Detection voltage output circuit of charge generation type sensing device

Abstract: A detection voltage output circuit of a charge genera­ tion type sensing device outputs the charges generated on the sensing device in the form of a voltage. The detection voltage output circuit comprises: a sensing device composed of a dielectric for generating charges in accordance with a change in a physical quantity applied from the outside;a capacitor at a first stage which is connected in parallel to the sensing device and stores the charges generated on the sensing device; a capacitor at a second stage which stores the charges transferred from the capacitor at the first stage; a transfer switch which is provided between the capacitor at the first stage and the capacitor at the second stage so as to transfer the charges stored in the capacitor at the first stage to the capacitor at the second stage; an operational amplifier which is connected to the capacitor at the second stage so as to convert the charges stored in the capacitor at the second stage into a voltage and outputting the thus-obtained voltage; and an output switch which is connected to the output side of the operational amplifier so as to be turned off while at least a part of the charges stored in the capacitor at the first stage are transferred to the capacitor at the second stage and turned on after the transfer switch is turned off to output the voltage from the operational amplifier. the charges generated on the sensing device are output in the form of a voltage.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a detection voltage output circuit of a charge generation type sensing device which converts the charges generated on the charge generation type sensing device due to a change in a physical quantity applied from the outside into a voltage and outputs the thus-obtained voltage. Description of the Prior Art It is generally known that some kinds of ferroelectric ceramic materials from among ceramic dielectrics, barium titanate (BaTiO₃) and lead zirconate titanate (Pb 1-x Zr x TiO₃), for example, have both piezo-electricity and pyro-electricity. Charge generation type sensing devices (hereinunder referred to as "sensing devices") utilizing these piezo-electricity and pyro-electricity are utilized for various purposes as piezoelectric sensors and pyroelectric sensors. For example, pyroelectric sensors are utilized as infrared sensors for detecting the infrared radiated from the human body, and an infrared sensor used as a human body detecting system such as that shown in Fig. 4 is known. In Fig. 4, a sensing device 1 constitutes an infrared sensor for converting a minute temperature change caused by infrared into an electrical change. The detection current generated on the sensing device 1 is converted into a voltage by a current-voltage converter 2, and the voltage output from the current-voltage converter 2 is amplified by an amplifier 3. A low-pass filter 4 is provided at the subsequent stage to the amplifier 3 so as to remove the noise other than the detection signals received from the human body. The output from the low-pass filter 4 is compared with a reference voltage by a comparator 5. The reference numeral 6 denotes an output circuit composed of a digital circuit, a relay circuit and the like, and when the sensing device 1 detects the infrared radiated from the human body, the output circuit 6 supplies, for example, a visitor notice signal in accordance with the output from the comparator 5. However, since the sensing device 1 generates charges when the spontaneous polarization of the pyroelectric changes due to a temperature change, the output current is as small as 10⁻¹¹ to 10⁻¹³ A, so that in order to apply the sensing device 1 to a human body detecting system, it is necessary to convert a current into a voltage by using a resistance as high as 10¹¹ to 10¹² Ω for the current-voltage converter 2. As such a current-voltage converter 2, a current-voltage converter 2A composed of a high resistor R H and an operational amplifier 2A₁ the inverting input terminal of which is connected to the sensing device 1 so as to produce an output voltage V out from the output terminal, as shown in Fig. 5, is conventionally known. As another current-voltage converter 2, an impedance converter 2B composed of the high resistor R H , a source resistor R S and a field-effect transistor 2B₁ the gate of which is connected to the sensing device 1 so as to supply an output voltage V out based on the voltage between both terminals of the source resistor R S from a source S, as shown in Fig. 6, is also known. However, both of the above-described current-voltage converters in the prior art have the following defects due to the use of the high resistor R H of 10¹¹ to 10¹² Ω. Firstly, a delay in electrical response is generated due to the capacity components such as the capacity of the sensing device and the input capacity which an active