U.S. patent application number 10/252239 was filed with the patent office on 2003-01-23 for voltage measurement circuit with ac-coupled diferential amplifier.
Invention is credited to Gray, David M., McMahon, Terrence A., Rigles, Ray V., Wood, William P..
Application Number | 20030016027 10/252239 |
Document ID | / |
Family ID | 26878459 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030016027 |
Kind Code |
A1 |
McMahon, Terrence A. ; et
al. |
January 23, 2003 |
Voltage measurement circuit with AC-coupled diferential
amplifier
Abstract
A voltage measurement circuit is provided, which is has an
AC-coupled differential amplifier coupled to first and second input
terminals for measuring a voltage across the first and second input
terminals. The AC-coupled differential amplifier blocks a DC
component of the voltage and presenting a low input bias current to
the first and second input terminals while isolating the first and
second input terminals from charge stored in capacitances in the
AC-coupled differential amplifier.
Inventors: |
McMahon, Terrence A.;
(Hudson, WI) ; Rigles, Ray V.; (Bloomington,
MN) ; Gray, David M.; (Bloomington, MN) ;
Wood, William P.; (Waconia, MN) |
Correspondence
Address: |
David D. Brush
WESTMAN CHAMPLIN & KELLY
International Centre
900 South Second Avenue, Suite 1600
Minneapolis
MN
55402-3319
US
|
Family ID: |
26878459 |
Appl. No.: |
10/252239 |
Filed: |
September 23, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10252239 |
Sep 23, 2002 |
|
|
|
09784782 |
Feb 15, 2001 |
|
|
|
60182826 |
Feb 16, 2000 |
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Current U.S.
Class: |
324/600 ;
G9B/20.014; G9B/20.051; G9B/5; G9B/5.145 |
Current CPC
Class: |
G01R 33/07 20130101;
G11B 5/455 20130101; G11B 5/4806 20130101; G11B 5/00 20130101; G11B
20/1816 20130101; G11B 11/10532 20130101; G11B 20/10527 20130101;
B82Y 25/00 20130101; G11B 5/3909 20130101; G11B 2005/0016 20130101;
B82Y 10/00 20130101; G11B 5/012 20130101 |
Class at
Publication: |
324/600 |
International
Class: |
G01R 027/00 |
Claims
What is claimed is:
1. A voltage measurement circuit comprising: first and second input
terminals; and voltage measurement means coupled to the first and
second input terminals for measuring a voltage across the first and
second input terminals with an AC-coupled differential amplifier,
which blocks a DC component of the voltage, and for presenting a
low input bias current to the first and second input terminals
while isolating the first and second input terminals from charge
stored in capacitances in the AC-coupled differential
amplifier.
2. The voltage measurement apparatus of claim 1 wherein the voltage
measurement means comprises: a first input stage comprising a first
buffer input, which is coupled to the first input terminal, a first
buffer output and a plurality of parallel-connected buffer
amplifiers coupled between the first buffer input and the first
buffer output; a second input stage comprising a second buffer
input, which is coupled to the second input terminal, a second
buffer output and a plurality of parallel-connected buffer
amplifiers coupled between the second buffer input and the second
buffer output; and the AC-coupled differential amplifier, which has
first and second amplifier inputs coupled to the first and second
buffer outputs through first and second capacitors, respectively,
and having a measurement output.
3. The voltage measurement apparatus of claim 2 wherein each buffer
amplifier in the first and second input stages comprises an
operational amplifier coupled to operate as a voltage follower.
4. The voltage measurement apparatus of claim 3 wherein the first
and second input stages have unity gain.
5. The voltage measurement apparatus of claim 1 wherein the
differential amplifier comprises an instrumentation amplifier.
6. The voltage measurement apparatus of claim 2 wherein the
differential amplifier comprises a plurality of individual
differential amplifiers coupled in parallel to one another between
the first and second amplifier inputs and the measurement
output.
