CIRCUIT ARRANGEMENT AND METHOD FOR CHARGE INTEGRATION

Description

[0001] The present patent application concerns a circuit arrangement for charge integration, a photon counting arrangement, an x-ray apparatus and a method for charge integration.

[0002] Circuit arrangements for charge integration are used to count charge pulses. The charge pulses are, for example, generated by x-ray photons hitting a detector material which converts one x-ray photon into a respective charge pulse. By counting these pulses, it is possible to precisely measure the amount of x-ray photons hitting the detector as the amount of charge generated by the detector material is proportional to the energy level of the x-ray. The spectral information can also be determined when measuring the size of the charge pulse.

[0003] Typically, the charge is integrated by a capacitance and converted into a voltage at the output of the charge integrator. To remove the charge of a respective charge pulse by discharging, a resistive feedback resistor is used. The RC time constant determines how fast the charge is removed.

[0004] In order to allow for a high dynamic range, very high count rates are desirable.

[0005] The document US 2010 / 187432 A1 shows a spectral photon counting detector with an integrator that produces a pulse having a peak amplitude indicative of the energy of a detected photon. First and second discharging circuitry discharge the integrator at a first and second discharging speed, respectively.

[0006] EP 3510928 A1 refers to a tomographic imaging apparatus including an X-ray detector comprising a plurality of dual mode pixels.

[0007] EP 3385756 A1 shows a pulse shaper circuit having an integrator for generating a pulse having a peak amplitude indicative of the energy of a detected photon and switchable discharge circuitry for discharging the integrator.

[0008] It is an object of the present patent application to increase the accuracy of the charge integration in varying conditions.

[0009] This object is solved by the subject-matter of the independent claims. Further aspects are covered by the respective dependent claims.

[0010] A circuit arrangement for charge integration comprises an integrating circuit for integrating charge pulses which is connected to an input of the circuit arrangement on its input side and to an output of the circuit arrangement on its output side. The integrating circuit comprises a resistive circuit and a capacitor. The capacitor is used for integrating the charge pulse applied at the input. The resistive circuit is used for removing the charge after integration. Both the resistive circuit and the capacitor define an RC time constant. A feedback control circuit is provided which is connected at its input side to the output of the circuit arrangement and provides a control signal at its output. The resistive circuit has a variable value depending on the control signal.

[0011] When the output voltage is small, the circuit is in a slow but accurate condition. To achieve this, the time constant is large. A large time constant also reduces the noise bandwidth and therefore reduces the input-related noise.

[0012] When the output voltage is increasing, the feedback control circuit reduces the RC time constant. In this way, so-called pile-up is avoided.

[0013] In other words, the circuit is configured such that at high count rates of charge pulses, the RC time constant is reduced and pile-up is avoided. In contrast, when the count rate of the charge pulses is slowing down, the RC time constant is increased.

[0014] In one embodiment, the resistive circuit comprises at least one transistor having a control terminal to which the feedback control signal is connected. In this way, the value of the resistive circuit can be controlled as a function of the control signal.

[0015] For example, the transistor can be realized as a metal oxide semiconductor transistor, for example a field-effect transistor, with its gate voltage depending on the control signal. This way, the resistor value of the drain source channel of the transistor is controlled by the control signal via the gate voltage.

[0016] The resistive circuit comprises an operational transconductance amplifier that includes the at least one transistor and provides an output current that depends on the control signal.

[0017] For example, the drain current of the operational transconductance amplifier can be controlled by the control voltage. Increasing the drain current, which is the effective current between the operational transconductance amplifier and, for example, the reference voltage node VSS, makes the equivalent feedback resistor smaller.

[0018] For example, the output of the circuit arrangement can be connected to a positive input of the operational transconductance amplifier and the output of it can be connected to the input of the circuit arrangement. The negative input of the operational transconductance amplifier can be connected to a reference potential, for example ground.

[0019] The operational transconductance amplifier is implemented using a differential pair of transistors, the controlled current paths of which are coupled to each other via a current mirror on one side and via a controlled current source to a reference potential on their other side. The control signal controls the current generated by the controlled current source. Of course, other implementations of an operational transconductance amplifier can be implemented as well according to examples not covered by the invention.

[0020] The controlled signal that is used to control the resistor value of the resistive circuit is an analog signal.

[0021] In this way, the resistor value can be controlled in a non-discrete manner depending on the output signal of the integrating circuit.

[0022] In one embodiment, the integrating circuit comprises an amplifier which has an input coupled to the input of the circuit arrangement and an output coupled to the output of the circuit arrangement. The capacitor couples the input of the amplifier to the output of the amplifier and a terminal of the resistive circuit is coupled to the input of the amplifier and a further terminal of the resistive circuit is coupled to the output of the amplifier.

