Integrated circuit and measurement method

Abstract

According to a first aspect of this disclosure, an integrated circuit including a current source and a reference capacitor is provided, the integrated circuit being configured to: inject a first current into an external measuring capacitor using the current source, and determine a first time duration in which the resulting voltage on the measuring capacitor reaches a voltage threshold; inject a second current into the reference capacitor using the current source, and determine a second time duration in which the resulting voltage on the reference capacitor reaches the voltage threshold; and detect a change in capacitance on the measuring capacitor using the difference between the first time duration and the second time duration. According to a second aspect of this disclosure, a corresponding measurement method is proposed.

Description

Technical Field

[0001] This disclosure relates to an integrated circuit. Furthermore, this disclosure relates to a measurement method. Background Technology

[0002] Capacitance-based measurement systems and methods can be used in a variety of applications. For example, they can be used in tamper detection devices. Although many tamper detection devices include resistance-based measurement systems, using capacitance-based systems can be advantageous. Since re-establishing a current connection is easier than restoring a capacitance value, capacitance-based measurement systems enable more reliable tamper detection. Capacitance-based measurement systems and methods can also be used in other areas, such as measuring physical parameters like humidity. The currently disclosed systems and methods can be applied to tamper detection devices and other types of devices requiring accurate capacitance measurements.

[0003] For example, a capacitance-based tamper detection device includes an integrated circuit (i.e., a chip) coupled to a capacitor. The integrated circuit may, for example, be included in a radio frequency identification (RFID) tag or a near field communication (NFC) tag. In operation, a change in capacitance on the capacitor can indicate a tampering attempt on the product, and information indicating such an attempt can be stored in the integrated circuit. This information can then be read out by an RFID reader or an NFC reader.

[0004] For example, a capacitor could be a plate capacitor consisting of two metal foils and a plastic foil in between as the dielectric. This capacitor could be attached to a bottle stopper to protect expensive wine from counterfeiting, and once the bottle is opened, the capacitor would be damaged or at least its capacitance would change significantly. Information indicating counterfeiting could be stored in the integrated circuit (IC) of a tag. Then, a mobile phone could read the RFID tag attached to the capacitor, and the phone could display a message indicating that someone had tampered with the bottle. In another example, tamper detection devices could be applied to envelopes, with paper as the dielectric. Thus, it would be possible to detect whether the envelope had been altered or manipulated.

[0005] It is important that capacitance measurements be as accurate as possible. Currently disclosed systems and methods contribute to improving this accuracy. Summary of the Invention

[0006] According to a first aspect of this disclosure, an integrated circuit including a current source and a reference capacitor is provided, the integrated circuit being configured to: inject a first current into an external measuring capacitor using the current source, and determine a first time amount at which the resulting voltage on the measuring capacitor reaches a voltage threshold; inject a second current into the reference capacitor using the current source, and determine a second time amount at which the resulting voltage on the reference capacitor reaches the voltage threshold; and use the difference between the first time amount and the second time amount to detect a change in capacitance on the measuring capacitor.

[0007] In one or more embodiments, the integrated circuit is further configured to inject the first current into the measuring capacitor while simultaneously injecting the second current into the reference capacitor.

[0008] In one or more embodiments, the first current is a configurable current, and the second current is a constant current.

[0009] In one or more embodiments, the first current is configured such that the voltage slope on the measuring capacitor is substantially the same as the voltage slope on the reference capacitor.

[0010] In one or more embodiments, the integrated circuit is configured to determine the value of the first current after the integrated circuit has been calibrated.

[0011] In one or more embodiments, the integrated circuit is configured to store a determined value of the first current in the non-volatile memory of the integrated circuit.

[0012] In one or more embodiments, the determined value of the first current is a value that results in the minimum difference between the first time quantity and the second time quantity, the minimum difference being stored as a calibration offset in the non-volatile memory.

[0013] In one or more embodiments, the integrated circuit further includes a counter for determining the first time amount and the second time amount.

[0014] In one or more embodiments, the counter is configured to start counting after a trigger voltage is reached and to stop counting after the voltage threshold is reached.

[0015] In one or more embodiments, the integrated circuit is further configured to: determine a third time amount at which the obtained voltage on the measuring capacitor reaches an additional voltage threshold; determine a fourth time amount at which the obtained voltage on the reference capacitor reaches the additional voltage threshold; and use the difference between the third time amount and the fourth time amount to detect a change in capacitance on the measuring capacitor.

