Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
  • H03M1/12—Analogue/digital converters
  • H03M1/34—Analogue value compared with reference values
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
  • H03M1/12—Analogue/digital converters
  • H03M1/18—Automatic control for modifying the range of signals the converter can handle, e.g. gain ranging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J1/00—Photometry, e.g. photographic exposure meter
  • G01J1/02—Details
  • G01J1/0238—Details making use of sensor-related data, e.g. for identification of sensor or optical parts
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J1/00—Photometry, e.g. photographic exposure meter
  • G01J1/10—Photometry, e.g. photographic exposure meter by comparison with reference light or electric value provisionally void
  • G01J1/16—Photometry, e.g. photographic exposure meter by comparison with reference light or electric value provisionally void using electric radiation detectors
  • G01J1/18—Photometry, e.g. photographic exposure meter by comparison with reference light or electric value provisionally void using electric radiation detectors using comparison with a reference electric value
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
  • H04N25/70—SSIS architectures; Circuits associated therewith
  • H04N25/76—Addressed sensors, e.g. MOS or CMOS sensors
  • H04N25/78—Readout circuits for addressed sensors, e.g. output amplifiers or A/D converters
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M1/00—Analogue/digital conversion; Digital/analogue conversion
  • H03M1/12—Analogue/digital converters
  • H03M1/60—Analogue/digital converters with intermediate conversion to frequency of pulses

Definitions

• the invention lies in the field of imaging devices comprising a detector generating electric charges upon receipt of a photon flux, and reading means for quantifying the amount of electrical charges generated. It relates to an analog-digital conversion circuit forming such reading means.
• the imaging devices concerned can in particular be made in CMOS technology and be intended for imaging, for example X-ray or gamma radiological imaging or visible or infrared imaging.
• An imaging device generally comprises a matrix of pixels and reading means. Each pixel comprises at least one photosensitive element generating electric charges in proportion to the quantity of photons received. These electric charges, also called photocharges, are processed by the reading means in order to provide information representative of the quantity of photons received by each photosensitive element.
• the reading means can be made in CMOS technology, which allows their integration in each pixel.
• the means for reading a pixel consist for example of an analog-to-digital conversion circuit called "charge injection”, which corresponds to the English term “charge feedback digital to analog convertor”.
• charge injection analog-to-digital conversion circuit
• load balancing load balanced circuit
• a charge-coupled analog-to-digital conversion circuit comprises at least an integration capacitor, a comparator, a counter-charge injection circuit, and a counter.
• the integration capacity is connected by one of its electrodes to the photosensitive element of the pixel in question. During an exposure phase of the photosensitive element under consideration, the latter converts the photons into electron-hole pairs. Electrical charges, electrons or holes, are collected by an electrode of the detector, then accumulate at the terminals of an integration capacitance, which causes a voltage variation across the capacitance.
• An input of the comparator is connected to the integration capacity collecting the electrical charges. The comparator compares the potential at this input, called the detection potential, with a threshold value.
• the output signal of the comparator switches from a first state to a second state.
• Each switch causes the incrementation of the counter and the injection of a quantity QO of counter-charges on the electrode of the integration capacitor in order to compensate for the charges generated by the photosensitive element. If the quantity QO of counter-charges is correctly calibrated, the detection potential again crosses the threshold value, and the output signal of the comparator switches from the second state to the first state. The switching of the output signal of the comparator and the injection of counter-charges are repeated a certain number of times depending on the total amount of charges generated by the photosensitive element.
• the number of counter-charge injections necessary to balance the detection potential thus makes it possible to give a numerical value representative of the total amount of charges generated by the photosensitive element during a given integration time.
• a disadvantage of this charge-coupled analog-to-digital conversion circuit is that it can only be adapted to a relatively limited range of photon doses received by the photosensitive member. Indeed, in order to allow the precise quantification of low doses of photons, the quantity QO, which corresponds to the least significant bit of the value coded by the counter, must be relatively small. However, when the quantity QO is relatively small, many injections must be made to be able to quantify a large dose of photons. Thus, the counter must have a large number of bits (16 bits for example) to be able to count all the injections.
• the photon flux is subjected to an intrinsic noise according to a Poisson law.
• the noise of the electric current generated by the photons is proportional to the square root of the number of photons received.
