Input circuit for evaluating a sensor current, having ESD protection and high current sensitivity

Abstract

The invention relates to an input circuit for an integrated component (1) for detecting a sensor current of a sensor (2) which can be connected to the integrated component (1) by means of a sensor connection point (3) and a reference potential connection point (4), the input circuit comprising: - a reference voltage source (7) which is designed to provide a reference voltage potential (VREF) at the reference voltage connection point (4); - a transimpedance amplifier (9) which is connected to the sensor connection point (3) and is designed to provide a measurement voltage on the basis of a sensor current (ISENSE); - a first ESD protection circuit (5) for protecting against overvoltage or undervoltage at the sensor connection point (3), wherein the first ESD protection circuit (5) is designed to derive the overvoltage or undervoltage from the reference voltage potential (VREF).

Description

[0001] Description

[0002] Input circuit for evaluating a sensor current with ESD protection and high current sensitivity

[0003] Technical field

[0004] The invention relates to an integrated sensor current evaluation circuit with ESD protection circuits (ESD: Electrostatic Discharge), and in particular with devices for reducing leakage currents.

[0005] Technical background

[0006] A number of electronic sensors, such as photodiodes for measuring light intensity or piezoelectric sensors for measuring mechanical force, provide an electrical current as their sensor parameter, the magnitude of which indicates the physical quantity to be measured. The sensor current is often measured using an integrated circuit to which the electronic sensor is connected via external terminals.

[0007] The external connections of the integrated circuit are typically equipped with ESD protection circuits connected to a reference potential, usually one or more supply potentials. For example, ESD protection diodes are often used, connected to a high supply potential and the ground potential (VSS). This type of connection causes a significant leakage current of several nanoamperes, especially at higher temperatures.

[0008] Especially with sensor currents that need to be measured with picoampere accuracy, these leakage currents are disruptive, difficult to compensate for, and in the worst case, can prevent accurate measurement of the sensor currents. Furthermore, leakage currents can also occur due to an offset in a transimpedance amplifier used to evaluate the sensor current.

[0009] The object of the present invention is to provide a sensor current evaluation circuit for an integrated component with which leakage currents can be reduced, and in particular sensor currents in the picoampere range can be evaluated.

[0010] Disclosure of the invention

[0011] This task is solved by the sensor current evaluation circuit according to claim 1.

[0012] Further details are specified in the dependent claims.

[0013] According to a first aspect, an input circuit for an integrated device for detecting a sensor current of a sensor connectable to the integrated device via a sensor terminal and a reference potential terminal is provided, comprising: a reference voltage source configured to provide a reference voltage potential at the reference voltage terminal; a transimpedance amplifier connected to the sensor terminal and configured to provide a measurement voltage dependent on the sensor current; a first ESD protection circuit for protection against an over- or undervoltage at the sensor terminal, wherein the ESD protection circuit is designed to dissipate the over- or undervoltage against the reference voltage potential.

[0014] To evaluate electronic sensors that supply a sensor current, an integrated circuit can be used that converts the sensor current into a corresponding measurement voltage via a transimpedance amplifier. Since the electronic sensor is usually a discrete component, such as a photodiode or a piezoelectric sensor, it must be connected to external pins of the integrated circuit.

[0015] The external connections of the integrated circuit are usually equipped with ESD protection circuits that allow static overvoltages to be dissipated and thus protect the integrated component.

[0016] The current strengths of the sensor currents are usually very low, i.e. in the range of pico- to nanoamperes, so that leakage currents caused by conventional ESD protection circuits or an offset of the transimpedance amplifier can significantly impair the measurement of the sensor currents.

[0017] The first ESD protection circuit may include a diode arrangement with one or more diodes connected in series in the forward direction between the sensor terminal and the reference voltage terminal, and a diode arrangement with one or more diodes connected in series in the reverse direction between the sensor terminal and the reference voltage terminal.

[0018] Furthermore, a second ESD protection circuit can be provided for the reference voltage connection, wherein the second ESD protection circuit connects the reference voltage connection to the high supply potential or the low supply potential VSS via respective diode arrangements with one or more diodes connected in series.

