Abstract:
A circuit includes at least one input node, at least one output node, a signal conditioning circuit, and an electrostatic discharge (ESD) circuit. The signal conditioning circuit is characterized by a transfer function and adapted to receive input signals from and provide conditioned output signals to the input and output node(s), respectively. The signal conditioning circuit processes the input signal and outputs the conditioned output signal as a function of the transfer function. The ESD protection circuit is adapted to the signal conditioning circuit (i) to suppress electrostatic discharge signals applied to at least one of the input or output node(s) to levels electrically non-destructive to the signal conditioning circuit and (ii) to interact with the signal conditioning circuit in a manner substantially maintaining the transfer function.

Description:
BACKGROUND  
         [0001]    Electrostatic discharge (ESD) causes circuit failures. ESD may be caused by current surges or arcing due to lightning, human contact (i.e., charged non-conductive element contacting a conductive element), or “hot” connect or disconnect of circuits via cable connectors, Typically, ESD reaches a circuit via pins at the cable level, circuit board level, or chip level. ESD pulses cause power, high voltage, or current spikes (i.e., power surges) that can damage electronics that are not equipped to dissipate the power or withstand peak voltages or currents.  
           [0002]    Some classes of ESD protection circuits include semi-conductor layering schemes (U.S. Pat. No. 6,091,082), voltage clamps (U.S. Pat. Nos. 6,259,573 and 5,903,415), and resistive bleed circuits (U.S. Pat. No. 5,539,598).  
         SUMMARY  
         [0003]    Typical sensors have been protected against ESD inadequately. Some protection techniques affect functionality of an ESD protected circuit in the sensors. Thus, there is a need for improved protection that effectively maintains the transfer function of the ESD protected circuit.  
           [0004]    According to the principles of the present invention, inventive circuitry includes at least one input node, at least one output node, a signal conditioning circuit, and an electrostatic discharge protection circuit. The signal conditioning circuit is characterized by a transfer function and is adapted to receive input signals from and provide conditioned output signals to the input and output node(s), respectively. The signal conditioning circuit processes the input signal and outputs the conditioned output signal as a function of the transfer function. The electrostatic discharge protection circuit is adapted to the signal conditioning circuit (i) to suppress electrostatic discharge signals applied to at least one of the input or output node(s) to levels (e.g., voltage, current, or frequency) electrically non-destructive to the signal conditioning circuit and (ii) to interface with the signal conditioning circuit in a manner that substantially maintains the transfer function.  
           [0005]    In one embodiment, the electrostatic discharge protection circuit suppresses electrostatic discharge signals greater than 2000 Vpp. In another embodiment, the electrostatic discharge protection circuit may suppress electrostatic discharge signals up to about 4000 Vpp.  
           [0006]    The circuitry may further include a transducer connected to the input node(s) to provide the input signal. In one embodiment, the transducer is an accelerometer.  
           [0007]    The signal conditioning circuit may include a high impedance input stage, such as a charge amplifier.  
           [0008]    The electrostatic discharge protection circuit may include at least one capacitor, which may be coupled to the signal conditioning circuit at the input or output node(s). The electrostatic discharge protection circuit may be absent non-linear circuit elements. At least one of the capacitors may have capacitance values matching a capacitance value of a transducer providing the input signal.  
           [0009]    The signal conditioning circuit may include an input stage having an operational amplifier, where the capacitor(s) may be electrically connected between the input terminals of the operational amplifier.  
           [0010]    The circuitry may be stimulated by the electrostatic discharge in a manner producing an oscillation having a high frequency. The oscillation may be measurable at the input or output terminal(s) or may occur within sections of or active circuits within. The electrostatic discharge protection circuit may reduce a peak amplitude of the oscillation or reduce the frequency of oscillation.  
