JPH10300572A - Infrared ray sensor reading-out circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To constitute an infrared ray sensor reading-out circuit so that it may be operated with a power source of low voltage without lowering a conversion gain. SOLUTION: The resistance value of a bolometer which constitutes the pixel of an infrared ray sensor is converted into a current value by a transistor 2, and then converted into a voltage value by an integration capacitor 7, then sent to a sample hold circuit at the next stage. When a pixel is selected, firstly the capacitor 7 is reset by a switch 6. At that time, the capacitor 7 is charged by a power source Vr to a voltage Vr. When the resetting is finished, an integral action with the capacitor 7 and a bolometer resistance value Rb starts. An electric charge is accumulated in the capacitor 7, and an output voltage Vi drops, and when the voltage falls below a reference voltage VL of a comparator 9, the comparator 9 comes to a high level to invert a toggle flip-flop 10, so that the connection direction of the capacity 7 is inverted by 180 deg.. Then, the voltage Vi rises to 2 Vr-VL without delay, for restarting the integral action. When the Vi reaches the VL, the above stated action is repeated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal infrared sensor used in an image pickup apparatus utilizing infrared rays, and more particularly to a circuit for reading out signals from the sensor.

[0002]

2. Description of the Related Art Conventionally, a readout circuit of this type of infrared sensor has been disclosed in Japanese Patent Application Laid-Open No. H8-10579, for example, with respect to a bolometer type infrared sensor.
It is used for reading the signal at the highest SN ratio.

FIG. 3 is a circuit diagram showing an example of a conventional infrared sensor reading circuit, and FIG. 4 is a timing chart of the reading circuit of FIG. In the infrared sensor 1, a bolometer is two-dimensionally arranged (this is called a pixel and its resistance is Rb), and a bolometer of an arbitrary pixel can be selected by a horizontal switch and a vertical switch by a pixel selection pulse. It has a structure.

The read circuit includes a power supply 3 for supplying a bias voltage Vb to the infrared sensor 1, a transistor 2 for reading a current flowing to the infrared sensor 1, a capacitor 7 for storing a signal current, and a switch 6 for resetting the capacitor 7. It is constituted by.

Next, the operation will be described with reference to FIGS. The bolometer is made of metal such as Ti or poly.
-A heat sensitive element using a semiconductor such as -Si as a base material, and its resistance value changes with temperature. Then, when infrared rays are condensed on the bolometer using a lens, the energy of the bolometer increases the temperature. Therefore, an infrared sensor can be realized by reading the temperature as a change in the resistance value Rb.

Now, it is assumed that a certain pixel is selected by the pixel selection pulse shown in FIG. The period of one pixel in FIG. 5 indicates the time during which this pixel is selected, and the signal from that pixel must be read during this period. For reading, a method of applying a constant voltage to the bolometer using the bias power supply 3 and the transistor 2 and reading a change in resistance value from a change in flowing current is adopted. The readout circuit performs this operation. The change in the current is converted into a voltage by the capacitor 7 and sent to the next stage (sample and hold circuit).

When a pixel is selected, first, the capacitor 6 is reset by the switch 6. That is, the switch 6 is turned on by the reset pulse ΦRST, and the capacitor 7 is charged to the voltage Vr by the power supply Vr. on the other hand,
The following current Ie flows through the bolometer, and this current is supplied from the power supply Vr during reset.

Ie = (Vb−Vbe) / Rb (1) Vbe: Base-emitter voltage of the transistor 2 When the switch 6 is turned off, the integration operation by the capacitor 7 and the resistance value Rb of the bolometer starts.

The collector current Ic of the transistor 2 as a constant current source is almost the same as the emitter current Ie, and this collector current Ic is supplied from the capacitor 7 during the integration period. As a result, the rate of change of the voltage Vi of the capacitor 7 varies depending on the magnitude of Ie (that is, Ic), that is, the magnitude of the resistance value Rb of the bolometer.

