Patent Publication Number: US-9838767-B2

Title: Method and means for regulating the electrical bias voltage at the measuring capacitor of a MEMS sensor element

Description:
BACKGROUND INFORMATION 
     The present invention relates to a method and device for regulating the electrical bias voltage at the measuring capacitor of a MEMS sensor element, in which a base voltage is applied to the measuring capacitor, and in which this base voltage is subsequently regulated in such a way that the potential difference between the two electrode sides of the measuring capacitor corresponds to the setpoint voltage. 
     The regulation of the electrical bias voltage at the measuring or microphone capacitor of MEMS microphones is of particular significance. These generally include a sound pressure-sensitive diaphragm and a fixed counter-element. The diaphragm and the counter-element act as supports for the flat electrodes of the microphone capacitor, so that the changes of the distance between the diaphragm and the counter-element caused by sound pressure are detectable as capacitance fluctuations of the microphone capacitor. To increase the sensitivity of such MEMS microphones, a mechanical preload is applied to the diaphragm by applying a direct voltage to the microphone capacitor. This draws the diaphragm toward the counter-element electrostatically, the electrostatic force of the direct voltage counteracting the spring force of the diaphragm. This direct voltage may only be increased to the so-called pull-in point, at which the electrostatic force is equal to the spring force of the diaphragm. If the pull-in voltage is exceeded, the diaphragm snaps abruptly against the counter-element, as a result of which the microphone capacitor is short-circuited. Since the diaphragm at the pull-in point is in equilibrium of forces, each external force effect results in a diaphragm deflection, which is counteracted by no or only a very slight spring force. Consequently, the sensitivity of the diaphragm is highest at the pull-in point. If a MEMS microphone is to be operated in the range of maximum sensitivity, the electrical bias voltage at the microphone capacitor must be continuously monitored and regulated to the pull-in voltage. The pull-in voltage of MEMS microphones typically lies in the range of 5 V through 8 V. For regulating the electrical bias voltage of MEMS microphones, regulators are therefore used in practice, the output stage of which is able to regulate voltages of this magnitude. 
     SUMMARY 
     The present invention relates to regulating the base voltage, which is applied to the measuring capacitor, in a low-voltage range. This makes it possible to omit a high-voltage output driver. Consequently, the electricity demand of the circuit is reduced as well as the ASIC area required for the circuit. 
     There are various possibilities for implementing such regulation of the base voltage at the measuring capacitor as well as for its circuitry-wise implementation. 
     In a first method variant, a predefined and non-variable base potential in the order of the setpoint voltage is applied to one electrode side of the measuring capacitor. A regulatable counter-potential, which is low compared to the base potential, is applied to the other electrode side of the measuring capacitor. This counter-potential is then regulated in such a way that the potential difference at the measuring capacitor corresponds to the setpoint voltage. 
     This first method variant may be implemented simply in analog circuitry using standard transistors. In one preferred specific embodiment, the means for regulating the electrical bias voltage at the measuring capacitor of a MEMS sensor element include in this case a first voltage source, which delivers a voltage in the order of the setpoint voltage and is connected to the first electrode side of the measuring capacitor as a predefined base potential n 1 , a second voltage source, which delivers a voltage which is low in comparison to this and is connected to the other second electrode side of the measuring capacitor as counter-potential n 2 , an operational amplifier A, the inverting input of which is connected to the second electrode side of the measuring capacitor and whose output is fed back to its inverting input via a defined capacitance C int , and a regulator connected downstream from the output of operational amplifier A, the second input of the regulator being connected to the first voltage source as reference voltage n 1 , and whose output is fed back to the non-inverting input of operational amplifier A. In this way, counter-potential n 2  present at the inverting input of operational amplifier A is regulated on the second electrode side of the measuring capacitor via the output signal of the regulator. 
     Very different regulators may be used in this specific embodiment. However, an analog PI controller having a processing logic connected upstream proves to be particularly suitable. Such a PI controller includes at least one operational amplifier A PI , which is fed back via a defined capacitance C 1  and a resistor R 2 , and a resistor R 1  is connected upstream of its inverting input. 