• the flux of photons can vary enormously, for example in a ratio of the order of 1 to 10 4 . Consequently, if the quantity Q0 is calibrated so as to correspond substantially to the most low dose can be received by the photosensitive member, then a large amount of charges generated by the photosensitive member is encoded with a noise equal to several tens of times the amount Q0. In other words, the quantity of charges is digitized with a precision much greater than the noise, which means that several bits of the counter are used unnecessarily.
• the patent application EP 1860778 A1 proposes a charge-coupled analog-to-digital conversion circuit in which the quantity Qc of counter-charges of each injection is modulated as a function of the photon flux, in this case as a function of the electric current generated. by this flow of photons.
• Control means are used to modulate the quantity Qc and to control a switch so as to increment the counter by a number of units depending on this quantity Qc. More precisely, the switch is controlled to increment the counter by a number of units. units equal to the multiple of the quantity of elementary counter-charges Q0. The accuracy of the digitization is therefore adapted to the detected photon fluxes.
• the order of magnitude of this accuracy is easily determined by the greater quantity Qc of counter-charges injected during a given integration time.
• the analog-to-digital conversion circuit described in this patent application has the drawback of requiring an input switch of the counter, as well as relatively complex control means.
• the counter in order to allow the digitization of photon doses over a wide range, the counter must always be as deep as that of the conversion circuit described above.
• An object of the invention is in particular to overcome all or part of the aforementioned drawbacks.
• the invention aims in particular to provide an analog-to-digital conversion circuit by counter-charge injection which is of simple design, and in which the counter of the number of counter-charge injections comprises a reduced number of bits while allowing digitize varying amounts of electrical charges with precision high for small amounts of electrical charges.
• the subject of the invention is an analog-digital conversion circuit for an imaging device comprising a detector generating electric charges under the effect of an incident photon radiation, the electric charges causing a variation of a potential of integration on an integration node, the analog-to-digital conversion circuit comprising:
• ⁇ a comparator adapted to switch according to the comparison between the integration potential and a predetermined threshold potential
• ⁇ a counter connected to an output of the comparator and incrementing each comparator tilting
• ⁇ means for controlling the charge-injection circuit against determining the quantity Qc against load-injected
• the analog-to-digital conversion circuit being characterized in that the control means determine the quantity Qc of counter-charges injected as a function of a value of the counter. According to a particular embodiment, the control means are configured to vary the quantity Qc each time the value of the counter reaches one or more predetermined threshold values.
• the counter may comprise a predetermined number of bits, in which case each predetermined threshold value corresponds, for example, to the first switching of one of the bits of the counter.
• the quantity Qc of counter-charges injected is for example doubled each time the value of the counter reaches a predetermined threshold value.
• the counter-charge injection circuit may comprise a plurality of counter-charge injectors each capable of injecting a predetermined quantity of counter-charges.
• the counter-charge injectors are each able to inject the same quantity QO of counter-charges, the quantity Qc of each injection of counter-charges on the integration node varying by the selection of one or more injectors of counter-charges.
• the counter-charge injectors are each capable of injecting a different quantity of counter-charges, the quantity Qc of each injection of counter-charges on the integration node varying by the selection of the one of the injectors of counter-charges.
• the invention also relates to an imaging device comprising a detector generating electric charges under the effect of incident photon radiation, the electrical charges causing a variation of a detection potential on an integration node, and a circuit analog-to-digital conversion as previously described.
• the invention has the particular advantage that it reduces the size of the counter while maintaining a precision in the same order of magnitude as the amount of noise to which the flow of photons is subjected, regardless of the amount of photons received.
• FIG. 1 shows the electrical diagram of a pixel in a first embodiment of an imaging device according to the invention
• FIG. 2 shows the electrical diagram of a pixel in a second embodiment of an imaging device according to the invention.
• FIG. 1 represents the electric diagram of a pixel 10 in a first exemplary embodiment of an imaging device according to the invention.
• the imaging device may comprise a plurality of pixels organized in line or in matrix. Each pixel forms a photosensitive point of the device imager.
• the pixel 10 comprises a photosensitive element 11 and an analog-digital conversion circuit 12.
• the photosensitive element 11 forms a pixel detector. It is for example formed by a photodiode, a phototransistor or, more generally, by any device generating electrical charges depending, for example proportionally, the amount of photons it receives.
• the photons considered have for example a wavelength in the visible range, infrared, X-rays or gamma rays.
