Reference generator for multilevel nonlinear resistivity memory storage elements

[0001] 1. Field of the Invention

[0002] This invention relates generally to electrical circuits for the generation of reference currents and/or voltages. More particularly, this invention relates to electrical circuits for the generation of multiple reference currents and/or voltages for non-linear resistive elements such as magnetic tunneling junctions (MTJ). Further, this invention relates to electrical circuits for the generation of multiple reference currents and/or voltages to provide the reference levels for the sensing of multiple bits of digital data stored within a Magnetic Random Access Memory (MRAM) cell.

[0003] 2. Description of Related Art

[0004] Magneto-electronic memories are emerging as important memory technologies. Presently, there are three types of magnetic memory devices that are grouped according to the physics of their operation. These categories are a hybrid ferromagnetic semiconductor structure, a metal spin transistor or spin valve, and a magnetic tunnel junction (MTJ). Writing to memories of each of the devices is essentially the same because the direction of a magnetic field determines the state of the digital data stored in a memory cell. However, reading each of the devices is different. The ferromagnetic semiconductor device essentially employs a Hall Effect to determine the state of the digital data stored in the memory cell. The metal spin transistor and the magnetic tunnel junction each employ a change in magneto-resistance to determine a state of the digital data stored in the memory cell. In a metal spin transistor, the relative resistance difference is approximately from 6% to 8%. The discrimination of the state of the digital data is difficult with the metal spin transistor. However, the MTJ has a resistance difference approximately 12%.

[0005] As shown in FIG. 1, a memory array 5 is generally formed of groups of MTJ cells 10 in columns and rows. Each MTJ cell 10 has an MTJ device 15 for retaining digital data as an orientation of the magnetic fields within the MTJ device 15. A sense voltage V_cell 20 is applied to a selected MTJ cell 15. The application of the sense voltage V_cell 20 causes a cell current I_cell 25 to flow through the MTJ device 15. The magnitude of the cell current I_cell 25 is dependent on the resistance of the MTJ device 15, which is dependent on the orientation of the magnetic fields of the MTJ device 15.

[0006] The cell current I_cell 25 flows through the cell load resistor 30 to develop the sense voltage V_sense 80, which is the input to the comparator 55. The sense voltage V_sense 80 is compared within the comparator 55 with the reference voltage V_REF 75 to determine the state of the digital data retained within the MTJ device 15. Ideally, the voltage V_REF 75 has a magnitude that is approximately the average of the sense voltage V_sense 80 with the MTJ device 15 at its maximum resistance (R_max) and the sense voltage V_sense 80 with the MTJ device 15 at its minimum resistance (R_min). The average of the sense voltage V_sense 80 with the MTJ device 15 at its maximum resistance (R_max) and the sense voltage V_sens 80 with the MTJ device 15 at its minimum resistance (R_min) is dependent upon the expression:
V_sense∝½(R_min+R_max)

[0007] A reference current source 35 generates the reference current I_REF 45 as function of the a biasing voltage 40 that is approximately twice the magnitude of the sense voltage V_cell 20 applied across two pair of serially connected reference resistance devices 36, 37, 38, and 39. The reference resistance devices 36, 37, 38, and 39 are MTJ devices that have their magnetic orientations set such that the reference resistance devices 36 and 38 have their resistance set to the minimum resistance values and the reference resistance devices 37 and 39 set to their maximum resistance values. It can be shown that the reference current I_REF 45 becomes the current generated by sense voltage V_cell 20 applied to a resistor that has an average value of the maximum resistance and the minimum resistance of the MTJ device 15.

[0008] However, it is apparent that the voltage V2 across the reference resistance devices 37 and 39 are greater than the voltage V1 across the reference resistance devices 36 and 38. The resistance of the MTJ devices 36, 37, 38, and 39 is not linear, as is shown in FIG. 2. FIG. 2 shows the biasing voltage dependence of the magneto-resistance (MR) ratio. The MR ratio is defined as the difference in resistance between the two states divided by the resistance in the low state expressed as a percentage. As shown, the voltage dependence indicates that the reference MTJ devices 36 and 38 are biased at the voltage level V1 65 and the reference MTJ devices 37 and 39 are biased at the voltage level V2 70 and that they are not really equal to the sense voltage V_cell 20. The reference MTJ devices 36, 37, 38, and 39 do not really have their maximum and minimum resistances equal to the values of the respective maximum and minimum resistance of the MTJ device 15. The equivalent resistance of the reference resistance devices 36, 37, 38, and 39 as configured is not equal to ½(R_min+R_max). This causes the reference current I_REF 45 to be unequal to the average between the cell current I_cell 25 with the MTJ device 15 at a maximum resistance (R_max) and the cell current I_cell 25 with the MTJ device 15 at a minimum resistance (R_min).

