Circuit arrangement and method for charge integration

Abstract

A circuit arrangement for charge integration comprises an input (1) for applying a signal representing a charge pulse, an output (2) for providing an integrated signal, and an integration circuit (3) connected between the input (1) and the output (2), comprising a resistance circuit (5) and a capacitor (6) and having an RC time constant which is a function of the resistance circuit (5) and the capacitor (6). The circuit arrangement further comprises 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, wherein at least one of the resistance circuit (5) and the capacitor (6) has a variable value which depends on the control signal.

Description

[0001] This patent application relates to a circuit device for charge integration, a photon counting device, an X-ray device, and a method for charge integration.

[0002] The circuitry used for charge integration is employed to count charge pulses. For example, charge pulses are generated by X-ray photons impacting detector material, which converts an X-ray photon into a corresponding charge pulse. By counting these pulses, the amount of X-ray photons impacting the detector can be accurately measured, since the amount of charge generated by the detector material is proportional to the energy level of the X-rays. When measuring the magnitude of the charge pulses, spectral information can also be determined.

[0003] Typically, charge is integrated through a capacitor and converted into a voltage at the output of the charge integrator. To remove the charge of the corresponding charge pulse by discharge, a resistive feedback resistor is used. The RC time constant determines the rate of charge removal.

[0004] To allow for a high dynamic range, a very high counting rate is required.

[0005] One objective of this patent application is to improve the accuracy of charge integration under different conditions.

[0006] This objective is addressed by the subject matter of the independent claim. Other aspects are covered by the corresponding dependent claims.

[0007] In one embodiment, the circuit arrangement for charge integration includes an integrating circuit for integrating a charge pulse, the integrating circuit being connected at its input side to an input of the circuit arrangement and at its output side to an output of the circuit arrangement. The integrating circuit includes a resistive circuit and a capacitor. The capacitor is used to integrate the charge pulse applied at the input. The resistive circuit is used to remove the charge after integration. 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. At least one of the resistive circuit and the capacitor has a variable value that depends on the control signal.

[0008] When the output voltage is low, the circuit operates in a slow but accurate state. To achieve this, a larger time constant is used. A larger time constant also reduces the noise bandwidth, thereby reducing input-related noise.

[0009] When the output voltage increases, the feedback control circuit reduces the RC time constant. This avoids what is known as voltage buildup.

[0010] In other words, the circuit is configured to reduce the RC time constant and avoid accumulation at high charge pulse counting rates. Conversely, the RC time constant is increased when the charge pulse counting rate decreases.

[0011] In one embodiment, the resistor circuit includes at least one transistor having a control terminal, to which a feedback control signal is connected. This allows the value of the resistor circuit to be controlled according to the control signal.

[0012] For example, a transistor can be implemented as a metal-oxide-semiconductor transistor (e.g., a field-effect transistor), whose gate voltage depends on a control signal. Thus, the control signal controls the resistance of the transistor's drain-source channel via the gate voltage.

[0013] In an alternative embodiment, the resistor circuit includes a transconductance operational amplifier that includes at least one transistor and provides an output current that depends on a control signal.

[0014] For example, the drain current of a transconductance operational amplifier can be controlled by a control voltage. Increasing the drain current (i.e., the effective current between the transconductance operational amplifier and, for example, a reference voltage node VSS) results in a smaller equivalent feedback resistance.

[0015] For example, the output of the circuit device can be connected to the positive input of a transconductance operational amplifier, and the output of the transconductance operational amplifier can be connected to the input of the circuit device. The negative input of the transconductance operational amplifier can be connected to a reference potential (e.g., ground).

[0016] In one embodiment, the transconductance operational amplifier can be implemented using differential pairs of transistors, whose controlled current paths are coupled to each other on one side via current mirrors and on the other side via controlled current sources to a reference potential. A control signal controls the current generated by the controlled current sources. Of course, other implementations of the transconductance operational amplifier are also possible.

[0017] In one embodiment, the controlled signal used to control the resistance value of the resistor circuit is an analog signal.