7. The voltage measurement apparatus of claim 2 wherein the voltage
measurement means further comprises: first and second amplifier
biasing resistors coupled between a voltage reference terminal and
the first and second amplifier inputs, respectively; and first and
second switches coupled in parallel with the first and second
amplifier biasing resistors, respectively, and each having a
control input for selectively shorting the respective first and
second amplifier bias resistors.
8. The voltage measurement apparatus of claim 1 wherein the voltage
measurement means further comprises a balanced, differential
current source coupled to the first and second tester input
terminals.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application No.
09/784,782, filed Feb. 15, 2001 and entitled "A TRASFER CURVE
TESTER FOR TESTING MAGNETIC RECORDING HEADS", which claims priority
from U.S. Provisional Application No. 60/182,826, filed Feb. 16,
2000, and entitled "HIGH PERFORMANCE TRANSFER CURVE TESTER AND
TEMPERATURE COMPENSATED HALL SENSOR."
FIELD OF THE INVENTION
[0002] The present invention relates to data storage systems and,
more particularly, to a transfer curve tester having a
bi-directional current source for testing magnetic recording heads
used in data storage systems.
BACKGROUND OF THE INVENTION
[0003] Many data storage systems use magnetic or magneto-optical
recording heads for writing information to and reading information
from a magnetic medium. For example, disc drives of the
"Winchester" type have one or more rigid discs, which are coated
with a magnetizable medium for storing digital information in a
plurality of circular, concentric data tracks. The discs are
mounted on a spindle motor, which causes the discs to spin and the
surfaces of the discs to pass under respective head suspension
assemblies. Head suspension assemblies carry transducers which
write information to and read information from the disc surface. An
actuator mechanism moves the head suspension assemblies from
track-to-track across the surfaces of the discs under control of
electronic circuitry. "Floppy-type" disc drives use flexible discs,
which also have circular, concentric data tracks. For a tape drive,
the information is stored along linear tracks on the tape
surface.
[0004] In these applications, several different types of
transducers have been used that rely on magnetic properties for
writing to and/or reading from the magnetic medium. For an
inductive-type transducer, the direction of current through the
transducer is controlled during a write operation to encode
magnetic flux reversals on the surface of the medium within the
selected data track. When retrieving data from the medium, the
inductive transducer is positioned over the data track to sense the
flux reversals stored in the data track and generate a read signal
based on those flux reversals. In a magnetoresistive type of
transducing head, the flux reversals cause a change in the
resistance of the head, which is sensed by a detector circuit.
Typically, a reference current is passed through the
magneto-resistive head and the change in resistance is sensed by
measuring changes in the voltage across the head. Other types of
detecting circuits can also be used.
[0005] In order to understand the basic physics of a magnetic
transducing head during development and manufacturing, it is common
to test the response of the head to an applied magnetic field. For
example, one series of tests is known as "Transfer Curve Testing".
To generate a transfer curve for a particular transducing head, the
head is placed in a magnetic field (steady state or time varying)
and the output signal from the transducing head is measured. The
transfer curve is simply a plot of the output signal versus the
applied magnetic field, where the field is varied from some
negative value to some positive value, which is usually the same
magnitude as the negative value. For a magneto-resistive type of
head, the output signal consists of a steady state voltage, which
is a function of the bias current applied to the head, the bulk
resistance of the head and the applied magnetic field. Typical
characteristics that can be measured from the transfer curve data
include read signal amplitude at maximum field, noise with zero
field, noise with applied field, linearity over some range of
field, and symmetry. Symmetry is a comparison of the read signal
amplitude with a maximum positive field and the read signal
amplitude with a maximum negative field.
[0006] The rapidly changing technology in magnetic recording heads
has created a wide range of operating requirements for the heads as
well as a wide range of head performances. For example, reference
bias current requirements for a transfer curve tester can vary from
tens of micro-Amperes to many tens of milli-Amperes, and the
transfer curve tester may require tens of volts to drive the
reference current. For magneto-resistive types of heads, the
amplitudes of output voltages that must be measured can range from
tens of micro-volts to tens of milivolts, while the resistance of
the head can range from tens of Ohms to hundreds of Ohms. Also, the
steady-state voltage output due to the reference bias current is
typically hundreds of millivolts, but can be as large as tens of
volts with special devices.