[0009] “Demonstration of a Four State Sensing Scheme for a Single Pseudo-Spin Valve GMR Bit”, Zhang et al., IEEE Transactions on Magnetics, Volume: 35, Issue: 5, September 1999, describes a simple and fast method for sensing four states from a single Pseudo-Spin Valve GMR device.

[0010] “Windowed MRAM Sensing Scheme”, Zhang, et al., “Memory Technology, Records of the 2000 IEEE International Workshop on Design and Testing,” August 2000, pp.: 47–55 details a method for allowing bits with unstable domains to be detected during reading. The method also allows for an improved sensing error rate by identifying read cycles with inadequate signal size caused for example by external noise.

[0011] “Novel Sensing Scheme for an MRAM with a 5% MR ratio”, Yamada, et al., “Digest of Technical Papers. 2001 Symposium on VLSI Circuits,” June, 2001, pp.: 123–124 provides a novel sensing scheme for a magneto-resistive random access memory (MRAM) with a twin cell structure. It operates by sensing the difference in voltage between a couple of magnetic tunnel junctions (MTJ) in a transitional state.

[0012] “Fully Integrated 64 Kb MRAM with Novel Reference Cell Scheme”, Jeong, et al., “Digest. International Electron Devices Meeting—IEDM '02”, December, 2002, pp.: 551–554, employs a new sensing scheme with a separated half-current source. The separated half current source is adopted for the reference bit line to increase the sensing signal.

[0013] U.S. Pat. No. 6,317,376 (Tran, et al.) and U.S. patent application 2001/0053104 (Tran, et al.) describe a Magnetic Random Access Memory (“MRAM”) device. The MRAM device includes an array of memory cells and generates reference signals that can be used to determine the resistance states of each memory cell in the array, despite variations in resistance due to manufacturing tolerances and other factors such as temperature gradients across the array, electromagnetic interference and aging.

[0014] U.S. Pat. No. 6,055,178 (Naji) teaches an MRAM device that includes a memory array and a reference memory array. Each reference memory cell has a magnetic memory cell and a transistor, that is coupled in series and has a reference resistance across the reference memory cell and the transistor. The transistor is controlled by a reference row line control, so as for the reference resistance to show a mid-value between the maximum resistance and the minimum resistance of the magnetic memory cell. A bit line current (Ib) and a reference bit current (Ir) are provided to the magnetic memory cell and the reference memory cell, respectively. Magnetic states alternate the bit line current, which is compared to the reference bit current to provide an output.

[0015] U.S. Pat. No. 6,169,689 (Naji) illustrates an MTJ stacked cell memory sensing method and apparatus. The memory array consists of stacks of cells in an addressable array with each stack including MTJ memory cells stacked together with current terminals connected in series, and including a first and second current terminals coupled through an electronic switch to a current source. Each stack includes 2ⁿ levels of memory. A voltage drop across an addressed stack is sensed. Reference voltages equal to the 2ⁿ memory levels are provided and the sensed voltage drop is compared to the reference voltages to determine the memory level in the addressed stack. Encoding apparatus is used to convert the voltage drop to a digital output signal.

[0016] U.S. Pat. No. 6,385,109 (Naji) and U.S. Pat. No. 6,496,436 (Naji) detail readout circuitry for a magnetic tunneling junction (MTJ) memory cell, or an array of MTJ memory cells, which require a varying reference voltage equal to (V_{bias 1/2}) (1+R_min/R_max). The V_bias1 is a clamping voltage applied to the readout circuitry, R_min is a minimum resistance of the magnetic tunneling junction memory cell, and R_max is a maximum resistance of the magnetic tunneling junction memory cell. A reference voltage generator generates the reference voltage and includes an operational amplifier and two MTJ memory cells connected to provide an output signal equal to (V_bias1/2) (1+R_min/R_max).

[0017] U.S. Pat. No. 6,426,907 (Hoenigschmid) describes a reference circuit for an MRAM array, including logic “1” reference MRAM cells and coupled in parallel with logic “0” reference MRAM cells. The reference current is coupled to a measurement resistor of a sense amplifier, which is adapted to determine the logic state of an unknown memory cell.

