Signal processing circuit and signal processing device

Abstract

A signal processing circuit includes first and second current sensor inputs (PD1, PD2) to receive respective sensor currents and a voltage sensor input to receive a sensor voltage. A first selection unit (SEL1) includes first and second current inputs coupled to the first and second current sensor inputs, respectively, and is configured to select one of the first and second current inputs as a first selected input connected to a first current output and to select one of the first and second current inputs as a second selected input connected to a second current output. A second selection unit (SEL2) includes a first voltage input coupled to the voltage sensor input (VIN) and a second voltage input, and is configured to connect one of the first and second voltage inputs to a voltage output. A current ADC (IADC) is coupled to the first current output and a voltage ADC (VADC) is coupled to the voltage output. A digital processing block (DIG) is coupled to respective outputs of the current ADC and the voltage ADC. A current voltage converter (TIA) is coupled between the second current output and the second voltage input.

Description

[0001] This disclosure relates to signal processing circuits and signal processing apparatus having such signal processing circuits.

[0002] This patent application claims priority to German Patent Application 10 2021 104 980.6, the disclosure of which is incorporated herein by reference. Background Technology

[0003] Various sensors provide voltage or current signals as their corresponding sensor signals. In order to process such sensor signals in the digital domain, it is desirable to provide an analog-to-digital converter (ADC) suitable for the sensor signal type (i.e., voltage or current signal).

[0004] For example, sensors used in photoplethysmography (PPG) typically provide current signals, while electrocardiogram (ECG) measurements are performed based on voltage. Therefore, conventional signal processing for such applications provides one ADC for processing the PPG signal and another ADC for processing the ECG signal. If another current-based sensor signal needs to be processed, the ADC used for PPG must be shared, or a separate current-based ADC must be provided. Summary of the Invention

[0005] The goal is to provide improved processing concepts for handling current-based and voltage-based sensor signals.

[0006] This objective is achieved through the subject matter of the independent claims. Implementations and improvements of the improved concept are defined in the dependent claims.

[0007] The improved processing concept is based on the idea of ​​processing two current-based sensor signals in parallel (e.g., simultaneously) without providing a dedicated second current-based ADC. Instead, one of the current sensor signals is converted to the voltage domain, and a voltage-based ADC, typically configured to process voltage-based sensor signals, is used to process this signal. For this purpose, a first selection unit is configured to select the two current sensor signals and route them to the corresponding ADC. Furthermore, a second selection unit is configured to select whether to route the voltage-based sensor signal or the current signal converted to the voltage domain to the voltage-based ADC. The outputs of the two ADCs can be processed in a common digital processing block.

[0008] For example, a signal processing circuit according to an improved processing concept includes: a first current sensor input for receiving a first sensor current, a second current sensor input for receiving a second sensor current, and a voltage sensor input for receiving a sensor voltage. A first selection unit includes a first current input coupled to the first current sensor input and a second current input coupled to the second current sensor input. The first selection unit is configured to: select one of the first and second current inputs as a first selected input to be connected to a first current output based on a first selection signal, and select one (e.g., the other) of the first and second current inputs as a second selected input to be connected to a second current output.

[0009] The second selection unit includes a first voltage input terminal, a second voltage input terminal, and a voltage output terminal. The first voltage input terminal is coupled to a voltage sensor input terminal. The second selection unit is configured to connect one of the first and second voltage input terminals to the voltage output terminal based on a second selection signal.

[0010] The signal processing circuit also includes: a current analog-to-digital converter (ADC) coupled to the first current output terminal, a voltage ADC coupled to the voltage output terminal, and digital processing blocks coupled to the corresponding output terminals of the current ADC and the voltage ADC. The current-to-voltage converter is coupled between the second current output terminal of the first selection unit and the second voltage input terminal of the second selection unit.

