Abstract:
A microphone includes a microelectromechanical system (MEMS) device, an amplifier, and an attenuation apparatus. The MEMS device converts acoustic energy into electrical signals. The amplifier is coupled to the MEMS device and receives an input signal from the MEMS device and performs amplification on the input signal to produce an output signal. The attenuation apparatus is coupled to the amplifier. Activation of the attenuation apparatus is effective to attenuate the output signal of the amplifier. A self-noise of the amplifier is attenuated and a sensitivity of the microphone is reduced such that a first signal-to-noise ratio is substantially the same as a second signal-to-noise ratio. The first signal-to-noise ratio occurs when the attenuation apparatus is not activated, and the second signal-to-noise ratio occurs when the attenuation apparatus is activated.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This patent claims benefit under 35 U.S.C. §119(e) to U.S. Provisional Application No. 62/078,624 entitled “Microphone with Trimming” filed Nov. 12, 2014, and also claims benefit to U.S. Provisional Application 62/237,165 entitled “Microphone with Trimming” filed Oct. 5, 2015 the contents of both of which are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to microphones and the operation and performance of these microphones. 
       BACKGROUND OF THE INVENTION 
       [0003]    Microphones are used to obtain sound energy and convert the sound energy into electrical signals. Once obtained, the electrical signals can be processed in a number of different ways. 
         [0004]    One example of a microphone is a Micro-Electro-Mechanical System (MEMS) microphone. MEMS microphones are typically composed of two main components: a MEMS device (including a diaphragm and a back plate) that receives and converts sound energy into an electrical signal, and an Application Specific Integrated Circuit (ASIC) (or other circuits such as buffers, amplifiers, and analog-to-digital converters). The ASIC receives the electrical signal from the MEMS device and performs post-processing on the signal and/or buffering the signal for the following circuit stages. The following circuit stages may include a codec or digital signal processor (DSP) to mention two examples. 
         [0005]    The MEMS component is typically desired to have a higher output than a customer&#39;s DSP or codec requires. Consequently, the sensitivity (i.e., the ratio of voltage output to incoming sound pressure) of the microphone is reduced. 
         [0006]    In previous approaches, microphone noise sensitivity was sometimes reduced, but at the cost of decreasing the signal-to-noise ratio of the microphone. The drawbacks associated with previous approaches have resulted in some general user dissatisfaction. 
     
    
     
       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 microphone according to various embodiments of the present invention; 
           [0009]      FIG. 2  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0010]      FIG. 3  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0011]      FIG. 4  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0012]      FIG. 5  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0013]      FIG. 6  comprises two graphs showing some of the advantages of the present approaches according to various embodiments of the present invention; 
           [0014]      FIG. 7  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0015]      FIG. 8  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0016]      FIG. 9  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0017]      FIG. 10  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0018]      FIG. 11  comprises a block diagram of a microphone according to various embodiments of the present invention; 
           [0019]      FIG. 12  comprises a block diagram of a microphone according to various embodiments of the present invention. 
       
    
    
       [0020]    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 
       [0021]    The present approaches provide microphones where the sensitivity of the microphone is adjusted (e.g., trimmed), but the signal-to-noise ratio (SNR) of the microphone is not reduced (or not substantially reduced). These advantages are accomplished in one aspect by disposing one or more attenuation components (e.g., resistors, capacitors, or active components) at the output of the microphone. In addition to disposing attenuation components at the output of the microphone, attenuation components may be placed at the input of the microphone. 
         [0022]    Referring now to  FIG. 1 , one example of a microphone  100  that is coupled to a customer electronic device is described. The microphone  100  includes a MEMS device  102  and an application specific integrated circuit (ASIC)  104 . The MEMS device  102  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  104 . In one example, the MEMS device  102  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0023]    The ASIC  104  may be any type of integrated circuit that includes an amplifier  106  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  104  also includes an output resistor  108  (R out ). 
