JP3736026B2 - Current-voltage conversion circuit in pyroelectric infrared detector - Google Patents

Description

として入力抵抗Ｒｉは測定出来ることがわかる。
【００１６】
なお、以上の例では、演算増幅器１の出力ノード電圧Ｖｏｕｔが、第１、第２の試験パッド４１，４２をグランドに接続した時点から第１の試験パッド４１に制御電圧Ｖｇを加えて、電源電圧に飽和するまでの時間ｔと、その場合の電位差ΔＶを求めて、入力抵抗の抵抗値を測定しているが、演算増幅器１の出力ノードの電圧が飽和するまでの途中における電位差（電圧変化）と、その電位差に至るまでの時間を求めることによって、入力抵抗の抵抗値を求めてもよく、この場合、抵抗値が極めて大きい場合でも迅速な測定が可能となる。
【００１７】
【発明の効果】
本発明によれば、第１、第２の試験パッドに電圧変化を加えるだけで、演算増幅器と、直流増幅回路との間に接続された抵抗値の高い、通常の抵抗測定器では測定できないような入力抵抗であっても、回路基板に実装した状態で容易に測定できる。
【図面の簡単な説明】
【図１】本発明の一実施例（請求項１）の回路図である。
【図２】本発明の一実施例（請求項２）の回路図である。
【図３】本発明の動作のシミュレーション結果を示すグラフである。
【図４】ＦＥＴバッファを用いた従来の電流電圧変換回路を示す図である。
【符号の説明】
１・・・演増幅器
２・・・焦電素子
３・・・短絡用スイッチ回路
４１・・・第１の試験パッド
４２・・・第２の試験パッド
５・・・直流帰還回路
５１・・・演算増幅器
Ｃｆ・・・帰還容量
Ｒｉ・・・入力抵抗（高抵抗）
Ｒ１・・・抵抗
Ｃ１・・・コンデンサ[0001]
BACKGROUND OF THE INVENTION
The present invention uses a pyroelectric element to detect infrared energy radiated from the human body, to detect the presence and movement of the human body, or to detect radiation energy and room temperature, thereby acting as a radiation thermometer. The present invention relates to an improvement of a type infrared detection device and a current-voltage conversion circuit used therein.
[0002]
[Prior art]
Conventionally, a current-voltage conversion circuit using an FET is used in this type of pyroelectric infrared detector.
As shown in FIG. 4, this current-voltage conversion circuit connects the pyroelectric element Cs and the high resistance Rg in parallel to the gate of the FET, and takes out a voltage signal from the output resistance Rs connected to the source and ground of the FET. The pyroelectric current output from the pyroelectric element Cs when a heat ray is sensed is converted into a voltage by the high resistance Rg, and is received by the gate of the FET, and a current is passed through the FET and the resistance Rs to allow the FET to flow. The source voltage is changed, and the voltage applied to the resistor Rs at that time is output to the signal amplifier circuit.
[0003]
However, in such a current-voltage conversion circuit using an FET buffer, a high resistance having a very high resistance value is used for impedance conversion in order to suppress the influence of thermal noise and improve the S / N ratio. However, if attention is paid to the vicinity of 1 Hz in human body detection, which is the largest application of pyroelectric elements, the stability of the operation in the pyroelectric sensor, that is, external noise, FET bias current fluctuation, high resistance itself resistance value Considering changes and the like, desiring further improvement of the S / N ratio is almost at the limit.
[0004]
Therefore, the present inventors have considered the current situation in consideration of the fact that the frequency characteristic of the impedance of the capacitor is used for current-voltage conversion from various viewpoints, and as a result, further improved the S / N ratio. A useful and feasible current / voltage conversion circuit is known and proposed in Japanese Patent Application No. 9-91047 to Japanese Patent Application No. 9-91049.
[0005]
[Problems to be solved by the invention]
By the way, when the current-voltage conversion circuit using the frequency characteristics of the capacitor impedance already proposed by the present inventors for current-voltage conversion is actually used, the basic operational amplifier to which the feedback capacitance is added is further connected to the DC. A feedback circuit is additionally connected via an input resistor. In this case, the input resistor is not used for impedance conversion, and therefore does not directly affect the S / N ratio as in a current-voltage conversion circuit using a conventional FET. However, a high resistance value is also used to suppress thermal noise.
Therefore, once such a high resistance is mounted on a circuit board or the like, a method that can not be measured by a normal resistance measuring instrument but can be easily measured has been desired.
[0006]
[Means for Solving the Problems]
The present invention responds to the above-mentioned demand, and a current-voltage conversion circuit proposed in claim 1 is provided with an operational amplifier to which a feedback capacitor to which a pyroelectric element is connected is added, and to the operational amplifier via an input resistor. And a short-circuit switch circuit which is connected in parallel with the feedback capacitor and short-circuits both ends of the feedback capacitor, and the short-circuit switch circuit is turned on. A first test pad for applying a control voltage to be turned off, and a second test pad is connected to a connection point between the DC feedback circuit and the input resistor, and is connected to an output terminal of the operational amplifier. Is provided with an output node.
This current-voltage conversion circuit has a configuration excluding a pyroelectric element, and the pyroelectric element is connected to an operational amplifier so that a resistance value having a high input resistance can be measured before mounting. .
[0007]
In the current-voltage conversion circuit proposed in claim 2, a pyroelectric element is connected to the operational amplifier to which a feedback capacitor is added, and a DC feedback circuit is additionally connected to the operational amplifier via an input resistor. The feedback capacitor is provided with a short-circuit switch circuit for short-circuiting both ends of the feedback capacitor, and a control voltage for turning on and off the short-circuit switch circuit is applied to the short-circuit switch circuit. A second test pad is connected to a connection point between the DC feedback circuit and the input resistor, and an output node is provided at the output terminal of the operational amplifier. It is configured.
In this current-voltage conversion circuit, a high resistance value can be measured with the pyroelectric element and the input resistance mounted.
[0008]
In the current-voltage circuit proposed in claim 3, when the first and second test pads are connected to the ground, the operational amplifier has a potential difference equal to the reference voltage of the operational amplifier at both ends of the input resistor. The configuration is to be generated.
