Patent Publication Number: US-2022229086-A1

Title: Real-time isolation of self-test and linear acceleration signals

Description:
BACKGROUND 
     Numerous items such as smartphones, smart watches, tablets, automobiles, aerial drones, appliances, aircraft, exercise aids, and game controllers utilize sensors during their operation (e.g., motion sensors, pressure sensors, temperature sensors, etc.). In commercial applications, microelectromechanical (MEMS) sensors such as accelerometers and gyroscopes capture complex movements and determine orientation or direction. For example, smartphones are equipped with accelerometers and gyroscopes to understand the movement of the smartphone, to augment navigation systems that rely on Global Position System (GPS) information, and to perform numerous other functions. Wearable devices and internet-of-things (IoT) devices constantly measure movement and other characteristics of a person, animal, or electronic device. In another example, drones and aircraft determines orientation based on gyroscope measurements (e.g., roll, pitch, and yaw) and vehicles of all types implement assisted driving to improve safety (e.g., to recognize skid or roll-over conditions). 
     MEMS sensors, such as MEMS accelerometers, exhibit certain sensitivities that when left unaddressed effectively degrade the quality of sensing operations, and even more so over time. Manufacturing tolerances, mechanical product wear, and operational electronic drift contribute to imprecise sense detection. During sensor manufacture as well as installation within end-use products, sensors may undergo product stress and deformation that cause sensitivity error effects. For example, capacitance-based MEMS sensors, such as accelerometers, may undergo a sense capacitor gap change and sensing circuit gain changes over time. To combat these issues manufacturers attempt to compensate for deviations from ideal or designed sensitivity. For example, manufacturing self-test procedures simulate real world operations by applying inertial forces to sensors or electrostatic forces to sensor proof masses and measuring the responsive sensor behavior against an expected result. The manufacturer then has the opportunity to adjust for sensitivity deviations. 
     In some instances, accelerometer products are tested for undesirable sensitivity changes by real-time monitoring. One of the ways to monitor MEMS accelerometer sensitivity changes in real time is by applying a self-test signal, designed to cause an expected physical movement of proof mass components of the MEMS accelerometer relative to sense electrodes, and measuring the proof mass acceleration response. The self-test procedure attempts to mimic a motion in response to an acceleration. 
     SUMMARY 
     A microelectromechanical (MEMS) accelerometer includes a first proof mass, a second proof mass, first electrodes, and second electrodes. The first electrodes are located adjacent to the first proof mass to sense movement of the first proof mass along a first axis in response to a linear acceleration along the first axis. The second electrodes are located adjacent to the second proof mass to sense movement of the second proof mass along the first axis. The first proof mass and the second proof mass move in-phase along the first axis in response to the linear acceleration along the first axis. A self-test drive circuitry is coupled to the first proof mass and the second proof mass. The self-test drive circuitry is configured to cause the first proof mass and the second proof mass to move out-of-phase along the first axis. Processing circuitry is coupled to the first electrodes and the second electrodes and configured to receive a sense signal and to extract from the sense signal a self-test signal corresponding to the out-of-phase movement of the proof masses and a linear acceleration signal corresponding to the in-phase movement of the proof masses. 
     A method of self-testing a microelectromechanical (MEMS) accelerometer includes applying a self-test drive signal at a first polarity to a first proof mass and applying the self-test drive signal at a second polarity to a second proof mass. The first polarity of the self-test drive signal is opposite to the second polarity of the self-test drive signal. Applying the first polarity of the self-test drive signal to the first proof mass and applying the second polarity of the self-test drive signal to the second proof mass causes the first proof mass and the second proof mass to move out-of-phase along a first axis. The method further includes detecting a sense signal comprising a linear acceleration signal and a self-test sense signal. The linear acceleration signal corresponds to the first proof mass and the second proof mass moving in-phase and a self-test sense signal corresponds to the first proof mass and the second proof mass moving out-of-phase. The self-test sense signal and the linear acceleration signal are extracted from the sense signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present disclosure, its nature, and various advantages will be more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts an exemplary motion sensing system in accordance with at least some embodiments of the present disclosure; 
         FIG. 2  depicts exemplary movements of proof masses of an out-of-plane MEMS accelerometer in accordance with at least some embodiments of the present disclosure; 
         FIGS. 3A-3D  depict exemplary MEMS accelerometer proof mass configurations for an in-plane MEMS accelerometer in accordance with at least some embodiments of the present disclosure; 
         FIG. 4A  depicts graphical representations of an exemplary MEMS accelerometer self-test response with in-phase self-test, in accordance with at least embodiments of the present disclosure; 
         FIG. 4B  depicts graphical representations of an exemplary MEMS accelerometer self-test response with out-of-phase self-test, in accordance with at least some embodiments of the present disclosure; 
         FIG. 5  depicts exemplary self-test and sensing circuitry in accordance with at least some embodiments of the present disclosure; 
         FIG. 6  depicts exemplary signals at different stages of self-test and sensing circuitry when no force is applied to MEMS accelerometer proof masses, in accordance with at least some embodiments of the present disclosure; 
         FIG. 7  depicts exemplary signals at different stages of self-test and sensing circuitry in response to a linear acceleration applied to MEMS accelerometer proof masses, in accordance with at least some embodiments of the present disclosure; 
         FIG. 8  depicts exemplary signals at different stages of self-test and sensing circuitry in response to an out-of-phase self-test force applied to MEMS accelerometer proof masses, in accordance with at least some embodiments of the present disclosure; 
         FIG. 9  depicts exemplary signals at different stages of self-test and sensing circuitry in response to both a linear acceleration and an out-of-phase self-test force applied to MEMS accelerometer proof masses, in accordance with at least embodiments of the present disclosure; and 
         FIG. 10  illustrates steps of an exemplary MEMS accelerometer self-test process in accordance with at least embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In some applications, such as in the automotive industry, consumer safety requirements encourage maintaining adherence to strict product specifications throughout a product lifetime. Sensor product sensitivity changes of MEMS sensor devices are monitored in real time and during the device in-field operations to identify changes in operation and provide for subsequent compensation or adjustment in light of those changes. This monitoring also allows ongoing reporting on the health of a product throughout the product lifecycle. Notification of product performance degradation, such as component displacements or wear, affords the opportunity to perform compensation for or adjustment of component operation. In accordance with some embodiments and methods, a real-time, power efficient, and robust approach to monitoring and compensating for sensitivity changes of an in-field MEMS sensor device is disclosed. 
     The MEMS sensor device may be a capacitance-based MEMS sensor, such as without limitation, a MEMS accelerometer. Performance of the MEMS sensor device is maintained and improved over the lifetime of the device by implementing a vibration-robust self-test processing circuitry. Further, the processing circuitry improves the functional safety of the MEMS sensor device over the device lifetime. 
     In some embodiments, self-test mechanisms are implemented to estimate the MEMS sensor device sensitivity by measuring an amplitude response to an applied electrostatic force to facilitate compensation for an undesirable sensor response (e.g., gain change) or to raise awareness of a potential electrical or mechanical component degradation, for example. The self-test routine stimulates an out-of-phase motion of proof masses (e.g., out-of-phase as compared to a typical in-phase movement of the proof masses in response to a measured force such as linear acceleration), generating a responsive signal distinguishable from signals generated by regular in-field device operations, such as linear acceleration or vibration-related signals. For example, in a MEMS accelerometer the motion applied to the accelerometer proof masses by the self-test routine is out-of-phase compared to the proof mass response to a sensed linear acceleration. Respective MEMS accelerometer proof masses may be designed to move in a particular manner with respect to each other (e.g., an in-phase response to linear acceleration). These same respective proof masses may be excited out-of-phase by the self-test motion, such that the out-of-phase self-test response can be distinguished from the in-phase response to linear acceleration. 