           [0011]    The circuitry may be used to measure vibration of an aircraft engine during operation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0013]    [0013]FIG. 1 is a diagram of a military aircraft in which an embodiment of the present invention may be employed;  
         [0014]    [0014]FIG. 2 is a diagram of a jet engine of the aircraft of FIG. 1 in which a vibration sensor, having an ESD protection circuit adapted to a circuit in the vibration sensor according to the principles of the present invention, measures vibration of the jet engine;  
         [0015]    [0015]FIG. 3 is a mechanical diagram of the circuitry of FIG. 2;  
         [0016]    [0016]FIG. 4 is a block diagram of the circuitry of FIG. 2;  
         [0017]    [0017]FIG. 5 is a wire diagram of a cable connecting to the circuitry of FIG. 2;  
         [0018]    [0018]FIG. 6A is a schematic diagram of the circuitry of FIG. 2 without the ESD protection circuit;  
         [0019]    [0019]FIG. 6B is a Bode plot representative of transfer function of the circuitry of FIG. 6A;  
         [0020]    [0020]FIG. 6C is a waveform diagram captured at an input node of the circuitry of FIG. 6A;  
         [0021]    [0021]FIG. 7A is the schematic diagram of the circuitry of FIG. 2 with an embodiment of the ESD protection circuit;  
         [0022]    [0022]FIG. 7B is a Bode plot representative of the transfer function of the circuitry of FIG. 7A; and  
         [0023]    [0023]FIG. 7C is a waveform diagram captured at the same input node as FIG. 6C that shows the voltage suppression provided by the ESD protection circuitry of FIG. 7A. 
     
    
     DETAILED DESCRIPTION  
       [0024]    A description of preferred embodiments of the invention follows. The preferred embodiments will be described herein in relation to an aircraft environment, and, in particular, a military aircraft environment.  
         [0025]    [0025]FIG. 1 is a graphical diagram of an example system in which an electrostatic discharge (ESD) circuit according to the principles of the present invention may be employed to protect an electronic circuit, such as environment sensor circuitry. The system, in this case, is a military aircraft  100 .  
         [0026]    The aircraft  100  includes a jet engine  105  that is subject to a wide dynamic of environmental conditions, including high temperature and vibration. The environmental conditions may be monitored by a system controller, which receives input from environmental sensors. One such environmental sensor is a vibration sensor.  
         [0027]    [0027]FIG. 2 is a graphical diagram of the jet engine  105  and a vibration sensor  200  used to measure the vibration of the jet engine  105 . The vibration sensor  200  includes a transducer  230  and signal conditioner  220 . The transducer  230 , such as an accelerometer, is mechanically connected to the jet engine  105 . The transducer  230  converts the sensed environmental condition (e.g., a mechanical vibration) to an electrical charge signal  227  in a typical manner. The electrical charge signal  227  is conveyed to a signal conditioner  220  via a cable pigtail  225 .  
         [0028]    A system controller  240  may communicate with the signal conditioner  220  via cables  235  and  210 . The interface cables  235  and  210  may be electrically connected via connectors  215  and  205  that are designed to operate in a harsh environment, such as experienced by the military aircraft  100 .  
         [0029]    The signal conditioner  220  includes electrical components subject to damage caused by electrostatic discharge. The electrostatic discharge may be generated in various ways, such as lightning, human contact, or “hot” connects or disconnects during system test and integration phases of building the aircraft  100 . For example, an ESD pulse  245  may be delivered to the signal conditioner  220  via the interface cables  235  and  210 . Alternatively, the ESD pulse  245  may occur prior to system assembly (e.g., circuit board functional testing) or during system test and integration of the signal conditioner  220  following interface cable  210  connection to the signal conditioner  220 .  
         [0030]    [0030]FIG. 3 is a mechanical diagram of the vibration sensor  200 . In this embodiment, the signal conditioner  220  resides in a protective box  315  that is electrically isolated from a larger case  310 .  
         [0031]    The electrical isolation may be provided by non-conductive standoffs, rubber, epoxy, and so forth. The electrical isolation typically provides at least 20 kV protection for the signal conditioner  220 .  
         [0032]    The pigtail cable  225  electrically connecting the transducer  230  to the signal conditioner  220  enters through the case  310  and the protective box  315 . The interface cable  210  that connects the signal conditioner  220  to the system controller  240  includes a connector  205  having interface pins  305 . The pins connect to associated wires in the interface cable  210  that provide the electrical conduction means through which signals, including an electrostatic discharge pulse  245 , travel from the system controller  240  through the cable  210  to cause damage to the signal conditioner  220 .  
         [0033]    [0033]FIG. 4 is a block diagram of the sensor  200 . The transducer  230  provides an electrical charge signal  227  to the signal conditioner  220 . The ESD protection circuit  215  may protect the signal conditioner  220  by processing the electrical charge signal  227  prior to being received by the signal conditioner  220 . The ESD protection circuit  215  may protect the signal conditioner  220  by processing the electrical charge signal  227  after being received by the signal conditioner  220 . The electrical charge signal  227  may travel a path not susceptible to damage by the ESD pulse  245 , so the electrical charge signal  227  may not be processed by the ESD protection circuit  215 . In other words, the ESD protection circuit  215  may precede the signal conditioner  220 , be an integral part of the signal conditioner  220 , or be applied to select areas or interface pins  305  of the signal conditioner  220 .  
