Abstract:
The present invention is related to a filter circuit that allows preamplifiers, such as those used in hard disk drives, to quickly recover from transients such transients caused by power switching, write to read switching, and thermal asperity. According to an embodiment of the present invention, under normal conditions, such as signals at 100 KHz to 10 MHz, signals at medium frequencies, such as signals at 100 KHz to 10 MHz, pass through a low pass filtering circuit. When a transient signal is received, then a change in voltage for V out  of the low pass filtering circuit increases by more than 100*V out . For example, during transient conditions, the low pass filter circuit may also pass signals ranging from 10 MHz to 100 MHz. Accordingly, under transient conditions, the low pass pole moves approximately 100 times or more. A feedback loop subtracts the resulting low pass signals from the original signal, acting as a high pass filter. The overall circuit recovers quickly from the transient by adjusting for the tail end of the transient. Transient conditions may include a signal that is low frequency or higher amplitude or both. As the transient condition disappears, then the filter circuit resumes normal operation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electronic circuits, in particular, the present invention relates to a circuit used in a preamplifier that allows fast recovery from electrical transients. 
     BACKGROUND OF THE INVENTION 
     Magnetic read head signal preamplifiers used in hard disk drives typically require fast recovery from electronic transients such as power switching, write to read switching, and thermal asperity. If the preamplifier is unable to recover quickly from transients, then performance of the hard disk drive may be compromised. Power switching occurs when the power is turned on or off. Write to read switching occurs when the disk drive changes functions from writing to the hard disk to reading from the hard disk. A transient such as thermal asperity occurs when a disk drive head hits the disk as the disk goes by. The contact with the disk causes heat on the disk drive head and this heat may show up as a voltage. A thermal asperity may generate a significant amount of energy and only a small fraction of that energy should be processed. These examples of electronic transients typically produce signals that are undesirable to process. If these electronic transients are processed, performance is typically reduced. 
     Fast recovery from electronic transients, such as power switching, write to read switching, and thermal asperity, is typically achieved via a complicated circuit performing complicated functions, such as dual peak detection and subtraction. Conventional preamplifier circuits that provide fast recovery from transients typically require a substantially large number of components. A large number of circuit devices potentially increase the risk of a device failure in addition to increasing the cost of the circuit. 
     It would be desirable to allow preamplifiers to quickly recover from electrical transients by integrating a simple circuit. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     The present invention is related to a filter circuit that allows preamplifiers, such as those used in hard disk drives, to quickly recover from transients such transients caused by power switching, write to read switching, and thermal asperity. According to an embodiment of the present invention, under normal conditions, such as signals at 100 KHz to 10 MHz, signals at medium frequencies, such as signals at 100 KHz to 10 MHz, pass through a low pass filtering circuit. When a transient signal is received, then a change in voltage for V out  of the low pass filtering circuit increases by more than 100*V out . For example, during transient conditions, the low pass filter circuit may also pass signals ranging from 10 MHz to 100 MHz. Accordingly, under transient conditions, the low pass pole moves approximately 100 times or more. A feedback loop subtracts the resulting low pass signals from the original signal, acting as a high pass filter. The overall circuit recovers quickly from the transient by adjusting for the tail end of the transient. Transient conditions may include a signal that is low frequency or higher amplitude or both. As the transient condition disappears, then the filter circuit resumes normal operation. 
     A system according to an embodiment of the present invention of filtering a signal in a preamplifier is presented. The system comprising a filter that passes a first range of signals during a first input signal condition and passes a second range of signals during a second input signal condition, wherein the filter produces a first signal, and wherein the second input signal condition includes a transient. The system also includes a differential amplifier coupled to the filter, wherein the differential amplifier subtracts the first signal from a second signal. 
