JP2930100B2 - Infrared sensor level adjustment circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a level adjustment circuit of an infrared sensor, and more particularly to a level adjustment circuit of an infrared sensor in an infrared imaging apparatus using an infrared FPA (Focal Plane Array) whose thermal element is a bolometer.

[0002]

2. Description of the Related Art Conventionally, an infrared imaging apparatus of this type is provided with an integrating circuit for converting a change in bolometer resistance into a voltage change and reading out the voltage change. The upper limit is restricted.

As a method of correcting the variation of the bolometer resistance, a method of performing level adjustment can be considered. As a circuit for performing such a level adjustment, there is, for example, an automatic gain control circuit disclosed in Japanese Patent Application Laid-Open No. 59-37716. It is used in a wide range of fields, taking advantage of the low cost of adjustment.

As shown in FIG. 8, an automatic gain control circuit, which is one of the level adjustment circuits, includes an input resistor 21, an operational amplifier 22, a rectifier circuit 23, an integration circuit 24, a reference voltage circuit 25, a comparator 26 and shift register 27
And a feedback resistor 28.

The operational amplifier 22 is a central amplifier for obtaining a gain. The input resistor 21 is one resistor for determining the gain, and the other resistor is a feedback resistor 28. Rectifier circuit 2
3 rectifies the operational amplifier output waveform. The integration circuit 24 integrates the rectified signal. The reference voltage circuit 25 generates a reference voltage.

[0006] The comparator 26 compares the reference voltage with the output voltage of the integration circuit, and outputs a high or low signal. The shift register 27 shifts the data left and right according to the comparator output signal, and turns the switch on or off based on the result.

The gain of the automatic gain control circuit is determined by the ratio between the resistor 21 and the feedback resistor 28. For example, when the gain is too large, the output of the operational amplifier 22 increases. As a result, the output of the rectifier circuit 23 also increases. Integrator 2
4 is necessary to stabilize this control system.

In the above case, the result of the comparison between the output of the integration circuit 24 and the reference voltage becomes high, and the shift register 27 is shifted rightward. The switch in the feedback resistor 28 is controlled according to the data in the shift register 27, and the feedback resistance value decreases. As a result, the gain of this circuit is reduced. As described above, the gain of the automatic gain control circuit is automatically adjusted.

[0009]

In the above-mentioned conventional infrared imaging apparatus, the upper limit of the conversion gain of the integration circuit is limited by the variation of the bolometer resistance, so that the conversion gain of the integration circuit cannot be increased. The subsequent circuit noise cannot be ignored, and the S / N ratio (Signal-t
o-Noise Ratio).

As a method of correcting the variation of the bolometer resistance, a method of adjusting the level can be considered. In the above automatic gain control circuit, the magnitude becomes constant based on the average value of the signal obtained by the rectifier circuit. Control the gain in such a way that the average value of the gain over time is controlled,
Inability to control short-time signal levels.

In this case, the integration circuit also causes a longer control response time. In addition, in this type of feedback control circuit, an integrating circuit is indispensable for ensuring control stability and accuracy, and makes high-speed response difficult in principle.

That is, it is difficult to use the above-described automatic gain control circuit for a signal such as an infrared sensor signal handled by an infrared imaging apparatus, which needs to be adjusted at high speed and for each pixel. is there.

Accordingly, an object of the present invention is to solve the above-mentioned problems, and to perform effective level adjustment for a signal such as an infrared sensor signal which requires high-speed and level adjustment for each pixel. An object of the present invention is to provide a level adjustment circuit for an infrared sensor.

[0014]

A level adjusting circuit of an infrared sensor according to the present invention selectively supplies a bias voltage from a constant voltage bias circuit to each of a plurality of bolometer type thermoelectric conversion elements arranged in a grid. A level adjustment circuit of an infrared sensor for selectively reading a current flowing through each of the plurality of bolometer-type thermoelectric conversion elements and obtaining data for each of the bolometer-type thermoelectric conversion elements, wherein a level of each of the plurality of bolometer-type thermoelectric conversion elements is Storage means for storing adjustment value data of each of the plurality of bolometer-type thermoelectric conversion elements obtained by performing measurement in advance, and the plurality of bolometer-type thermoelectric elements based on the adjustment value data stored in the storage means; A digitally controlled variable current source for supplying a level adjustment current corresponding to each of the conversion elements; And means for performing addition and subtraction of the level adjusting current supplied from the current that is selectively read from the child each said digital control variable current source.

