JP2005121607A - Current/voltage converting circuit for infrared ray detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a current/voltage converting circuit for an infrared ray detector shortening the inspection time of a leak current performed before shipment. <P>SOLUTION: This converting circuit comprises: an operation amplifier 2 in which a pyroelectric element 1 is connected to a reversal input terminal; a capacitor Cf connected between the reversal input terminal and output terminal of the operation amplifier 2; a series circuit comprising a resistance Ri, a switch SW1 and a DC feedback circuit DF connected in parallel with the capacitor Cf; a series circuit comprising a switch SW2 of which the one end is connected to the reversal input terminal of the operation amplifier 2, and the other end is grounded, and a reference voltage source E3; and a control circuit 4 controlling the turn-on/turn-off of the switch SW2. The control circuit 4 turns on the switch SW2, and turns off the switch SW2 after predetermined time in the state where the switch SW1 is turned off by a control signal from a TEST terminal at the time of leak current inspection. When the amount of change of the potential of a So terminal during Time T is set to ΔV and the leak current is set to I1, the leak current can be calculated by the formula I1=Cf×ΔV/T. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a current-voltage conversion circuit for an infrared detector.

  In an infrared detector that detects the infrared energy radiated from the human body using a pyroelectric element and detects the presence and movement of the human body, the output signal of the pyroelectric element is a very weak current output. This output current is converted into a current voltage using a resistance element. The signal thus converted is amplified and then compared with a preset threshold value in the comparator unit to determine the presence or movement of the human body. However, thermal noise due to the resistance element is a main factor that deteriorates the S / N ratio of the infrared detection device.

  Therefore, a current-voltage conversion circuit using the impedance component of the capacitor has been proposed, and an example of the circuit configuration is shown in FIG. The pyroelectric element 1 has one end grounded via a DC voltage source E1, and the other end connected to the inverting input terminal of the operational amplifier 2 via the Si terminal. A capacitor Cf that forms a feedback capacitance is connected between them. In addition, a series circuit of a DC feedback circuit DF and a resistor Ri is connected in parallel to the capacitor Cf. A reference voltage source E2 for setting the operating point to a predetermined level is connected to the non-inverting input terminal of the operational amplifier 2, and an output terminal is connected to the So terminal.

  The DC feedback circuit DF is composed of an integrating circuit composed of an operational amplifier 3 in which one end of a resistor R1 is connected to the inverting input terminal and a capacitor C1 is connected between the output terminal and the inverting input terminal. The output of the operational amplifier 2 is connected to the input terminal, and the reference voltage source E2 is connected to the other end of the resistor R1.

  Such a current-voltage conversion circuit operates as a band-pass filter, and the impedance Z (S) viewed from the pyroelectric element 1 and its center frequency ω0 in the circuit of FIG. 4 are expressed by [Equation 1].

FIG. 5 shows the frequency characteristic of this impedance (conversion impedance). In the frequency region A higher than the center frequency f0 (= ω0 / 2π), the impedance characteristic of the capacitor Cf is reflected and Z (ω) = 1 / ( The impedance increases as the frequency decreases. However, since the DC feedback circuit DF operates in a region lower than the center frequency f0, the impedance decreases conversely.

Here, since the detection frequency in human body detection is centered at 1 Hz, a frequency band in the vicinity of 0.1 to 10 Hz centered on 1 Hz is selectively amplified in a subsequent voltage amplification unit (not shown). Therefore, in order to improve the S / N ratio, it is necessary to perform current-voltage conversion in this frequency band with the impedance characteristics of the capacitor Cf. That is, the center frequency ω0 <2π × 0.1 Hz must be satisfied. Further, since the output current from the pyroelectric element 1 is very weak, the conversion impedance value is also very high, and the resistors Ri and R1 need to have very high resistance values of several T (tera) Ω. However, in general, when such a high resistance is realized, variations in resistance values and temperature characteristics become very poor. For example, when an impurity non-diffusing polysilicon resistor element is used for circuit integration, the variation and the minimum value of the temperature characteristic at the time of change of 25 to 60 ° C. are about 0.5 times the variation and the temperature characteristic is about 0.1 times. Even in this case, in order to set the center frequency ω0 <2π × 0.1 Hz, it is necessary to set the center frequency f0 = several mHz (ω0 = 2π × f0). Therefore, this current-voltage conversion circuit has frequency characteristics with a central frequency of several mHz. (For example, see Patent Document 1)
Japanese Patent Laid-Open No. 10-281866

