Abstract:
A method and circuit for reducing correlated noise in imagers is disclosed. According to an aspect, correlated noise is reduced by coupling the reference inputs of imager amplifiers to common voltage sources. The reference inputs of differential amplifiers on the imager can be coupled to common noise sources such as the imager low gate voltage and array bias voltage through suitably chosen capacitances.

Description:
BACKGROUND AND SUMMARY 
     The invention relates to imaging devices, such as flat panel imagers. An imager is a device that receives electromagnetic radiation, e.g., light or x-rays, from the direction of something to be imaged in which an image is formed based the detected pattern of the radiation received at the imager. A flat panel imager is a type of imager that comprises a matrix/array of detection elements, with each detection element providing a separate item of image data that is usable to reconstruct an image. For light-sensitive imagers, each detection element comprises a photosensitive device. For x-ray sensitive imagers, each detection element comprises an x-ray sensitive device. 
     FIG. 1 depicts one configuration of electrical components for a flat panel imager, showing selectable wiring connections to transmit the voltage, current, or charge created by detection elements on the imager. Each image element  104  in the imager of FIG. 1 comprises a photodiode  106  that generates an electrical signal in response to a light input. In an x-ray imager, the photodiode  106  receives light input from an x-ray scintillator element that generates light in response to x-rays. A transistor  108  (such as a thin-film N-type FET) functions as a switching element for the image element  104 . When it is desired to capture image data from image element  104 , control signals  114  are sent to gate driver  112  to “select” the gate of transistor  108 . Electrical signals from the photodiode  106  are passed through line  116  to a charge amplifier  110 . The output of charge amplifier  110  is sent to a “sample and hold” stage for further image processing/display. While FIG. 1 only shows four image elements  104   a - 104   d , it is likely that the typical flat panel imager includes many such image elements  104  depending upon the size and resolution of the imaging device. 
     Many imagers perform simultaneous sampling of image data from multiple image elements in a correlated manner. For example, the imager of FIG. 1 collects image data from an entire line of image elements (i.e., row or column) at the same time. To form an entire image frame, each line of image data is collected on a line-by-line basis until all lines for the image has been sampled. To obtain image data for a line of image elements, all the switching transistors (e.g.,  108   a  and  108   b ) for image elements (e.g.,  104   a  and  104   b ) on the same line are tied to the same control line  126  extending from gate driver  112 . When the image data for a particular line of image elements is desired, control signals  114  are sent to the gate driver  112  to select the transistor gates for the desired line of image elements. The electrical signals from the entire line of image elements are passed to their corresponding charge amplifiers, which output signal data to the subsequent sampling stage. 
     The photodiodes  106  of FIG. 1 are connected to a common node  122  to supply a reverse bias voltage for the image element array. The gate driver  112  is connected to a node  124  to supply low gate voltage to drive the gate control lines. Parasitic capacitance may exist in the imager, such as C gd  to the gate control line and C ad  to the common array bias line for each image element. Each amplifier ( 110   a ,  110   b , etc.) may gain noise (e.g., AC noise) present on the low gate voltage and array bias voltage depending upon the ratio of the capacitances. If an entire line of image data is sampled at the same time, then the same noise offset may exist for every pixel in that line, which causes the corresponding line of pixels in the final image to appear markedly different from other lines of pixels. This type of “image artifact” is created in the example of FIG. 1 because the low gate voltage and the array bias voltage are common for the amplifiers in the circuit. Compared to random pixel noise, this correlated noise is often relatively more visible and may significantly degrade image quality. 
     The present invention is directed to a method and mechanism for reducing correlated noise. According to an embodiment, the invention reduces correlated noise in imagers induced by noise in common supply voltages to the imager electronics, in which the reference input of each coordinated amplifier is connected to common voltage sources. An aspect of an embodiment is directed to coupling the reference input of differential amplifiers on the imager to common noise sources such as the imager low gate voltage and array bias voltage through suitably chosen capacitances. Further details of aspects, objects, and advantages of the invention are described below in the detailed description, drawings, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention and, together with the Detailed Description, serve to explain the principles of the invention. 
     FIG. 1 shows a simplified schematic of imager electronics. 
     FIG. 2 depicts an equivalence schematic for components shown in FIG.  1 . 