[0007] These wide ranges of operating requirements and head
performances set very challenging requirements for the measurement
electronics. For example, in order to measure the noise of a 50 Ohm
head, the noise introduced by the transfer curve tester should be
less than 1 nV/{square root}Hz. Bias currents of 10 micro-Amperes
require a current source with an accuracy of better than 100 nAmps,
and the input bias currents drawn by the measurement electronics
should be similar to prevent measurement errors. All of these
requirements, when coupled with a potentially large DC bias
voltage, present a difficult design challenge for the measurement
electronics in the transfer curve tester. Typically one or more of
these requirements is substantially compromised.
[0008] Thus, a transfer curve tester having improved measurement
electronics and an accurate current source is desired.
SUMMARY OF THE INVENTION
[0009] One embodiment of the present invention is directed to a
voltage measurement circuit, which is has an AC-coupled
differential amplifier coupled to first and second input terminals
for measuring a voltage across the first and second input
terminals. The AC-coupled differential amplifier blocks a DC
component of the voltage and presenting a low input bias current to
the first and second input terminals while isolating the first and
second input terminals from charge stored in capacitances in the
AC-coupled differential amplifier.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a perspective view a typical head suspension
assembly for a rigid disc drive.
[0011] FIG. 2 is a simplified diagram of a head testing apparatus
according to one embodiment of the present invention.
[0012] FIG. 3 is a diagram of a head transfer curve tester having a
balanced, bi-directional current source and a low-noise measurement
circuit according to one embodiment of the present invention.
[0013] FIG. 4 is a diagram of an input stage used in the tester
shown in FIG. 3, according to one embodiment of the present
invention.
[0014] FIG. 5 is a diagram of a differential amplifier stage used
in the tester shown in FIG. 3, according to one embodiment of the
present invention.
[0015] FIG. 6 is a diagram of a differential amplifier stage
according to an alternative embodiment of the present
invention.
[0016] FIG. 7 is a diagram of the balanced, bidirectional current
source shown in FIG. 3, according to one alternative embodiment of
the present invention.
[0017] FIG. 8 is a diagram of a current control circuit used in the
current source shown in FIG. 7, according to one alternative
embodiment of the present invention.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0018] FIG. 1 is a perspective view of a typical head suspension
assembly for a rigid disc drive. Head suspension assembly 100
includes suspension 102, flexure 104 and slider 106. Slider 106
carriers a transducer or "head" for writing to and/or reading from
a disc surface in a disc drive. Slider 106 can carry a variety of
different types of transducers in alternative embodiments, such as
an inductive-type transducer, a magneto-resistive type transducer,
a giant magneto-resistive transducer, a spin tunnel junction
transducer or a magneto-optical transducer. During operation,
suspension 100 is attached to an actuator mechanism (not shown),
which moves suspension 100 and the transducer carried by slider 106
from track-to-track across the surface of the disc under control of
electronic circuitry.
[0019] In order to understand the basic physics of a magnetic
recording head during development and manufacturing, it is common
to test the response of the head to an applied magnetic field,
prior to assembling the head in a drive. For example, one series of
tests is known as "Transfer Curve Testing". To generate a transfer
curve for a particular recording head, the head is placed in a
magnetic field and the output signal from the head is measured. The
transfer curve is simply a plot of the output signal versus the
applied magnetic field, where the field is varied from some
negative value to some positive value, which is usually the same
magnitude as the negative value.