[0018] In this way, the resistance value can be controlled in a non-discrete manner based on the output signal of the integrator circuit.

[0019] In one embodiment, the comparator is positioned downstream of the integrator circuit. Thus, the integrator circuit is connected to the comparator at its output. At least one predeterminable detection level is provided at another input of the comparator. A digital integration signal is provided at the output of the comparator.

[0020] In one embodiment, multiple of these comparators are used. For example, the comparators are used to perform energy detection by comparing a charge pulse with a respective energy level. This enables the detection of the energy level of the charge pulse and also allows for the implementation of a digital control loop to control the RC time constant of the integrator circuit.

[0021] In one embodiment, the integrating circuit includes an amplifier having an input coupled to an input of the circuit device and an output coupled to an output of the circuit device. A capacitor couples the amplifier's input to its output, and a resistor circuit has terminals coupled to the amplifier's input, while other terminals of the resistor circuit are coupled to the amplifier's output.

[0022] For example, resistors and capacitors can be connected in parallel between the amplifier's output and negative input to form a negative feedback loop.

[0023] The circuit device described in the above embodiments can be integrated on a semiconductor substrate. For example, the circuit device can be integrated on a CMOS chip.

[0024] In one embodiment, a photon counting device is provided, which includes circuitry for charge integration. A converter is provided at the input side of the circuitry for charge integration, which converts photon pulses into corresponding charge pulses at its output. The charge pulses are then integrated by the circuitry for charge integration as described above.

[0025] The advantageous effect of a circuit arrangement for charge integration based on an integrator circuit output with a controllable RC time constant is particularly useful in photon counting applications, such as X-ray photon counting. This arrangement allows for faster circuitry without compromising noise and spectral resolution at low counting rates. Spectral resolution decreases at high counting rates anyway because X-ray photons will strike the detector with small time differences, causing them to overlap. However, using this proposed principle, signal information can still be measured because the recovery time is reduced in this case.

[0026] The photon counting device described above can be applied in, for example, X-ray equipment (such as CT scanners, flat panel X-ray scanners, and security X-ray scanners).

[0027] In one embodiment, a method for charge integration includes applying a signal representing a charge pulse to an integrating circuit, the integrating circuit including a resistive circuit and a capacitor, and having an RC time constant that is a function of the resistive circuit and the capacitor. The integrating circuit provides an integrated signal based on the input charge pulse and the RC time constant.

[0028] Furthermore, a control signal is provided via a feedback control circuit based on the integral signal or a signal derived from the integral signal. The variable values ​​of the resistor circuit and / or capacitor are then controlled according to the control signal.

[0029] Other aspects of the method are described with reference to the following embodiments.

[0030] The method for charge integration can be implemented, for example, using a circuit arrangement according to one of the above embodiments.

[0031] Other aspects of this patent application will now be described with reference to the accompanying drawings, which illustrate exemplary embodiments.

[0032] Figure 1 An embodiment of a circuit arrangement for charge integration according to an example is shown;

[0033] Figure 2 An example of a controllable resistor is shown;

[0034] Figure 3 An example of a signal according to one embodiment is shown;

[0035] Figure 4A An embodiment of a feedback control circuit is shown;

[0036] Figure 4B It shows the reflection Figure 4A An example of the signal function of a feedback control circuit;

[0037] Figure 5 It shows relative to Figure 1 An alternative embodiment;

[0038] Figure 6 Another embodiment of the circuit device is shown; and

[0039] Figure 7 An example of a photon counting device is shown.

[0040] Figure 1 A circuit arrangement for charge integration based on the proposed principle is shown. The circuit arrangement includes an input 1 for applying a signal representing a charge pulse and an output 2 for providing an integration signal. The integration circuit 3 includes an amplifier 4, a resistor circuit 5, and a capacitor 6.

[0041] Feedback control circuit 7 is connected to output 2 at its input side and provides a control signal at its output side, which is supplied to resistor circuit 5. Resistor circuit 5 has a variable value that depends on the control signal provided by feedback control circuit 7.