     A method according to an embodiment of the present invention for filtering a signal in a preamplifier is also presented. The method comprising providing a first signal and determining whether the signal is a transient signal. The method also filters the signal at a first range of frequencies if the signal is not a transient signal, resulting in a first filtered signal; and filters the signal at a second range of frequencies if the signal is a transient signal, resulting in a second filtered signal. One of the first filtered signal and the second filtered signal is subtracted from a second signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a filter circuit according to an embodiment of the present invention for allowing a preamplifier to quickly recover from a transient event. 
     FIGS. 2A-2B is are schematic diagrams of a low pass filter circuit according to an embodiment of the present invention for allowing a preamplifier to quickly recover from a transient event. 
     FIG. 3 is a flow diagram of a method according to an embodiment of the present invention for filtering a signal in a preamplifier. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is presented to enable one of ordinary skill in the art to make and to use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     FIG. 1 is a schematic diagram of a circuit according to an embodiment of the present invention for quickly recovering from transients. The circuit  200  is shown to include a differential amplifier  202 , a transconductor  204 , and a low pass filter circuit  100 . The differential amplifier  202  receives a signal  206  and subtracts a low pass signal  208  produced by circuit  100  from the signal  206 . When the low pass signal  208  is subtracted from the original signal  206 , then the result is equivalent to a high pass filter. A high pass filter is a selective filter that transmits frequencies above a critical (cut-off) frequency and blocks frequencies below the cut-off value. The critical cut-off frequency is commonly referred to as a high pass pole. In the example shown in FIG. 1, the high pass pole is equivalent to the low pass pole of the low pass filter circuit  100 . An example of a low pass pole of circuit  100  and a high pass pole of circuit  200  is approximately 100 KHz. As the name suggests, a low pass filter is a selective filter that transmits frequencies below a critical (cut-off) frequency and blocks frequencies above the cut-off value. The cut-off frequency is commonly referred to as a low pass pole. 
     The range of the resulting high pass signal depends on the range of signals passed by the low pass filter circuit  100 . The high pass signal is then sent out via connection  250 . For example, the high pass signal sent out through connection  250  may be the output of a preamplifier in a disk drive. 
     In addition to being sent out through connection  250 , the high pass signal is also sent into the transconductor  204 . The transconductor  204  outputs a current as a linear function of an input voltage. For example, the transconductor  204  may have a multiplier of −100 microAmps per volt. The transconductor  204  produces an I in  of the low pass filter circuit  100 . An example of the AC signal of I in  under normal conditions is approximately 2 micro Amps at 10 MHz. The I in  of circuit  100  flows into circuit  100  via connection  210 . The V out  of circuit  100  is sent out of circuit  100  and into the differential amplifier  202  via connection  208 . 
     Under normal conditions, the circuit  100  acts as a low pass circuit that sends the resulting low pass signal through connection  208  and into a differential amplifier  202 . For example, under normal conditions, the low pass circuit may send low pass signals ranging from 100 KHz to 10 MHz. 
     A transient, such as thermal asperity, may occur when a disk drive head hits the disk as the disk goes by. The contact with the disk causes heat on the disk drive head and this heat may show up as a voltage. A thermal asperity may generate a significant amount of energy and only a small fraction of that energy should be processed. Accordingly, a high pass filter may be used to subtract out the low frequencies of the thermal asperity signal. 
     Under normal conditions, the circuit  100  passes low pass signals such as 100 KHz to 10 MHZ and it is subtracted from the signal in the differential amplifier  202 , thus producing a high pass result. For example, the high pass signal may be a signal ranging above 100 KHz to above 10 MHz, depending on the low pass signal produced by circuit  100 . However, under non-normal conditions, i.e., during receipt of transient signals such as those caused by thermal asperity, the circuit  100  moves its low pass pole by a significant amount, such as more than 100 times the low pass pole during normal conditions. For example, during transient conditions, the low pass filter circuit  100  may also pass signals ranging from 10 MHz to 100 MHz. Accordingly, the high pass pole of the differential amplifier  202  is effectively moved by a significant amount, such as more than 100 times the high pass pole during normal conditions. When the high pass pole is moved, then the remainder of the transient is filtered out and the circuit  100  moves back into normal conditions. 