[0016] In one or more embodiments, the radio frequency identification (RFID) tag or near field communication (NFC) tag includes an integrated circuit of the described type.

[0017] In one or more embodiments, the measurement system includes an integrated circuit of the described type, and a measuring capacitor.

[0018] In one or more embodiments, a tamper detection system includes a measurement system of the type described.

[0019] According to a second aspect of this disclosure, a measurement method is proposed, the method comprising: injecting a first current into a measuring capacitor by means of a current source in an integrated circuit, and determining a first time amount at which the resulting voltage on the measuring capacitor reaches a voltage threshold; injecting a second current into a reference capacitor by means of the current source, and determining a second time amount at which the resulting voltage on the reference capacitor reaches the voltage threshold; and detecting a change in capacitance on the measuring capacitor by means of the difference between the first time amount and the second time amount using the integrated circuit.

[0020] In one or more embodiments, the integrated circuit injects the first current into the measuring capacitor while simultaneously injecting the second current into the reference capacitor. Attached Figure Description

[0021] The embodiments will be described in more detail with reference to the accompanying drawings, in which:

[0022] Figure 1 An example of a measurement system is shown;

[0023] Figure 2 A schematic embodiment of the measurement system is shown;

[0024] Figure 3 A schematic embodiment of the measurement method is shown;

[0025] Figure 4 Another illustrative embodiment of the measurement system is shown. Detailed Implementation

[0026] Figure 1 An example of a measurement system 100 is shown. System 100 includes a passive RFID tag having a sensor interface 102 and a device-under-test capacitor 108 (i.e., the measuring capacitor). As mentioned above, tamper detection devices typically include an integrated circuit (i.e., a chip) coupled to the capacitor. Sensors for physical parameters such as humidity may also be included. The integrated circuit may, for example, be included in a radio frequency identification (RFID) tag or a near field communication (NFC) tag. In operation, a change in capacitance on the capacitor can indicate an attempt to tamper with the product or a sensed change in a physical parameter, and information indicating such an attempt to tamper can be stored in the integrated circuit. This information can then be read out by an RFID reader or an NFC reader. Alternatively, the reader can trigger the tag to measure the device-under-test capacitor 108 by sending a command to the tag; the tag can then transmit the measurement result back to the reader without storing the measurement result in the integrated circuit. A change in capacitance can be detected by measuring the voltage slope across the capacitor. When the capacitance value changes due to tampering or a change in a physical parameter, the voltage slope changes because the current injected during the measurement remains substantially the same: ΔV / Δt = I / C. Figure 1 The passive RFID tag 102 shown includes an integrated circuit 104 (i.e., a chip) of the type described, and an antenna 106. Furthermore, the device-under-test capacitor 108 is an example of a measuring capacitor of the type described. An external RFID reader (not shown) can be configured to read data from the RFID tag via the antenna 106 of the RFID tag 102. It should be noted that the coupling between the capacitor and the chip can be referred to as a sensor interface.

[0027] The change in capacitance on capacitor 108 of the device under test is measured using charging current and a reference voltage. However, these charging current and reference voltage can be susceptible to variations (i.e., tolerances), such as process-, voltage, temperature, and time-related variations between different integrated circuits. These variations limit the accuracy achievable in the measurement. The currently disclosed systems and methods contribute to improving the accuracy of the measurement.

[0028] Figure 2 A schematic embodiment of a measurement system 200 is shown. The measurement system 200 includes a measurement capacitor 202 (i.e., a device-under-test capacitor) and an integrated circuit 204. The integrated circuit 204 includes a current source 206 and a reference capacitor 208. The measurement capacitor 202 may be included in or attached to an external product (not shown), such as a bottle cap (which may be tampered with), or the measurement capacitor 202 may be configured to sense physical or environmental parameters. According to this disclosure, the integrated circuit 204 is configured to inject a first current into the measurement capacitor 202 using the current source 206 and determine a first amount of time during which the resulting voltage on the measurement capacitor 202 reaches a voltage threshold. Furthermore, the integrated circuit 204 is configured to inject a second current into the reference capacitor 208 using the same current source 206 and determine a second amount of time during which the resulting voltage on the reference capacitor 208 reaches the same threshold voltage. Additionally, the integrated circuit 204 is configured to use the difference between the first and second amount of time to detect a change in capacitance on the measurement capacitor.

