Patent Description:
Capacitive MEMS microphones use a source follower based readout amplifier, as this is the optimum choice for such an application in order to optimize the signal to noise ratio (SNR) , the total harmonic distortion (THD), the consumption, the area, etc.. The MEMS (micro-electro-mechanical system) used is of differential type and therefor generates a positive and a negative signal which gets buffered and or amplified by the readout circuit. It can happen that the MEMS generated signals have a different amplitude up to several decibel. This results in a lower acoustic overload point since the larger signal will define the clipping of the signal. Such a clipping should be avoided.

One straightforward solution would be to make the MEMS more symmetric or sort out devices with too high asymmetry. But this of course causes a not wanted yield issue. Another obvious solution is to use a gain amplifier with e.g. different gain resistors for positive or negative output. This solution suffers from a not optimum SNR, since the gain resistors contribute to the noise budget or their values have to be that small that they cause a high dynamic current when signal is applied.

<CIT> discloses a system and method for signal read-out, and, in particular embodiments, to a system and method for signal read-out using source follower feedback. According to an embodiment, circuit includes an amplifier and a programmable capacitor coupled between an output of the first non-inverting and the input of the first amplifier.

Generally, there is a need in the art for an approach to implement an improved differential MEMS-readout circuit and an improved system comprising a MEMS-device coupled to the proposed MEMS-readout circuit and a method of using a MEMS-readout circuit, so that a higher acoustic overload point results.

Such a need can be solved for the subject-matter of the independent claims. Further specific implementations of the present concept (method and apparatus) for providing an improved differential MEMS-readout circuit is subject of the dependent claims. A MEMS-device for connection to a MEMS-readout circuit and to a system comprising the MEMS-readout circuit and the MEMS device and a method of using a MEMS-readout circuit are also disclosed.

In an embodiment, a differential MEMS-readout circuit comprises a first input bonding pad, which represents a first capacitance C1; wherein the first capacitance comprises the first input bonding pad as a first contact pin and a second contact pin. The differential MEMS-readout circuit comprises further a second input bonding pad, which represents a second capacitance C2; wherein the second capacitance comprises the second input bonding pad as a first contact pin and a second contact pin.

Furthermore, the differential MEMS-readout circuit comprises a differential-readout amplifier section that comprises a first input connected to the first contact pin of the first capacitance and a second input connected to the first contact pin of the second capacitance. The differential-readout amplifier section comprises a first and a second transistor circuit and each of the second contact pins of the first and second capacitance, in particular of the input bonding pads, is either connected to an output of the first transistor circuit (<NUM>) or to an ouput of the second transistor circuit (<NUM>) by using switches. Each of the first and second transistor circuits comprises a source follower. This in turn means, that the differential-readout amplifier section comprises two source followers that are coupled with one another.

The proposed differential MEMS-readout circuit makes use of the always available input bonding pads, which represents capacitances. One input (i.e. the first contact pin) of each capacitance is connected to the MEMS output, and the second contact pin of each capacitance is connected either to a positive or a negative output of the used readout circuit. The connection between the second contact pins of each capacitance and the outputs is done with switches.

By using switches either the positive or negative output can be chosen. If the capacitance at the positive MEMS output is connected to the positive readout output the capacitance acts, as it is not present since the first and the second contact pins of the corresponding capacitance are in phase. If the capacitance is connected to the negative (ASIC) output it can act as attenuator of the input signal. With this method it is possible to achieve a gain symmetry of up to <NUM> to 2dB (depending on the capacitance value), resulting in a better THD of high sound pressure signals also of plus <NUM> to 2dB.

According to an example, a MEMS-device for connection to a MEMS-readout circuit is disclosed wherein the MEMS device is configured to act as a differential type sensor to provide at the first contact pins differential signals, wherein the MEMS-readout circuit is configured to receive the differential signals and to reduce a difference in the amplitude of the differential signals at the second outputs. For connection with the MEMS-device a MEMS-readout circuit as proposed is used. The proposed MEMS-device connected to the proposed MEMS-readout circuit form a proposed system. Such a system may be implemented by a MEMS-microphone. The System, in particular such a MEMS-microphone or the like, have a higher acoustic overload, which is measured at the customer.

According to an example, a method of using a proposed MEMS-readout circuit is disclosed. The method comprises providing a MEMS-device and connecting the MEMS-device with the MEMS-readout circuit. The method comprises connecting a first input to the first contact pin of the first input bonding pad and connecting a second input to the first contact pin of the second bonding pad, and coupling each of the second contact pins of the first and second input capacitances either to an output of the first transistor circuit (<NUM>) or to an ouput of the second transistor circuit (<NUM>) by means of switches (<NUM>).

In particular, if the amplitudes of the signals of the MEMS-device do not differ essentially from each other, the method comprises connecting the first, in particular positive, contact pin INP of the first capacitance C1 to the positive output of the MEMS-device and the second contact pin of the first capacitance to the first, in particular positive, output of the first transistor circuit of the readout amplifier section, so that the first capacitance acts as it is not present, because the first and second contact pins of the first capacitance C1 are switched in phase, or connecting the first, in particular positive, contact pin INP of the first capacitance C1 to the positive output of the MEMS-device and the second contact pin of the first capacitance to the first, in particular negative, output of the second transistor circuit of the readout amplifier section or to ground, so that the first capacitance C1 acts as attenuator of the input signal in order to reduce a difference between the differential output signals.

Accordingly the method may comprise connecting the first, in particular positive, contact pin of the second capacitance C2 to the negative output of the MEMS-device and the second contact pin of the second capacitance C2 to the first, in particular negative, output of the second transistor circuit of the readout amplifier section, so that the second capacitance C2 acts as it is not present, because the first and second contact pins of the second capacitance C2 are switched in phase, or connecting the first, in particular positive, contact pin of the second capacitance C2 to the positive output of the MEMS-device and the second contact pin of the second capacitance C2 to the first, in particular positive, output of the first transistor circuit of the readout amplifier section or to ground, so that the second capacitance C2 acts as attenuator of the input signal in order to reduce a difference between the differential output signals.

In particular, if the amplitudes of the signals of the MEMS-device differ essentially from each other, the method may comprise connecting the first contact pin of the first capacitance to the positive output of a MEMS-device and connecting the second contact pin of the first capacitance to the first output or to the second output of the first transistor circuit of the readout amplifier section, and connecting the first contact pin of the second capacitance to the negative output of a MEMS-device and connecting the second contact pin of the second capacitance to the first output or to the second output of the first transistor circuit of the readout amplifier section, a difference between amplitudes of signals at the first contact pins of the first and second capacitances is compensated.

Alternatively, the method may comprise connecting the first contact pin of the first capacitance to the positive output of a MEMS-device and connecting the second contact pin of the first capacitance to the first output or to the second output of the second transistor circuit of the readout amplifier section, and connecting the first contact pin of the second capacitance to the negative output of a MEMS-device and connecting the second contact pin of the second capacitance to the first output or to the second output of the second transistor circuit of the readout amplifier section, a difference between amplitudes of signals at the first contact pins of the first and second capacitances is compensated.

The latter two options allow for compensating a difference of the amplitudes of the signals of the MEMS-device.

The core of the invention is to use the bonding pad capacitance to correct a possible amplitude asymmetry of the MEMS. The correction can be done in positive or negative direction.

Embodiments of the present device for providing a differential MEMS-readout circuit, and providing a MEMS-device coupled to the differential MEMS-readout circuit, an a method of using a MEMS-readout circuit are described herein making reference to the appended drawings and figures.

Before discussing the present embodiments in further detail using the drawings, it is pointed out that in the figures and the specification identical elements and elements having the same functionality and/or the same technical or physical effect are usually provided with the same reference numbers or are identified with the same name, so that the description of these elements and of the functionality thereof as illustrated in the different embodiments are mutually exchangeable or may be applied to one another in the different embodiments.