[0020] FIG. 2 is a simplified diagram of an apparatus for applying
a magnetic field to a recording head during transfer curve testing,
according to one embodiment of the present invention. The
particular testing apparatus shown in FIG. 2 is provided as an
example only. It should be understood that any suitable apparatus
can be used to generate a magnetic field for testing a magnetic
head or other magnetically responsive device in accordance with
alternative embodiments of the present invention. Magnetic field
generating apparatus 200 includes base 202, magnetic cores 204 and
206, windings 208 and 210 and air gap 212. Cores 204 and 206 are
arranged to generate a magnetic field (simple or complex) across
air gap 212 in orthogonal directions when excited by windings 208
and 210. Air gap 212 forms a test volume for receiving a magnetic
transducer under test. The magnetic transducer is inserted into air
gap 212 along axis 214 by a suitable positioning device. In one
embodiment, air gap 212 is sized to receive the distal end of a
head suspension assembly, such as that shown in FIG. 1, so as to
position slider 106 and is attached transducer between opposing
faces of cores 204 and 206. However, air gap 212 can be sized to
receive a plurality of head suspension assemblies, such as those
carried by an E-block actuator assembly in alternative
embodiments.
[0021] When the head being tested is positioned within air gap 212,
windings 208 and 210 are excited to generate a selected magnetic
field in air gap 212 according to a predetermined test pattern. The
response of the transducer being tested is then measured and
analyzed. Hall sensors 220 and 222 are positioned relative to air
gap 212 to measure the strength of the magnetic field that is
applied across the air gap. In one embodiment, Hall sensors 220 and
222 are supported by a magnetically permeable material 224, which
is positioned about air gap 212 between cores 204 and 206. Hall
sensor 220 measures the magnetic field generated between the
opposing faces of core 204, and Hall sensor 222 measures the
magnetic field generated between the opposing faces of core
206.
[0022] FIG. 3 is a diagram of a circuit 300 for electrically
biasing the recording head and measuring its response to the
magnetic field applied by the apparatus shown in FIG. 2, according
to one embodiment of the present invention. Circuit 300 includes
input terminals 301 and 302 for coupling to a recording head 303
under test. In one embodiment, input terminals 301 and 302 include
electrical sockets that are configured for quickly exchanging one
recording head 303 for another within circuit 300 as the transfer
curve or other characteristics of each recording head is
measured.
[0023] Circuit 300 further includes a current source 304, which is
coupled in parallel with input terminals 301 and 302 for applying a
reference bias current, I.sub.HEAD, to recording head 303. In one
embodiment, current source 304 is a balanced, bi-directional
current source that is capable of supplying large ranges of
currents at low noise. A measurement circuit 305 is coupled to
input terminals 301 and 302 for measuring the response of head 303,
as biased by I.sub.HEAD and excited by the magnetic field applied
by magnetic field generating apparatus 200 (shown in FIG. 2).
Measurement circuit 305 has a measurement output 306.
[0024] In one embodiment, a control circuit 310 controls the
sequence of tests performed on each recording head 303 through
control outputs 311 and 312 while measuring the resulting head
response on input 313. Control output 311 is coupled to a current
control input 314 of current source 304 for controlling the level
of bias current, I.sub.HEAD, applied to recording head 303 through
input terminals 301 and 302. The control signal applied to current
control input 314 can include a reference voltage or current, for
example. In an alternative embodiment, current source 304 is a
fixed, not adjustable current source. Output 311 is coupled to
magnetic field generating apparatus 200 (shown in FIG. 2) for
controlling the magnetic field applied to recording head 303. Input
313 is coupled to measurement output 306 for receiving a measure of
the head response for each level of applied magnetic field.
[0025] For each level of applied magnetic field, control circuit
310 stores the measured head response in memory 316 or supplies the
measured head response to user interface 318. The stored response
data, when plotted as a function of magnetic field, forms a
transfer curve for the respective recording head 303 under test as
the magnetic field is varied from some negative value to some
positive value. Typical characteristics that can be measured from
the stored transfer curve data include read signal amplitude at
maximum field, noise with zero field, noise with applied field,
linearity over some range of field, and symmetry. Symmetry is a
comparison of the read signal amplitude with a maximum positive
field and the read signal amplitude with a maximum negative field.
Control circuit 310 can be implemented with a discrete analog
control circuit, a discrete digital control circuit or state
machine, or a programmed computer such as a microcontroller, for
example. Other types of control circuits can also be used.
