JP2012013578A - Infrared detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an infrared detector for improving the SN ratio of a current/voltage conversion part while suppressing unnecessary influence of low frequency components to the output of the current/voltage conversion part.SOLUTION: A current/voltage conversion part 3 has a first arithmetic amplifier 31 in which a pyroelectric element 2 is connected to an inverted input port. Between an output port and the inverted input port of the arithmetic amplifier 31, a capacitor C1 as a capacitance element for AC feedback is connected. Between the output port and the inverted input port of the arithmetic amplifier 31, a reset switch 33 is connected in parallel to the capacitor C1. The reset switch 33 is on-off controlled by a reset signal from a control section 6 and, when the reset switch is on, functions as a discharging part forming a discharge path for discharging the electric charge accumulated in the capacitor. The control section 6 has an oscillator generating a clock signal at the intervals so predetermined as to remove low frequency components equal to or lower than a predetermined frequency, and outputs a reset signal on the basis of the clock signal.

Description

  The present invention relates to an infrared detector using a pyroelectric element.

  2. Description of the Related Art In recent years, various electric devices that detect the movement of a human body and perform an efficient operation have been proposed for the purpose of saving energy. For example, such an electric device incorporates an infrared detector using a pyroelectric element as an infrared detector. A general infrared detection device collects infrared rays from the detection area using a lens or the like into a pyroelectric element, and a current signal output from the pyroelectric element in response to a change in the amount of infrared light received by the pyroelectric element. Changes.

  As this type of infrared detection device 1, for example, as shown in FIG. 10, a pyroelectric element 2, a current-voltage converter 3 that converts a current signal output from the pyroelectric element 2 into a voltage signal, and a current-voltage converter 3 There is known an apparatus including a voltage amplifying unit 300 that amplifies the output. In this infrared detector 1, the current signal output from the pyroelectric element 2 is converted into a voltage signal by the current-voltage converter 3, then amplified by the voltage amplifier 300, and input to a downstream detection circuit (not shown). Is done. In the configuration of FIG. 10, the voltage amplifying unit 300 has a function as a band-pass filter having a pass band in the frequency band (for example, about 0.1 Hz to 10 Hz) of a current signal generated during human body detection.

  By the way, the current signal output from the pyroelectric element 2 may include an unnecessary low-frequency component that is not related to the detection target (human body) due to, for example, a change in ambient temperature. Therefore, as the infrared detector 1, a DC feedback circuit 200 is connected between the output terminal and the input terminal of the current / voltage converter 3 in order to suppress the influence of unnecessary low frequency components on the output of the current / voltage converter 3. An apparatus having the above configuration has been proposed (see, for example, Patent Document 1).

  In the infrared detection device 1 described in Patent Document 1, the current-voltage conversion unit 3 includes a first operational amplifier 31 having the inverting input terminal connected to the pyroelectric element 2. Between the output terminal and the inverting input terminal of the first operational amplifier 31, a capacitor C1 as a capacitive element for AC feedback is connected. A reference power source 202 that generates a reference voltage is connected to the non-inverting input terminal of the first operational amplifier 31.

  The DC feedback circuit 200 includes an integrating circuit in which a capacitor C200 and a resistor R200 are added to the operational amplifier 201, and the output terminal of the first operational amplifier 31 is connected to the non-inverting input terminal of the operational amplifier 201. . The capacitor C200 is connected between the output terminal and the inverting input terminal of the operational amplifier 201. A reference power source 202 that generates a reference voltage is connected to the inverting input terminal of the operational amplifier 201 via a resistor R200. The output terminal of the operational amplifier 201 is connected to the inverting input terminal of the first operational amplifier 31 via the input resistor R201.

  The infrared detection device 1 having the above-described configuration causes unnecessary low frequency components to flow through the input resistor R201 in accordance with the output of the operational amplifier 201, so that an unnecessary low frequency to the voltage signal output from the current-voltage converter 3 is obtained. The influence of a component can be suppressed.

Japanese Patent No. 3472906 (paragraphs 0037 to 0044, FIG. 9)

  However, in the infrared detection device 1 having the above-described configuration, since the input resistor R201 is connected to the input terminal of the current-voltage conversion unit 3, noise components generated at the input resistance R201 are input to the current-voltage conversion unit 3. This leads to a decrease in the SN ratio of the current-voltage converter 3. In particular, in order to reduce the cut-off frequency (for example, less than 0.1 Hz) or suppress the thermal noise of the input resistor R201, the input resistor R201 has a high resistance value of, for example, T (Tera) Ω order. There is a need.

  However, from the viewpoint of miniaturization of the infrared detecting device 1, the input resistor R201 is usually composed of a resistor element built in an IC (integrated circuit). And the variation of the resistance value becomes large. When the resistance value of the input resistor R201 varies and the resistance value decreases, the thermal noise of the input resistor R201 increases. As a result, the noise component increases and the SN ratio of the current-voltage conversion unit 3 decreases.

  The present invention has been made in view of the above-described reason, and an infrared detection device capable of improving the SN ratio of the current-voltage conversion unit while suppressing the influence of unnecessary low-frequency components on the output of the current-voltage conversion unit. The purpose is to provide.

  The infrared detection device of the present invention includes a pyroelectric element and a current-voltage conversion unit that converts a current signal output from the pyroelectric element into a voltage signal using an operational amplifier to which a capacitive element for feedback is connected. The current-voltage converter has a discharge part that forms a discharge path for discharging the charge accumulated in the capacitor element, and the output of the current-voltage converter by controlling the discharge part And a controller that resets the capacitive element at a timing such that a low-frequency component equal to or lower than a predetermined frequency is removed.

  In this infrared detection apparatus, the control unit includes an oscillator that generates a clock signal at a predetermined cycle so as to remove the low-frequency component, and generates a reset signal based on the clock signal. It is desirable that the capacitive element is reset by the reset signal.

  In this infrared detection apparatus, the infrared detector further includes a filter unit that allows a signal component of a predetermined frequency band to pass through the output of the current-voltage conversion unit, and the control unit receives the output of the filter unit as a feedback signal, More preferably, after the clock signal is generated, the reset signal is generated at the first zero cross point of the feedback signal to reset the capacitive element.

  In this infrared detection apparatus, the control unit, when the zero-cross point of the feedback signal does not occur for a predetermined extension time after the generation of the clock signal, when the extension time has elapsed from the generation of the clock signal. More preferably, the reset signal is generated to reset the capacitive element.

  In the infrared detection device, the control unit is configured to perform an abnormality that resets the capacitive element when an absolute value of an output value of the current-voltage conversion unit exceeds a predetermined threshold continuously for a predetermined allowable time or more. It is more desirable to have a protective part.

  In this infrared detection device, the absolute value of the value corresponding to the output value of the current-voltage converter is compared with a predetermined first threshold value, and detection is performed based on whether the absolute value exceeds the first threshold value. A determination unit configured to determine whether or not there is a target, wherein the control unit is configured to be configured in a first operation mode capable of resetting the capacitive element when the absolute value is within a second threshold value that is lower than the first threshold value; It is more desirable to operate in the second operation mode in which the capacitor element is not reset when the absolute value exceeds the second threshold value.

  The infrared detection apparatus further includes an AD conversion unit that converts an output value of the current-voltage conversion unit into a digital value, and a digital processing unit that passes a signal component in a predetermined frequency band out of the output of the AD conversion unit. It is more desirable.

  In the infrared detection device, the control unit may set a third threshold value, in which the absolute value of the input of the AD conversion unit is set to be equal to or less than an upper limit value of a corresponding range that can be converted into a digital value by the AD conversion unit. If it exceeds, it is more desirable to have an AD conversion protection unit that resets the capacitive element.

  In this infrared detection apparatus, it is more desirable that the AD conversion unit is a ΔΣ method.

  In this infrared detection apparatus, it is more preferable that the digital processing unit outputs a digital value in a serial manner.

  In this infrared detection apparatus, it is more preferable that the digital processing unit uses a BMC encoding method in which an output is inverted for each cell as the serial method.

  In this infrared detection device, the current-voltage converter includes the first operational amplifier connected to one end of the pyroelectric element and the second operational amplifier connected to the other end of the pyroelectric element. More preferably, the amplifier has an amplifier and outputs a signal corresponding to the difference between the output voltage of the first operational amplifier and the output voltage of the second operational amplifier as the voltage signal.

  In this infrared detection apparatus, the control unit is configured so that an output voltage of at least one of the first operational amplifier and the second operational amplifier is determined by a predetermined fourth threshold and a predetermined fifth threshold. It is more desirable to have an abnormal voltage protection unit that resets the capacitive element when out of range.

  The present invention has an advantage that the SN ratio of the current-voltage converter can be improved while suppressing the influence of unnecessary low-frequency components on the output of the current-voltage converter.

It is a schematic block diagram which shows the structure of the infrared rays detection apparatus of Embodiment 1. It is a time chart which shows operation | movement same as the above. It is a time chart which shows operation | movement same as the above. It is a time chart which shows other operation | movement same as the above. It is a time chart which shows operation | movement same as the above. It is a time chart which shows operation | movement same as the above. It is a time chart which shows operation | movement same as the above. It is explanatory drawing which shows the output format of a digital processing part same as the above. It is a schematic circuit diagram which shows the structure of the infrared rays detection apparatus of Embodiment 2. It is a schematic circuit diagram which shows the structure of the conventional infrared rays detection apparatus.