[0023] For example, the resistive circuit and the capacitor can be connected to each other in parallel between the output and a negative input of the amplifier, thus creating a negative feedback loop.

[0024] The circuit arrangement according to the embodiments described above may be integrated on a semiconductor substrate. For example, the circuit arrangement is integrated on a CMOS chip.

[0025] In one embodiment a photon counting arrangement is provided which comprises the circuit arrangement for charge integration. At the input side of the circuit arrangement for charge integration, a converter is provided which converts a photon pulse into a corresponding charge pulse at its output. The charge pulse is then integrated by the circuit arrangement for charge integration as described above.

[0026] The advantageous effects of the circuit arrangement for charge integration with a controllable RC time constant based on the output of the integrating circuit are especially useful in photon counting applications, for example x-ray photon counting applications. In this way, the arrangement allows for speeding up the circuit without degrading the noise and spectral resolution at low count rates. The spectral resolution at high count rates is degraded anyway because the x-ray photons will hit the detector with such a small time difference that they start to overlap. But with this proposed principle signal information can still be measured because the recovery time is reduced in such cases.

[0027] The photon counting arrangement proposed above can be applied, for example, in x-ray apparatuses like CT scanners, flat panel x-ray scanners and security x-ray scanners.

[0028] In one embodiment, a method for charge integration comprises applying a signal representing charge pulses to an integrating circuit, comprising a resistive circuit and a capacitor and having an RC time constant which is a function of the resistive circuit and the capacitor. The integrating circuit provides an integrating signal based on incoming charge pulses and based on the RC time constant.

[0029] Furthermore, a control signal is provided by a feedback control circuit depending on the integrated signal or a signal derived from the integrated signal. Depending on the control signal, the variable value of the resistive circuit and / or the capacitor is controlled.

[0030] Further aspects of the method are described with reference to the embodiments below.

[0031] The method for charge integration may be implemented e.g. with the circuit arrangement according to one of the embodiments described above.

[0032] Further aspects of the present patent application are explained below with reference to the drawings which show exemplary embodiments. Figure 1 shows an embodiment of a circuit arrangement for charge integration according to an example; Figure 2 shows an example of a controllable resistor; Figure 3 shows an example of signals according to an embodiment; Figure 4A shows an embodiment of a feedback control circuit; Figure 4B shows an example of signals reflecting the function of the feedback control circuit of Figure 4A; Figure 5 shows an alternative embodiment with respect to Figure 1, not covered by the invention. Figure 6 shows another embodiment of a circuit arrangement not covered by the invention; and Figure 7 shows an example of a photon counting arrangement.

[0033] Figure 1 shows a circuit arrangement for charge integration according to the proposed principle. The circuit arrangement comprises an input 1 for applying a signal representing charge pulses and an output 2 for providing an integrated signal. An integrating circuit 3 comprises an amplifier 4, a resistive circuit 5 and a capacitor 6.

[0034] A feedback control circuit 7 is connected, at its input side, to the output 2 and provides, at its output side, a control signal which is supplied to the resistive circuit 5. The resistive circuit 5 has a variable value depending on the control signal provided by the feedback control circuit 7.

[0035] The resistive circuit 5 comprises an operational transconductance amplifier OTA with a positive input which is in that case connected to the output 2 and with a negative input which is connected to ground potential.

[0036] The resistive circuit 5 has a resistor value R and the capacitor 6 has a capacitance value C which together represent an RC time constant of the integrating circuit 3.

[0037] The feedback control circuit 7 works such that it reduces the RC time constant at increasing values of the integrated signal during operation and wherein the feedback control circuit increases the RC time constant at decreasing values of the integrated signal.

[0038] A charge pulse arriving at input 1 is integrated by the capacitance 6 and converted into a voltage at the output 2 of the integrator. The resistive circuit 5 is for removing the charge or, in other words, discharging the capacitor between consecutive incoming charge pulses.

[0039] The RC time constant, which is a product of the value of the resistive circuit and the capacitor, defines how fast the charge is removed. The pulse duration may be, for example, approximately 10 nanoseconds. The charge size of a charge pulse may be approximately 2.5 Femtocoulomb and the value of the capacitor may be in the range of around 10 Femtofarad. The resistor value may be in the range of 1 to 2 Megaohm.

[0040] In other embodiments, the capacitor value may be between 1 and 100 fF.

[0041] In other embodiments, the resistor value may be in the range of 0.1 MΩ to 10 MΩ.

[0042] With an optional, additional preamplifier in the signal path, e.g. between a detector and a shaper circuit, the values can be multiplied by the gain for the capacitance respectively divided by the gain for the resistor value.

[0043] It is an advantage of the resistor being implemented as an active circuit, not only that it is easily controlled by the feedback control circuit, but also that it has better performance with respect to parasitic capacitance.