     In response to a self-test signal applied to the proof masses of the MEMS sensor device, an electrostatic force sets the proof masses of the device in an out-of-phase motion. Even in the presence of an external linear acceleration that also causes movement of the proof masses, the in-phase component due to linear acceleration can be distinguished and isolated from the out-of-phase self-test component. The sensed linear acceleration and the monitored self-test response are extracted for further processing, for example, to perform compensation and/or to generate a monitoring signal (e.g., representative of sensor health). 
     In an exemplary embodiment of differential sensing of in-phase linear acceleration by a MEMS accelerometer, the sense signal induced by the out-of-phase self-test motion is a common-mode signal. Unlike conventional self-test techniques where the self-test motion imparted on respective proof masses results in similar respective directions of movement as with linear acceleration, the self-test motion is observable even in the presence of an unknown linear acceleration or other in-phase vibration. 
     Exemplary processing circuitry of a differential sensing MEMS accelerometer may detect a linear acceleration signal based on the opposite polarities (+/−) of changes in the values of the respective capacitors formed between MEMS sensor device proof masses and corresponding sense electrodes due to linear acceleration, and may monitor a self-test output signal based on a common polarity (+/+or −/−) of changes in the values of the respective capacitors formed between the proof masses and corresponding sense electrodes due to the self-test movements. In some embodiments, the differential response and the common-mode response are effectively multiplexed at the sense capacitors to effect power conservation and circuit component savings and subsequently demultiplexed to extract each portion of the sensed response. The responses are further processed by separate circuit paths after demultiplexing/extraction. Alternatively, the differential (linear acceleration) response and the common-mode (self-test) response may be sensed from the sense capacitors in parallel therefore avoiding multiplexing and demultiplexing the responses, with a tradeoff of larger circuit footprint and increased power consumption because parallel signal conversion (e.g., capacitance-to-voltage conversion) may require additional circuit components in the processing circuitry. 
     The common-mode response is processed to identify a sensitivity change (e.g., a change in mechanical or electrical response, such as a gain change) in the system to facilitate safety monitoring and sensitivity compensation. Operational parameters such as gain scaling, filtering, offset values, and sensitivity scaling can be modified based on the sensitivity change. In some embodiments, errors and warnings may be provided to other circuitry, facilitating alternative compensation or calculation techniques (e.g., based on outputs from other sensors or components). 
     In some embodiments, drive carrier signals applied to respective proof masses may have different signal characteristics (e.g., frequency and/or phase). For example, applying a frequency multiple (e.g., 2×) to a subset of the proof masses facilitates multiplexing of the linear acceleration and self-test signals (e.g., because during a portion of a measurement cycle the proof mass drive signals are in-phase, and during other portions the proof mass drive signals are out-of-phase). In some embodiments, the frequency of the periodic carrier signals is the same, but the signals are out of phase. In some embodiments, the drive carrier signals are periodic and the frequency of one periodic carrier signal is a multiple of the frequency of the other periodic carrier signal to achieve separated differential and common mode sensing. 
       FIG. 1  depicts an exemplary motion sensing system  100  in accordance with some embodiments of the present disclosure. Although particular components are depicted in  FIG. 1 , it will be understood that other suitable combinations of sensors, processing components, memory, and other circuitry may be utilized as necessary for different applications and systems. In an embodiment as described herein, the motion sensing system may include at least a MEMS accelerometer  102  (e.g., a single- or multi-axis accelerometer for measuring linear acceleration along one or more axes) and supporting circuitry, such as processing circuitry  104  and memory  106 . In some embodiments, one or more additional sensors  108  (e.g., MEMS gyroscopes, additional MEMS accelerometers, MEMS microphones, MEMS pressure sensors, and a compass) may be included within the motion processing system  100  to provide an integrated motion processing unit (“MPU”) (e.g., including 3 axes of MEMS gyroscope sensing,  3  axes of MEMS accelerometer sensing, microphone, pressure sensor, and compass). 
     Processing circuitry  104  may include one or more components providing necessary processing based on the requirements of the motion processing system  100 . In some embodiments, processing circuitry  104  may include hardware control logic that may be integrated within a chip of a sensor (e.g., on a substrate or capacitor of a MEMS accelerometer  102  or other sensor  108 , or on an adjacent portion of a chip to the MEMS accelerometer  102  or other sensor  108 ) to control the operation of the MEMS accelerometer  102  or other sensors  108  and perform aspects of processing for the MEMS accelerometer  102  or other sensors  108 . In some embodiments, the MEMS accelerometer  102  and other sensors  108  may include one or more registers that allow aspects of the operation of hardware control logic to be modified (e.g., by modifying a value of a register). In some embodiments, processing circuitry  104  may also include a processor such as a microprocessor that executes software instructions, e.g., that are stored in memory  106 . The microprocessor may control the operation of the MEMS accelerometer  102  by interacting with the hardware control logic, and process signals received from MEMS accelerometer  102 . The microprocessor may interact with other sensors in a similar manner. 
     Although in some embodiments (not depicted in  FIG. 1 ), the MEMS accelerometer  102  or other sensors  108  may communicate directly with external circuitry (e.g., via a serial bus or direct connection to sensor outputs and control inputs), in an embodiment the processing circuitry  104  may process data received from the MEMS accelerometer  102  and other sensors  108  and communicate with external components via a communication interface  110  (e.g., a SPI or I 2 C bus, in automotive applications a controller area network (CAN) or Local Interconnect Network (LIN) bus, or in other applications suitable wired or wireless communications interfaces as is known in the art). The processing circuitry  104  may convert signals received from the MEMS accelerometer  102  and other sensors  108  into appropriate measurement units (e.g., based on settings provided by other computing units communicating over the communication bus  110 ) and perform more complex processing to determine measurements such as orientation or Euler angles, and in some embodiments, to determine from sensor data whether a particular activity (e.g., walking, running, braking, skidding, rolling, etc.) is taking place. In some embodiments, some or all of the conversions or calculations may take place on the hardware control logic or other on-chip processing of the MEMS accelerometer  102  or other MEMS sensors  108 . 
     In some embodiments, certain types of information may be determined based on data from multiple MEMS inertial sensors  102  and other sensors  108 , in a process that may be referred to as sensor fusion. By combining information from a variety of sensors it may be possible to accurately determine information that is useful in a variety of applications, such as image stabilization, navigation systems, automotive controls and safety, dead reckoning, remote control and gaming devices, activity sensors, 3-dimensional cameras, industrial automation, and numerous other applications. 
