CN204944423U - A kind of capacitive MEMS sensor detection circuit - Google Patents

Abstract

The utility model discloses a kind of capacitive MEMS sensor detection circuit.This circuit is used for detecting the pole plate change in displacement of MEMS, mainly comprises: one produces sinusoidal wave high-frequency oscillating circuits; Two C ^-1/ V testing circuit; Differential amplifier circuit; Phase-shift network; Mlultiplying circuit; Low-pass filter circuit and amplification output circuit.High-frequency oscillating circuits produces the sine wave of three road homophases, and wherein two-way is supplied to two C ^-1/ V testing circuit, to complete the C caused the pole plate displacement of MEMS ^-1the detection of change; The impact that differential amplifier circuit brings in order to eliminate body electric capacity, improves the precision detected; Phase-shift network is used for compensating through C ^-1the phase shift of/V circuit, guarantees that the signal phase inputting mlultiplying circuit is identical; Low-pass filter circuit filters the higher hamonic wave of multiplier output signal, extracts the signal of reflection MEMS pole plate change in displacement, thus completes the detection to MEMS pole plate change in displacement.

Description

A kind of capacitive MEMS sensor detection circuit

Technical field

The utility model relates to the Detection of Weak Signals field of MEMS sensor, more specifically relates to a kind of capacitive MEMS sensor detection circuit.

Background technology

In recent years, along with the continuous popularization of capacitive MEMS application, new requirement be it is also proposed to MEMS interface circuit.But the interface circuit of the capacitive MEMS of present stage, major part is all the metering circuit based on C, and only when in static state, polar plate spacing just can obtain certain minimizing much larger than dynamic change spacing is non-linear, otherwise clearly non-linear.Simultaneously, the chopped wave stabilizing method generally adopted at present and two-phase close sampling method has certain limitation to MEMS capacitance detecting: chopped wave stabilizing method, the relative delay of circuit can make a big impact to measurement, and the harmonic wave brought of square wave can make stacking in baseband signal of the noise in each frequency range; The limitation such as two-phase closes sampling method, but makes moderate progress and still there is clock feedthrough, charge injection, and noise aliasing input bandwidth is not enough.Above-mentioned capacitance determining method can produce very big error because of the ghost effect of interface simultaneously.

Utility model content

For overcoming above-mentioned shortcoming, the utility model proposes a kind of capacitive MEMS sensor detection circuit.

In order to realize above-mentioned utility model object, the utility model adopts following technical scheme.

A kind of capacitive MEMS sensor detection circuit, changes corresponding electric signal for exporting with MEMS polar plate spacing, and detect the pole plate change in displacement of MEMS, it comprises: high-frequency oscillating circuits (1), C ^-1/ V circuit (2); Differential amplifier circuit (3), multiplier circuit (4), low-pass filter (5), amplification output circuit (6) and phase-shift network (7); Wherein, high-frequency oscillating circuits (1) produces three tunnel sinusoidal signals, and wherein two-way sinusoidal signal exports two C respectively to ^-1/ V testing circuit (2), two C ^-1/ V testing circuit exports two input terminals of differential amplifier circuit (3) respectively to, to two-way C ^-1/ V signal carries out differential amplification, to eliminate the impact that MEMS interface body electric capacity brings, exports an input terminal of multiplier circuit (4) to; And an other road of three tunnel sinusoidal signals exports phase-shift network circuit (7) to, after phase shift, export the another one input terminal of multiplier to; Multiplier is to the C after differential amplification ^-1/ V signal carries out demodulation and exports low-pass filter circuit (5) to, exports amplification output circuit (6) to, export after the buffering of amplifier after low-pass filtering.

Further, described two C ^-1/ V testing circuit, is different from traditional C/V circuit, for detecting C ^-1but not C value, C ^-1linear relationship is presented with polar plate spacing.And traditional C/V testing circuit is more utilize to be similar to much smaller than one during static substrate spacing substrate pitch change.

Further, described two-way sinusoidal signal passes through C ^-1after/V testing circuit, through differential amplifier circuit, differential amplification is carried out to two-way measuring-signal, to eliminate the impact of this body capacitance, improve measuring accuracy.

Further, described phase-shift network, is mainly used in making through C ^-1the signal in orthogonal of mlultiplying circuit is directly sent on the signal of/V testing circuit and another road, carries out phase compensation by described phase-shift network to the sinusoidal signal that high-frequency oscillating circuits produces.

Further, described mlultiplying circuit, by being multiplied by the C demodulating and will measure mutually by two paths of signals ^-1signal, instead of carry out peakvalue's checking simply; Compared with peak-value detection method, the signal to noise ratio (S/N ratio) that the utility model outputs signal is higher.

