CN102624397B - High-linearity fully differential digital micro-accelerometer interface circuit system - Google Patents

Abstract

The invention belongs to the field of MEMS (Micro-Electro-Mechanical Systems) inertia devices, and particularly relates to a capacitance type micro-accelerometer digital output fully differential digital micro-accelerometer interface circuit system. The system comprises a drive signal generating part, a fully differential charge integrator, a fully differential post-amplifier, a fully differential pre-distorter, a relative dual-sampling and sampling retaining circuit, a fully electric integrator, a dynamic comparator, a D/A (Digital/Audio) converter and an electrostatic force feedback device. The system improves the linearity of an accelerometer system, effectively lowers the quantization noise of D/A conversion, restrains zero shift, reduces common mode interference generated by switching charge injection and a substrate noise, improves power supply rejection ration (PSRR), and reduces harmonic distortion.

Description

A high-linearity full-differential digital micro-accelerometer interface circuit system

technical field

The invention belongs to the field of MEMS inertial devices, and in particular relates to a fully differential digital accelerometer interface circuit system with digital output of a capacitive micro accelerometer.

Background technique

MEMS (Micro Electromechanical System) is developed on the basis of microelectronics technology. Devices made with MEMS technology are widely used in aerospace due to their advantages of miniaturization, intelligence, integration, and high reliability. , automobiles, biomedicine, environmental monitoring and almost all fields that people come into contact with. Micromachined accelerometers are one of the most important MEMS devices, second only to pressure sensors in terms of market sales. Micro-accelerometers have the advantages of small size, low power consumption, good stability, high reliability, and convenience for mass production, and are widely used in the automotive industry, aerospace fields, and consumer electronics.

At present, many inertial sensor applications use computers, microprocessors and other digital devices. In order to process signals in the digital field, it is necessary to integrate the analog-to-digital conversion and sensor signal processing circuits on the same chip. The integrated digital Sensors not only provide more functionality, but also reduce the cost of the overall system. Sigma-Delta (ΣΔ) modulation technology is an important way to realize analog-to-digital conversion. With the development of MEMS technology, ΣΔ modulation technology has been introduced into the design of micro-mechanical accelerometers. The sensitive structure of the micro accelerometer can be used as a second-order ΣΔ modulator. However, due to the very low DC gain of the sensitive structure, it is difficult to achieve low quantization noise in the second-order structure. High-order structures are the most effective in reducing system quantization noise. way.

In addition, since the sensitive structure is a three-terminal device, that is, two fixed electrodes and an intermediate variable electrode as the output, the single-ended detection method is generally used to process the output signal, which has the advantages of simple structure, less control clocks, and feedback It is easy to implement, but its disadvantage is that the zero point drift is relatively serious, and the noise of the driving signal has a serious impact on the output. In fact, two fixed electrodes can also be used as the output of the sensitive structure, and a fully differential detection circuit is used to process the signal, which can reduce the common-mode interference caused by switch charge injection and substrate noise, improve the power supply rejection ratio, and reduce the Harmonic distortion.

However, for the fully differential ΣΔ signal processing structure, there is a main problem: in the differential structure, the driving signal is applied to the active electrode, so the feedback voltage can only be applied to the fixed electrode. When feedback, the active electrode is grounded, and the two fixed electrodes One of the plates is connected to the feedback voltage V _fb , and the other is also connected to 0 potential to form an electrostatic force opposite to the inertial force. The resultant electrostatic force on the movable electrode is in a quadratic relationship with the feedback voltage, which reduces the linearity of the system.

Contents of the invention

The purpose of the present invention is to provide a fully differential digital accelerometer interface circuit system with high linearity and high precision digital output.