     An exemplary MEMS accelerometer  102  may include one or more movable proof masses that are configured in a manner that permits the MEMS sensor to measure a desired force (e.g., linear acceleration) along an axis. In some embodiments, the MEMS accelerometer may be configured for simultaneous self-test and measurement of linear acceleration. The MEMS accelerometer proof masses, sense electrodes, and sense circuitry of the MEMS accelerometer may perform differential sensing based on in-phase movements of the proof masses in response to linear acceleration. By applying out-of-phase self-test movements to the proof masses, the sensing of the self-test movements is a common mode sensing. The movements in response to linear acceleration and self-test are mechanically multiplexed based on the movement of the proof masses. Either through distinct sense circuitry (e.g., common-mode and differential coupled C2V circuitry) or demultiplexing of output signals from common mode sense circuitry, the respective outputs due to linear acceleration and self-test can be isolated and processed to measure both linear acceleration and accelerometer sensitivity in real time. 
       FIG. 2  shows exemplary movement of proof masses of an exemplary out-of-plane MEMS accelerometer in accordance with some embodiments of the present disclosure. The proof masses of  FIG. 2  and subsequent figures depict nonlimiting examples of MEMS accelerometer devices.  FIG. 2  shows exemplary proof masses of an out-of-plane accelerometer (e.g., a z-axis accelerometer) in response to different applied forces in accordance with some embodiments of the present disclosure. MEMS accelerometer  200  is shown to include two proof mass components, a proof mass  210  and a proof mass  212 , positioned about an axis of rotation  220  within the plane of a MEMS layer of MEMS accelerometer  200 . Each of the proof masses  210  and  212  is offset with respect to axis of rotation  220 , such that a z-axis acceleration causes opposite ends of the respective proof masses to move in unison (i.e., in phase). 
     At  202 , MEMS accelerometer  200  is shown with proof mass  210  and proof mass  212  in at rest in-plane configuration along axis of rotation  220 . At  202 , proof masses  210  and  212  are static, exhibiting no movement since linear acceleration along the sense axis (e.g., z-axis) remains absent. At  204 , MEMS accelerometer  200  is shown with proof mass  210  and proof mass  212  moving in-phase about axis of rotation  220  in response to a linear acceleration, caused by application of a linear acceleration force (“F accel ”). In this configuration, movement of proof masses  210  and  212  is in-phase along the linear acceleration direction. 
     At  206 , MEMS accelerometer  200  is shown with proof mass  210  and proof mass  212  having an in-phase self-test signal applied to cause movement of the proof masses  210  and  212 . As depicted at  206 , the force (“F st ”) of the exemplary self-test movement (e.g., caused by electrodes, not depicted) is in-phase about the axis of rotation  220  similar to an applied linear acceleration. In the presence of an applied linear acceleration as depicted in  204 , the movement of the proof masses due to self-test and the movement of the proof masses due to linear acceleration may be difficult to distinguish. 
     At  208 , MEMS accelerometer  200  is shown with proof mass  210  and proof mass  212  moved out-of-phase about axis of rotation  220  in response to a self-test signal. Because proof masses  210  and  212  are offset with respect to the axis of rotation  220 , they should normally rotate about the axis of rotation  220  in-phase as depicted at  204  and  206  in response to the in-phase external force imparted upon the proof masses. In the exemplary embodiment of  208  however, the self-test signals are applied to cause the proof masses to move contrary to their designed response to linear acceleration (e.g., respective self-test drive electrodes cause movements of the proof masses that are out-of-phase with respect to their designed motion due to linear acceleration). In this manner, as depicted at  230 , the self-test and acceleration motions impart countervailing movements on the respective proof masses, which can be measured and extracted for independent processing as described herein. 
       FIGS. 3A-3D  show exemplary MEMS accelerometer proof mass configurations for an in-plane MEMS accelerometer in accordance with some embodiments of the present disclosure. With reference to  FIGS. 3A-3D , each figure shows MEMS accelerometer  300  including two proof masses (PM 1  and PM 2 ) and four sets of electrodes, with two sets of electrodes positioned adjacent to each respective proof mass. Each pair of electrodes includes an electrode with a positive polarity (a positive electrode) and an electrode with a negative polarity (a negative electrode). For example, a pair of self-test drive electrodes includes a positive self-test drive electrode and a negative self-test drive electrode, and a pair of sense electrodes includes a positive sense electrode and a negative sense electrode. 
     In continued reference to  FIGS. 3A-3D , in some embodiments, two sets of the four sets of electrodes are self-test drive electrodes and a remaining two sets of the four sets of electrodes are sense electrodes. A first pair of sense electrodes is generally located adjacent to a first proof mass and a second pair of sense electrodes is generally located adjacent to a second proof mass. A first pair of self-test electrodes is generally located adjacent to a first proof mass and a second pair of self-test electrodes is generally located adjacent to a second proof mass. 
     In  FIGS. 3A-3D , a positive sense electrode of the first pair of sense electrodes is generally positioned on a first adjacent side of the first proof mass in a first direction along an axis and a negative sense electrode of the first pair of sense electrodes is generally positioned on an adjacent side of the first proof mass opposite to the first adjacent side of the first proof mass, in a direction opposite to the first direction along the same axis. A positive sense electrode of the second pair of sense electrodes is generally positioned on a first adjacent side of the second proof mass in a first direction along an axis and a negative sense electrode of the second pair of sense electrodes is generally positioned on an adjacent side of the second proof mass opposite to the first adjacent side of the second proof mass, in a direction opposite to the first direction along the same axis. 
     In  FIGS. 3A-3D , a positive self-test drive electrode of the first pair of self-test drive electrodes is generally positioned on a first adjacent side of the first proof mass in a first direction along an axis and a negative self-test drive electrode of the first pair of self-test drive electrodes is generally positioned on an adjacent side of the first proof mass opposite to the first adjacent side of the first proof mass, in a direction opposite to the first direction along the same axis. In some embodiments, the axis is in plane with a MEMS layer of MEMS accelerometer  300 . In  FIG. 3D , a positive self-test drive electrode of the second pair of self-test drive electrodes is generally positioned on an adjacent side of the second proof mass, at a side opposite to a like side of the first adjacent side of the first proof mass and a negative self-test drive electrode of the second pair of self-test drive electrodes is generally positioned on an adjacent side of the second proof mass, at a like side of the first adjacent side of the first proof mass. In  FIG. 3D , the polarity of the second self-test drive electrode pair is opposite to the polarity of the first test drive electrode pair. Each pair of self-test drive electrode applies a self-test drive signal to a respective proof mass. In response to the application of the self-test drive signal to the first and second proof masses, the proof masses move out-of-phase relative to one another. 
     More specifically, as shown in each configuration of  FIGS. 3A-3D , MEMS accelerometer  300  includes a proof mass (PM 1 )  302  and a proof mass (PM 2 )  304  and four sets of electrodes including a pair of self-test drive electrodes  310 , a pair of sense electrodes  312 , a pair of sense electrodes  314 , and a pair of self-test drive electrodes  316 . In each configuration, a first pair of sense electrodes and a first pair of self-test drive electrodes are generally located adjacent to proof mass  302  and a second pair of sense electrodes and a second pair of self-test drive electrodes are generally located adjacent to proof mass  304 . Each pair of electrodes includes a positive electrode and a negative electrode. Self-test drive electrodes  310  includes a positive self-test drive electrode  310   a  and a negative self-test drive electrode  310   b;  sense electrodes  312  includes a positive sense electrode  312   a  and a negative sense electrode  312   b;  sense electrodes  314  includes a positive sense electrode  314   a  and a negative sense electrode  314   b;  and self-test drive electrodes  316  includes positive self-test drive electrode  316   a  and negative self-test drive electrodes  316   b.    