[0029] In this way, the accuracy of capacitance measurement can be improved. Specifically, the application of differential measurement cancels out variations related to process, voltage, temperature, and time, because both measurements are affected by these variations in the same way. Accordingly, the two measurement paths use current from the same current source: the external measurement capacitor 202 is charged through the first measurement path, while the internal reference capacitor 208 is charged through the second measurement path. Since both measurements are driven by the same source current, variations in the measurement current will affect both measurements in the same way. The same applies to the voltage threshold. Therefore, these effects cancel each other out. The capacitance measurement on the measurement capacitor 202 and the reference capacitor 208 is performed by determining the amount of time required to reach the voltage threshold. Because the capacitance on the reference capacitor 208 within IC 204 remains substantially the same as it is unaffected by external events that should be detected (e.g., tampering or changes in physical parameters), changes in the capacitance on the measurement capacitor 202 can be easily detected by comparing the amount of time required to measure the capacitor 202 with the amount of time required to measure the reference capacitor 208. For example, if the difference is less than a predetermined threshold, it will be assumed that the capacitance on the measured capacitor has not changed, and if the difference is greater than the predetermined threshold, it will be assumed that the capacitance on the measured capacitor has changed.

[0030] Figure 3 A schematic embodiment of measurement method 300 is shown. Method 300 includes the following steps. At 302, an integrated circuit (IC) injects a first current into a measuring capacitor using a current source and determines a first time amount at which the resulting voltage across the measuring capacitor reaches a voltage threshold. At 304, the IC injects a second current into a reference capacitor using the same current source and determines a second time amount at which the resulting voltage across the reference capacitor reaches the same voltage threshold. Furthermore, at 306, the IC uses the difference between the first and second time amounts to detect a change in capacitance across the measuring capacitor. For example, these steps can be performed or controlled by a controller of the integrated circuit. As explained above, this approach improves the accuracy of capacitance measurement.

[0031] In one or more embodiments, the integrated circuit is configured to inject a first current into the measuring capacitor while simultaneously injecting a second current into the reference capacitor. By charging both the measuring and reference capacitors simultaneously, the measurement system can be unaffected by current variations or noise coupling over time. Therefore, the accuracy of capacitance measurements can be further improved.

[0032] In one or more embodiments, the first current is a configurable current, and the second current is a constant current. In other words, the first current can be adjustable, which helps to match the charging slope of the internal capacitor (i.e., the reference capacitor) with the charging slope of the external capacitor (i.e., the measuring capacitor) whose capacitance is to be measured. Therefore, in one or more embodiments, the first current is configured such that the voltage slope across the measuring capacitor is substantially the same as the voltage slope across the reference capacitor. In this way, the accuracy of capacitance measurement can be further improved. In a practically effective implementation, the value of the first current is determined after the integrated circuit is calibrated. Furthermore, in one or more embodiments, the determined value of the first current is stored in the non-volatile memory of the integrated circuit. This facilitates the retrieval of the determined value when power is off. It should be noted that since the tag may be a field-powered device, power may only be available for a short period of time. Furthermore, in one or more embodiments, the determined value of the first current is the value that results in the minimum difference between a first time quantity and a second time quantity, wherein said minimum difference is stored as a calibration offset in the non-volatile memory. In this way, the accuracy of capacitance measurement can be further improved. Specifically, the calibration offset can reflect the quantization error caused by the finite granularity of the adjustable current. Therefore, calibration steps can be performed to discover the correct current setting required to match the measurement slope (i.e., the charging slope or voltage slope of the corresponding capacitor) and to detect quantization errors caused by the finite granularity of the adjustable current. In a practically effective implementation, the integrated circuit further includes a counter for determining a first time quantity and a second time quantity. Furthermore, in one or more embodiments, the counter starts counting after a trigger voltage is reached and stops counting after a voltage threshold is reached. (See reference below.) Figure 4 The explanation is that counting after the trigger voltage is reached reduces the inaccuracy of the ground level, thereby further improving the measurement.

[0033] In one or more embodiments, the integrated circuit is further configured to: determine a third time amount at which the obtained voltage on the measuring capacitor reaches an additional voltage threshold, determine a fourth time amount at which the obtained voltage on the reference capacitor reaches the additional voltage threshold, and use the difference between the third and fourth time amounts to detect a change in capacitance on the measuring capacitor. In this way, the voltage slopes of the measuring capacitor and the reference capacitor can be segmented, which can further improve measurement accuracy. Specifically, the additional voltage threshold can be an intermediate voltage threshold between the trigger voltage and the (final) voltage threshold. Thus, two segments are formed. Those skilled in the art will understand that more segments can be formed by extending this embodiment, with regard to the use of an additional intermediate threshold voltage. In this case, the additional time amounts are compared with each other, particularly the time amount at which the obtained voltage on the corresponding capacitor (i.e., the voltage generated by the injected first and second currents) reaches the additional intermediate threshold voltage. It should be noted that the principle of segmentation has already been described in the tampering detection content of the European patent application No. 18192899.5, entitled "System and method for detecting tampering with a product" filed on September 6, 2018.