[0019] The electronic sensor is typically connected to the integrated circuit via a sensor terminal and a reference potential terminal. The sensor terminal is coupled to a first ESD protection circuit, and the reference potential terminal to a second ESD protection circuit. The reference potential terminal is connected to a predefined reference potential and is protected, in a known manner, by ESD protection diodes of the second ESD protection circuit with respect to the high supply potential and the low supply potential (Vss) or ground potential. It is also common to couple the sensor terminal to the supply potentials via corresponding ESD protection diodes. The leakage currents occurring through the diodes then add to the sensor current and distort the measurement result.

[0020] To prevent leakage currents via the sensor connection, the sensor connection is not coupled to one of the supply potentials via corresponding ESD protection diodes as is conventional, but rather to the reference potential at the reference potential terminal. This ensures that any leakage currents flowing through the ESD protection diodes at the sensor connection are dissipated via the reference potential. The entire potential at the sensor connection has no pn junction or transmission gate that is not connected to the reference voltage potential. Therefore, any potential leakage current is automatically zero.

[0021] It is therefore planned to provide an improved first ESD protection circuit to protect a sensor connection of the integrated component against static overvoltages.

[0022] The first ESD protection circuit features a forward-biased diode array to dissipate a positive terminal voltage, i.e., a positive overvoltage between the sensor terminal and the reference potential terminal that exceeds a predetermined diode voltage. An antiparallel diode is also provided to dissipate a negative terminal voltage, i.e., an undervoltage between the sensor terminal and the reference potential terminal, to the reference potential. When using a photodiode as the electronic sensor, whose diode voltage is typically between 0.6 and 0.8 V, the forward-biased diode array can usually be configured as a series connection of two or more diodes to prevent the photodiode's diode voltage from being dissipated through the ESD protection circuit during normal operation, i.e., when the photodiode is connected.

[0023] The reference potential itself is protected against the high and low supply potentials by a conventionally designed second ESD protection circuit. This circuit essentially consists of diodes designed to divert over- or undervoltages to the supply potentials of the integrated circuit. Furthermore, a detection circuit can be provided with a voltage measuring device and a current source, configured to induce a current in the sensor terminal and to measure the voltage at the sensor terminal. A missing or faulty sensor at the sensor terminal is detected when the voltage at the sensor terminal corresponds to the voltage resulting from the diode voltages of the first ESD circuit and, in particular, the second ESD protection circuit.

[0024] Furthermore, it may be provided that a control unit is configured to impress a diagnostic current onto the sensor terminal; to detect a voltage at the sensor terminal, to determine that an electronic sensor is connected when the detected voltage corresponds to a diode voltage of the sensor, or that an electronic sensor is not connected when the voltage of one of: o differs from the diode voltage of the sensor, and o corresponds to a diode voltage of the diode arrangement.

[0025] According to another aspect, a method for monitoring and diagnosing a connected or disconnected electronic sensor is provided, wherein the sensor has a diode characteristic, in particular a photodiode, with the above input circuit comprising the following steps:

[0026] Imprinting a diagnostic current onto the sensor connection;

[0027] Detecting a voltage at the sensor terminal,

[0028] Determining whether an electronic sensor is connected if the detected voltage corresponds to a diode voltage of the sensor, or whether a sensor is not connected if the voltage differs from the diode voltage of the sensor or corresponds to a diode voltage of the diode arrangement.

[0029] The first ESD protection circuit, connected to the sensor terminal, consists of a diode circuit with two diodes connected in series in the forward direction. A detection circuit allows the system to determine whether an electronic sensor is connected to the sensor terminal and reference potential terminal. For this purpose, a diagnostic current is applied to the sensor terminal, and the resulting voltage is measured on the sensor terminal line. If the voltage on the sensor terminal line is approximately two diode voltages, no sensor is connected to the sensor terminal and reference potential terminal. If the voltage is approximately 0.6–0.8 V, it can be determined that a photodiode is connected, and the corresponding diode voltage drops across the sensor terminal and reference potential terminal.

