JP2014006211A - Sensor circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sensor circuit that allows highly accurate measurement without increasing a circuit scale.SOLUTION: A sensor circuit includes: a piezoelectric element X as a sensor element; an oscillator 11 in which frequency fvco of an oscillation signal is controlled by a control signal Vlp; a PLL circuit 10 that returns a control signal Vlp according to a phase difference Δθ between an oscillation signal (oscillation frequency fvco) and a reference signal (reference frequency fr) to the oscillator 11; and an output part 20 that outputs a variation amount of the sensor element according to a temporal change of the control signal Vlp. Thus, a frequency change in the oscillation signal of the oscillator 11 is detected by voltage variation of the control signal Vlp so as to estimate a sensor variation amount.

Description

  The present invention relates to analog technology of a semiconductor LSI circuit, and more particularly to a sensor circuit using a vibrating body as a sensor element.

Conventionally, as an example of using a piezoelectric element as a sensor, for example, there is a technique described in Patent Document 1. This technique is a technique (QCM: Quarts Crystal Microbalance) that uses a crystal oscillator to detect a minute amount of substance from its piezoelectric effect.
When a substance is adsorbed on the electrode surface of the crystal oscillator, the resonance frequency changes according to the weight of the adsorbed substance. Therefore, if this is used, the weight of a very small amount of substance can be read from the frequency change.

  FIG. 3 is a conceptual diagram of QCM. For the QCM, a crystal oscillator having a reaction film on the surface as shown in FIG. At the time of detection, the substance to be analyzed is attracted to the reaction film side as shown in FIG. 3B, and is adsorbed on the surface of the crystal oscillator as shown in FIG. As described above, when the substance to be analyzed is adsorbed, a sensor mutation (a change in mass or stress of the piezoelectric element) occurs, which affects the resonance characteristics of the piezoelectric element, and the oscillation frequency changes before and after the sensor mutation. .

  The observer can estimate the amount of sensor variation by measuring the amount of frequency change associated with the sensor variation with a frequency counter. In this case, it is assumed that the frequency sensitivity of the oscillator is known.

JP 2008-102118 A

However, in the technique described in Patent Document 1, since the frequency change amount is measured by the frequency counter, it becomes difficult to measure the sensor variation amount with higher accuracy as the frequency becomes higher.
Therefore, an object of the present invention is to provide a sensor circuit that can perform highly accurate measurement without increasing the circuit scale.

  In order to achieve the above object, a first form of a sensor circuit according to the present invention includes a piezoelectric element as a sensor element, and has an oscillator in which the frequency of an oscillation signal is controlled by a control signal. A PLL circuit that feeds back the control signal according to a phase difference between a signal and a reference signal to the oscillator, and an output unit that outputs a variation amount of the sensor element based on a temporal change of the control signal. It is characterized by that.

In this way, by configuring the PLL with an oscillator using a piezoelectric element serving as a sensor element, it is possible to detect an oscillation frequency change accompanying a sensor variation by detecting a control signal change (voltage change) fed back to the oscillator. . Feedback is applied so that the oscillation frequency is always constant, and it is not necessary to measure the amount of frequency change as in the conventional method, so that highly accurate measurement is possible.
Furthermore, it can be realized with a relatively simple circuit such as a circuit that constitutes a PLL or a circuit that detects a temporal change in a control signal, and can be easily integrated on a semiconductor substrate, thereby reducing the circuit scale. Can be planned.

  In the second embodiment, the output unit converts an analog control signal into a digital signal and outputs it at regular time intervals, and a non-volatile memory that sequentially stores output values from the AD converter And an arithmetic circuit for calculating a difference between the current output value from the AD converter and the previous output value from the AD converter stored in the nonvolatile memory, and a difference calculated by the arithmetic circuit, And a signal processing circuit that calculates a frequency change amount of the oscillation signal based on the sensitivity of the oscillator and calculates a variation amount of the sensor element based on the frequency change amount.

In this way, it is possible to detect a temporal change in the control signal fed back to the oscillator in a relatively simple manner. At this time, if the difference value from the previous value is always stored in the nonvolatile memory as the output value from the AD converter, the memory capacity can be reduced. Further, in this case, the temporal change of the sensor can be reproduced by integrating the memory numerical values.
Furthermore, using the fact that there is a relationship of Δf = Kvco · ΔV (where Kvco is the frequency sensitivity) between the frequency change amount Δf of the oscillator and the change amount (voltage change amount) ΔV of the control signal, Based on the temporal change of the control signal and the sensitivity Kvco of the oscillator, the frequency change amount of the oscillator can be obtained with high accuracy by calculation. Therefore, the sensor variation amount can be easily and appropriately estimated based on the frequency change amount thus obtained.

The third mode is characterized in that a low-pass filter is provided between the PLL circuit and the output unit, and the control signal is input to the output unit via the low-pass filter. .
Thereby, the noise component can be effectively removed, the temporal change of the control signal can be appropriately detected, and the sensor variation amount can be measured with high accuracy.

Furthermore, the fourth embodiment is characterized in that the PLL circuit excluding the piezoelectric element and the output unit are integrated on the same semiconductor substrate.
As described above, since the sensor detection circuit can be integrated on the same semiconductor substrate, the circuit scale can be reduced and the sensor detection circuit can be easily mounted. In particular, mounting suitable for the application is facilitated by mounting the piezoelectric element exposed to the external environment and mounting the sensor detection circuit other than the piezoelectric element from the external environment.

The fifth embodiment is characterized in that the PLL circuit excluding the piezoelectric element and the output unit are integrated on the same semiconductor substrate, and the semiconductor substrate and the piezoelectric element are mounted in the same package.
Thus, since the piezoelectric element and the sensor detection circuit are mounted in the same package, the operating environment can be stabilized. Further, it is not necessary to separately mount the piezoelectric element and the sensor detection circuit at the time of assembly, and the sensor circuit can be easily assembled.

The sixth embodiment is characterized in that the PLL circuit including the piezoelectric element and the output unit are integrated on the same semiconductor substrate.
As a result, the circuit scale can be reduced and the sensor circuit can be easily assembled.

  According to the present invention, a PLL is constituted by an oscillator using a piezoelectric element serving as a sensor element, and the amount of variation of the sensor element is measured based on the temporal change of the control signal fed back to the oscillator. High-precision measurement can be performed without increasing.

It is a block diagram which shows the structure of the sensor circuit in embodiment of this invention. It is a figure which shows the structure of an oscillator. It is a conceptual diagram of QCM.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of a sensor circuit in an embodiment of the present invention.
In the figure, reference numeral 1 denotes a sensor circuit. The sensor circuit 1 includes a PLL circuit (phase lock loop circuit) 10 including an oscillator 11 including a piezoelectric element serving as a sensor element, an output unit 20 that outputs a sensor variation, and a PLL circuit 10 and an output unit 20. And a low-pass filter 30 installed therebetween.

As shown in FIG. 2A, the oscillator 11 includes a piezoelectric element X that is a vibration element. The piezoelectric element X is made of, for example, crystal, SAW, BAW or the like, and is connected to an oscillation circuit composed of a negative gm having an amplifier circuit Amp, a resistor R, and capacitors CL1 and CL2.
Specifically, one terminal of the capacitor CL1 is connected to one terminal of the piezoelectric element X, and the other terminal of the capacitor CL1 is grounded. Further, one terminal of the capacitor CL2 is connected to the other terminal of the piezoelectric element X, and the other terminal of the capacitor CL2 is grounded. Furthermore, one terminal of the amplifier circuit Amp and the resistor R is connected to one terminal of the piezoelectric element X, and the other terminal of the amplifier circuit Amp and the resistor R is connected to the other terminal of the piezoelectric element X. To do.