         [0024]    In addition, the ASIC  104  includes a bank of attenuation resistors: a small value resistor  120 , a medium value resistor  122 , and a large value resistor  124 . In one aspect, the values of the resistors are sufficiently small so as to add relatively little noise to the microphone  100 . A switch  110  is used to selectively switch in appropriate resistors  120 ,  122 , and  124  to the microphone circuit. This may be accomplished by a user selection  111 , which in one example is manually performed, but in some cases may be automatically performed. The number and value of resistors to be added to the circuit depend upon the amount of attenuation needed by a customer device (e.g., a codec or DSP of a customer) that is coupled to V out . Consequently, although the switch  110  is shown as being open in this and the other figures herein, it will be appreciated that, in fact, it will couple to one or more of the resistors  120 ,  122 , and  124 . Together, whichever resistors  120 ,  122 , and  124  are selected, the selected resistors form an equivalent resistance R atten  that is coupled to V out . 
         [0025]    In this case, V out =(R atten /(R out +R atten )) V signal , where Rotten is the equivalent value of the resistors  120 ,  122 , and  124  selected by switch  110 , and V signal  is the voltage of the signal received from the MEMS device  102 . The resistors  120 ,  122 , and  124  are disposed at the output of the amplifier  106  to perform attenuation. In this case, the amplifier noise is attenuated, so the original SNR of the microphone is preserved even after one or more of the resistors  120 ,  122 , and  124  are added to the ASIC  104 . Again, the exact value of R atten  will vary, depending upon the resistors selected and the values of these resistors. Moreover, although three possible resistors  120 ,  122 , and  124  are shown, it will be appreciated that any number of resistors may be used depending on the trim resolution and range desired by the designer. It should also be noted that in some instances, it may be advantageous to add a buffer at the output of the circuit to prevent the trimming network from effecting the output impedance of the microphone as seen by any following circuitry (e.g. a codec, a DSP, to mention two examples) 
         [0026]    Referring now to  FIG. 2 , another example of a microphone  200  that is coupled to customer electronics is described. The microphone  200  includes a MEMS device  202  and an application specific integrated circuit (ASIC)  204 . The MEMS device  202  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  204 . In one example, the MEMS device  202  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0027]    The ASIC  204  may be any type of integrated circuit that includes an amplifier  206  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  204  also includes an output resistor  208  (R out ). 
         [0028]    In addition, the ASIC  104  includes a bank of attenuation resistors: a small value resistor  220 , a medium value resistor  222 , and a large value resistor  224 . In one aspect, the values of the resistors  220 ,  222 , and  224  are sufficiently small so as to add negligible noise to the microphone. A switch  210  is used to switch in appropriate resistors  220 ,  222 , and  224  to the circuit. This may be accomplished by a user selection  211 , which in one example is manually performed, but in some cases may be automatically performed. The number of resistors to be added to the circuit depend upon the amount of attenuation needed by a customer device (e.g., a codec or DSP of a customer) that is coupled to V out . Consequently, although the switch  210  is shown as being open in this and the other figures herein, it will be appreciated that in fact it will couple to one or more of the resistors  220 ,  222 , and  224 . Together, whichever resistors  220 ,  222 , and  224  are selected, the selected resistors form an equivalent resistance R atten  that is coupled to V out . 
         [0029]    In this case, V out =(R atten /(R out +R atten )) V signal , where R atten  is the equivalent value of the resistors  220 ,  222 , and  224  selected by switch  210 , and V signal  is the voltage of the signal received from the MEMS device  202 . The resistors  220 ,  222 , and  224  are disposed at the output of the amplifier  206  to perform attenuation. In this case, the amplifier noise is attenuated, so the original SNR of the microphone is preserved even after one or more of the resistors  220 ,  222 , and  224  are added to the ASIC  204 . Again, the exact value of R atten  will vary, depending upon the resistors selected and the values of these resistors. Moreover, although three possible resistors  220 ,  222 , and  224  are shown, it will be appreciated that any number of resistors may be used. It should also be noted that in some instances, it may be advantageous to add a buffer at the output of the circuit to prevent the trimming network from effecting the output impedance of the microphone as seen by any following circuitry (e.g. codec, DSP, to mention two examples). 
