Abstract:
Structures and methods are disclosed for parallel biasing and testing, at dc and high frequency, of two or more read heads within a two dimensional magnetic recording (TDMR) slider. Testing of heads comprises both conventional tests of the individual heads within the slider, as well as tests for interactions between the heads which are very closely spaced within the slider and thus may exhibit various magnetic, capacitive, ohmic, and stress-related interactions not seen in non-TDMR heads having only a single read head. Overall testing times are nearly the same as for single head testing.

Description:
TECHNICAL FIELD 
     The present invention relates to structures and methods for testing read heads for use in magnetic data storage systems such as hard disk drives or tape drives, and in particular to structures and methods for testing multiple read heads within sliders configured for two dimensional magnetic recording. 
     BACKGROUND 
     Data storage devices employ rotating data storage media such as hard disk drives or moving magnetic tape. In a hard drive, for example, data is written to the disk medium using a write head which generates a high localized magnetic field which aligns magnetic domains within the disk in one of two directions. In some cases, the magnetization direction is up or down relative to the plane of the disk (perpendicular magnetic recording, or PMR). In other cases, the magnetization direction is within the plane of the disk. In all cases, this data may then be read-out with a read head. The write and read heads are typically integrated within a single assembly. To achieve steadily increasing data storage densities (typically measured in bits/inch 2 ), which are now achieving levels near or beyond 10 12  bits/in 2  (1 Tb/in 2 ), larger numbers of tracks are being written on each disk. Since disk diameters have remained relatively unchanged, this increase in the number of tracks has necessitated the use of narrower tracks, spaced more closely together. In the past, the read heads used to read data from these tracks were typically narrower than the track width so it was practical to achieve good signal-to-noise ratios (SNRs) using a single head to read the data from each track. 
     However, track widths are now becoming smaller than the widths of practical magnetic read heads (which are fabricated using methods similar to those in semiconductor manufacturing), with the result that a single read head may pick up increasing amounts of inter-track noise (i.e., the head senses data written on the two neighboring tracks to the track which the head is supposed to be reading). A technology called “Two Dimensional Magnetic Recording” (TDMR) is being applied to address this problem through the use of multiple read heads integrated within a single slider assembly. Another term for TDMR is Multiple-Input/Multiple-Output (MIMO) recording. A slider assembly may typically comprise one or more (for TDMR) read heads as well as a write coil, magnetic pole pieces, and in some embodiments thermal fly height control heaters, and optical waveguides or microwave sources. 
     The predominant sources of noise in the signal were found to be from adjacent track noise and track edge curvature distortion arising from fringing of the write head and from the fact that the slider does not move in a linear radial motion, but rather along an arc. In a TDMR slider, multiple read heads may be configured in various arrangements, either along-track one in front of the other, or side-to-side in a direction perpendicular to the track, or in some other arrangement—details of the configuration of the plurality of read heads within the TDMR slider are not part of the present invention. 
     With multiple read heads per slider, testing requirements during data storage system manufacturing become more complex. The difficult economics for the data storage device industry, however, require that testing times cannot substantially increase while still maintaining acceptable manufacturing costs. Thus it would be advantageous in a read head testing system to test TDMR sliders in approximately the same time as non-TDMR sliders are tested. 
     It would also be advantageous in a TDMR slider testing system to test multiple read heads simultaneously, thereby enabling the testing time for each read head within a TDMR slider to remain approximately the same as the testing time for the single read head in a non-TDMR slider. 
     It would be further advantageous in a TDMR read head testing system to measure any additional noise sources or read-signal coupling between read heads integrated within single sliders, and to perform this additional testing function with minimal increase in the overall testing time per slider. 
     It would also be advantageous to employ testing structures and methods for multiple read heads which are modifications of existing testing structures and methods employed for testing non-TDMR sliders, thereby minimizing the efforts required to implement TDMR slider testing in manufacturing. 
     It would also be advantageous to be able to independently set the bias conditions for each head being tested. 
     It would be further advantageous to configure the testing structures and methods for TDMR sliders to be compatible with also testing non-TDMR sliders, thereby avoiding the need for dedicated TDMR and dedicated non-TDMR slider testing systems within manufacturing. 
     Additionally, it would be advantageous to have the test structure be able to test sliders with either common lead or separate lead connections. 
     It would be advantageous to configure a testing system to be compatible with testing multiple read heads in non-TDMR sliders within a single row bar, thereby decreasing overall read head testing times and improving manufacturing efficiencies. 
     SUMMARY 
     Embodiments of the present invention provide structures and methods for testing multiple read heads within TDMR sliders simultaneously. This testing comprises determining the required bias currents/voltages for the individual read heads within single sliders, and other noise and performance characterization steps. The various measurements and performance characterizations of the individual read heads within a TDMR slider are familiar to those skilled in the art and are not part of the present invention. Due to possible inter-head coupling (magnetic, capacitive, ohmic, mechanical stresses, etc.) between the pluralities of read heads within each TDMR slider, additional measurements and performance characterizations are required for TDMR heads which are not required for non-TDMR sliders—these additional measurements and performance characterizations are part of embodiments of the present invention. 
     A goal of some embodiments is to test TDMR sliders in approximately the same time as non-TDMR sliders (with a single read head) are tested. 
     A goal of some embodiments is to test multiple read heads simultaneously, thereby enabling the testing time for each read head within a TDMR slider to remain approximately the same as the testing time for the single read head in a non-TDMR slider. 
     A goal of some embodiments is to measure any additional noise sources or read-signal coupling between read heads integrated within single sliders, and to perform this additional testing function with minimal increase in the overall testing time per slider. 
     A goal of some embodiments is to employ testing structures and methods for multiple read heads which are modifications of existing testing structures and methods employed for testing non-TDMR sliders, thereby minimizing the efforts required to implement TDMR slider testing in manufacturing. 
     A goal of some embodiments is to configure the testing structures and methods for TDMR sliders to be compatible with also testing non-TDMR sliders, thereby avoiding the need for dedicated TDMR and dedicated non-TDMR slider testing systems within manufacturing. 
     A goal of some embodiments is to be able to independently set the bias conditions for each head being tested. 
     A goal of some embodiments is to have the test structure be able to test sliders with either common lead or separate lead connections. 
     A goal of some embodiments is to have the testing system be able to test multiple read heads in non-TDMR sliders within a single row bar. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a testing configuration for TDMR read heads; 
         FIG. 2  is an electrical model for the testing configuration in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a circuit for parallel testing of multiple read heads in a TDMR slider in a common lead configuration during DAC set-up; 
         FIG. 4A  is the circuit from  FIG. 3  during setting of the bias and dc testing on a single read head in a slider; 
         FIG. 4B  is the circuit from  FIG. 3  during noise testing of a single read head in a slider; 
         FIG. 5A  is the circuit from  FIG. 3  during parallel setting of the bias and dc testing of four read heads in a slider; 
         FIG. 5B  is the circuit from  FIG. 3  during parallel noise testing of four read heads in a slider; 
         FIG. 6  is a flow chart of an algorithm for setting the bias on multiple read heads simultaneously; 
         FIG. 7  is the circuit from  FIG. 3  during setting of the bias and dc testing of two read heads in a slider in a series configuration; 
         FIG. 8  is a flow chart of an algorithm for setting the bias on two read heads simultaneously in a series configuration; 
         FIG. 9  is a schematic diagram of a circuit for parallel testing of multiple read heads in a TDMR slider in either a common lead or a separate lead electrical configuration; and 
         FIG. 10  is a flow chart of a procedure for sequentially testing a multiplicity of N sliders in a nest, wherein each slider has a plurality of M read heads. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments can provide one or more advantages over previous methods for testing read heads in sliders designed for application to two-dimensional magnetic recording (TDMR). Not all embodiments may provide all the benefits. The embodiments will be described with respect to these benefits, but these embodiments are not intended to be limiting. Various modifications, alternatives, and equivalents fall within the spirit and scope of the embodiments herein and as defined in the claims. 
     Testing Configuration for TDMR Read Heads 
       FIG. 1  is a schematic diagram  100  of a testing configuration for TDMR read heads. TDMR read heads may be employed in hard disk drives, tape drives, or similar magnetic data storage systems. A slider  102  is shown affixed to a movable nest  104 . In some testing configurations, slider  102  may be one of a multiplicity N of sliders, wherein each of the N sliders would comprise a plurality M of read heads, as described in the Background section above for TDMR technology. At least three different testing configurations for sliders are possible within the scope of the invention:
     1) Testing of single sliders, mounted in a “nest” (as shown in  FIG. 1 )   2) Testing of a single bar, sliced out of a wafer, which may contain up to 60 sliders, and   3) Testing of Head Gimbal Assemblies (HGAs), which are sliders already attached to a suspension.   