This oscillation circuit is an oscillation circuit with a varicap input, and the capacitances of the capacitors CL1 and CL2 can be controlled by the control voltage Vc. The oscillator 11 having the configuration shown in FIG. 2A is represented by a circuit symbol shown in FIG. That is, in FIG. 1, the oscillator 11 has an oscillation frequency that varies depending on the control voltage Vlp.
The PLL circuit 10 includes the above-described oscillator 11, frequency divider (1 / N) 12, reference frequency generation circuit 13, frequency phase comparator 14, charge pump circuit 15, and loop filter 16.

The frequency divider 12 includes a prescaler and a divider, and outputs a frequency-divided signal (frequency fs) in which the frequency (oscillation frequency fvco) of the oscillation signal output from the oscillator 11 is 1 / N. Here, N is a frequency division ratio.
The reference frequency generation circuit 13 is constituted by TCXO, OCXO, or the like, and outputs a highly accurate reference signal (reference frequency fr). The frequency phase comparator 14 compares the phase of the frequency-divided signal output from the frequency divider 12 with the reference signal output from the reference frequency generation circuit 13 at a constant period, and generates these phase comparison signals Δθ. To the charge pump circuit 15.

The charge pump circuit 15 outputs a current Iout corresponding to the input phase comparison signal Δθ. The output current Iout is a charge / discharge current that flows in and out of the loop filter 16 by charging or discharging the charge current.
The loop filter 16 smoothes the current Iout from the charge pump circuit 15 and outputs a control voltage Vlp. This control voltage Vlp is applied to the oscillator 11.

Next, the circuit operation of the PLL circuit 10 configured as described above will be analytically described.
The oscillation frequency fvco of the oscillation signal output from the oscillator 11 is expressed by the following equation based on the frequency sensitivity Kvco of the oscillator 11 and the control voltage Vlp.
fvco = Kvco · Vlp (1)
The oscillation signal having the oscillation frequency fvco output from the oscillator 11 is converted to a signal having a frequency of 1 / N by the frequency divider 12. That is, the frequency fs of the frequency-divided signal output from the frequency divider 12 is expressed by the following equation.
fs = fvco / N (2)

Next, the phase difference Δθ between the reference signal (reference frequency fr) and the divided signal (frequency fs) is detected by the frequency phase comparator 14. At this time, if the divided signal fs is delayed with respect to the reference signal, an up signal for increasing the input voltage to the oscillator 11 is output from the frequency phase comparator 14 to the charge pump circuit 15, and the divided signal fs is If it is advanced with respect to the reference signal, the frequency phase comparator 14 outputs a down signal for lowering the input voltage to the oscillator 11 to the charge pump circuit 15. As a result, a current Iout corresponding to the phase difference Δθ is output from the charge pump circuit 15.
Iout = Ip · Δθ / (2 · π) (3)
Here, Ip is a charge pump current.

The output current Iout of the charge pump circuit 15 is smoothed by the loop filter 16 and output as the control voltage Vlp. At this time, if the transfer function of the loop filter 16 is Hlp, the control voltage Vlp is expressed by the following equation.
Vlp = Hlp · Iout (4)
Then, by inputting the control voltage Vlp to the oscillator 11, an oscillation signal having a frequency corresponding to the control voltage Vlp is output from the oscillator 11. By repeating this, a phase shift between the reference signal and the frequency-divided signal is corrected, and the phases of both signals coincide with each other to enter a locked state. Thus, in the PLL circuit 10, feedback is applied so that the oscillation frequency fvco always matches the reference frequency fr.

  When sensor variation occurs due to environmental changes such as pressure, mass, and temperature, and the oscillation frequency fvco changes, the frequency fs of the frequency-divided signal changes according to the above equation (2), and the phase relationship with the reference signal changes. This change causes a change in the output current Iout of the charge pump circuit 15 by the above equation (3), and finally appears as a change in the control voltage Vlp by the above equation (4).

That is, since negative feedback is applied to the PLL circuit, frequency changes caused by environmental changes are suppressed and appear as changes in the DC voltage that is the output of the loop filter 16. Therefore, the sensor variation can be detected by the temporal voltage difference of the control voltage Vlp fed back to the oscillator 11.
The control voltage Vlp fed back to the oscillator 11 is also input to the output unit 20 via the low-pass filter 30.

The output unit 20 outputs a sensor variation amount based on a temporal change in the control voltage Vlp (sensor output) that has passed through the low-pass filter 30, and includes an AD converter (ADC) 21 and an arithmetic circuit ( ALU) 22, a non-volatile memory 23, and a signal processing circuit (DSP) 24.
The ADC 21 converts an analog sensor output input at regular intervals into a digital signal and outputs the digital signal to the ALU 22.

The ALU 22 receives a digital signal (ADC output) output from the ADC 21 at regular intervals. The ALU 22 calculates a difference between the current ADC output and the previous ADC output, stores the difference in the nonvolatile memory 23, and outputs the difference to the DSP 24.
In addition to the ADC output difference calculated by the ALU 22, the nonvolatile memory 23 stores the initial value of the ADC output (ADC output when the sensor is clean) and the frequency sensitivity Kvco of the oscillator 11. That is, when the ADC output is input from the ADC 21, the ALU 22 first reads the difference between the initial value of the ADC output and the ADC output stored so far from the nonvolatile memory 23, and sets the ADC output to the initial value of the ADC output. By integrating the difference, the previous ADC output is calculated. Then, the difference between the calculated previous ADC output and the currently input ADC output is calculated and stored in the nonvolatile memory 23 and output to the DSP 24.

Thus, the storage capacity of the nonvolatile memory 23 can be reduced by storing the difference from the previous ADC output instead of storing the ADC output in the nonvolatile memory 23 as it is. Further, as described above, by integrating the numerical values stored in the nonvolatile memory 23, it is possible to easily reproduce the temporal change in the sensor output.
The DSP 24 calculates the frequency change amount of the oscillation signal from the oscillator 11 based on the difference between the ADC outputs input from the ALU 22 and the oscillator sensitivity Kvco. Based on the calculated frequency change amount, sensor variations such as mass change and pressure (stress) change are estimated.

Assuming that the ADC output at an arbitrary time (n-1) is ADC (n-1) and the ADC output at the time (n) is ADC (n), the relationship between the ADC output and the frequency fosc is as follows. Is established.
fosc (n−1) = Kvco · ADC (n−1) (5)
fosc (n) = Kvco · ADC (n) (6)
Therefore, the frequency change amount Δfosc (n) is calculated from the above equations (5) and (6) as follows.
Δfosc = fosc (n) −fosc (n−1)
= Kvco · (ADC (n) −ADC (n−1))
= Kvco · ΔADC (n) (7)
Here, the voltage difference ΔADC (n) can be easily calculated by the ALU 22 as described above.

Thus, by storing the ADC output difference ΔADC and the frequency sensitivity Kvco in the nonvolatile memory 23, the frequency change amount Δfosc can be easily calculated. Then, the sensor variation amount can be easily estimated from the calculated frequency change amount Δfosc by using, for example, the Sauerbrey equation.
By the way, as a sensor circuit using a piezoelectric element as a sensor, there is a sensor circuit that estimates a sensor variation amount by measuring a frequency change amount of an oscillator accompanying a sensor variation with a frequency counter. However, in this case, since it is necessary to measure the frequency with a frequency counter, it becomes more difficult to measure with higher accuracy as the frequency becomes higher.

  On the other hand, in the present embodiment, a PLL is constituted by the oscillator 11 with a varicap input, so that a change in the frequency of the oscillator 11 due to the sensor variation is detected by the difference in the control signal Vlp fed back to the oscillator 11. be able to. Therefore, the frequency change is not directly detected as in the above method, and the sensor variation amount can be estimated with high accuracy even at a high frequency.