     In  FIGS. 3A-3C , all positive sense electrodes,  312   a  and  314   a,  and all positive self-test drive electrodes,  310   a  and  316   a,  are located adjacent to and on the left side of respective proof masses  302  and  304 . Also in  FIGS. 3A-3C , all negative sense electrodes,  312   b  and  314   b,  and all negative self-test drive electrodes,  310   b  and  314   b,  are located adjacent to and on the right side of respective proof masses  302  and  304 . In  FIG. 3D , self-test drive electrode  310   a  and positive sense electrode  312   a  are located adjacent to and on the left side of proof mass  302  and negative self-test drive electrode  310   b  and negative sense electrode  312   b  are located adjacent to and on the right side of proof mass  302 . Positive sense electrode  314   a  is located on the left side and adjacent to proof mass  304  and negative sense electrode  314   b  is located on the right side and adjacent to proof mass  304 . In contrast to  FIGS. 3A-3C , in  FIG. 3D  positive self-test drive signal  316   a  is located on the right side of and adjacent to proof mass  304  and negative self-test drive signal  316   b  is located on the left side of and adjacent to proof mass  304 . The polarity of self-test drive electrodes  316  is opposite to the polarity of self-test drive electrodes  310  in  FIG. 3D . 
     Depending on the applied forces, each of the proof masses  302  and  304  of MEMS accelerometer  300  can move separately and distinctly relative to one other and in-phase and out-of-phase relative to a common axis. Differential sensing and common-mode sensing are performed based on the movements of proof masses  302  and  304 . In an exemplary embodiment, differential sensing is performed in response to in-phase movements of the proof masses in a direction consistent with linear acceleration. Common-mode sensing is performed in response to out-of-phase movements of the proof masses. 
     In the configuration  310  of  FIG. 3A , similar to a corresponding out-of-plane configuration at  202  of  FIG. 2 , proof masses  302  and  304  are static in the absence of any linear acceleration force, self-test force, or vibration. In the configuration  320  of  FIG. 3B , similar to a corresponding out-of-plane configuration at  204  of  FIG. 2 , proof masses  302  and  304  move in-phase or in unison, as shown by arrows  342  and  344 , respectively, in response to a linear acceleration, caused by application of a linear acceleration force (F accel ). In this configuration, differential sensing is performed in response to an in-phase proof mass movement in the presence of acceleration. The in-phase movement of proof masses  302  and  304  is along the linear acceleration direction. As proof mass  302  moves towards sense electrode and  312   b  and away from sense electrode  312   a,  the capacitance between proof mass  302  and sense electrode  312   b  increases while the capacitance between proof mass  302  and sense electrode  312   a  decreases. Similarly, as proof mass  302  moves towards sense electrode and  312   b  and away from sense electrode  312   a,  the capacitance between proof mass  302  and sense electrode  312   b  increases while the capacitance between proof mass  302  and sense electrode  312   a  decreases. 
     In the configuration  330  of  FIG. 3C , similar to a corresponding out-of-plane accelerometer configuration at  206  in  FIG. 2 , differential sensing is achieved in response to the application of an in-phase self-test drive signal to self-test drive electrodes  310  and  316 . In response to application of a common self-test drive signal to self-test drive electrodes  310  and  316  (e.g., with the positive polarity self-test drive electrodes  310   a  and  316   a  on the same side of the proof masses and the negative polarity and negative-test drive electrodes  310   a  and  316   a  on the opposite side of the proof masses), the self-test drive electrodes  310  and  316  cause movement of proof masses  302  and  304 , respectively, in a manner similar to the manner in which acceleration causes movement of proof masses  302  and  304 . Proof masses  302  and  304  move in-phase to an applied linear acceleration. 
     In the configuration  330 , proof masses  302  and  304  move in unison toward respective negative sense electrodes and away from respective positive electrodes. Proof mass  302  moves toward negative sense electrode  312   b  while moving away from positive sense electrode  312   a  and proof mass  304  moves toward negative sense electrode  314   b  while moving away from positive sense electrode  314   a.  In this respect, the capacitance formed between proof mass  302  and negative sense electrode  312   b  increases, as does the capacitance formed between proof mass  304  and negative sense electrode  314   b  while the capacitance formed between proof mass  302  and positive sense electrode  312   a  decreases, as does the capacitance formed between proof mass  304  and positive sense electrode  314   a.  Accordingly, sense electrodes  312  and  314  respond in a similar manner to the in-phase self-test motion as they do to a response to linear acceleration. 
     In the configuration  340  of  FIG. 3D , similar to a corresponding out-of-plane accelerometer configuration at  208  in  FIG. 2 , in response to a self-test drive signal, an out-of-phase self-test force is applied to proof masses  302  and  304  causing proof masses  302  and  304  to move out-of-phase, as shown by the opposite direction of movement of the proof masses by arrows  350  and  352 , respectively. In the configuration  340  of  FIG. 3D , the polarity of self-test drive electrodes  316  is flipped relative to the polarity of self-test drive electrodes  310  and the polarity of self-test drive electrodes of the configuration  330 , causing out-of-phase movement of proof masses  302  and  304  and common-mode sensing due to the unchanged polarity of sense electrodes  314  relative to sense electrodes  312 . Unlike the configuration  330 , the motion of proof masses  302  and  304  are out-of-phase relative to the direction of acceleration. 
     As previously discussed, in the configuration  330 , the capacitance formed between proof mass  302  and negative sense electrode  312   b  increases, as does the capacitance formed between proof mass  304  and negative sense electrode  314   b.  In  330 , the capacitance formed between proof mass  302  and positive sense electrode  312   a  decreases, as does the capacitance formed between proof mass  304  and positive sense electrode  314   a.  Whereas, in the configuration  340 , the opposite occurs in that proof mass  302  moves toward sense electrode  312   b  and away from positive sense electrode  312   a  while proof mass  304  moves toward positive sense electrode  314   a  and away from negative sense electrode  314   b.  The capacitance formed between proof mass  302  and negative sense electrode  312   b  increases while the capacitance formed between proof mass  304  and negative sense electrode  314   b  decreases. In the same manner, the capacitance formed between proof mass  302  and positive sense electrode  312   a  decreases while the capacitance formed between proof mass  304  and positive sense electrode  314   a  increases. Accordingly, in the configuration  330 , the sense electrodes perform differential sensing in response to an in-phase self-test motion, and in the configuration  340 , the sense electrodes perform common-mode sensing in response to an out-of-phase self-test motion. 
       FIG. 4A  shows graphical representations  400  of an exemplary MEMS accelerometer self-test response with an in-phase self-test movement, in accordance with at least some embodiments of the present disclosure.  FIG. 4B  shows graphical representations  450  of an exemplary MEMS accelerometer self-test response with an out-of-phase self-test movement, in accordance with at least some embodiments of the present disclosure. 
       FIG. 4A  shows a graphical representation  402  of an in-phase self-test drive signal applied to a MEMS accelerometer under test. Graphical representation  402  includes two self-test drive signals (ST 1  and ST 2 ), both shown, one overlaid by another, by  416 . The self-test drive signals are active at a frequency common to the frequency of the self-test motion, as shown by the square wave shape portions of graph  416 . Otherwise, the self-test drive signals are inactive as reflected at the flat portions of graph  416 . The in-phase self-test drive signals, ST 1  and ST 2 , are indistinguishable at graph  416 . Graphical representation  402  further shows an acceleration (or vibration) signal at graph  418 . The acceleration signal, at graph  418 , has a sinusoidal shape in an exemplary embodiment. It is understood that for the purpose of simplicity, acceleration graph  418  is an approximated representation of vibration behavior, which may be any suitable pattern of acceleration applied to the MEMS accelerometer. 