     The plurality of read heads in each slider may be configured in a common lead circuit with one common connection (i.e., a single pad) among the M read heads within a single slider, or the M read heads may be configured in a separate lead circuit wherein each read head is connected to two pads separate from the pads for the other M−1 read heads in the slider. 
     For testing, the slider is connected to a probe card  110  by a plurality of probe wires  112 , one per pad on the slider being tested. In addition to the probe wires  112  connecting to the read heads, in some embodiments there may be additional probe wires connecting to additional pads to control the write coil, thermal fly height control heaters, etc. In some embodiments, the write coil may be energized to exert a magnetic stress on the read heads during testing. In some embodiments, the thermal fly height control heater may be energized to apply thermal stress to the read heads during testing. Two magnetic poles  106  and  108  are energized by a magnetic coil (not shown) to generate a transverse magnetic field as shown by the arrows—this magnetic field simulates the magnetic field which will be induced at the read heads by the magnetic media when the slider has been assembled into a hard disk drive. The polarity and strength of this magnetic field are determined by the current in the magnet coil which is regulated by the test system controller (not shown). 
     After testing of slider  102  is complete, nest  104  would be moved as shown schematically by arrows  114  to position another slider at the probe wires  112 . Arrows  114  represent a multi-step motion, typically consisting of the steps of: 1) motion away from the probe card, then 2) motion perpendicular to the plane of figure (parallel to the probe card), and finally 3) motion back towards the probe card until the probe wires are in contact with pads in the next slider. The flowchart in  FIG. 10  describes this overall testing procedure for a multiplicity N of sliders contained in the nest  104 .  FIG. 2  is an electrical model  200  for the testing configuration in  FIG. 1 . Section  216  represents the testing system, including the current monitor  212 , the voltage monitor  214  and probe wires  210 . Section  218  represents the magnetic field generated between the two pole pieces  106  and  108  in  FIG. 1 , passing through the slider to simulate the magnetic fields induced by the magnetic storage medium above which the slider will “fly” during normal operation within a data storage system such as a hard disk drive or tape drive. Section  220  represents the slider under test, comprising slider pads  208  (contacted by probe wires  210 ) and the read head  202 , represented here by a variable resistance. As is well known in the art, and not part of the invention, the resistance of a magneto-resistive read head is a function of the local magnetic field at the head, represented here by the arrow through resistor  202 . By applying a bias current  212  flowing through read head  202 , a time-varying voltage  214  is generated by application of Ohm&#39;s Law—this voltage then generates the signal from the read head, either during read head testing, or during actual operation of the read head within a data storage device. 
     Parallel Testing of Multiple Read Heads in a Slider Wired in a Common Lead Configuration 
       FIGS. 3-5  show a circuit for simultaneous testing of up to four read heads within a single TDMR slider (or a single read head within a non-TDMR slider) in various operational modes selected by the opening and closing of relays as shown in  FIGS. 4 and 5 .  FIG. 7  (see section below) shows this same circuit operating in a series mode, testing two sliders simultaneously. 
     Before initiating testing of read heads, in some embodiments the leakage currents through the ESD diodes (the pairs of diodes seen in  FIG. 3  in parallel with each read head  341 - 344 ) may be measured. Prior to connecting the probe card wires to the pads on a slider, the ESD diode leakage currents may be measured using the following procedure:
         1) Close relays  311  and  315 , open relays  312 - 314 ,  381 - 385 .   2) Ramp DAC  301  over a range of voltages corresponding to typical read head bias voltages and record the current measured by amplifier  351 .   3) Close relays  312  and  315 , and open relays  311 ,  313 ,  314 ,  381 - 385 .   4) Ramp DAC  302  over a range of voltages corresponding to typical read head bias voltages and record the current measured by amplifier  352 .   5) Close relays  313  and  315 , and open relays  311 ,  312 ,  314 ,  381 - 385 .   6) Ramp DAC  303  over a range of voltages corresponding to typical read head bias voltages and record the current measured by amplifier  353 .   7) Close relays  314  and  315 , and open relays  311 - 313 ,  381 - 385 .   8) Ramp DAC  304  over a range of voltages corresponding to typical read head bias voltages and record the current measured by amplifier  354 .
 