Compared with prior art, the utility model tool has the following advantages and technique effect:

A kind of capacitive MEMS sensor detection circuit that the utility model proposes, the pole plate micro-displacement change realized for the electric capacity becoming space type MEMS detects, there is the advantage that output signal is consistent with polar plate spacing highly linear, the C-1/V circuit that the utility model proposes is by detecting detected capacitive impedance values, and unconventionally carry out Detection capacitance by discharge and recharge, carry out C-1/V accuracy of detection by contrast higher, thus realize the accurate detection of micro-displacement change between the capacitor plate to MEMS.The circuit structure that the utility model proposes, compared with existing switch sample circuit, has circuit structure simple, easily implements, the advantage that signal to noise ratio (S/N ratio) is high.

Accompanying drawing explanation

Fig. 1 is structure and the Signal transmissions block diagram of capacitive MEMS pole plate displacement detecting circuit in example.

Fig. 2 is a kind of typical pectination MEMS structure schematic diagram.

Fig. 3 is the C of the utility model example ^-1/ V circuit.

Fig. 4 is the differential amplifier circuit of the utility model example.

Fig. 5 is the phase-shift network of the utility model example.

Fig. 6 is the multiplier circuit of the utility model example.

Fig. 7 is the low-pass filter circuit of the utility model example.

Fig. 8 is the low-pass filter circuit frequency domain response figure of the utility model example.

Fig. 9 is the C of the utility model distance variable condenser type MEMS ^-1the contrast of/V and traditional C/V testing circuit.

Embodiment

Below in conjunction with embodiment and accompanying drawing, the utility model is described in further detail, but embodiment of the present utility model is not limited thereto.

With reference to Fig. 1, the capacitive MEMS pole plate displacement detecting circuit of this example comprises: high-frequency oscillating circuits 1; C ^-1/ V circuit 2; Differential amplifier circuit 3; Multiplier circuit 4; Low-pass filter 5; Amplification output circuit 6; Phase-shift network 7.

High-frequency oscillating circuits, for generation of high frequency string ripple, exports C to ^-1/ V circuit and phase-shift network;

C ^-1/ V circuit, detects by detecting C ^-1change detects MEMS pole plate change in displacement;

Differential amplifier circuit, for by two-way C ^-1/ V signal carries out differential amplification and exports mlultiplying circuit demodulation to, and each inter-stage electricity consumption is simultaneously held and connected, to reduce the impact of prime 1/f noise;

Mlultiplying circuit, is used for the C of difference output ^-1/ V signal and the non-orthogonal sinusoidal signal mixing of high-frequency agitation source same frequency are to demodulate the collection signal of sensor;

Phase-shift network, ensure that two sinusoidal signals participating in mixing are non-orthogonal, because through C ^-1the sine of/V circuit has phase shift, so need to carry out certain phase shift compensation to an other road.

Amplification output circuit because the dynamic range considering mlultiplying circuit, before differential amplification signal out can not be too large, so must be amplified by circuit at the corresponding levels, export as circuit buffer simultaneously.

Low-pass filter, filters higher hamonic wave, exports the signal of reflection MEMS pole plate change in displacement.

If a certain movable polar plate of MEMS is subjected to displacement Δ d, for the MEMS of differential configuration, so have relative pole plate generation-Δ d displacement, its medium relative dielectric coefficient is ε simultaneously, and time static, fore-aft travel pole plate is all d with fixed polar plate spacing.The electric capacity that fore-aft travel pole plate and fixed polar plate are formed is respectively c ₁, c ₂, and ε ₀be the specific inductive capacity in vacuum, S is the area that two-plate is relative, then now:

$c_1=\frac{\mathrm{\ϵ}.{\mathrm{\ϵ}}_0.S}{d+\mathrm{\Δ}d}---\left(1\right)$

$c_2=\frac{\mathrm{\ϵ}.{\mathrm{\ϵ}}_0.S}{d-\mathrm{\Δ}d}---\left(2\right)$

With reference to shown in Fig. 1, be two equivalent resistance before operational amplifier, be used for controlling size of current.If the sinusoidal wave time-domain expression that high-frequency oscillating circuits produces is A.sin (w.t) (A sinusoidal signal amplitude, w is sinusoidal signal frequency, wherein w > > measured signal frequency), so two-way C ^-1the two-way of/V testing circuit exports respectively:

(R connects high-frequency oscillating circuits and C ^-1the current-limiting resistance of/V) (3)

(R connects high-frequency oscillating circuits and C ^-1the current-limiting resistance of/V) (4)

So C ^-1the difference Δ U of/V output signal voltage is:

$\mathrm{\Δ}U=U_{C^{-1}/V-}-U_{C^{-1}/V+}=\frac{A.sin(w.t)}{R}(\frac{1}{{\mathrm{jwc}}_1}-\frac{1}{{\mathrm{jwc}}_2})---\left(5\right)$