         [0030]    A capacitor  232  is coupled in series with the switch  210 . The capacitor  232  prevents DC current flow through resistors  220 ,  222 , and  224 . In some aspects, the capacitor  232  has a value such to define a cutoff frequency of f hpf =1/(2*pi*C*R atten ), where signal attenuation occurs only above the cutoff frequency. 
         [0031]    Referring now to  FIG. 3 , another example of a microphone  300  that is coupled to customer electronics is described. The microphone  300  includes a MEMS device  302  and an application specific integrated circuit (ASIC)  304 . The MEMS device  302  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  304 . In one example, the MEMS device  302  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0032]    The ASIC  304  may be any type of integrated circuit that includes an amplifier  306  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  304  also includes an output capacitor  308  (C out ). 
         [0033]    In addition, the ASIC  304  includes a bank of attenuation capacitors: a small value capacitor  320 , a medium value capacitor  322 , and a large value capacitor  324 . In one aspect, the values of the capacitors have values selected so as to add negligible noise to the microphone  300 . A switch  310  is used to switch in appropriate capacitors  320 ,  322 , and  324  to the circuit. This may be accomplished by a user selection  311 , which in one example is manually performed, but in some cases may be automatically performed. The capacitors to be added to the circuit depend upon the amount of attenuation needed by a customer device (e.g., a codec or DSP of a customer) that is coupled to V out . Consequently, although the switch  310  is shown as being open in this and the other figures herein, it will be appreciated that in fact it will couple to one or more of the capacitors  320 ,  322 , and  324 . Together, whichever capacitors  320 ,  322 , and  324  are selected, the selected capacitors form an equivalent capacitance C atten  that is coupled to V out . 
         [0034]    In this case, V out =(C out /(C out +C atten )) V signal , where C atten  is the equivalent value of the capacitance  320 ,  322 , and  324 , and V signal  is the voltage of the signal received from the MEMS device  302 . The number and value of capacitors are used at the output of the amplifier  306  to perform attenuation depend on the level of attenuation desired. In this case, the amplifier noise is attenuated, so the original SNR of the microphone is preserved even after one or more of the capacitance  320 ,  322 , and  324  are added to the ASIC  304 . Again, the exact value of C atten  will vary, depending upon the capacitors selected and the values of these capacitors. Moreover, although three possible capacitors  320 ,  322 , and  324  are shown, it will be appreciated that any number of capacitors may be used. It should also be noted that in some instances, it may be advantageous to add a buffer at the output of the circuit to prevent the trimming network from effecting the output impedance of the microphone as seen by any following circuitry (e.g. codec, DSP, to mention two examples). 
         [0035]    Referring now to  FIG. 4 , another example of a microphone  400  that is coupled to customer electronics is described. The microphone  400  includes a MEMS device  402  and an application specific integrated circuit (ASIC)  404 . The MEMS device  402  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  404 . In one example, the MEMS device  402  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0036]    The ASIC  404  may be any type of integrated circuit that includes an amplifier  406  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  404  also includes an output resistor  408  (R out ). 
         [0037]    In addition, the ASIC  404  includes an active attenuation circuit  422  that includes which has variable gain settings which can be selected by a switch  410 . This may be accomplished by a user selection  411 , which in one example is manually performed, but in some cases may be automatically performed. Consequently, although the switch  410  is shown as being open in this and the other figures herein, it will be appreciated that in fact it will couple to one or more active elements within active attenuation circuit  422 . Together, whichever active attenuation elements are selected, the selected elements form an equivalent less than unity gain, A v , that is coupled to V out . 