The stored table of diode leakage currents as a function of the voltage across the diodes may then be used to account for the diode leakage current during all dc bias settings and dc performance characterization steps below. The above procedure for measuring ESD diode leakage currents may in some embodiments be applied to single read heads in either TDMR sliders, or non-TDMR sliders.
       

       FIG. 3  is a schematic diagram  300  of a circuit for parallel testing of multiple read heads in a TDMR slider in a common lead configuration during digital-to-analog converter (DAC) set-up. This figure corresponds to block  602  in  FIG. 6 —where relays  311 - 315  are open, disconnecting the outputs of the five DACs  301 - 305  from the read heads  341 - 344  (within a single TDMR slider) to be tested. Also, relays  381 - 385  are open, disconnecting the high frequency preamps used for noise measurements (see  FIGS. 4B, 5B and 7 ). Because relays  311 - 315  are open, no currents can flow due to the output voltages of the DACs  301 - 305 , and thus no currents are shown here (compare with  FIGS. 4, 5, and 7 ). With no currents flowing, there will be no voltage drops across any of resistors  321 - 325 ,  331 - 335 , or magneto-resistive sensors (GMR or TMR—which appear electrically as resistors)  341 - 344 . To protect the sensitive read heads  341 - 344  from electrostatic discharge damage, the function of resistors  321 - 325  (typically with resistances in the 1 Mohm range—adequate to bleed off accumulated charges, but too large to have any significant effect on the testing system operation) is to drain any charges to ground, thereby protecting the heads against damage when the circuits are disconnected. The output voltages of DACs  301 - 305  are set to 0 V, as determined by the binary input signals to each of DACs  301 - 305 . By zeroing out the DAC voltages before closing any of relays  311 - 315 , any possible impulse voltage and/or current stresses arising from rapid current spikes on read heads  341 - 344  during relay closing are minimized. The outputs of amplifiers  361 - 365  should also be zero since their positive and negative inputs will be equal in the absence of currents flowing through read heads  341 - 344 . Amplifiers  351 - 355  are connected across current sense resistors  331 - 335 , respectively—the outputs of amplifiers  351 - 355  measure the currents flowing in both the sense resistors  331 - 335  as well as the read heads  341 - 344 , as discussed in  FIG. 4 , below. 
     Set-Up and Testing of a Single Read Head 
       FIGS. 4A and 4B  illustrate two operating modes of the circuit in  FIG. 3 , which essentially comprises two combined test circuits: 1) a dc set-up and test circuit employing DACs  301 - 304  to apply either voltage or current biases to read heads  341 - 344 , and 2) a high frequency test circuit employing preamps  371 - 374  to measure noise and instabilities of the heads  341 - 344 .  FIG. 4A  shows dc set-up and testing, while  FIG. 4B  illustrates high frequency testing. In both cases, a single read head  341  is being tested. In  FIG. 4A , the testing system is configured for setting the dc bias on read head  341  and then to perform dc testing, such as 1) resistance measurements, 2) quasi amplitude and asymmetry, and 3) the transfer curve and kink. In  FIG. 4B , the testing system is configured for high frequency noise and instability measurements. Details of both dc and high frequency testing are discussed further in the section below on “Procedure for Sequentially Testing Multiple Sliders”. 
       FIG. 4A  shows the circuit  400  from  FIG. 3  during bias set up and dc testing of a single read head  341  in a slider. Before examining the parallel testing of multiple (four) read heads in  FIGS. 5A and 5B , the testing of a single read head is first illustrated here. This may correspond either to a single read head in a TDMR slider, or to the only read head in a non-TDMR slider—thus embodiments of the invention are capable of testing both multiple read head TDMR sliders, as well as non-TDMR sliders—this ensures backwards compatibility in a manufacturing test environment. Relay  311  is closed, allowing currents to flow in resistors  321  and  331 , as well as through read head  341 . Specifically, current  451  flows through the current-sense resistor  331  and then through read head  341  with resistance R ( 341 ) as shown. Amplifier  351  is connected across sense resistor  331  to measure the voltage drop and thus the current flowing  451  (since the resistance of resistor  331  is known). Since no currents flow into the positive and negative inputs of amplifiers  361 - 365 , the currents through read heads  341 - 344  and ESD (electro-static discharge) diodes (leakage current) are always equal to the currents through sense resistors  331 - 334 , respectively. The leakage currents through the ESD diodes induce non-linearities in measurements of the resistivities of read heads  341 - 344  which must be accounted for. The current  455  through sense resistor  335  is measured by amplifier  355  and is always equal to:
 
