Abstract:
A pseudo-differential accelerometer resistant to EMI is disclosed that includes a device with a sensor core connected to an integrated circuit including a chopper, differential amplifier, and dummy core. The chopper swaps input to output connections during different states. The dummy core is coupled to a dummy chopper input. Three bond wires coupling the sensor output to a sensor chopper input, a first chopper output to a first sensor input, and a second chopper output to a second sensor input can connect the sensor and integrated circuit. The device can include a dummy pad and dummy bond wire connecting the dummy pad to the dummy chopper input. This configuration requires four bond wires connecting the sensor and integrated circuit. A neutralization core can be connected to the sensor chopper input. The chopper can change states to smear noise across a wide range, or away from a band of interest.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This patent relates to capacitive transducers, and more particularly to techniques for overcoming electromagnetic interference in capacitive sensors. 
         [0002]    In inertial sensors, electromagnetic disturbance or interference (EMI) occurs primarily due to capacitive coupling between bond wires and nearby cables, plates, circuitry, etc.  FIG. 1  illustrates an exemplary scenario of EMI. In  FIG. 1 , a microelectromechanical structure (MEMS) device  102  is coupled to an application specific integrated circuit (ASIC)  104  by a plurality of bond wires  106 . A source of EMI  110  that is near the bond wires  106  creates capacitive coupling  112  between the EMI source  110  and the bond wires  106 . Capacitor symbols are shown in  FIG. 1  to illustrate the capacitive coupling  112 , but this simply illustrates the parasitic capacitance between the electromagnetic disturbance source  110  and the bond wires  106 , no actual electrical component is present. The bond wires coupling capacitive nodes are the most sensitive to EMI, as opposed to nodes driven by a voltage source or amplifier. 
         [0003]    In environments with a high density of electronics, there can be numerous sources of EMI, and these EMI sources can be significant. The electromagnetic disturbances can also occur at substantially a single frequency, which upon sampling can get folded into a DC component. These electromagnetic disturbances can land on top of a desired sensor signal and obliterate the desired signal. For example, if a desired signal is sampled at a 100 kHz clock frequency, and the disturbance is at 100 kHz, then when sampling the disturbance at the clock frequency it can appear as a substantially DC signal. Thus, it is important to protect desired sensor signals, especially along capacitive paths, from EMI. The EMI problem is especially important to solve in safety critical applications that are in harsh environments, for example the sensors used for electronic stability in an automobile. 
         [0004]    Two commonly used solutions to EMI are shielding the sensor with metal, and using a differential approach. Shielding the sensor with metal includes creating a Faraday cage to block external electric fields which can cause EMI. However, shielding can be bulky and expensive, especially when there are numerous sensors to be shielded. 
         [0005]    The differential approach takes the differences between signals on parallel wires which can substantially subtract out the electromagnetic disturbance as a common mode signal.  FIG. 2  illustrates the differential approach with an exemplary differential sensor and amplifier system  200  that includes a MEMS device  220  coupled to an ASIC  240  by bond wires  260 ,  262 . Each of the bond wires  260 ,  262  experiences EMI from external EMI sources  210 . Capacitive coupling  250  between the EMI source  210  and the first bond wire  260  creates a first disturbance capacitance C1, and capacitive coupling  252  between the EMI source  210  and the second bond wire  262  creates a second disturbance capacitance C2. If the disturbance capacitances C1 and C2 between the EMI sources  210  and the bond wires  260 ,  262  are the same, then the electromagnetic disturbance can be rejected due to the common mode rejection of the differential amplifier of the ASIC  240 . However, in order to achieve the desired cancellation, the disturbance capacitances C1 and C2 between the EMI sources  210  and the bond wires  260 ,  262  should be closely matched, for example a difference of less than 0.5%. This matching can be very difficult to achieve in practice. Even if the matching is achieved initially, bond wires can be disturbed or warped, for example by an automobile accident. This movement of the bond wires can cause asymmetry between the bond wires, which can cause an unwanted mismatch in the disturbance capacitances and reduce the effectiveness of the differential approach. For this reason additional techniques can be used to smear the EMI energy over a wide frequency range. 