element such as the operational amplifier 2A₁ and the field-effect transistor 2B₁ has and the stray capacity, and the high resistor R H . Especially, since the high resistor R H has a resistance of 10¹¹ to 10¹² Ω, the time constant determined by the capacity components and the resistance is the order of 0.1 to 1 second, so that the responsiveness of the detecting system as a whole is deteriorated and the S/N of the signal obtained is lowered in the case of using the detecting system for the purposes other than the human body detection. Secondly, in spite of the demand for a monolithic IC for the purpose of reduction in the size of a signal processing circuit and realization of a one-chip signal processing circuit, it is very difficult to produce a monolithic circuit in the prior art because the high resistor R H has a very high resistance. Thirdly, since the high resistor R H has a very high resistance, it is difficult to reduce the deviation of resistance and there is a problem in the stability. Johnson noise produced by the high resistance is another problem. On the other hand, a charge generation type sensing device intrinsically acts when an external force or external heat is applied thereto, so that noise is apt to be caused by a temperature change, vibration, etc. in the ambience, the noise containing various kinds of frequency components. In order to prevent malfunction caused by such noise and enhance the reliability, some known sensing devices are composed of a plurality of sensing devices, as shown in Figs. 7 to 9. The sensing device shown in Fig. 7 is composed of sensing devices 1A, 1A' which are connected with each other in series so as to have the opposite polarities and are connected to the impedance converter 2B. The sensing device shown in Fig. 8 is composed of sensing devices 1B, 1B' which are connected with each other in parallel so as to have the opposite polarities and are connected to the impedance converter 2B. Such structures can enhance the reliability to a certain extent, but the effect on the noise signals produced on the sensing devices separately from each other is disadvantageously small. In order to enhance the reliability with respect to noise, the sensing device shown in Fig. 9 is composed of two pairs of sensing devices 1B, 1B' connected to each other in parallel and opposition, each pair 1B, 1B' being connected to a current-voltage converter 2 independently of another pair, and the respective output voltages are supplied through an AND circuit 7. Such structure, however, is disadvantageous not only in that it is difficult to realize a monolithic IC of the current-voltage converter 2 but also in that the number of parts is so large as to result in rise in cost. A switched capacitor circuit is known from J. Millmam & A. Grabel "Microelectronics", pages 751-755, 1987. This known switched capacitor circuit may be fabricated in monolithic form and comprises an operational amplifier, the inverting input and the output of which are bridged by a capacitor, a further capacitor being connected to ground and to the inverting input of the operational amplifier via a switch. A further switch may be provided at the output of the operational amplifier. SUMMARY OF THE INVENTION It is the object of the present invention to provide a detection voltage output circuit of the kind defined by the precharacterizing features of claim one which is adapted to effectively eliminate noise signals from the input to the circuit. This object is attained by the characterizing features of claim one. Advantageous developments of the invention are given by the subclaims. Since a high resistor is dispensed with, it is possible to realize a monolithic IC in the detection voltage output circuit. In addition, since it is possible to shorten the outputting time in comparison with the detecting time, improvement of the detecting accuracy is enabled by eliminating the influence of a background noise containing a high-frequency component, and it is further possible to constitute the detection voltage output circuit as a switched capacitor filter. The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram of a preferred embodiment of a detection voltage output circuit according to the present invention, in which the electrostatic capacity of the sensing device is used as the capacitor at the first stage and a sensing device for correction and a resent switch are provided; Fig. 2 is the equivalent circuit diagram of the embodiment shown in Fig. 1; Fig. 3 shows the relationship between the switching operation timing and the output voltage in the embodiment shown in Fig. 1; Fig. 4 is a block diagram of a conventional human body sensing system using a sensing device; Fig. 5 is a circuit diagram of a current-voltage converter using a high resistor as the current-voltage converter in the conventional sensing system shown in Fig. 4; Fig. 6 is a circuit diagram of an impedance converter using a field-effect transistor as the current-voltage converter in the conventional sensing system shown in Fig. 4; Fig. 7 is a circuit diagram of a conventional system shown in Fig. 6 in which a pair of sensing devices are connected in series and opposition; Fig. 8 is a circuit diagram of another modification of the conventional sensing system shown in Fig. 6 in which a pair of sensing devices are connected in parallel and opposition; and Fig. 9 is a block diagram of the structure of a conventional detection voltage output circuit having duplex sensing device and current-voltage converter. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be explained in more detail with reference to Figs. 1 to 3. Fig. 1 is a circuit diagram of a second embodiment of the present invention. The equivalent circuit thereof is shown in Fig. 2. To a sensing device 110 having an electrostatic capacity of C₁₁₀ is connected a switched capacitor circuit 112, which incorporates a transfer switch 116, an operational amplifier 120, an output switch 122, a sensing device 124 for correction having an electrostatic capacity of C₁₂₄ and a reset switch 126. The sensing device 110 is composed of a constant-current source 110A for current i₁₁₀ and a capacitor 110B having an electrostatic capacity of C₁₁₀, and changes Q₁₁₀ generated on the sensing device 110 due to a change in a physical quantity applied from the outside are stored in the capacitor 110B. That is, the capacitor 110B functions as the capacitor at the first stage of the switched capacitor circuit 112. The sensing device 124 for correction is composed of a constant-current source 124A for current i₁₂₄ and a capacitor 124B having an electrostatic capacity of C₁₂₄, and charges Q₁₂₄ generated on the sensing device 124 for correction due to a background noise and the charges Q₁₁₀ transferred from the capacitor 110B are stored in the capacitor 124B. That is, the capacitor 124B functions as the capacitor at the second stage of the switched capacitor circuit 112. The reset switch 126 is connected to the sensing device 124 for correction having the capacitor 124B. The reset switch 126 is composed of, for example, an analog switch and when the reset switch 126 is turned ON, a short-circuit is caused between the electrodes of the capacitor 124B, thereby discharging the stored charges. The operation of this embodiment having the above-described structure will be explained with reference to Fig. 3. In Fig. 3, the symbol t₀ represents the time at which the reset switch 126 is turned OFF, t s the time at which the transfer switch 116 is turned ON, t s ' the time at which the transfer switch 116 is turned OFF and the output switch 122 is turned ON, and t s '' the time at which the output switch 122 is turned OFF and the reset switch 126 is turned ON. The charges Q₁₁₀ stored in the sensing device 110 due to a change in a physical quantity applied from the outside during the period Ts from the time at which the reset switch 126 is turned OFF to the time at which the transfer switch 116 is turned ON, namely, when t₀ ≦ t ≦ t s , are represented by the following formula (1): The charges Q₁₂₄ stored in the sensing device 124 due to a background noise during the period Ts' from the time at which the reset switch 126 is turned OFF to the time at which the output switch 122 is turned ON are represented by the following formula (2): Since the sensing device 110 is not connected to the operational amplifier 120 during the period Ts in which the transfer switch 116 is OFF, the charges Q₁₁₀ represented by the formula (1) does not influence the output voltage V out . On the other hand, since the sensing device 124 is constantly connected to the operational amplifier 120, the output voltage V out of the operational amplifier 120 changes in accordance with the charges Q₁₂₄ in such a manner as to have a characteristic exhibited when t₀ ≦ t ≦ t s '' shown in Fig. 3 during the period Ts'' in which the reset switch 126 is OFF. When the transfer switch 116 is turned ON at the time ts, all the charges Q₁₁₀ stored in the sensing device 110 are transferred to the sensing device 124, and when the output switch 122 is turned ON at the time t s ', the output voltage V out is supplied to the circuit at the subsequent stage. The output voltage V out produced by the operational amplifier 120 during the period at which t₀ ≦ t ≦ t s '' are represented as follows: The output voltage V out shows the characteristic as a whole such as that shown in Fig. 3. In this embodiment, the sensing device 110 and the sensing device 124 (for correction) are connected with each other so as to have the opposite polarities with respect to the inverting input terminal of the operational amplifier 120. As a result, the charges produced on the sensing device 110 and the sensing device 124 for correction due to a change in a physical quantity such as an external heat and an external force are cancelled out by each other immediately after