[0026] For a magneto-resistive type of head, the head response
consists of a steady state voltage on tester inputs 301 and 302,
which is a function of the reference bias current I.sub.HEAD
applied to recording head 303, the bulk resistance of the head and
the magnetic field applied by the apparatus shown in FIG. 2. As
described in more detail below, measurement circuit 305 quickly
measures changes in the voltage developed across recording head 303
while drawing a very low input bias current, introducing a very low
noise level into the measurement, and isolating the recording head
from potentially harmful capacitances within the measurement
circuit.
[0027] Measurement circuit 300 includes input stages 320 and 322,
AC-coupling capacitors 324 and 326, analog switches 328 and 330,
bias resistors 332 and 334 and differential amplifier 336. Input
stages 320 and 322 have inputs coupled to input terminals 301 and
302, respectively, and outputs coupled to capacitors 324 and 326,
respectively. Each input stage 320 and 322 includes at least one
buffering amplifier, such as an operational amplifier coupled as a
voltage follower. In one embodiment, the buffering amplifiers have
unity gain, but can apply other gain factors in alternative
embodiments. A variety of common operational amplifiers can be
used, such as LT1128 operational amplifiers available from Linear
Technology Corporation, which draw input bias currents on the
nano-Ampere level.
[0028] The use of buffering amplifiers in input stages 320 and 322
therefore provides low input bias currents to input terminals 301
and 302, which are significantly lower than the current that is
used to bias recording head 303. This allows for greater accuracy
in the measured voltage across terminals 301 and 302. Input stages
320 and 322 also protect head 303 from any stray charges that might
be present on capacitors 324 and 326 as successive heads 303 are
inserted and removed from the test socket. As recording heads
continue to become smaller, these heads can be damaged more easily
by such stray charges. Another function of input stages 320 and 322
is to isolate input terminals 301 and 302 from differential
amplifier 336 so that the amplifier will not alter the operating
point of the recording head 303 under test. In addition, input
stages 320 and 322 provide a strong drive current to quickly charge
and discharge capacitors 324 and 326 and thus the inputs to
differential amplifier 336 when the DC operating point of head 303
is changed. This quickens the settling time at measurement output
306. Even higher drive and lower noise can be achieved by coupling
multiple operational amplifiers in parallel with one another within
each input stage 320 and 322.
[0029] Capacitors 324 and 326 are coupled between input stages 320
and 322 and inputs 340 and 342 of differential amplifier 336 to
block any DC voltage component of the head response from reaching
the amplifier inputs. Bias resistors 332 and 334 are coupled
between respective amplifier input terminals 340 and 342 and ground
terminal GND for biasing each input of amplifier 336. Differential
amplifier 336 has an amplifier output 348, which is coupled to
measurement output 306 of measurement circuit 305. Capacitors 324
and 326 and bias resistors 332 and 334 together form a high-pass
filter. The capacitances of AC coupling capacitors 324 and 326 and
the resistances of resistors 332 and 334 are selected to achieve a
desired high-pass corner frequency for the filter as well as a
desired noise level introduced by the filter. The high-pass corner
frequency is determined by 1/(2.pi.RC), where R is the resistance
of resistors 332 and 334 and C is the capacitance of AC coupling
capacitors 324 and 326. In a typical transfer curve tester, this
corner frequency is often less than 1 Hz. In order to prevent noise
introduced by biasing resistors 332 and 334 from affecting the
composite noise of the filter, resistors 332 and 334 are made as
small as possible and the AC coupling capacitors 324 and 326 are
made as large as possible. With the drive that is available from
input stages 320 and 322, resistances of less than 100 ohms can
easily be used. The large AC coupling capacitors provide a low
impedance path through input stages 320 and 322 to shunt the
resistor noise to ground.
[0030] Differential amplifier 336 can include any discrete or
commercial differential amplifier that meets the desired noise and
bandwidth requirements. For example, a suitable commercial
instrumentation amplifier having low noise (1 nV/{square root}Hz)
and high gain is the SSM2017 amplifier, which is available from
Analog Devices. For low noise at lower gains, an instrumentation
amplifier made from multiple operational amplifiers such as the
LT1128 or AD797 can be used. The AD797 is also available from
Analog Devices. For extremely low noise (less than 0.5 nV/{square
root}Hz), multiple operational or instrumentation amplifiers can be
coupled together in parallel with one another.
