Abstract:
An the acoustic apparatus comprising a first MEMS motor that includes a first diaphragm and a first back plate, and a second MEMS motor that includes a second diaphragm and a second back plate. The first motor is biased with a first electrical polarity and a second motor is biased with a second electrical polarity such that the first electrical polarity and the second electrical polarity are opposite. At the first motor, a first signal is created that is representative of received sound energy. At the second motor, a second signal is created that is representative of the received sound energy. A differential output signal that is the representative of the difference between the first signal and the second signal is obtained. In obtaining the differential output signal, common mode noise between the first motor and the second motor is rejected.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent claims benefit under 35 U.S.C. §119 (e) to U.S. Provisional Application No. 61810387 entitled “Differential Outputs in Multiple Motor MEMS Devices” filed Apr. 10, 2013, the content of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to MEMS devices and, more specifically to MEMS devices that utilize differential amplifiers. 
       BACKGROUND OF THE INVENTION 
       [0003]    Microelectromechanical System (MEMS) microphones have been used throughout the years. These devices include a back plate (or charge plate), a diaphragm, and other components. In operation, sound energy moves the diaphragm, which causes an electrical signal to be created at the output of the device and this signal represents the sound energy that has been received. 
         [0004]    These microphones typically use amplifiers or other circuitry that further processes the signal obtained from the MEMS component. In some examples, a differential amplifier is used that obtains a difference signal from the MEMS device. 
         [0005]    In these applications, the Signal-To-Noise ratio (SNR) is desired to be high since a high SNR signifies that less noise is present in the system. However, achieving a high SNR ratio is difficult to achieve. For example, different sources of noise are often present (e.g., power supply noise, RF noise, to mention two examples). In systems that use differential amplifiers, it is possible to reduce correlated (common mode) noise as well as increasing signal to noise ratio via the subtraction of the signals from the differential pair. 
         [0006]    In previous systems, various attempts to negate noise in have generally been unsuccessful. As a result, user dissatisfaction with these previous systems has resulted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
           [0008]      FIG. 1  comprises a block diagram of a system that has two single ended inputs on two chips to an external differential stage according to various embodiments of the present invention; 
           [0009]      FIG. 2  comprises a block diagram of a system that has single ended inputs on two chips to an external differential flipped motor according to various embodiments of the present invention; 
           [0010]      FIG. 3  comprises a block diagram of a system that has single ended inputs in a single chip to internal differential stage according to various embodiments of the present invention; and 
           [0011]      FIG. 4  comprises a block diagram of a system with single ended inputs to one ASIC to internal differential stage flipped motor according to various embodiments of the present invention. 
       
    
    
       [0012]    Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. 
       DETAILED DESCRIPTION 
       [0013]    The present approaches provide MEMS microphone arrangements that eliminate or substantially reduce common mode noise and/or other types of noise. By “common mode noise,” it is meant noise that is common to both devices feeding the inputs of the differential stage. Common mode noise is unlike the intended signal generated by the devices because it is in phase between devices. The presented approaches may be provided on single or multiple substrates (e.g., integrated circuits) to suit a particular user or particular system requirements. 
         [0014]    When these approaches are provided on a single substrate or integrated circuit, less elimination of common mode noise is typically provided, but this allows that the provision of an integrated amplifier and microphone assembly that it is more economical and user friendly than approaches are not provided on the single substrate or integrated circuit. 
         [0015]    In some aspects, two MEMS devices are used together to provide differential signals. The charge plate of the one MEMS device may be disposed or situated on the top, the diaphragm on the bottom, and the charge plate supplied with a positive bias. Alternatively, the charge plate of the same MEMS device may be disposed on the bottom, the diaphragm disposed on the top, and the diaphragm supplied with a negative bias. These two arrangements will supply the same signal that is  180  degrees out of phase with a second MEMS device that has a diaphragm on the top, a charge plate on the bottom, and the diaphragm being positively biased. 