 I (335)= I (331)+ I (332)+ I (333)+ I (334)
 
Current  455  [I( 455 )] flowing through resistor  335  must equal I( 451 ) in this configuration of testing a single read head. Relays  312 - 314  remain open (from  FIG. 3  and block  602  in  FIG. 6 ) since there are no read heads connected to these other three individual test circuits. Also, relays  381 - 385  (connecting to high frequency preamps  371 - 374 ) remain open since in this mode we are not making high frequency measurements. The voltage output from amplifier  361  indicates the voltage drop due to the current  451  flowing through read head  341 , by applying Ohm&#39;s Law using the value of current  451 , which was measured by amplifier  351 . The output voltages of amplifiers  362 - 364  are unimportant since there are no read heads connected to their corresponding test circuits.
 
       FIG. 4B  shows the circuit from  FIG. 3  which was illustrated in  FIG. 4A , except that the circuit  450  is now configured to perform high frequency noise and instability testing. Relays  311  and  315  are now open, and relays  381  and  385  have been closed, connecting both inputs of high frequency preamp  371  to read head  341 . Note that now the bias current I( 491 )=I( 455 ) [which can be measured by amplifier  361 ] for read head  341  is supplied by preamp  371 . 
     Set-Up and Testing of a Four Read Heads in Parallel 
       FIGS. 5A and 5B  are similar to  FIGS. 4A and 4B , respectively, except now the parallel bias set-up, dc testing, and high frequency testing of four read heads is illustrated. 
       FIG. 5A  is the circuit  500  from  FIG. 3  during parallel bias set-up and dc testing of four read heads  341 - 344  in a TDMR slider. Now, according to block  604  in  FIG. 6 , all of relays  311 - 315  are closed allowing currents  551 - 554  to flow through sense resistors  331 - 334  and magneto-resistive read heads  341 - 344 , respectively. Current  555  is the summation of currents  551 - 554 , as indicated in the formula below. 
     There are two alternative approaches to setting the bias on the read heads:
     1) Bias Current—in this approach, the target current flowing through the read head (e.g., current  551  through read head  341 ) is specified, and the output of DAC  301  is adjusted to achieve the desired current. Typical bias currents may be around 50 μA. Since the actual resistance of read head  341  is not known exactly, a multiple step procedure (see  FIG. 8 ) is followed:
       a. The output of DAC  301  [V( 301 )] is set based on the assumption that head  341  has a resistance (Rinit) towards the lower end of the nominal resistance range. This ensures that if the head happens to have a low resistance that it will not inadvertently be damaged by overstressing, which might occur if a higher current were to flow through the head.   b. The voltage drop across sense resistor  331  (measured by amplifier  351 ) is used to measure the current  551 .   c. Amplifier  361  measures the voltage drop, V( 341 ), across head  341 , and Ohm&#39;s law then gives the resistance of head  341 : R( 341 )=V( 341 )/I( 551 ).   d. If the current I( 551 ) differs from the desired bias current, the output of DAC  301  is adjusted to correct the bias current. This is done by adjusting DAC  301  to a new value of DAC(updated)=[(Bias target)/(Bias measured)]*DAC (current)   
       2) Bias Voltage—in this approach, the voltage across the read head [V( 341 )] is specified. Typical bias voltages may be around 125 mV with typical head resistance in the range of 1 kohm. Again, a multiple step procedure is followed:
       a. The output of DAC  301  is set based on the assumption that head  341  has a resistance [R( 341  init)=Rinit] towards the lower end of the nominal resistance range. This ensures that if the head happens to have a low resistance that it will not inadvertently be damaged by overstressing, which might occur if a higher current were to flow through the head.   b. The voltage drop across head  341  is measured by amplifier  361 .   c. If the voltage V( 341 ) differs from the desired bias voltage, the output of DAC  301  is adjusted to correct the bias voltage. This is done by adjusting DAC to a new value of DAC(updated)=[(Bias target)/(Bias measured)]*DAC (current)   
       