  In the infrared detection device, there is a shot noise due to a leakage current of the input unit as a noise source which becomes a problem in addition to the high resistance thermal noise of the input unit. In general, the operational amplifier 2 is constituted by a CMOS circuit, so that the input leakage current to the operational amplifier 2 can be reduced to a negligible level. For example, when the circuit is integrated, the input of the operational amplifier 2 as shown in FIG. It is necessary to connect a protective diode D1 for preventing static electricity between the terminal and the ground level. However, the leakage current Id through the diode D1 is a value that cannot be ignored, and it is necessary to inspect and manage the leakage current value at the time of shipping inspection. However, since these current values are weak levels of several f (femto) ampere level, it is difficult to directly measure the current values with a general measuring instrument.

  Therefore, in order to measure such a leakage current, the potential of the So terminal connected to the output terminal of the operational amplifier 2 and the potential of the Sf terminal connected to the output terminal of the operational amplifier 3 are measured when the circuit operation is stable. By dividing this potential difference by the resistance value of the resistor Ri, the leakage current value was measured equivalently (assuming that the potential at the So terminal is equal to the potential at the Si terminal).

  However, in order to measure the leakage current value by the above method, it is necessary to perform measurement after supplying power to the circuit and the operation is stabilized. As described above, this circuit has a time constant of several mHz. Therefore, it takes several hundred seconds to stabilize the circuit operation, and there is a problem that the inspection time of the circuit becomes very long.

  The present invention has been made in view of the above reasons, and an object of the present invention is to provide a current-voltage conversion circuit for an infrared detection device that shortens the inspection time of leakage current performed at the time of shipment.

  According to a first aspect of the present invention, there is provided an operational amplifier having a pyroelectric element connected to the inverting input terminal and a first reference voltage source connected to the non-inverting input terminal, and connected between the inverting input terminal and the output terminal of the operational amplifier. A series circuit of a capacitor, a resistor having one end connected to the inverting input terminal of the operational amplifier and the first switch, a DC circuit having one end connected to the other end of the series circuit and the other end connected to the output terminal of the operational amplifier A charging circuit including a feedback circuit, a second switch for charging the potential of the inverting input terminal of the operational amplifier to the same potential as that of the non-inverting input terminal, and conducting / cutting off the charging path; Means for turning off the first switch and means for turning off the second switch after turning on the second switch at the time of leak current inspection of the operational amplifier.

  According to the present invention, the leakage current can be inspected in a short time from the slope of the change in the output voltage of the operational amplifier determined only by the leakage current and the feedback capacity of the capacitor.

  According to a second aspect of the present invention, in the first aspect, the charging unit includes a series circuit of a second reference voltage source that generates the same voltage as the first reference voltage source and a second switch, and the series circuit One end of the circuit is connected to the inverting input terminal of the operational amplifier, and the other end is grounded.

  According to this invention, the specific circuit of a charging means can be comprised.

  According to a third aspect of the present invention, in the first aspect, the charging unit is configured such that the second switch has one end connected to the inverting input terminal of the operational amplifier and the other end connected to the output of the DC feedback circuit. It is characterized by that.

  According to the present invention, the circuit stabilization time shortening circuit at the time of power-on is also used for shortening the leakage current inspection time, thereby reducing both the circuit stabilization time and the leakage current inspection time at the time of power-on in a small circuit. It becomes possible to do.