         [0016]    As has been mentioned, the MEMS motors could be disposed on one substrate (e.g., an integrated circuit or chip) or on multiple substrates. “Bias” as used herein is defined as the electrical bias (positive or negative) of diaphragm with respect to the back plate. By “MEMS motor,” it is meant a compliant diaphragm/backplate assembly operating under a fixed DC bias/charge. 
         [0017]    Referring now to  FIG. 1 , a system  100  includes a first MEMS device  102  (including a first diaphragm  106  and a first back or charge plate  108 ) and a second MEMS device  104  (including a second diaphragm  110  and a second back or charge plate  112 ). The diaphragms and charge plates mentioned herein are those that are used in typical MEMS devices as known to those skilled in the art and will be discussed no further detail herein. 
         [0018]    The output of the MEMS devices  102  and  104  is supplied to a first integrated circuit  114  and a second integrated circuit  116 . The integrated circuits, can in one example be application specific integrated circuits (ASICS). These circuits perform various processing functions such as amplification of the received signals. 
         [0019]    The integrated circuits  114  and  116  include a first preamp circuit  118  and a second preamp circuit  120 . The purpose of the preamp circuits  114  and  116  is to provide an extremely high impedance interface for a capacitive transducer which is generally high impedance source in the bandwidth of interest. 
         [0020]    The outputs of the circuits  114  and  116  are transmitted to an external differential stage  122  (that includes a difference summer  124  that takes the difference of two signals from the circuits  114  and  116 ). In one example, the external differential stage  122  is either an integrated circuit on a microphone base PCB, or external hardware provided by the user. 
         [0021]    A positive potential is supplied to first diaphragm  106  and a negative potential is applied to the second diaphragm  110 . This creates a differential signal at leads  126  and  128  as illustrated in graphs  150  and  152 . The differential signals in these graphs and as described elsewhere herein are out of phase by approximately  180  degrees with respect to each other. An output  130  of stage  122  is the difference between signals  127  and  129  and is shown in graph  154 . 
         [0022]    Common mode noise of the whole system is rejected by the stage  122 . Common mode noise occurs between both of the MEMS motors and both ASICs in the example of  FIG. 1 . As can be seen in the graphs, an increased SNR is achieved at the output  130  and as mentioned, common mode noise is significantly reduced or eliminated. Both of these aspects provide for improved system performance. Common mode noise is significantly reduced or eliminated in the example of  FIG. 1  because the common noise components are subtracted from one another. Because they have 0 degree phase difference, the differential amplifier will reject some or all of the common mode signal. 
         [0023]    Referring now to  FIG. 2 , a system  200  includes a first MEMS device  202  (including a first diaphragm  206  and a first back or charge plate  208 ) and a second MEMS device  204  (including a second diaphragm  210  and a second back or charge plate  212 ). The output of the MEMS devices  202  and  204  are supplied to a first integrated circuit  214  and a second integrated circuit  216 . The integrated circuits, can in one example be application specific integrated circuits (ASICS). These circuits perform various processing functions such as amplification of the received signals. 
         [0024]    The integrated circuits  214  and  216  include a first preamp circuit  218  and a second preamp circuit  220 . The purpose of the preamp circuits  214  and  216  is to provide an extremely high impedance interface for a capacitive transducer which is generally high impedance in the bandwidth of interest. A difference between the circuits  214  and  216  is in regard to the diaphragm/back plate orientation (i.e., one circuit  214  or  216  is “upside down,” thus causing 180 degree phase shift without negative bias). 
         [0025]    The outputs of the circuits  214  and  216  are transmitted to an external differential stage  222  (that includes a difference summer  224  that takes the difference of two signals from the circuits  214  and  216 ). 
         [0026]    A positive potential is supplied to the first diaphragm  206 . A positive potential is applied to the second back plate  212 . This creates a differential signal at leads  226  and  228  as illustrated in graphs  250  and  252 . Here, the second diaphragm and second back plate are flipped mechanically as compared to the example shown in  FIG. 1 . This creates signals that are 180 degrees out of phase with respect to each other. An output  230  of stage  222  is the difference between signals  227  and  229  and is shown in graph  254 . 