     For the case of setting up four bias currents [I( 551 ) to I( 554 )] in parallel, the voltages on DACs  301 - 304  may be set according to the equations [where R( 341  init)=Rinit, . . . , R( 344  init)=Rinit, and Rinit is near the lower end of the expected resistance range for read heads]:
 
 V (301)= I (551) I (bias551 target)[ R (331)+ R (341 init)]+ V ref
 
 V (302)= I (552) I (bias552 target)[ R (332)+ R (342 init)]+ V ref
 
 V (303)= I (553) I (bias553 target)[ R (333)+ R (343 init)]+ V ref
 
 V (304)= I (554) I (bias554 target)[ R (334)+ R (344 init)]+ V ref
 
 I (555)= I (551)+ I (552)+ I (553)+ I (554)= I (bias551 target)+ I (bias552 target)+ I (bias553 target)+ I (bias554 target).
 
 V (305)=− I (555) R (335)+ V ref
 
It is preferred that the sensors are biased symmetrically around 0 V (i.e., ground potential), thus we apply the additional constraint:
 
 V ref=−0.5 V (bias average)
 
 V (bias average)==[ I (551 target) R (341 init)+ I (552 target) R (342 init)+ I (553 target) R (343 init)+ I (554 init) T (344 init)]/4
 
     For the case of setting up four bias voltages [V( 341 ) to V( 344 )] in parallel, the voltages on DACs  301 - 304  [V( 301 )-V( 304 )] may be set according to these equations [where Rinit=the initial assumption for the resistances R( 341 )-R( 344 ) of the read heads  341 - 344 , and where V( 341  target) is the desired voltage bias for read head  341 , etc.]:
 
 V (301)=[ V (341 target)/ R (341 init)]*[ R (331)+ R (341 init)]+ V ref
 
 V (302)=[ V (342 target)/ R (342 init)]*[ R (332)+ R (342 init)]+ V ref
 
 V (303)=[ V (343 target)/ R (343 init)]*[ R (333)+ R (343 init)]+ V ref
 
 V (304)=[ V (344 target)/ R (344 init)]*[ R (334)+ R (344 init)]+ V ref
 
 I (555)= V (341 target)/ R (341 init)+ V (342 target)/ R (342 init)+ V (343 target)/ R (343 init)+ V (344 target)/ R (344 init)
 
 V (DAC 305)=− I (555) R (335)+ V ref
 
     It is preferred that the sensors are biased symmetrically around 0 V (i.e., ground potential), thus we apply the additional constraint:
 
 V ref=−0.5  V (bias average)
 
 V (bias average)=[ V (341 target)+ V (342 target)+ V ( R 43 target)+ V (344 target)]/4
 
In general, current sense resistors  331 - 335  may have the same value, typically in the range of 1 to 2 kohms. Charge bleed-off resistors  321 - 325  also will have the same value, typically approximately 1 Mohm.
 
     The output voltages of amplifiers  361 - 364  represent the Ohmic voltage drops across read heads  341 - 344 , respectively. These voltages represent the output signals from the four read heads  341 - 344  being measured simultaneously by means of the four parallel voltage outputs from amplifiers  361 - 364 . 
     Other testing configurations for two or three read heads are also possible with the appropriate closing and opening of relays  311 - 314 . 
     Once the bias currents or voltages for read heads  341 - 344  have been set by the above procedures, dc testing may be performed, including 1) resistance measurements, 2) quasi amplitude and asymmetry, and 3) the transfer curve and kink—details of these procedures are provided in the section “Procedure for Sequentially Testing Multiple Sliders”, below. 
       FIG. 5B  shows the circuit from  FIG. 3  which was illustrated in  FIG. 5A , except that the circuit  550  is now configured to perform high frequency noise and instability testing. Relays  311 - 315  are open, and relays  381 - 385  have been closed, connecting high frequency preamps  371 - 374  to read heads  341 - 344 , respectively. Note that now the bias currents for read heads  341 - 344  are supplied by preamps  371 - 374  as illustrated by currents  591 - 594 , respectively, shown coming through relays  381 - 384  from preamps  371 - 374 . Current  555  is the combination of currents  591 - 594 . 
     Algorithm for Setting the Bias on Multiple Read Heads Simultaneously 
       FIG. 6  is a flow chart  600  of an algorithm for setting the bias on multiple read heads simultaneously. Block  602  is illustrated in  FIG. 3 , where the output voltages of DACs  301 - 305  are all initialized to 0 V. 
     In block  604 , relays are left open or closed depending on the various read heads to be tested. Thus if there is a read head  341  connected to the probe card, then relay  311  will be closed. If there is a read head  342  connected, relay  312  will be closed. Similarly for read heads  343  and  344 , with respect to relays  313  and  314 . Any combination of read heads is allowable, from one head (as in  FIG. 4 ) up to four heads (as in  FIG. 5 ). Read heads do not have to be sequential—some read head test locations out of the set of four ( 341 - 344 ) may be skipped or omitted)—e.g., heads  341  and  344  may be tested while heads  342  and  343  are absent. 
     In block  606 , the initial DAC settings are calculated and then applied to the inputs of DACs  301 - 305 . The equations in the previous section for V( 301 )-V( 305 ) are used, with the initial values for R( 331 )-R( 335 ) and the assumed initial values for R( 341 )-R( 344 ) are at the low end of the expected head resistance range (as discussed above) unless the actual values for R( 341 )-R( 344 ) have been measured previously. 
     Next, in block  608 , the bias values (currents or voltages) for each read head are measured. Due to manufacturing tolerances, in general these bias values will differ from the desired (“target”) values. An adjustment factor is then calculated:
 