         [0034]    The signal conditioner  220  may include a charge amplifier  510 , integrator  515 , low pass filter  520 , and high pass filter  525 . In this exemplary signal conditioner  220 , the output signal from the high pass filter  525  is the output signal from the signal conditioner  220 . The output signal may pass through the ESD protection circuit  215  before being provided to the system controller  240 . The output signal is among multiple interface signals  530  communicated to or provided by the system controller  240 , including, for example, a common (i.e., ground reference), power (e.g., +/−15V), and test signal.  
         [0035]    In this embodiment, the output signal from the signal conditioner  220  is a velocity representation for the case where the transducer  230  is an accelerometer. Transformation from acceleration to velocity is provided through the use of the integrator  515 . The charge amplifier  510  converts the electrical charge signal  227  provided by the transducer  230  into a voltage for the integrator  515  to integrate. The low pass filter  520  and high pass filter  525  condition the velocity signal to remove electrical noise and provide proper amplification in a predetermined frequency range. The signal conditioner  220  may be more vulnerable to the ESD pulse  245  because of the high gains provided by the charge amplifier  510 , integrator  515 , high impedance of the charge amplifier  510 , and low output impedance provided by the high pass filter  525 .  
         [0036]    Because the transducer  230  is connected to the ESD protection circuit  215  and signal conditioner  220  in a “closed” circuit, there is little chance for an ESD pulse to be sourced by the transducer  230 . It is more likely that an ESD pulse  245  contacts the electronics of the sensor  200  via one of the interface signals  530 . This is shown more clearly in a wiring diagram in FIG. 5.  
         [0037]    [0037]FIG. 5 is a wiring diagram for the interface cable  210  and pigtail cable  225  that connects the signal conditioner  220  and ESD protection circuit  215  to the system controller  240  and transducer  230 , respectively. In this example, the transducer  230  connects to pins  3  and  9 . The system controller  240  connects to pins  1 ,  2 , and  4 - 8  via wires  505  to the interface pins  305 . Signals carried by the wires  505  connected to the interface pins  305  to which the system controller  240  connects are a signal out, power (i.e., +Vss and −Vss), common, built-in-test (BIT) and signal return. It should be understood that these signals are merely exemplary and may include both analog and digital signals.  
         [0038]    The interface cable  210  may be more than  12  inches long to provide some physical distance between the vibration sensor  200  and the system controller  240 . The pigtail cable  225 , in one embodiment, extends about 0.5 inches from the signal conditioner  220 . This allows the transducer  230  to be vibrationally isolated from the signal conditioner  220 , but not so long as to have electrical characteristics of the pigtail cable  225  affect the electrical charge signal  227  produced by the transducer  230 .  
         [0039]    [0039]FIG. 6A is a schematic diagram of the vibration sensor  200  that includes the signal conditioner  220  and transducer  230 . The transducer  230  connects to the signal conditioner  220  via the pigtail cable  225 . The electrical charge signal  227  produced by the transducer  230  is received at connector J 1 , pins  3  and  5 . The electrical charge signal  227  at pin  3  may be capacitively coupled (i.e., high pass filtered) to the charge amplifier  510  by capacitors C 14  and C 12 , which remove bias drift effects of the transducer  230 , for example.  
         [0040]    The charge amplifier  510  converts the electrical charge signal  227  to a voltage between +/−5V. The voltage produced by the charge amplifier  510  is electrically integrated by the integrator  515 , converting the acceleration signal, for example, to a velocity signal. The integrated signal may be low and high pass filtered by a low pass filter  520  and high pass filter  525 , respectively. The output signal is presented at connector J 1 , pin  2 , which is capacitively coupled through capacitor C 18 , which was previously employed to provide ±2 kV ESD protection at the output, to a signal return at connector J 1 , pin  8 . The signal conditioner  220  may also include an on-board power conditioner  605  to convert input power voltages presented at connector J 1 , pin  4  and J 1 , pin  7  from a high voltage to a lower voltage, while at the same time filtering the input power voltages to protect the circuitry in the signal conditioner  220 .  