(Embodiment 1)
As shown in FIG. 1, the infrared detection device 1 of the present embodiment includes a pyroelectric element 2, a current-voltage conversion unit 3, an AD conversion unit 4, a digital processing unit 5, and a control unit 6. . In the present embodiment, the infrared detection device 1 used for human body detection in the detection area will be described as an example. However, this is not intended to prevent the infrared detection device 1 from being used for purposes other than human body detection such as gas detection.

  The pyroelectric element 2 receives infrared rays from the detection area and outputs a current signal according to a change in the amount of received infrared rays.

  The current-voltage conversion unit 3 is basically the same as the current-voltage conversion unit 3 of FIG. 10 described in the background art section, and is a first operational amplifier in which the pyroelectric element 2 is connected to the inverting input terminal. 31. A capacitor C1 as a capacitive element for AC feedback is connected between the output terminal and the inverting input terminal of the operational amplifier 31. A reference power supply 32 that generates a reference voltage is connected to the non-inverting input terminal of the operational amplifier 31.

  According to the capacity-conversion-type current-voltage conversion unit 3 configured as described above, the current signal from the pyroelectric element 2 is converted into a voltage signal using the impedance of the capacitor C1. Therefore, the voltage output from the operational amplifier 31 is a value obtained by subtracting the voltage across the capacitor C1 from the reference voltage generated by the reference power supply 32. In short, the output of the current-voltage converter 3 changes from the operating point according to the change of the current signal caused by the pyroelectric element 2 receiving infrared rays, with the reference voltage as the operating point.

  In the following description, for simplicity of explanation, the output of the current-voltage conversion unit 3 at the operating point (reference voltage) is assumed to be zero. That is, the output of the current-voltage conversion unit 3 means the amount of change from the operating point of the voltage output from the operational amplifier 31.

  Here, a reset switch 33 is connected between the output terminal and the inverting input terminal of the operational amplifier 31 in parallel with the capacitor C1. The reset switch 33 is ON / OFF controlled by a reset signal from the control unit 6, and functions as a discharge unit that forms a discharge path for discharging the charge accumulated in the capacitor C <b> 1 when ON. That is, when the reset switch 33 is turned on, the voltage across the capacitor C1 is reset to zero, and the output value of the current-voltage converter 3 is reset to zero (operating point).

  The AD conversion unit 4 converts the voltage value (analog value) input from the current-voltage conversion unit 3 into a digital value and outputs the digital value to the digital processing unit 5. In the AD converter 4, a corresponding range (here, −Vth0 to Vth0) representing a range of analog values that can be converted into a digital value is set in advance according to the magnitude of a reference voltage given from the outside. The AD converter 4 saturates when an analog signal having an amplitude outside the corresponding range is input.

  The digital processing unit 5 determines the presence / absence of a human body in the detection area based on the digital signal input from the AD conversion unit 4. That is, the digital processing unit 5 compares the output value of the AD conversion unit 4 (corresponding to the output of the current-voltage conversion unit 3) with a predetermined first threshold value, thereby comparing the human body in the detection area. It has the determination part (not shown) which determines presence or absence. The determination unit determines that there is a person in the detection area and outputs an H level detection signal during a period in which the absolute value of the output value of the AD conversion unit exceeds the first threshold value. If there is no person in the detection area, the detection signal is set to L level.

  Further, the digital processing unit 5 is a digital bandpass filter (hereinafter referred to as a bandpass filter) having a passband that is a frequency band (here, about 0.1 Hz to 10 Hz) of a current signal generated by the pyroelectric element 2 during human body detection. Is called “BPF”).

  Here, in the case of using an analog BPF as in the infrared detecting device 1 of FIG. 10 described in the background art section, a capacitor having a relatively large circuit constant is required to pass a signal of about 0.1 Hz to 10 Hz. Etc. are required. Since such an element is externally attached to an IC (integrated circuit), the circuit portion of the infrared detecting device 1 cannot be formed into a single chip with this configuration. On the other hand, the infrared detection device 1 of the present embodiment has an advantage that the circuit part can be made into one chip because an external component is not required by using the digital BPF as described above.

  In the infrared detection device 1 having the configuration described above, the current signal output from the pyroelectric element 2 is converted into a voltage signal by the current-voltage conversion unit 3, and then converted into a digital value by the AD conversion unit 4. Input to the processing unit 5. The digital processing unit 5 determines the presence / absence of a human body in the detection area based on the input digital value, and outputs the determination result to a subsequent microcomputer (not shown) or the like.