[0030] The sensor connection can also be connected to a transimpedance amplifier, preferably via a first transmission gate. The transimpedance amplifier converts the sensor current flowing through the transmission gate into a corresponding sensor voltage signal, which can be further processed internally by the integrated circuit and represents the magnitude of the sensor current.

[0031] Furthermore, the transimpedance amplifier can be designed with an operational amplifier and a feedback resistor to convert the sensor current into a measuring voltage, wherein the transimpedance amplifier is connected to the sensor terminal in particular via a switchable first transmission gate.

[0032] Alternatively, a feedback diode can be used instead of the feedback resistor to obtain logarithmic rather than linear behavior of the transimpedance amplifier. The feedback diode can be connected in the forward direction between the output and the inverting input of the operational amplifier.

[0033] The sensor connection can be connected to an inverting input of the operational amplifier via the first transmission gate. The reference voltage potential can be connected to the non-inverting input of the operational amplifier. The output of the operational amplifier can be connected to the inverting input via the feedback resistor or feedback diode.A compensation block may be provided which is designed to compensate for an offset of the operational amplifier of the transimpedance amplifier by connecting a low-resistance compensation resistor between the inverting input of the operational amplifier and the reference voltage potential, using a compensation voltage source that applies the reference voltage potential and is connected to the non-inverting input of the operational amplifier, to vary the voltage in order to set a compensation voltage when the voltage at the output of the operational amplifier changes sign.

[0034] Furthermore, a control unit can be configured to: disconnect the transimpedance amplifier from the sensor connection; connect an inverting input of the operational amplifier to the reference voltage potential via a compensation resistor; vary compensation voltages added to the reference voltage potential at a non-inverting input of the operational amplifier; and use the compensation voltage at which the voltage at the output of the operational amplifier changes sign for measurement operation of the input circuit.

[0035] According to another aspect, a method for offset compensation of the operational amplifier of the transimpedance amplifier in the above input circuit is provided, with the following steps:

[0036] Disconnect the transimpedance amplifier from the sensor connection;

[0037] Connecting an inverting input of the operational amplifier to the reference voltage potential via a compensation resistor;

[0038] Varying compensation voltages added to the reference voltage potential at a non-inverting input of the operational amplifier;

[0039] Using the compensation voltage, at which the voltage at the operational amplifier's output reverses polarity, for measuring the input circuit. For a (temperature-induced) leakage current to flow through a diode, a voltage must be applied across the diode. At low voltages in the range of a few mV, the leakage current is almost proportional to the voltage, and the direction of the leakage current also depends on the polarity of the applied voltage. This small voltage can result from the offset of the operational amplifier or the transimpedance amplifier. This offset voltage manifests externally as a small voltage between the sensor input and the reference potential. To minimize the diode leakage current between the sensor input and the reference potential, this voltage must be as small as possible. This is achieved through appropriate offset compensation of the transimpedance amplifier.

[0040] When using a photodiode, a sensor current flows even without a voltage across the photodiode, essentially acting as a short-circuit current. A small bias voltage across the photodiode does not change the magnitude or polarity of the photocurrent. The compensation block allows the amplifier's offset voltage to be minimized. This results in only a negligible leakage current flowing through the ESD diodes, as there is virtually no voltage between the reference terminal and the reference voltage terminal. There is no diode from the sensor input to VDD or VSS; there are only diodes connected to the reference potential.

[0041] To compensate for an operational amplifier offset in the transimpedance amplifier, which can lead to leakage current affecting the sensor current, a compensation voltage source can be provided. The compensation voltage source is configured to apply the reference voltage potential, allowing the reference voltage at the non-inverting input of the operational amplifier to be adjusted to compensate for the operational amplifier offset. This allows, either alternatively or additionally, the prevention of leakage currents through the sensor line, thus enabling highly precise sensor current measurements in the picoampere range.