The purpose of the present invention is achieved like this:

The present invention comprises a driving signal generation part (101), a fully differential charge integrator (102), a fully differential post-stage amplifier (103), a fully differential predistorter (104), a correlated double sampling and sample-and-hold circuit (105), Fully differential electrical integrator (106), 1-bit dynamic comparator (107), 1-bit digital-to-analog converter (108) and 1-bit electrostatic force feedback (109) are characterized in that:

The driving signal generating part (101) generates a two-phase high-frequency driving square wave signal, which is respectively loaded on two fixed electrodes of the sensitive structure;

The fully differential charge integrator (102) converts the tiny capacitance change of the sensitive structure into a differential voltage signal and outputs it to the fully differential post-stage amplifier (103);

The differential post-amplifier (103) inverts and amplifies the signal and outputs it to the fully differential predistorter (104);

The fully differential predistorter (104) provides the prephase for the high frequency part of the signal and outputs it to the correlated double sampling and sampling and holding (105);

Correlated double sampling and sample-and-hold circuit (105) eliminates signal high-frequency 1/f noise and operational amplifier offset, compensates operational amplifier for limited bandwidth and limited gain effects, and sends the fully differential electrical integrator (106) after demodulating the high-frequency signal;

The fully differential electrical integrator (106) provides low-frequency gain to the signal, reduces the quantization noise of the signal and sends it to a 1-bit dynamic comparator (107);

1-bit digital-to-analog converter (108), receives the output signal of 1-bit dynamic comparator (107), and judges the feedback voltage of electrical integrator;

The 1-bit electrostatic force feedback (109) receives the output signal of the 1-bit dynamic comparator (107), and judges the direction of the feedback force fed back to the sensitive structure.

The beneficial effects of the present invention are:

The present invention proposes a double-sided electrostatic force feedback method, which greatly improves the linearity of the accelerometer system compared with the currently commonly used single-sided electrostatic force feedback method; uses the second-order low-pass filter characteristics of the micro-mechanical structure itself combined with electrical The integrator effectively reduces the quantization noise of analog-to-digital conversion; the signal processing method of the fully differential structure suppresses zero drift, reduces the common-mode interference caused by switch charge injection and substrate noise, improves the power supply rejection ratio, and reduces harmonics distortion.

Description of drawings

Figure 1 is a block diagram of the high linearity full differential digital accelerometer interface circuit system;

Figure 2 is a schematic diagram of unilateral electrostatic force feedback;

Figure 3 is a schematic diagram of bilateral electrostatic force feedback.

Detailed ways

FIG. 1 shows a block diagram of the high linearity fully differential digital accelerometer interface circuit system in this embodiment.

As shown in Fig. 1, the interface circuit system has: a drive signal generating unit 101, a fully differential charge integrator 102, a fully differential post-stage amplifier 103, a fully differential predistorter 104, a correlated double sampling and sample-and-hold circuit 105, a fully differential Differential electrical integrator 106 , 1-bit dynamic comparator 107 , 1-bit analog-to-analog (D/A) converter 108 , 1-bit electrostatic force feedback 109 , clock signal generator 110 .

The integrator in the ΣΔ modulator is composed of the sensitive structure 100 of the micro accelerometer and the fully differential electrical integrator 106. Under the control of the clock signal generator, it provides high low-frequency gain, obtains a high-precision 1-bit digital output, and A fully differential closed-loop structure is formed by a 1-bit digital-to-analog D/A converter 108 and a 1-bit electrostatic force feedback 109 . The fully differential predistorter 104 determines the depth of predistortion by the number of stages of the fully differential charge integrator and the parameters of the mechanical structure, so as to compromise between accuracy and stability. The 1-bit digital-to-analog converter determines whether the feedback voltage of the fully differential charge integrator 1 is positive or negative according to the output level of the 1-bit dynamic comparator. The 1-bit electrostatic force feedback determines the direction of the electrostatic force applied to the sensitive mass according to the output level of the 1-bit dynamic comparator. The feedback adopts the double-sided electrostatic force feedback method to improve the linearity of the system.