[0031] The settling time of amplifier 336 to a change in the input
DC levels at input terminals 301 and 302 is determined by the RC
time constant of the filter formed by AC coupling capacitors 324
and 326 and bias resistors 332 and 334. This settling time can be
very long, which can impact the time required to fully test each
head 303 if there are a large number of measurement levels for each
head. To circumvent this problem, analog switches 328 and 330 are
coupled in parallel across bias resistors 332 and 334,
respectively. Switches 328 and 330 have switch control inputs 344
and 346, which are coupled to and controlled by control circuit
310. When control circuit 310 changes the head bias current
I.sub.HEAD through current control input 314, control circuit 310
closes switches 328 and 330 through switch control inputs 344 and
346 for a time sufficient for capacitors 324 and 326 to fully
charge. When switches 328 and 330 are closed, the RC time constant
is reduced by several orders of magnitude, allowing the
differential amplifier 336 to settle quickly to the new DC
operating point. Control circuit 310 then opens switches 328 and
330 and records the response on measurement output 306 in memory
316.
[0032] FIG. 4 is a diagram illustrating one of the input stages 320
and 322 in greater detail. Each input stage 320 and 322 includes an
input 400, an output 402 and a plurality of buffer amplifiers 404,
which are coupled in parallel with one another. Each buffer
amplifier 404 is coupled to operate as a voltage follower. The
output of each amplifier 404 is coupled to buffer output 402
through a respective summing resistor 406. Input stages 320 and 322
can also be designed to provide gain from buffer input 400 to
buffer output 402 or can be simple buffers with unity gain as shown
in FIG. 4. By coupling a plurality of amplifiers 404 in parallel
with one another, an input stage having a low noise level, such as
less than 0.5 nV/{square root}Hz, an input bias current in the
nano-amp range and a moderately large drive current can be
realized.
[0033] The input bias current drawn through buffer input 400 and
the current driven through buffer output 402 go up in proportion to
the number of devices used, i.e.:
[0034] Total I.sub.BIAS=N*I.sub.BIAS of each device; and
[0035] Total I.sub.OUT=N*I.sub.OUT of each device
[0036] where N is the number of parallel-connected amplifiers in
each stage.
[0037] Conversely, the noise of input stages 320 and 322 go down by
the square root of the number of devices used, i.e.:
V.sub.NOISE TOTAL=[V.sub.NOISE OF EACH DEVICE]/{square root}N
[0038] FIG. 5 is a diagram illustrating differential amplifier 336
in greater detail according to one embodiment of the present
invention. Amplifier 336 includes a plurality of individual
amplifiers 500 coupled in parallel with one another. Summing
resistors 502 are coupled in series between the outputs of
respective amplifiers 500 and differential amplifier output 348.
Again, the use of multiple operational or instrumentation
amplifiers in parallel with one another can provide for extremely
low noise (less than 0.5 nV/{square root}Hz) within differential
amplifier 336.
[0039] FIG. 6 is a diagram illustrating differential amplifier 336
in greater detail according to an alternative embodiment of the
present invention, which uses multiple operational amplifiers 600
for providing low noise at a lower gain. The same reference
numerals are used in FIG. 6 as were used in FIG. 3 for the same or
similar elements. Other types of differential amplifier circuits
can also be used in accordance with the present invention.
[0040] FIG. 7 is a block diagram which illustrates current source
304 in greater detail according to one embodiment of the present
invention. Current source 304 is a balance, bidirectional current
source that includes current control circuit 700, amplifiers 702
and 704, sense impedances 706 and 708, analog voltage inverter 710
and current output terminals 712 and 714. Current control circuit
700 includes current control input 314, feedback inputs 720 and 722
and control output 724. Current control circuit 700 generates a
control voltage on control output 724 based on the voltage received
on current control input 314 from control circuit 303 (shown in
FIG. 3) and a voltage developed across feedback inputs 720 and 722.