     FIG. 3 is a schematic of an embodiment of the invention in which capacitance components are coupled between common noise sources and a reference input of an amplifier. 
     FIG. 4 is a schematic of an embodiment of the invention in which a capacitance component is coupled between ground and the reference input of an amplifier. 
     FIG. 5 is a schematic of an embodiment of the invention in which a capacitance component is coupled between the low gate voltage source and the array bias voltage source. 
     FIG. 6 is a schematic of an embodiment of the invention in which capacitance components are coupled between common noise sources and a reference input of an amplifier, a capacitance component is coupled between the common noise sources, and a capacitance component is coupled between ground and the reference input of the amplifier. 
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a method and mechanism for reducing correlated noise in imagers. The invention is useful for reducing line correlated noise in flat panel imagers induced by noise in common supply voltages to the imager electronics. To illustrate the invention, FIG. 2 shows an equivalence schematic for an amplifier (e.g.,  110   a ,  110   b , etc.) shown in FIG. 1. C GD  and C AD  represent equivalents for the capacitances of the input line  116  for each amplifier  110 , which is a summation of capacitances for the input line for the example of FIG.  1 . If the integrating capacitor C i  is approximately 1 pF and the parasitic capacitances C GD and C AD  are approximately 30 pF each, then the approximate gain for each noise source in this example is 30. 
     The present invention reduces correlated noise by coupling an input of the amplifiers to one or more of the common noise sources. In an embodiment, this coupling is performed through additional capacitance elements introduced in the imager electronics. FIG. 3 shows a schematic of a first embodiment of the invention, in which amplifier  310  is a differential amplifier (e.g.,  110   a ,  110   b , etc.). A component C A    308  is electrically coupled between the array bias voltage source  300  and the reference input  304  for amplifier  310 . A component C G    306  is electrically coupled between the low gate voltage source  302  and the reference input  304 . By placing these components between the noise sources and the reference input of the differential amplifiers, the noise from voltage sources  300  and  302  is passed to the non-inverting input of the differential amplifiers. This compensates, in whole or part, for noise signals applied to each amplifier  310  through the imager panel. 
     The additional capacitance components are preferably coupled directly to the common voltage sources shared by all amplifiers in the imager circuit. Other suitable approaches to connect common noise sources to the circuit amplifiers, other than direct coupling of all amplifiers to common voltage sources, may be employed in the invention to reduce correlated noise. 
     FIG. 4 shows a schematic of another embodiment of the invention, in which an additional component C R    402  is coupled between ground and the reference input  304  of amplifier  310 . Capacitor C R    402  reduces the common mode signal to amplifier  310 . This additional component further reduces noise gain by reducing the common mode gain of amplifier  310 . 
     FIG. 5 depicts a schematic of an embodiment of the invention where an additional component C o    502  is coupled between the array bias voltage  300  and the low gate voltage  302 . C o    502  reduces the differences between the two noise sources  300  and  302 , making the noise on both supply lines relatively equivalent to each other. 
     FIG. 6 shows an embodiment of the invention in which capacitors C A    308 , C G    306 , C R    402 , and C o    502  are all introduced into the circuit to reduce noise from voltage sources  300  and  302 . 
     In a preferred embodiment, the ratio (C GD /C i ) is equal to (C G /C R ) and the ratio (C AD /C i ) is equal to (C A /C R ). The capacitance of component C o  is preferably chosen to be relatively large compared to the rest of the circuit. The specific frequency range in which the noise reduction is effective depends upon the output impedances of the voltage sources and the value of the added capacitors. 
     To illustrate the invention, the foregoing explanation was directed to a method and system for reducing line correlated noise that occurs for an entire line (row or column) of image elements for an imager. The invention may also be applied to reduce correlated noise for other groupings or granularities of image elements in addition to lines, depending upon the particular grouping of circuit image elements for which data is sampled in a correlated manner. For example, the invention may be applied to reduce correlated noise for correlated sampling of partial lines of data or for correlated sampling of blocks of image data. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, the invention has been described with reference to imager schematics having a common low gate voltage and array bias voltage, in which the reference input of each charge amplifier is connected to these common voltage sources through suitably chosen capacitors. However, the disclosed principles of the invention are equally applicable to address correlated noise in imagers having other common voltage sources. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.