In the following description, embodiments and examples are discussed in detail, however, it should be appreciated that the embodiments and examples provide many applicable concepts that can be embodied in a wide variety of MEMS devices. The specific embodiments and examples discussed are merely illustrative of specific ways to make and use the present concept, and do not limit the scope of the embodiments. In the following description of embodiments and examples, the same or similar elements having the same function have associated therewith the same reference signs or the same name, and a description of the such elements will not be repeated for every embodiment. Moreover, features of the different embodiments described hereinafter may be combined with each other, unless specifically noted otherwise.

It is understood that when an element is referred to as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or intermediate elements may be present. Conversely, when an element is referred to as being "directly" connected to another element, "connected" or "coupled," there are no intermediate elements. Other terms used to describe the relationship between elements should be construed in a similar fashion (e.g., "between" versus "directly between", "adjacent" versus "directly adjacent", and "on" versus "directly on", etc.).

It is further understood that an output OutP may also be abbreviated by outp or outP, or OutP. The same abbreviation apply to all the other output and inputs in an analogous fashion, i.e. it does not matter if capital letters or small letter are used.

Simulation results as for example shown in <FIG>, <FIG>, and <FIG> are shown on the y-axis in a logarithmic scale [dB] against different settings.

<FIG> shows a schematic of a default input configuration of a differential MEMS-readout circuit <NUM>, not according to the invention but useful for understanding the invention, <NUM>- wherein the differential MEMS-readout circuit <NUM> is coupled to a MEMS-device <NUM>. The MEMS-device <NUM> and the MEMS-readout circuit <NUM> together form a system <NUM>. <FIG> shows, for example, different signals <NUM> outputted by a MEMS-device <NUM>; the signals <NUM> having different amplitudes <NUM>, <NUM>, <NUM>. For example, if the MEMS-device <NUM> provide at his inputs INP, INN signals <NUM> having a similar amplitude <NUM> with respect to its absolute maximal and/or minimal values (and not with respect to its actual phase), it is not necessary to compensate the difference of the signals <NUM>. In this case one of the configurations called settings a, a', d or d' shown in the <FIG> may be used. <FIG> show in each case a schematic of a differential MEMS-readout circuit <NUM>, wherein the differential MEMS-readout circuit <NUM> of <FIG> is not according to the invention but useful for understanding the invention, wherein the coupling to a MEMS-device <NUM> is indicated by input the signals <NUM>. In all <FIG> the differential MEMS-readout circuit <NUM> comprises a first input bonding pad <NUM>, which represents a first capacitance C1, wherein the first capacitance C1 comprises the first input bonding pad <NUM> as a first contact pin INP and a second contact pin C1-<NUM>.

The differential MEMS-readout circuit <NUM> comprises furthermore a second input bonding pad <NUM>, which represents a second capacitance C2. The second capacitance C2 comprises the second input bonding pad <NUM> as a first contact pin (INN) and a second contact pin C2-<NUM>. Furthermore, the differential MEMS-readout circuit <NUM> comprises a differential-readout amplifier section <NUM> comprising a first input INP1 connected to the first contact pin INP of the first input bonding pad <NUM> and a second input INN1 connected to the first contact pin INN of the second bonding pad <NUM>. The differential-readout amplifier section <NUM> comprises a first and a second transistor circuit <NUM>, <NUM>. Each of the first transistor circuit <NUM> and the second transistor circuit <NUM> functions as a source follower. This means the differential-readout amplifier section <NUM> comprises two source followers. The first transistor circuit <NUM> and the second transistor circuit <NUM> are coupled with one another in the differential-readout amplifier section <NUM>. This implementation of the differential MEMS-readout circuit <NUM> is common in all of the differential MEMS-readout circuits <NUM> shown in the <FIG>. <FIG>, for example, show configurations of settings b, b', c or c', that may be used, if the signals <NUM> provided by the MEMS-device <NUM> at the first contact pins INP, INN have a different amplitude <NUM>, <NUM> with respect to its absolute maximal and/or minimal values. The configurations shown in <FIG>, i.e. settings b, b', c or c', may be used for compensating a difference in the amplitudes. This will be explained in more detail with respect to the <FIG>.

As shown in the <FIG> each of the second contact pins C1-<NUM>, C2-<NUM> of the first and second input bonding pads <NUM>, <NUM> is coupled to one of the first and the second transistor circuits <NUM>,<NUM> or is coupled to one of the first and the second transistor circuits <NUM>, <NUM> and/or to ground Gnd.

For example, <FIG> and <FIG>, not according to the invention but useful for understanding the invention, show that each of the second contact pins C1-<NUM>, C2-<NUM> of the first and second input bonding pads <NUM>, <NUM> is coupled to ground gnd. In the configuration as shown in <FIG> and <FIG> the first and second capacitance C1, C2 attenuate the input signal <NUM> (or in short signal(INP) or signal(INN) respectively). The signals at the first outputs is given by <MAT> and <MAT>.

The capacitance Cm is the corresponding capacitance of the used Mems-device <NUM>, in particular defined by the area of the Mems, and a distance of the read out plates.

At the second output (OutN, OutP) the signal is amplified according to an amplification factor A. This means <MAT> and <MAT> is valid. The amplification factor A may be equal to <NUM> or may be greater.

The option that one of the second contact pins C1-<NUM>, C2-<NUM> of the first and second input bonding pads <NUM>, <NUM> is coupled to one of the first and the second transistor circuits <NUM>, <NUM> and/or one of the second contact pins C1-<NUM>, C2-<NUM> is coupled to ground Gnd is not shown in the <FIG>. A basic idea of the concepts described herein is to compensate different amplitudes provided by the MEMS-device <NUM>. The most effective is to attenuate the too large signal <NUM> , in particular, if the signal <NUM> at input INP, for example, gets too large signal, put the first capacitance C1 to one of the outputs outn1/outn, and amplify the too small signals <NUM>, in particular, if the signal <NUM> at input INN, for example, gets too large signal, put the second capacitance C2 to outp1/outp. Such configurations are for example shown in <FIG>. If the amplitude difference is in a smaller range, a connection to gnd could be also sufficient, in particular in the example above the first capacitance C1 may be connected to gnd, while the second capacitance may be connected with the output outN1 or outN (not shown).

As shown in <FIG> each of the first and second transistor circuit <NUM>, <NUM> comprises a first output OutP1, OutN1 and a second output Outp, Outn. The second contact pins C1-<NUM>, C2-<NUM> are impinged with one of the signals from the first output OutP1, OutN1 and/or second output Outp, Outn and/or ground gnd.

The first output OutP1, OutN1 of each of the first and second transistor circuits <NUM>, <NUM> is an intermediate output of the differential-readout amplifier section <NUM>. In particular, the first output OutP1, OutN1 of each of the first and second transistor circuits <NUM>, <NUM> provides a buffered input signal. This means, the input signal <NUM> is stabilized by the source followers. The input signal is buffered, but not amplified. <FIG> and <FIG> show a switching of the differential MEMS-readout circuit <NUM> providing a buffered, but not amplified input signal <NUM> at the first intermediate outputs OutP1, OutN1.

As shown in <FIG>, the second contact pins C1-<NUM> of the first capacitance C1 of the first input bonding pad <NUM> is coupled to the first output OutP1 of the first transistor circuit <NUM>. Further, the second contact pin C2-<NUM> of the second capacitance C2 of the second input bonding pad <NUM> is coupled to the first output OutN1 of the second transistor circuit <NUM>. In the configuration as shown in <FIG>, that corresponds to a setting a, the first and second capacitance C1, C2 act as they were not present, because the first and the second capacitance C1, C2 are switched in phase. The signals at the first inputs InP1, InN1 (in short signal (InP1)/signal (InN1)) correspond to the signals at the first outputs OutP1, OutN1, in particular signal(lnP1)=signal(OutP1) and signal (InN1)=signal (outN1) is valid. At the second output (OutN, OutP) the signal is amplified according to an amplification factor A. This means signal(outn)=A * signal(OutN1) and signal(outp)=A * signal(OutP1) is valid. The amplification factor A may be equal to <NUM> or may be greater.