[0011] Such signal processing circuitry allows for selectable operation in at least two configurations. For example, in a first configuration, one of the current signals received at the first and second current inputs is processed by a current ADC, while the voltage signal at the voltage sensor input is processed by a voltage ADC, wherein the digital outputs of both the current ADC and the voltage ADC are processed by a digital processing block.

[0012] In the second configuration, one of the current sensor signals is processed using a current ADC, while the other current sensor signal is converted to voltage using a current-to-voltage converter and then processed using a voltage ADC. This allows for flexible operation of the signal processing circuitry.

[0013] For example, the first selection unit provides two current paths: from a first selected input terminal to a first current output terminal and from a second selected input terminal to a second current output terminal. Therefore, the first selection unit is configured to simultaneously provide the corresponding current from the first selected input terminal to the first current output terminal and from the second selected input terminal to the second current output terminal. The simultaneous existence of current paths allows both current signals to be converted to the digital domain simultaneously, ensuring no time delay between the corresponding digital current samples.

[0014] For example, the signal processing circuit is configured to have photodiodes connected to the input terminals of a first current sensor and a second current sensor. Therefore, two different photoelectric signals can be processed simultaneously, allowing for combined evaluation of the corresponding photodiode signals.

[0015] In various implementations, at least one of the current ADC and voltage ADC includes a Δ-Σ modulator. For example, an ADC with a Δ-Σ modulator allows noise shaping to be used to improve the corresponding conversion results.

[0016] Current-to-voltage converters can be implemented using various types of circuits that provide such functionality. For example, a current-to-voltage converter includes a transimpedance amplifier, which is primarily composed of an operational amplifier having at least a resistive element in its feedback path.

[0017] In some implementations, the signal processing circuitry also includes at least one offset current source connected to either the first or second current output terminal. This involves connecting a separate offset current source to each of the first and second current output terminals. Therefore, the current offset in the corresponding current sensor signal can be compensated for by the corresponding offset current. Specifically, the offset current is combined with the sensor current prior to analog-to-digital conversion.

[0018] In various implementations, the digital processing block includes a buffer element for buffering digital values ​​provided by the current ADC and voltage ADC. For example, the buffer element is implemented as a first-in-first-out (FIFO) element. Values ​​stored in the buffer element can be processed directly within the digital processing block or in a separate processor or other entity connected to the digital processing block. For this purpose, the digital processing block may include a corresponding interface.

[0019] The improved processing concept is not limited to exactly two current sensor inputs (which will still be an option), but can also include three or more current sensor inputs. In such an implementation, the first selection unit can select from any available current sensor input.

[0020] For example, the signal processing circuit also includes at least one additional current sensor input for receiving at least one additional sensor current. In such a configuration, the first selection unit is configured to: select one of the first current input, the second current input, and at least one additional current input as a first selected input based on a first selection signal, and select one of the first current input, the second current input, and at least one additional current input, particularly another, as a second selected input.

[0021] While choosing two different input terminals is a reasonable application, it should not be excluded that the same current input terminal can be connected to both the first current output terminal and the second current output terminal.

[0022] Similarly, the second selection unit may include an additional voltage input terminal, which may be connected to an external voltage input terminal or an internal voltage terminal to allow for various other measurements.

[0023] The signal processing circuit according to one of the above implementations can be used, for example, in various types of signal processing devices. For instance, a signal processing device according to an improved processing concept includes: a signal processing circuit according to one of the above embodiments, a first photosensitive element connected to a first sensor input terminal, a second photosensitive element connected to a second current sensor input terminal, and a sensor element connected to a voltage sensor input terminal for providing at least one sensor voltage. For example, the photosensitive element is implemented as one or more photodiodes.

[0024] Therefore, such a signal processing device can process two photosensitive elements in parallel or process one of the photosensitive elements together with a sensor element that provides the sensor voltage.