Adj( n )=(Bias Target)/(Bias Measured)
 
Where n=341 to 344, corresponding to the particular read head, and the “Bias Target” and “Bias Measured” may be either voltages or currents. The Biases applied to each read head are then multiplied by the Adj(n) factors in block  610 , and the output settings of DACs  301  to  305  are recalculated in block  612  and applied to the DAC digital inputs.
 
     Typically, when the bias correction factors Adj(n) are less than ±20%, only a single bias adjustment cycle is required, and block  614  will respond with a “No” leading to completion block  616 . In cases of larger corrections, a second cycle may be desirable, so that block  614  will respond with a “Yes”, leading back to block  608  for a remeasurement of the bias and the calculation of modified adjustment factors Adj(n). For all passes through the loop after the first pass, the same values for R( 341  init)-R( 344  init) are used as in block  606 . 
     Testing of Two Read Heads in a Slider in a Series Configuration 
       FIG. 7  is similar to  FIG. 4A , except now the set-up, dc testing, and high frequency testing of two read heads in a series configuration is illustrated.  FIG. 7  is the circuit  700  from  FIG. 3  during set-up and dc testing of two read heads  341  and  342  in a series configuration. In this operating mode, the reference circuit is not used, and instead current  751  flows through current sense resistor  331  (where the current is measured by amplifier  351 ), then through read heads  341  and  342  (in series) and finally through current sense resistor  332  (where it is labelled current  752  which is equal in magnitude but opposite in polarity to current  751  and is measured by amplifier  352 ). This testing configuration enables the measurement of interactions between the two heads  341  and  342  which are in close physical proximity within the slider and thus may have correlated noise and signal coupling or signal shunting. Relays  381 - 385  remain open to isolate the high frequency preamps from the read heads. 
     Algorithm for Setting the Bias on Two Read Heads Simultaneously 
       FIG. 8  is a flow chart  800  of an algorithm for setting the bias on two read heads simultaneously in a series configuration, as shown in  FIG. 7 . Block  802  is illustrated in  FIG. 3 , where the output voltages of DACs  301 - 305  are all initialized to 0 V. Series measurements are assumed to be performed after the read head resistances have already been measured, for example using the procedure of  FIG. 6 . 
     Referring to  FIG. 7 , in block  804 , relays  313 - 315  and  381 - 385  are left open while relays  311  and  312  are closed. The bias current is calculated in block  806 :
 
 I (bias)= V (bias total)/[ R (341)+ R (342)]
 
where we use previously-measured values for R( 341 ) and R( 342 ). If the head resistances have not been measured, then the assumed values of R( 341  init)=R( 342  init)=Rinit would be used instead, where Rinit is chosen near the lower end of the expected resistance range for the read heads (approximately 1 kohm) to avoid possible damage to the heads due to overstressing in the event that their resistances are below nominal.
 
     In block  808 , the initial settings for DACs  301  and  302  are calculated:
 
 V (DAC 301)=0.5 I (bias)( R (331)+ R (332)+ R (341)+ R (342)]
 
 V (DAC 302)=− V (DAC 301)
 
and then applied to the inputs of DACs  301 - 302 .
 
     Next, in block  810 , the bias values (currents or voltages) for read heads  341  and  342  are measured. Due to manufacturing tolerances, in general these bias values will differ from the desired (“target”) values. A adjustment factor is then calculated:
 
Adj( n )=(Bias target)/(Bias measured)
 
Where n=341 and 342, corresponding to two read heads being tested. The Biases applied to each read head are then multiplied by the Adj(n) factors in block  812 , and the output settings of DACs  301  and  302  are recalculated in block  814  and applied to the DAC digital inputs. Block  816  is the completion block—typically only one cycle is required for this procedure, like the case in  FIG. 6 . The bias currents (voltages) are multiplied by Adj(n) and the same values are used for R( 331 ), R( 332 ), R( 341 ), and R( 342 ) as in block  806 .
 