[0018] U.S. Pat. No. 6,445,612 (Naji) specifies an MRAM with midpoint generator reference and method for readout. The MRAM includes a data column of memory cells and a reference column, including a midpoint generator, positioned adjacent the data column on a substrate. The memory cells and the midpoint generator include similar magneto-resistive memory elements, e.g. MTJ elements. The MTJ elements of the generator are each set to one of R_max and R_min and connected together to provide a total resistance of a midpoint between R_max and R_min. A differential read-out circuit is coupled to the data column and to the reference column for differentially comparing a data voltage to a reference voltage.

[0019] An object of this invention is to provide a reference generator with multiple reference levels.

[0020] Another object of this invention is to provide a reference generator that proportionally tracks nonlinear resistive elements.

[0021] To achieve at least one of these objects, a multilevel reference generator has a plurality of standard resistive elements such as multilevel magnetic tunnel junctions set to differing parallel and anti-parallel magnetic orientations. Each resistive element is biased at a constant level to impart a resultant level from each resistive element. If the constant level is a constant voltage, the resultant levels of the plurality of resistive elements and the mirrored replications are currents. Alternately, the constant level is a constant current and the resultant levels of the plurality of resistive elements and the mirrored replications are voltages.

[0022] The multilevel reference generator has a plurality of mirror sources. Each mirror source is in communication with the one of the plurality of resistive elements such that each mirror source receives the resultant level from the one standard resistive element and provides a mirrored replication of the resultant level.

[0023] The multilevel reference generator has a plurality of reference level combining circuits. Each reference level combining circuit is connected to receive a first mirrored replication of one resultant level from one mirror source and a second mirrored replication of the resultant level from a second mirror source. From a combination of the one mirrored replication from the one mirror source and the second mirrored replication from the second mirror source each reference level combining circuit creates one of the reference levels.

[0024] The reference level combining circuit includes a summing circuit to additively combine the first and second mirrored replication and a scaling circuit to create a scaling of the combined first and second mirrored replications. This creates a reference level. The reference level maybe a reference current. In the alternative, the reference level is a voltage developed across a reference resistor with the reference current flowing through the reference resistor. Therefore, the reference generator may include a plurality of reference resistors. Each reference resistor associated with one of the plurality of reference level combining circuits to receive the reference current for creation of the voltage that is the reference level.

[0035] Referring to FIG. 3, a multilevel MTJ cell 105 in an MRAM memory array 100 consists of two MTJ devices 110 and 115 and a gating transistor 120. The bit line 125 provides a biasing voltage to generate a read current I_RD 145 through the two MTJ devices 110 and 115. The word line 130 provides the control signal to activate the gating transistor 120 during either the programming (writing) or reading of the multilevel MTJ cell 105. The primary program line 135 is connected to the source of the gating transistor 120. The second program line 140 is connected to the MTJ device 110. The third program line 142 is connected to the MTJ device 115. The second program line 140 and the third program line 142 provide the programming currents necessary to determine the orientation of the magnetic fields of the two MTJ devices 110 and 115. The primary program line 135 provides the necessary source/return path for these programming currents.

[0036] The multilevel memory cell 105, when programmed, contains two bits of digital data or four different states. The MTJ device 110 is sized to have differing maximum and minimum resistivity than that of the MTJ device 115. Thus, if the MTJ device 110 and the MTJ device 115 are both programmed to have parallel orientations, the multilevel MTJ cell 105 has a cell resistance state that is the minimum total resistance. Alternately, if the MTJ device 110 and the MTJ device 115 are both programmed to have anti-parallel orientations, the multilevel MTJ cell 105 has a cell resistance state that is the maximum total resistance. If the MTJ device 110 and the MTJ device 115 are both programmed to have opposite (one parallel and one anti-parallel) orientations, the multilevel MTJ cell 105 has two resistance states that are between the maximum and minimum resistance.

[0037] For a read operation, the bit line 125 is set to a constant biasing voltage for generation of the read current I_RD 145. The primary program line 135 is connected to provide a path for the flow of the read current I_RD 145 for sensing. During the read operation, the word line is activated to turn on the gating transistor 120 to connect the two MTJ devices 110 and 115 to the primary program line 135.