     Another method variant for the regulation of the base voltage at the measuring capacitor according to the present invention provides for determining the difference between the capacitance of the measuring capacitor and a reference capacitance, the reference capacitance corresponding to the capacitance of the measuring capacitor when the setpoint voltage is applied. The base voltage applied to the measuring capacitor is subsequently regulated as a function of the determined capacitance difference. 
     This second method variant may advantageously be implemented simply with the aid of circuit means for digitizing the output signal. Thus, in a preferred circuitry-wise implementation of this method variant, the means for regulating the electrical bias voltage at the measuring capacitor include a voltage source, which is used as a voltage supply for a Wheatstone bridge. In this Wheatstone bridge, the measuring capacitor is interconnected with a reference capacitance C ref  and two additional capacitances C 1  and C 2 , and specifically in such a way that the output signal of the Wheatstone bridge corresponds to the deviation of the potential difference at the measuring capacitor from the setpoint voltage. The output signal of the Wheatstone bridge is supplied to an operational amplifier A, downstream from which are connected a filter and a quantizer. The output signal of the quantizer is fed back to the Wheatstone bridge, so that the potential difference at the measuring capacitor is regulated via the bit stream of the quantizer. 
     In terms of circuitry, it is in particular simple if reference capacitance C ref  corresponds to the capacitance of the measuring capacitor when the setpoint voltage is applied and the two capacitances C 1  and C 2  are essentially identical. 
     The regulation according to the present invention may be based on an arbitrary setpoint voltage. The setting or regulation of the pull-in voltage at the measuring capacitor of a MEMS sensor element represents only one particularly advantageous application of the measures according to the present invention. These may be used in any stress-sensitive capacitive sensor element, even if they prove to be advantageous in particular in connection with MEMS microphones. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       As discussed above, there are various options for developing and refining the present invention in an advantageous manner. For this purpose, reference is made to the following description of two exemplary embodiments of the present invention based on the figures. 
         FIG. 1  shows the circuitry-wise design of a MEMS microphone including a first circuitry variant for regulating the electrical bias voltage at microphone capacitor C MIC . 
         FIG. 2  shows the circuit diagram of a regulator  10  for the circuitry variant shown in  FIG. 1 . 
         FIG. 3  shows the schematic design of a MEMS microphone including a second circuitry variant for regulating the electrical bias voltage at microphone capacitor C MIC . 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     The exemplary embodiments described below relate in each case to a MEMS microphone including one microphone capacitor C MIC  for signal detection, an electrical bias voltage being applied to it for increasing the microphone sensitivity. This bias voltage is intended to be regulated to the pull-in voltage of the MEMS microphone element. For that purpose, a base voltage is applied to microphone capacitor C MIC  in both exemplary embodiments and regulated in such a way that the potential difference between the two electrode sides of microphone capacitor C MIC  corresponds to the pull-in voltage. According to the present invention, this regulation of the base voltage is implemented in the low-voltage range. 
     In the first exemplary embodiment shown in  FIG. 1 , the base voltage at microphone capacitor C MIC  is implemented with the aid of two voltage sources, each of which is connected to one electrode side of microphone capacitor C MIC . The one voltage source delivers a voltage in the order of the setpoint voltage, i.e., the pull-in voltage, and is connected to one electrode side of microphone capacitor C MIC  as a predefined non-variable base potential n 1 . The other voltage source delivers a voltage which is low in comparison to this and is connected to the other electrode side of microphone capacitor C MIC  as counter-potential n 2 . The base voltage at microphone capacitor C MIC  results as the difference between base potential and counter-potential (n 1 −n 2 ). At least base potential n 1  is applied in the form of a modulation voltage. This may be, for example, a square-wave voltage, which may be implemented simply using two voltages and one selection switch. 
     In the specific embodiment of the present invention described here, base voltage (n 1 −n 2 ) is regulated to the pull-in voltage, in that the first electrode side of microphone capacitor C MIC  is held at high base potential n 1 , while low counter-potential n 2  on the second electrode side of microphone capacitor C MIC  is regulated accordingly. In MEMS microphones, the pull-in voltage normally lies in the range of 5 V through 8 V. Voltage n 1  applied on the first electrode side must be accordingly high. 
     In the exemplary embodiment represented here, the regulation of lower counter-potential n 2  takes place with the aid of an operational amplifier A used as a charge integrator and a regulator  10 , the circuitry-wise integration of which is explained in greater detail in connection with  FIG. 2 . 