[0042] Resistor circuit 5 includes a transconductance operational amplifier OTA, which in this case has a positive input connected to output 2 and a negative input connected to ground potential.

[0043] The resistor circuit 5 has a resistance value R and the capacitor 6 has a capacitance value C. The resistance value and capacitance value together represent the RC time constant of the integrator circuit 3.

[0044] The feedback control circuit 7 operates such that during operation, when the value of the integral signal increases, the feedback control circuit decreases the RC time constant, and when the value of the integral signal decreases, the feedback control circuit increases the RC time constant.

[0045] The charge pulse arriving at input 1 is integrated through capacitor 6 and converted into a voltage at output 2 of the integrator. Resistor circuit 5 is used to remove the charge, or in other words, to discharge the capacitor between consecutive input charge pulses.

[0046] The RC time constant is the product of the values ​​of the resistor and the capacitor, and it defines the rate at which charge is removed. The pulse duration can be, for example, approximately 10 nanoseconds. The charge magnitude of the charge pulse can be approximately 2.5 femtocoulombs, and the capacitor value can be in the range of approximately 10 femtofarads. The resistor value can be from 1 megohm to 2 megohms.

[0047] In other embodiments, the capacitor value can be from 1 fF to 100 fF.

[0048] In other embodiments, the resistance value can be from 0.1 MΩ to 10 MΩ.

[0049] Optional additional preamplifiers are used in the signal path (e.g., between the detector and the shaping circuit), and these values ​​can be multiplied by the gain of the capacitor and divided by the gain of the resistor value, respectively.

[0050] The advantage of implementing a resistor as an active circuit is that it is not only easily controlled by feedback control circuits, but also has better performance than parasitic capacitance.

[0051] The following is combined with Figure 3 Further explanation Figure 1 The technical effects and advantages achieved by the embodiments.

[0052] Figure 2 It shows Figure 1 An example of a transconductance operational amplifier. The source terminals of differential pair transistors M1 and M1' are connected together and connected to the VSS potential via current source 8. Current source 8 delivers drain current according to a control signal. The gate of transistor M1 forms the negative input of OTA and is connected to ground potential. The gate terminal of transistor M1' is connected to output 2. Each transistor M1 and M1' is connected in series in the same current path to complementary transistors M2 and M2' of different conduction types. Transistors M2 and M2' are themselves connected as current mirrors.

[0053] Thus, a signal can be obtained at output 1 of the OTA, representing the voltage difference between the gate voltages of transistors M1 and M1' and having a current output. The OTA provides a resistor equal to 1 / gm. The gm of the OTA is equal to the gm of transistor M1. gm is the transfer rate.

[0054] Figure 3 It shows that according to Figure 1 and 2 Example graphs of multiple parameters over time in an embodiment.

[0055] Reading the figure from top to bottom, the top shows the X-ray photons as a function of time t, followed by the charge at input 1 as a function of time, then gm at OTA as a function of time according to the proposed principle, and the bottom shows the output voltage at output 2 as a function of time.

[0056] Regarding X-ray photons, please refer to the following: Figure 7 As explained therein, the detector material is used to convert X-ray photons into corresponding charge pulses, which are shown in the second figure. Figure 3 The image shows two incident events: first, one X-ray photon impacts the detector, and then three X-ray photons impact the detector.

[0057] As shown in the second figure, at input 1, the first incident generates a small charge pulse, and the second incident generates a larger charge pulse.

[0058] A single charge pulse does not affect the transfer rate gm because the feedback control circuit does not actively change the resistance value of the integrator during the first incident. In contrast, during the second incident (i.e., a large number of charge pulses in a short period), the transfer rate gm increases, meaning that the resistance value decreases and the RC time constant also decreases in this case. This increase in gm is triggered by a large number of charge pulses in a short period and is only temporary. After the event, gm returns to its previous value (called the default value).