By formula (1), (2) substitute into formula (5) and obtain

$\mathrm{\Δ}U=U_{C^{-1}/V-}-U_{C^{-1}/V+}=\frac{A.sin(w.t)}{R}.\frac{1}{jw}.\left(\frac{2.\mathrm{\Δ}d}{\mathrm{\ϵ}.{\mathrm{\ϵ}}_0.S}\right)---\left(6\right)$

With reference to shown in Fig. 4, this is this differential amplifier circuit of a symmetrical structure, will to C ^-1the signal difference that/V circuit exports is amplified, and each inter-stage electricity consumption of circuit holds coupling, the impact that the drift which eliminating prime brings.If the gain of this differential amplifier circuit is G, then export the signal U of multiplier to ₁for:

$U_1=U_{C-1/V+}-U_{C^{-1}/V-}=G.\frac{A.sin(w.t)}{R}.\frac{1}{jw}.\left(\frac{2.\mathrm{\Δ}d}{\mathrm{\ϵ}.{\mathrm{\ϵ}}_0.S}\right)---\left(7\right)$

With reference to shown in Fig. 1, by by C ^-1/ V circuit signal the other road homogenous frequency signal mixing with oscillation source to demodulate frequency measurement segment signal.Consider C ^-1the phase shift effect (being assumed to be Δ φ) of/V and differential amplifier circuit, simultaneously the sinusoidal signal intensity U on an other road ₂for B.sin (w.t+ Δ φ) (B is the amplitude of sinusoidal signal after phase-shift network that high-frequency oscillating circuits produces, and Δ φ is the compensating phase shift of phase-shift network), when multiplier gain is 0dB, multiplier exports as U _{mixer_Out}:

U ₂＝B.sin(w.t+Δφ)(8)

$U_{Mixer\_Out}=U_2.U_1.e^{j\mathrm{\φ}}=B.\sin (w.t+\mathrm{\Δ}\mathrm{\φ}).G.\frac{A,.\sin (w.t+\mathrm{\Δ}\mathrm{\φ})}{R}.\frac{1}{jw}.\left(\frac{2.\mathrm{\Δ}d}{\mathrm{\ϵ}.{\mathrm{\ϵ}}_0.S}\right)---\left(9\right)$

After abbreviation:

$U_{Mixer\_Out}=U_1.U_2=B.A.G.\frac{\[\mathrm{\Δ}d-cos(2.w.t+2.\mathrm{\Δ}\mathrm{\φ}).\mathrm{\Δ}d\]}{jw.\mathrm{\ϵ}.{\mathrm{\ϵ}}_0.S.R}---\left(10\right)$

Mixer output signal comprises cos (2.w.t+ Δ φ). Δ d, Δ d item, wherein cos (2.w.t+ Δ φ). Δ d is higher hamonic wave item, will be low-pass filtered device and filter, when in pass band, gain is 0dB, then low-pass filter exports U _lpfor:

$U_{Lp}=B.A.G.\frac{\mathrm{\Δ}d}{jw.\mathrm{\ϵ}.{\mathrm{\ϵ}}_0.S.R}---\left(11\right)$

If buffer amplifier gain is C, so last amplifier exports U _out:

$U_{out}=C.B.A.G.\frac{-\mathrm{\Δ}d}{jw.\mathrm{\ϵ}.{\mathrm{\ϵ}}_0.S.R}---\left(12\right)$

Derive from above, the impact that the ghost effect that this circuit can eliminate the machining precision error brought of deficiency and circuit to greatest extent by symmetric difference structure brings, the stray capacitance of such as interface can regard two electric capacity that front and back pole plate increases as, to greatly eliminate the impact of interface stray capacitance after differential configuration, detection signal can avoid the impact that 1/f noise brings well by being multiplied with high-frequency signal simultaneously.

The structure of typical pectination MEMS sensor and its size are as shown in Figure 2.Utilize capacitive detection circuit of the present utility model to detect its electric capacity, its structural parameters are as follows: electrode logarithm n=12, static spacing d=10um, capacitor plate relative area S=(455X120) um ².

Specific inductive capacity in vacuum gets ε ₀=8.85*10^ (-12) F/m, silicon relative dielectric coefficient gets ε=12, and often pair of stationary plate spacing of electric capacity is d=10um, and its dynamic range is (-5um, 5um), and oscillatory circuit exports two-way C to ^-1the amplitude of/V testing circuit is A=1mv.

Adopt the first resistance R in circuit shown in Fig. 3 ₂₀₁=1KOhm, the second resistance R ₂₀₂the equivalent capacity of=1KOhm, detected MEMS press connection shown in Fig. 3: respectively by the two poles of the earth downlink connection of equivalent capacity at the first operational amplifier U ₂₀₁, the second operational amplifier U ₂₀₂output and counter terminal between.