As shown in <FIG>, the second contact pins C1-<NUM> of the first capacitance C1 of the first input bonding pad <NUM> is coupled to the first output OutN1 of the second transistor circuit <NUM>. Further, the second contact pin C2-<NUM> of the second capacitance C2 of the second input bonding pad <NUM> is coupled to the first output OutP1 of the first transistor circuit <NUM>. In the configuration as shown in <FIG>, that corresponds to a setting d, the first and second capacitance C1, C2 attenuate the input signal <NUM>, because the first and the second capacitance C1, C2 are no longer switched in phase. This behavior is similar to the resulting behavior of the configurations as shown in <FIG> and <FIG>. The signal at the first outputs is given by <MAT> and <MAT>.

The capacitance Cm is the corresponding capacitance of the used MEMS-device <NUM>, in particular defined by the area of the MEMS, and the distance of the read out plates.

At the second output (OutN, OutP) the signal is amplified according to an amplification factor A. This means signal(outn)=A * signal(OutN1) and signal(outp)=A * signal(OutP1) is valid. The amplification factor A may be equal to <NUM> or may be greater.

As shown in the <FIG> the second output OutP, OutN of each of the first and second transistor circuits <NUM>, <NUM> is an external output of the differential-readout amplifier section <NUM>. In particular, the second output OutP, OutN of each of the first and second transistor circuits <NUM>, <NUM> is amplified in an arbitrary ratio of amplification A. The ratio of amplification A or the amplification factor A is setting dependent. For example, the amplification factor A may be dependent on resistances R1, R2, R3, R4. The resistances R1, R2, R3, R4 may be used in the first and second transistor circuits <NUM>, <NUM>.

As shown in <FIG>, the second contact pins C1-<NUM> of the first capacitance C1 of the first input bonding pad <NUM> is coupled to the second output OutP of the first transistor circuit <NUM>. Further the second contact pin C2-<NUM> of the second capacitance C2 of the second input bonding pad <NUM> is coupled to the second output OutN of the second transistor circuit <NUM>. In the configuration as shown in <FIG>, that corresponds to a setting a' (compare with <FIG>), the signal at the second output (OutN, OutP) is amplified according to an amplification factor A. This means signal(outn)=A * signal(OutN1) and signal(outp)=A * signal(OutP1) is valid. The amplification factor A may be equal to <NUM> or may be greater. Again, the first and second capacitance C1, C2 act as they were not present, because the first and the second capacitance C1, C2 are switched in phase. The settings a (<FIG>) and a' (<FIG>) are of similar connection resulting both in an in-phase switching of the capacitances C1, C2. The settings a and a' respectively each provide a positive connection, which may be represented in short by {INP-OutP, and INN-OutN} or by {INP-OutP <NUM>, and INN-OutN1} (see <FIG> and <FIG>).

As shown in <FIG>, the second contact pins C1-<NUM> of the first capacitance C1 of the first input bonding pad <NUM> is coupled to the second output OutN of the second transistor circuit <NUM>. Further the second contact pin C2-<NUM> of the second capacitance C2 of the second input bonding pad <NUM> is coupled to the second output OutP of the first transistor circuit <NUM>. In the configuration as shown in <FIG>, that corresponds to a setting d' (compare with <FIG>), the first and second capacitance C1, C2 attenuate the input signal <NUM>, because the first and the second capacitance C1, C2 are no longer switched in phase. According to the settings d and d', the first and the second capacitance C1, C2 act as twice implemented (<NUM>*A). At the second output (OutN, OutP) the signal is amplified according to an amplification factor A. This means signal(outn)=A * signal(OutN1) and signal(outp)=A * signal(OutP1) is valid. The amplification factor A may be equal to <NUM> or may be greater. The settings d and d' respectively each provide a negative connection, which may be represented in short by {INP-OutN, and INN-OutP} or by {INP-OutN1, and INN-OutP1} (see <FIG> and <FIG>). The negative connection may also be considered a cross connection or an out-of-phase connection. A cross connection because one contact pin of one capacitance is coupled with the first transistor circuit <NUM> and the other contact pin of the same capacitance is coupled with the second transistor circuit <NUM>. Said connection effects that the first and the second capacitance C1, C2 act as twice implemented (<NUM>*A). Therefore, the negative connection may be considered as out-of-phase connection.

The examples according to the setting a, a', d, d' described above are affecting both second outputs OutP, OutN in the same direction. A possible asymmetry of the signals <NUM> arising at the contact pins INP and INN from a connection between the MEMS-device <NUM> and the MEMS-readout circuit <NUM> may be handled as described with respect to the <FIG> next. The handling of an asymmetry of the signals <NUM> describe an important part of the technical concept described herein. The technical concept described herein allows for using the differential MEMS-readout circuit <NUM>, when coupled to a MEMS-device, no matter what kind of signals <NUM> occur at the contact pins INN, INP.

<FIG> and <FIG> are described together, since <FIG> and <FIG> show similar settings b and b'. For example, if the input signal <NUM> at INN is larger compared to the input signal <NUM> at INP (see for example, reference signs <NUM>, <NUM> in <FIG> or <FIG> and <FIG>), than the second contact pin C2-<NUM> of the second capacitance C2 is connected to first output Outp1 or the second output Outp of the first transistor circuit <NUM>. In this case, the second capacitance C2 acts as two times present. The second contact pin C1-<NUM> of the first capacitance C1 is also connected to the first output Outp1 or the second output OutP of the first transistor circuit <NUM>. The first capacitance C1 is acting as it was not present, since the first and second contact pins INP, C1-<NUM> of the first capacitance C1 are impinged with the same in phase signal.

<FIG> and <FIG> are described together, since <FIG> and <FIG> show similar setting c and c'. For example, if the input signal <NUM> at INP is larger compared to the input signal <NUM> at INN (see for example, reference signs <NUM>, <NUM> in <FIG> or <FIG> and <FIG>), than the second contact pin C1-<NUM> of the first capacitance C1 is connected to the first Outn1 or second output Outn of the second transistor circuit <NUM>. In this case, the first capacitance C1 acts as two times present. The second contact pin C2-<NUM> of second capacitance C2 is also connected to the first Outn1 or second output Outn of the second transistor circuit <NUM> (compare with <FIG> and <FIG> showing the settings c and c'). The second capacitance C2 is acting as it was not present, since the first and second contact pins INN, C2-<NUM> of the second capacitance C2 are impinged with the same in-phase signal.

For the configuration as shown in the <FIG> and <FIG>, not according to the invention but useful for understanding the invention, a connection of the first inputs INP1, INN1, the first outputs OutP1, OutN1 and/or the second outputs OutP, OutN with the corresponding bonding pad <NUM>, <NUM> is made by metal wires <NUM>. If only metal wires <NUM> are used, the configuration of the MEMS-readout circuit <NUM> is fixed and will be set during manufacture of the MEMS-readout circuit <NUM>.

The solution is to have switches <NUM> as shown in the <FIG>, wherein the switches <NUM> provide a connection from the first input INP of the first capacitance <NUM>, C1 to the first or second output outN1, outN of the second transistor circuit <NUM> (see <FIG> and <FIG>, i.e. settings c and c'), or from the first input INP of the first capacitance <NUM>, C1 to the first or second output outP1, outP of the first transistor circuit <NUM> (see <FIG> and <FIG>, i.e. settings d and d'), from the first input INN of the second capacitance C2, <NUM> to the first or second output outN1outN of the second transistor circuit <NUM> (see <FIG> and <FIG>, i.e. settings a and a'), or from the first input INN of the second capacitance C2, <NUM> to the first or second output outP1, outP of the first transistor circuit <NUM> (see <FIG> and <FIG>, i.e. settings b and b'). During calibration of a system <NUM> comprising the differential readout circuit <NUM> and the MEMS-device <NUM> the switches <NUM> will be set accordingly, so that the a possible amplidute difference from the input signals <NUM> of the MEMS-device <NUM> will be compensated. Thus a connection of the first outputs OutP1, OutN1 and/or the second outputs OutP, OutN with the corresponding bonding pad <NUM>, <NUM> is made by switches <NUM>. Depending on the preferred setting , the switches <NUM> are opened or closed so that the connections between the second contact pins C1-<NUM>, C2-<NUM> to one of the first and/or second outputs OutP1, OutN1, OutP, OutN is obtained as just described with respect to the <FIG>. Using switches <NUM> instead of metal wires <NUM> allows for amending in situ the setting of the configuration, i.e. during a calibration of a system <NUM>.