[0042] As described above, the transimpedance amplifier is implemented with an operational amplifier, which is configured with a feedback resistor between the output of the operational amplifier and the inverting input of the operational amplifier. The non-inverting input of the operational amplifier is set to a potential derived from a compensation voltage and the reference voltage potential. The sensor current is then impressed into the inverting input of the operational amplifier.

[0043] To calibrate the transimpedance amplifier's offset, the sensor current is disconnected from the amplifier by opening the first transmission gate, and a low-value resistor is connected between the op-amp's inverting input and the reference voltage potential. This results in a very high gain for the op-amp, which then acts as a comparator. By adjusting the compensation voltage source in small steps, the value of the compensation voltage can be determined when a sign change occurs at the op-amp's output. Thus, adjusting the compensation voltage source at the sign change provides offset voltage compensation for measurement operation (i.e., for the sensor's regular operation). This allows for efficient compensation of any leakage current caused by the transimpedance amplifier's offset.

[0044] Brief description of the drawings

[0045] The embodiments are explained in more detail below with reference to the accompanying drawings. These show:

[0046] Figure 1 shows a circuit diagram for an input circuit of an integrated circuit for reading an electronic sensor, in particular a photodiode;

[0047] Figure 2 is a flowchart illustrating a method for diagnosing a connected, disconnected, or defective photodiode; and Figure 3 is a flowchart illustrating a method for performing offset compensation for a transimpedance amplifier of the input circuit of Figure 1.

[0048] Description of embodiments

[0049] Figure 1 shows a section of an input circuit for an integrated circuit 1, which is designed to read an electronic sensor 2, in particular in the form of a photodiode. The electronic sensor 2 is connected to the integrated circuit 1 via a sensor terminal 3 and a reference potential terminal 4. The external connections are protected against over- and undervoltages by a first ESD protection circuit 5 in the integrated circuit and a second ESD protection circuit 6.

[0050] The reference voltage potential VREF is provided by a reference potential source 7 and is usually between the low and high supply potentials Vss, VDD.

[0051] A first ESD protection circuit 5 is connected to the sensor terminal 3. The first ESD protection circuit 5 has a forward-biased diode arrangement 51 consisting of two series-connected diodes 52, which are connected between the sensor terminal and the reference potential terminal. A reverse-bias diode 53, i.e., an antiparallel diode, is also provided, which is connected between the sensor terminal 3 and the reference potential terminal 4 for dissipating undervoltages.

[0052] To prevent a node from floating between the two diodes 52 of the diode arrangement 51, a drain device 54 is provided which drains any potential that may arise there in the direction of the reference voltage potential VREF.

[0053] The second ESD protection circuit 6 is connected to the reference potential terminal 4 and includes an ESD protection diode 61, 62, respectively, facing the high supply potential DD and the low supply potential Vss to dissipate the charge of an overvoltage or undervoltage. A voltage above the high supply potential VDD is dissipated via the ESD protection diode 61 towards the high supply potential VDD, and a voltage below the low supply potential Vss is dissipated via the second ESD protection diode 62 to the low supply potential Vss.

[0054] Depending on the incoming light intensity, photodiode 2 supplies a photodiode current as sensor current Lense. The sensor current Lense is passed via a first transmission gate 8 to a transimpedance amplifier 9.

[0055] The transimpedance amplifier 9 converts the sensor current ISENSE into an equivalent measurement voltage V™, which is further processed in the integrated component.

[0056] The transimpedance amplifier 9 is constructed with an operational amplifier 91, the output of which is connected to an inverting input via a relatively high-impedance feedback resistor 92. The non-inverting input of the operational amplifier 91 is essentially connected to the reference voltage potential VREF.

[0057] To perform offset compensation using the compensation block 10, the non-inverting input of the operational amplifier 91 is connected to the reference voltage potential VREF via a compensation voltage source 101, via which offset compensation can be set.

[0058] To detect a detached or missing electronic sensor 2, the sensor connection 3 can be connected to a switchable power source 11 and the sensor current line 31 can be connected to a voltage measuring device 13 via a second transmission gate 12.