The acquisition of high linearity of the system is illustrated in conjunction with Fig. 2 and Fig. 3 . Figure 2 is the commonly used unilateral electrostatic force feedback method suitable for fully differential ΣΔ interface circuits. During feedback, the movable electrode is grounded, and one of the two fixed plates is connected to the feedback voltage V _fb , and the other is also connected to 0 potential, forming Electrostatic force F _fb opposite to inertial force

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; fb</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; fb</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

It can be seen from the above formula that the resultant electrostatic force on the movable electrode has a quadratic relationship with the feedback voltage, which reduces the linearity of the system.

Fig. 3 is a schematic diagram of the double-sided electrostatic force feedback proposed in the present invention. During feedback, the active electrode is connected to the negative power supply voltage V _SS , and the feedback voltage V _fb and -V _fb are respectively applied to the two plates. Therefore, the resultant electrostatic force on the mass block is:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; fb</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; SS</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; fb</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; SS</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; fb</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; SS</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; fb</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

It can be seen that this method improves the linearity of the system. During feedback, the active electrode can also be connected to the positive power supply voltage V _CC . At this time, it is only necessary to exchange the voltage fed back to the fixed electrode.

The invention utilizes a drive signal generating circuit to generate a two-phase non-overlapping square wave drive signal, which is respectively applied to the upper and lower fixed electrode plates of the sensitive structure, and combined with a charge integrator to convert the capacitance change of the sensitive structure caused by the applied acceleration into a full differential voltage signal output. The weak signal is amplified by the post-stage amplifier and sent to the post-stage circuit for processing. The phase compensation of the system is carried out through the predistorter to improve the stability of the system. Correlative double sampling and sample-hold circuits are used to eliminate circuit high-frequency 1/f noise and operational amplifier offset, and at the same time realize demodulation of high-frequency signals. The output of the comparator is not only used as the final 1-bit digital output, but also as the control signal of the electrical integrator feedback and 1-bit electrostatic force feedback. The electrostatic force feedback part adopts a double-sided feedback method to greatly improve the linearity of the system.

Claims (1)

1. a high linearity fully differential digital accelerometer interface circuitry, comprise drive singal generating unit (101), fully differential charge integrator (102), fully differential post-amplifier (103), fully differential predistorter (104), correlated-double-sampling and sampling hold circuit (105), fully differential electricity integrator (106), 1 dynamic comparer (107), 1 figure place weighted-voltage D/A converter (108) and 1 electrostatic force feedback (109), it is characterized in that:

Drive singal generating unit (101), produces two-phase high-frequency drive square-wave signal, is carried in two fixed electrodes of sensitive structure respectively;

The change of the small capacitance of sensitive structure is converted to differential voltage signal and exports to fully differential post-amplifier (103) by fully differential charge integrator (102);

Signal is carried out anti-phase amplification and exports to fully differential predistorter (104) by fully differential post-amplifier (103);

Correlated-double-sampling and sampling hold circuit (105) is exported to after the HFS that fully differential predistorter (104) is signal provides preceding phase;

Correlated-double-sampling and sampling hold circuit (105) erasure signal high-frequency noise and amplifier are lacked of proper care, and compensate amplifier finite bandwidth and finite gain impact, send to fully differential electricity integrator (106) after demodulation high-frequency signal;

Fully differential electricity integrator (106) provides low-frequency gain to signal, sends to 1 dynamic comparer (107) after reducing signal quantization noise;

1 figure place weighted-voltage D/A converter (108), receives the output signal of 1 dynamic comparer (107), judges the feedback voltage of fully differential electricity integrator;

1 electrostatic force feedback (109), receives the output signal of 1 dynamic comparer (107), judges the feedback force direction feeding back to sensitive structure;

Described electrostatic force feedback, during feedback, float electrode meets negative supply voltage V _sS, two-plate applies feedback voltage V respectively _fbwith-V _fb.