Furthermore, since the above-described operation can be realized with a relatively simple circuit such as a circuit constituting a PLL or a circuit that detects a temporal change in the control signal Vlp, the circuit scale is not significantly increased.
In the present embodiment, it is easy to integrate the sensor detection circuit on the same semiconductor substrate. Here, the PLL circuit 10 excluding the piezoelectric element X and the output unit 20 are integrated on the same semiconductor substrate and mounted on the same package as the piezoelectric element X.

Thus, by integrating the sensor detection circuit on the same semiconductor substrate, the circuit scale can be reduced. In addition, mounting suitable for the application can be facilitated by mounting the piezoelectric element X exposed to the external environment and mounting the sensor detection circuit other than the piezoelectric element X from the external environment.
Furthermore, since the piezoelectric element and the sensor detection circuit are mounted in the same package, the operating environment can be stabilized. Further, it is not necessary to separately mount the piezoelectric element and the sensor detection circuit at the time of assembly, and the sensor circuit can be easily assembled. Furthermore, by providing the variable capacitors CL1 and CL2 on the same semiconductor substrate, the oscillation frequency of the oscillator 11 can be easily adjusted.

Thus, a sensor circuit using a piezoelectric element can be constructed on a semiconductor substrate without increasing the circuit scale or the number of parts.
Here, the case where the PLL circuit 10 excluding the piezoelectric element X and the output unit 20 are integrated on the semiconductor substrate and mounted in the same package as the piezoelectric element X has been described. However, the PLL circuit including the piezoelectric element X is described. 10 and the output unit 20 can be integrated on the same semiconductor substrate.

  Also in this case, the circuit scale can be reduced, and the sensor circuit can be easily assembled.

  DESCRIPTION OF SYMBOLS 1 ... Sensor circuit, 10 ... PLL circuit, 11 ... Oscillator, 12 ... 1 / N frequency divider, 13 ... Reference frequency generation circuit, 14 ... Frequency phase comparator, 15 ... Charge pump circuit, 16 ... Loop filter, 20 ... Output unit, 21 ... AD converter (ADC), 22 ... arithmetic circuit (ALU), 23 ... non-volatile memory, 24 ... signal processing circuit (DSP), 30 ... low-pass filter, Amp ... amplifier circuit, CL1, CL2 ... capacitor, R ... resistor, X ... piezoelectric element

Claims (6)

A PLL having a piezoelectric element as a sensor element and having an oscillator in which the frequency of an oscillation signal is controlled by a control signal, and feeding back the control signal according to the phase difference between the oscillation signal and a reference signal to the oscillator Circuit,
An output unit that outputs a variation amount of the sensor element based on a temporal change of the control signal. The output unit is
An AD converter for converting the analog control signal into a digital signal and outputting it at regular intervals;
A non-volatile memory for sequentially storing output values from the AD converter;
An arithmetic circuit for calculating a difference between an output value from the AD converter this time and an output value from the previous AD converter stored in the nonvolatile memory;
Based on the difference calculated by the arithmetic circuit and the sensitivity of the oscillator, a frequency change amount of the oscillation signal is calculated, and based on the frequency change amount, a signal processing circuit that calculates a variation amount of the sensor element;
The sensor circuit according to claim 1, comprising: A low-pass filter is provided between the PLL circuit and the output unit,
The sensor circuit according to claim 1, wherein the control signal is input to the output unit via the low-pass filter.   The sensor circuit according to claim 1, wherein the PLL circuit excluding the piezoelectric element and the output unit are integrated on the same semiconductor substrate.   The sensor circuit according to claim 4, wherein the PLL circuit excluding the piezoelectric element and the output unit are integrated on the same semiconductor substrate, and the semiconductor substrate and the piezoelectric element are mounted in the same package.   The sensor circuit according to claim 1, wherein the PLL circuit including the piezoelectric element and the output unit are integrated on the same semiconductor substrate.

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