     FIGS. 2A-2B are schematic diagrams of a low pass filter circuit, such as circuit  100  of FIG. 1, according to an embodiment of the present invention for filtering out unwanted low frequency signals and providing fast recovery from a transient, such as thermal asperity. The low pass filtering circuit  100  according to an embodiment of the present invention includes a resistor  102 , transistors  106   a - 106   b , and capacitors  104   a - 104   c . An example of the type of transistor for transistor  106   a  is an NMOS transistor, and for transistor  106   b  is a PMOS transistor. An example of the threshold voltages of transistor  106   a  and  106   b  is approximately 0.4V. An example of the capacitance of capacitors  104   a ,  104   b , and  104   c  are approximately 4×10 −12  farad, 10×10 −15  farad, and 200×10 −12  farad, respectively. An example of the AC signal of I in  under normal conditions is approximately 2 micro Amps at 10 MHz. An example of a range of the resistance of resistor  102  is approximately 10 kilo-Ohms to approximately 500 kilo-Ohms. The primary function of this circuit  100  is to reach mid-point operating frequencies and then quickly become nonresponsive to fluctuations in the frequencies. Circuit  100  allows a quick recovery to fluctuations in the frequencies such as those caused by thermal asperity. 
     An example of normal condition for the circuit  100  is approximately 20 milli-volts of input voltage (V in ) at 10 MHz with an input current (I in ) of approximately 2 micro Amps. An example of the circuit  100  operating under normal conditions is when I in &lt;[(V T )*(capacitance of capacitor  104   a )*(2Π)*(frequency of the input signal)]. Wherein V T  is a threshold voltage of either one of transistors  106   a  and  106   b . Under normal conditions, the function of circuit  100  is to perform a low pass filter function that allows medium frequencies, such as 100 KHz to 10 MHz, to pass through the circuit  100 . 
     FIG. 2A shows the path of the current in circuit  100  during normal conditions. I in  flows into the circuit  100  and through resistor  102 . The resistor  102  keeps the circuit  100  from oscillating back and forth between a state of the transistors  106   a - 106   b  being turned on and off and on again. Most of the current, such as 90% to 99% of I in  flows into capacitor  104   a , while the remaining current flows through capacitor  104   b  and into capacitor  104   c . Most of the current will flow through capacitor  104   a  because transistors  106   a  and  106   b  are off under normal conditions and capacitor  104   a  is bigger than capacitor  104   b.    
     In this example, normal conditions may be defined as I in  being&lt;[(V T )*(capacitance of capacitor  104   a )*(2Π)*(frequency of I in )]. If V T  is 0.4V, and capacitance of capacitor  104   a  is 4×10 −12  farad, at a frequency of 10 MHz, an I in  of approximately 100 micro-Amps would satisfy the condition of being less than [(V T )*(capacitance of capacitor  104   a )*(2Π)*(frequency of I in )]. The I in  under normal conditions is not high enough to turn on either transistor  106   a  or transistor  106   b.    
     Under normal conditions, the circuit  100  performs a low pass filter function that allows medium frequencies, such as 100 KHz to 10 MHz, to pass through the circuit  100 . When a signal that is a lower frequency or of higher amplitude or both, such as a thermal asperity, is received, then the conditions are considered not normal. An example of a non-normal condition signal is a signal of approximately 100 KHz and 25 milli-volts. Under these conditions, the difference in voltage between gate to source voltage on transistors  106   a - 106   b  will turn on transistor  106   b  if the voltage difference is positive, and turn on transistor  106   a  if the voltage difference is negative. The difference in voltage between gate to source voltage on a transistor, such as transistor  106   a  or  106   b , is commonly referred to as a threshold voltage of the transistor. An example of the threshold voltage of transistors  106   a  and  106   b  is approximately 500 milli-volts. 