  By the way, in the infrared detection device 1 of the present embodiment, the control unit 6 appropriately turns on the reset switch 33 to reset the capacitor C1, so that an unnecessary low-frequency component (not more than a predetermined frequency) from the output of the current-voltage conversion unit 3 ( Hereinafter, the “unnecessary component”) is removed. The unnecessary component is a low-frequency fluctuation component that occurs regardless of the detection target (human body) due to, for example, a change in ambient temperature with respect to the current signal output from the pyroelectric element 2.

  That is, the control unit 6 outputs a reset signal at a timing such that unnecessary components are removed from the output of the current-voltage conversion unit 3, and turns on the reset switch 33 to reset the voltage across the capacitor C1. More specifically, the control unit 6 has an oscillator (not shown) that generates a clock signal at a predetermined period so as to remove unnecessary components, and generates a reset signal based on the clock signal. To do.

  The period for generating the clock signal is determined by the frequency that is the upper limit of the unnecessary component. Here, as an example, it is assumed that the unnecessary component is a low frequency component of 0.1 Hz or less from the relationship with the frequency band (about 0.1 Hz to 10 Hz) of the current signal generated by the pyroelectric element 2 during human body detection.

  That is, if the upper limit frequency of the unnecessary component is 0.1 Hz, the time of 10 seconds corresponding to this frequency is a cycle for generating the clock signal. If the control unit 6 resets the capacitor C <b> 1 with the period determined in this way, unnecessary components are removed from the output of the current-voltage conversion unit 3. In short, the control unit 6 and the current-voltage conversion unit 3 constitute a high-pass filter having an upper limit frequency of unnecessary components as a cutoff frequency.

  In the present embodiment, the output of the digital processing unit 5 is input to the control unit 6 as a feedback signal. Since the digital processing unit 5 has a function as a digital BPF (filter unit) having a pass band of 0.1 Hz to 10 Hz as described above, at least unnecessary components are removed from the output of the current-voltage conversion unit 3 in the feedback signal. Signal.

  As shown in FIG. 2, the controller 6 outputs a reset signal at the first zero-cross point of the feedback signal after the generation of the clock signal generated at a cycle of 10 seconds, and turns on the reset switch 33 to reset the capacitor C1. . The zero-cross point of the feedback signal here means a time point when the output of the digital processing unit 5 becomes zero (operating point). In FIG. 2, (a) is a current signal input to the current-voltage conversion unit 3, (b) is a voltage signal output from the current-voltage conversion unit 3, and (c) is an output (feedback signal) of the digital processing unit 5. , (D) represents a reset signal. In FIG. 2C and the drawings described below, the output value of the digital processing unit 5 that is actually a digital value is shown as an analog value.

  For example, as shown in FIG. 2, when a current signal consisting only of an unnecessary component of 0.1 Hz or less is input to the current-voltage conversion unit 3, the digital processing unit 5 removes the unnecessary component as shown in FIG. The feedback signal is output to the control unit 6. Here, since the feedback signal includes a noise component, the feedback signal has fluctuation as shown in FIG. Noise components included in the feedback signal mainly include circuit noise generated in the current-voltage conversion unit 3 before AD conversion and noise generated in the digital processing unit 5 after AD conversion. The control unit 6 outputs a reset signal at the zero cross point of the feedback signal (that is, the point at which the noise component becomes zero) after the generation of the clock signal generated at a cycle of 10 seconds to reset the capacitor C1.

  Therefore, the control unit 6 resets the capacitor C1 when the output of the digital processing unit 5 becomes zero, and the fluctuation that may occur in the output of the digital processing unit 5 due to the potential difference at the time of reset can be suppressed. .

  In addition, as shown in FIG. 3, the control unit 6 resets when the extended time elapses from the generation of the clock signal if the zero cross point of the feedback signal does not occur for a predetermined extended time after the generation of the clock signal. The switch 33 is turned on to reset the capacitor C1. In FIG. 3, (a) represents the output (feedback signal) of the digital processing unit 5, and (b) represents the reset signal.

  Here, the extension time is a time shorter than at least the period in which the oscillator generates the clock signal, and when the period is 10 seconds, for example, the extension time is set to 3 seconds. The extended time may be set to a time (for example, 20 seconds) longer than the cycle for generating the clock signal.

  According to this configuration, even if the output of the digital processing unit 5 does not become zero even if the extended time elapses after the generation of the clock signal due to some abnormality occurring in the infrared detection device 1, the capacitor C1 is reset and is unnecessary. The component will be removed.