Therefore, according to an example, each input bonding pad <NUM>, <NUM> is connected to two switches <NUM> for switching between the first outputs OutP1, OutN1, in particular of the first and second transistor circuits <NUM>, <NUM>, and/or the second outputs OutP, OutN, in particular of the first and second transistor circuits <NUM>, <NUM>, and the corresponding bonding pad <NUM>, <NUM>. In each switching state one of the two switches <NUM> associated with one of the input bonding pads <NUM>, <NUM> is opened while the other is closed (compare the switching states as shown in <FIG>).

The MEMS-readout circuit <NUM> as shown in the <FIG>, <FIG> is configured to provide a signal transfer function to reduce a difference between the signals <NUM> at the second output OutP and at the second output OutN of the readout amplifier section <NUM>. For, example, the second output OutP, in particular of the first transistor circuit <NUM>, is configured to be a positive output, while the second output OutN, in particular of the second transistor circuit <NUM>, is configured to be a negative output. According to an example, one of the second inputs is a positive input INP while the other of the second inputs is a negative input INN or wherein one of the second input is a negative input INN while the other of the second input is a positive input INP. As shown in the <FIG>, <FIG> the second input INN1 of the second transistor circuit <NUM> is a negative input INN1 while the second input INP1 of the first transistor circuit <NUM> is a positive input INP1. However, both options as just disclosed are possible.

With regard to the <FIG> and/or <FIG>, for example, the MEMS-readout circuit <NUM> behave as follows: If the first contact pin INP, which in particular can be a positive contact pin, of the first capacitance C1 is connected to the positive output of the MEMS-device and the second contact pin C1-<NUM> of the first capacitance C1 is connected to the first output OutP1, which in particular can be a positive first output, or to the second output OutP of the first transistor circuit <NUM> of the readout amplifier section <NUM>, the first capacitance C1 acts as it is not present, because the first and second contact pins INP, C1-<NUM> of the first capacitance C1 are switched in phase. Accordingly, the same situation can be applied to the second capacitance C2. Then, if the first contact pin INN, which in particular can be a negative contact pin, of the second capacitance C2 is connected to the negative output of the MEMS-device and the second contact pin C2-<NUM> of the second capacitance C2 is connected to the first output OutN1, which in particular can be a negative first output, or to the second output OutN of the second transistor circuit <NUM> of the readout amplifier section <NUM>, the second capacitance C2 acts as it is not present, because the first and second contact pins INN, C2-<NUM> of the second capacitance C2 are switched in phase.

With regard to the <FIG> and/or <FIG>, for example, the MEMS-readout circuit <NUM> behave as follows: If the first, in particular positive, contact pin INP of the first capacitance C1 is connected to the positive output of the MEMS-device and the second contact pin C1-<NUM> of the first capacitance C1 is connected to the first, in particular negative, output OutN1 or to the second output OutN of the second transistor circuit <NUM> of the readout amplifier section <NUM> or to ground gnd, the first capacitance C1 acts as attenuator of the input signal. Accordingly, the same situation can be applied to the second capacitance C2. Then, if the first contact pin INN of the second capacitance C2 is connected to the negative output of the MEMS-device and the second contact pin C2-<NUM> of the second capacitance C2 is connected to the first, in particular positive, output OutP1 or to the second output OutP of the first transistor circuit <NUM> of the readout amplifier section <NUM> or to ground gnd, the second capacitance C2 acts as attenuator of the input signal.

With regard to the <FIG> and/or <FIG> for example, the MEMS-readout circuit is configured as follows. If the first contact pin INP of the first capacitance C1 is connected to the positive output of a MEMS-device <NUM> and the second contact pin C1-<NUM> of the first capacitance C1 is connected to the first output OutP1 or to the second output OutP of the first transistor circuit <NUM> of the readout amplifier section <NUM>, and if first contact pin INN of the second capacitance C2 is connected to the negative output of the MEMS-device <NUM> and the second contact pin C2-<NUM> of the second capacitance C2 is connected to the first output OutP1 or to the second output OutP of the first transistor circuit <NUM> of the readout amplifier section <NUM>, a difference between amplitudes of signals at the first contact pins INP, INN of the first and second capacitances C1, C2 is compensated. These configurations describe the settings b and b', as illustrated in <FIG> and <FIG>.

With regard to the <FIG> and/or <FIG> for example, the MEMS-readout circuit is configured as follows. If the first contact pin INP of the first capacitance C1 is connected to the positive output of a MEMS-device <NUM> and the second contact pin C1-<NUM> of the first capacitance C1 is connected to the first output OutN1 or to the second output OutN of the second transistor circuit <NUM> of the readout amplifier section <NUM>, and if first contact pin INN of the second capacitance C2 is connected to the negative output of a MEMS-device <NUM> and the second contact pin C2-<NUM> of the second capacitance C2 is connected to the first output OutN1 or to the second output OutN of the second transistor circuit <NUM> of the readout amplifier section <NUM>, a difference between amplitudes of signals at the first contact pins INP, INN of the first and second capacitances C1, C2 is compensated. These configurations describe the settings c and c', as illustrated in <FIG> and <FIG>.

The settings b, b', c, and c' are preferably used, if the signals <NUM> at the inputs INN and INP have a differing amplitude.

With this method i.e. way of coupling the output of the capacitance C1, C2 with the output of the transistor circuits <NUM>, <NUM>, it is possible to achieve a gain symmetry of up to <NUM> to 2dB depending on the capacitance value, resulting in a better THD of high sound pressure signals also of plus <NUM> to 2dB. Thus according to an example, a gain symmetry of up to <NUM> to 2dB is achieved, depending on the capacitance of the input bonding pad <NUM>, <NUM>.

According to an example, the differential-readout amplifier section <NUM> comprises a number of resistors R1, R2, R3, R4 that are configured to be switched depending on the received differential signals so that a difference between the received differential signals is reduced, in particular at the second outputs OutP, OutN of the differential-readout amplifier section <NUM>. A reduction of the difference between the received differential signals is indicated in the <FIG> for example by the output signals <NUM> compared to the input signal <NUM>. Such a reduction of the difference between the received differential signals is technically achieved by the differential-readout amplifier section <NUM>, in particular by the settings b, b', c and c' (<FIG>), which a preferably chosen if asymmetric input signals <NUM> occur. The differential-readout amplifier section <NUM> comprises the number of resistors R1, R2, R3, R4 that are switched accordingly for obtaining output signals <NUM> that do have a comparable, in particular similar, scale. The resistors R1, R2, R3, R4 are used for applying gain to the output signals <NUM> at the second outputs Outp and Outn according to: <MAT> <MAT>.

The input signals <NUM>, i.e. signal (inp) and signal (inn), may be buffered signals. For R1=R2 and R3=R4 the same gains output for the output signals <NUM> at the second output OutN, OutP is obtained. For R1≠R2 and/or R3≠R4 the gain obtained for the output signals <NUM> at the second output OutN, OutP can be controlled separately. The gains control may e achieved by choosing the resistance values of the resistors R1, R2, R3, R4 accordingly and/or by coupling the resistors R1, R2, R3, R4 with the output outN1, outP1 as already described accordingly.

The <FIG> show simulated values obtained for a coupling of the first and second capacitances C1, C2 coupled to one of the first and/or second outputs OutN1, OutN, OutP1, OutP according to the settings a, a', b, b', c, c', d and d'. The simulated values are shown as x and are connected with one another.

It is noted that the results of setting a are similar to the results of setting a', The results of setting b are similar to the results of setting b'. The results of setting c are similar to the results of setting c'. The results of setting d are similar to the results of setting d'. That is why the settings a and a' etc. are shown in the <FIG> at the same point.