         [0027]    Common mode noise of the whole system is rejected by the stage  222 . Common mode noise occurs between both of the MEMS motors and both ASICs in the example of  FIG. 2 . As can be seen in the graphs, an increased SNR is achieved at the output  230  and as mentioned, common mode noise is significantly reduced or eliminated. Both of these aspects provide for improved system performance. Common mode noise is significantly reduced or eliminated in the example of  FIG. 1  because the common noise components are subtracted from one another. Because they have 0 degree phase difference, the differential amplifier will reject some or all of the common mode signal. 
         [0028]    Referring now to  FIG. 3 , a system  300  includes a first MEMS device  302  (including a first diaphragm  306  and a first back or charge plate  308 ) and a second MEMS device  304  (including a second diaphragm  310  and a second back or charge plate  312 ). The output of the MEMS devices  302  and  304  are supplied to an integrated circuit  314 . The integrated circuit, can in one example be application specific integrated circuit (ASIC). These circuits perform various processing functions such as amplification of the received signals. 
         [0029]    The integrated circuit  314  includes a first preamp circuit  318  and a second preamp circuit  320 . The purpose of the preamp circuits  318  and  320  is to provide an extremely high impedance interface for a capacitive transducer which is generally high impedance in the bandwidth of interest. 
         [0030]    The outputs of the preamps  318  and  320  are transmitted to a difference summer  324  that takes the difference of two signals from the preamps. 
         [0031]    A positive potential is supplied to first diaphragm  306 . A negative potential is applied to the second diaphragm  310 . This creates a differential signal at leads  326  and  328  as illustrated in graphs  350  and  352 . An output  330  of ASIC  314  is the difference between signals  327  and  329  and is shown in graph  354 . 
         [0032]    Common mode noise of the system in  FIG. 3  is rejected by the summer  354 . Common mode noise occurs between the two MEMS motors in the example of  FIG. 3 . As can be seen in the graphs, an increased SNR is achieved at the output  330  and as mentioned, common mode noise is significantly reduced or eliminated. Both of these aspects provide for improved system performance. Common mode noise is significantly reduced or eliminated in the example of  FIG. 1  because the common noise components are subtracted from one another. Because they have 0 degree phase difference, the differential amplifier will reject some or all of the common mode signal. 
         [0033]    Referring now to  FIG. 4 , a system  400  includes a first MEMS device  402  (including a first diaphragm  406  and a first back or charge plate  408 ) and a second MEMS device  404  (including a second diaphragm  410  and a second back or charge plate  412 ). The output of the MEMS devices  402  and  404  are supplied to an integrated circuit  414 . The integrated circuit, can in one example be an application specific integrated circuits (ASIC). The integrated circuit can perform various functions such as signal amplification. 
         [0034]    The integrated circuits  414  include a first preamp circuit  418  and a second preamp circuit  420 . The purpose of the preamp circuits is to provide an extremely high impedance interface for a capacitive transducer which is generally high impedance in the bandwidth of interest. The outputs of the circuits  414  that takes the difference of two signals from the preamps  414  and  418 . 
         [0035]    A positive potential is supplied to first diaphragm  406 . A positive potential is applied to the second back plate  412 . This creates a differential signal at leads  426  and  428  as illustrated in graphs  450  and  452 . An output  430  of ASIC  414  is the difference between signals  427  and  429  and is shown in graph  454 . 
         [0036]    Common mode noise of system of  FIG. 4  is rejected by the ASIC  414 . Common mode noise occurs between the two MEMS motors in the example of  FIG. 4 . As can be seen in the graphs, an increased SNR is achieved at the output  430  and as mentioned, common mode noise is significantly reduced or eliminated. Both of these aspects provide for improved system performance. Common mode noise is significantly reduced or eliminated in the example of  FIG. 1  because the common noise components are subtracted from one another. Because they have 0 degree phase difference, the differential amplifier will reject some or all of the common mode signal. 
         [0037]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention.