# Patent

## Title
MEMS device front-end charge amplifier

## Abstract

This document discusses, among other things, apparatus and methods for a front-end charge amplifier. In certain examples, a front-end charge amplifier for a microelectromechanical system (MEMS) device can include a charge amplifier configured to couple to the MEMS device and to provide sense information of a proof mass of the MEMS device, a feedback circuit configured to receive the sense information and to provide feedback to an input of the charge amplifier, and wherein the charge amplifier includes a transfer function having a first pole at a first frequency, a second pole at a second frequency, and one zero at a third frequency.

## Background
CLAIM OF PRIORITY
This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/620,622, filed Apr. 5, 2012, hereby incorporated by reference herein in its entirety.


OVERVIEW
This document discusses, among other things, apparatus and methods for a front-end charge amplifier. In certain examples, a front-end charge amplifier for a microelectromechanical system (MEMS) device can include a charge amplifier configured to couple to the MEMS device and to provide sense information of a proof mass of the MEMS device, a feedback circuit configured to receive the sense information and to provide feedback to an input of the charge amplifier, and wherein the charge amplifier includes a transfer function having a first pole at a first frequency, a second pole at a second frequency, and one zero at a third frequency.
This overview is intended to provide a general overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application.



BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.
FIG. 1 illustrates generally an example front-end amplifier.
FIG. 2 illustrates generally example amplifier stages of an example front-end amplifier.
FIG. 3 illustrates generally an example front-end amplifier employing a transconductor and current attenuator in the feedback loop.
FIGS. 4A and 4B illustrate general comparisons of a single-pole charge amplifier and an example front-end charge amplifier having two poles and a single zero



DETAILED DESCRIPTION
The present inventors have recognized an improved front-end amplifier for use with microelectromechanical system (MEMS) device such as a MEMS gyroscope. The front end amplifiers can convert charge from MEMS device electrodes to voltage for further processing to assist in providing feedback for driving the MEMS device or to assist in sensing movement of the MEMS device. The improved front-end amplifiers can reduce noise, reduce distortion, reduce offset, and provide improved temperature stability of gain drift and phase drift over existing amplifiers that convert charge from capacitive MEMS sensors to voltage.
FIG. 1 illustrates generally an example front-end amplifier 100. In certain examples, the front-end amplifier 100 can be used to convert charge of a capacitive MEMS device sensor to voltage to assist in driving a proof mass 102 of a MEMS device such as a MEMS gyroscope 101, for example, or to assist in providing an indication of movement of the MEMS gyroscope 101. In certain examples, the front-end amplifier 100 can receive a differential sense signal from a pair of capacitive sensor electrodes 103 of the MEMS gyroscope 101, and can provide a differential voltage (VOUT) that is representative of the charge on the capacitive sensor electrodes 103. In certain examples, the capacitive sensor electrodes 103 can be associated with a proof mass of the MEMS device and the sense signal can provide sense information about position and movement of the proof mass.
In an example, the front-end amplifier 100 can include a feedback loop 104 coupled between the input of the front-end amplifier 100 and the output of the front-end amplifier 100. In certain examples, the feedback loop 104 can include an adjustable capacitance (CF) such as an adjustable capacitor. In an example, the feedback loop 104 can include a resistor network 105 and a current-based reference resistor 106 that can include a current mirror 107. In certain examples, the resistor network 105 and current-based reference resistor 106 can be coupled in parallel with the adjustable capacitance (CF). In certain examples, the overall gain of the front-end amplifier 100 can be set using the adjustable capacitance (CF). In an example, the value of current supplied by the current-based reference resistor 106 can also change with the adjustable capacitance (CF) such that closed loop pole locations of the front-end amplifier 100 do not change, thereby preserving phase relationship at a predetermined frequency or frequency range such as the resonant frequency of the MEMS gyroscope 101. In certain examples, the front-end amplifier 100 can include multiple amplifier stages, such as a first stage 108, a second stage 109, and a third stage 110.
FIG. 2 illustrates generally example amplifier stages of an example multistage front-end charge amplifier 200. In certain examples, the multistage front-end amplifier 200 can include a first amplifier stage 208, a second amplifier stage 209, and a third amplifier stage 210. In some examples, a transfer function of the three amplifier stages 208, 209, 210 can include two poles and one zero. In an example, the first amplifier stage 208 can provide a straight, or fixed, gain with low noise. In an example, the gain of the first amplifier stage 208 can be about 10. In certain examples, the second amplifier stage 209 can provide a transfer function including a pole. In some examples, the second amplifier stage 209 can include a variable gain configured to accommodate loop gain factors and the ensure feedback loop stability. In an example, the second amplifier stage 209 can have a gain of about 25 micro-amps per volt (uA/V) to about 250 uA/V. In an example, the second amplifier stage 209 can include an amplifier 212 with a feedback loop with a capacitive element 211. It is understood that the gains provided above are example gains and that other gains are possible without departing from the scope of the present subject matter. The third amplifier stage 210 can provide a transfer function including a pole and one zero. In an example, the third amplifier stage 210 can include an amplifier 214 with a first capacitive element 215 in a feedback loop, and a second capacitive element 216 coupled to the input of the third amplifier stage 210 and configured to receive the input signal to the third amplifier stage 210. In certain examples, the second capacitive element 216 can be coupled to the input signal of the third amplifier stage 210 through a resistive element. In an example, the third amplifier stage 210 can receive the output of the second amplifier stage 209, and the second amplifier stage can receive the output of the first amplifier stage 208. It is understood that different sequential arrangements of the amplifier stages, including different sequential arrangements of the amplifier stages including the poles and the zero, are possible without departing from the scope of the present subject matter. It is understood that a front-end amplifier can include more or less amplifier stages without departing from the scope of the present subject matter.
In addition to the above improvements, the example front-end amplifier can provide low Δgain/gain drift versus temperature (40 dB additional open loop gain). In certain examples, the front-end amplifier can provide low variation of Δphase drift versus temperature (5-50 times improvement over a single pole front-end amplifier). In certain examples, the front-end amplifier can provide low output offset. In some examples, the multistage front-end charge amplifier 200 can provide at least 60 dB of gain, a high pass filter corner at  ̃2 Hz, and a closed loop bandwidth up to 20 MHz, to allow phase at 20 kHz to not drift much due to RC variation vs. temperature. Gain of each stage in the multistage front-end charge amplifier 200 can be set by a ratio of resistors to provide constant gain vs. temp.
FIG. 3 illustrates generally an example front-end amplifier 300 employing a transconductor 317 and current attenuator 318 in the feedback loop 304. The front-end amplifier 300 can include a three stage amplifier 319 having a transfer function that includes two poles and one zero. The front-end amplifier can be used to process capacitive signals received from a MEMS device, such as a MEMS gyroscope (not shown). The use of the transconductor 317 to convert the voltage signal to a current signal can improve performance of the front-end amplifier 300 over the example front end amplifier of FIG. 1 because the current based reference resistor 106 can distort an input signal non-linearly whereas the transconductor 317 does not distort the sensor signal in a non-linear fashion near the frequency of interest. In certain examples, the front-end amplifier 300 can include a common mode feedback circuit 320 to stabilize common mode voltages of the differential input of the front-end amplifier 300. In some examples, the common mode feedback circuit 320 can set the common mode voltage to the common mode reference (ref).
In certain examples, a front-end charge amplifier according to the present subject matter can provide improved processing performance of the MEMS device sense signals. For example, a front-end amplifier having a transfer function including two poles and one zero can reduce signal attenuation and phase shift of the sense signal at or near a frequency of interest, or frequency range of interest, over existing front-end amplifiers employing a single pole transfer function. FIGS. 4A and 4B illustrate general comparisons of a single-pole charge amplifier 401 compared to an example front-end charge amplifier 402 having two poles and a single zero. In an example, such as with a MEMS gyroscope having a gyroscope resonant frequency at about 20 kHz, an example front-end charge amplifier can provide 100 dB DC gain compared to 80 dB DC gain of a single pole front-end amplifier, as illustrated in FIG. 4A. The plots of FIGS. 4A and 4B are taken from an example front-end charge amplifier having two poles at 20 kHz and one zero at 2 MHz and a single-pole front-end amplifier having a single pole at 2 kHz. It is understood that other pole and zero frequencies are possible, including but not limited to, pole frequencies below or near the resonant frequency of the MEMS device, without departing from the scope of the present subject matter. In certain examples, such pole and zero frequencies can be selected to provide improved phase and gain stability. In some examples, the pole and zero frequencies can be selected to reduce a noise bandwidth. FIG. 4B illustrates comparisons of open loop frequency phase characteristics of the single-pole charge amplifier 401 and an example front-end charge amplifier 402 having two poles and a single zero. With respect to phase shift, the example front-end amplifier can reduce the phase shift to about −1.2 millidegrees (mdeg) compared to about −57.3 mdeg for the single-pole front-end amplifier.
Additional Notes
In Example 1, and apparatus con include a charge amplifier configured to couple to a microelectromechanical system (MEMS) device and to provide sense information of a proof mass of the MEMS device, a feedback circuit configured to receive the sense information and to provide feedback to an input of the charge amplifier; and wherein the charge amplifier includes a transfer function having a first pole at a first frequency, a second pole at a second frequency, and one zero at a third frequency.
In Example 2, the first frequency and the second frequency of Example 1 are at or below a resonance frequency of the proof mass.
In Example 3, the third frequency of any one or more of Examples 1-2 optionally is substantially higher than the first frequency and the second frequency.
In Example 4, the two poles and the zero of any one or more of Examples 1-3 optionally are configured to reduce phase offset of the sense information to substantially zero over a predetermined frequency range.
In Example 5, the charge amplifier of any one or more of Examples 1-4 optionally includes a multistage charge amplifier.
In Example 6, the multistage charge amplifier of any one or more of Examples 1-5 optionally includes a first amplifier stage configured to provide a fixed gain.
In Example 7, the multistage charge amplifier of any one or more of Examples 1-6 optionally includes a second amplifier stage having a transfer function including the first pole.
In Example 8, a gain of the second amplifier stage of any one or more of Examples 1-7 optionally is a variable gain.
In Example 9, the multistage charge amplifier of any one or more of Examples 1-8 optionally includes a third amplifier stage having a transfer function including the second pole and the zero.
In Example 10, the feedback circuit of any one or more of Examples 1-9 optionally includes an adjustable capacitance.
In Example 11, the feedback circuit of any one or more of Examples 1-10 optionally includes a resistor network, the resistor network including a current mirror coupled to the input of the charge amplifier; a first feedback resistor coupled to an output of the charge amplifier and coupled in series with the current mirror to the input of the charge amplifier, and a second feedback resistor having a first terminal coupled to a voltage supply and a second terminal coupled to the current mirror and the first feedback resistor.
In Example 12, the feedback circuit of any one or more of Examples 1-11 optionally includes a current attenuator coupled to the input of the charge amplifier, and a transconductor coupled to the current attenuator and an output of the charge amplifier, the transconductor configured provide a current signal representative of the sense information.
In Example 13, a method can include receiving a sense signal from a microelectromechanical system (MEMS) device, and providing a voltage representative of a capacitance of the MEMS device using the sense signal and an amplifier having a first transfer function pole at a first frequency, a second transfer function pole at a second frequency, and one transfer function zero at a third frequency.
In Example 14, the receiving a sense signal of any one or more of Examples 1-13 optionally includes receiving a sense signal associated with a proof mass of the MEMS device, and wherein the first frequency and the second frequency are below or about equal to a resonance frequency of the proof mass.
In Example 15, the providing a voltage representative of the capacitance of any one or more of Examples 1-14 optionally includes providing substantially zero phase offset over a predetermined frequency range between the sense signal and the voltage using the two transfer function poles and the transfer function zero.
In Example 16, the method of any one or more of Examples 1-15 optionally includes providing first feedback to an input of the amplifier using an adjustable capacitance coupled to an output of the amplifier.
In Example 17, the method of any one or more of Examples 1-16 optionally includes providing second feedback to the input of the amplifier using a current signal representative of the sense information.
In Example 18, the providing the second feedback of any one or more of Examples 1-17 optionally includes converting the sense information from a voltage signal to a current signal using a transconductor, and attenuating the current signal using a current attenuator to provide the second feedback to the input.
In Example 19, a system can include a microelectromechanical system (MEMS) sensor, and a front-end charge amplifier configured to provide sense information of the MEMS sensor. The front-end charge amplifier can include a charge amplifier configured to couple to the MEMS sensor and to provide the sense information of a proof mass of the MEMS sensor, a feedback circuit configured to receive the sense information and to provide feedback to an input of the charge amplifier, and wherein the charge amplifier includes a transfer function having a first pole at a first frequency, a second pole at a second frequency, and one zero at a third frequency.
In Example 20, the MEMS sensor of any one or more of Examples 1-19 optionally includes a MEMS gyroscope.
Example 21 can include, or can optionally be combined with any portion or combination of any portions of any one or more of Examples 1 through 20 to include, subject matter that can include means for performing any one or more of the functions of Examples 1 through 20, or a machine-readable medium including instructions that, when performed by a machine, cause the machine to perform any one or more of the functions of Examples 1 through 20.
The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein.
All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.
In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.
Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like.
The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

## Claims

1. An apparatus comprising:
a charge amplifier configured to couple to a microelectromechanical system (MEMS) device and to provide sense information of a proof mass of the MEMS device;
a feedback circuit configured to receive the sense information and to provide feedback to an input of the charge amplifier; and
wherein the charge amplifier includes a transfer function having a first pole at a first frequency, a second pole at a second frequency, and one zero at a third frequency; and
wherein the first frequency and the second frequency are at or below a resonance frequency of the proof mass.

2. The apparatus of claim 1, wherein the third frequency is substantially higher than the first frequency and the second frequency.

3. The apparatus of claim 1, wherein the two poles and the zero are configured to reduce phase offset of the sense information to substantially zero over a predetermined frequency range.

4. The apparatus of claim 1, wherein the charge amplifier include a multistage charge amplifier.

5. The apparatus of claim 4, wherein the multistage charge amplifier includes a first amplifier stage configured to provide a fixed gain.

6. The apparatus of claim 5, wherein the multistage charge amplifier includes a second amplifier stage having a transfer function including the first pole.

7. The apparatus of claim 6, wherein a gain of he second amplifier stage is a variable gain.

8. The apparatus of claim 6, wherein the multistage charge amplifier includes a third amplifier stage having a transfer function including the second pole and the zero.

9. The apparatus of claim 1, wherein the feedback circuit includes an adjustable capacitance.

10. The apparatus of claim 1, wherein the feedback circuit includes a resistor network, the resistor network including:
a current mirror coupled to the input of the charge amplifier
a first feedback resistor coupled to an output of the charge amplifier and coupled in series with the current mirror to the input of the charge amplifier; and
a second feedback resistor having a first terminal coupled to a voltage supply and a second terminal coupled to the current mirror and the first feedback resistor.

11. The apparatus of claim 1, wherein the feedback circuit includes;
a current attenuator coupled to the input of the charge amplifier; and
a transconductor coupled to the current attenuator and an output of the charge amplifier, the transconductor configured provide a current signal representative of the sense information.

12. A method comprising:
receiving a sense signal from a proof mass of a microelectromechanical system (MEMS) device; and
providing a voltage representative of a capacitance of the MEMS device using the sense signal and an amplifier having a first transfer function pole at a first frequency, a second transfer function pole at a second frequency, and one transfer function zero at a third frequency; and
wherein the first frequency and the second frequency are below or about equal to a resonance frequency of the proof mass.

13. The method of claim 12, wherein providing a voltage representative of the capacitance includes providing substantially zero phase offset over a predetermined frequency range between the sense signal and the voltage using the two transfer function poles and the transfer function zero.

14. The method of claim 12, including providing first feedback to an input of the amplifier using an adjustable capacitance coupled to an output of the amplifier.

15. The method of claim 14, including providing second feedback to the input of the amplifier using a current signal representative of the sense information.

16. The method of claim 15, wherein the providing the second feedback includes:
converting the sense information from a voltage signal to a current signal using a transconductor; and
attenuating the current signal using a current attenuator to provide the second feedback to the input.

17. A system comprising:
a microelectromechanical system (MEMS) sensor; and
a front-end charge amplifier configured to provide sense information of the MEMS sensor, the front-end charge amplifier comprising:
a charge amplifier configured to couple to the MEMS sensor and to provide the sense information of a proof mass of the MEMS sensor;
a feedback circuit configured to receive the sense information and to provide feedback to an input of the charge amplifier; and
wherein the charge amplifier includes a transfer function having a first pole at a first frequency, a second pole at a second frequency, and one zero at a third frequency; and
wherein the first frequency and the second frequency are at or below a resonance frequency of the proof mass.

18. The system of claim 17, wherein the MEMS sensor includes a MEMS gyroscope.