In synopsis of the <FIG>, a simulation of the signal transfer function for four settings {a=<NUM>:<NUM>; b=<NUM>:<NUM>; c=<NUM>:<NUM>; d=<NUM>:<NUM>} or {a'=<NUM>:<NUM>; b'=<NUM>:<NUM>; c'=<NUM>:<NUM>; d'=<NUM>:<NUM>} respectively of the switches <NUM> for the first and second outputs OutN1, OutN, OutP1, OutP is presented. The signal transfer function comprises signals at the outputs (OutN1, OutN, OutP1, OutP , and a differential output signal. The settings {<NUM>:<NUM>; <NUM>:<NUM>; <NUM>:<NUM>; <NUM>:<NUM>} are associated with switching states of the switches <NUM>, in particular, the settings a, a', b, B', c, c', d, d' that are illustrated in <FIG>. The settings as shown in the <FIG> are summarized in table <NUM>.

<FIG> shows a simulation of the output signal <NUM> (signal (OutN) at the second output OutN of the signal transfer function. <FIG> shows a simulation of the output signal <NUM> (signal (OutP) at second output OutP of the signal transfer function. The difference between both signals as shown in <FIG> and <FIG> is shown in <FIG>. Thus, <FIG> shows a simulation of the differential output (~ OutP-OutN) derived from the output signals <NUM> (signal (OutP), signal (OutN)) at second outputs OutP, OutN. The settings as summarized in table <NUM> are used for resulting the simulations results shown in <FIG>.

The settings a, a' and d, d' do not change the amplitudes from the MEMS-device <NUM>. With respect to setting a, a', the differential output signal determined according to <NUM>*log10 (outP-outN) and shown in <FIG> has a value deviating from the differential output signals of settings c, c', b, b'. Also, with respect to setting d, d', the differential output signal determined according to <NUM>*log10 (outP-outN) and shown in <FIG> has a value deviating from the differential output signals of settings c, c', b, b'. Compared to the differential output signals for settings b, b', c, c' (--<NUM>,<NUM>) the differential output signal for setting d, d' (~-<NUM>) is smaller with respect to absolute values than for settings b, b', c, c' (~-<NUM>,<NUM>) (both shown in <FIG>), while the differential output signal for setting a, a' (--<NUM>,<NUM>) is larger with respect to absolute values than for settings b, b', c, c' (--<NUM>,<NUM>) (both shown in <FIG>). With setting a, a' the bond pad capacitances C1, C2 act as they were not present. That is why the differential output signal is absolutely larger than for setting d, d'. With setting d, d' the bond pad capacitances C1, C2 act as twice implemented.

When considering the values shown in <FIG> and <FIG>, the output signals at outN and outP for settings a, a' both are proportional to ~-<NUM>,<NUM>, while output signals at outN and outP for settings d, d' both are proportional to ~-<NUM>. This is different for the settings b, b', c, c'. The output signal at OutN (<FIG>) for setting b, b' is proportional to ~-<NUM>,<NUM>, while the output signal at OutP (<FIG>) for setting b, b' is proportional to ~-<NUM>,<NUM>. With respect to the setting c, c' this behavior is vice versa, i.e. the output signal at OutN (<FIG>) for setting c, c' is proportional to --<NUM>,<NUM>, while the output signal at OutP (<FIG>) for setting c, c' is proportional to ~-<NUM>,<NUM>. That is why the resulting differential output signal shown in <FIG> is essentially constant for setting b, b', c, c'. This mean, with settings b, b', c, c' an asymmetric MEMS-signal from the MEMS-device <NUM> can be compensated.

<FIG> a and <FIG> show each the ratio of the signals at output OutP and at output OutP according to the function <NUM>*log10 (outP/outN) for a bond capacitance of C=C2=500fF (<FIG>) and a bond capacitance of C1=C2=<NUM> fF (<FIG>). <FIG> a and <FIG> show each the differential output signals of the signals at output OutP and at output OutP according to the function <NUM>*log10 (outP-outN) for a bond capacitance of 500fF (<FIG>) and a bond capacitance of <NUM> fF (<FIG>). For asymmetric signals and settings b, b', c, c' the ration may be up to 3dB for a bond pad capacitance of 500fF (see <FIG>). With the settings d, d', there is less signal at both outputs OutN, OutP of the ASOC, because the corresponding bond pad capacitance acts two times implemented (<FIG>). For setting a, a', a larger differential output signal is available at the output OutN, OutP (see <FIG>), The settings a, a', d, d' may be chosen, if the MEMS-device <NUM> delivers signals <NUM> of the same amplitude. If there is a difference of the signal amplitudes of the MEMS-device <NUM> at INP, INN, settings b, b' or c, c' may be used to get zero difference at the outputs outP, outN (see the constant differential output signal in <FIG> for settings b, b', c, c').

As already mentioned, <FIG> shows the ratio of the signal at outp to the signal at outn according to <NUM>*log10(outp/outn). Therewith, <FIG>, each shows the possible range for changing the amplitude of a signal <NUM>. <FIG> shows the single ended output at OutN or at outP in <FIG>. The difference is zero for the settings a, a', d, d' (as described above). With settings b, b', c, c' either too large signals on input INP could be increased or decreased, same holds for input INN.

<FIG> and <FIG> show the difference at outp and outn in dB, i.e. the differential output signal determined according to <NUM>*log10(outp-outn). Here it is only shown that more signal is achieved for setting a, a' than for setting d, d'. The <FIG> and <FIG> give no information, if the signals at output outp, outn are equal or if they differ.

For example, for the setting at <NUM>:<NUM>, i.e. setting b, b', of <FIG> the second capacitance C2 was coupled to the first output outP1 and the first capacitance C1 was coupled to the first output outP1 (see also table <NUM>). This means both capacitances C1 and C2 were coupled to the first output outP1. Also for the setting at <NUM>:<NUM>, i.e. setting c, c', of <FIG> both capacitances C1 and C2 were coupled to the first output outN1. For the setting <NUM>:<NUM>, i.e. setting b, b' the signal at output OutP1 is obtained by: <MAT>.

C1 is the first capacitance that is in phase with the input signal.

The signal at output OutN <NUM> is obtained by: <MAT>.

C2 is the second capacitance that is connected to the <NUM>° shifted output outP1/outP and therefor doubles.

For the setting <NUM>:<NUM>, i.e. the setting c, c', the signal at output OutP1 is obtained by: <MAT>.

The signal at output OutN was attenuated for setting <NUM>:<NUM> (b, b'), and the signal at output OutP was attenuated for setting <NUM>:<NUM> (c, c'). For the settings at <NUM>:<NUM> (a, a') and <NUM>:<NUM> (d, d') the first capacitance C1 was either coupled to OutN1 or to OutP1, while the second capacitance C2 was either coupled to OutP1 or to OutN1 (see <FIG> and table <NUM>). The differential output signal (<NUM>*log10 signal(OutP-OutN)) was derived from the second outputs OutP, OutN.

For the settings <NUM>:<NUM> and <NUM>:<NUM> the logarithmic ratio of signals <NUM> at the outputs outp, outn is close to zero( <FIG>), while for the settings <NUM>:<NUM> and <NUM>:<NUM> the difference of the differential output signals <NUM> is unequal to zero. The greater the capacitance C1 and C2 are chosen, the more the differential output signals <NUM> is away from zero (compare the setting at <NUM>:<NUM> and <NUM>:<NUM> in <FIG>).

The simulation results shown in <FIG> were obtained by coupling the first and the second capacitances C1, C2 with the first outputs OutP1, OutN1. A coupling of the capacitances C1, C2 with the first outputs OutP, OutN is also possible. That is why the setting a', b', c', and d' are also indicated in <FIG>. The results obtained for the settings a', b', c', and d' are similar, i.e. essentially equal, than that obtained for the settings, a, b, c, and d.

With the settings b, b', c and c as described with respect to the Fig- <NUM> to <NUM> asymmetric input signals <NUM> at INN and INP may be compensated, In order to have the flexibility to choose one of the settings, a, a', b, b', c, c', d or d' (according to one of the <FIG>) the switches <NUM> are accordingly switched.

Alternatively, in order to compensate asymmetric input signals <NUM> the first outputs outP1, outN1 or the second outputs outP, outN of the first and second transistor circuits <NUM>, <NUM> are connected to a voltage node Vmic of the MEMS-device <NUM> for generating a feedback lined back to the MEMS-readout circuit <NUM>. In <FIG> and <FIG> the connection to the second outputs outP, outN is shown, while the connection to the first outputs outP1, outN1 is not shown in <FIG> and <FIG>. Then a difference between amplitudes of signals <NUM> at the first contact pins INP, INN of the first and second capacitances (C1, C2) is compensated by switching capacitances Coutn, Coutp. The capacitances Coutn, Coutp are provided in a path between the second outputs outP, outN and the voltage node Vmic. The capacitances Coutn, Coutp are switched accordingly, in particular by switching switches <NUM> that are associated with the capacitances Coutn, Coutp and that are associated with ground gnd. This is, for example, shown in <FIG>.

Both outputs, the first outputs outP1, outN1 or the second outputs outP, outN, are needed for compensation (as shown in <FIG> and <FIG>), since a capacitance from outp/outp1 to vmic can only increase the ratio outP/outN or outp1/outn1 respectively, whereas a capacitance from outn or outn1 to vmic decreases the ratio outp/outn or outp1/outn1 respectively. If the MEMS delivers a symmetric signal no or the same capacitances between vmic and outp1/outp, vmic and outn1/outn will be connected. There would be one case that only one capaitcanes needs to be changed. If a fixed capacitance Cfix is connected between vmic and e.g. outn/outn1, then only the capacitance at vmic and outp/outn1 needs to be changed. The switchable capacitance needs to be either larger than Cfix or lower than Cfix to change the ratio of the MEMS input amplitudes to values larger and smaller than <NUM>.

The voltage at the voltage node Vmic is a bias voltage for the MEMS-device <NUM>, in particular the bias voltage is generated with a charge pump <NUM>. For example, the bias voltage is generated at an ASIC. For example, the bias voltage is generated with an charge pump <NUM>. This is, for example, shown in <FIG>.

A further aspect of the present technical teaching is to provide a MEMS-device <NUM> for connection to a MEMS-readout circuit <NUM> as described herein. The MEMS device <NUM> is configured to act as a differential type sensor to provide at the first contact pins (INP, INN) differential signals, wherein the MEMS-readout circuit <NUM> is configured to receive the differential signals and to reduce a difference in the amplitude of the differential signals at the second outputs OutP, OutN as already described herein.

The proposed MEMS-device <NUM> together with the proposed MEMS-readout circuit <NUM> build a proposed system <NUM> according to a further aspect of the present technical teaching. <FIG> shows a system <NUM>. For example, the MEMS-readout circuit <NUM> is implemented on an ASIC <NUM>.

A further aspect of the present technical teaching is to provide a method of using a MEMS-readout circuit <NUM> as described herein. <FIG> presents a block diagram of a method of using a MEMS-readout circuit <NUM>. The method comprises a step190 of providing a MEMS-device <NUM>. In step <NUM> a connecting of the MEMS-device <NUM> with the a MEMS-readout circuit <NUM> is performed. The step of connecting <NUM> is performed by step <NUM> and <NUM>.

The step <NUM> comprises connecting a first input INP1 to the first contact pin INP of the first input bonding pad <NUM> and connecting a second input INN1 to the first contact pin INN of the second bonding pad <NUM>. The step <NUM> comprises coupling each of the second contact pins C1-<NUM>, C2-<NUM> of the first and second input bonding pads <NUM>, <NUM> to one of the first and the second transistor circuits <NUM>, <NUM>. Alternatively, the step <NUM> comprises coupling each of the second contact pins C1-<NUM>, C2-<NUM> of the first and second input bonding pads <NUM>, <NUM> to one of the first and the second transistor circuits (<NUM>, <NUM>) and/or to ground (Gnd).

For example, the step <NUM> may comprise: connecting the first contact pin INP of the first capacitance C1 to the positive output of the MEMS-device <NUM> and the second contact pin C1-<NUM> of the first capacitance C1 to the first output OutP1 of the first transistor circuit <NUM> of the readout amplifier section <NUM>, so that the first capacitance C1 acts as it is not present, because the first and second contact pins INP, C1-<NUM> of the first capacitance C1 are switched in phase. Step <NUM> may comprise connecting the first contact pin INN of the second capacitance C2 to the negative output of the MEMS-device <NUM> and the second contact pin C1-<NUM> of the second capacitance C2 to the first output OutN1 of the second transistor circuit <NUM> of the readout amplifier section <NUM>, so that the second capacitance C2 acts as it is not present, because the first and second contact pins INN, C2-<NUM> of the second capacitance C2 are switched in phase. In this way setting a or a' is obtained.

Alternatively, for example, the step <NUM> may comprise: connecting the first contact pin INP of the first capacitance C1 to the positive output of the MEMS-device <NUM> and the second contact pin C1-<NUM> of the first capacitance C1 to the first output OutN1 of the second transistor circuit <NUM> of the readout amplifier section <NUM> or to ground gnd, so that the first capacitance C1 acts as attenuator of the input signal in order to reduce a difference between the differential output signals. The step <NUM> may then comprise connecting the first contact pin INN of the second capacitance C2 to the negative output of the MEMS-device <NUM> and the second contact pin C2-<NUM> of the second capacitance C2 to the first output OutP1 of the first transistor circuit <NUM> of the readout amplifier section <NUM> or to ground gnd, so that the second capacitance C2 acts as attenuator of the input signal in order to reduce a difference between the differential output signals. In this way setting d or d' is obtained.

Alternatively, for example, the step <NUM> may comprise: connecting the first contact pin INP of the first capacitance C1 to the positive output of a MEMS-device <NUM> and connecting the second contact pin C1-<NUM> of the first capacitance C1 to the first output OutP1 or to the second output OutP of the first transistor circuit <NUM> of the readout amplifier section <NUM>. Then, step <NUM> may comprise connecting the first contact pin INN of the second capacitance C2 to the negative output of a MEMS-device <NUM> and connecting the second contact pin C2-<NUM> of the second capacitance C2 to the first output OutP1 or to the second output OutP of the first transistor circuit <NUM> of the readout amplifier section <NUM>. In doing so, a difference between amplitudes of signals at the first contact pins INP, INN of the first and second capacitances C1, C2 is compensated. In this way setting b or b' is obtained.

Alternatively, for example, the step <NUM> may comprise: connecting the first contact pin INP of the first capacitance C1 to the positive output of a MEMS-device <NUM> and connecting the second contact pin C1-<NUM> of the first capacitance C1 to the first output OutN1 or to the second output OutN of the second transistor circuit <NUM> of the readout amplifier section <NUM>. Then, step <NUM> may comprise connecting the first contact pin INN of the second capacitance C2 to the negative output of a MEMS-device <NUM> and connecting the second contact pin C2-<NUM> of the second capacitance C2 to the first output OutN1 or to the second output OutN of the second transistor circuit <NUM> of the readout amplifier section <NUM>. In doing so, a difference between amplitudes of signals at the first contact pins INP, INN of the first and second capacitances C1, C2 is compensated. In this way setting c or c' is obtained.

In particular the alternatives of steps <NUM> and <NUM> may be combined in any combination, so that the output of the capacitance C1, C2 are connected to any output of the MEMS-device and to any output of the differential-readout amplifier section <NUM>, i.e. outN1, outN, outP1 and/or outN1.

According to an example, the method <NUM> may comprise a step of applying gain to the second output OutP of the first transistor circuit <NUM> by using a number of resistors R1, R2, R3, R4 , wherein an input signal <NUM> is converted at the second output (outP) into an output signal according to signal (Outp) = signal(inP) * (R1 + R3)/R1; wherein the input signal is a combined signal from the first output (outP1) and the first input (INP1) of the of the first transistor circuit <NUM>.

