SE503643C2 - Sensor device comprising a matrix of detectors, each detector being a radiation sensitive component - Google Patents

Abstract

The invention relates to a sensor device comprising an array of detectors. In such a detector array, it is difficult to read out weak electric signals, especially if there are many detectors. The invention solves the contemplated problem by using a device in which each detector consists of a radiation-sensitive component included in an oscillator, for example a resistor or a capacitor included in an RC oscillator, whose frequency is used as a measure of the detected quantity.

Description

vfii. sos 643 2 - In the following, the invention is described for the sake of simplicity with reference to a matrix with detectors in rows and columns. However, the invention is not limited to a detector array of such a design. What is said below about rows and columns can be easily replaced by the person skilled in the art with other groups of detectors which in these contexts appear to be obvious to the person skilled in the art.

The basic idea of the invention is to use an oscillator for integrating and reading the detector signal. The detector signal is converted to frequency, which is then normally read in parallel one row or column at a time. The invention reduces the noise at the pixel level through the integration during the time between the readings. A suitable way of realizing the invention is to use as an oscillator an RC oscillator, where either a resistor, R, or a capacitor, C, constitutes the detector itself. However, it is possible to use other types of oscillators instead. One type of oscillator that can be used in the invention instead of an RC oscillator is a ring oscillator.

Fig. 5 shows, an example with one, built up of three inverters and a capacitor as the radiation-sensitive component.

In a first embodiment of the invention, each detector is permanently connected to an oscillator, i.e. there is one oscillator per pixel.

Each detector is thus permanently assigned one or more non-radiation-sensitive supplementary components, which together with the detector are arranged to form the oscillator.

Figure 1 shows an example with an array of 100x100 detectors. To explain the function, a numerical example of this array is given below with the line reset every 20 ms. Since the capacitive, C, or resistive, R, detector is included in an RC oscillator, its frequency will depend on the detected magnitude. The oscillator frequency is assumed to be nominally l0kHz, ie the period time is 0.1 ms.

In the interval 19.8-20.0 ms from the reset of the line, an observation window opens where the time for the approximately 200 periods of the oscillator signal since the reset is observed (here is the integration), see figure 2. The time from the beginning of the window to the first positive edge is read.

In addition, the period time is read by measuring the time between two consecutive positive edges. The purpose of the later reading is to determine the number of pulses since the reset.

The reading takes place in this example with a frequency of 10 MHz. An external counter is started at the beginning of the observation window. At each reading of the window no action is taken if the oscillator signal is low. The first time the signal is high, a memory cell belonging to the current pixel is set equal to the value of the counter.

Because the reading takes place at 10 MHz in a window after almost 20 ms, the relative resolution is about 1: 200 000. The dynamics within a window will be 1: 1000 (the number of readings in the period time is 0.1ms). By reading the period time, the number of pulses after the reset can be determined and thereby you can obtain a dynamics that is a multiple of 1000. If, for example, you have a variation of +/- 4 pulses since the reset, it means a dynamics of 1:16 000 corresponding to 14 bits .

By using the remaining 99 0.2 ms intervals, we can read the rest of the columns or rows in a l00x100 image with the same hardware.

Reading frequency and reading window, respectively, can be selected depending on the oscillator frequency and requirements for resolution and dynamics. For larger arrays, you can choose to read out several lines at a time.

In the given example with resistance bolometer, a capacitor is added to each pixel as part of the matrix. The capacitance and thus the surface output can be kept low if the detector resistance is high or if a high oscillator frequency can be used. In addition to the noise reduction that takes place through the integration at pixel level, a noise-free reading is obtained from the sensor matrix because the reading takes place digitally.

This embodiment of the invention provides a system that entails 1) AD conversion at pixel level, which provides digital, ie noise-free, readout from the pixel, 2) noise reduction through integration at pixel level and so on 643 4 - 3) that the capacitor's capacitance and thus surface consumption on the chip can be kept down by using repeated recharging or discharging in the oscillator.

In a second embodiment of the invention, each group of detectors is assigned one or more non-radiation sensitive supplementary components. A wiring network with switches is arranged to successively connect different detectors in the group with the additional component or components and thereby form the oscillator. Such a group may suitably consist of a column in the matrix.