         [0006]    Accelerometers are often implemented in harsh vibration-ridden environments, for example automotive or industrial environments. In these environments, it is desirable to have accelerometers with good linearity, low drift performance and large full scale range. Self-balanced accelerometers are usually chosen for these applications. In self-balanced accelerometers, the capacitance C is proportional to 1/d, where d is the distance between the capacitive plates; and the measured output voltage V 0  is proportional to (C 1 −C 2 )/(C 1 +C 2 ). Combining these two relationships provides: 
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         [0000]    where x is the displacement value, d0 is the zero displacement value, d1=d0−x is the distance between the plates of capacitor C 1 , and d2=d0+x is the distance between the plates of capacitor C 2 . Equation (1) shows that in the ideal case the output voltage V 0  of the self-balanced accelerometer is a linear function of the displacement x. Unfortunately, in actual implementations, there are sources of non-linearity not taken into account in Eq. (1). 
         [0007]    Though there are several ways to build self-balanced accelerometers to obtain a reading that is proportional to the displacement of the proof mass, to achieve a highly linear accelerometer it is desirable to have a topology that results in zero residual force upon the application of sensor excitation voltages. There are two main sources of non-linearity in self-balanced accelerometers: feed-through capacitance, and mismatch between the two sensor cores. The dominant source is feed-through capacitance, and it is present in both single ended (using only one core) and differential (using two cores) topologies. Feed-through capacitance (Cft) is any fixed capacitance between the proof mass and the sense electrodes. The feed-through capacitances Cft arise due to parasitics in the sensor element and due to capacitance between the bond wires. 
         [0008]    To achieve robustness to EMI and spurious vibration, a fully differential accelerometer is typically used for automotive applications. A fully differential self balanced accelerometer for first-order EMI reduction typically has two capacitive cores as described below with reference to  FIG. 3 . However, two capacitive cores on a MEMS device coupled by bond wires to an integrated circuit requires numerous bonding pads and bond wires, which requires a relatively large area just for connections. It would be desirable to reduce the number of bonding pads and bond wires to reduce the area needed for connections. 
         [0009]    It would be desirable to have a robust technique for reducing electromagnetic interference that also overcomes some of the disadvantages of shielding and differential circuits with reduced connections to reduce the area needed for connections. It would also be desirable to reduce or eliminate the nonlinearity due to feed-through capacitances. 
       SUMMARY OF THE INVENTION 
       [0010]    A pseudo-differential accelerometer resistant to electromagnetic interference is disclosed. The pseudo-differential accelerometer includes a microelectromechanical device connected to an integrated circuit by bond wires. The microelectromechanical device includes a capacitive sensor core having a first sensor core input, a second sensor core input and a sensor core output. The integrated circuit includes a chopper system, a differential amplifier, a dummy core and a reference voltage. The differential amplifier has an inverting input, a non-inverting input and produces an amplifier output voltage. The chopper system has a plurality of chopper inputs and a plurality of chopper outputs, wherein during a chop state 0 the chopper system connects a first set of the plurality of chopper inputs to a first set of the plurality of chopper outputs and during a chop state 1 the chopper system connects a second set of the plurality of chopper inputs to a second set of the plurality of chopper outputs. The dummy core is coupled to a dummy core chopper input of the chopper system. A sensor core bond wire couples the sensor core output of the capacitive sensor core to a sensor core chopper input of the chopper system. A first feedback bond wire couples a first feedback signal to the first sensor core input of the capacitive sensor core, and the first feedback bond wire is coupled to a first chopper feedback output. A second feedback bond wire couples a second feedback signal to the second sensor core input of the capacitive sensor core, and the second feedback bond wire is coupled to a second chopper feedback output. When the chopper system is in the chop state 0, the chopper system connects the sensor core chopper input to the inverting input of the differential amplifier, connects the dummy core chopper input to the non-inverting input of the differential amplifier, connects the first chopper feedback output to a difference of the amplifier output and reference voltages, and connects the second chopper feedback output to a sum of the amplifier output and reference voltages. When the chopper system is in the chop state 1, the chopper system connects the sensor core chopper input to the non-inverting input of the differential amplifier, connects the dummy core chopper input to the inverting input of the differential amplifier, connects the first chopper feedback output to the inverse of the difference of the amplifier output and reference voltages, and connects the second chopper feedback output to the inverse of the sum of the amplifier output and reference voltages. The inverse of the difference of the amplifier output and reference voltages having the same magnitude and opposite polarity as the difference of the amplifier output and reference voltages, and the inverse of the sum of the amplifier output and reference voltages having the same magnitude and opposite polarity as the sum of the amplifier output and reference voltages. 