The method <NUM> may alternatively or additionally comprise a step of applying gain to the second output (OutN) ) of the second transistor circuit <NUM> by using the number of resistors R1, R2, R3, R4, wherein an input signal is converted at the second output (outN) into an output signal according to signal(OutN) = signal (InN) * (R<NUM> + R<NUM>)/R<NUM>, wherein the input signal (InN) is a combined signal from the first output (outN1) and the first input (INN1) of the of the second transistor circuit <NUM>.

The signals at the first output (outN1) and the first input (INN1) and/or the signals at the first output (outP1) and the first input (INP1) each depends on the input signal <NUM> provided by the MEMS-device <NUM>.

According to an example, the method <NUM> may comprise a step of obtaining the same amount of gain at the second output (outP) of the first transistor circuit <NUM> and at the second output (outN) of the second transistor circuit <NUM>, if R1=R2 and R3 =R4 is valid. The resistance value of R1, R2, R3, R4 may be chosen at the manufacture process of the readout amplifier section <NUM>. It is also possible to provide the readout amplifier section <NUM> with more resistors than with resistors R1, R2, R3, R4, for example additional resistors R5, R6, R7, R8 etc. For example, the additional resistors may be switched in or off in the readout amplifier section <NUM> by switches and depending on the needed resulting resistance of the resistors R1, R2, R3, R4. This allows to provide more flexibility. The more resistors are implemented, the more various gains settings may be chosen. In doing so or by just providing fixed resistor values that are unequal to one another, in particular if R1 is unequal to R2 and/or R3 is unequal to R4, the method <NUM> may comprise controlling the gain at the second output (outP) of the first transistor circuit <NUM> and at the second output (outN) of the second transistor circuit <NUM> separately.

According to an example, the method comprises connecting the second outputs outP, outN of the first and second transistor circuits <NUM>, <NUM> to a voltage node Vmic of the MEMS-device <NUM> for generating a feedback lined back to the MEMS-readout circuit <NUM>. A connection of the second outputs outP, outN of the first and second transistor circuits <NUM>, <NUM> to an voltage node Vmic of the MEMS-device <NUM> is shown in <FIG> and <FIG>. <FIG> shows a schematic of a differential MEMS-readout circuit <NUM>, wherein the differential MEMS-readout circuit <NUM> is coupled to a MEMS-device <NUM> and is coupled to an voltage node Vmic of the MEMS-device <NUM> for generating feedback to the voltage node. In <FIG> and <FIG> the circuit between the MEMS-device <NUM>, the capacitances C1, C2 and the readout amplifier section <NUM> is shown only generally, wherein the connection between the capacitances C1, C2 and the readout amplifier section <NUM> is indicated only by switches <NUM>. For a person skilled in the art, it is clear that any circuit between the capacitances C1, C2 and the readout amplifier section <NUM> described herein, preferably the ones shown in <FIG>, may be used for the configuration as shown in <FIG> and <FIG> in order to generate a feedback to the voltage node Vmic of the MEMS-device <NUM>.

<FIG> shows schematically a circuit with which an asymmetric gain with use of feedback of an output to Vmic may be generated. To generate said feedback the second output Outp is coupled with a capacitance Coutp, and the second output OutN is couples with a capacitance Coutn, which in turn is coupled to the voltage node Vmic of the MEMS-device <NUM>. In doing so, an output signal at the second outputs OutP, OutN may be feed back to the MEMS-device <NUM>. The feedback then obtained at the seconds outputs OutN, OutP due to Coutn and Coutp is given by: <MAT> and <MAT>.

Cvmic is the capacitance of a bonding pad of the voltage node Vmic, The Vmic bonding pad shown in <FIG> and <FIG> may use a filter capacitance, for example of <NUM> pF. Because a desired filter capacitance of for example 8pF to <NUM> pF is not reached by the bonding pad Vmic alone, an additional capacitance may be added, in order to achieve the desired filter capacitance, for example of 8pF to <NUM> pF. An additional capacitance is not shown in <FIG> and <FIG>. The capacitance together with a high ohmic output resistance, for example of <NUM>-100GΩ, creates a lowpass to cut off significantly below the audioband of <NUM>. A value of 8pF to <NUM> pF corresponds to a lower end value of the filter capacitance. A greater value of the capacitance of for example <NUM> pF may be beneficial. A greater value of the capacitance may be related to a greater area of the capacitance. The factor A is the amplification factor described herein. If Coutp=Coutn is valid, no gain for the signal at outp and attenuation for the signal at outn or vice versa is possible.

As shown in <FIG>, the capacitance Coutp is connected to the second output Outp, if the switch <NUM> between CoutP and Outp is closed. In this case the capacitance Coutn is connected to ground gnd. The capacitance Coutn may act as additional capacitance on the Vmic node. The ASIC <NUM> may a have a charge pump <NUM>. The charge pump <NUM> provides a voltage Vmic for the MEMS-device <NUM>. With the voltage Vmic the MEMS-devices' <NUM> sensitivity may be set. A higher voltage Vmic results in larger signals <NUM> at the MEMS-device outputs Vtop and Vbot. As shown in <FIG>, the MEMS-device output Vtop is connected to the first contact pin INP of the first capacitance C1, and the output Vbot is connected to the first contact pin INN of the second capacitance C2. The feedback then obtained at the seconds outputs OutN, OutP due to Coutn and Coutp is given by: <MAT> and <MAT>.

Since signal(INP)=Ue and signal (INN)=-Ue is valid, wherein lie is the voltage provided by the voltage source of the MEMS-device <NUM>. Thus, the differential output signal is given by: <MAT> wherein A is again the amplification factor described herein. For the method of using a MEMS-readout circuit <NUM> described above the Vmic pad is not used to change a possible asymmetry of the signals at INP, INN.

However, the Vmic pad may also be used to set the signals at INP and INN to the same amplitude levels. If a capacitance Coutp is connected to a capacitance Cvmic of the Vmic pad then the capacitance Coutp creates a positive feedback from the output Outp to first contact pin INP of the first capacitance C1 (see <FIG> or <FIG>). The signal at Outp is amplified. It is important that the capacitance Coutn is connected between Cvmic to ground,i.e. switch <NUM> to Outn should be opened, while the switch <NUM> to ground should be closed (in <FIG> switches <NUM> are all shown opened in order to indicate the switch as such). Otherwise the feedback of Coutn to Outn has the same effect as Coutp and no adjustment of asymmetric MEMS amplitudes would be possible (see also <FIG>).

If Coutn=Coutp is valid, the ratio of the signals at Outp andOutn determined according to <NUM>'log10(outP/outN) is fixed and keeps as it is. The amplification A at Outp is A=(<NUM>+Coutp/(Cvmic+Coutn)). Since the output Vbot (connected to INN) of the MEMS-deive <NUM> is connected to Vmic a negative feedback to Outn is generated. The signal Outn is attenuated with the same order as the signal Outp was amplified. The voltage attenuation at Outn is A=((<NUM>-Coutp/(Cvmic+Coutn)). Several Capacitors (not shown in <FIG>) with switches <NUM> between Vmic and/or Outp/gnd can be implemented to have a wider tuning range. If a signal attenuation at Outp is needed, than the capacitor Coutp is switched to ground and at the same time Coutn is connected with a switch to Outn. The amplification A for signal Outn is A=(<NUM>+Coutn/(Cvmic+Coutp)). The attenuation of the signal Voutp is A=(<NUM>-Coutn/(Cvmic +Coutp)). The attenuation is an amplification smaller than <NUM>, therefore amplification and attenuation are abbreviated by letter A.

<FIG> shows the ratio of the output signals <NUM> determined according to <NUM>*log10(OutP/OutN) for capacitances Coutp, Coutn having different values and Cvmi=10pF (compare with <FIG>). <FIG> shows a principle circuit, if capacitors Coutp, Coutn are connected between outn to Vmic and outp to Vmic. If the same values for Coutp and Coutn are chosen, i.e. for example Coutn=Coutp=1fF, the ratio of outp/outn is <NUM> or <NUM> as shown in <FIG>, if the functions is logarithmic according to <NUM>*log10(outp/outn). The first and the <NUM>nd values at 1f: 1f (= Coutn=Coutp=1fF) and 1p:1p (=Coutn=Coutp=1pF) of <FIG> show this. However, because the ratio of OutP7OutN is shown in a logarithmic scale, the first and the <NUM>nd values at 1f:1f and 1p:1p are at zero, according to log10(<NUM>)=<NUM>,.