Sinusoidal signal passes through C ^-1after/V, through the differential amplifier circuit as Fig. 4.Wherein the first operational amplifier is U ₃₀₁, the second operational amplifier is U ₃₀₂, the 3rd operational amplifier is U ₃₀₃, the first electric capacity C ₃₀₁=1uf, the second electric capacity C ₃₀₂=1uf, the 3rd electric capacity C ₃₀₃=1uf, the 4th electric capacity C ₃₀₄=1uf is every straight coupling capacitance.First resistance R ₃₀₁=100Ohm, the second resistance R ₃₀₂=100Ohm, the 3rd resistance R ₃₀₃=1KOhm, the 4th resistance R ₃₀₄=1KOhm, the 5th resistance R ₃₀₅=100Ohm, the 6th resistance R ₃₀₆=100Ohm, the 7th resistance R ₃₀₇=10KOhm, the 8th resistance R ₃₀₈=10KOhm, makes this gain per stage G=60dB.

Oscillatory circuit exports phase-shift network to and passes through C to compensate ^-1the phase shift of the generation of/V and differential amplifier circuit.Adopt circuit, wherein U shown in Fig. 5 ₄₀₁be the first operational amplifier, the first resistance R ₄₀₁=10KOhm, the second resistance R ₄₀₂=10kOhm, the first electric capacity C ₄₀₁=1nf, oscillatory circuit is by the first resistance R ₄₀₁connect the first operational amplifier U ₄₀₁inverting input, simultaneously the first operational amplifier U ₄₀₁inverting input and output terminal by the second resistance R ₄₀₂and the first electric capacity C ₄₀₁be connected in parallel the phase compensation to the other road input of mlultiplying circuit.

In Fig. 6 multiplier circuit, the first electric capacity C ₅₀₁=1uf, the second electric capacity C ₅₀₂=1uf is every straight coupling capacitance, the first operational amplifier U ₅₀₁, the first NPN type triode U ₅₀₂, the second NPN type triode U ₅₀₃, the first resistance R ₅₀₁=the six resistance R ₅₀₆, the second resistance R ₅₀₂=the seven resistance R ₅₀₇, as the biasing circuit of multiplier, and the 3rd resistance R ₅₀₃=530Ohm, the 4th resistance R ₅₀₄=10KOhm, the 5th resistance R ₅₀₅=530Ohm, makes multiplier gain be close to 0dB.

The 6 rank Butterworth LPF of Fig. 7 to be a passband be DC-34KHz, U in figure ₆₀₁the first operational amplifier, U ₆₀₂the second operational amplifier, U ₆₀₃the 3rd operational amplifier, and the first resistance R ₆₀₁=9.5KOhm, the second resistance R ₆₀₂=18KOhm, the 3rd resistance R ₆₀₃=8.3KOhm, the 4th resistance R ₆₀₄=12KOhm, the 5th resistance R ₆₀₅=2.4KOhm, the 6th resistance R ₆₀₆=5KOhm, the first electric capacity C ₆₀₁, the second electric capacity C ₆₀₂, the 3rd electric capacity C ₆₀₃, pass is C ₆₀₁=C ₆₀₂=C ₆₀₃=330pf, the 4th electric capacity C ₆₀₄=390pf, the 5th electric capacity C ₆₀₅=680pf, the 6th electric capacity C ₆₀₆=6nf, its frequency domain response as shown in Figure 8.

Finally export wave filter and carry out amplification Buffer output, the gain of amplifier is C=40dB, finally circuit parameter at different levels is substituted into formula (11) and can draw the relation that output voltage changes with this detection MEMS polar plate spacing, as shown in Figure 9.

Claims (1)

1. a capacitive MEMS sensor detection circuit, changes corresponding electric signal for exporting with MEMS polar plate spacing, detects the pole plate change in displacement of MEMS, it is characterized in that comprising: high-frequency oscillating circuits (1), C ^-1/ V circuit (2); Differential amplifier circuit (3), multiplier circuit (4), low-pass filter (5), amplification output circuit (6) and phase-shift network (7); Wherein, high-frequency oscillating circuits (1) produces three tunnel sinusoidal signals, and wherein two-way sinusoidal signal exports two C respectively to ^-1/ V testing circuit (2), two C ^-1/ V testing circuit exports two input terminals of differential amplifier circuit (3) respectively to, to two-way C ^-1/ V signal carries out differential amplification, to eliminate the impact that MEMS interface body electric capacity brings, exports an input terminal of multiplier circuit (4) to; And an other road of three tunnel sinusoidal signals exports phase-shift network circuit (7) to, after phase shift, export the another one input terminal of multiplier to; Multiplier is to the C after differential amplification ^-1/ V signal carries out demodulation and exports low-pass filter circuit (5) to, exports amplification output circuit (6) to, export after the buffering of amplifier after low-pass filtering.