     The second electrode side of microphone capacitor C MIC  is connected to the inverting input of operational amplifier A, so that counter-potential n 2  is thus applied here. Output n 3  of operational amplifier A is on the one hand fed back to its inverting input via a defined integration capacitance C int . On the other hand, output n 3  of operational amplifier A is supplied to one input of regulator  10 . The second input of regulator  10  is connected to the first voltage source as reference voltage. Fixed base potential n 1  is thus present at this input, the first electrode side of microphone capacitor C MIC  being held on this base potential. Output n 4  of regulator  10  is fed back to the non-inverting input of operational amplifier A. 
     Since the difference of the inputs of operational amplifier A is regulated to zero, the inverting input does not follow the non-inverting input. In this way, counter-potential n 2  on the second electrode side of microphone capacitor C MIC  may be controlled via output signal n 4  of regulator  10  and consequently also the base voltage (n 1 −n 2 ) present at microphone capacitor C MIC . 
       FIG. 2  represents an embodiment variant for regulator  10 . The core element is a PI controller made up of an amplifier API, a capacitance C 1  and two resistors R 1  and R 2 . Resistor R 1  is connected upstream from the inverting input of amplifier API. 
     The non-inverting input of amplifier API is connected to a reference potential V ref , which corresponds to 0 V in the exemplary embodiment represented here. Output n 4  of amplifier API is fed back to its inverting input via resistor R 2  and capacitance C 1 . Three subtractors S 1 , S 2 , S 3  and one factor N or 1/N are connected upstream from resistor R 1 , so that a zero is output at the output of subtractor S 3 , when the following condition is met: C MIC =N·C int ·C MIC  denotes here the capacitance of the microphone capacitor. Its capacitance at the pull-in point may be determined simply by reducing the base distance of the capacitor electrodes to ⅔ in the capacity calculation. Since integration capacitance C int  of operational amplifier A is known, factor N may be calculated simply and implemented accordingly in the circuitry. 
     Amplifier API subsequently delivers an output voltage N 4 , which is used to set counter-potential n 2  on one electrode side of microphone capacitor C MIC  in such a way that the base voltage (n 1 −n 2 ) corresponds to the pull-in voltage of microphone capacitor C MIC . 
     In the second exemplary embodiment shown in  FIG. 3 , microphone capacitor C MIC  is interconnected with a reference capacitance C ref  and two additional capacitances C 1  and C 2  in a Wheatstone bridge. This bridge circuit is fed from a separate fixed voltage source U 0 , which delivers a voltage in the order of double the pull-in voltage. Reference capacitance C ref  and additional capacitances C 1  and C 2  are selected and interconnected in such a way that the output signal of the Wheatstone bridge corresponds to the deviation of the voltage present at microphone capacitor C MIC  from the corresponding pull-in voltage. In the simplest case, reference capacitance C ref  corresponds to the capacitance of microphone capacitor C MIC  when the pull-in voltage is applied and the two capacitances C 1  and C 2  are generally identical. 
     In this second exemplary embodiment, the base voltage present at microphone capacitor C MIC  is not regulated directly, but instead indirectly, in that the voltage present at the Wheatstone bridge is regulated as a function of its output signal. For this purpose, the output signal of the Wheatstone bridge is fed to an operational amplifier A. A filter F and a quantizer Q are connected downstream from it. This is advantageously a delta-sigma modulator for digitizing the output signal. The digitized output signal is fed back to the Wheatstone bridge, so that U Wheat =U 0 +ΔU Q , ΔU Q  corresponding to the bit stream of quantizer Q. This causes voltage U Wheat  at the Wheatstone bridge to be regulated in such a way that the capacitance of microphone capacitor C MIC  corresponds to reference capacitance C ref . This means that the potential difference at microphone capacitor C MIC  is regulated to the pull-in voltage via bit stream ΔU Q  of quantizer Q. For this purpose, the pulse density of bit stream ΔU Q  is adjusted in such a way that on average the voltage, which is required for the pull-in operation of microphone capacitor C MIC , is generated. 
     Since the voltage contribution of bit stream ΔU Q  is very small compared to U 0  and also to the base voltage at microphone capacitor C MIC , the regulation of the base voltage at microphone capacitor C MIC  takes place here also in a low-voltage range.