If Coutp is bigger than Coutn than the signal at outp gets larger whereas the signal at outn gets smaller. This is shown for settings at 1p:1f (for CoutP=1pF and Coutn=1fF) and 2p:1f (for CoutP=2pF and Coutn=1fF). The settings at 1f:1p (for CoutP=1fF and Coutn=1pF) and 1f:2p (for CoutP=1fF and Coutn=2pF) show the effect when Coutn is larger than Coutp. In particular, the capacitances Coutp and Coutn are different, preferably one is Zero whereas the other value is chosen to get the preferred amplitude change. For the final version of the asic the capacitors are connected with switches <NUM> either to gnd or to outn (Coutn) or outp (Coutp)(see <FIG>).

In order to compensate a difference in the amplitudes of the signals <NUM> a voltage node (Vmic) and capacitances Coutn, Coutp, that may be connected to the second outputs OutN, OutP may be providet, in particular the voltage node (Vmic) and the capacitances Coutn, Coutp are provided at an ASIC. By connecting the first outputs outP1, outN1 or the second outputs outP, outN of the first and second transistor circuits <NUM>, <NUM> to the voltage node Vmic of the MEMS-device <NUM> for generating a feedback lined back to the MEMS-readout circuit <NUM>, a difference between amplitudes of signals at the first contact pins INP, INN of the first and second capacitances C1, C2 is compensated by switching capacitances Coutn, Coutp accordingly. The Coutn, Coutp are provided in a path between the second outputs outP, outN and the voltage node Vmic.

With respect to <FIG>, the following settings Aa and Bb are preferred. According to a setting Aa, the capacitance Coutp is connected to the second output OutP, and the capacitance Coutn is connected to ground gnd, Thus, the corresponding switches <NUM> in <FIG> should be closed. However, in <FIG> all switches <NUM> are shown on an opened state for simplicity. With setting Aa the signal at Outp is amplified with the factor <MAT> whereas the signal at outn is attenuated according to <MAT>.

The setting Aa may be used, if the MEMS-device <NUM> deliver lager signals at Inn compared to the signal at Inp.

According to a setting Bb, the capacitance Coutn is connected to the second output Outn, and the capacitance Coutp is connected to ground gnd. Thus, the corresponding switches <NUM> in <FIG> should be closed. With setting Bb the signal at Outn is amplified with the factor <MAT> whereas the signal at outn is attenuated according to <MAT>.

The setting Bb may be used, if the MEMS-device <NUM> deliver lager signals at Inp compared to the signal at Inn. To have a higher trimming range several capacitances (not shown in <FIG>) may be implemented, such that the capacitances may be connectable via switches <NUM> to the second output Outp and the Vmic-node and/or may be connectable via switches <NUM> to the second output Outn and the Vmic-node and/or to ground gnd.

With the MEMS-readout circuit <NUM> described herein it is favorable that the differential output (OutP-OutN) is close to zero. When choosing the capacitance CoutN and CoutP such that the one is twice as great as the other, the simulation results show a maximum, for example at 2p:1f, and a minimum, for example at 1f:2p, of the output signal (outP-OutN). For the implementation of the readout circuit <NUM> and the circuits of <FIG> and <FIG> switches <NUM> are used. If more signal at outp is needed, than a feedback capacitance is activated with a switch <NUM>. If less signal is needed at outp the corresponding capacitance is connected to gnd with an additional switch <NUM>. For more signal at outn the same procedure is done.

If the capacitance Coutp is connected to the Vmic node, than the feedback results in an increased signal at the second output outp, in particular according to: <MAT>.

The Vmic node keeps its voltage that is why the second output outnhas to attenuate the signal, in particular according to: <MAT>.

As shown in the <FIG>the differential output stays constant, whereas at the same time the signal at output outp is amplified and the signal at the output outn is attenuated. This can be applied also the other way round (see <FIG>). If the capacitance Coutn is connected to Vmic, then the voltage at the second output outn is amplified, in particular according to: <MAT> and the voltage at the second output outp is attenuated at the same time, in particular according to: <MAT>.

The <FIG> and <FIG> show the compensation range. If the Mems-device <NUM> provides an amplitude mismatch inp/inn of ~+<NUM>. 8dB, then setting <NUM> (see table <NUM> and <FIG>) will be used. The setting <NUM> gives -<NUM>. 7dB, together with the -<NUM>. 8dB of the MEMS-device <NUM> the difference will be 0dB. With this setting the best THD and highest sound pressure level can be achieved.

<FIG> shows simulation results of the differential output signal (outp-outn) with asymmetric input signals. The settings <NUM> to <NUM> used to obtain the simulation result presented in <FIG> is summarized in table <NUM>.

As presented in <FIG>, the signal transfer function (outp-outn) keeps essentially constant. According to <FIG> the transfer function moves between -<NUM> dB and -<NUM> dB, i.e. in a range of <NUM>,<NUM> dB, that is essentially constant. This means, the transfer function (outp-outn) keeps constant, although the single ended outputs were amplified or attenuated, as presented in <FIG> and <FIG>.

<FIG> shows simulation results of the signal at output outP used to determine the results shown in <FIG> and in <FIG>. <FIG> shows simulation results of the signal at output outN used to determine the results shown in <FIG> and in <FIG>. The settings <NUM> to <NUM> used to calculate the values presented in <FIG> are summarized in table <NUM>. <FIG> shows simulation results of the ratio of signals at output outP and outN, i.e. the ratio of the signals as presented in <FIG> and <FIG>.

The voltage node Vmic is derived from the charge pump <NUM> and a voltage of the voltage node Vmic is used to set a matching sensitivity of the MEMS-device <NUM>.

Although some aspects have been described as features in the context of an apparatus it is clear that such a description may also be regarded as a description of corresponding features of a method. Although some aspects have been described as features in the context of a method, it is clear that such a description may also be regarded as a description of corresponding features concerning the functionality of an apparatus.

Additional embodiments and examples are described which may be used alone or in combination with the features and functionalities described herein.

Depending on certain implementation requirements, embodiments of the processing device can be implemented in hardware or in software or at least partially in hardware or at least partially in software. Some embodiments comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

Generally, embodiments of the processing device can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. In other words, an embodiment of the method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.

A further embodiment of the methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or non-transitory.

In the foregoing Detailed Description, it can be seen that various features are grouped together in examples for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed examples require more features than are expressly recited in each claim. Rather, as the following claims reflect, subject matter may lie in less than all features of a single disclosed example. Thus the following claims are hereby incorporated into the Detailed Description, where each claim may stand on its own as a separate example.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the claims.

Claim 1:
A differential MEMS-readout circuit (<NUM>) comprising:
a first input bonding pad (<NUM>), which represents a first capacitance (C1); wherein the first capacitance (C1) comprises the first input bonding pad (<NUM>) as a first contact pin (INP) and a second contact pin (C1-<NUM>);
a second input bonding pad (<NUM>), which represents a second capacitance (C2);
wherein the second capacitance (C2) comprises the second input bonding pad (<NUM>) as a first contact pin (INN) and a second contact pin (C2-<NUM>); and
a differential-readout amplifier section (<NUM>) comprising a first input (INP1) connected to
the first contact pin (INP) of the
first capacitance (C1)and a second input (INN1) connected to the first contact pin (INN) of the second capacitance (C2), wherein the differential-readout amplifier section (<NUM>) comprises a first and a second transistor circuit (<NUM>, <NUM>) and
characterized in that
each of the second contact pins (C1-<NUM>, C2-<NUM>) of the first and second capacitances (C1, C2) is either connected to an output of the first transistor circuit (<NUM>) or to an ouput of the second transistor circuit (<NUM>) by using switches (<NUM>).