Bridge sensor linearization circuit and method

Details Bridge sensor linearization circuit and method Bridge sensor linearization circuit and method

1999-01-14

2001-03-06

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Using a particular bridge circuit

C324S601000, C702S086000, C307S131000

Reexamination Certificate

active

06198296

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a circuit for precisely correcting positive and negative linearity errors of a voltage-excited bridge sensor by a technique utilizing a minimum amount of circuitry and a minimum number of external package leads.
Resistive bridge circuits, i.e., bridge sensors, have nonlinearities due to mismatches in values of the bridge circuit elements. Many bridge sensors are inherently non-linear. It is possible to compensate for such non-linearity by varying the bridge excitation voltage proportionally to the output unbalance signal of the bridge. The following equation represents the bridge excitation voltage V
EXCITE
:
V
EXCITE
=V
EXCITE(0)
±V
BROUT
×K
LIN
,  (Equation 1)
where V
BROUT
is the bridge circuit output voltage, K
LIN
is a linearization constant, and V
EXCITE(0)
is an initial value of V
EXCITE
.
The uncorrected signal results in a non-linear curve for V
BROUT
, as indicated by curve A in FIG.
5
. Curve B in
FIG. 5
represents the usually parabolic relative non-linearity of the bridge transducer that results in the nonlinear output of the bridge circuit indicated by curve A. Curve C represents the non-linearity after correction or linearization by varying the excitation voltage V
EXCITE
, and curve D represents the corrected bridge output voltage obtained as a result of correcting the excitation voltage by means of a feedback circuit coupled between the bridge output and V
EXCITE
.
A very effective technique for “linearizing” a bridge circuit is to modulate its “excitation source”, i.e., the reference voltage which is applied to the bridge circuit. U.S. Pat. Nos. 4,190,796, 4,362,060, 4,492,122, 5,122,756 and 5,764,067 are illustrative of the state of the art. The known linearization circuits generally are used in conjunction with conventional instrumentation amplifiers which provide amplified outputs to suitable utilization circuits.
The above mentioned known linearization circuits generally require four external package leads to allow a user to determine both the polarity and magnitude of linearity corrections required for each individual bridge sensor circuit. However, the user often has no way of knowing in advance whether the polarity of linearity correction needed for a particular bridge sensor circuit is positive or negative. Consequently, the user may have to swap connections between two external leads of the bridge linearization circuit to get the correct polarity of linearization correction, which is inconvenient. Furthermore, it usually is undesirable to have to use more external package leads than is genuinely necessary, and it would be better to be able to adjust the magnitude of the needed correction with one, rather than two external package leads.
Accordingly, there is an unmet need for an improved bridge linearity correction technique which requires a reduced amount of circuitry and a reduced number of external package leads for setting both the polarity and magnitude of the linearity corrections required for each different bridge sensor.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a bridge sensor linearization circuit and technique for providing a correction to the excitation voltage of a bridge sensor circuit using a reduced number of circuit components.
It is another object of the invention to provide a bridge linearization circuit and technique for providing a correction in the bridge excitation voltage of the desired polarity and magnitude requiring only two external package leads, one to select the polarity of the needed correction and the other to establish the magnitude of the needed correction.
It is another object of the invention to avoid the need to swap package lead connections to establish the correct polarity of a correction to a bridge excitation voltage produced by a linearization circuit.
It is another object of the invention to avoid the need for a user to construct “build-your-own” circuitry to obtain the needed linearization of a bridge sensor circuit.
It is another object of the invention to avoid dependance of the linearization constant K
LIN
on variations of absolute resistances of on-chip integrated circuit resistors.
Briefly described, and in accordance with one embodiment thereof, the invention provides a linearization circuit including a sensor circuit having a first terminal receiving an excitation voltage, and second and third terminals producing a sensor output voltage therebetween, a differential amplifier circuit coupled to the second and third terminals and producing a linearization current in response to the sensor output voltage, a current direction switch circuit producing a bi-directional correction current proportional to the linearization current, the current direction switch circuit having a fourth terminal receiving the linearization current, a fifth terminal conducting the correction current, and a control terminal receiving a polarity control signal to determine the direction of flow of the correction current through the fifth terminal in response to the sensor output voltage, and an amplifier circuit receiving and amplifying a reference voltage to produce the excitation voltage. The amplifier circuit includes a feedback circuit, the feedback circuit being coupled to the fifth terminal and modulating the feedback circuit in response to the correction current to cause the amplifier circuit to produce the excitation voltage equal to the reference voltage plus or minus a positive or negative correction, respectively, according to the level of the polarity control signal and according to the magnitude of the sensor output voltage.
In one embodiment, the linearization circuit includes a scaling circuit operative to produce a scaled linearization current in response to the linearization current. The linearization circuit includes a first resistor coupled to the fifth terminal to develop a voltage change on the fifth terminal proportional to the correction current, and further includes a band gap circuit producing the reference voltage. The amplifier circuit includes a differential amplifier having an output coupled to the first terminal, and a feedback resistor coupled between an inverting input of the differential amplifier and the output of the differential amplifier, the inverting input being coupled to the fifth terminal, a non-inverting input of the differential amplifier being coupled to receive the reference voltage. In one embodiment, the current direction switch circuit includes a first switch operatively connecting the fourth terminal to the fifth terminal during a first level of the polarity control signal to conduct the scaled linearization current as the correction current in a first direction through the fifth terminal. A current mirror, a second switch operatively conducts the scaled linearization current through a current mirror control transistor of the current mirror during a second level of the polarity control signal, and a current mirror output transistor of the current mirror producing a replica of the scaled linearization current as the correction current flowing in a second direction through the fifth terminal.
In one embodiment of the invention, the differential amplifier circuit includes a first operational amplifier having a non-inverting input coupled to the second terminal, an output coupled to a control terminal of a first output transistor having a first main terminal coupled to a first output conductor and a second main terminal coupled to an inverting input of the first operational amplifier. The inverting input of the first operational amplifier is coupled to a first terminal of a transconductance control resistor. A second operational amplifier includes an inverting input coupled to a second terminal of the transconductance control resistor and to a first main terminal of a second output transistor having a control terminal coupled to an output of the second operational amplifier. The second operational amplifier has a non-inverting input coupled to the third ter

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