  Furthermore, as another example of the present embodiment, the control unit 6 may be configured to output a reset signal at the timing when the oscillator outputs a clock signal and reset the capacitor C1, as shown in FIG. Good. In FIG. 4, (a) represents the output (feedback signal) of the digital processing unit 5, and (b) represents the reset signal. In this case, the control unit 6 only needs to generate a clock signal regardless of the output of the digital processing unit 5, so that a feedback signal from the digital processing unit 5 to the control unit 6 becomes unnecessary, and the circuit configuration is simplified. Can do.

  Incidentally, the control unit 6 outputs a reset signal to reset the capacitor C1 when the absolute value of the output value of the current-voltage conversion unit 3 exceeds the first threshold continuously for a predetermined allowable time or longer. An abnormality protection unit (not shown) is provided.

  In short, if the infrared detection device 1 is normal, the absolute value of the output value of the digital processing unit 5 periodically exceeds the first threshold value Vth1 as shown in FIG. As shown in FIG. 5B, the H level and the L level are repeated. On the other hand, if there is an abnormality in the infrared detection device 1, the detection signal may maintain the H level as shown in FIG. Therefore, when the permissible time elapses while the detection signal is maintained at the H level as shown in FIG. 5C, the control unit 6 outputs a reset signal and resets the capacitor C1 as shown in FIG. 5D.

  According to this configuration, when an abnormality occurs in the operation of the infrared detection device 1 such that the detection signal becomes H level for an allowable time or longer, the control unit 6 determines that the abnormality is abnormal and resets the capacitor C1. Therefore, it becomes possible to perform human body detection normally thereafter.

  The control unit 6 resets when the absolute value of the input of the AD conversion unit 4 exceeds a third threshold set before the corresponding range that can be converted into a digital value by the AD conversion unit 4. An AD conversion protection unit (not shown) that outputs a signal and resets the capacitor C1 is provided.

  That is, as shown in FIG. 6, the third threshold Vth3 that is the upper limit is set at a position slightly lower than the upper limit Vth0 of the corresponding range, and the lower limit of the corresponding range is −the third threshold that is the lower limit at a position slightly higher than Vth0−. Vth3 is set. In FIG. 6, (a) represents the output value of the current-voltage converter 3, and (b) represents the reset signal. The control unit 6 monitors the output value of the current-voltage conversion unit 3 (the input value of the AD conversion unit 4), and when this value reaches the third threshold value Vth3 (or -Vth3), the threshold values Vth3 and -Vth3 are set. It is judged that it has exceeded, a reset signal is output, and the capacitor C1 is reset.

  According to this configuration, the control unit 6 resets the capacitor C1 when the input of the AD conversion unit 4 is likely to be out of the corresponding range, whereby the output value of the current-voltage conversion unit 3 that is the input of the AD conversion unit 4 is reset. To zero (operating point). Therefore, the input of the AD conversion unit 4 does not deviate from the corresponding range, and the infrared detection device 1 can avoid the saturation of the output of the AD conversion unit 4 and can always maintain a human body detectable state. The third threshold values Vth3 and -Vth3 may be equal to the upper limit Vth0 and the lower limit -Vth0 of the corresponding range.

  The control unit 6 operates in the first operation mode when the absolute value of the output value of the current-voltage conversion unit 3 is within a second threshold that is lower than the first threshold for human body detection. When the threshold value is exceeded, the operation is performed in the second operation mode. The control unit 6 can output a reset signal in the first operation mode, but does not output a reset signal in the second operation mode.

  Specifically, as shown in FIG. 7A, the control unit 6 sets the output (feedback signal) of the digital processing unit 5 corresponding to the output value of the current-voltage conversion unit 3 to the second threshold values −Vth2 and Vth2. Compare. When the output of the digital processing unit 5 goes out of the range (-Vth2 to Vth2) defined by the second threshold values -Vth2 and Vth2, the control unit 6 starts from the first operation mode as shown in FIG. A second mode switching signal for switching to the second operation mode is output. On the other hand, when the output of the digital processing unit 5 returns to the range (-Vth2 to Vth2) defined by the second threshold values -Vth2 and Vth2, the control unit 6 performs the second operation as shown in FIG. A first mode switching signal for switching from the mode to the first operation mode is output.

  In FIG. 7, (a) is the output (feedback signal) of the digital processing unit 5, (b) is the reset signal, (c) is the second mode switching signal, (d) is the first mode switching signal, (e ) Represents a detection signal.

  In the first operation mode, the control unit 6 outputs a reset signal at the first zero-cross point of the feedback signal after the generation of the clock signal generated at a cycle of 10 seconds as described above, and turns on the reset switch 33. The capacitor C1 is reset. On the other hand, in the second operation mode, the control unit 6 stops the clock signal and stops the operation for resetting the capacitor C1. Further, when the second operation mode is switched to the first operation mode by the first mode switching signal, the control unit 6 outputs a reset signal at the zero-cross point of the first feedback signal after switching the operation mode, and the reset switch 33 Is turned on to reset the capacitor C1.