     Graphical representation  404  shows a graph of the self-test operation output in response to the in-phase differential sensing of graph  402 . The differential output  404  includes linear acceleration  420 , superimposed by the self-test drive signal, shown as  426 . Accordingly, the MEMS accelerometer self-test output and linear acceleration output interfere with each other at portions where both movements are active. A common mode output  406  in response to these linear acceleration signals does not have any output  422 . 
       FIG. 4B  shows graphical representations of a MEMS accelerator configuration similar to the configuration of  FIG. 3D  and configuration  208  of  FIG. 2 . More specifically,  FIG. 4B  shows a graphical representation  408  of an anti-phase self-test drive signal applied to a MEMS accelerometer under test, in accordance with various embodiments and methods of the disclosure. Graphical representation  408  includes two (anti-phase) self-test drive movements (ST 1  and ST 2 ), shown by signals  432  and  434 , respectively. The self-test output signals are active at a common frequency during a self-test duration, shown by the square wave shape portions of graphs  434  and  432 , and otherwise inactive during non-self-test durations, shown by the remaining flat portions of the signals. Graphical representation  408  further includes an acceleration signal at the self-test output signal, shown as signa 1430 . As shown by graphical representations  410  and  412 , the linear acceleration signal and the self-test signals are both observable and distinct within the sense signal. 
     Graphical representation  410  shows a differential portion  436  of a sense signal in response to the combined movements depicted in  408 . The differential signal  436  corresponds to the in-phase movement  430  of the proof masses in response to linear acceleration. In the exemplary embodiment of  FIG. 4B , linear acceleration  430  and differential sense signal  436  are generally sinusoidal-shaped for ease of illustration, although it will be understood that an applied acceleration may have any suitable signal patterns and may not be periodic. As depicted in  410 , the differential sense signal  436  corresponds only to the acceleration signal is therefore observable without interference by the self-test signal. 
     Graphical representation  412  shows a common mode portion  438  of a sense signal in response to the combined proof mass movements depicted in  408 . A self-test output is generated in response to the MEMS accelerometer anti-phase self-test input (anti-phase proof mass movement). Signal  438  has a square wave shape portion corresponding to where the anti-phase self-test drive signal is active and is otherwise inactive, as shown by the flat portions of signal  438 . Although particular self-test drive signal patterns are depicted in  FIGS. 4A and 4B , it will be understood that a variety of other patterns (e.g., non-square wave signal patterns, non-periodic, pseudo-noise, CDMA, etc.) may be applied as self-test drive signals. In the exemplary embodiment of  FIG. 4B , as depicted at  412 , because the self-test drive signal is out-of-phase with the movement due to linear acceleration, the common mode portion  438  of the sense signal includes only the out-of-phase self-test signal  438 , without interference from the in-phase (differentially sensed) linear acceleration signal. 
       FIG. 5  depicts exemplary self-test and sensing circuitry in accordance with at least some embodiments of the present disclosure. The self-test and sensing circuitry of  FIG. 5  may be configured, in part or in whole, as processing circuitry. In  FIG. 5 , an exemplary MEMS accelerometer including the self-test and sensing circuitry includes a proof mass  502  (PM 1 ), a proof mass  504  (PM 2 ), a pair of self-test drive electrodes  506 , a pair of a sense electrodes  508 , a pair of a sense electrodes  510 , a pair of self-test drive electrodes  512 , self-test drive circuitry  514 , carrier drive circuitry  544 , and sensing circuitry  516 , in accordance with some embodiments of the disclosure. As previously noted, relative to preceding figures, each pair of sense electrode and each pair of self-test drive electrode includes a respective positive electrode and a respective negative electrode. Accordingly, self-test drive electrodes  506  include negative self-test drive electrode  506   a  and positive self-test drive electrode  506   b ; sense electrodes  508  include positive sense electrode  508   a  and negative sense electrode  508   b;  sense electrodes  510  include positive sense electrode  510   a  and negative sense electrode  510   b;  and self-test drive electrodes  512  include positive self-test drive electrodes  512   a  and negative self-test drive electrodes  512   b.    
     PM 1  and PM 2  move in-phase in response to a sensed linear acceleration. As described herein (e.g., based on the respective polarity and location of the applied self-test drive signals), self-test drive circuitry  514  causes an out-of-phase movement onto PM 1  and PM 2 . In the exemplary embodiment of  FIG. 5 , the motion of PM 1  and PM 2  in response to the linear acceleration and the self-test movement is sensed as a sense signal on a shared set of sense electrodes, and thus, is effectively multiplexed on the sense signal. Processing circuitry extracts from the sense signal a linear acceleration signal corresponding to the in-phase movement due to linear acceleration and a self-test sense signal corresponding to the out-of-phase movement due to the self-test drive signal. 
     Sense electrodes  508   a  and  508   b  are located adjacent to and on either side of PM 1 , in a manner similar to sense electrodes  312   a  and  312   b,  respectively, of configuration  340  of  FIG. 3D . Sense electrodes  510  are located adjacent to and on either side of PM 2 , in a manner similar to sense electrodes  314   a  and  314   b,  respectively, of configuration  340  of  FIG. 3D . 
     Self-test drive circuitry  514  is shown to generate SelftestN (drive) and SelftestP (drive) signals. Carrier drive circuitry  544  is shown to include PM 1 _Drive and PM 2  Drive signals. Sensing circuitry  516  is shown to include a C2V  522 , a demodulator  524 , a demultiplexer  526 , a self-test monitor path  528 , and an acceleration sense path  530 . Self-test monitor path  528  is shown to include a self-test filter  538  and a self-test digital signal processor (DSP)  540 . Acceleration sense path  538  is shown to include an acceleration filter  532 , an acceleration DSP  534 , and a gain/offset/sensitivity (GOS) block  536 . 
     PM 1 , PM 2 , self-test drive electrodes  506 , sense electrodes  508 , sense electrodes  510  and self-test drive electrodes  512  may collectively form a MEMS accelerometer  552 , an example of a MEMS sensor product undergoing real time and continuous self-test monitoring during the product operational lifetime, in accordance with various disclosed embodiments and methods. MEMS accelerometer  552  includes a MEMS layer including a suspended spring-mass system including proof masses that move with respect self-test drive electrodes  506 , sense electrodes  508 , sense electrodes  510  and self-test drive electrodes  512 . 
     In some embodiments, in-phase movement of PM 1  and PM 2  includes simultaneous movement of the proof masses toward all the positive sense electrodes or all the negative sense electrodes. An in-phase movement example is a PM 1  and a PM 2  simultaneous movement toward an electrode of each of the electrodes  506 ,  508 ,  510  and  512  with a common polarity. The out-of-phase movement of PM 1  and PM 2  includes movement of one of the proof masses, either PM 1  or PM 2 , in a direction toward a corresponding positive sense electrode (at an adjacent side) of the proof mass and simultaneous movement of the other proof mass in a direction toward a corresponding negative sense electrode (an adjacent opposite side) of the proof mass. 