As a result, when a control voltage for turning off the short-circuit switch circuit is applied to the second test pad, the current flowing through the input resistor is made constant when charge is released from the feedback capacitor.
[0009]
According to the fourth aspect of the present invention, there is proposed a DC feedback circuit configured by an integration circuit configured by connecting and adding a capacitor and a resistor to another operational amplifier different from the operational amplifier to which a feedback capacitor is additionally connected.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
[Circuit embodiment]
FIG. 1 shows a basic configuration of a current-voltage conversion circuit which forms a main part of the present invention.
As shown in the figure, the operational amplifier 1 has an input terminal having an input node connected to the pyroelectric element 2 and a reference terminal for connecting a reference voltage Vr.
In the illustrated example, the input terminal of the operational amplifier 1 is an inverting input terminal, and the output terminal is a non-inverting input terminal. A feedback capacitor Cf is connected between the input terminal and the output terminal of the operational amplifier 1, and this feedback. The capacitance Cf can be short-circuited at both ends by a short-circuit switch circuit 3 controlled by a control voltage applied to the first test pad 41.
[0011]
An output node is provided at the output terminal of the operational amplifier 1, and a non-inverting input terminal of another operational amplifier 51 constituting the DC feedback circuit 5 is connected to the output node, and the inverting input of the operational amplifier 51 is connected. The terminal is connected to a resistor R1 and a reference voltage Vr having the other end connected to the ground. The resistor R1 is further connected to a connection point between the output end of the operational amplifier 51 and the input resistor via another feedback capacitor C1. A second test pad 42 is provided at the connection point.
[0012]
Here, the DC feedback circuit 5 is configured as an integrating circuit in which a capacitor C1 and a resistor R1 are added to the operational amplifier 51. Most of such a current-voltage conversion circuit is integrated into an IC on a semiconductor substrate. The pyroelectric infrared detector can be made small and thin.
According to the current-voltage conversion circuit having such a configuration, when converting the pyroelectric current generated when the pyroelectric element senses a change in heat amount into the voltage signal using the impedance characteristic of the feedback capacitor Cf, the conventional FET buffer is used. Since the impedance conversion is not performed with a high resistance as described above, the influence of thermal noise due to the high resistance is remarkably eliminated.
Therefore, the noise source can be fundamentally eliminated, the total noise can be reduced, and as a result, the S / N can be improved. This point is as disclosed in the aforementioned application.
[0013]
Next, the measurement principle of the input resistance mounted on the current-voltage conversion circuit of the present invention will be described.
When measuring the input resistance Ri with reference to FIG. 1, first, the first test pad 41 and the second test pad 42 are connected to the ground, and the operational amplifier 1 is powered on in this state.
As a result, the short-circuit switch circuit 3 short-circuits the feedback capacitor Cf, so that the operational amplifier 1 with negative feedback acts as a buffer.
As a result, the voltages at the input terminal and the output terminal of the operational amplifier 1 are both equal to the reference voltage Vr, and the connection point between the output terminal of the operational amplifier 51 and the input resistance is also connected to the ground . A reference voltage Vr generated at the input terminal is applied, and the circuit is stabilized in that state.
[0014]
Next, when the control voltage of the first test pad 41 is increased to the power supply voltage VDD, as shown in the simulation result of FIG. 3 (described later), a current required by I = Vr / Ri flows through the input resistance Ri, and feedback is performed. A potential difference ΔV = (I · t) / Cf occurs between both ends of the capacitor Cf, and the potential of the output node of the operational amplifier 1 finally reaches VDD.
Accordingly, if the time t required for the voltage of the output node to reach VDD from Vr and the potential difference at that time are measured, the input resistance Ri is obtained from the equation Ri = (Vr · t) / (Cf · ΔV). Can be measured.
[0015]
FIG. 2 shows an example of a current-voltage conversion circuit to which the pyroelectric element 2 is connected. In this example, since the equivalent resistance R2 of the pyroelectric element is connected in parallel to the input resistance Ri, the combined resistance can be measured. Therefore, after measuring the combined resistance in the same manner as described above, Needless to say, only the input resistance Ri can be calculated except for the equivalent resistance R2 of the connected pyroelectric element.
[Example of simulation]
FIG. 3 shows a simulation result when VDD = 2V, Vr = 1V, and Cf = 10 pF in the current-voltage conversion circuit of the present invention.
Vg in FIG. 2 indicates the control voltage applied to the first test pad 41, and Vout indicates the potential of the output node.
The first and second test pads 41 and 42 are connected to the ground, the circuit is stabilized for t = 10 seconds, and then the control voltage Vg of the first test pad 41 is raised from 0V to 2V. As a result, the short-circuit switch circuit 3 is turned off.
At the time when the switch circuit 3 is turned off, the voltage at the output node is equal to the reference voltage Vr of the operational amplifier 1 and becomes 1V. However, after t = 10 seconds, the voltage Vout at the output node becomes the reference voltage Vr = 1V. The power supply voltage reaches VDD = 2V, and is saturated.
That is, in FIG. 3, ΔV = 1V and t = 10 seconds.