[0025] For example, the sensor element includes at least two electrodes for electrocardiogram (ECG) measurement. The signal processing device also includes a processing block connected between the voltage sensor input and the first voltage input. For example, such a processing block includes elements for leakage compensation and / or lead drop detection and / or an amplifier for processing the corresponding voltage signal. Thus, the voltage signal provided to the second selection unit, and if selected, to the voltage ADC, realizes the ECG signal to be evaluated.

[0026] Therefore, the signal processing device supports processing ECG signals in parallel with one of the photocurrents or as a substitute for one or more photocurrents.

[0027] In various implementations, the first photosensitive element and the second photosensitive element are suitable for at least one of the following configurations:

[0028] - The first and second photosensitive elements are configured for photoelectric volumetric plethysmography (PPG);

[0029] - A first photosensitive element is configured for PPG, and a second photosensitive element is configured for detecting oxygen saturation;

[0030] - A first photosensitive element is configured for PPG, and a second photosensitive element is configured for ambient light detection.

[0031] When both the first and second photosensitive elements are configured for PPG, multi-wavelength PPG measurements can be performed, thereby improving the quality of PPG results and / or allowing for improved applications.

[0032] The configuration of a first photosensitive element for PPG and a second photosensitive element for oxygen saturation detection enables the parallel measurement and evaluation of both types of measurement signals.

[0033] When one photosensitive element is configured for PPG and another photosensitive element is configured for ambient light detection, the measurement quality of the PPG signal can be improved by combining appropriate compensation for the simultaneously measured ambient light signal, which is included in the PPG measurement, but this degrades the PPG measurement.

[0034] As discussed above, if two or more current sensor inputs are available, such as three or four, then a photosensitive element of a corresponding specific configuration can be connected to these inputs, thereby allowing for selectable configurations (e.g., from one of the configurations described above). Furthermore, selectable measurement of ECG signals or other voltage-based signals remains possible.

[0035] In various implementations, the signal processing device can be implemented as a wearable device, such as a smartwatch, wristband, or glasses. It can also be implemented as a sensor patch, for example, by using an adhesive to attach the signal processing device to the skin of a person or animal. Attached Figure Description

[0036] The improved processing concept will now be described in more detail with the aid of the accompanying drawings. Elements with the same or similar functions have the same reference numerals throughout the drawings. Therefore, their description need not be repeated in the following drawings.

[0037] In the attached diagram:

[0038] Figure 1 A schematic diagram of an example signal processing circuit is shown;

[0039] Figure 2 Another schematic diagram of the example signal processing circuit is shown;

[0040] Figures 3A to 3C Various examples of signal processing devices are shown; and

[0041] Figure 4A and Figure 4B An example configuration of a photosensitive element is shown. Detailed Implementation

[0042] Figure 1An example implementation of a signal processing circuit based on an improved processing concept is shown. The signal processing circuit includes: a first current sensor input terminal PD1, a second current sensor input terminal PD2, and additional current sensor input terminals PD3, PD4, PD5, and PD6. Furthermore, a current reference terminal PDREF may be included, which can be internally connected to a reference potential terminal, such as a ground terminal. A first selection unit SEL1 has corresponding current input terminals, with a first current input terminal connected to the first current sensor input terminal PD1, a second current input terminal coupled to the second current sensor input terminal PD2, and additional current input terminals connected to the other current sensor input terminals PD3 through PD6. The first selection unit SEL1 has a first current output terminal connected to a current ADC IADC and a second current output terminal coupled to a current-to-voltage converter TIA.

[0043] The signal processing circuitry also includes a digital processing block (DIG), which is shown schematically for better overview. For example, the output of the current ADC (IADC) is coupled to the digital processing block (DIG).

[0044] The signal processing circuit also includes a voltage sensor input terminal VIN and a second selection unit SEL2. The second selection unit SEL2 includes a first voltage input terminal coupled to the voltage sensor input terminal VIN and a second voltage input terminal coupled to the output terminal of the current-to-voltage converter TIA. The voltage output terminal of the second selection unit SEL2 is coupled to the voltage ADC VADC, and the output terminal of the voltage ADC VADC is coupled to the digital processing block DIG.