         [0038]    In this case, V out =A v *V signal , where A v  is the equivalent gain of the active attenuation circuit  422 , and V signal  is the voltage of the signal received from the MEMS device  402 . The active attenuation circuit  422  is used at the output of the amplifier  406  to perform attenuation. In this case, the amplifier noise is attenuated, so the original SNR of the microphone is preserved even after the active attenuation circuit  422  added to the ASIC  404 . Again, the exact value of Av will vary, depending upon the resistors selected and the values of these resistors. A skilled artisan will appreciate that there are many ways to implement the less-than-unity gain circuit in practice. Several examples of which will be illustrated in  FIGS. 7-10 . 
         [0039]    It will be appreciated that the examples of  FIGS. 1-4 and 7-11  present approaches of trimming the sensitivity of a microphone in ways which preserve the SNR of the sensor by moving the trimming to occur after the sensor buffering or amplification in the signal chain. This approach is referred to as “back-end trimming” and refers to sensitivity trimming, which is performed after the signal has been passed through the input stage of the amplifier or buffer circuitry. 
         [0040]    In other approaches, sensitivity trimming is implemented prior to the first stage of the buffer or amplifier circuit and performed after the signal has been passed through the input stage of the amplifier or buffer circuits. Referring now to  FIG. 5 , another example of a microphone  500  that is coupled to customer electronics is described. The microphone  500  includes a MEMS device  502  and an application specific integrated circuit (ASIC)  504 . The MEMS device  502  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  504 . In one example, the MEMS device  502  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0041]    The ASIC  504  may be any type of integrated circuit that includes an amplifier  206  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  504  also includes an output resistor  508  (R out ). 
         [0042]    In addition, the ASIC  504  includes a bank of attenuation resistors: a small value resistor  520 , a medium value resistor  522 , and a large value resistor  524 . In one aspect, the values of the resistors are sufficiently small so as to add relatively little noise to the microphone  500 . A switch  510  is used to switch in appropriate resistors  520 ,  522 , and  524  to the circuit. The setting of the switch  510  may be accomplished by a user selection  511 , which in one example is manually performed, but in some cases may be automatically performed. The number and resistance values of resistors to be added to the circuit depend upon the amount of attenuation needed by a customer device (e.g., a codec or DSP of a customer) that is coupled to V out . Consequently, although the switch  510  is shown as being open in this and the other figures herein, it will be appreciated that in fact it will couple to one or more of the resistors  520 ,  522 , and  524 . Together, whichever resistors  520 ,  522 , and  524  are selected, the selected resistors form an equivalent resistance R atten  that is coupled to V out . 
         [0043]    In this case, V out =(R atten /(R out +R atten ))V signal , where R atten  is the equivalent value of the resistors  520 ,  522 , and  524 , and V signal  is the voltage of the signal received from the MEMS device  502 . The resistors are used at the output of the amplifier  506  to perform attenuation. In this case, the amplifier noise is attenuated, so the original SNR of the microphone is preserved even after one or more of the resistors  520 ,  522 , and  524  are added to the ASIC  504 . Again, the exact value of R atten  will vary, depending upon the resistors selected and the values of these resistors. Moreover, although three possible resistors  520 ,  522 , and  524  are shown, it will be appreciated that any number of resistors may be used. 
         [0044]    A capacitor  532  is coupled in series with the switch  510 . The capacitor  532  prevents current flow through resistors  520 ,  522 , and  524 . In some aspects, the capacitor  532  has a value such the F hpf =1/(2*pi*C*R atten ). 