That is, the level adjustment circuit of the infrared sensor of the present invention comprises a memory for holding data of a level adjustment value for each pixel, a level shifter for converting an output voltage of the memory into a voltage capable of driving an analog switch, It has a digital control current source that controls the current with a value, and a sequencer that operates each unit at a predetermined timing.

In the memory, level adjustment value data for each pixel of the infrared sensor is measured and written in advance.
The digital control current source generates a level adjustment current according to the level adjustment value data. The level adjustment is performed for each pixel (bolometer-type thermoelectric conversion element) using the level adjustment current. As described above, since the level can be adjusted in the open loop, high-speed adjustment can be performed.

[0017]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing a level adjustment circuit of an infrared sensor according to one embodiment of the present invention. In the figure, an infrared imaging apparatus according to one embodiment of the present invention includes an infrared sensor 1, a readout circuit 2, a digital control current source 3, level shifters 6, 7, a memory 8, a sequencer 9, a sample and hold circuit 10, It is comprised including. Here, the digital control current source 3 has R-2R
Digital variable resistor (Rx) using ladder resistor network 5
4 are provided.

In the infrared sensor 1, a plurality of bolometers 1a are arranged two-dimensionally (in a lattice), and each of the plurality of bolometers 1a forms a pixel and has a resistor Rb. In this infrared sensor 1, a vertical switch 1
The structure is such that the bolometer 1a of an arbitrary pixel can be selected by b and the horizontal switch 1c.

A read circuit 2 supplies a bias voltage to the infrared sensor 1, a transistor Q 1 for reading a current flowing to the infrared sensor 1, a capacitor C for storing a signal current, and a reset circuit for resetting the capacitor C. FET (Field Effect Transistion)
tor) Q2.

The digital control current source 3 comprises a digital variable resistor 4, a voltage source Vc and transistors Q3 and Q4 for generating currents corresponding to both values. The digital variable resistor 4 includes an R-2R ladder resistor network 5 and analog switches S1 to Sk.

In the memory 8, adjustment value data obtained by measuring in advance for each pixel (bolometer 1a) is written. The level shifter 6 generates a voltage necessary to drive the analog switches S1 to Sk of the digital variable resistor 4. The level shifter 7 generates a voltage necessary to drive the FET Q2 of the read circuit 2. The sequencer 9 generates a pulse for operating each unit at an appropriate timing. The sample hold circuit 10 has a role of shaping the output waveform of the read circuit 2.

FIG. 2 is a diagram showing a configuration example of the analog switches S1 to Sk of the digital variable resistor 4 of FIG. In the figure, the analog switches S1 to Sk are FET Qa,
Qb.

FIG. 3 is a diagram for explaining the relationship between the output data of the memory 8 of FIG. 1 and the output current of the digital control current source 3. FIG. 4 is a timing chart of the level adjustment function according to one embodiment of the present invention. FIG. 5 is a timing chart for explaining the operation of level adjustment according to one embodiment of the present invention. The operation of level adjustment according to one embodiment of the present invention will be described with reference to FIGS.

Here, the bolometer 1a is a heat-sensitive element whose base material is a metal such as Ti or a semiconductor such as poly-Si, and its resistance value changes with temperature. When infrared light is condensed on the bolometer 1a using a lens, the temperature of the bolometer 1a rises due to the energy. Therefore, the infrared sensor 1 can be realized by reading the rise in temperature as a change in resistance. .

In FIG. 1, a method is adopted in which a constant voltage is applied to the bolometer 1a to be read, and a change in the resistance value is read from a change in the flowing current. This is done by the read circuit 2
Then, the change in the current is converted into a voltage using the capacitor C.

The bolometers 1a are arranged in a lattice pattern, and have different resistance values. Assuming that the resistance value of the bolometer 1a is Rb, the current Ib flowing through the bolometer 1a
Ib = (Vb-Vbe1) / Rb Vbe1: Base-emitter voltage of Q1. FIG. 4 shows this as a one-dimensional model.

When the resistance value Rb of the bolometer 1a is further decomposed, Rb = Rbo + ΔRb is obtained. Rbo is a fixed value that does not change, and ΔRb is a component that changes according to infrared energy focused on the bolometer 1a.