The time from when the switch 6 is turned off to when the sample and hold is performed by the next stage circuit is represented by the integration time Ti
Then, the voltage Vo of the capacitor 7 at the end of the integration is
Is as follows: Vo = Vr−Ic · Ti / C (2) (C is the capacitance value of the capacitor 7). Assuming that the variation of Vo is ΔVo, IcｅIe, and ΔVo = ｛(Vb−Vbe) / Rb｝ · Ti / C (3)

The voltage applied to the bolometer (Vb-V
If the ratio of ΔVo to be) is called an integral gain G, the integral gain G is given by G = Ti / C / Rb (4)

As described above, the resistance value Rb of the bolometer can be converted to a voltage value by using the integration type readout circuit.

[0013]

The characteristic of this integration type readout circuit is that a signal (I) containing information on the intensity of infrared rays is included.
e) can be read from the infrared sensor 1 and converted to a voltage at the same time as amplification, and the larger the conversion gain (integral gain) G, the larger the output signal amplitude can be. The effect can be reduced.

However, when the integral gain G is increased, the variation ΔVo of the voltage of the capacitor 7 is also increased, and a high-voltage power supply corresponding to the change is required, resulting in an increase in size, cost, and power consumption of the device. There is a problem.

For example, Rb = 3 kΩ, c = 100 pF,
Considering the case where Ti = 1 μs and Vb−Vbe = 6 V, the conversion gain G is 3.3 from the above equation, and ΔVo = 2
It becomes 0V. Therefore, the reset voltage Vr is: Vr> (Vb−Vbe) + ΔVo + Vce + Vs> 2
8.5 V Vce: Collector-emitter voltage mi of transistor 2
n about 0.5 V Vs: signal voltage is about 2 V, and usually requires a voltage of 30 V or more. If the conversion gain G is increased by 1, the reset voltage Vr can operate as an integrating circuit even at about 15 V, but deterioration of the SN ratio is inevitable.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to configure a reading circuit of a sensor so that it can operate with a lower-voltage power supply without lowering the conversion gain.

[0017]

An infrared sensor readout circuit according to the present invention comprises a comparator for detecting an integrated amount and comparing it with a reference amount, a toggle flip-flop for inverting a level based on a comparator output, and a toggle flip-flop output. It has two switches for reversing the connection direction of the integration capacitor.

The amount of integration of the capacitor, that is, the voltage across the capacitor, is constantly monitored and never exceeds a predetermined voltage. Therefore, there is no need to set the reset voltage, that is, the power supply voltage to a high voltage.

[0019]

FIG. 1 is a circuit diagram showing an embodiment of an infrared sensor reading circuit according to the present invention.
Is a timing chart for explaining the operation of the present invention.
In FIG. 1, the infrared sensor 1, the transistor 2, and the bias power supply 3 have the same configuration as that of the conventional example.

The comparator 9 constantly compares the voltage Vi at the lower end of the capacitor 7 with the reference voltage VL of the reference voltage source 8,
The output level is changed according to the magnitude. The toggle flip-flop 10 receives the output of the comparator 9 and inverts the output. The inverting switches 4 and 5 uniquely determine the connection point corresponding to the output level of the toggle flip-flop 10. When the reset switch 6 is turned on, the electric charge accumulated in the capacitor 7 is discharged.

Next, the operation of the present invention will be described with reference to FIGS. Now, it is assumed that a certain pixel is selected by the pixel selection pulse in FIG. Also in the circuit of the present invention, the period of one pixel in FIG. 2 indicates the time during which this pixel is selected, and the signal from that pixel must be read during this period. For reading, a method of applying a constant voltage to the bolometer using the bias power supply 3 and the transistor 2 and reading a change in resistance value from a change in flowing current is adopted. The change in the current is converted into a voltage by the capacitor 7 and sent to the next stage (sample and hold circuit).