     The circuit  100  shown in FIG. 2B illustrates the current path when transistor  106   a  or transistor  106   b  is turned on. If the voltage difference between the gate and source of a transistor is positive, then transistor  106   b  turns on. Accordingly, I in  flows through transistor  106   b  and directly into capacitor  104   c , bypassing capacitors  104   a ,  104   b  and resistor  102 . Accordingly, rather than capacitor  106   c  receiving a very small percentage of the incoming current, such as {fraction (1/400)} I, capacitor  104   c  now receives a large portion of the incoming current, such as 50% to 90%. Hence, there is an effective current gain from I in  to V out  of an increase greater than 100 times than during normal conditions. Accordingly, the low pass pole moves up to pass the signals caused by the thermal asperity. For example, during transient conditions, the low pass filter circuit may also pass signals ranging from 10 MHz to 100 MHz. 
     Likewise, if the voltage difference between the source and drain of transistors  106   a  and  106   b  is negative for an incoming signal that is a transient, such as a thermal asperity, then transistor  106   a  is turned on. The incoming current then flows through transistor  106   a  and directly into capacitor  104   c , bypassing capacitors  104   a ,  104   b , and resistor  102 . 
     Under non-normal conditions, such as when signals caused by a thermal asperity are received, the function of circuit  100  changes to increase V out  from the normal condition V out  of approximately one micro volt to approximately twenty-five milli-volt. 
     Transistors  106   a  and  106   b  may turn on when V in =2V T , wherein V T  is the threshold voltage of either transistor  106   a  or transistor  106   b . If I in  is operating at a voltage where both transistor  106   a  and transistor  106   b  are both off, then the V in  may remain unknown since I in  would be under normal conditions and V out  would also remain under normal conditions, such as 1 micro volt. If, however, either transistor  106   a  or transistor  106   b  is on, then V out  is adjusted and transistors  106   a  and  106   b  are turned off. Transistor  106   a  is turned off when the absolute value of the gate to source voltage of transistor  106   a  is less than V T , such as less than approximately 500 milli-volts. Likewise, transistor  106   b  is turned off when the absolute value of the gate to source voltage of transistor  106   b  is less than V T . 
     For example, if transistor  106   b  is on, then V out  is pulled down to the same value as the received signal, such as a change of voltage of approximately 25 milli-volts. The differential amplifier  202  of FIG. 1 then lowers its high pass pole, such as by one hundred times its previous pole. When I in  flows through capacitor  104   c , the output voltage of circuit  100  is lowered, which in turn moves the voltage at the differential amplifier  202  to a nominal voltage, such as the reference voltage. The change in voltage at differential amplifier  202  down to the nominal voltage then causes the current at transconductor  204  to become zero and the charge at low pass bleeds into capacitor  104   a  sufficient to shut off transistor  106   b . Likewise, if transistor  106   a  is on, then V out  is pulled up and the feedback loop of FIG. 1 pulls down I in  and turns off transistor  106   a.    
     According to an embodiment of the present invention, V in  does not need to be exactly determined. If the V in  is within an operating range, then the response of the circuit  100  is within desired specification, and if the V in  is outside of the operating range, then circuit  100  pulls the V in  back into operating range. 
     FIG. 3 is a flow diagram of a method according to an embodiment of the present invention for filtering a signal in a preamplifier. A first signal is provided (step  300 ), and it is determined whether the signal is a transient signal (step  302 ). A transient signal is expected to have low frequencies with a direct current (DC) component. If the signal is not a transient signal, then the signal is filtered at a predetermined low pass pole to produce a first range of frequencies (step  304 ). A non-transient signal is expected to have alternating pulses (AC) returning to a zero base line, free of DC. If, however, the signal is a transient signal, then the signal is filtered at a low pass pole that is significantly higher, such as 10 to 100 times higher, than the predetermined low pass pole to produce a second range of frequencies if the signal is a transient signal (step  306 ). The resulting filtered signal is then subtracted from an original signal (such as signal  206 ). 
     Although the present invention has been described in accordance with the embodiment shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiment and these variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.