[0038] The read current I_RD 145 is transferred to a sense amplifier that must now distinguish between the four different levels of read current I_RD 145. A multilevel reference source must provide separate references that are ideally midway between the levels of current of the read current I_RD 145 so as to distinguish the four levels of the read current I_RD 145. Referring now to FIG. 4, the representation of the four cell resistance states will generate the four read currents I_RD1 150a, I_RD2 150b, I_RD3 150c, and I_RD4 150d. The multilevel reference source must provide reference currents I_REF1 155a, I_REF2 155b, and I_REF3 155c that have magnitudes that are midway between the magnitudes of the four read currents I_RD1 150a, I_RD2 150b, I_RD3 150c, and I_RD4 150d.

[0039] The fundamental circuits that form the multilevel reference generator of this invention are shown in FIGS. 5 and 6. The multilevel reference generator has a nonlinear resistive element 200 for each of the reference levels provided by the multilevel reference generator. The nonlinear resistive element 200 is biased to a constant voltage level V_A 220 to generate the current I_n 235. The nonlinear resistive element 200 in the preferred embodiment consists of two MTJ devices 205 and 210. The two MTJ devices 205 and 210 are connected in parallel and attached between the constant voltage level V_A 220 and the substrate biasing voltage Vss. The two MTJ devices 205 and 210 are programmed to have their orientation of the magnetic fields as shown in FIGS. 10a–10d. The orientation of the magnetic fields of the two MTJ devices 205 and 210 determine the total resistance of the resistance of the nonlinear resistive element 200. In FIG. 10a, the orientation of the magnetic fields is set to be anti-parallel and the resistance of the nonlinear resistive element 200 is set to the maximum resistance and the current I_n 235 is at its minimum or equivalent to the read current I_RD4 150d of FIG. 4. In FIG. 10b, the orientation of the magnetic field of the MTJ device 205 is set to be anti-parallel such that its resistance is at the maximum and the MTJ device 210 is set to be parallel such that its resistance is at the minimum. This makes the resistance of the nonlinear resistive element 200 such that the current I_n 235 is at an intermediate level that is equivalent to the read current I_RD3 150c of FIG. 4. In FIG. 10c, the orientation of the magnetic field of the MTJ device 205 is set to be parallel such that its resistance is at the minimum and the MTJ device 210 is set to be anti-parallel such that its resistance is at the maximum. This makes the resistance of the nonlinear resistive element 200 such that the current I_n 235 is at an intermediate level that is equivalent to the read current I_RD2 150b of FIG. 4. In FIG. 10d, the orientation of the magnetic fields is set to be parallel and the resistance of the nonlinear resistive element 200 is set to it minimum resistance and the current I_n 235 is at its maximum or equivalent to the read current I_RD1 150a of FIG. 4.

[0040] Returning to FIG. 5, the source of the biasing transistor 225 is connected to establish the constant voltage level V_A 220. The gate of the biasing transistor 225 is connected to a biasing voltage source V_BIAS 230, thus establishing the constant voltage level V_A 220 as the voltage level of the biasing voltage source V_BIAS 230 plus the threshold voltage (Vt) of the biasing transistor 225. The current I_n 235 is then determined by the formula: $l_n=\frac{\left(V_A\right)}{\left(R_{\mathrm{MTJ1}}{R}_{\mathrm{MTJ2}}\right)/\left(R_{\mathrm{MTJ1}}+R_{\mathrm{MTJ2}}\right)}$

[0041] The MOS transistors M3 240 and M4 245 are connected as a current mirror source. The gates of the MOS transistors M3 240 and M4 245 are connected together and to the drain of the biasing transistor 225 such that the current I_n 235 flows through the MOS transistor M3 240. The mirror current I_n MIRROR 250 is source from the source of the MOS transistor M4 245.

[0042] The multilevel reference generator of this invention will have a resistive element as will the current mirror formed by the MOS transistors M3 240 and M4 245 for each of the four read currents I_RD1 150a, I_RD2 150b, I_RD3 150c, and I_RD4 150d of FIG. 4. The structure of the nonlinear resistive element 200 is equivalent to that of the multilevel MTJ cell 105 of FIG. 3. The equivalent structure and the constant voltage level V_A 220 ensure that the mirror currents I_n 235 are approximately equal to the read currents I_RD 145.