         [0045]    A capacitor bank is also provided at the input of the amplifier. This capacitor bank includes a small capacitor  552 , a medium capacitor  554 , and a large capacitor  556 . A switch  560  is set to selectively switch in selected ones of the capacitors  552 ,  554 , and  556 . This may be accomplished by a user selection  513 , which in one example is manually performed, but in some cases may be automatically performed. Consequently, although the switch  560  is shown as being open in this and the other figures herein, it will be appreciated that in fact it will couple to one or more of the capacitors  552 ,  554 , and  556 . The use of the capacitors  552 ,  554 , and  556  controls microphone distortion (improves total harmonic distortion (THD)), while the use of resistors  520 ,  522 , and  524  maintains or improves the SNR of the microphone. Moreover, although three possible resistors  520 ,  522 , and  524  and three possible capacitors  552 ,  554 , and  556  are shown, it will be appreciated that any number of resistors and capacitors may be used. 
         [0046]    While  FIG. 5 . shows how a bank of selectable input capacitors can be combined with a selectable back of attenuation resistors after the input buffer to optimize a trade-off between SNR and THD, a user will appreciate that any of the back-end attenuation methods illustrated in  FIGS. 1-4  and  FIGS. 7-11  can be combined with a selectable bank of input capacitors to similar effect. 
         [0047]    Referring now to  FIG. 6 , some of the advantages of the present approaches are described. A first graph  602  shows various characteristics of a microphone before attenuation is performed. A second graph  604  shows the effects of performing the present approaches at a microphone. 
         [0048]    The first graph  602  shows MEMS self-noise  622 , amplifier self-noise  624 , total noise  626  a response to tone  627 , and signal to noise ratio (SNR)  628 . Before attenuation, the MEMS self-noise is shown here as being above the amplifier self-noise, as is often the case, and the SNR is at its maximum value. 
         [0049]    The second graph  604  shows MEMS self-noise  632 , amplifier self-noise  634 , total noise  636  a response to tone  637 , and signal to noise ratio (SNR)  638 . After attenuation is performed, the MEMS self-noise  632  is reduced (from original MEMS self-noise  622 ) and the sensitivity (V out /sound pressure) of the microphone is reduced. The amplifier self-noise  634  is also reduced (in comparison to previous approaches where it was not reduced). As a result, the SNR  638  is the same as (or approximately the same as) the SNR  628 . Therefore, a microphone is provided where sensitivity can be adjusted through attenuation of its output signal but without degradation of the SNR of the microphone. 
         [0050]    Referring now to  FIG. 7 , another example of a microphone  700  that is coupled to customer electronics is described. The microphone  700  includes a MEMS device  702  and an application specific integrated circuit (ASIC)  704 . The MEMS device  702  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  704 . In one example, the MEMS device  702  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0051]    The ASIC  704  may be any type of integrated circuit that includes a first amplifier  706  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  704  also includes an output resistor  708  (R out ). 
         [0052]    In addition, the ASIC  704  includes a second operational amplifier  722 , a first capacitor (C 1 )  724 , a resistor  726 , and a second capacitor (C 2 )  728 . The first capacitor  724  may be a variable capacitor whose value is set during manufacturing or by either user input or automatically similar to the cases in  FIGS. 1-4 . The resistor  726  is large (e.g. 1 G ohm) and keeps output from becoming unstable. In this case, V out =(−C 2 /C 1 )*V signal . It will be appreciated that this circuit functions as a charge amplifier. While a capacitor  724  is drawn as a single, variable capacitor, it should be appreciated that capacitor  724  could be replaced with a bank of selectable capacitors to similar effect. It should also be appreciated that capacitor  728  could be made variable rather than or in addition to capacitor  724 , either by means of a single variable capacitor or a bank of selectable capacitors. It should also be noted that in some instances, it may be advantageous to add a buffer at the output of the circuit to prevent the trimming network from effecting the output impedance of the microphone as seen by any following circuitry (e.g. codec, DSP, to mention two examples). 
         [0053]    Referring now to  FIG. 8 , another example of a microphone  800  that is coupled to customer electronics is described. The microphone  800  includes a MEMS device  802  and an application specific integrated circuit (ASIC)  804 . The MEMS device  802  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  804 . In one example, the MEMS device  802  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0054]    The ASIC  804  may be any type of integrated circuit that includes a first amplifier  806  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  804  also includes a variable output resistor  808  (R 2 ). 