Usually, ΔRb is equal to or less than 1/10000 of Rbo. Therefore, most of the variation in the resistance value Rb of the bolometer 1a shown in FIG.
and the variation of the current Ib is also a fixed value R
This can be attributed to the variation in bo.

The digital control current source 3 is a variable current source that can be controlled by digital data. R-
A 2R ladder resistor network 5, analog switches S1 to Sk, and a resistor Re constitute a digital variable resistor (Rx) 4 that can be controlled by digital data, and a voltage source Vc
And the transistor Q3, a current source is formed. Assuming that the level adjustment current that can be generated by the digital control current source 3 is Ic, Ic = (Vc−Vbe3) / RxVbe3: Base-emitter voltage of Q3. The level adjustment current Ic is a function of digital data, and its characteristics are shown in FIG.

The minimum level adjustment current Icmin and the maximum level adjustment current Imax are as follows: Icmin = (Vc-Vbe3) / ReImax = (Vc-Vbe3) / (Re // R) In this case, the resistance R
e and R may be selected.

In the memory 8, level adjustment value data for each pixel (bolometer 1 a) of the infrared sensor 1 obtained in advance by measurement is written. The memory 8 receives the memory address and read pulse of the pixel from the sequencer 9 at the timing shown in FIG. 5 and outputs level adjustment value data corresponding to the pixel to be read from the infrared sensor 1.

The level adjustment value data is stored in the level shifter 6.
Are converted to voltages that can drive the analog switches S1 to Sk, and the level adjustment current Ic is generated as described above. The level adjustment current Ic is, for example, as shown in FIG. The level adjustment current Ic is
1 is injected into the collector of the capacitor C.
Is stored as Ii = Ib-Ic, and if the level adjustment value data is appropriate, the variation between pixels is corrected as shown in FIG.

Referring to the output waveform of the readout circuit 2, as shown in FIG. 5, if the level adjustment is not performed, the readout circuit output voltage ΔVa greatly varies from pixel to pixel. However, when the level is adjusted, the output voltage of the readout circuit can be set to a substantially constant ΔVb.

Imax-Icmin is defined as the pe of the variation.
If it is adjusted to ak-to-peak, the variation can be reduced to 1 / (2 ^k -1) by performing this correction. Here, k is the number of bits of the digital variable resistor 4.

The level adjustment according to the embodiment of the present invention is an open loop system in which adjustment is performed based on the level adjustment value data stored in the memory 8 in advance. It can also be applied to the level adjustment of. The level adjustment according to the embodiment of the present invention is characterized in that the adjustment is performed not by the gain of the amplifier but by the addition and subtraction of an offset.

FIG. 6 is a circuit diagram showing a level adjusting circuit of an infrared sensor according to another embodiment of the present invention. In the figure, transistors Q3 and Q
4 is combined with the operational amplifiers AMP1 and AMP2. In the digital control current source 3 shown in FIG. 1, the emitter potential of the transistor Q3 and the emitter potential of the transistor Q4 must be the same.

However, the analog switches S1 to S
Depending on the state of k, the collector current of the transistor Q3 and the collector current of the transistor Q4 differ,
As a result, the base-emitter voltage of the transistor Q3 is different from the base-emitter voltage of the transistor Q4, and the emitter potentials of the transistor Q3 and the transistor Q4 do not become the same. Then, the characteristics as shown in FIG. 3 cannot be obtained, resulting in a problem that accuracy is deteriorated and distortion is increased.

In the digitally controlled current source 11 according to another embodiment of the present invention, the operational amplifier A
Since the emitter potentials of the transistors Q3 and Q4 are fixed to Vdd-Vc by MP1 and AMP2, the above problem can be solved.

FIG. 7 is a circuit diagram showing a level adjusting circuit of an infrared sensor according to another embodiment of the present invention. In the figure, an R-2R ladder resistor network 5 and analog switches S1 to S1 are shown.
Sk is in the base of the transistor Q3. Since this is considered to be a voltage source that can be controlled by digital data, if this voltage is Vbb, then Ic = (Vbb-Vbe3) / Re, and the digitally controlled current source 12 can be realized.

In the digital control current source 12, the number of transistors can be reduced by one, and at the same time, the two transistors Q3 and Q3 of the digital control current source 3 shown in FIG.
Problems due to imbalance of Q4 can be avoided.