[0044] The technical effects and advantages achieved with the embodiment of Figure 1 are further explained below with reference to Figure 3.

[0045] Figure 2 shows an example of the operational transconductance amplifier of Figure 1. Differential pair transistors M1, M1' have their source terminals connected together and, via a current source 8 to VSS potential. The current source 8 delivers a drain current as a function of the control signal. The gate of transistor M1 forms the negative input of the OTA and is connected to ground potential. A gate terminal of transistor M1' is connected to the output 2. Each transistor M1, M1' is, in the same current path, serially connected to a complementary transistor M2, M2' with respect to conductivity type. The transistors M2, M2' themselves are connected in the way of a current mirror.

[0046] In this way, at the output 1 of the OTA a signal can be derived which is representative of the difference in voltage between the gate voltages of the transistors M1, M1' and which has a current output. This OTA provides a resistance which is equal to 1 / gm of the OTA. The gm of the OTA is equal to the gm of the transistor M1. The gm is the transfer rate.

[0047] Figure 3 shows exemplary diagrams of several parameters over time according to the embodiments of Figures 1 and 2.

[0048] Reading this diagram from top to bottom, shown on top are the x-ray photons over time t, then the charge at the input 1 over time, next the gm of the OTA over time which is varied according to the proposed principle and at the bottom the output voltage at the output 2 over time.

[0049] Regarding the x-ray photons, it is referred to Figure 7 below. As explained therein, a detector material is used to convert x-ray photons into respective charge pulses which are shown in the second diagram. In Figure 3, two incidents are shown, namely at first one x-ray photon hitting the detector and then three x-ray photons hitting the detector.

[0050] The first incident creates one small charge pulse and the second incident creates a larger amount of charge pulses at the input 1, as shown in the second diagram.

[0051] The single charge pulse does not affect the transfer rate gm because the feedback control circuit will not be active to change the resistor value of the integrating circuit during the first incident. In contrast to this, with the second incident, namely a large number of charge pulses in a short time, the transfer rate gm is increased which means that the resistive value is decreased and also the RC time constant is decreased in this scenario. This increase of gm is triggered by the large number of charge pulses in a short time period and is only temporarily maintained. After the incident, the gm settles back to the value it had before, referred to as a default value.

[0052] As can be seen in the output diagram, for the first incident with a single charge pulse the output voltage ramps up and then the charge is removed by the resistive circuit based on the RC time constant. In the second incident, however, the signal without the active feedback control circuit is shown in a dashed line and with the active feedback control circuit is shown in the solid line. It can be seen that the recovery time is reduced significantly by temporarily increasing gm of the OTA.

[0053] In this way an accurate circuit in combination with a fast recovery time at high count rates is achieved according to the proposed principle. Moreover, pile-up at the output 2 is avoided.

[0054] Figure 4A shows an exemplary embodiment of the feedback control circuit 7 of Figure 1. This circuit is basically a multiplier cell where two pairs of differential transistors are cascaded over one pair of differential transistors. The two pairs are cross-coupled to each other on their sides. In more detail, a first pair of transistors T1, T1' is provided with their drain terminals connected to each other, and, over a current source IB to VSS potential.

[0055] On their drain sides, the transistors T1, T1' are connected to a respective common source node of two further transistor pairs T2, T2' and T3, T3'. The gate terminals of transistors T1', T2' and T3 are connected to the output 2 of the circuit arrangement for charge integration. The gate terminal transistor T1 is connected to reference potential, for example ground. The drain terminals of the two transistor pairs T2, T2' and T3, T3' are cross-coupled to each other, thus forming a differential output. The positive one of them forms the output of the feedback control circuit 7.

[0056] In this way, as can be seen from the diagram of Figure 4B, with increasing output voltage at the output 2 of the integrating circuit, a control signal is created which also increases in an analog, which means non-discrete, way.

[0057] Figure 5 shows an alternative embodiment not covered by the invention with respect to the one shown in Figure 1. Insofar as the two embodiments are identical, the embodiment of Figure 5 is not described again. In contrast to Figure 1, the operational transconductance amplifier is replaced by a single field-effect transistor 9. The source drain channel of transistor 9 which forms a controllable resistor is connected in parallel to capacitor 6. At the gate terminal of transistor 9 the output of the feedback control circuit 7 is connected, thus applying the control signal to the gate of transistor 9 that forms the resistive circuit of the integrating circuit.

[0058] Using just one controlled MOSFET is a very simple implementation of the controllable resistive circuit for implementing the present principle.

[0059] Another embodiment, not covered by the invention, to control the value of the resistive circuit is shown in Figure 6. Figure 6 is another alternative embodiment with respect to Figure 1 and insofar as the embodiments of Figures 1 and 6 are identical, Figure 6 is not described again.