         [0055]    In addition, the ASIC  804  includes a second operational amplifier  822 , a first capacitor (C 1 )  824 , a variable resistor  826  (R 1 ), and a second capacitor (C 2 )  828 . The value of the variable resistor  826  may be set during manufacturing or by either user input or automatically similar to the cases in  FIGS. 1-4 . The resistor  826  is small (e.g. 1 k ohm) and keeps output from becoming unstable. Capacitor  828  is included to provide rejection of any DC offset present at the output of amplifier or buffer  806 . Capacitor  828  may be omitted in some cases, or if included, is selected such to provide DC rejection without interfering with the desired frequency response of the sensor. In this case, V out =(−R 1 /R 2 )*V signal . This circuit functions as an inverting amplifier. While R 1  is drawn as a single, variable resistor, it should be appreciated that resistor  826  could be replaced with a bank of selectable resistors to similar effect. It should also be appreciated that resistor  808  could be made variable rather than or in addition to resistor  826 , either by means of a single variable resistor or a bank of selectable resistors. It should also be noted that in some instances, it may be advantageous to add a buffer at the output of the circuit to prevent the trimming network from effecting the output impedance of the microphone as seen by any following circuitry (e.g. codec, DSP, to mention two examples). 
         [0056]    Referring now to  FIG. 9 , another example of a microphone  900  that is coupled to customer electronics is described. The microphone  900  includes a MEMS device  902  and an application specific integrated circuit (ASIC)  904 . The MEMS device  902  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  904 . In one example, the MEMS device  902  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0057]    The ASIC  904  may be any type of integrated circuit that includes a first amplifier  906  (e.g., with an op-amp having a gain). The ASIC  904  also includes a variable output resistor  908  (R 2 ). 
         [0058]    In addition, the ASIC  904  includes a second operational amplifier  922 , a resistor  926  (R 1 ), and a capacitor (C 1 )  928 . The variable resistors  908  may be a variable resistor whose value is set during manufacturing. The resistor  926  is small (e.g. 1 k ohm) and keeps output from becoming unstable. The value of capacitor  928  (C 1 ) is typically approximately 1 μF, but will vary by design and may be excluded in some instances. In this case, V out =(−R 1 /R 2 )*V signal . It will be appreciated that this circuit functions as an inverting amplifier. It should also be noted that in some instances, it may be advantageous to add a buffer at the output of the circuit to prevent the trimming network from effecting the output impedance of the microphone as seen by any following circuitry (e.g. codec, DSP, etc.) 
         [0059]    Referring now to  FIG. 10 , another example of a microphone  1000  that is coupled to customer electronics is described. The microphone  1000  includes a MEMS device  1002  and an application specific integrated circuit (ASIC)  1004 . The MEMS device  1002  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  1004 . In one example, the MEMS device  1002  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0060]    The ASIC  1004  may be any type of integrated circuit that includes a first amplifier  1006  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  1004  also includes an output resistor  1008  (R out ). 
         [0061]    In addition, the ASIC  1004  includes a second operational amplifier  1022 . The ASIC  1004  includes a switch  1024  that selects between one or more of three resistors  1026 ,  1028 ,  1030  in a resistor bank  1025 . In this case, V out =(R atten /(R out +Rotten)) V signal , where Rotten is the equivalent value of the resistors  1026 ,  1028 ,  1030  selected by switch  1024 , and V signal  is the voltage of the signal received from the MEMS device  1002 . The range of resistor values selected will depend on the requirements of the designer, but in typical applications might range from 10&#39;s of ohms to 10&#39;s of kohms, although this range may not be inclusive of the preferred values for all designs. This circuit functions as a resistance trimming before a unity buffer. The resistor bank allows better control of the output impedance of the microphone 
         [0062]    The resistors are used at the output of the amplifier  1006  to perform attenuation. In this case, the amplifier noise is attenuated, so the original SNR of the microphone is preserved even after one or more of the resistors  1026 ,  1028 , and  1030  are added to the ASIC  1004 . Again, the exact value of the attenuation resistance (R atten ) will vary, depending upon the resistors selected and the values of these resistors. Moreover, although three possible resistors  1026 ,  1028 , and  1030  are shown, it will be appreciated that any number of resistors may be used. 
         [0063]    Referring now to  FIG. 11 , another example of a microphone  1100  that is coupled to customer electronics is described. The microphone  1100  includes a MEMS device  1102  and an application specific integrated circuit (ASIC)  1104 . The MEMS device  1102  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  1104 . In one example, the MEMS device  1102  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0064]    The ASIC  1104  may be any type of integrated circuit that includes a first amplifier  1106  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  1104  also includes an output capacitor  1108  (Cout). 
         [0065]    In addition, the ASIC  1104  includes a second operational amplifier  1122 . The ASIC  1104  includes a switch  1124  that selects between one of three capacitors  1126 ,  1128 ,  1130  in a capacitor bank  1125 . Capacitor  1126  may be of a small value (e.g., 0.5 pF), resistor  1128  may be of medium value (e.g., 1 pF) and Capacitor  1130  may be of large value (e.g., 10 pF). In this case, V out =Cout/(Cout+Cattten)*V signal . This circuit functions as a capacitor trimming before a unity buffer. The capacitor bank allows better control of the output impedance of the microphone. 
         [0066]    The capacitors are used at the output of the amplifier  1106  to perform attenuation. In this case, the amplifier noise is attenuated, so the original SNR of the microphone is preserved even after one or more of the capacitors  1126 ,  1128 , and  1130  are added to the ASIC  1104 . Again, the exact value of the attenuation capacitance (C atten ) will vary, depending upon the capacitors selected and the values of these capacitors. Moreover, although three possible capacitors  1126 ,  1128 , and  1130  are shown, it will be appreciated that any number of capacitors may be used. 
         [0067]    Referring now to  FIG. 12 , another example of a microphone  1200  is described. The microphone  1200  includes a MEMS device  1202  and an application specific integrated circuit (ASIC)  1204 . The MEMS device  1202  (or in some cases other sensing elements such as a piezoelectric sensor) converts sound energy into an electrical signal, which is sent to the ASIC  1204 . In one example, the MEMS device  1202  includes a diaphragm and back plate. In other examples, other devices (e.g., piezoelectric sensors) may be used that do not include a diaphragm and back plate. 
         [0068]    The ASIC  1204  may be any type of integrated circuit that includes a first amplifier  1206  (e.g., with an op-amp having a gain) or impedance buffer. The ASIC  1204  also includes an output resistor  1208  (R out ). 
         [0069]    In addition, the ASIC  1204  includes a second operational amplifier  1222 . The ASIC  1204  includes a switch  1224  that selects between one or more of three resistors  1226 ,  1228 ,  1230  in a resistor bank  1225 . 
         [0070]    This example is similar to the example that depicted in  FIG. 10 , except in this case, the resistors in bank  1225  are connected to a reference voltage rather than ground. The reference voltage can be set to any value, including ground. In an ideal case, the reference voltage will be set to match the DC level at the node labeled node  1 . By matching Vref to node  1 , DC current flow through the resistor(s) defining R atten  is eliminated, while signal attenuation is still achieved by an apparent AC ground at the Vref node. This embodiment serves a similar purpose as the example shown in  FIG. 2 , which is to achieve signal attenuation while preventing DC current flow through the attenuation resistors, which adds to the power consumption of the microphone. 
         [0071]    It will be understood that the number and values of resistors included in bank  1225  may vary depending on the needs of the designer, and that the buffer  1222  may not be necessary in all designs, depending on the following circuitry and the needs of the designer. 
         [0072]    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.