• the analog-digital conversion circuit 12 is of the so-called "charge injection” type. It comprises an integration capacitor 121, a comparator 122, a counter 123, a counter-charge injection circuit 124 and control means 125.
• the integration capacitor 121 is connected by one of its electrodes to the photosensitive element 1 1 and its other electrode to a reference potential, for example the electrical mass of the imaging device.
• the point of connection between the photosensitive element 11 and the integration capacitor 121 is called the integration node A and the potential at this point is called the integration potential Va.
• This node is also connected to a first input of the comparator 122, a second input being connected to a reference potential, called the threshold potential Vthreshold.
• the comparator 122 delivers on an output a signal noted Scomp taking, for example, either a first value or a second value depending on the result of the comparison between the integration potential Va and the threshold potential Vthreshold.
• the signal Scomp is for example a voltage.
• the output of the comparator 122 is connected, on the one hand, to an input of the counter 123 and, on the other hand, to an input of the injection circuit 124.
• the counter 123 comprises, for example, 12 flip-flops each representing a bit of the counter 123. It can of course include a larger number of flip-flops depending on the maximum value to be encoded. Moreover, the counter can be made by any other device capable of coding a positive integer value.
• the counter-charge injection circuit 124 comprises 6 counter-charge injectors 1240 to 1245, and a switch 126.
• the switch 126 is for example formed of six controlled switches 1260 to 1265. More generally, the injection circuit 124 may comprise K counter-charge injectors, denoted 124k, where K is a strictly positive integer, and the switch may comprise as many controlled switches, denoted 126k, as counter-injectors. 124k loads. Each injector 124k is connected to the output of the comparator 122 and can deliver counter charges in an amount of 2 k .Q0. Thus, the injector 1240 can inject a quantity Q0 of counter-charges, the injector 1241 can inject a quantity 2.Q0, and so on.
• Each controlled switch 126k is connected between an output of one of the injectors 124k and the integration node A.
• the control means 125 make it possible to control the controlled switches 126k according to the value stored in the counter 123.
• the analog-digital conversion circuit 12 comprises an integration capacitor 121.
• the photosensitive element 11 may possibly have a parasitic capacitance sufficient to fulfill the function of the integration capacitor 121. In such a case, the analog-digital conversion circuit 12 may not include an integration capacitor.
• the analog-to-digital conversion circuit described with reference to FIG. 1 operates in the following manner.
• the photosensitive element 1 1 At the reception of photons, the photosensitive element 1 1 generates electric charges, for example electrons, which accumulate on the electrode of the integration capacitor 121 connected to the integration node A. This results in a decrease integration potential Va.
• the output signal Scomp switches from a first value, for example ' ⁇ ', to a second value, for example ⁇ '.
• the signal Scomp is received by the counter 123 and the injection circuit 124. In particular, the signal Scomp can be received by each countercurrent injector 124k.
• each counter-charge injector 124k delivers a calibrated quantity of counter-charges.
• the integration node A receives a quantity of counter-charges from one of the injectors 124k.
• This amount of counter-charges denoted Qc, increases the integration potential Va to a value greater than the threshold potential Vthreshold.
• the Scomp output signal then switches from the second to the first value. The process of digitization described above is repeated a number of times depending on the total amount of electric charges generated by the photosensitive element 1 1.
• the state of the switch 126, and therefore the quantity Qc of counter-charges of each injection, depends on the value of the counter 123.
• the value of the counter 123 Prior to the reception of photons, the value of the counter 123 is initialized to zero.
• the switch 126 is then controlled to close the controlled switch 1260, the other controlled switches 1261-1265 being open.
• each switching of the output signal Scomp triggers the incrementation of the value of the counter 123 by one unit and the injection of a quantity Qc of counter-charges equal to Q0.
• the quantity Qc remains equal to Q0 until the value of the counter 123 reaches a first predetermined threshold value.
• This first threshold value corresponds, for example, to the first switchover of the sixth bit (Bit5 in FIG. 1) of the counter 123, that is to say to the value 32.
• the means of control 125 controls switch 126 to close the controlled switch 1261, the other controlled switches 1260 and 1262-1265 being open.
• each switching of the output signal Scomp always triggers the incrementation of the value of the counter 123 by one unit, but the injection of a quantity Qc of counter-charges equal to 2.Q0.