         [0011]    The chopper system can be varied between the chop state 0 and the chop state 1 at frequencies that smear noise away from a frequency band of interest. Alternatively, the chopper system can be varied between the chop state 0 and the chop state 1 at frequencies that smear noise substantially evenly across a wide frequency range. 
         [0012]    The dummy core can include a first dummy core input, a second dummy core input and a dummy core output, where the dummy core output is coupled to the dummy core chopper input of the chopper system. When the chopper system is in the chop state 0, the chopper system connects the reference voltage to the first dummy core input, and connects the inverse reference voltage to the second dummy core input. When the chopper system is in the chop state 1, the chopper system connects the inverse reference voltage to the first dummy core input, and connects the reference voltage to the second dummy core input. The inverse reference voltage has the same magnitude and opposite polarity as the reference voltage. The dummy core can include a first dummy capacitor having a first dummy capacitor input and a first dummy capacitor output, and a second dummy capacitor having a second dummy capacitor input and a second dummy capacitor output, where the first dummy capacitor input is the first dummy core input, the second dummy capacitor input is the second dummy core input, and a common node receives the first and second dummy capacitor outputs and is the dummy core output. The dummy core can also include a dummy parasitic capacitor that connects the dummy core output to ground. In this configuration, the pseudo-differential accelerometer only requires the sensor core bond wire, and the first and second feedback bond wires to fully connect the capacitive sensor core of the microelectromechanical device to the integrated circuit. 
         [0013]    The microelectromechanical device can also include a dummy pad and a dummy bond wire can connect the dummy pad to the dummy core chopper input. In this configuration, the pseudo-differential accelerometer only requires the sensor core bond wire, the dummy bond wire, and the first and second feedback bond wires to fully connect the capacitive sensor core of the microelectromechanical device to the integrated circuit. 
         [0014]    The pseudo-differential accelerometer can also include a neutralization core connected to the sensor core chopper input of the chopper system. A neutralization core is a set of capacitors that are used to cancel the charge injected into the amplifier terminal because of unwanted parasitic capacitances in parallel with the MEMS element. The charge cancellation is done by using an opposite polarity voltage to what is used to excite the sensor. The neutralization core can include a first neutralization core input, a second neutralization core input and a neutralization core output, where the neutralization core output is coupled to the sensor core chopper input of the chopper system. When the chopper system is in the chop state 0, the chopper system connects the first neutralization core input to the inverse of the difference of the amplifier output and reference voltages, and connects the second neutralization core input to the inverse of the sum of the amplifier output and reference voltages. When the chopper system is in the chop state 1, the chopper system connects the first neutralization core input to a difference of the amplifier output and reference voltages, and connects the second neutralization core input to a sum of the amplifier output and reference voltages. The neutralization core can include a first neutralization capacitor having a first neutralization capacitor input and a first neutralization capacitor output, and a second neutralization capacitor having a second neutralization capacitor input and a second neutralization capacitor output, where the first neutralization capacitor input is the first neutralization core input, the second neutralization capacitor input is the second neutralization core input, and a common node receiving the first and second neutralization capacitor outputs is the neutralization core output. The neutralization core can also include a neutralization parasitic capacitor connecting the neutralization core output to ground. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0016]      FIG. 1  illustrates electromagnetic disturbance or interference (EMI) due to capacitive coupling between bond wires and nearby cables, plates, circuitry, etc.; 
           [0017]      FIG. 2  illustrates a differential approach to overcome electromagnetic disturbances; 
           [0018]      FIG. 3  illustrates an exemplary one-axis fully symmetric differential accelerometer which includes a MEMS device coupled to an ASIC by six bond wires; 
           [0019]      FIG. 4  illustrates the bond wire connections between a MEMS device and an ASIC for a 3-axis (X, Y, Z) fully differential accelerometer; 