     As shown in the embodiment of  FIG. 5 , self-test drive circuitry  514  is coupled to PM 1  and PM 2  through the SelftestN and SelftestP drive signals, the negative and positive polarities of a “self-test drive signal”, respectively. In  FIG. 5 , SelftestN drive signal is shown coupled to self-test drive electrode  506   a  and to self-test drive electrode  512   a  and SelftestP drive signal is shown coupled to self-test drive electrode  506   b  and self-test drive electrode  512   b.  The SelftestN and SelftestP drive signals are applied at opposite sides to PM 1  and PM 2 , therefore, self-test drive circuitry  514  causes PM 1  and PM 2  to move out-of-phase. More specifically, the negative polarity of self-test drive signal, SelftestN signal, is applied to positive self-test drive electrode  506   a  adjacent to PM 1  and to positive self-test drive electrode  512   a  adjacent to PM 2  while the positive polarity of self-test drive signal, SefltestP signal, is applied to negative self-test drive electrode  512   b  adjacent to PM 2  and to negative self-test drive electrode  506   b  adjacent to PM 1 . The polarities of the self-test drive electrodes  506  and  512  are opposite to cause an out-of-phase proof mass movement. But because the polarity of sense electrodes  508  and  510  are not in opposite, the sensing of the out-of-phase movement is common mode. 
     The sensed movements of the proof masses are represented as capacitance changes between the proof masses and respective sense electrodes based on an in-phase and an out-of-phase movement of PM 1  and PM 2 . As earlier noted, this dual motion feature of PM 1  and PM 2  is effectively multiplexed in the form of combined sense signal include a linear acceleration signal and a self-test signal, implementing a design architecture with fewer circuit components and lower power consumption. In other embodiments (not depicted in  FIG. 5 ), a second C2V amplifier similar to  522  may be connected for common mode sensing. 
     Sensing circuitry  516  is coupled to MEMS accelerometer  552  at the proof mass outputs through the SenseP and SenseN signals of self-test drive circuitry  514 . SenseP signal is coupled to sense electrodes  510   a  and  508   a  and SenseN signal is coupled to sense electrodes  510   b  and  508   b.  In some embodiments, a capacitance to voltage (C2V) converter  522  of processing circuitry  516  is a differential amplifier coupled, at a positive input, to the combined SenseP signal, and at a negative input, to the combined SenseN signal. In response to changes in the SenseN and SenseP signals, C2V  522  generates a differential output, represented by a sense signal, that is detected by sensing circuitry  516 . 
     More specifically, C2V  522  generates the sense signal based on the differential inputs of the SenseN and SenseP signals, with embedded linear acceleration and self-test signals that are subsequently demultiplexed into two distinct signals, such as shown at graphs  410  and  412 , of  FIG. 4B , respectively. As earlier described, the self-test output signal is generated in response to an out-of-phase movement of PM 1  and PM 2  and the linear acceleration signal is generated in response to the in-phase movement of PM 1  and PM 2 . 
     In the embodiment of  FIG. 5 , C2V  522  is configured as a capacitance-to-voltage C2V) converter. The capacitance measured based on the movements of PM 1  and PM 2  relative to respective sense electrodes, is converted to a proportional voltage by the C2V converter and further processed by the processing circuitry of  FIG. 5 . 
     During the MEMS accelerometer in-field operation of the exemplary processing circuitry of  FIG. 5 , a relative in-phase proof mass movement causes differential sensing and a relative out-of-phase proof mass movement causes common-mode sensing. During PM 1  and PM 2  in-phase movement, for example, PM 1  is caused to move to the right toward a respective sense electrode located to the right of PM 1 , and PM 2  is made to simultaneously move to the right toward a respective adjacent sense electrodes located to the right of PM 2 , such as shown by the direction of movement of PM 1  and PM 2  in  FIG. 3B . This in-phase movement has the effect of increasing the capacitance between each proof mass and a respective electrode positioned to the adjacent right of the proof mass in response to a force (e.g., linear acceleration) applied to the proof mass. More specifically, the capacitance between PM 1  and sense electrode  508   b  increases while the capacitance between PM 1  and sense electrode  508   a  decreases, and the capacitance between PM 2  and sense electrode  510   b  increases while the capacitance between PM 2  and sense electrode  510   a  decreases. The in-phase proof mass movement simultaneously decreases the capacitance between each proof mass and a respective electrode positioned to the adjacent left of the proof mass. 
     A relative out-of-phase movement of PM 1  and PM 2 , for example, PM 1  moving toward sense electrode  508   b  and away from sense electrode  508   a  and PM 2  moving toward sense electrode  510   a  and away from sense electrode  510   b,  is shown by the direction of the arrows  350  and  352  associated with PM 1  and PM 2  in  FIG. 3D , respectively, and results in common-mode sensing in the exemplary configuration of  FIG. 5 . That is, in response to the proof mass self-test drive signal (SelftestP signal and SelftestN signal) applied to self-test drive electrodes  506   a  and  506   b,  PM 1  moves closer to sense electrode  508   b  and the capacitance between PM 1  and sense electrode  508   b  increases while the capacitance between PM 1  and sense electrode  508   a  decreases. In the meanwhile, in response to the proof mass self-test drive signal applied to self-test drive electrodes  512   a  and  512   b,  PM 2  moves closer to sense electrode  510   a  and the capacitance between PM 2  and sense electrode  510   a  increases while the capacitance between PM 2  and sense electrode  510   b  decreases. 
     In an exemplary embodiment, the PM 1 _Drive and PM 2 _Drive signals are periodic carrier signals. In some embodiments, the PM 1 _Drive and PM 2 _Drive signals have a common frequency. In some embodiments, the PM 1 _Drive and PM 2 _Drive signals are in phase and a frequency of the PM 1 _Drive and PM 2 _Drive signals is a multiple of a frequency of the other one of the PM 1 _Drive and PM 2 _Drive signals. In the examples to follow, the PM 2 _Drive signal is presumed to have a frequency twice that of the frequency of the PM 1 _Drive signal. As earlier noted, embodiments with drive signals of different frequencies facilitate separating differential sensing from common-mode sensing and can possibly be effective power consumption measures. 
     PM 1 _Drive and PM 2 _Drive signals are similar to the SelftestN and SelftestP signals in that both sets of signals are drive signals, but unlike the SelftestN and SelftestP signals, each of the PM 1 _Drive and PM 2 _Drive signals acts as a respective carrier of the linear acceleration signal and the self-test signal, illustrative by the example graphs of  FIGS. 6-9 . Additionally, unlike the SelftestN and SelftestP signals, the PM 1 _Drive and PM 2  Drive signals do not cause physical movement of the proof masses PM 1  and PM 2 . 
     Demodulator  524  receives the sense signal, output of C2V  522 , and demodulates the sense signal to remove the carriers, PM 1 _Drive and PM 2  Drive signals, outputting the raw sense signal due to proof mass movement (e.g., including embedded linear acceleration signal and self-test signals) for further processing. 
     Although not depicted in  FIG. 5 , in some embodiments the processing circuitry of  FIG. 5  may include additional circuitry such as an analog-to-digital converter (ADC) for digitizing the analog signals generated by C2V  522  or other as processed by other subsequent circuitry. For example, an ADC may be implemented between demodulator  524  and demultiplexer  526 , or between demultiplexer  526  and each of acceleration sense path  530  and self-test monitor path  528 , or in each of acceleration sense path  530  and self-test monitor path  528 . 