[0060] Instead of the feedback control circuit 7 which generates an analog control signal, a comparator 10 is implemented in Figure 6. The comparator 10 has a positive input, a negative input and an output. At the output 10, a digital output signal is provided which, on the one hand, represents the integrated signal in digital form and, on the other hand, is fed back to the control input of the resistive circuit 5 implemented as an OTA similar to Figure 1. The positive input of the comparator 10 is connected to the output of the integrating circuit 3. At the negative input a predetermined detection level can be supplied. If the detection level of the comparator is exceeded, the gm, namely the transfer rate, of the OTA will be increased to reduce the value of the resistive circuit and to reduce the RC time constant. In this way, a digital control loop can be implemented. If the integrated signal in digital form drops again below the detection level, then the gm will be decreased to increase the recovery time again.

[0061] In other embodiments, multiple comparators may be connected in parallel to compare the charge pulse with different energy levels. This way a digital output signal may be generated that has more than one bit.

[0062] To summarize, with the presented principle according to all embodiments it is possible to avoid the tradeoff between speed and accuracy and to have speed and accuracy at the same time.

[0063] The RC time constant is dynamically controlled during operation.

[0064] Figure 7 shows an embodiment of a photon counting arrangement having a converter 11 converting a photon pulse, for example an X-Ray photon pulse, into a charge pulse at its output where the output is connected to the input 1 of the circuit arrangement as shown in Figure 1.

[0065] In preferred applications, the photon counting arrangement is provided in an x-ray apparatus such as a CT scanner, a flat panel apparatus or a security x-ray scanner, both for scanning individuals and / or luggage, or other security applications.

[0066] In one embodiment, the converter 11 comprises a conversion material that can be Si, GaAs, CdTe or CdZnTe (CZT).

[0067] The conversion material may be a direct conversion semiconductor material.List of reference numerals:

[0068] 1input 2output 3integrating circuit 4amplifier 5resistive circuit 6capacitor 7feedback control circuit 8current source 9MOSFET 10comparator 11converter

Claims

1. Circuit arrangement for charge integration comprising: - an input (1) for applying a signal representing charge pulses, - an output (2) for providing an integrated signal, - an integrating circuit (3) connected between the input (1) and the output (2), comprising a resistive circuit (5) and a capacitor (6) and having an RC time constant which is a function of the resistive circuit (5) and the capacitor (6), and - a feedback control circuit (7) connected, at its input, to the output (2) of the circuit arrangement and providing, at its output, a control signal, where the resistive circuit (5) has a variable value depending on the control signal; - in which the resistive circuit (5) comprises an operational transconductance amplifier that includes a controlled current source (8) and a differential pair of transistors, the controlled current paths of which being coupled to each other via a current mirror on one side and via the controlled current source to a reference potential on their other side and provides an output current that depends on the control signal; the control signal controlling the current generated by the controlled current source (8); in which the control signal is an analog signal.

2. Circuit arrangement according to claim 1, wherein the feedback control circuit (7) reduces the RC time constant at increasing values of the integrated signal during operation of the integrating circuit, and wherein the feedback control circuit (7) increases the RC time constant at decreasing values of the integrated signal during operation of the integrating circuit.

3. Circuit arrangement according to claim 1 or 2, in which the integrating circuit (3) comprises an amplifier (4) having an input coupled to the input (1) of the circuit arrangement and an output coupled to the output (2) of the circuit arrangement, wherein the capacitor (6) couples the input of the amplifier to the output of the amplifier, wherein a terminal of the resistive circuit (5) is coupled to the input of the amplifier and a further terminal of the resistive circuit is coupled to the output of the amplifier (4).

4. Photon counting arrangement, comprising a converter converting a photon pulse into a corresponding charge pulse at its output, and the circuit arrangement for charge integration according to one of claims 1 to 3 which is coupled, at its input, to the output of the converter.

5. X-Ray apparatus comprising the photon counting arrangement of claim 4.

6. X-Ray apparatus according to claim 5, which is one of the following: a computed tomography x-ray scanner, a flat-panel x-ray apparatus, and a security x-ray scanner.

7. Method for charge integration, comprising - applying a signal representing charge pulses to an integrating circuit (3), comprising a resistive circuit (5) and a capacitor (6) and having an RC time constant which is a function of the resistive circuit (5) and the capacitor (6), - providing an integrated signal by the integrating circuit (3), and - providing a control signal by a feedback control circuit (7) depending on the integrated signal or a signal derived from the integrated signal, where the resistive circuit (5) has a variable value depending on the control signal, the resistive circuit (5) comprising an operational transconductance amplifier that includes a controlled current source (8) and a differential pair of transistors, the controlled current paths of which being coupled to each other via a current mirror on one side and via the controlled current source to a reference potential on their other side and provides an output current that depends on the control signal; the control signal controlling the current generated by the controlled current source (8), in which the control signal is an analog signal.