[0059] As can be seen from the output diagram, for the first incident of a single charge pulse, the output voltage ramps up, and then the resistor circuit removes the charge based on the RC time constant. However, in the second incident, the signal without active feedback control circuitry is shown as a dashed line, while the signal with active feedback control circuitry is shown as a solid line. It can be seen that by temporarily increasing the OTA's gm, the recovery time is significantly reduced.

[0060] Thus, based on the proposed principle, a precise circuit combining high counting rate and fast recovery time was achieved. Furthermore, accumulation at output 2 was avoided.

[0061] Figure 4AIt shows Figure 1 An exemplary embodiment of the feedback control circuit 7 is provided. This circuit is essentially a multiplier unit in which two pairs of differential transistors are cascaded on top of a single differential transistor. The two pairs of differential transistors are cross-coupled to each other on their sides. More specifically, a first pair of transistors T1, T1' are provided, their drain terminals connected to each other and connected to the VSS potential above the current source IB.

[0062] Transistors T1 and T1' are connected at their drain sides to the respective common source node of two other transistor pairs T2, T2' and T3, T3'. The gate terminals of transistors T1', T2', and T3 are connected to the output 2 of a circuit device for charge integration. The gate terminal of transistor T1 is connected to a reference potential (e.g., ground). The drain terminals of the two transistor pairs T2, T2' and T3, T3' are cross-coupled to each other, thereby forming a differential output. The positive one forms the output of the feedback control circuit 7.

[0063] Thus, from Figure 4B As can be seen from the figure, as the output voltage at output 2 of the integrator circuit increases, a control signal is generated that increases in an analog (meaning non-discrete) manner.

[0064] Figure 5 It shows relative to Figure 1 An alternative embodiment of the illustrated embodiment. The common parts between these two embodiments are... Figure 5 The embodiments will not be described further.

[0065] and Figure 1 In contrast, the transconductance operational amplifier is replaced by a single field-effect transistor 9. The source-drain channel of transistor 9, forming a controllable resistor, is connected in parallel to capacitor 6. The output of feedback control circuit 7 is connected at the gate terminal of transistor 9, thereby applying a control signal to the gate of transistor 9, which forms the resistive circuit of the integrator circuit.

[0066] Using only a single controlled MOSFET is a very simple implementation of a controllable resistor circuit to achieve this principle.

[0067] Figure 6 Another embodiment of the control resistor circuit is shown in the figure. Figure 6 It is relative to Figure 1 Another alternative embodiment, and Figure 1 and Figure 6 The same parts as in the embodiments, Figure 6 No further description.

[0068] exist Figure 6Instead of a feedback control circuit 7 that generates analog control signals, a comparator 10 is implemented. Comparator 10 has positive input, negative input, and output. A digital output signal is provided at output 10, which, on the one hand, represents the integral signal in digital form, and on the other hand, feeds back to the control input of the resistor circuit 5, which is implemented similarly to... Figure 1 The OTA (Over-The-Air) is achieved by connecting the positive input of comparator 10 to the output of integrator 3. A predetermined detection level can be provided at the negative input. If the detection level of the comparator is exceeded, the OTA's gm (i.e., the transfer rate) will increase to decrease the value of the resistor circuit and the RC time constant. This enables a digital control loop. If the integrated signal, in digital form, falls below the detection level again, gm will decrease to increase the recovery time again.

[0069] In other embodiments, multiple comparators can be connected in parallel to compare charge pulses with different energy levels. This allows the generation of a digital output signal with more than one bit.

[0070] In summary, by utilizing the principles presented according to all embodiments, it is possible to avoid the trade-off between speed and accuracy while simultaneously achieving both.

[0071] The RC time constant is dynamically controlled during operation.

[0072] Figure 7 An embodiment of a photon counting device with a converter 11 is shown, which converts photon pulses (e.g., X-ray photon pulses) into charge pulses at its output, wherein the output is connected to, for example, Figure 1 Input 1 of the circuit device shown.

[0073] In preferred applications, the photon counting device is incorporated into X-ray equipment such as CT scanners, flat panel devices for scanning individuals and / or luggage, security X-ray scanners, or other security applications.