           [0020]      FIG. 5  illustrates an exemplary one-axis pseudo-differential accelerometer which includes a MEMS device coupled to an ASIC by four bond wires where the ASIC includes a dummy sensor core; 
           [0021]      FIG. 6  illustrates the bond wire connections between a MEMS device and an ASIC for a 3-axis (X, Y, Z) pseudo-differential accelerometer; 
           [0022]      FIG. 7A  illustrates the connections made by the chopper systems of the exemplary pseudo-differential accelerometer of  FIG. 5  during an exemplary chop state 0; 
           [0023]      FIG. 7B  illustrates the connections made by the chopper systems of the exemplary pseudo-differential accelerometer of  FIG. 5  during an exemplary chop state 1; 
           [0024]      FIG. 8  shows how a chopping pattern can be used to reduce an electromagnetic disturbance by smearing it across a wide frequency range; 
           [0025]      FIG. 9  shows how a shaped chopping pattern can be used to smear the error due to the offset difference in the two chop states away from DC as shaped noise; 
           [0026]      FIG. 10  shows a potential tradeoff between a shaped chopping pattern and an unshaped random pattern; 
           [0027]      FIG. 11A  illustrates the connections made by the chopper systems of the exemplary pseudo-differential accelerometer of  FIG. 5  with a neutralization core during an exemplary chop state 0; and 
           [0028]      FIG. 11B  illustrates the connections made by the chopper systems of the exemplary pseudo-differential accelerometer of  FIG. 5  with a neutralization core during an exemplary chop state 1. 
       
    
    
       [0029]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates embodiments of the invention, in several forms, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed. 
       DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0030]      FIG. 3  illustrates an exemplary fully symmetric differential accelerometer  300  which includes a MEMS device  310  coupled to an ASIC  340  by six bond wires  321 - 326 . The MEMS device  310  includes two capacitive cores C A  and C B . Each of the capacitive cores C A  and C B  includes two variable capacitors having an input side and an output side, the input sides of the two variable capacitors forming the two inputs to the capacitive core and the output sides of the two variables capacitors being coupled to a common node which forms the single output of the capacitive core. 
         [0031]    The ASIC  340  includes a differential amplifier  342 , a first chopper system  344  and a second chopper system  346 . The differential amplifier  342  has inverting and non-inverting inputs and one or more outputs. The inputs of the first chopper system  344  are coupled to the bond wires  321 ,  322  which are coupled to the outputs of the two capacitive cores C A , C B , and the outputs of the first chopper system  344  are coupled to the inverting and non-inverting inputs of the differential amplifier  342 . The input of the second chopper system  346  are coupled to the ASIC feedback signals, and the outputs of the second chopper system  346  are coupled to the bond wires  323 - 326  which are coupled to the inputs of the two capacitive cores C A , C B . As shown in the exemplary embodiment of  FIG. 3 , the ASIC feedback signals can be a combination of the output, V O , of the differential amplifier  342  and a reference voltage, V S . 
         [0032]    The first chopper system  344  swaps the connections on its inputs and outputs back and forth such that during one time slice the signal on bond wire  321  is coupled to the inverting input of the differential amplifier  342  and the signal on bond wire  322  is coupled to the non-inverting input of the differential amplifier  342 , and during the next time slice the signal on bond wire  321  is coupled to the non-inverting input of the differential amplifier  342  and the signal on bond wire  322  is coupled to the inverting input of the differential amplifier  342 . The second chopper system  346  swaps the connections on its inputs and outputs back and forth such that during one time slice the feedback signals with the inverted amplifier output, V S −V O  and −V S −V O , are coupled to bond wires  323 ,  324  which are coupled to the inputs of capacitive core C B  and the feedback signals with the non-inverted amplifier output, −V S +V O  and V S +V O , are coupled to bond wires  325 ,  326  which are coupled to the inputs of capacitive core C A ; and during the next time slice the feedback signals with the non-inverted amplifier output, −V S +V O  and V S +V O , are coupled to bond wires  323 ,  324  which are coupled to the inputs of capacitive core C B  and the feedback signals with the inverted amplifier output, V S −V O  and −V S −V O , are coupled to bond wires  325 ,  326  which are coupled to the inputs of capacitive core C A . The chopper systems  344 ,  346  can swap the bond wire signals according to a pattern to cancel electromagnetic interference from external sources, and maintain a substantially zero average voltage across the sensor capacitors in cores C A  and C B . 