Parallel Testing of Multiple Read Heads in a Separate Lead Configuration
 
       FIG. 9  is a schematic diagram  900  of a circuit for parallel testing of multiple read heads in a TDMR slider in either a common lead or a separate lead electrical configuration. DACs  901 - 905  are analogous to DACs  301 - 305 , however DACs  901 - 905  are configured with symmetrical opposite polarity outputs A and B (i.e., when output A is +4 V, then B will be −4 V, and if output A is −2 V, then B will be +2 V). Read heads  981 - 984  are analogous to heads  341 - 344 . Current sense resistors  971 - 974  are analogous to resistors  331 - 334 . Current sense amplifiers  985 - 988  are analogous to amplifiers  351 - 354 . Output amplifiers  991 - 995  are analogous to amplifiers  361 - 365 . High frequency preamps  941 - 944  are analogous to preamps  371 - 374 . Relays  911 - 915  are analogous to relays  311 - 315 . Relays  921 - 924  are equivalent to relays  381 - 384 . To enable dc and high frequency testing of read heads with separate leads, relays  916 - 920 ,  931 - 934 ,  951 - 959 ,  965 - 969  have been added in comparison to circuit  300  in  FIG. 3 . Relays  951 - 954  are connected in common at “Node A”, which also connects to the common current sense circuit comprising amplifier  989  and resistor  975 . Relays  955 - 959  are connected in common at “Node B”, which also connects to the positive input of amplifier  995 . Relays  965 - 969  are connected in common at “Node C”, which also connects to the negative input of amplifier  995 . 
     Setting of Bias Currents or Voltages and DC Measurements in a Separate Lead Configuration. 
     When the following relay opens and closes are implemented, the circuit in  FIG. 9  will enable parallel setting of bias currents and performing dc measurements for separate lead read heads  981 - 984 : 
     Close relays:  911 - 919   
     Open relays:  921 - 924 ,  931 - 934 ,  951 - 954 ,  955 - 959 ,  965 - 969   
     This test arrangement separately biases each of the four read heads  981 - 984  [although one, two, or three read heads can be biased as well] using the symmetrical outputs A and B of DACs  901 - 904 . The current through read head  981  is measured using amplifier  985  to measure the ohmic voltage drop across sense resistor  971 , and similarly for read heads  982 - 984 . The previously-measured leakage currents through the ESD diodes may be subtracted off the measured currents, based on the measured voltage across the read head, since these leakage currents bypass the read heads, but pass through the sense resistors. 
     Setting of Bias Currents or Voltages and DC Measurements in a Common Lead Configuration (Including for Separate Lead Read Heads). 
     When the following relay opens and closes are implemented, the circuit in  FIG. 9  will enable parallel setting of bias currents and performing dc measurements for separate lead read heads  981 - 984  in the same way that the circuit in  FIG. 3  did for common lead read heads  341 - 344  in  FIG. 4A : 
     Close relays:  911 - 915 ,  951 - 954 ,  959 ,  969   
     Open relays:  916 - 920 ,  921 - 924 ,  931 - 934 ,  955 - 958 ,  965 - 968   
     This test arrangement provides some additional data about the performance of the multiple read heads in a TDMR slider, including measurements of the interactions between heads, such as leakage currents, etc. 
     Series DC Measurements in a Separate Lead Configuration. 
     When the following relay opens and closes are implemented, the circuit in  FIG. 9  will enable dc measurements for two separate lead read heads  981  and  982  in series: 
     Close relays:  911 ,  912 ,  951 - 952 ,  959 ,  969   
     Open relays:  913 - 920 ,  921 - 924 ,  931 - 934 ,  953 - 954 ,  955 - 958 ,  965 - 968   
     Set V( 901 )=−V( 902 ), i.e., the two DAC outputs are equal magnitude and opposite sign 
     This test arrangement provides some additional data about the performance of the neighboring read heads in a TDMR slider, including measurements of the interactions between heads, such as leakage currents, etc. 
     High Frequency Noise and Instability Measurements in a Separate Lead Configuration. 
     When the following relay opens and closes are implemented, the circuit in  FIG. 9  will enable parallel measurement of high frequency noise and instability for read heads  981 - 984  in the same way that the circuit in  FIG. 3  did using a common-lead circuit configuration for read heads  341 - 344  in  FIG. 5B : 
     Close relays:  921 - 924 ,  931 - 934 ,  951 - 954   
     Open relays:  911 - 915 ,  916 - 920 ,  955 - 959 ,  965 - 969   
     This test arrangement allows the measurement of high frequency noise and instability using the preamps  941 - 944 . 
     Measurement of Resistances Between Read Heads in a Slider. 
     When the following relay opens and closes are implemented, the circuit in  FIG. 9  will enable of the resistances between read heads in a slider (in this example, heads  981  and  982 )—this enables a detection of defective sliders. The method works by applying voltages to the two read heads, and then measuring the current and voltage difference between the heads—note that this is not measuring the resistance of either of heads  981  or  982 , but rather the resistance between them within the slider structure: 
     Close relays:  911 ,  912 ,  955 ,  966   
     Open relays:  913 - 920 ,  921 - 924 ,  931 - 934 ,  951 - 954 ,  956 - 959 ,  965 ,  967 - 969   
     Set V( 901 )=−V( 902 ), i.e., the two DAC outputs are equal magnitude and opposite sign 
     This test arrangement does not apply voltages across any read heads, but instead applies voltages to only one end of each of two read heads and then measures the current flowing and the voltage difference. In this example, amplifier  985  would measure the ohmic voltage drop across resistor  971  and amplifier  986  would measure the ohmic voltage drop across resistor  972 —both these measurements should be comparable. The amplifier  995  measures the voltage difference between read heads  981  and  982 . The resistance between heads  981  and  982  is then:
 
 R (leakage)= V (995)/ I (971)
 
where I( 971 )=V( 985 )/R( 971 ).
 