[0059] The flowchart in Figure 2 describes the detection of a detached or missing electronic sensor 2 if it exhibits diode characteristics, as is the case with a photodiode as sensor 2. The method is controlled by a control unit, which can be located internally or externally of the integrated circuit.

[0060] In step S1, a diagnostic current loss is first applied to the sensor current line 31. This is done by closing a switch of the power source 11 using a control signal Ss.

[0061] In step S2, the second transmission gate 12 is closed by a corresponding control signal S2, thus activating the voltage measuring device 13.

[0062] By providing the first ESD protection circuit 5 with a diode arrangement 51 of series-connected, forward-oriented diodes 52 between the sensor terminal 3 and the reference potential terminal 4, a voltage drop can be achieved at the sensor terminal 3 or on the sensor current line 31 by injecting the diagnostic current Loss, which results from the diode characteristics. The voltage drop corresponds to a voltage drop of one diode voltage of the photodiode 2 when the photodiode 2 is connected, and to a voltage drop of two diode voltages of the diode arrangement 51 when the photodiode is not connected to the sensor terminal 3 and the reference potential terminal 4.

[0063] Thus, in step S3, the magnitude of the voltage drop is checked. If a voltage drop of one diode voltage is detected (alternative: Yes), step S4 signals that the sensor / photodiode 2 is connected or functioning correctly. If a voltage drop of more than one diode voltage is detected (alternative: No), or if the voltage drop deviates from the photodiode's diode voltage, step S5 signals that no photodiode 2 is connected or that it is defective. Specifically, a voltage drop of a multiple of the diode voltage can be detected if no sensor 2 is connected, where this multiple corresponds to the number of diodes in the diode array of the first ESD protection circuit 5.

[0064] The measurement is performed using the voltage measuring device 13 with the second transmission gate 12 closed and can be evaluated accordingly in the integrated module 1. To measure a sensor current ISENSE, the first transmission gate 8 is switched on. To compensate for leakage currents due to an offset of the operational amplifier 91, offset compensation can be performed. Thus, a procedure can be carried out according to the flowchart in Figure 3, which can, for example, be executed during a measurement pause. The procedure is controlled by a control unit, which can be arranged internally or externally within the integrated module.

[0065] In step S11, the first transmission gate 8 is first switched to blocking in order to disconnect the transimpedance amplifier 9 from the sensor current.

[0066] Now, in step S12, the inverting input of the operational amplifier is connected to the reference voltage potential VREF via a compensation switch 102 and a comparatively low-impedance compensation resistor 103 (RCOMP_OFS) using the compensation block 10.

[0067] Furthermore, in step S13, varying compensation voltages are set using the compensation voltage source 101. The variation can be made in voltage steps of mV or a few tens of pV.

[0068] In step S14, during the variation of the compensation voltages, the output of operational amplifier 91 is checked for a sign change, and the compensation voltage is fixed at this sign change, also for measurement operation. Since operational amplifier 91 has a very high gain due to the high resistance ratio between the feedback resistor 92 and the compensation resistor 103, the compensation voltage to be set can be determined and adjusted by finding a compensation voltage at which a sign change occurs at operational amplifier 91. The compensation voltage then achieved at the compensation voltage source 101 corresponds to the permanently set offset compensation.

Claims

Claims 1. Input circuit for an integrated device (1) for sensing a sensor current of a sensor (2) connectable to the integrated device (1) via a sensor terminal (3) and a reference potential terminal (4), comprising: a reference voltage source (7) configured to provide a reference voltage potential (VREF) at the reference voltage terminal (4); a transimpedance amplifier (9) connected to the sensor terminal (3) and configured to provide a measurement voltage dependent on a sensor current (ISENSE); a first ESD protection circuit (5) for protection against an over- or undervoltage at the sensor terminal (3), wherein the first ESD protection circuit (5) is designed to dissipate the over- or undervoltage against the reference voltage potential (VREF).

2. Input circuit according to claim 1, wherein the first ESD protection circuit (5) comprises a diode arrangement (51) with one or more series-connected diodes (52) in the forward direction between the sensor terminal (3) and the reference voltage terminal (4) and a diode arrangement with one or more series-connected diodes (53) in the reverse direction between the sensor terminal (3) and the reference voltage terminal (4).