[0043] To generate the reference currents I_REF1 155a, I_REF2 155b, and I_REF3 155c of FIG. 4, the multilevel reference generator combines two of the mirrored current and effectively averages the combined sum to form one of the reference currents. Referring to FIG. 6, the drain of the MOS transistors M5 260 is connected to receive two of the mirrored currents I_n 250 and I_n+1 255. The MOS transistors M5 260 and M6 265 are connected as a current source. In the preferred embodiment the MOS transistors M5 260 and M6 265 are scaled in size such that the reference current I_REF—n 270 is determined by the formula: $l_{\mathrm{REF\_n}}=\frac{\left(l_n+l_{n+1}\right)}{\mathrm{sf}}$

• • [0044] where:
  • [0045] sf is a scaling factor that in the preferred embodiment is two (2) to achieve the midpoint between the two mirrored currents I_n 250 and I_n+1 255.

[0046] The reference current I_REF—n 270 may be applied directly to the sense amplifier as a reference current. Alternately, as shown in FIG. 7, the reference current I_REF—n 270 of the reference current generator 275 may be applied within the multilevel reference generator of this invention to a reference resistor 280. The voltage across the reference resistor 280 then becomes the reference voltage V_REF—n.

[0047] FIG. 8 illustrates the whole reference generation sub-circuit 300 necessary to provide one of the reference levels of a multilevel reference generation circuit of this invention. The reference generation sub-circuit 300 as shown creates the first reference voltage V_REF—1 380 that is developed from the reference current I_REF—1 370. The reference generation sub-circuit 300 includes a first nonlinear resistive element 305. The first nonlinear resistive element is formed by combining two MTJ devices 307 and 309 connected in parallel. The two MTJ devices 307 and 309 have their magnetic fields set to be parallel. This makes the resistance level of the two MTJ devices 307 and 309 at their minimum values and any current through them at the maximum value when they are biased at a constant voltage level V_A 315.

[0048] The biasing transistor 325 has its gates set to a biasing voltage V_BIAS to set the constant voltage level V_A 315 at the constant level of V_BIAS plus the threshold voltage level V_T of the biasing transistor 325. The resultant current I₁ 320 flowing through the two MTJ devices 307 and 309 is approximately equivalent to the first read current level I_RD1 of FIG. 4.

[0049] A first mirror source 330 is connected to provide the resultant current I₁ 320 and a first mirrored replication current I₁MIRROR 335 of the resultant current I₁ 320. The first mirror source is formed by connecting the MOS transistors 332 and 334 such that their gates are connected together and to the drain of the biasing transistor 325 and the drain of the MOS transistor 332. The first mirrored replication current I₁MIRROR 335 flows from the drain of the MOS transistor 334.

[0050] The reference generation sub-circuit 300, further, includes a second nonlinear resistive element 310. The second nonlinear resistive element 310 is formed of two MTJ devices 312 and 314 connected in parallel. The two MTJ devices 312 and 314 have their magnetic fields set to be such that the MTJ device 312 is parallel and the MTJ device 314 is anti-parallel. This makes the resistance level of the MTJ device 312 at its minimum value and the resistance level of the MTJ device 314 at its maximum value and any current through them at an intermediate value when they are biased at a constant voltage level V_A 315.

[0051] The biasing transistor 350 has its gates set to a biasing voltage V_BIAS to set the constant voltage level V_A 340 at the constant level of V_BIAS plus the threshold voltage level V_T of the biasing transistor 350. The resultant current I₂ 345 flowing through the two MTJ devices 312 and 314 is approximately equivalent to the second read current level I_RD2 of FIG. 4.

[0052] A second mirror source 355 is connected to provide the resultant current I₂ 345 and a second mirrored replication current I₂MIRROR 360 of the second resultant current I₂ 345. The second mirror source 355 is formed of the MOS transistors 357 and 359 having their gates connected together and to the drain of the biasing transistor 350 and the drain of the MOS transistor 357. The second mirrored replication current I₂MIRROR 360 flows from the drain of the MOS transistor 359.

[0053] A reference level combining circuit 365 is connected to receive the first mirrored replication current I₁MIRROR 335 from the first mirror source 330 and the second mirrored replication current I₂MIRROR 360 from the second mirror source 355. A combination current of the first mirrored replication current I₁MIRROR 335 and the second mirrored replication current I₂MIRROR 360 creates the reference current I_REF—1 370. The reference level combining circuit 365 is formed by the MOS transistors M5 367 and M6 369 which form a current source. The MOS transistors M5 367 and M6 369 are scaled such that the reference current I_REF—1 370 is at the midpoint of the level of the first mirrored replication current I₁MIRROR 335 and the level of the second mirrored replication current I₂MIRROR 360 as described above. In the preferred embodiment, the scaling of the MOS transistors M5 367 and M6 369 is a factor of two to provide the averaging of the first mirrored replication current I₁MIRROR 335 and the second mirrored replication current I₂MIRROR 360.