         [0041]    The signals from the system controller  240  (FIG. 2) are received at connector J 1  via the interface cable  210 . This means that if the ESD pulse  245  is received via the interface cable  210 , it may affect some or all of the circuitry of the signal conditioner  220 . Potentially, because pins  4 ,  5 , and  8  are coupled directly to a ground or common  610  of the signal conditioner  220 , the ESD pulse  245  can damage any of the circuitry, particularly the active circuitry. Thus, in certain circuits, such as this signal conditioner  220 , the electrostatic discharge protection circuit  215  may be strategically placed in a manner that protects the circuitry, yet maintains the functionality and transfer function of the signal conditioner  220 . For example, if the ESD protection circuit  215  were to affect the transfer function of the signal conditioner  220 , the output signal at J 1 , pin  2  may cause error in the output signal provided to the system controller  240 .  
         [0042]    [0042]FIG. 6B is a Bode plot of a transfer function corresponding to the signal conditioner  220  of FIG. 6A. In this particular example, a velocity signal corresponding to the vibration acceleration of the jet engine  105  is anticipated to be between about 70 Hz and 800 Hz. Accordingly, the transfer function has gain in that expected range of velocity signals.  
         [0043]    [0043]FIG. 6C is a waveform capture measured across J 1 , pins  3  and  8  (transducer-HI and SIG RTN, respectively) of the signal conditioner  220  resulting from the ESD pulse  245  of ±4 kV applied to J 1 , pin  6  (BIT N). The ±4 kV ESD pulse  245  causes an oscillation of about 7.7 MHZ, with peak voltage about 130-200 Vpp at about 200 nsec after the application of the ESD pulse  245 . Moreover, the oscillation may cause heating in active devices, such as the charge amplifier  510 . The heating may cause second order breakdown (i.e., failure) of semi-conductor devices in the operational amplifier of the charge amplifier  510 .  
         [0044]    Engine environmental sensors for the aircraft  100  (FIG. 1) contain state of the art electronics that are specified to be capable of performing up to +225° C. and withstand ESD pulses up to ±4000 V ESD. Existing sensors, such as the vibration sensor  200  employing the signal conditioner  220  of FIG. 6A, do not meet this ESD requirement. Existing environmental sensors are typically capable of withstanding not more than ±2000 V ESD. Numerous tests with ±4000 V ESD pulses applied to the vibration sensor  200  of FIG. 6A and associated connector pins have led to damage of several components in the signal conditioner  220 , which may be implemented in a Pin Grid Array (PGA) package in the vibration sensor  200 .  
         [0045]    During failure testing and analysis of the vibration sensor  200  of FIG. 6A, application of ±4000 V ESD pulses to each sensor and connector pin of the unprotected circuit were found to have damaged various electronic components, such as resistors and operational amplifiers. Failure analysis of the signal conditioner  220  showed that several components were damaged by the ±4 kV ESD pulse  245 . Those components included R 3 , R 11 , R 9 , R 10 , and U 1 . Measuring the circuit at pins  3  and  8  during application of the ESD pulse  245  revealed the waveform of FIG. 6C. This 7.7 MHZ oscillation frequency, with amplitudes ranging as high as 120 to 200 volts peak-to-peak (Vpp) depending upon the air gap between the connector pins and the end of an ESD gun tip used to produce the ESD pulses, appears to have been a contributing factor to the damage of the aforementioned circuit elements. This high frequency oscillation may inject power into the active circuitry, such as U 1 A, that the active circuitry cannot dissipate rapidly enough. The high-frequency oscillations may damage amplifiers, such as the charge amplifier  510 , having high open- or closed-loop gain at high frequencies.  
         [0046]    Further investigation showed that all pins of the connector were affected, not just certain pins, because cable capacitance between each wire and cable case has a value of 200-230 pF. This is enough capacitance coupling between all pins, except spare pins, for the 7.7 MHZ oscillations to pass from each pin of the cable connector to the unprotected circuit. The ESD protection circuit  215  has been developed to protect the signal conditioner  220  and transducer  230  through the use of additional circuit elements. In one embodiment, the ESD protection circuit  215  includes five capacitors for the circuitry of FIG. 6A. These capacitors prevent circuit damage during ESD testing, where the maximum amplitude of oscillation is decreased from a damaging level of as high as 200 Vpp to a safe level of about 67 Vpp. At this amplitude, all components of the signal conditioner  220  are safe.  
         [0047]    This solution has proven to be successful. Three vibration sensors  200  were tested with +4 kV and −4 kV ESD pulses applied five times on each connected pin with no damage to the sensors.  