[0034] More specifically, segmentation can be achieved by using different threshold voltages (e.g., start, intermediate, and end voltages). The current injected into the corresponding capacitor remains the same, but the counter value can be read multiple times. In this way, the voltage slope is segmented. Specifically, if the threshold voltages are equidistantly distributed, then for each voltage slope, an equal counter value is expected. By means of a comparator, it is possible to detect whether a specific threshold voltage has been reached. Specifically, the comparator can be configured to take different threshold voltages (i.e., reference voltages) as inputs, or an additional comparator can be provided that takes a specific intermediate threshold voltage as input.

[0035] Figure 4Another schematic embodiment of the measurement system 400 is shown. The system 400 can be divided into an analog domain 402 and a digital domain 414. The analog domain 402 includes a measurement capacitor (i.e., the device-under-test capacitor) and analog components of an integrated circuit coupled to the measurement capacitor via a sensing pad. The digital domain 414 includes digital components of the integrated circuit. The analog domain 402 includes a current reference 404, a current mirror 406 connected to the power supply voltage, a voltage reference 408, and comparators 410 and 412. The current reference 404 and the current mirror 406 implement current sources of the types described. The digital domain 414 includes digital logic 416 and a time reference 418. The integrated circuit can be in a calibration state and a measurement state. In the calibration state, the integrated circuit is calibrated. More specifically, in this state, the current setting for measuring the capacitor is determined, and a calibration offset is determined. In the measurement state, changes in capacitance on the measurement capacitor are detected.

[0036] In a practically effective implementation, the integrated circuit operates as follows in measurement mode. Measurement can be initiated by discharging both capacitors to zero. Once the discharge switch is released, the internal reference capacitor is charged with a reference current (i.e., the second current), while the external capacitor is charged with a previously defined external current (i.e., the first current determined in calibration mode). Once a certain reference voltage (i.e., the trigger voltage) is reached, a counter begins counting until a final reference voltage (i.e., the voltage threshold) is reached. Independent counters are used for the two measurement paths. Not starting the counters simultaneously with releasing the discharge switch reduces the inaccuracy of the ground level, thereby further improving the measurement. Since both measurements are driven by the same source current, changes in the measurement current will affect both measurements in the same way. The same applies to the voltage reference. Therefore, those effects cancel each other out. Additionally, multiple reference voltages (i.e., additional voltage thresholds) can be selected to further segment the measurement slope. When the measurement is complete, the difference between the two counter values, plus the calibration offset, can be evaluated. A change detection threshold can be defined, indicating which counter difference is assumed to have changed the capacitance on the measurement capacitor. Because the components are not perfectly matched, only a certain resolution can be obtained, and the change detection threshold should be selected to be higher than the minimum achievable accuracy.

[0037] In a practically effective implementation, the integrated circuit operates as follows in calibration mode. During calibration, measurements are performed and the measurement current is adjusted until the difference between the two counters is minimized. It should be noted that the successive approximation algorithm will provide the minimum number of measurement steps required to obtain the minimum difference between the two counters. The current setting used to charge the external measurement capacitor (DUT) is stored in non-volatile memory, as is the difference between the two counters. This difference can be used as a calibration offset to compensate for quantization errors caused by the limited amount of available current setting.

[0038] It should be noted that the above embodiments have been described with reference to different subjects. Specifically, some embodiments may have been described with reference to claims of the method class, while other embodiments may have been described with reference to claims of the device class. However, those skilled in the art will conclude from the foregoing that, unless otherwise indicated, any combination of features related to different subjects, particularly features of claims of the method class and features of claims of the device class, is also considered to be disclosed in this document, except for any combination of features belonging to one type of subject matter.