[0054] If the reference level of the multilevel generator is to be the reference voltage V_REF—1 380, then the resistor R₁ 375 is provided such that the reference current reference current I_REF—1 370 flows through the resistor R₁ 375 to develop the reference voltage V_REF—1 380.

[0055] Refer now to FIG. 9 for a discussion of the structure and operation of an MRAM utilizing a reference generator of this invention. The MRAM contains a memory array 100 having rows and columns of multilevel MTJ cells 105 as is described in FIG. 3. Bit lines 125 of columns of the multilevel MTJ cells 105 are connected to the biasing transistor such that the voltage V_A 447 is a constant voltage equal to the constant level of V_BIAS plus the threshold voltage level V_T of the biasing transistor 435. The voltage V_BIAS is the same biasing voltage that establishes the constant voltage V_A 220 of FIG. 5. The word line 130 is set to turn on the transistor 120. The primary program 135 is connected such that it is set to the substrate voltage source V_ss. The cell current I_CELL 445 flows through the MTJ devices 110 and 115 to primary program line 135. The magnitude of the cell current I_CELL 445 is determined by the magnetic orientation of the MTJ devices 110 and 115 and is equal to the four read currents I_RD1 150a, I_RD2 150b, I_RD3 150c, and I_RD4 150d of FIG. 4. The cell current I_CELL 445 flows through the load resistor R_L 440 to develop the cell voltage V_CELL 442. The cell voltage V_CELL 442 is applied to the comparators 450, 455, and 460. The cell voltage V_CELL 442 is compared in the comparators 450, 455, and 460 to the reference voltages V_REF—1 465, V_REF—2 470, and V_REF—3 475 to derive the digital data contents of the memory cell 105. The outputs of the comparators 450, 455, and 460 provide a barometer code that is interpreted by the barometer coder 480 to develop the digital data bits b0 485 and b1 490.

[0056] The reference generator 400 is connected to provide the reference voltages V_{REF—1 465, VREF—2} 470, and V_REF—3 475 to the comparators 450, 455, and 460. The reference voltages V_REF—1 465, V_REF—2 470, and V_REF—3 475 are produced by the reference generator 400 from the reference current generators I_REF—1 405, I_REF—2 415, and I_REF—3 425. The reference currents I_REF—1, I_REF—2, and I_REF—3 respectively flow through the load resistors R_1—1 410, R_1—2 420, and R_1—3 430 to generate the reference voltages V_REF—1 465, V_REF—2 470, V_REF—3 475. The reference current generators I_REF—1 405, I_REF—2 415, and I_REF—3 425 reference current generators I_REF—1 405, I_REF—2 415, and I_REF3 425 are structured and operate as described for the reference generator 300 of FIG. 8.

[0057] In summary the process for generating multiple reference levels of this invention begins with providing nonlinear resistive elements such as magnetic tunneling junctions to act as reference elements for the generation of the reference levels. Each resistive element is biased at a constant voltage or current level to impart a resultant current or voltage level from each resistive element. This resultant current or voltage level has a resistance that is a nonlinear function of the applied voltage or current. With the biased voltage as applied to each resistive element being equal to a sensing voltage for determining the stored contents of a memory employing similar resistive elements for the storage elements.

[0058] The resultant current or voltage level developed from each nonlinear resistive element is replicated to provide a mirrored replication current or voltage of the resultant current or voltage level from each resistive element. Two of the mirrored replication currents or voltages are effectively combined in a fashion to create each of the multiple reference levels. Normally the combination of the mirrored replication current or voltage levels set the reference level as a midpoint between the two mirrored replication current or voltage levels.

[0059] As described for the preferred embodiment, the constant level is a voltage across the resistive element (two MTJ's having particular magnetic orientations). The resultant level is a current from the constant voltage across the resistive element. The resultant current level is then mirrored. Two of the mirror current levels from two resistive elements (two set of two MTJ's, each having differing orientations) are added. The resulting sum current is the then scaled to form the reference current level.

[0060] If the desired reference level is a voltage, the reference current flows through a provided reference resistor. The reference current flowing through the reference resistor develops the reference voltage.

[0061] While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.