         [0048]    In an analog circuit, such as the signal conditioner  220 , ESD protection circuits may have a deleterious effect on the performance of the analog circuit, as measured by comparing the transfer function before and after the ESD protection circuit  215  is applied. An example of a circuit that may change the operational characteristics of the signal conditioner  220  are non-linear circuits, including elements such as diodes, variacs, or transistors. These circuit elements may be used to clip peak pulse amplitudes, but such clipping may cause a system level oscillation, or these non-linear circuit elements may have capacitive and inductive characteristics that may interact with the signal conditioner  220 , causing a change in the transfer function of the signal conditioner  220 .  
         [0049]    The following describes an embodiment of the ESD protection circuit  215  as applied to the signal conditioner  220 .  
         [0050]    [0050]FIG. 7A is a schematic diagram of the signal conditioner  220  to which an embodiment of the ESD protection circuit  215  has been applied to protect the signal conditioner  220  from ±4 kV ESD pulses  245  from damaging the circuitry. Specifically, in this example circuit, resistors R 3 , R 11 , R 9 , and R 10  are protected, and operational amplifier U 1  is also protected.  
         [0051]    To protect these circuit elements, capacitor C 16  is electrically connected between pins  6  and  5 ; capacitor C 17  is electrically connected between pins  9  and  5 ; and capacitor C 15  is electrically connected between pins  3  and  5 ; and capacitor C 18  is connected between pins  2  and  8 . Capacitor C 15  has a capacitance of 1 nF, which is equivalent to the capacitance of the transducer  230  so as not to upset the balance of the associated circuit(s) while protecting the circuit(s) against high oscillation peak voltages or frequencies. Similarly, capacitors C 16  and C 17  may be selected to reduce the peak of the oscillation signal or the frequency at which the oscillation signal oscillates, while interfacing with the signal conditioner  220  in a manner substantially maintaining the transfer function of the signal conditioner  220 .  
         [0052]    In addition to the aforementioned capacitors being applied to the signal conditioner  220  as part of the ESD protection circuit  215 , capacitor C 13  is also applied to the circuit between pins  2  and  3  of U 1 A. Applying C 13  to the operational amplifier U 1 A provides a low-pass filter in combination with the input resistor R 3  and the feedback elements R 1  and C 1 . Thus, the low-pass filter provided by applying C 13  to pins  2  and  3  of operational amplifier U 1 A reduces the amplitude and frequency of the oscillation caused by applying a ±4 kV ESD pulse  245  to the pins (e.g., pin  6 ) of the signal conditioner  220 . Further, frequency at the charge amplifier  510  may cause oscillation in the feedback loop configuration, which may create thermal heating inside the operational amplifier U 1 A composing the charge amplifier  510  leading to component failure.  
         [0053]    As mentioned in reference to FIG. 6A, the capacitor C 18  between pins  2  and  8  is part of a previous ESD protection solution that helps to protect the high pass filter  525  (i.e., output stage) from experiencing a high voltage or high oscillation frequency resulting from an application of a ±2 kV ESD pulse  245  at pin  2 . In combination with capacitors C 16 , C 17 , C 15 , and C 13 , or a subset thereof, C 18  protects against a ±4 kV ESD pulse.  
         [0054]    [0054]FIG. 7B is a Bode plot of the frequency response of the signal conditioner  220  with the ESD protection circuit  215  applied, as discussed in reference to FIG. 7A. As can be determined by comparison with the frequency response of FIG. 6B for the signal conditioner  220  without the ESD protection circuit  215 , the transfer function is substantially unaffected by the application of the ESD protection circuit  215 . Note the smoothing of the frequency response between about 10 kHz and 100 kHz caused by the addition of the capacitors composing the ESD protection circuit  215  in this embodiment. The frequency response of FIG. 7B, as in the case of FIG. 6B, is measured between pin  2  and pin  3  of connector J 1  (i.e., SIG OUT vs. SIG IN).  
         [0055]    [0055]FIG. 7C is a waveform capture of the oscillation between pins  3  and  8  caused by an application of ±4 kV on pin  6  of connector J 1 . As can be seen in comparison with the corresponding waveform capture of FIG. 6C, the peak of the oscillation has also been reduced from 65 Vpk to 37 Vpk, and the frequency of oscillation has been reduced, thereby reducing the amount of thermal heating in the active component U 1 . Additionally, because the peak voltage is reduced, the likelihood of damaging other passive or active elements in the signal conditioner  220  is also reduced.  
         [0056]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.