  According to this configuration, the infrared detection device 1 does not reset the capacitor C1 when the absolute value of the output value of the current-voltage conversion unit 3 exceeds the second threshold value. The output value of the converter 3 is not reset to zero (operating point). Therefore, there is an advantage that the detection signal delay and misreporting due to the reset of the output value of the current-voltage converter 3 during the human body detection operation can be prevented, and the sensitivity of human body detection is improved.

  According to the infrared detection device 1 having the above-described configuration, the control unit 6 controls the reset switch 33 serving as a discharge unit to reset the voltage across the capacitor C1, thereby removing unnecessary components from the output of the current-voltage conversion unit 3. Can be removed. That is, the output current output from the pyroelectric element 2 may include an unnecessary low-frequency component that is not related to the detection target (for example, the human body) due to, for example, a change in ambient temperature. On the other hand, in the infrared detection device 1 having the above-described configuration, such an unnecessary low frequency component can be removed as an unnecessary component, so that it is possible to avoid occurrence of erroneous detection due to the unnecessary component. .

  In addition, since the infrared detecting device 1 has only the pyroelectric element 2 connected to the input end of the current-voltage converter 3, the input resistance is the input end of the current-voltage converter 3 as described in the background art section. The SN ratio of the current-voltage conversion unit 3 is improved as compared with the configuration connected to. That is, in the configuration in which the input resistance is connected to the input terminal of the current-voltage conversion unit 3, a noise component generated by the input resistance is input to the current-voltage conversion unit 3, and the SN ratio of the current-voltage conversion unit 3 Decreases. In particular, from the viewpoint of miniaturization of the infrared detection device 1, the input resistance is usually composed of a resistance element built in an IC (integrated circuit), and when trying to achieve high resistance with such a resistance element, the temperature characteristics are Large variation in resistance value. When the resistance value of the input resistance varies and the resistance value decreases, thermal noise of the input resistance increases, and the SN ratio of the current-voltage conversion unit 3 decreases.

  On the other hand, the infrared detecting device 1 having the above configuration removes unnecessary components by controlling the timing at which the capacitor C1 of the current-voltage converting unit 3 is reset, so that the direct current including the input resistance is included. The feedback circuit can be omitted. Therefore, it is possible to eliminate the influence of noise caused by the DC feedback circuit including the input resistance, and to improve the SN ratio of the current-voltage conversion unit 3.

  Note that the infrared detection device 1 of the present embodiment has a DC feedback circuit as compared with a configuration in which a DC feedback circuit is added between the output terminal and the input terminal of the current-voltage conversion unit 3 as described in the background art section. There is also an advantage that the circuit scale is reduced as much as it becomes unnecessary.

  By the way, in the present embodiment, a ΔΣ (delta sigma) type AD converter is used as the AD conversion unit 4. Thereby, the AD converter 4 having a relatively small size and high accuracy can be realized.

  The digital processing unit 5 outputs a digital signal by a serial method. Specifically, as shown in FIG. 8A, the digital processing unit 5 adopts a signal format including a start bit 101, a main filter output 102, a detection signal state 103, an operation mode determination result 104, and a stop bit 105. . Similarly to the feedback signal, the main filter output 102 represents an instantaneous value of a signal obtained by removing at least unnecessary components from the output of the current-voltage conversion unit 3 by passing the digital BPF. The detection signal state 103 represents the state of the detection signal (H level or L level), and the operation mode determination result 104 represents the operation mode.

  The digital processing unit 5 transmits a 16-bit digital signal (10 bits for the main filter output 102, 3 bits for the stop bit 105, and 1 bit each for the other) in one communication, as shown in FIG. 8C. Output in serial communication in synchronization with (for example, 1 MHz). As a result, the digital processing unit 5 can superimpose the clock and various types of data and transmit the data through a single signal line, so that there is an advantage that the number of terminals can be reduced and the infrared detection device 1 can be downsized.

  Furthermore, in the present embodiment, the digital processing unit 5 converts the output using a BMC (Biface Mark Code) encoding method in which the output is inverted for each cell. That is, as shown in FIG. 8B, the digital processing unit 5 encodes the data “1” into “01” or “10” by the BMC and the data “0” by the BMC as “00” or “11”. And invert the output for each cell. Here, the cell means a time slot for outputting data for one bit before encoding.

  As described above, since the digital processing unit 5 always inverts the output for each cell by adopting the BMC encoding method, the signal does not include a low-frequency component, and the influence of the wraparound on the input of the current-voltage conversion unit 3 is affected. There is an advantage of being small. Therefore, when the infrared detecting device 1 is downsized, the chattering phenomenon that may occur due to the potential difference between the input of the current-voltage conversion unit 3 and the output of the digital processing unit 5 can be suppressed.