Using only one oscillator per column instead of one oscillator per pixel has both advantages and disadvantages.

* In the following, we assume that we use a detector array with NxN resistive or capacitive detectors. The electrical integration then takes place only during a nzte part of the image period. The noise suppression of the oscillator and detector therefore decreases by a factor VÛT compared with integration during the entire image period. Against this must be weighed the benefits obtained.

With only one oscillator per column instead of one in each pixel, each oscillator can be allowed to occupy a significantly larger chip area and consume N times higher power. This can be used to design oscillators with lower phase noise, since larger surfaces and bias currents for critical transistors can be used as well as a larger capacitance value in the RC product. In addition, a lower phase noise can be obtained by incorporating in the construction a lower sensitivity to noise on the supply voltage generated by, among other things, other oscillators, but also by incorporating a more effective compensation of the oscillator's 1 / f noise.

However, the effects of the oscillator 1 / f noise which is lower than the frame rate can also be compensated together with the thermal compensation of the silicon wafer. This occurs if a series of separate radiation-protected resistances at the upper and lower edges of the sensor matrix are used as references for each image. The frequency change occurring in columns - A 5 - is seen 643 due to the 1 / f noise of the respective oscillator can then not be distinguished from temperature variations and will therefore be compensated in the same way.

A better separation between the oscillators is also possible because supply and bias lines may be large. In addition, there will be N times fewer oscillators connected at the same time, which is why mutual influence should be greatly reduced. An advantage in frequency to the digital conversion is achieved by the oscillator being constantly connected to the subsequent logic. The conversion then becomes safer, as the risk of miscalculating the number of cycles disappears.

An example of an IR sensor with NxN resistive detectors is shown in Figure 3. Each pixel contains a resistive detector element, R, and two switches for contacting the detector resistance of the oscillator. Each pixel also has two vertical bus lines and a horizontal address line that connects the detector resistance to the oscillator via the switches. The connected detector resistance is included together with a capacitance, C, as a frequency-determined component in the RC oscillator. This example also includes reference detectors at the top and bottom edges of the sensor array. These are shielded from IR radiation and only react to the silicon's own temperature, so that its change can be compensated away from the signals from the other elements. The placement in the upper and lower edges also enables compensation of a linear temperature gradient over the detector matrix.

The RC oscillators are connected for one detector line at a time. Let 1 / Tb be the frame rate. The reading time T] for each detector line then becomes lb / N if the two reference detectors are neglected. With an oscillator frequency, fo, approximately 2fOT] half cycles will be registered per reading. It will normally only be in the order of 100-1000 cycles and in itself gives a too low resolution. With a well-defined starting point by resetting the oscillator at the beginning of the reading period, a reading of the oscillator's final value provides additional resolution. Figure 4 illustrates the procedure. A dynamic around 15 bits should be possible. see sas ï 6 - In order to be able to use the entire line time for reading the detector, the pipe line must be used in the A / D conversion of the oscillator's set value. This can be done so that the oscillator's side value is sampled and held, whereby the A / D conversion can take place in parallel with the reading of the next row. Note that up? and the discharge process is non-linear, so one must correct for this.

Claims (4)

Claims: A sensor device comprising an array of detectors, each detector being a radiation sensitive component which together with a plurality of permanently arranged non-radiation sensitive competing components comprises an oscillator, for example a radiation sensitive resistor or a capacitor oscillating in an RC capacitor. is used as a measure of the detected quantity, characterized in that it comprises a control and calculation device which successively calculates and reads out oscillator signals in groups by, during an observation window extending to the time of the next group's reading, measuring the time to high resolution. osciiiatorsignaien's first positive fiank and time meiian two consecutive fianker and from the latter calculate antaiet puiser then the previous observation window for the current group and overall get an integrated measure from the previous observation window. Sensor device according to claim 1, characterized in that the groups consist of a number of rows of detectors. 3. A sensor device according to claim 1 or 2, characterized in that there are a number of radiation shielded oscillators arranged to provide a reference signal which is used to compensate for temperature operation. 4. A sensor device according to claim 3, characterized in that there are a plurality of radiation shielded oscillators arranged on either side of the detector array, seen in one direction, which are arranged to provide a reference signal used to compensate for a superintendent temperature. .