  According to a fourth aspect of the present invention, there is provided the control circuit according to any one of the first to third aspects, wherein the first switch is turned off and the second switch is turned on a predetermined time after turning on the leak current of the operational amplifier. It is characterized by.

  According to the present invention, since the on / off of the switch is automatically controlled by the control circuit, the inspection can be performed in a short time without special setting at the time of the leakage current inspection.

  As described above, according to the present invention, there is an effect that it is possible to shorten the inspection time for leakage current performed at the time of shipment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 is a circuit diagram of a current-voltage conversion circuit for an infrared detector according to this embodiment. The pyroelectric element 1 has one end grounded via a DC voltage source E1, and the other end connected to the inverting input terminal of the operational amplifier 2 via the Si terminal. A capacitor Cf that forms a feedback capacitance is connected between them. Further, a series circuit of a resistor Ri, a switch SW1 (first switch) and a DC feedback circuit DF is connected in parallel between the inverting input terminal and the output terminal of the operational amplifier 2, and the resistor Ri is connected to the inverting input terminal side. The DC feedback circuit DF is connected to the output terminal side, and the switch SW1 is connected between the resistor Ri and the DC feedback circuit DF and is on / off controlled by a control signal from the TEST terminal. A reference voltage source E2 for setting the operating point to a predetermined level is connected to the non-inverting input terminal of the operational amplifier 2, and an output terminal is connected to the So terminal.

  The DC feedback circuit DF is composed of an integrating circuit including an operational amplifier 3 in which one end of a resistor R1 is connected to an inverting input terminal and a capacitor C1 is connected between the output terminal and the inverting input terminal. The output of the operational amplifier 2 is connected to the input terminal, and the reference voltage source E2 (first reference voltage source) is connected to the other end of the resistor R1.

  The inverting input terminal of the operational amplifier 2 is grounded through a series circuit of a switch SW2 (second switch) and a reference voltage source E3 (second reference voltage source). The switch SW2 is turned on / off by the control circuit 4. Controlled off. Here, the power supply voltages of the reference voltage sources E2 and E3 are set to the same voltage Vr.

  Next, the leak current measurement operation will be described. When measuring the leakage current, first, the switch SW1 is turned off by a control signal from the TEST terminal, and the switch SW2 is turned on by the control circuit 4. When the power is turned on in this state, the potential of the Si terminal is instantaneously charged to the voltage Vr by the reference voltage source E3. Thereafter, the switch SW2 is turned off, and it is sufficient that the switch SW2 is turned on and off for about 10 msec. When the switch SW2 is turned off, all the paths for supplying current to the Si terminal are cut off, so that the potential of the Si terminal is discharged only by the leakage current. At this time, the Si terminal is kept at the potential Vr by the operation of the operational amplifier 2, the charge Q is accumulated in the capacitor Cf, and the potential of the So terminal rises. Here, if the amount of change in the potential of the So terminal during time T is ΔV and the leakage current is I1, this voltage is applied to both ends of the capacitor Cf, so that Q = Cf × ΔV = I1 × T. That is, the leakage current I1 can be calculated from I1 = Cf × ΔV / T. Since it is sufficient if the time T is about 100 msec, the leakage current can be measured in about 110 msec even if the on time of the switch SW2 is added.

  Even when the circuit is integrated, since a TEST terminal for inputting a control signal for turning on / off the switch SW1 is provided, the switch SW1 can be easily controlled via the TEST terminal when inspecting a leakage current. Can do.

(Embodiment 2)
FIG. 2 is a circuit diagram of the current-voltage conversion circuit for the infrared detection device of the present embodiment. In the circuit of the first embodiment, the switch SW2 is connected between the inverting input terminal of the operational amplifier 2 and the output terminal of the operational amplifier 3. The reference voltage source E3 is removed (that is, connected in parallel to the series circuit of the resistor Ri and the switch SW1). In addition, the same code | symbol is attached | subjected to the structure similar to Embodiment 1, and description is abbreviate | omitted.