(Embodiment 2)
The infrared detection device 1 of the present embodiment is different from the infrared detection device 1 of the first embodiment in that the current-voltage conversion unit 3 employs a differential circuit system.

  That is, in the present embodiment, as shown in FIG. 9, the current-voltage converter 3 includes a first operational amplifier 311 connected to one end of the pyroelectric element 2 and a first operational amplifier 311 connected to the other end of the pyroelectric element 2. 2 operational amplifiers 312. A first capacitor C11 as a capacitive element for AC feedback is connected between the output terminal and the inverting input terminal of the first operational amplifier 311, and an AC is connected between the output terminal and the inverting input terminal of the second operational amplifier 312. A second capacitor C12 as a feedback capacitive element is connected. A reference power supply 32 for generating a reference voltage is connected to the non-inverting input terminals of both operational amplifiers 311 and 312.

  Further, a first reset switch 331 is connected between the output terminal and the inverting input terminal of the first operational amplifier 311 in parallel with the capacitor C11, and a capacitor is connected between the output terminal and the inverting input terminal of the second operational amplifier 312. A second reset switch 332 is connected in parallel with C12. The first and second reset switches 331 and 332 are on / off controlled by a reset signal from the control unit 6.

  Further, the current-voltage conversion unit 3 includes a differential amplifier circuit using a third operational amplifier 34. This differential amplifier circuit outputs a signal corresponding to the difference between the output voltage of the first operational amplifier 311 and the output voltage of the second operational amplifier 312 as a voltage signal. Specifically, the output terminal of the first operational amplifier 311 is connected to the inverting input terminal of the operational amplifier 34 via the resistor R11, and the output terminal of the second operational amplifier 312 is connected to the operational amplifier 34 via the resistor R12. It is connected to the non-inverting input terminal. A resistor R13 is connected between the output terminal and the inverting input terminal of the third operational amplifier 34, and a non-inverting input terminal of the third operational amplifier 34 is connected to a reference power source that generates a reference voltage via the resistor R14. Yes.

  In FIG. 9, the AD conversion unit 4 and the digital processing unit 6 are collectively illustrated, and a serial interface 7 for serially outputting the output of the digital processing unit 6 is illustrated in the subsequent stage. In FIG. 9, the current-voltage conversion unit 3, the AD conversion unit 4, the digital processing unit 5, the control unit, and the serial interface 7 are integrated into a single chip by an IC (integrated circuit) 8 and are housed in a case 9. Yes.

  According to this configuration, the current-voltage conversion unit 3 outputs a voltage signal corresponding to the difference between the output voltages of the first and second operational amplifiers 311 and 312, and therefore leaks from the terminal of the pyroelectric element 2 to the substrate. And in-phase components caused by disturbance noise can be canceled out. Furthermore, according to the above configuration, the chattering phenomenon that can be caused by the potential difference between the input of the current-voltage conversion unit 3 and the output of the digital processing unit 5 can be suppressed.

  In the present embodiment, the current-voltage conversion unit 3 includes the abnormality detection unit 35 that detects an abnormal value of at least one of the output voltages of the first operational amplifier 311 and the second operational amplifier 312. The abnormality detection unit 35 compares the output voltage of at least one of the operational amplifier 311 and the operational amplifier 312 with a predetermined fourth threshold and a predetermined fifth threshold (> fourth threshold). The abnormality detection unit 35 determines that the output voltage of the operational amplifiers 311 and 312 is an abnormal value when the output voltage of the operational amplifiers 311 and 312 is out of the normal range determined by the fourth and fifth thresholds. An abnormal signal is output to the control unit 6.

  When the control unit 6 receives an abnormality signal from the abnormality detection unit 35, the control unit 6 outputs a reset signal, turns on the first and second reset switches 331 and 332, and resets both capacitors C11 and C12 (an abnormal voltage protection unit ( (Not shown). That is, when the output voltage of at least one of the first operational amplifier 311 and the second operational amplifier 312 deviates from the normal range determined by the fourth and fifth thresholds, the abnormal voltage protection unit causes the capacitors C11, Reset C12.

  In the present embodiment, the abnormality detection unit 35 compares the output voltage of the second operational amplification unit 312 with the fourth and fifth thresholds by monitoring the potential at the connection point between the resistor R12 and the resistor R14. ing.