• the quantity Qc remains equal to 2.Q0 until the value of the counter 123 reaches a second predetermined threshold value.
• This second threshold value corresponds, for example, to the first changeover of the eighth bit (Bit7 in FIG. 1) of the counter 123, that is to say to the value 128.
• each switching of the output signal Scomp triggers the incrementation of the value of the counter 123 by one unit, and the injection of a quantity Qc of counter-charges equal to 4.Q0.
• the quantity Qc incrementally increases when the value of the counter 123 reaches the threshold values 256, 512, and 1024, that is, when the ninth, tenth, and eleventh bits, respectively, switch for the first time.
• the quantity Qc corresponds in fact to the step of digitization (or no quantification) of the total quantity Qt of counter-charges injected on the integration node A, this total quantity Qt being, for example, proportional to the dose of photons received by the photosensitive element 11 from the beginning of the photon reception.
• the scanning accuracy therefore changes step by step with the value of the counter 123.
• the total quantity Qt can therefore be directly determined from the value of the counter 123.
• a correspondence table can be used to determine the total quantity Qt from the value of the counter 123.
• the following table presents an extract of Such a look-up table for a pixel of an exemplary X-ray imaging device.
• a first column indicates the value of the counter 123.
• a second column indicates the corresponding quantity Qc for each injection from this value.
• a third column indicates the total quantity Qt of counter-charges injected on the integration node A until the last switchover of the signal Scomp.
• a fourth column indicates the amount of noise to which the flow of photons X is subjected.
• This table shows in particular that the amount of noise is in the same order of magnitude as the scanning step (Qc) of the total quantity Qt. Moreover, it shows that the total quantity Qt can be determined by the analog conversion circuit.
• -numerical number according to the invention with a 12-bit counter is equal to 43743, a value much greater than the value that can be stored in a 12-bit counter, namely 2047.
• a total quantity Qt equal to 43743 requires a counter on 16 bits. The invention makes it possible to reduce the size of the counter, and therefore the number of digital data to be transferred as well as the number of connections between the counter 123 and other elements of the imaging device.
• the decrease in the number of connections can be used either to decrease the size of the pixels of the imaging device, or to increase the area of each photosensitive element.
• the fact of using a device according to the invention makes it possible to maintain a high measurement dynamic while reducing the electrical activity of the circuit, and therefore the power consumption.
• FIG. 2 represents the wiring diagram of a pixel 20 in a second exemplary embodiment of an imaging device according to the invention.
• the pixel 20 differs from the pixel 10 shown in FIG. 1 by the counter-charge injection circuit.
• all the injectors 224k are able to deliver the same quantity Q0 of electrical counter-charges.
• each controlled switch 126k of the switch 126 is connected between the output of the comparator 122 and one of the injectors 224k.
• the controlled switches 126k are always controlled by the control means 125 as a function of the value of the counter 123.
• an injector 224k delivers counter-charges only if the controlled switch 126k to which it is connected is controlled. in the passing state.
• control means 125 progressively control a larger number of controlled switches 126k in the on state.
• the number of controlled switches 126k controlled in the on state increases in a power progression of two. The quantity Qc is then doubled each time the value of the counter reaches one of the threshold values.
• the quantity Qc of counter-charges of each injection increases when certain bits of the counter 123 switch for the first time.
• the counter may be realized by means other than flip-flops and the quantity Qc may increase to any threshold values.
• the quantity Qc is doubled each time the value of the counter reaches one of the threshold values.
• the quantity Qc can evolve differently. It can in particular be determined so as to substantially follow the amount of noise associated with the total quantity Qt of counter-charges injected.

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• 2011
  • 2011-11-24 FR FR1160739A patent/FR2983371B1/fr active Active
• 2012
  • 2012-11-14 EP EP12784280.5A patent/EP2783466A1/de not_active Ceased
  • 2012-11-14 JP JP2014542776A patent/JP6019486B2/ja active Active
  • 2012-11-14 CN CN201280066391.6A patent/CN104040895B/zh active Active
  • 2012-11-14 CA CA2856777A patent/CA2856777A1/en not_active Abandoned
  • 2012-11-14 WO PCT/EP2012/072643 patent/WO2013075993A1/fr active Application Filing
  • 2012-11-14 US US14/360,581 patent/US9379730B2/en active Active

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