         [0033]    A fully symmetric differential accelerometer like the exemplary embodiment  300  can significantly reduce electromagnetic interference by common mode rejection but some remnant interference energy may still be present. A random chopping scheme using the chopping systems  344 ,  346  can be used to push any remnant interference energy to frequencies outside the frequencies of interest to the system. The fully symmetric differential accelerometer  300  also requires two sensor cores per accelerometer. The two sensor cores provide twice the signal (hence twice the signal-to-noise ratio), but also require many bonding pads and bond wires.  FIG. 4  illustrates the bond wire connections between a MEMS device  410  and an ASIC  440  for a 3-axis (X, Y, Z) fully differential accelerometer. The illustrated 3-axis fully differential accelerometer requires twenty (20) bonding pads which can significantly add to required chip dimensions. 
         [0034]    The required number of bonding pads for a differential accelerometer can be reduced by using a dummy bonding pad in place of one of the capacitive cores as shown in  FIG. 5 .  FIG. 5  illustrates an exemplary pseudo-differential accelerometer  500  which includes a MEMS device  510  coupled to an ASIC  540  by four bond wires  521 - 524 . The MEMS device  510  includes a single capacitive core C A  and a dummy bond pad  512  which is not coupled to a capacitive core or an active signal generator on the MEMS device  510 . 
         [0035]    The ASIC  540  includes a differential amplifier  542 , a first chopper system  544 , a second chopper system  546 , a third chopper system  548  and a dummy ASIC sensor  550 . The dummy ASIC sensor  550  includes capacitors  552 ,  554  to mimic a MEMS sensor and a capacitor  556  to mimic parasitic capacitance on the MEMS sensor. The differential amplifier  542  has inverting and non-inverting inputs and one or more outputs. The first chopper system  544  has a dummy core input  543  and a MEMS core input  545 . The MEMS core input  545  is coupled to the bond wire  521  which is coupled to the output of the capacitive core C A  of the MEMS device  510 . The dummy core input  543  is coupled to the bond wire  522  which is coupled to the dummy bond pad  512  of the MEMS device  510 . The dummy core input  543  is also coupled to the output of the dummy ASIC sensor  550 . The outputs of the first chopper system  544  are coupled to the inverting and non-inverting inputs of the differential amplifier  542 . The inputs of the second chopper system  546  are coupled to the ASIC feedback signals, and the outputs of the second chopper system  546  are coupled to the bond wires  523 ,  524  which are coupled to the inputs of the capacitive core C A . The inputs of the third chopper system  548  are coupled to a non-inverted and inverted reference voltage, V S  and −V S , the magnitude of the reference voltages being substantially the same and the polarities being opposite. The outputs of the third chopper system  548  are coupled to the inputs of the dummy ASIC sensor  550 . As shown in the exemplary embodiment of  FIG. 5 , the ASIC feedback signals can be a combination of the output voltage, V O , of the differential amplifier  542  and the reference voltage, V S . 