[0039] Furthermore, it should be noted that the drawings are schematic. Similar or identical elements are represented by the same reference numerals in different drawings. Additionally, it should be noted that, in order to provide a concise description of illustrative embodiments, implementation details that are customary to those skilled in the art may not be described. It should be understood that in the development of any such implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developer's specific objectives, such as complying with system-related and business-related constraints, which may vary depending on the implementation. Furthermore, it should be understood that such development work may be complex and time-consuming, but is merely a routine task of design, manufacture, and production for those of ordinary skill in the art who benefit from this disclosure.

[0040] Finally, it should be noted that those skilled in the art should be able to devise numerous alternative embodiments without departing from the scope of the appended claims. Any reference numerals placed between parentheses in the claims should not be construed as limiting the claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in the claims. The words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements. The measures recited in the claims can be implemented by means of hardware comprising several distinct elements and / or by means of a suitably programmed processor. In device claims enumerating several components, several of these components can be implemented by one and the same hardware. The mere fact that certain measures are recited in different appendix claims does not imply that combinations of these measures cannot be advantageously used.

[0041] List of reference numerals

[0042] 100 Measurement System

[0043] 102 RFID tags with sensor interfaces

[0044] 104 Chip (Integrated Circuit)

[0045] 106 antenna

[0046] 108 Device under Test (DUT) Capacitors

[0047] 110 Capacitor Electrodes

[0048] 112 Capacitor Electrodes

[0049] 200 Measurement System

[0050] 202 Measurement Capacitor (DUT Capacitor)

[0051] 204 Integrated Circuits (ICs)

[0052] 206 Current Source

[0053] 208 Reference Capacitor

[0054] 300 Measurement Method

[0055] The 302 IC uses a current source to inject a first current into a measurement capacitor included in or attached to the product, and determines a first time that the resulting voltage across the measurement capacitor reaches a voltage threshold.

[0056] The 304 IC uses the current source to inject a second current into the reference capacitor and determines a second time amount at which the resulting voltage across the reference capacitor reaches the voltage threshold.

[0057] The 306 IC uses the difference between a first time value and a second time value to detect and measure changes in the capacitance of a capacitor.

[0058] 400 Measurement System

[0059] 402 Analog Domain

[0060] 404 Current Reference

[0061] 406 Current Mirror

[0062] 408 Voltage Reference

[0063] 410 comparator

[0064] 412 comparator

[0065] 414 Number Field

[0066] 416 Digital Logic

[0067] 418 Time reference.

Claims

1. An integrated circuit comprising a current source and a reference capacitor, characterized in that, The integrated circuit is configured to: inject a first current in a measurement capacitor using the current source, and determine a first amount of time for a resulting voltage on the measurement capacitor to reach a voltage threshold; inject a second current in a reference capacitor using the current source, and determine a second amount of time for a resulting voltage on the reference capacitor to reach the voltage threshold; detect a change in capacitance on the measurement capacitor using a difference between the first amount of time and the second amount of time, wherein the first amount of time is measured independently of the second amount of time.

2. The integrated circuit of claim 1, wherein, further configured to inject the second current in the reference capacitor while injecting the first current in the measurement capacitor.

3. The integrated circuit according to claim 1 or 2, characterized in that, The first current is a configurable current, and the second current is a constant current.

4. The integrated circuit of claim 1, wherein, The first current is configured such that a voltage slope on the measurement capacitor is substantially the same as a voltage slope on the reference capacitor.

5. The integrated circuit of claim 1, wherein, configured to determine a value of the first current after the integrated circuit is calibrated.

6. The integrated circuit of claim 1, wherein, further configured to: determine a third amount of time for the resulting voltage on the measurement capacitor to reach a further voltage threshold; determine a fourth amount of time for the resulting voltage on the reference capacitor to reach the further voltage threshold; detect a change in capacitance on the measurement capacitor using a difference between the third amount of time and the fourth amount of time.

7. A radio frequency identification, RFID, tag or near field communication, NFC, tag, characterized in that An integrated circuit according to any preceding claim.

8. A measurement system characterized by, An integrated circuit according to any preceding claim, and the measurement capacitor.

9. A tamper detection system characterized by, A measurement system according to claim 8.

10. A method of measurement, characterized by, comprising: inject a first current in a measurement capacitor using a current source in an integrated circuit, and determine a first amount of time for a resulting voltage on the measurement capacitor to reach a voltage threshold; inject a second current in a reference capacitor using the current source, and determine a second amount of time for a resulting voltage on the reference capacitor to reach the voltage threshold; detect a change in capacitance on the measurement capacitor using a difference between the first amount of time and the second amount of time by the integrated circuit, wherein the first amount of time is measured independently of the second amount of time.

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