     Demultiplexer  526  extracts the linear acceleration signal and the self-test (output) signal from the sense signal, for example, by selectively outputting the differential and common-mode components of the combined sense signal. Demultiplexer  526  may output the linear acceleration signal onto accelerometer sense path  530  (e.g., at particular time periods associated with a demux clock  550 ) and output the self-test signal onto self-test monitor path  528  (e.g., at other particular time periods associated with the demux clock  550 ). In the graphs of  FIGS. 6-9 , to follow, during the negative (or low) periods of the demux clock cycles, the linear acceleration signal is extracted onto self-test monitor path  528  and during the positive (or high) periods of the demux clock cycles, the self-test signal is extracted onto accelerometer sense path  530 . It is understood that alternatively, the self-test signal and the linear acceleration signals may be extracted at opposite half clock periods. It is also understood that references to a particular polarity and polarity coupling herein are merely example polarities and opposite polarities and polarity couplings is contemplated. 
     In some embodiments, self-test filter  538  of self-test monitor path  528  is configured as a low pass filter to remove noise caused by frequency harmonics that may be associated with sampling among other causes. Analogously, acceleration filter  532  of acceleration sense path  530  is configured as a low pass filter for a similar reason. It is understood that filters  532  and  538  may each be a suitably different type of filter. Each of DSPs  534  and  540  digitally processes respective filtered outputs from filters  532  and  538 . In some embodiments, DSP  540  generates a sensor health monitor output  554 , in common-mode sensing, in response to detection of a degraded component or component feature, for example. In a nonlimiting example, output  554  may identify errors from the measured sensitivity based on absolute changes (e.g., compared to a baseline) or changes over time (e.g., abrupt changes in sensitivity). 
     In some embodiments, the outputs of DSPs  540  and  534  are received by GOS block  536 . The gain, offset, and scaling parameters of GOS block  536  collectively modify the measured value based on characteristics of the MEMS accelerometer. The output of GOS block  536  generates signal  546  corresponding to a sensed linear acceleration, which may be further processed by other analog and/or digital processing circuitry. 
     In some embodiments, any of the self-test drive signal pattern and/or the acceleration signal may correspond to code-division multiple access (CDMA) signals to further assist in distinguishing self-test and acceleration signals, for example, as is described in commonly owned U.S. patent application Ser. No. 16/874,418, filed on May 14, 2020, and entitled “MEMS SENSOR MODULATION AND MULTIPLEXING”, which is incorporated by reference as though set forth in full herein. 
     In some embodiments, the self-test (output) signal may be recovered from the sense signal with other implementations. For example, notch filters may notch the signal at the self-test signal frequency. In some embodiments, the processing circuitry may include a second C2V where one C2V, for example, amplifier  522 , performs differential sensing exclusively and at all times and another C2V, simultaneously performs common-mode sensing exclusively and at all times. In this configuration, a demultiplexer is not needed because the linear acceleration signal and the self-test signal are separated at sensing, not embedded in a common sense signal. 
       FIGS. 6-9  show exemplary signals at different stages of self-test and sensing circuitry, in response to different applied linear acceleration and self-test signals. Each of the figures of 
       FIGS. 6-9  includes six graphs, a graph of a PM 1  signal (PM 1 _Drive in  FIG. 5 ) and a PM 2  signal (PM 2 _Drive in  FIG. 5 ), a C2V output graph, a demodulated signal graph, a demux clock graph, a linear acceleration signal graph, and a self-test signal graph. Further, in each figure, the PM 2  signal is presumed to have a frequency twice that of the frequency of the PM 1  signal. In the first graph of each of the  FIGS. 6-9  (top-most graph), “PM 1 ” represents the signal behavior of PM 1 _Drive of  FIG. 5  and “PM 2 ” represent the signal behavior of PM 2 _Drive of  FIG. 5 . Each of the PM 1  and PM 2  signals is presumed a square wave shaped periodic signal. It is understood that each of signals PM 1   604  and PM 2   602  may have a suitably different signal shape. PM 1  and PM 2  are further presumed to have opposite polarity relative to one another and the frequency of PM 2  is presumed twice the frequency of PM 1 . In each figure, the first (top) graph shows PM 1  with a solid line and PM 2  is shown with a dashed line. 
     In each of  FIGS. 6-9 , the horizontal axis represents time in milliseconds (ms) and the vertical axis represents a range of normalized signal amplitude values. In each figure, a time range is shown on the horizontal axis from 0 ms to 0.05 ms in 0.005 ms intervals. Signal amplitude ranges are shown on the vertical axis and vary based on the signal. For example, at graph  610 , the signal amplitude ranges may vary across figures and across graphs of a figure. Graph  610  of  FIG. 6  and graph  910  of  FIG. 9  both show a graph of PM 1  and PM 2  yet in  FIG. 6 , the signal amplitude ranges from −0.1 to +0.1 and in  FIG. 9 , the signal amplitude ranges from −0.2 to +0.2 and at graph  610 , the signal amplitude range is −0.1 to +0.1 while at graph  620 , the signal amplitude range is 0.2 to +0.2. 
     In a second top-most graph of each figure, “C2V output”, a graph of the output of C2V  522  (sense signal) is shown; the third top-most graph of each figure is a graph of the output of demodulator  526 ; the fourth top-most graph of each figure is a graph of the demux clock  550 ; the fifth top-most graph of each figure is a graph of the linear acceleration signal (extracted from the sense signal in  FIG. 5 ); and the last graph of each figure is a graph of the self-test (output) signal (extracted from the sense signal in  FIG. 5 ). 
     In the exemplary embodiment depicted in  FIG. 6 , the MEMS accelerator  552  proof masses (proof mass  502  and proof mass  504  of  FIG. 5 , for example) are static and not in motion either due to linear acceleration or a self-test motion. Accordingly, the proof mass  502  output and the proof mass  504  output are not modulated by a self-test signal or an acceleration signal. 
     With continued reference to  FIG. 6 , at graph  610  (output of C2V  522 ), during the periods when PM 1   604  is high and PM 2   602  is low or vice versa, the output of C2V  522  is at a maximum high (shown at  612  of graph  610 ), +0.2, and a maximum low (shown at  614  of graph  610 ), −0.2, respectively, a differential of 0.1 and −0.1, i.e., +0.1 minus (+0.1)=+0.2, and −0.1 minus (−0.1)=−0.2, respectively. Because PM 2   602  has double the frequency of PM  604 , where PM 1   604  and PM 2   602  have the same amplitude (both are at −0.1 or both are at +0.1), the net output is zero, as shown at  616  of graph  610 . Accordingly, graph  610  is made of two essential portions, a differential portion where the frequency of PM 2   602  is double that of PM 1   604  and graph  610  is 0, at  616 , common-mode carrier, and another portion where the C2V output is a differential output at  612  and  614 , a differential carrier. 
     The output of C2V  522  is demodulated (by demodulator  524 ) with PM 2   602  to generate a demodulated signal with behavior shown by graph  620 . Where PM 2   602  is high (at 0.1) and graph  610  is at 0 (shown at  616 ), the demodulated signal is at high (at 0), as shown by  622  at graph  620 , where PM 2   602  is low and graph  610  is at +0.2, graph  620  is low, shown at  624 , at graph  620 , and where PM 2   602  is high or low and graph  610  is at 0, graph  620  is at high, as shown at  626  at graph  620 . Graph  630  shows the behavior of the demux clock  650 . At high half periods of demux clock  650 , shown at  632  of graph  630 , acceleration signal  642 , at graph  640 , is output by demux  626  and at low half periods of demux clock  650 , shown at  634  of graph  630 , self-test signal  652 , at graph  650 , is output by demux  626 . Both the acceleration signal  642  and self-test signal  652  are flat at 0, due to the inactivity of proof masses  502  and  504 . 
     In  FIG. 7 , the proof mass  502  and proof mass  504  movements are consistent with the movement of the MEMS accelerometer due to linear acceleration and sensing is performed on the differential path, path  530  in  FIG. 5 . A (linear) accelerometer signal  706 , shown at graph  700 , modulates both PM 1   704  and PM 2   702  and the way the signals add together in response to an in-phase movement of proof masses  502  and  504 . Accordingly, PM 1   704  and PM 2   702 , proof mass  502  and proof mass  504  outputs, respectively, move together, enveloped in the same envelope of accelerometer signal  706 . When the frequencies of PM 1   704  and PM 2   702  align, their sums are differential values, as shown at  714  of graph  710 . When PM 1   704  and PM 2   702  overlap, their sum is 0, as shown at  716  of  FIG. 710 . The amplitude of the differential portions of C2V  522  change in accordance with the amplitude of accelerometer signal  706 , an enveloped amplitude change. The demodulated C2V output with PM 2   702  is shown at graph  720 . As shown by graph  720 , the demodulation output follows the amplitude of acceleration signal  706 . Demux clock  550 , shown at graph  730 , switches between a differential output, shown at graph  740  and a common-mode output, shown at graph  750 . The common-mode output—the sensed self-test signal—has a value of 0 because no self-test signal is applied in the configuration of 
       FIG. 7  and the differential output is the sensed acceleration signal as shown at graph  740 . 
       FIG. 8 , a self-test only (no acceleration) configuration, proof masses  502  and  504  are moved out-of-phase. Therefore, sensing is performed on the common-mode path, path  528 , and not the differential path, path  530 . Whereas if the movement of the two proof masses were in the same direction as the linear acceleration, such as shown and discussed relative to  FIG. 7 , sensing is performed on the differential path. 
     In  FIG. 8 , PM 1   804  and PM 2   802  (shown at graph  800 ) have the same frequency but their movements are lined up differently because the self-test drive signals force proof masses  502  and  504  to move in opposite directions. Accordingly, at the output of C2V  522 , shown by graph  810 , the differential parts of graph  810  become 0, as shown at  812 , and the common-mode parts of graph  810 , show the PM 1   804  and PM 2   802  differences, as shown at  814 . Accordingly, on path  530 , no signal is observed, as shown at graph  840 , and on the self-test path, path  528 , the self-test movement is observed, as shown at graph  850 . 
     In the configuration of  FIG. 9 , the acceleration and the self-test signals are both present. With reference to graph  900 , where PM 2   902  is periodic but with a small magnitude, acceleration moves one of the proof mass (of proof masses  502  and  504 ) in a first direction but while acceleration attempts to move the remaining proof mass in the same direction, the self-test signal forces the remaining proof mass to move in a direction opposite to the direction of the acceleration movement (or the direction of the other proof mass movement) by a displacement amount smaller than the displacement amount the first proof mass moves the direction of acceleration. Accordingly, while the periodic self-test movement exist, but it is at a smaller amplitude amount because it is in a direction opposite to the acceleration movement direction. Therefore, when the self-test movement and acceleration movement are both present, both proof masses  502  and  504  move at a location very close to a corresponding sense electrode and the varying small magnitude signal to a large magnitude signal effect results in PM 2   902 , as shown at graph  900 . The large magnitude parts of PM 2   902 , as shown at  906 , show the behavior of PM 2   902  when acceleration and self-test move together. At the same time, the magnitude of PM 1   904  decreases significantly because self-test is working against acceleration, as earlier explained, shown at  908 . The demodulated signal follows the amplitude envelope of PM 2   902 , as shown at graph  920 . At half periods  932  (of graph  930 ) of demux clock  550 , acceleration data is acquired. The behavior of the acquired acceleration data is shown at graph  940 . During other half periods  934  of demux clock  550 , self-test data is acquired with the behavior of the acquired self-test data shown at graph  950 . Demultiplexer  526  can extract the two types of data because they are out-of-phase relative to one another. 
       FIG. 10  depicts exemplary steps for self-test operation of an exemplary MEMS accelerometer in accordance with at least some embodiments of the present disclosure. Although  FIG. 10  is described in the context of the particular structures and components of the present disclosure, it will be understood that the methods and steps described in  FIG. 10  may be applied to a variety of MEMS accelerometer designs, self-test techniques, processing circuitry, and compensation techniques. Although a particular order and flow of steps is depicted in  FIG. 10 , it will be understood that in some embodiments one or more of the steps may be modified, moved, removed, or added, and that the flow depicted in  FIG. 10  may be modified. 
     The self-test operation of  FIG. 10  is described with reference to the processing circuitry of  FIG. 5  for the benefit of simplicity. It is understood that alternate processing circuitry suitable for carrying out the steps of  FIG. 10  may be employed. At step  1002  of the self-test operation, a self-test drive signal, at a first polarity, is applied to a first proof mass. For example, in  FIG. 5 , SelftestN and SelftestP signals are applied to PM 1  at a certain polarity. Simultaneously, at step  1004 , the self-test drive signal is applied at a polarity opposite to the first polarity to a second proof mass. For example, a SelftestN/SelftestP polarity is presumed applied to PM 1 , a SelftestP/SelftestN polarity is presumed applied to PM 2 . In response to applying the self-test drive signal to PM 1  and PM 2 , a sense signal is generated. At step  1006 , the sense signal is detected. For example, the sense signal may be generated by C2V  522  and detected at the output of C2V  522 . The sense signal has two embedded signals, each is a function of a distinct PM 1 -PM 2  movement. That is, the detected sense signal carries a linear acceleration signal in response to a PM 1  and PM 2  in-phase movement and a self-test output signal in response to a PM 1  and PM 2  out-of-phase movement. Next, at step  1008 , the linear acceleration signal and the self-test (sense) signals are extracted from the sense signal. For example, demultiplexer  526  outputs the linear acceleration signal at first half periods of demux clock  550  cycles and outputs the self-test sense signal at second half periods of the demux clock  550  cycles. The linear acceleration signal may be further processed by the acceleration sense path components and the self-test signal is further processed by the self-test monitor path components as earlier discussed and shown relative to preceding figures. The foregoing description and  FIG. 10  describe a self-test operation in accordance with the implementation of  FIG. 5 . In alternate implementations where two C2Vs are employed, one for acceleration sensing and another for self-test sensing, the sense signal is two distinct signals, each generated by a distinct C2V, alleviating the need for a demultiplexer and demux clocking. The accelerometer and self-test signals can be detected in parallel. 
     The foregoing description includes exemplary embodiments in accordance with the present disclosure. These examples are provided for purposes of illustration only, and not for purposes of limitation. It will be understood that the present disclosure may be implemented in forms different from those explicitly described and depicted herein and that various modifications, optimizations, and variations may be implemented by a person of ordinary skill in the present art, consistent with the following claims.