         [0036]    In the embodiment of  FIG. 5 , the core and dummy bond wires  521 ,  522  connecting the MEMS device  510  and the ASIC  540  are coupled to the inputs  543 ,  545  of the first chopper system  544 . The dummy bond wire  522  is exposed to similar electromagnetic interference as the core bond wire  521  and the signal on the dummy bond wire  522  can be used to help reject the EMI on the core bond wire  521 . The dummy input  543  of the first chopper system  544  is also connected to the output of the dummy ASIC core  550  which loosely mimics a sensor core on the MEMS device  510 . The first chopper system  544  swaps the connections between the signals at the dummy core input  543  and the MEMS core input  545  and its output signals to the inputs of the differential amplifier  542 . During one time slice the first chopper system  544  connects the signal at the MEMS core input  545  to the inverted input of the amplifier  542  and connects the signal at the dummy core input  543  to the non-inverted input of the amplifier  542 , and then during the next time slice the first chopper system  544  swaps the connections to connect the signal at the MEMS core input  545  to the non-inverted input of the amplifier  542  and connects the signal at the dummy core input  543  to the inverted input of the amplifier  542 . 
         [0037]    The second chopper system  546  switches the ASIC feedback signals fed back on the feedback bond wires  523 ,  524  to the inputs of the capacitive core C A . The feedback signals can be a combination of the output voltage V O  of the differential amplifier  542  and the reference voltage V S . In the embodiment of  FIG. 5 , during one time slice the second chopper system  546  connects the feedback signals with the inverted amplifier output, V S −V O  and −V S −V O , to the feedback bond wires  523 ,  524 , and then during the next time slice the second chopper system  546  swaps the connections to connect the feedback signals with the non-inverted amplifier output, −V S +V O  and V S +V O , to the feedback bond wires  523 ,  524 . 
         [0038]    The third chopper system  548  switches the dummy feedback signals fed back to the inputs of the dummy ASIC sensor  550 . In the embodiment of  FIG. 5 , during one time slice the third chopper system  548  connects the non-inverted reference voltage, V S , to the dummy sensor capacitor  552  and connects the inverted reference voltage, −V S , to the dummy sensor capacitor  554 , and then during the next time slice the third chopper system  548  swaps the connections to connect the inverted reference voltage, −V S , to the dummy sensor capacitor  552  and connect the non-inverted reference voltage, V S , to the dummy sensor capacitor  554 . 
         [0039]    Using the dummy ASIC capacitor  550  on the ASIC  540  in place of a capacitive core on the MEMS device  510  significantly reduces the number of required bonding pads.  FIG. 6  illustrates the bond wire connections between a MEMS device  610  and an ASIC  640  for a 3-axis (X, Y, Z) pseudo-differential accelerometer. The illustrated 3-axis pseudo-differential accelerometer requires fourteen (14) bonding pads which is 30% less than the number of bonding pads required by a fully differential accelerometer as illustrated in  FIG. 4 . If the electromagnetic rejection requirement is not constraining, then the dummy pad  512  and dummy bond wire  522  can be eliminated which brings the required number of boning pads down to eleven (11) for a 3-axis (X, Y, Z) pseudo-differential accelerometer. 
         [0040]    The chopper systems  544 ,  546 ,  548  can swap the signals according to fixed and random patterns to significantly reduce electromagnetic interference from external sources, and maintain a substantially zero average voltage across the sensor capacitors. The chopper systems  544 ,  546 ,  548  can switch between two states based on a certain shaped pseudo-random pattern.  FIGS. 7A and 7B  illustrate exemplary chop states for the exemplary pseudo-differential accelerometer  500 . 
         [0041]      FIG. 7A  illustrates the connections made by chopper systems  544 ,  546 ,  548  of the exemplary pseudo-differential accelerometer  500  during an exemplary chop state 0. In chop state 0, the first chopper system  544  connects the signal from the MEMS capacitive core C A  on the core bond wire  521  to the inverted input of the amplifier  542  and connects the combined signals from the dummy ASIC core  550  and from the dummy pad  512  on the dummy bond wire  522  to the non-inverted input of the amplifier  542 . In chop state 0, the second chopper system  546  connects the feedback signals with the inverted amplifier output, V S −V O  and −V S −V O , to the feedback bond wires  523 ,  524  to be fed back to the MEMS capacitive core C A . In chop state 0, the third chopper system  548  connects the non-inverted reference voltage, V S , to the dummy sensor capacitor  552  and connects the inverted reference voltage, −V S , to the dummy sensor capacitor  554 . 