Procedure for Sequentially Testing Multiple Sliders
 
       FIG. 10  is a flow chart  1000  of procedure for sequentially testing a multiplicity of N sliders in a nest, where each slider has a plurality of M read heads. As discussed in the section “Testing Configuration for TDMR Read Heads”, in some cases, N may be one slider, and in other cases, N may range up to approximately sixty sliders. The plurality of heads in each slider, M, may range from one (as in a non-TDMR slider) up to at least four (a TDMR slider with four read heads). 
     In block  1002 , a nest (e.g.,  104  in  FIG. 1 ) for holding sliders or bars during testing is loaded with N sliders for testing. Next, in block  1004  the nest is loaded either manually or automatically into the tester between the pole pieces (e.g.,  106  and  108  in  FIG. 1 ) which will immerse the sliders in the nest in an approximately uniform magnetic field to simulate the magnetic field at the read head induced by the magnetization of the data storage medium (typically either a magnetic disk or magnetic tape). 
     In block  1006 , the mechanical stage supporting the nest moves the nest into position so that the set of probe pins (one per pad on the slider) can electrically contact the pads on the slider. 
     In block  1008 , parallel testing of the M individual heads on the slider is performed, as well as testing for interactions between the M heads. Examples of the kinds of tests performed are described in the following four sections. 
     [ FIGS. 4A, 5A, 7, 9 ] Resistance Measurement. 
     This is a DC measurement of the resistance of all the slider transducer elements. This may include the read sensor, write coil, thermal fly-height control (TFC) heater, ECS (Electrical Contact Sensor), and other electrical elements. It may also include electrical isolation measurement between the elements. For the write coil, it may include an impedance (Inductance) measurement. For embodiments of this invention for TDMR, it will include making read sensor measurement on all the read elements at the same time ( FIGS. 3-6 ). It may also include resistance measurements of two read sensors in series ( FIGS. 7 and 8 ). Ideally the series measurement should sum to the individual measurements. If it does not, this indicates that there may be internal shunts (undesirable low resistance paths) between the heads. Also for differential TDMR lead design, the DC isolation between the different sensors will be measured. 
     [ FIGS. 4A, 5A, 7, 9 ] Quasi Amplitude and Asymmetry. 
     This is a DC measurement, with the bias condition subtracted. The magnetic field is switched between a positive field, a negative field, and a zero state while measuring the head response. This may also include stress conditions, such as TFC and Write current. For embodiments of this TDMR invention, it will include making quasi measurements on all the read elements at the same time and checking that they are similar, and can also include making a quasi-series measurement on two sensors. Again, the amplitudes of the two sensors should sum together. If not, again this is an indication of shunting/defect occurring in the structure, which should be detected at test. 
     [ FIGS. 4A, 5A, 7, 9 ] Transfer Curve and Kink. 
     This test is a DC magnetic field stepped in small increments to examine the linearity of the transfer curve. Again, this may contain stress conditions, such as TFC on during measurements (causing heating of the other elements in the slider). For TDMR, this test may include the same comparison algorithm, and series summing looking for defects. 
     [ FIGS. 4B, 5B, 9 ] Noise and Instability Measurement. 
     This is an AC measurement, typically from low hundred kHz to 50 MHz or higher. The noise signal is captured from the head in a specified frequency range. The magnetic field may also be slowly swept (few hundred Hz, below the preamp range) to provide a stress. The signal capture can be done in either the frequency domain (using spectrum analyzer) or time domain with a high speed digitizer (like an oscilloscope). For time domain analysis, will capture noise on M heads simultaneously. The captured data is corrected for the preamp gain. From this data, many different parameters can be calculated. If the noise is pure random Gaussian, a noise value can be calculated, which can then be used in signal to noise (SNR) calculation. However, the noise can have structure, such as mode hopping between states. Many enhanced algorithms have been developed to extract information on unstable heads. For TDMR embodiments of the invention enable measurement of two or more heads at the same time. The individual noise components are measured as is currently done. However, embodiments of the invention also measure the coherent noise (i.e. noise that is the same) between heads. 
     Stress Conditions for the Measurements. 
     Some of the above measurements may contain stress conditions such as activating the Thermal Fly Height Control (TFC) [thermal stress], write current [magnetic domain stress] or Temperature [thermal stress]. 
     Block  1010  determines if all the sliders in the nest have been tested already. If “No”, the nest is moved in block  1012  to position the next slider for testing at the probe card wires. If “Yes”, then “Done” block  1014  is entered. 
     TERMINOLOGY 
     In the above descriptions of embodiments of the invention, the term “digital-to-analog converter”, or “DAC” is used to denote any type of programmable voltage supply. The term “relay” is used to denote any type of switching mechanism, including, but not limited to, relays, mechanical switches, pneumatic switches, CMOS switches, etc. The term “current sensing circuit” includes, but is not limited to a sense resistor in series with a read head, with a differential amplifier being connected across the sense resistor. The term “electrical connector” is synonymous with “probe wire” or “probe pin” for connecting to read head pads in the slider. Also note that a slider assembly may typically comprise one or more (for TDMR) read heads as well as a write coil, magnetic pole pieces, and in some embodiments thermal fly height control heaters, and optical waveguides or microwave sources. 
     ALTERNATIVE EMBODIMENTS 
     Although embodiments have been described in the context of hard disk drive testing structures and methods, it should be understood that various changes, substitutions and alterations can be made. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, or composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of embodiments, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.