As is known, differential amplifiers tend to have an undesirable offset voltage that results in a non-zero output voltage from the differential amplifier when the differential amplifier receives a differential input signal having a value of zero.
In addition, it is known that some types of sensing elements, including, but not limited to, magnetic field sensing elements, also tend to have undesirable offset voltages that result in a non-zero output voltage from the sensing element when the sensing element experiences a sensed parameter having a value of zero, e.g., a magnetic field with a value of zero.
When a sensing element is coupled to input nodes of a differential amplifier, the resulting offset voltage of the combination can be larger than the offset voltage of the differential amplifier or of the sensing element alone.
Some techniques have been used to reduce the offset voltage of a differential amplifier and/or of a combination of a sensing element with a differential amplifier.
Referring to FIG. 1, an offset reduction circuit 10 includes four magnetoresistance elements 12, each having a resistance proportional to a magnetic field, which are arranged in a Wheatstone bride 12. The bridge 12 generates a differential voltage signal 12a, 12b coupled to differential inputs of a differential amplifier 16, and, in particular, to a differential transistor arrangement 18, 20 within the differential amplifier 16.
Symbolically, in order to represent an offset voltage adjustment by way of input adjustment, a voltage source 14 is shown to be inserted into one of the couplings of the differential voltage signal 12a, 12b as shown. The voltage source 14 can be adjusted to cause an offset voltage of the combination of the differential amplifier 16 with the magnetoresistance element bridge 12 to be at or near zero. In other words, a voltage value of the voltage source 14 can be adjusted to cause an output signal 16a generated by the differential amplifier 16 to be zero when the magnetoresistance element bridge 12 experiences a magnetic field having a value of zero.
Also shown, in place of the magnetoresistance element bridge 12, a Hall Effect element 24 coupled between a current generator 22 and a voltage reference (e.g., ground), can generate a differential signal 24a, 24b that can be coupled to the input nodes of the differential amplifier. Similarly, a voltage value of the voltage source 14 can be adjusted to cause the output signal 16a generated by the differential amplifier 16 to be zero when the Hall Element 24 experience a magnetic field having a value of zero.
Though two types of magnetic field sensing elements are shown, the voltage source adjustment of the offset voltage is applicable to any type of sensing element, magnetic or not. Furthermore, the voltage source adjustment of the offset voltage is applicable to circuits that do not use a sensing element at all.
With further regard to the magnetoresistance element bridge 12, the magnetoresistance elements can be positioned in a magnetic field so that a pair of diagonally opposed magnetoresistance elements senses a field +B while the other pair of magnetoresistance elements senses an inverse field −B. This produces corresponding changes in a typical giant magnetoresistance (GMR) element resistance (+/−ΔRB) and generates a differential signal, VBRIDGE, 12a, 12b at the bridge output terminals. The bridge output voltage is subsequently processed by a differential amplifier, often using a bipolar junction transistor (BJT) differential pair 18, 20 as shown.
GMR device mismatch due to fabrication variations produces GMR resistance variation (ΔRMMn) that remains when no magnetic field is applied. This leads to significant bridge output offset, which is defined herein as the bridge output voltage with a zero value input magnetic field. Offset voltage of the GMR elements tends to degrade accuracy performance a magnetic field sensor that uses GMR magnetoresistance elements.
Offset in GMR bridges is problematically large. When experiencing substantial magnetic fields, the resistance of a GMR device changes only about 5% before saturation; and a usable linear range is even smaller in many applications. Nominal resistance values of typical GMR elements match to 0.1 to 1% accuracy, depending on device size and fabrication technology. Thus, bridge offset can be large compared to usable signal range, up to (1%/5%)=20% of the signal range for these values. For very small magnetic fields generated, for example, in a highly accurate GMR current sensor, offset voltage can be larger than a detected signal.
Trimming, e.g., with the voltage source 14, is a conventional method used to remove or reduce bridge offset voltage. A digital-to-analog converter (DAC) (not shown) can be used to adjust the voltage source 14. In production testing, to accomplish the trimming, a zero magnetic field can be applied to the magnetic field sensor that has the magnetoresistance element bridge 12 and DAC input codes can be searched to find a code that generates the sensor output voltage 16a closest to zero.
Referring now to FIG. 2, an electronic circuit 70 shows a particular differential offset adjustment arrangement. The electronic circuit 70 includes a differential operational amplifier 72. Resistors R1, R2, R3, R4 are coupled around the operational amplifier 72 to provide a differential amplifying circuit 80, which is coupled to receive a differential input signal in+, in−, and which is configured to generate a differential output signal out+, out−. Offset adjustment can be implemented by injecting adjustment currents into input nodes 74, 76 of the differential amplifier 72.
It will be understood that input nodes 74, 76 of the differential amplifier 72 within the differential amplifying circuit 80 are so-called “virtual ground” nodes, i.e., nodes that have very low input impedances by virtue of feedback. The nodes 74, 76 are known to act as so-called “summing nodes.” Equal currents injected into the nodes 74, 76 (with equal impedances) will have no effect upon output offset voltage. However, unequal currents injected into the nodes 74, 76 can be used to adjust output offset voltage.
Two cross-coupled R-2R digital-to-analog converters 78 (DACs) can be coupled to the input nodes 74, 76, and can provide an offset adjustment to the differential amplifying circuit 80 by injecting unequal currents into these nodes. The DACs 78 are shown here to be a switched resistor type of DACs.
The two DACs 78 can be coupled to receive two respective reference currents, I. Via cross-coupled switches, an expanded view of which is shown as element 82, currents can be injected from the two DACs 78 into the nodes 74, 76. The relative current received by each one of the input nodes 74, 76 is controlled by switching positions of the cross-coupled switches, which are, in turn, controlled by control bits b0 to bN-1.
It is known that R-2R DACs coupled as shown to the nodes 74, 76 can generate a binary-weighted offset adjustment, but without a typical DAC drawback of incurring exponential growth in area for each added bit of resolution.
The offset adjustment circuit of FIG. 2 requires the sensor signal, in+, in−, to pass through the same summing junctions 74, 76 as the offset adjustment signal generated by the DACs 78. These summing junctions 74, 76 perform the addition function described above in conjunction with FIG. 1, shown as a VTRIM voltage source 14 in FIG. 1. However, it may be undesirable to process the differential sensor signal, in+, in−, in this way. For example, the differential sensor signal, in+, in−, must drive input resistors R1, Depending on the desired offset adjustment range, resistors R1 may be small, presenting a low input resistance to the differential sensor signal, in+, in−. In particular, the resistor bridge of FIG. 1 is generally unable to drive a small input resistance, thus an additional buffer or amplifier stage may be required between the resistor bridge and the electronic circuit 70. An additional amplifier or buffer stage would negatively impact critical system parameters such as noise, offset, and bandwidth.
It would be desirable to provide a circuit for adjusting an offset voltage of a differential amplifier, or of a combination of a differential amplifier coupled to a magnetic field sensing element, for example, a GMR element or GMR bridge, but without adding extra amplifier stages in the magnetic field sensing element signal path.