It can be seen that the input resistance Ri can be measured.
[0016]
In the above example, the output node voltage Vout of the operational amplifier 1 applies the control voltage Vg to the first test pad 41 from the time when the first and second test pads 41 and 42 are connected to the ground, and the power supply The time t until the voltage is saturated and the potential difference ΔV in that case are obtained and the resistance value of the input resistance is measured, but the potential difference (voltage change) in the middle until the voltage at the output node of the operational amplifier 1 is saturated. ) And the time until the potential difference is obtained, the resistance value of the input resistance may be obtained. In this case, even when the resistance value is extremely large, quick measurement is possible.
[0017]
【The invention's effect】
According to the present invention, it is not possible to measure with a normal resistance measuring instrument having a high resistance value connected between the operational amplifier and the DC amplifier circuit, simply by applying a voltage change to the first and second test pads. Even a simple input resistance can be easily measured while mounted on a circuit board.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of an embodiment (claim 1) of the present invention.
FIG. 2 is a circuit diagram of one embodiment (claim 2) of the present invention.
FIG. 3 is a graph showing a simulation result of the operation of the present invention.
FIG. 4 is a diagram showing a conventional current-voltage conversion circuit using an FET buffer.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Amplifier 2 ... Pyroelectric element 3 ... Short circuit switch circuit 41 ... 1st test pad 42 ... 2nd test pad 5 ... DC feedback circuit 51 ... Operational amplifier Cf ... feedback capacitance Ri ... input resistance (high resistance)
R1 ... resistor C1 ... capacitor

Claims (4)

An operational amplifier with an added feedback capacitance to which the pyroelectric element is connected;
A DC feedback circuit additionally connected to the operational amplifier via an input resistor;
A short-circuit switch circuit connected in parallel with the feedback capacitor and short-circuiting both ends of the feedback capacitor; and a control voltage for turning on and off the short-circuit switch circuit is applied to the short-circuit switch circuit. A configuration in which a first test pad is connected, a second test pad is connected to a connection point between the DC feedback circuit and the input resistor , and an output node is provided at an output terminal of the operational amplifier A current-voltage conversion circuit in a pyroelectric infrared detector. A pyroelectric element is connected to the operational amplifier to which the feedback capacitance is added,
A DC feedback circuit is additionally connected to this operational amplifier via an input resistor.
The feedback capacitor is provided in parallel with a short-circuit switch circuit for short-circuiting both ends of the feedback capacitor, and the short-circuit switch circuit is applied with a control voltage for turning on and off the short-circuit switch circuit. The test pad is connected, the second test pad is connected to the connection point between the DC feedback circuit and the input resistor, and the output node is provided at the output terminal of the operational amplifier . A current-voltage conversion circuit in a pyroelectric infrared detector. In any one of Claims 1, 2.
The operational amplifier is a current-voltage conversion circuit configured to generate a potential difference equal to a reference voltage of the operational amplifier at both ends of the input resistance when the first and second test pads are connected to the ground. In any one of Claims 1, 2.
The DC feedback circuit is a current-voltage conversion circuit configured by an integration circuit configured by adding a capacitor and a resistor to another operational amplifier different from the operational amplifier to which the feedback capacitor is additionally connected.

Images