When a pixel is selected, first, the capacitor 6 is reset by the switch 6. However, the reset is performed by turning on the switch 6 by the reset pulse ΦRST and discharging the electric charge accumulated in the capacitor 7. That is, the switch 6 is activated by the reset pulse φRST.
Is turned on, the electric charge accumulated in the capacitor 7 is discharged, and the output voltage Vi is set to the power supply voltage Vr.

On the other hand, the bolometer has the current Ie of the equation (1).
During reset, this current is supplied from the power supply Vr via the reset switch 6.

When the switch 6 is turned off, the capacitor 7
And the integration operation based on the resistance value Rb of the bolometer starts. The collector current Ic of the transistor 2 is supplied from the power supply Vr via the capacitor 7 during the integration period. As a result, a charge is accumulated in the capacitor 7 to generate a voltage, and the potential of Vi decreases (FIG. 2A). When this potential falls below the reference voltage VL, the comparator 9 goes high (FIG. 2C), and the toggle flip-flop 1
0 is inverted (FIG. 2D), and the connection direction of the capacitor 7 is inverted by 180 degrees. Then, Vi instantaneously (2V
r-VL) (FIG. 2A), and the integration operation is started again. When Vi reaches VL, the above operation is repeated.

The output voltage Vi does not exceed a predetermined voltage range (VL to 2Vr-VL).
The output voltage sampled and held is also VL to 2Vr-VL
, The change range of the resistance value Rb of the bolometer,
By appropriately setting the relationship between the capacitance value of the capacitor 7 and the reading period of one pixel, the resistance value Rb of the bolometer and the sample-hold output voltage value in the range of VL to 2Vr-VL correspond one-to-one. Can be.

Also, in the case of the circuit of the present invention, the integral gain D can be expressed by equation (4). Therefore, the infrared sensor readout circuit of the present invention can lower the voltage of the power supply Vr while securing the required integral gain. For example, if VL = 10 V, it is sufficient to set Vr to 15 V even under the above-mentioned conditions, and this kind of infrared sensor readout circuit can be realized by a low-voltage power supply without lowering the SN ratio. it can.

[0027]

The infrared sensor readout circuit according to the present invention comprises:
Since the power supply voltage can be lowered, the size, cost, and power consumption of the device can be reduced.

[0028]

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a timing chart illustrating the operation of FIG.

FIG. 3 is a circuit diagram of a conventional example.

FIG. 4 is a timing chart illustrating the operation of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Infrared sensor 2 Transistor 3 Bias power supply 4,5 Inversion switch 6 Reset switch 7 Capacitor 8 Reference power supply 9 Comparator 10 Toggle flip-flop

Claims (4)

[Claims] 1. An infrared sensor reading circuit for reading an output from an infrared sensor using a bolometer-type thermoelectric conversion element, wherein the reading circuit includes a transistor for converting a resistance value of the infrared sensor into a current value, and an output terminal of the transistor. And a power supply, which are connected in series and convert the current value to a voltage value, a reset switch connected between both ends of the integration capacitor, and a connection point between the transistor and the integration capacitor. An infrared sensor reading circuit, comprising: a comparator for comparing an output integrated output voltage with a predetermined reference voltage; and a switch for inverting a connection direction of the integrating capacitor based on an output of the comparator. 2. The infrared sensor, wherein a plurality of bolometer-type thermoelectric conversion elements are arranged two-dimensionally as pixels, and an arbitrary element can be selectively connected to the readout circuit by pixel selection means. The infrared sensor readout circuit according to claim 1, wherein: 3. A switch according to claim 1, further comprising a toggle flip-flop for inverting a level based on an output of said comparator, wherein said switch inverts a connection direction of said integrating capacitor according to an output of said toggle flip-flop. 3. The infrared sensor reading circuit according to 1 or 2. 4. The infrared sensor reading circuit according to claim 1, further comprising a sample and hold circuit for inputting the integrated output voltage, and reading out the resistance value as a voltage value.