the transfer switch 116 is turned ON, so that even if the output switch is turned ON, no output voltage V out is generated. In other words, in this embodiment, the output voltage V out is produced only when charges are generated on the sensing devices 110 and the sensing devices 124 for correction. Accordingly, if the sensing device 110 is so designed that a change in a physical quantity is applied from the outside only to the sensing device 110, it is possible to remove the noise voltage caused by the noise charges Q₁₁₀ produced on the sensing device 124 for correction due to a background noise by utilizing said noise charge Q₁₂₄, thereby enabling stable sensing operation. It is also possible in this embodiment to exclude the influence of a background noise containing a high-frequency component and stabilize the output voltage V out transferred to the subsequent stage by sufficiently reducing the period Td during which t s ≦ t ≦ t s '' and the period Tm in which the output switch 122 is ON in comparison with the period Tm during which t₀ ≦ t ≦ t s . As described in the above embodiment, according to the present invention, it is possible to provide a detection voltage outputting circuit of a charge generation type sensing device which uses a switched capacitor type charge-voltage converter. It is also possible to eliminate the influence of a background noise on a sensing device and hence, to improve, the detecting accuracy, as described in the third and fourth embodiments. Thus, the present invention offers the following advantages. 1. Since a charge-voltage converter is composed of a switched capacitor circuit, the high resistor which is required in the prior art is dispensed with, and (1) it is possible to improve the responsiveness at the outputting time, (2) it is possible to realize a monolithic IC for the detection voltage circuit, and (3) since it is possible to shorten the outputting time in comparison with the detecting time, improvement of the detecting accuracy by eliminating the influence of a background noise containing a high-frequency component is enabled, and it is further possible to constitute the detected voltage outputting circuit as a switched capacitor filter. 2. It is possible to simplify the circuit structure by using the electrostatic capacity of a sensing device or a sensing device for correction as a capacitor at a first or second stage of a switched capacitor circuit. 3. It is possible to remove or correct the noise charges caused by a background noise or a noise voltage converted from the noise charges and improve the detecting accuracy by providing a sensing device for noise elimination or a sensing device for correction.

Claims: A detection voltage circuit of a charge generation type sensing device, comprising: a first sensing device (110) having an electrostatic capacity and composed of a dielectric for generating electrical charges in accordance with a change in a physical quantity applied from the outside; a second sensing device (124) for noise elimination which device has an electrostatic capacity and is composed of a dielectric for generating electrical charges in accordance with the same physical quantity applied from the outside; a converter including an operational amplifier (120) for converting the electrical charges into a voltage to be output; characterized in that the second sensing device (124) is connected to the input side and the output of said operational amplifier (120) so as to have the same polarity to the polarity of the first sensing device (110) with respect to the input side of the operational amplifier (120), and a transfer switch (116) is provided between said first sensing device (110) and said second sensing device (124) and an output switch (122) is connected to the output of the operational amplifer (120), said output switch (122) being adapted to be turned off while at least a part of said charges stored in the first sensing device (110) are transferred to the second sensing device (124) and to be turned on after the transfer switch (116) is turned off. The detection voltage output circuit according to claim 1, characterized in that the electrostatic capacity of the first sensing device (110) is used as said first capacitor (110B). The detection voltage output circuit according to claim 1 or 2, characterized in that the electrostatic capacity of the second sensing device (124) is used as said second capacitor (124B). The detection voltage output circuit according to one of claims 1 to 3, characterized in that the first and second sensing devices comprise an equivalent constant-current source. The detection voltage output circuit according to one of claims 1 to 4, characterized in that a reset switch (126) is connected in parallel to the second sensing device (124).

Inventors: KUMADA AKIRA, KUMADA, AKIRA
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1989
Main classification: G01R29
IPC codes: H03K3, H03F3, G01R29