  According to this configuration, the infrared detecting device 1 detects this abnormality when the output voltage of the operational amplifiers 311 and 312 abnormally increases or decreases due to the leakage from the terminal of the pyroelectric element 2 to the substrate or the influence of disturbance noise. It can be detected immediately at the first stage of the circuit (current-voltage converter 3). The infrared detection device 1 resets the capacitors C11 and C12 by the abnormal voltage protection unit when an abnormal voltage step-up or abnormal step-down of the output voltages of the operational amplifiers 311 and 312 is detected. Therefore, there is an advantage that it is possible to avoid saturation of the output of the current-voltage conversion unit 3 due to the influence of the in-phase component.

  Other configurations and functions are the same as those of the first embodiment.

DESCRIPTION OF SYMBOLS 1 Infrared detector 2 Pyroelectric element 3 Current-voltage conversion part 4 AD conversion part 5 Digital processing part (filter part)
6 Control Unit 31 Operational Amplifier 33 Reset Switch (Discharge Unit)
311 1st operational amplifier 312 2nd operational amplifier C1 Capacitor (capacitance element)
C11 First capacitor (capacitance element)
C12 Second capacitor (capacitance element)
Vth1, -Vth1 first threshold Vth2, -Vth2 second threshold Vth3, -Vth3 third threshold

Claims (13)

  A pyroelectric element and a current-voltage conversion unit that converts a current signal output from the pyroelectric element into a voltage signal using an operational amplifier connected with a feedback capacitive element, and is stored in the capacitive element. The current-voltage conversion unit has a discharge unit that forms a discharge path for discharging the generated electric charge, and by controlling the discharge unit, a low frequency component having a predetermined frequency or less from the output of the current-voltage conversion unit An infrared detecting device, comprising: a control unit that resets the capacitive element at a timing such as to remove the capacitor.   The control unit includes an oscillator that generates a clock signal at a predetermined cycle so as to remove the low-frequency component, generates a reset signal based on the clock signal, and generates the reset signal based on the reset signal. The infrared detection device according to claim 1, wherein the capacitive element is reset.   A filter unit that allows a signal component in a predetermined frequency band to pass through the output of the current-voltage conversion unit; and the control unit receives the output of the filter unit as a feedback signal, and after the generation of the clock signal The infrared detection device according to claim 2, wherein the reset signal is generated at the first zero cross point of the feedback signal to reset the capacitive element.   If the zero cross point of the feedback signal does not occur for a predetermined extension time after the generation of the clock signal, the control unit generates the reset signal when the extension time has elapsed since the generation of the clock signal. 4. The infrared detection device according to claim 3, wherein the capacitance element is reset.   The control unit includes an abnormality protection unit that resets the capacitive element when the absolute value of the output value of the current-voltage conversion unit continuously exceeds a predetermined threshold for a predetermined allowable time or longer. The infrared detection device according to any one of claims 1 to 4, wherein the infrared detection device is characterized.   The absolute value of the value corresponding to the output value of the current-voltage converter is compared with a predetermined first threshold, and the presence / absence of a detection target is determined based on whether the absolute value exceeds the first threshold. A determination unit, and the control unit operates in a first operation mode capable of resetting the capacitive element when the absolute value is within a second threshold lower than the first threshold, and the absolute value 6. The infrared detection device according to claim 1, wherein when the frequency exceeds the second threshold value, the infrared detection device operates in a second operation mode in which the capacitance element is not reset. 6. .   An AD conversion unit that converts an output value of the current-voltage conversion unit into a digital value, and a digital processing unit that passes a signal component in a predetermined frequency band among outputs of the AD conversion unit. The infrared detection device according to any one of claims 1 to 6.   When the absolute value of the input of the AD conversion unit exceeds a third threshold value set to be equal to or lower than an upper limit value of a corresponding range that can be converted into a digital value by the AD conversion unit, the control unit The infrared detection device according to claim 7, further comprising an AD conversion protection unit that resets the signal.   The infrared detection apparatus according to claim 7, wherein the AD conversion unit is a ΔΣ system.   The infrared detection apparatus according to claim 7, wherein the digital processing unit outputs a digital value in a serial manner.   The infrared detection apparatus according to claim 10, wherein the digital processing unit uses a BMC encoding method in which an output is inverted for each cell as the serial method.   The current-voltage conversion unit has a first operational amplifier connected to one end of the pyroelectric element and a second operational amplifier connected to the other end of the pyroelectric element as the operational amplifier. 12. The method according to claim 1, wherein a signal corresponding to a difference between an output voltage of the first operational amplifier and an output voltage of the second operational amplifier is output as the voltage signal. The infrared detection device according to item. When the output voltage of at least one of the first operational amplifier and the second operational amplifier deviates from a normal range defined by a predetermined fourth threshold and a predetermined fifth threshold, the control unit The infrared detection device according to claim 12, further comprising an abnormal voltage protection unit that resets the capacitive element.

Images