Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-12-07
2001-06-26
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C348S308000
Reexamination Certificate
active
06252217
ABSTRACT:
FIELD OF INVENTION
This invention relates to radiation imaging using a semiconductor imaging device consisting of an array of image cells.
BACKGROUND TO INVENTION
This invention describes a semiconductor imaging device for radiation imaging. The imaging device is an array of image cells, which consists of an array of radiation detector cells and an array of image cell circuits. An example of an imaging system configuration is shown in
FIG. 1
of the accompanying drawings. All cells in the detector cell array are connected to respective electronics cells in the array of image cell circuits. With appropriate processing technology, it is possible to implement both detector cells and circuit cells on the same substrate. Another possibility is to have two substrates, one for the detector and one for the cell circuits and, by using a bump-bonding or other technique connect them mechanically and electrically together so that each detector cell is connected to the corresponding cell circuit. A cross-section of a part of an imaging device made of two substrates, which are bump-bonded together, is shown in
FIG. 2
of the accompanying drawings.
In many radiation imaging applications, a need for different image resolutions exist. In single exposure images, the resolution should usually be relatively high. On the other land, the same imaging system could be used for displaying live image by continuously reading the image from the imaging device and updating the display in real time. However, if the imaging system is designed for high resolution, the data bandwidth for a live image at, for example, 30 frames per second may be so high that the requirements for the readout electronics for handling the data stream may become unreasonable. A readout system fast enough to capture and process the images could become unreasonably expensive compared to the total cost of the imaging system. Furthermore, a high image resolution required for single exposure images may not even be required for a live display of images.
Therefore, a method for effectively reducing the resolution and thus the data bandwidth on chip would solve the problem. Another problem is the scalability of the imaging system for large or small area imaging systems. If single imaging devices with relatively small area could be easily linked together to form a seamlessly connected array of imaging devices for large area imaging system, the same imaging devices could easily be used for either large and small area applications.
A further problem encountered with imaging devices is that distributed capacitance associated with the on-chip read-out lines can result in current spikes during read-out resulting in longer settling times for the read-out circuitry and consequently reducing the speed at which data can be read from the imaging device.
This invention tries to solve at least one of the problems referred to above.
Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from dependent claims may be combined with those of the independent claims in any appropriate manner and not merely in the specific combinations enumerated in the claims.
SUMMARY OF INVENTION
In accordance with a first aspect of the invention, there is provided an imaging device for radiation imaging, said device comprising an array of detector cells for generating a charge in response to incident radiation, an array of cell circuits for accumulating charge generated, and control circuitry for controlling output of signals from said cell circuits, said control circuitry comprising pre-charge circuitry for reducing current spikes at an output of said device. An embodiment of the invention advantageously reduces current spikes during read-out of said device.
In a preferred embodiment, said pre-charge circuitry is adapted to charge a parasitic capacitance associated with an output for a column of cell circuits, prior to reading data from said column. Thus, the parasitic capacitance is at least partially charged, such that when a read out is initiated a reduced current level is drawn from the output circuitry.
Preferably, the control circuitry is adapted to control said pre-charge circuitry to pre-charge said capacitance one clock cycle prior to reading data from said column.
Suitably, the control circuitry further comprises switch means for controllably applying a current signal for charging said parasitic capacitance.
In a preferred embodiment, the imaging device comprises an output switch means at an output of the device, for electronically separating the output from said array of cell circuits in order to reduce the capacitance seen at said output of said device when the device is not being read.
Advantageously, signals output from said cell circuits are current signals representative of charge accumulated in said cell circuits.
In a second aspect of the invention, there is provided an imaging system comprising a plurality of imaging devices as before described.
In an embodiment of the invention, there is provided an imaging device for radiation imaging, the device comprising an array of detector cells for generating a charge in response to incident radiation, an array of cell circuits for accumulating charge generated, and control circuitry controlling output of signals from the cell circuits programmably to adjust the resolution of the imaging device.
The array of detector cells and the array of cell circuits form an array of pixels. As a result of the programmable resolution, an imaging device according to an embodiment of the invention can provide different operational modes giving different pixel resolutions for different target applications.
In a further embodiment, the programmability in that the control circuitry is arranged to select a group of cell circuits and to produce an output signal representative of a sum of charge accumulated in all cell circuits in a group. Thus, the control circuitry enables grouping of several pixels together to form a larger area super pixel for lower resolution imaging.
In a yet further embodiment the control circuitry averages signals representative of charge accumulated in all cell circuits in a group. For example, the output signal is representative of the total charge for all of the cell circuits of a group divided by the number of cell circuits in the group. Preferably, the number of cell circuits in a group is selectable from a set of possible numbers.
In a still yet further embodiment, the output signal representative of charge accumulated is a current value. The use of a current output facilitates circuitry required to combine and average signal levels.
The control circuitry, for selecting a group of cell circuits, comprises a shift register arranged to select a plurality of columns or rows concurrently and to advance in steps of more than one row or column. The control circuitry can additionally comprise logic arranged simultaneous to select a plurality of rows and columns and a step size larger than one.
The control circuitry is arranged to average currents from a group of cell circuits by connecting current outputs of each cell circuit into a common output node and dividing the resulting sum of currents by the number of pixels in the group using a current mirror. The common output node can hold a current of the selected cell circuit(s).
Alternatively, for implementing group modes, each cell circuits in a group can be arranged to produce a scaled output signal representative of charge accumulated in the cell circuit divided by a number of cell circuits in the group. In order to be operable in a plurality of group modes, where each group mode has associated with it a predetermined number of cell circuits, the cell circuits can be arranged to include an output transistor for each group mode, which output transistor produces a scaled output signal according to the number of cells in a selected group mode. The output signal from all cell circuits in the group can then be averaged by summing the signals together.
In one embodiment of the invention, the r
Eräluoto Markku Tapio
Pyyhtia Jouni Ilari
Kenyon & Kenyon
Lee John R.
Pyo Kevin
Simage Oy
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