3. Input circuit according to claim 1 or 2, wherein a second ESD- Protection circuit (6) is provided for the reference voltage connection (4), wherein the second ESD protection circuit (6) in particular protects the Reference voltage connection (4) via respective diode arrangements with one or more series-connected diodes (61, 62) with the high Supply potential (DD) or low supply potential (Vss) is linked.

4. Input circuit according to one of claims 2 to 3, wherein a detection circuit is provided with a voltage measuring device (13) and a current source (11) configured to induce a diagnostic current (Loss) in the sensor terminal (3) and to perform a voltage measurement at the sensor terminal (3), wherein a missing or faulty sensor (2) at the sensor terminal (3) is detected when a voltage (VSENSE) at the sensor terminal (3) corresponds to the voltage resulting from the diode voltages of the first ESD circuit (5) and, in particular, the second ESD protection circuit (6).

5. Input circuit according to claim 4, wherein a control unit is configured to impress a diagnostic current (Loss ) onto the sensor terminal (3); to detect a voltage at the sensor terminal (3), to determine that an electronic sensor (2) is connected when the detected voltage corresponds to a diode voltage of the sensor, or that an electronic sensor (2) is not connected when the voltage of one of: o deviates from the diode voltage of the sensor (2), and o corresponds to a diode voltage of the diode arrangement (51).

6. Input circuit according to one of claims 1 to 5, wherein the transimpedance amplifier (9) is configured with an operational amplifier (91) and a feedback resistor (92) to convert the sensor current (LENSE) into a measuring voltage (VTIA), wherein the transimpedance amplifier (9) is in particular connected to the sensor terminal (3) via a switchable first transmission gate (8).

7. Input circuit according to claim 6, wherein a compensation block is provided which is configured to compensate for an offset of the operational amplifier (91) of the transimpedance amplifier (9) at 16 ELM5051WO decoupled sensor connection to connect a low-resistance compensation resistor (103) between the inverting input of the operational amplifier (91) and the reference voltage potential (VREF), using a compensation voltage source (10) that applies to the reference voltage potential (VREF) and is connected to the non-inverting input of the operational amplifier (91) to vary the voltage in order to set a compensation voltage (10) when the polarity of the voltage at the output of the operational amplifier (91) changes.

8. Input circuit according to claim 7, wherein a control unit is configured to disconnect the transimpedance amplifier (9) from the sensor connection (3); to connect an inverting input of the operational amplifier (91) to the reference voltage potential (VREF) via a compensation resistor (103); to vary compensation voltages added to the reference voltage potential at a non-inverting input of the operational amplifier (91); the compensation voltage at which a sign change of the voltage at the output of the operational amplifier (91) occurs; to use for a measurement operation of the input circuit.

9. A method for monitoring and diagnosing a connected or disconnected electronic sensor (2), wherein the sensor (2) has a diode characteristic, in particular a photodiode, with an input circuit according to claim 4 comprising the following steps: Imprinting (S1) a diagnostic current (Loss ) onto the sensor connection (3); Detecting (S2) a voltage at the sensor terminal (3), identifying (S3) a connected electronic sensor when the detected voltage corresponds to a diode voltage of the sensor, or an unconnected sensor when the voltage differs from the diode voltage of the sensor (2) or corresponds to a diode voltage of the diode arrangement. 17 ELM5051WO 10. Method for offset compensation of the operational amplifier (91) of the transimpedance amplifier (9) in the input circuit according to claim 6, comprising the following steps: Disconnect (S11) the transimpedance amplifier (9) from the sensor connection; connect (S12) an inverting input of the operational amplifier (91) via a compensation resistor (103) to the reference voltage potential (VREF); Varying (S13) compensation voltages added to the reference voltage potential at a non-inverting input of the operational amplifier (91); Use (S14) of the compensation voltage at which a sign change of the voltage at the output of the operational amplifier (91) occurs; for a measurement operation of the input circuit.

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