  Since the circuit of the conventional example shown in FIG. 4 takes a very long time to stabilize the circuit operation when the power is turned on as described above, the switch SW2 and the control circuit 4 shown in FIG. It has been proposed to shorten the circuit stabilization time when the power is turned on by turning on the switch SW2 only for a certain period of time. Therefore, in the present embodiment, the circuit stabilization time shortening circuit at the time of power-on is also used for shortening the leakage current inspection time. That is, at the time of leak current inspection, when the switch SW1 is turned off by the control signal from the TEST terminal and the power is turned on, the control circuit 4 turns on the switch SW2 for a certain period of time and charges the Si terminal to the potential Vr. The operation after the switch SW2 is turned off after a certain time is the same as that in the first embodiment, and the leakage current can be measured in a short time.

  In this way, the circuit stabilization time reduction circuit at power-on can also be used to reduce the leakage current inspection time, thereby reducing both the circuit stabilization time and leakage current inspection time at power-on in a small circuit. It becomes possible.

(Embodiment 3)
FIG. 3 is a circuit diagram of the current-voltage conversion circuit for an infrared detection device of the present embodiment. In the circuit of the second embodiment, both the switch SW1 and the switch SW2 are controlled by the control circuit 4, and the same as in the second embodiment. The same reference numerals are given to the configurations of and the description is omitted.

  In the present embodiment, the switch SW1 is automatically turned off and the switch SW2 is turned on automatically by the control circuit 4 when the power is turned on. The switch SW1 is controlled to turn on. Therefore, the leakage current can be measured using a specific time zone (for example, between 10 msec and 200 msec) when the power is turned on without performing any special setting at the time of measuring the leakage current.

  Also in the circuit of the first embodiment, if both the switch SW1 and the switch SW2 are controlled by the control circuit 4, the same effect as described above can be obtained.

1 is a diagram illustrating a configuration of a first embodiment. 6 is a diagram illustrating a configuration of a second embodiment. FIG. FIG. 6 is a diagram illustrating a configuration of a third embodiment. It is a figure which shows the structure of a prior art example. It is a figure which shows the frequency characteristic of conversion impedance same as the above.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Pyroelectric element 2, 3 Operational amplifier 4 Control circuit DF DC feedback circuit Cf, C1 Capacitor Ri, R1 Resistance SW1, SW2 Switch E1 DC voltage source E2, E3 Reference voltage source

Claims (4)

An operational amplifier in which a pyroelectric element is connected to the inverting input terminal and a first reference voltage source is connected to the non-inverting input terminal; a capacitor connected between the inverting input terminal and the output terminal of the operational amplifier; A series circuit of a resistor and one end connected to the inverting input terminal, a DC feedback circuit having one end connected to the other end of the series circuit and the other end connected to the output terminal of the operational amplifier, Charging means comprising a second switch for charging the potential of the inverting input terminal to the same potential as that of the non-inverting input terminal and conducting / cutting off the charging path, and means for turning off the first switch when checking the leakage current of the operational amplifier And a means for turning off the second switch after a predetermined time from turning on the second switch at the time of leak current inspection of the operational amplifier. The charging means comprises a series circuit of a second reference voltage source that generates the same voltage as the first reference voltage source and a second switch, and one end of the series circuit is connected to the inverting input terminal of the operational amplifier. The current-voltage conversion circuit for an infrared detecting device according to claim 1, wherein the other end is grounded. 2. The infrared detection according to claim 1, wherein the charging means is configured such that the second switch has one end connected to the inverting input terminal of the operational amplifier and the other end connected to the output of the DC feedback circuit. Current-voltage conversion circuit for equipment. 4. An infrared detection apparatus according to claim 1, further comprising a control circuit that turns off the first switch and turns off the second switch after a predetermined time from turning on the second switch when checking the leakage current of the operational amplifier. Current voltage converter circuit.

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