[0074] In one embodiment, the converter 11 includes a conversion material that can be Si, GaAs, CdTe, or CdZnTe (CZT).

[0075] The conversion material can be a direct conversion of semiconductor materials.

[0076] List of reference numerals

[0077] 1 Input

[0078] 2 Output

[0079] 3 Integrating Circuit

[0080] 4 Amplifiers

[0081] 5. Resistor Circuit

[0082] 6. Capacitors

[0083] 7 Feedback Control Circuit

[0084] 8 Current Source

[0085] 9 MOSFET

[0086] 10 Comparators

[0087] 11 Converters

Claims

1. A circuit arrangement for charge integration, the circuit arrangement comprising: - Input (1), which is used to apply a signal representing a charge pulse, - Output (2), which is used to provide the integration signal, - An integrating circuit (3), connected between the input (1) and the output (2), includes a resistor circuit (5) and a capacitor (6), and has an RC time constant that is a function of the resistor circuit (5) and the capacitor (6), and - A feedback control circuit (7), which is connected at its input to the output (2) of the circuit device and provides a control signal at its output. The resistor circuit (5) has a variable value that depends on the control signal; - Wherein, the resistor circuit (5) includes a transconductance operational amplifier, the transconductance operational amplifier includes a controlled current source (8) and a differential pair of transistors, the controlled current paths of the differential pair of transistors are coupled to each other via a current mirror on one side and coupled to a reference potential via a controlled current source on the other side, and the transconductance operational amplifier provides an output current that depends on the control signal; the control signal controls the current generated by the controlled current source (8); - Wherein, the control signal is an analog signal.

2. The circuit device according to claim 1, in, During the operation of the integrator circuit, the feedback control circuit (7) decreases the RC time constant when the value of the integral signal increases, and increases the RC time constant when the value of the integral signal decreases.

3. The circuit device according to claim 1, in, The resistor circuit (5) includes at least one transistor with a control terminal, and a feedback control signal is connected to the control terminal.

4. The circuit device according to claim 3, in, The at least one transistor is implemented as a metal-oxide-semiconductor transistor (9), and the gate voltage of the metal-oxide-semiconductor transistor depends on the control signal.

5. The circuit device according to claim 1, in, The integrating circuit (3) includes an amplifier (4) having an input coupled to the input (1) of the circuit device and an output coupled to the output (2) of the circuit device. Wherein, the capacitor (6) couples the input of the amplifier to the output of the amplifier. The terminals of the resistor circuit (5) are coupled to the input of the amplifier, and the other terminals of the resistor circuit are coupled to the output of the amplifier (4).

6. A photon counting device, comprising: A converter that converts photon pulses into corresponding charge pulses at its output, and The circuit arrangement for charge integration according to any one of claims 1 to 5, wherein the circuit arrangement is coupled at its input to the output of the converter.

7. An X-ray apparatus comprising the photon counting device according to claim 6.

8. The X-ray device according to claim 7, wherein it is one of: a computed tomography X-ray scanner, a flat panel X-ray device, and a safety X-ray scanner.

9. A method for charge integration, the method comprising: - A signal representing a charge pulse is applied to an integrating circuit (3), which includes a resistor circuit (5) and a capacitor (6) and has an RC time constant that is a function of the resistor circuit (5) and the capacitor (6). - An integration signal is provided through the integration circuit (3), and - Based on the integral signal or a signal obtained from the integral signal, a control signal is provided through the feedback control circuit (7). The resistor circuit (5) has a variable value that depends on the control signal. - Wherein, the resistor circuit (5) includes a transconductance operational amplifier, the transconductance operational amplifier includes a controlled current source (8) and a differential pair of transistors, the controlled current paths of the differential pair of transistors are coupled to each other via a current mirror on one side and coupled to a reference potential via a controlled current source on the other side, and the transconductance operational amplifier provides an output current that depends on the control signal; the control signal controls the current generated by the controlled current source (8); - Wherein, the control signal is an analog signal.

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