         [0042]      FIG. 7B  illustrates the connections made by chopper systems  544 ,  546 ,  548  of the exemplary pseudo-differential accelerometer  500  during an exemplary chop state 1. In chop state 1, the first chopper system  544  connects the signal from the MEMS capacitive core C A  on the core bond wire  521  to the non-inverted input of the amplifier  542  and connects the combined signals from the dummy ASIC core  550  and from the dummy pad  512  on the dummy bond wire  522  to the inverted input of the amplifier  542 . In chop state 1, the second chopper system  546  connects the feedback signals with the non-inverted amplifier output, −V S +V O  and V S +V O , to the feedback bond wires  523 ,  524  to be fed back to the MEMS capacitive core C A . In chop state 1, the third chopper system  548  connects the inverted reference voltage, −V S , to the dummy sensor capacitor  552  and connects the non-inverted reference voltage, V S , to the dummy sensor capacitor  554 . 
         [0043]    In the exemplary chopping method, the system can alternate between the chop 0 state ( FIG. 7A ) and the chop 1 state ( FIG. 7B ) based on a certain shaped pseudo-random pattern. The voltages used to excite the sensor core C A  and the dummy sensor  550  have opposite polarity in the two chop phases. By moving the sensor core C A  between the inverting and non-inverting inputs of the differential amplifier  542 , a pseudo-differential effect is obtained. 
         [0044]    A random chopping pattern can be used to smear an electromagnetic disturbance across a wide frequency range.  FIG. 8  shows the smearing of the electromagnetic disturbance across a wide frequency range. In  FIG. 8 , the top plots are in the time domain and the bottom plots are in the frequency domain. FIG.  8 A 1  shows a random pattern for the chopping signal in the time domain, and FIG.  8 A 2  shows the random chopping signal spread across a wide range in the frequency domain. The energy of the random chopping pattern is distributed substantially equally across frequencies.  FIG. 8B   1  shows an exemplary sinusoidal electromagnetic disturbance (ΔV emc ) in the time domain, and FIG.  8 B 2  shows the exemplary electromagnetic disturbance in the frequency domain. The energy of the exemplary electromagnetic disturbance is concentrated at a single frequency. FIG.  8 C 1  shows the result of combining the random chopping signal with the exemplary electromagnetic disturbance in the time domain, and FIG.  8 C 2  shows the result of combining these two signals in the frequency domain. The energy of the resulting disturbance signal is smeared substantially equally across a wide frequency range. 
         [0045]    This technique can achieve a significant improvement in dealing with electromagnetic disturbances. As shown in  FIG. 8 , a large electromagnetic disturbance at a single frequency can be distributed across a wide frequency range. For example, by using a clock frequency of 1 MHz and a desired bandwidth of 50 Hz, this technique provides an improvement in electromagnetic robustness of 10 log (1 MHz/(50 Hz*2))=40 dB which is a significant benefit. 
         [0046]    The shape of a chopping pattern can be selected to achieve the right compromise between EMI robustness and tolerance to MEMS non-idealities. In some cases, a flat spectrum chopping sequence like that shown in FIGS.  8 A 1  and  8 A 2  may not be the best choice. For example, if due to sensor non-idealities (for example parasitic capacitances), the offsets in the low and high phases of the chop signal are different, then it may be better to use a shaped chopping sequence. Plain random chopping smears the offset difference as white noise which puts some noise around DC and raises the noise floor. A shaped chopping sequence can be used to smear the noise away from a particular frequency band. For example, if the frequency band of interest is at DC or low frequencies, a shaped chopping sequence can be used that smears the noise to higher frequencies. 
         [0047]      FIG. 9  shows how a shaped chopping pattern can be used to smear the error due to the offset difference in the two chop states away from DC as shaped noise.  FIG. 9A  shows a chopping pattern in the frequency domain. The chopping pattern has substantially no DC or low frequency component and starts ramping up at higher frequencies.  FIG. 9B  shows an exemplary DC error due to a difference in the offsets between the chop states.  FIG. 9C  shows the result in the frequency domain of combining the shaped chopping pattern of  FIG. 9A  with the exemplary DC error due to the offset difference of  FIG. 9B . The error in output due to the offset difference is shaped as noise away from DC and low frequencies, the frequency band of interest, and into higher frequencies. 