The control voltage on control output 724 is coupled to the input
of amplifier 702 and to the input of analog voltage inverter 710.
Analog voltage inverter 710 inverts the control voltage on control
output 724 and provides the inverted control voltage to the input
of amplifier 704. In an alternative embodiment, analog voltage
inverter 710 is removed, and amplifier 704 is replaced with an
inverting amplifier.
[0041] Amplifiers 702 and 704 can include operational amplifiers or
class A/B bi-polar amplifiers, for example. Other types of
amplifiers can also be used. In one embodiment, amplifiers 702 and
704 have unity gain, but can have other gain values in alternative
embodiments. To provide a balanced, bidirectional current to
current output terminals 712 and 714, amplifiers 702 and 704 are
matched to one another, with each amplifier having the same input
bias current, the same output drive current and the same gain from
input to output.
[0042] The outputs of amplifiers 702 and 704 are coupled to sense
impedances 706 and 708. Sense impedance 706 is coupled in series
between the output of amplifier 702 and current output terminal
712. Similarly, sense impedance 708 is coupled between the output
of amplifier 704 and current output terminal 714. In one
embodiment, sense impedances 706 and 708 each include a resistance
coupled in series between the respective amplifier output and the
respective current output terminal 712 and 714. Sense impedances
706 and 708 are matched to one another to provide a balanced
differential output current through current output terminals 712
and 714.
[0043] The voltage developed across sense impedance 706 is fed back
to feedback inputs 720 and 722 of current control circuit 700.
Current control circuit 700 measures the voltage developed across
the sense impedance at feedback inputs 720 and 722 and compares
this voltage to the reference voltage provided on current control
input 314. Based on this comparison, current control circuit 700
adjusts control output 724 such that the desired current level is
supplied through current output terminals 712 and 714, as
represented by the voltage drop across sense impedance 706.
[0044] FIG. 8 is a schematic diagram, which illustrates current
control circuit 700 in greater detail, according to one embodiment
of the present invention. Current control circuit 700 includes
buffer and signal conditioning circuit 800, summation node 802,
integrator 804, which is formed by amplifier 806 and capacitor 808,
and differential amplifier 810. The reference voltage provided on
current control input 314 is coupled to an addend input 812 of
summation node 802. Feedback inputs 720 and 722 are coupled to
respective inputs of differential amplifier 810. The output of
differential amplifier 810 is coupled to a subtrahend input 814 of
summation node 802. The output 816 of summation node 802 is coupled
to the non-inverting input of amplifier 806 within integrator 804.
The inverting input of amplifier 806 is coupled to ground terminal
GND. Capacitor 808 is coupled between the output and the
non-inverting input of amplifier 806. The output of amplifier 806
forms control output 724 of current control circuit 700. In
operation, summation node 802 compares the reference voltage
received on current control input 314 to the voltage measured
across sense impedance 706 (shown in FIG. 7) and provides the
difference to integrator 804. In response, integrator 806 adjusts
the voltage on control output 724.
[0045] Current source 304 provides a balanced, differential output
current with a large voltage output using off-the-shelf operational
amplifiers. A balanced differential output, while doubling the
available current for driving high-resistance recording heads, also
assures that the measurement system has low noise by providing true
"4-point" measurement capability.
[0046] It is to be understood that even though numerous
characteristics and advantages of various embodiments of the
invention have been set forth in the foregoing description,
together with details of the structure and function of various
embodiments of the invention, this disclosure is illustrative only,
and changes may be made in detail, especially in matters of
structure and arrangement of parts within the principles of the
present invention to the full extent indicated by the broad general
meaning of the terms in which the appended claims are expressed.
For example, the current source and measurement circuit can be used
together or independently in applications other than testing
recording heads. The current source can be used in any application
in which a bi-directional current source is useful. The measurement
circuit can be used in any application requiring or benefiting from
a low input bias current, low noise and high accuracy. Also,
individual components can be implemented with analog circuit
elements, digital circuit elements or a combination of both.
* * * * *