists in the absence of a variation in the sensed parameter is commonly referred to as an “offset” voltage. This offset is typically removed by “paralleling” selected piezoresistors with trimmable (non-piezo) resistors and then production-trimming one or more of the resistors until the offset is zeroed. Although this technique provides good correction of an initial offset, it disadvantageously introduces higher-order error terms into response signals. The differential voltage becomes VccX(Δ+higher order terms), for constant voltage excitation, and Ic(ΔRo+higher order terms), for constant current excitation. Further, this trim-resistor technique cannot be used to make corrections to offset drifts that occur during operation, nor can it be used to correct for span drift. Furthermore, this technique cannot linearize the response of the bridge.
Therefore, the need exists for an apparatus and method that corrects undesired signal characteristics in an electric circuit. The teachings provide such an apparatus.
An improved apparatus for correcting undesired signal characteristics in an electrical circuit is disclosed. In one embodiment, a multi-wire sensing bridge circuit is disclosed. In this embodiment, a first impedance element and a second impedance element are arranged to sense an incoming signal and subsequently correct for at least one of a plurality of potential undesired signal characteristics.
Embodiments of the present disclosure are more readily understood by reference to the following figures, in which like reference numbers and designations indicate like elements.
The present teachings disclose an improved method and circuit for correcting undesired characteristics in an incoming electrical signal.
An improved multi-wire sensing bridge circuit is illustrated in
In the embodiment shown in
Referring now to
The half-bridge circuits shown in
The first wire 224 has a first voltage (“V1”) associated therewith. Similarly, the second wire 222 has a second voltage (“V2”) associated therewith. In one embodiment, wherein the impedance elements 226, 228 comprise piezoresistors, the differential voltage is defined by the following equation:
V2−V1=(Ic2)*(Ro2)−(Ic1)*(Ro1); Eq. 1
Wherein Ro1 comprises a resistance value associated with the first impedance element 228 and Ro2 comprises a resistance value associated with the second impedance element 226.
If Ro1 equals Ro2, for Ic1=Ic2, the measured offset is zero.
If the sensed parameters are at their reference zero for the first impedance element 228, Ro1=Ro1(0), and second impedance element 226, Ro2=Ro2(0), are not exactly equal, but differ by an amount, “r”, such that Ro2(0)=Ro1(0)−r=Ro(0)−r. And if Ic1=Ic2=Ic, then V2−V1 will be offset from zero by an amount V2(0)−V1(0)=−Ic*r.
An offset in the sensed parameters can be corrected, straightforwardly, by adjusting Ic2(0) such that Ic2(0)=[Ro(0)/(Ro(0)−r)]*Ic1(0)=[V1(0)/V2(0)]*Ic(0).
An adjusted offset ADJ[V2(0)−V1(0)] then becomes ADJ[V2(0)−V1(0)]={[[Ic(0)]*[Ro(0)]]/[Ro(0)−r]}*[Ro(0)−r]−(Ico(0))*(Ro(0))=0.
If a variation in the sensed parameters of an incoming electrical signal causes Ro2=Ro2(0)(1+δ) and Ro1=Ro1(0)*(1−δ), then the result is:
V2−V1={[[Ic(0)]*Ro(0)]]/Ro(0)−r}*[Ro(0)−r]*[1+δ]−[Ic(0)]*R(0)*(1−δ)=2*Ic(0)*Ro(0)*δ
This expression gives a simple, linear relationship between V2−V1 and δ, with higher-order error terms (e.g., second order error terms) absent. As such, the improved multi-wire sensing bridge advantageously eliminates higher-order terms from a measurement of the sensed parameters.
If δvaries with temperature (“T”), according to a known, or detectable linear or non-linear pattern, δ(T)=(1+B(T))*δ, then the bridge can be corrected directly for span drift by modifying Ic1 and Ic2, each by a factor, (1+B(T))−1. Span drift is caused by two main factors; changes in temperature and sensor deterioration.
The foregoing description illustrates exemplary implementations, and novel features, of aspects of an apparatus for correcting undesired characteristic associated with an incoming electrical signal. Alternative implementations are suggested, but it is impractical to list all alternative implementations of the apparatus. Therefore, the scope of the presented disclosure should be determined only by reference to the appended claims, and should not be limited by features illustrated in the foregoing description except insofar as such limitation is recited in an appended claim.
While the above description has pointed out novel features of the present disclosure as applied to various embodiments, the skilled person will understand that various omissions, substitutions, permutations, and changes in the form and details of the methods and systems illustrated may be made without departing from the scope of the present teachings.
Each practical and novel combination of the elements and alternatives described hereinabove, and each practical combination of equivalents to such elements, is contemplated as an embodiment of the present teachings. Because many more element combinations are contemplated as embodiments of the present teachings than can reasonably be explicitly enumerated herein, the scope of the present teachings is properly defined by the appended claims rather than by the foregoing description. All variations coming within the meaning and range of equivalency of the various claim elements are embraced within the scope of the corresponding claim. Each claim set forth below is intended to encompass any apparatus or method that differs only insubstantially from the literal language of such claim, as long as such apparatus or method is not, in fact, an embodiment of the prior art. To this end, each described element in each claim should be construed as broadly as possible, and moreover should be understood to encompass any equivalent to such element insofar as possible without also encompassing the prior art. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising.”