         [0048]    However, the use of a shaped pattern can result in slightly more EMI induced disturbance for certain EMI frequencies.  FIG. 10  illustrates the potential tradeoff between a shaped chopping pattern and an unshaped random pattern.  FIG. 10  shows the frequency spectrum of an unshaped random chopping pattern  1002  and of an exemplary shaped chopping pattern  1004 . If the aliased EMI frequency is less than frequency fa, for example at frequency f emi1 , then the shaped pattern  1004  folds less noise onto DC than the unshaped pattern  1002 . However, if the aliased EMI frequency is greater than frequency fa, for example at frequency f emi2 , then the unshaped pattern  1002  folds less noise onto DC than the shaped pattern  1004 . System level considerations can be used to decide the desired chopping pattern. 
         [0049]    Feedthrough capacitance can be a dominant source of nonlinearity in an accelerometer. Feedthrough capacitance can cause vibration induced offset drift. A neutralization core can be implemented on the ASIC, such as ASIC  540  of the exemplary pseudo-differential accelerometer  500  shown in  FIG. 5 , to neutralize the feedthrough capacitance.  FIGS. 11A and 11B  illustrate an exemplary implementation of a neutralization core  800  during chop states 0 and 1. The neutralization core  800  includes dummy sensor capacitors  802 ,  804  and a dummy parasitic capacitor  806 . The neutralization core  800  is preferably implemented on the ASIC  540  and, as shown in  FIGS. 11A and 11B , receives the outputs from the second chopper system  546  that are not being put on the bond wires  523 ,  524  and sent to the sensor core C A . Thus, the neutralization core  800  receives the opposite voltages as the sensor core C A . 
         [0050]      FIG. 11A  illustrates the connections made by chopper systems  544 ,  546 ,  548  of the exemplary pseudo-differential accelerometer  500  with the neutralization core  800  during an exemplary chop state 0. In chop state 0, the first chopper system  544  connects the combined signals from the MEMS capacitive core C A  on the core bond wire  521  and from the neutralization core  800  to the inverted input of the amplifier  542 , and connects the combined signals from the dummy ASIC core  550  and from the dummy pad  512  on the dummy bond wire  522  to the non-inverted input of the amplifier  542 . In chop state 0, the second chopper system  546  connects the feedback signals with the inverted amplifier output, V S −V O  and −V S −V O , to the feedback bond wires  523 ,  524  to be fed to the MEMS capacitive core C A . In chop state 0, the second chopper system  546  also connects the feedback signals with the non-inverted amplifier output, −V S +V O  and V S +V O , to the sensor capacitors  802 ,  804  of the neutralization core  800 . In chop state 0, the third chopper system  548  connects the non-inverted reference voltage, V S , to the dummy sensor capacitor  552  and connects the inverted reference voltage, −V S , to the dummy sensor capacitor  554 . 
         [0051]      FIG. 11B  illustrates the connections made by chopper systems  544 ,  546 ,  548  of the exemplary pseudo-differential accelerometer  500  with the neutralization core  800  during an exemplary chop state 1. In chop state 1, the first chopper system  544  connects the combined signals from the MEMS capacitive core C A  on the core bond wire  521  and from the neutralization core  800  to the non-inverted input of the amplifier  542 , and connects the combined signals from the dummy ASIC core  550  and from the dummy pad  512  on the dummy bond wire  522  to the inverted input of the amplifier  542 . In chop state 1, the second chopper system  546  connects the feedback signals with the non-inverted amplifier output, −V S +V O  and V S +V O , to the feedback bond wires  523 ,  524  to be fed to the MEMS capacitive core C A . In chop state 1, the second chopper system  546  also connects the feedback signals with the inverted amplifier output, V S −V O  and −V S −V O , to the sensor capacitors  802 ,  804  of the neutralization core  800 . In chop state 1, the third chopper system  548  connects the inverted reference voltage, −V S , to the dummy sensor capacitor  552  and connects the non-inverted reference voltage, V S , to the dummy sensor capacitor  554 . 
         [0052]    While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles.