Abstract:
A scheme is described to switch the power supply to the MEMS microphone on and off in a cyclic manner that is synchronized with the associated ADC sampling rate. In this way the MEMS microphone amplifier, whether it is a J-FET transistor or an operational amplifier, is off most of the cycle time, and is turned on only for a few micro-seconds prior to the sample-and-hold timing of the ADC device. By this method, the average power consumption of an existing analog MEMS microphone can be reduced by a factor of 10 or more.

Description:
RELATED APPLICATIONS 
     This application claims the priority of U.S. provisional patent Ser. No. 61/933,316 filing date Jan. 30, 2014 which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of microphones such as but not limited to MEMS (Micro Electrical-Mechanical System) microphones. 
     BACKGROUND OF THE INVENTION 
     A MEMS microphone is also called a microphone chip or silicon microphone. MEMS microphones are usually referred to as being of two main types: analog and digital. Both types are based on a membrane or diaphragm that is combined with a permanently charged capacitor that changes its capacitance according to the pressure derived from acoustic waves. This is commonly known as an electret microphone. The pressure-sensitive diaphragm is etched directly into a silicon chip by MEMS techniques, and is usually accompanied with a preamplifier; this is referred to as an ‘analog MEMS microphone’. To be more readily integrated with modern digital products an external analog-to-digital converter (ADC) is usually used together with the analog MEMS microphone. “Digital MEMS microphones” include an ADC circuit in the same package. 
     An external power supply is required by both the analog and digital MEMS microphone. In the case of an analog MEMS microphone, the power is consumed by the integrated preamplifier. A typical electret microphone preamplifier circuit uses an Field Effect Transistor (FET) in a common_source configuration which must be externally powered by a supply voltage. 
     It is becoming more common, however, for the preamplifier to be a low power operational amplifier. The analog MEMS microphone represents the lowest power consumption case. Digital MEMS microphones consume more power as they also contain an integrated ADC in addition to the amplifier. The use case is that the microphone in a mobile or wearable device needs to be active, even when the device is sleeping, such that it can detect any wake up voice commands. Hence it is highly desirable that the MEMS microphone consumes ultra-low power. Target power consumption is in the order of less than 25 microwatts whereas the usual power consumption of an analog MEMS microphone is in the order of 200 microwatts and digital MEMS microphones consume more. 
     SUMMARY OF THE INVENTION 
     There is provided a device and a method for minimizing a power consumption of the analog parts of an analog microphone down to few micro watts (and even below) while still using existing analog microphones. A first power supply coupled to the analog microphone is repetitively provided (turned on) or prevented from being provided (turned off) in a cyclic manner that is synchronized with the ADC sampling rate. In this way the microphone amplifier, whether it is a J-FET transistor or an operational amplifier, is off most of the cycle time, and is turned on only for a few micro-seconds prior to the sample-and-hold timing of the ADC device. By this method, the average power consumption of an existing analog microphone can be reduced by a factor of 10 or more. A microphone amplifier is fed by a first power supply V+. An ADC amplifier that precedes the ADC is fed by a second power supply V0. A capacitor is coupled between an output of the analog microphone and an input of the ADC and provides DC isolation. 
     The device may include a switching circuit. The switching circuit may be controlled by the same digital timing control unit that provides the sampling rate for the ADC or may be affected by the control signals generated by the timing control unit. 
     The analog switches are controlled by the digital timing control unit to close a settling time, Ts, before the ADC starts its sample-and-hold phase of an analog to digital conversion operation, and then to open again after a hold time, Th, which is the time required by the ADC for the sample-and-hold phase. The settling time Ts is to ensure that after the V+ supply has been switched on, the output of the microphone internal amplifier has reached its final (desired) DC voltage and also that the ADC amplifier&#39;s input junction has reached its final (desired) DC voltage level. 
     The duration of the setting time, Ts, can typically be very short, in the order of 1 or 5 microseconds, as neither of the microphone amplifier and the ADC amplifier contains capacitors or inductors. The sample-and-hold time, Th, for an ADC in this application is typically in the order of 7 μs. Hence, every sampling period of a duration of Tsampling, the analog switches are closed for a time of Ts+Th. 
     Hence, taking the example of an ADC sampling rate of 8000 samples per second, the sampling time, Tsampling will be 125 μs and for values of Ts and Th of 5 μs and 7 μs respectively, the analog switches will be closed for 12 μs every 125 μs. Power is therefore applied to the microphone amplifier for only about 1/10 th  of the time and hence the power consumption is reduced by a factor of 10. To further reduce the power consumption of the total system, an additional power switch may be added to the ADC amplifier. 
     According to an embodiment of the invention there may be provided a device that may include an analog microphone; an analog to digital converter (ADC); an ADC amplifier; a digital timing control circuit; and a switching circuit. The digital timing control circuit may be configured to repetitively trigger analog to digital conversion operations according to a sampling rate; wherein the sampling rate corresponds to a sampling period. The switching circuit may be configured to selectively provide power to the analog microphone and to the ADC amplifier in response to the sampling rate. 
     The switching circuit may be configured to prevent the supply of power to the microphone and to the ADC amplifier during power prevention periods that have a duration that equals a majority of the sampling period. 
     The device may include a capacitor coupled between an output of the analog microphone and an input of the ADC amplifier. 
     The switching circuit may be configured to disconnect the capacitor from the ADC amplifier during at least a majority of power prevention periods. 
     The switching circuit may be configured to (a) start powering the analog microphone and the ADC amplifier a settling period before a beginning of each digital to analog conversion operation and (b) stop powering the analog microphone and the ADC amplifier after the ADC completed a sample and hold phase of the analog to digital conversion operation; wherein during each settling period the analog microphone and the ADC amplifier are expected to settle. 
     The settling period may have a duration that is a fraction (for example between less than one percent till sixty percent) of the sampling period. 
     The switching circuit may include (i) a first switch that is coupled between the analog microphone and a first power supply, and (ii) a second switch that is coupled between the capacitor and the ADC amplifier. 
     The switching circuit may include (i) a first switch that is coupled between the analog microphone and a first power supply, (ii) a second switch that is coupled between the capacitor and the ADC amplifier, and (iii) a third switch that is coupled between a second power supply and the ADC amplifier. 
     The first switch and the third switch may be configured to be closed an intermediate period before the second switch is closed. 
     The switching circuit may be configured to disconnect the capacitor from the ADC amplifier during each power prevention period and each intermediate period that follows each power prevention period, wherein during each additional period a direct current (DC) level of an output of the analog microphone and a DC voltage level of an input port of the ADC amplifier is expected to settle. 
     The duty cycle of the analog microphone does may not exceed 10 percent. 
     The microphone may be a Micro Electrical-Mechanical System (MEMS) microphone. 
     According to an embodiment of the invention there may be provided a method, may include repetitively triggering, by a digital timing control circuit that is coupled to an analog to digital converter (ADC), analog to digital conversion operations according to a sampling rate; wherein the sampling rate corresponds to a sampling period; and selectively providing power, by a switching circuit, to an analog microphone and to an ADC amplifier in response to the sampling rate; wherein the ADC amplifier is coupled to the ADC. These stages may be executed concurrently—a triggering of the ADC occurs in parallel (or almost in parallel) to the provision of power to the ADC amplifier and the analog microphone (especially to a microphone amplifier). The ADC may be powered constantly or almost constantly. 
     The selectively providing of power may include preventing the supply of power to the microphone and to the ADC amplifier during power prevention periods that have a duration that equals a majority of the sampling period. 
     The method may include a capacitor coupled between an output of the analog microphone and an input of the ADC amplifier. 
     The method may include disconnecting the capacitor from the ADC amplifier during at least a majority of power prevention periods. 
     The method may include starting to power the analog microphone and the ADC amplifier a settling period before a beginning of each digital to analog conversion operation; stopping to power the analog microphone and the ADC amplifier after the ADC completed a sample and hold phase of the analog to digital conversion operation; wherein during each settling period the analog microphone and the ADC amplifier are expected to settle. 
     The settling period may have a duration that is a fraction of the sampling period. 
     The switching circuit may include (i) a first switch that is coupled between the analog microphone and a first power supply, and (ii) a second switch that is coupled between the capacitor and the ADC amplifier. 
     The switching circuit may include (i) a first switch that is coupled between the analog microphone and a first power supply, (ii) a second switch that is coupled between the capacitor and the ADC amplifier, and (iii) a third switch that is coupled between a second power supply and the ADC amplifier. 
     The method may include closing the first switch and the third switch an intermediate period before closing the second switch. 
     The method may include disconnecting the capacitor from the ADC amplifier during each power prevention period and each intermediate period that follows each power prevention period, wherein during each additional period a direct current (DC) level of an output of the analog microphone and a DC level of an input port of the ADC amplifier is expected to settle. 
     The duty cycle of the analog microphone does may not exceed 10 percent. 
     The microphone may be a Micro Electrical-Mechanical System (MEMS) microphone. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
         FIG. 1  is a schematic of a device according to an embodiment of the invention; 
         FIG. 2  is a timing diagram according to an embodiment of the invention; 
         FIG. 3  is a schematic of a device according to an embodiment of the invention; 
         FIG. 4  is a schematic of a device according to an embodiment of the invention; 
         FIG. 5  is a schematic of a device according to an embodiment of the invention; 
         FIG. 6  is a timing diagram according to an embodiment of the invention; 
         FIG. 7  is a schematic of a device according to an embodiment of the invention; 
         FIG. 8  is a schematic of a device according to an embodiment of the invention; and 
         FIG. 9  is flow chart of a method according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention. 
     The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings. 
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
     Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 
     Any reference in the specification to a method should be applied mutatis mutandis to a device capable of executing the method. 
     Any reference in the specification to a device should be applied mutatis mutandis to a method that may be executed by the device. 
     The following examples refer to a Micro Electrical-Mechanical System (MEMS) microphone. It is noted that the MEMS microphone is merely an example of a microphone and that the invention is applicable to any type of microphone such as but not limited to non-MEMS microphones including a condenser microphone, an electret-condenser microphone, a dynamic microphone, a ribbon microphone, a Carbon microphone, a Piezoelectric microphone, a Fiber microphone, a Laser microphone, a Liquid microphone, and the like. 
       FIG. 1  illustrates a device  91  according to an embodiment of the invention. 
     Device  91  may be a digital MEMS microphone, may include a digital MEMS microphone, may be a mobile communication device, a headset, a voice triggered device, and the like. 
     Device  91  may include analog MEMS microphone  10 , capacitor  22 , switching circuit  33 , ADC  60 , ADC amplifier  50  and digital timing control  70 . 
     Analog MEMS microphone  10  includes an electret microphone  11  and an MEMS microphone amplifier  12 . 
     The MEMS microphone amplifier  12  may include a variety of different circuits including an operational amplifier or a single J-FET stage. The MEMS microphone amplifier  12  may be of any design. 
     The MEMS microphone amplifier  12  is powered by a positive voltage  15  and its function is to amplify and isolate the signal from the electret microphone  11 . 
     First analog switch  30  is coupled between first power supply  40  V+ and MEMS microphone amplifier  12  (or analog MEMS microphone  11 ). 
     An output signal  20  that is outputted from analog MEMS microphone  10 , is provided to capacitor  22 . Capacitor is followed by second analog switch  35 . Capacitor  22  is configured to block the DC voltage at the output of the MEMS microphone amplifier  12 . The other side of the second analog switch  35  is connected to the input of the ADC Amplifier  50 . 
     First and second analog switches  30  and  35  form a switching circuit  33 . Any type of switching circuit  33  may be used instead of first and second analog switches  30  and  35 . 
     The input stage of ADC amplifier  50  is modeled in  FIG. 1  as including input impedance  25  that is fed by virtual voltage supply V0  45 . 
     The signal  20  from the MEMS microphone amplifier  12  is sent to capacitor  22 , passes through second analog switch  35  (when the second analog switch  35  is closed) and then is amplified by the ADC Amplifier  50  and applied to ADC  60  where an analog to digital conversion takes place. 
     A digital timing control block  70  is used to provide (a) control signals according to a sampling rate for the ADC  60  and (b) control signals to the first and second analog switches  30  and  35 . 
     Output  75  from the digital timing control block  70  is applied to the ADC  60  and is used to set the sampling rate and the timing of the sampling and hold phase of the ADC  60 . 
     Output  80  from the digital timing control block  70  is applied to the first and second analog switches  30  and  35  and is used to switch them both to the closed and open conditions simultaneously. It is noted that separate control signals may be sent to the first and second analog switches—allowing an independent control of these analog switches. 
     When first analog switch  30  is in the closed position, the first supply voltage  40  is applied to the MEMS microphone amplifier  12  and the MEMS microphone amplifier  12  is active. When first analog switch  30  is in the open position, the MEMS microphone amplifier  12  is inactive. As both analog switches  30  and  35  are controlled by the same signal  80 , they will both be in the same open or closed condition. When the MEMS microphone amplifier  12  is active both analog switches  30  and  35  are closed (connected). The amplified analog signal from the electret microphone  11  is therefore applied via capacitor  22  and the closed first analog switch  35  to the input of the ADC amplifier  50 . 
     The output signal  20  from the analog MEMS microphone  10  is usually an AC voltage that rides on a DC level that is much bigger than the amplitude of the AC voltage. Thus—without DC isolation—DC leakage that is much bigger than the AC voltage may mask the AC voltage or otherwise be interpreted as a valid AC voltage. 
     Second analog switch  35  is included so that DC blocking capacitor  22  is disconnected from the input to the ADC Amplifier  50  and therefore isolates the DC voltage step that results from the on off switching of the first supply voltage  40  at the MEMS microphone amplifier  12  from the ADC amplifier  50 . The output of ADC amplifier  50  is connected to the input of the ADC  60 . The sampling timing signal  75  is applied to the ADC such that the sample-and-hold phase and an analog to digital conversion phase in the ADC  60  is synchronized with the closing of the two analog switches  30  and  35 . This is further explained in  FIG. 2 . 
       FIG. 2  is a diagram according to an embodiment of the invention. 
     The upper part of  FIG. 2  illustrates analog switch connected period  115  and analog switch connected period  135 . It is noted that analog switch connected periods may equal to periods (ADC amplifier activation periods) during which the ADC amplifier is activated or may differs from ADC amplifier activation periods. For example, an ADC amplifier may be opened slightly after the analog switches enter their connected period. 
     The upper part of  FIG. 2  illustrates values of control signal  80  and corresponding states of the first and second analog switches according to an embodiment of the invention. 
     Referring to a first analog switch open period  115 —it starts at point of time t0  110  during which the first and second analog switches  30  and  35  are closed by control signal  80 . At time t1  120 , the two analog switches  30  and  35  are both set to the open position  125 . The first analog switch disconnect period  115  spans between t0 and t1. 
     At time t2,  130 , the two analog switches,  30  and  35  in  FIG. 1 , are both again set to the closed position during a second analog switch open period  135 . 
     Analog switch disconnected period Toff  160  spans between t1  120  and t2  130 . 
     The ADC samples at a sampling rate that has a sampling period Tsampling  140 . Tsampling  140  spans between t0  110  and t2  130 . 
     The lower part of  FIG. 2  illustrates in greater detail the first analog switch connected period  115  and also samples the actions of the ADC  60 . 
     At time t0,  110  the two analog switches  30  and  35  are both set to the closed position. A settling time Ts  250  after t0  110 , at point in time t0′  220  the ADC  60  starts to perform an ADC sample-and-hold operation  270 . The sample-and-hold operation  270  ends at point in time t0″  240 , before the end (t1  120 ) of the analog switch connected period  115 . 
     The ADC  60  then proceeds with the analog to digital conversion process  280 . The analog to digital conversion process  280  may end after t1  120 , before t1  120  or at t1  120 . 
     Settling time Ts  250 , may be set to allow the DC conditions of the MEMS microphone amplifier  12  and ADC Amplifier  50  to settle. 
     An ADC on period 
       FIG. 3  illustrates device  93  according to an embodiment of the invention. Device  93  of  FIG. 3  differs from device  91  of  FIG. 1  by not including resistor  25  that represents the input resistance of ADC amplifier  50 . 
       FIG. 4  illustrates device  94  according to an embodiment of the invention. Device  94  of  FIG. 4  differs from device  91  of  FIG. 1  by including a third analog switch  38  that is connected between the second supply voltage V0  45  and the ADC amplifier  50 . The third analog switch  38  is controlled by the same control signal  80  as the first and second analog switches  30  and  35  and may be opened and closed at the same time as the first and second analog switches. 
       FIG. 5  illustrates device  95  according to an embodiment of the invention. Device  95  of  FIG. 5  differs from device  91  of  FIG. 1  by including a third analog switch  38  that is connected between the second supply voltage V0  45  and the ADC amplifier  50 . 
     Each one of the first, second and third analog switches  30 ,  35  and  38  is controlled by a separate control signal— 81 ,  82  and  83  respectively—allowing an independent control of each of these switches. Accordingly—the connection period of each one of the first, second and third analog switches may be equal to or may differ from a connection period of any other analog switch. 
       FIG. 6  is a timing diagram of control signals  81 ,  82  and  83  according to an embodiment of the invention. 
       FIG. 6  shows that the second analog switch  35  that is fed by second control signal  82  may be closed an intermediate period ( 290 ) after first and third analog switches are closed. 
       FIG. 7  illustrates device  97  according to an embodiment of the invention. Device  97  of  FIG. 7  differs from device  91  of  FIG. 1  by not including second analog switch  35 . 
       FIG. 8  illustrates device  98  according to an embodiment of the invention. Device  98  of  FIG. 8  differs from device  91  of  FIG. 1  by not including second analog switch  35  and by not including capacitor  22 . 
       FIG. 9  is flow chart of method  200  according to an embodiment of the invention. 
     Method  200  includes stage  210  and  220 . 
     Stage  210  may include repetitively triggering, by a digital timing control circuit that is coupled to an analog to digital converter (ADC), analog to digital conversion operations according to a sampling rate; wherein the sampling rate corresponds to a sampling period. 
     Stage  220  may include selectively providing power, by a switching circuit, to an analog microphone (such as but not limited to a Micro Electrical-Mechanical System (MEMS) microphone) and to an ADC amplifier in response to the sampling rate. The ADC amplifier is coupled to the ADC. 
     According to various embodiments of the invention method  700  may include at least the following steps:
         a. Preventing the supply of power to the MEMS microphone and to the ADC amplifier during power prevention periods that have a duration that equals a majority of the sampling period.   b. Disconnecting by the switching circuit a capacitor that is coupled between an output of the analog MEMS microphone and an input of the ADC amplifier.   c. Disconnecting the capacitor from the ADC amplifier during at least a majority of power prevention periods.   d. Starting to power the analog MEMS microphone and the ADC amplifier a settling period before a beginning of each digital to analog conversion operation.   e. Stopping to power the analog MEMS microphone and the ADC amplifier after the ADC completed a sample and hold phase of the analog to digital conversion operation. During each settling period the analog MEMS microphone and the ADC amplifier are expected to settle. The settling period may have a duration that is a fraction of the sampling period.   f. Repetitively closing and opening (i) a first switch that is coupled between the analog MEMS microphone and a first power supply, and (ii) a second switch that is coupled between the capacitor and the ADC amplifier.   g. Repetitively closing and opening (i) a first switch that is coupled between the analog MEMS microphone and a first power supply, (ii) a second switch that is coupled between the capacitor and the ADC amplifier, and (iii) a third switch that is coupled between a second power supply and the ADC amplifier.   h. Closing the first switch and the third switch an intermediate period before closing the second switch.   i. Disconnecting the capacitor from the ADC amplifier during each power prevention period and each intermediate period that follows each power prevention period, wherein during each additional period a direct current (DC) level of an output of the analog MEMS microphone and a DC level of an input port of the ADC amplifier is expected to settle.       

     It is noted that the mentioned above figures provide only various examples of embodiments of the invention and they illustrate discrete components to illustrate the blocks. Any of the devices mentioned above may be embodied (or may be) a part of an audio processing integrated circuit 
     Different analog microphones and different amplifiers may impose other switching related issues to be solved when switching on and off the microphone DC voltage but such issues can be solved by those skilled in the art, by the addition of analog switches in the appropriate junctions of the analog circuits. 
     Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures may be implemented which achieve the same functionality. 
     Any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality. 
     Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments. 
     Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner. 
     However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense. 
     In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.