As described above, the bolometers 1a of the infrared sensor 1 are opened in an open loop by using the level adjustment currents supplied from the digitally controlled variable current sources 3, 11, and 12.
By performing the (pixel) level adjustment, it is possible to perform effective level adjustment for a signal requiring a level adjustment for each pixel at a high speed, such as a signal from the infrared sensor 1.

[0042]

As described above, according to the present invention, when a bias voltage is supplied from a constant voltage bias circuit to each of a plurality of bolometer-type thermoelectric conversion elements arranged in a lattice, a plurality of bolometer-type thermoelectric conversion elements are provided. In a level adjustment circuit of an infrared sensor for selectively reading a current flowing through each of them and obtaining data for each of the bolometer-type thermoelectric conversion elements, a plurality of measurement results obtained in advance for performing the level adjustment of each of the plurality of bolometer-type thermoelectric conversion elements The adjustment value data of each of the bolometer-type thermoelectric conversion elements is stored, and a level adjustment current corresponding to each of the plurality of bolometer-type thermoelectric conversion elements is supplied based on the stored adjustment value data. Addition and subtraction of the current selectively read from each thermoelectric conversion element and the level adjustment current supplied from the digitally controlled variable current source Ukoto by, there is an effect that it is also possible to perform effective level adjustments to high speed and the required signal level of the adjustment for each pixel as an infrared sensor signal.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a level adjustment circuit of an infrared sensor according to one embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration example of an analog switch of the digital variable resistor in FIG. 1;

FIG. 3 is a diagram for explaining a relationship between output data of a memory of FIG. 1 and an output current of a digital control current source.

FIG. 4 is a timing chart of a level adjustment function according to an embodiment of the present invention.

FIG. 5 is a timing chart illustrating an operation of level adjustment according to an embodiment of the present invention.

FIG. 6 is a circuit diagram showing a level adjustment circuit of an infrared sensor according to another embodiment of the present invention.

FIG. 7 is a circuit diagram showing a level adjustment circuit of an infrared sensor according to another embodiment of the present invention.

FIG. 8 is a block diagram showing a configuration of a conventional automatic gain adjustment circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Infrared sensor 1a Bolometer 1b Vertical switch 1c Horizontal switch 2 Readout circuit 3,11,12 Digital control current source 4 Digital variable resistor 5 R-2R ladder resistance network 6,7 Level shifter 8 Memory 9 Sequencer 10 Sample hold circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01J 1/44 G01J 5/22 H01L 27/14 H04N 5/33 G01J 1/02 G01J 5/02 G01J 5 / 48

Claims (4)

(57) [Claims] 1. A bias voltage is selectively supplied from a constant voltage bias circuit to each of a plurality of bolometer-type thermoelectric conversion elements arranged in a lattice, so that a current flowing through each of the plurality of bolometer-type thermoelectric conversion elements is selectively selected. A level adjustment circuit of the infrared sensor for obtaining data for each of the bolometer-type thermoelectric conversion elements, wherein the plurality of bolometers obtained by measuring in advance to perform level adjustment of each of the plurality of bolometer-type thermoelectric conversion elements Means for storing adjustment value data for each of the thermoelectric conversion elements, and digital control for supplying a level adjustment current corresponding to each of the plurality of bolometer-type thermoelectric conversion elements based on the adjustment value data stored in the storage means Type variable current source, a current selectively read from each of the plurality of bolometer type thermoelectric conversion elements, and the digital control type variable current source. Means for performing addition and subtraction with the level adjustment current supplied from a current source. 2. A method for selectively reading a current flowing through each of the plurality of bolometer-type thermoelectric conversion elements, comprising: reading means for reading the adjustment value data corresponding to the bolometer-type thermoelectric conversion elements from the storage means; 2. The digitally controlled variable current source supplies the level adjusting current to the bolometer-type thermoelectric conversion element based on the adjustment value data read from the storage unit by the reading unit. Level adjustment circuit as described. 3. The level adjustment circuit according to claim 1, wherein the digitally controlled variable current source includes a variable resistor that varies in accordance with the adjustment value data. 4. The level adjusting circuit for an infrared sensor according to claim 3, wherein an R-2R ladder resistor network is used as said variable resistor.