[0045] The solid black lines, including the current sensor inputs PD1 to PD6, the current reference terminal PDREF, the voltage sensor input VIN, and the voltage reference terminal VINREF, resemble the boundaries of a signal processing circuit.

[0046] Figure 1 Additionally, an array of photodiodes is shown as an example of various photosensitive elements connected between the respective current sensor input terminals PD1 to PD6 and the current reference terminal PDREF. Furthermore, Figure 1 A schematic representation of a sensor element VS connected to the voltage sensor input terminal VIN and the voltage reference terminal VINREF is shown. The photosensitive element and the sensor element VS can be part of a signal processing device that includes signal processing circuitry.

[0047] The first selection unit SEL1 is configured to select one of its current input terminals (e.g., one of the first and second current input terminals) as the first selected input terminal to be connected to the first current output terminal and thus to the current ADC IADC. Furthermore, the first selection unit SEL1 is configured to select one of its current input terminals (e.g., one of the first and second current input terminals) as the second selected input terminal to be connected to the second current output terminal and thus to the current-to-voltage converter TIA. Preferably, the first selected input terminal and the second selected input terminal are different from each other.

[0048] For example, the selection within the first selection unit SEL1 is based on a first selection signal provided via the first selection input SIN1. This first selection signal may be provided by the digital processing block DIG, but it could also be provided by an entity external to the signal processing circuitry.

[0049] The second selection unit is configured to connect one of the first voltage input terminal and the second voltage input terminal to the voltage output terminal based on a second selection signal that can be provided via the second selection input terminal SIA2. For example, the second selection signal is also provided by the digital processing block DIG, but it can also be provided by an entity external to the signal processing circuitry.

[0050] The current-to-voltage converter TIA is configured to generate a voltage corresponding to the respective input current supplied from the first selection unit SEL1. Therefore, the second selection unit SEL2 provides one of the respective input voltages at its output to the voltage ADC VADC.

[0051] Therefore, various configurations of the input signal to be processed are available during the operation of the signal processing circuit.

[0052] For example, in a configuration determined by a corresponding selection signal, one of the currents supplied at the current sensor inputs PD1 to PD6 (correspondingly the current inputs of the first selection unit SEL1) is provided to the current ADC IADC to convert the current value into its digital representation. The second selection unit SEL2 provides the voltage supplied by the sensor element VS to the voltage ADC VADC to generate a digital representation of the corresponding voltage value. In such a configuration, the selection of the second selected input within the first selection unit SEL1 can be ignored. For example, in such a case, even no selection is an option, so that no current is therefore supplied to the current-to-voltage converter TIA.

[0053] Ultimately, the described configuration allows for parallel processing of the sensor signals from the selected current sensor input (and correspondingly the selected photosensitive element) and the sensor element VS.

[0054] In another configuration, as in the configuration described above, one of the current input terminals (correspondingly current sensor input terminals PD1 to PD6) is also selected as the first selected input terminal in the first selection unit SEL1. However, in this configuration, the second selected input terminal is determined by the first selection signal, such that the current supplied at the selected input terminal is provided from the second selected input terminal to the current-to-voltage converter TIA for converting the current into a corresponding voltage signal. Furthermore, in the second selection unit SEL2, a second voltage input terminal connected to the output terminal of the current-to-voltage converter TIA is selected via the second selection signal, such that the output voltage of the current-to-voltage converter is provided to the voltage ADC VADC.

[0055] Ultimately, in this configuration, signals from two current sensors, implemented as photosensitive elements in this example, can be processed simultaneously.

[0056] Specifically, in the first selection unit SEL1, two parallel current paths are established between the corresponding selected input terminal and the first and second current output terminals. This avoids the need to switch the input terminals to provide current through a single current path. Furthermore, providing the corresponding sensor currents in parallel at the first and second current output terminals allows for simultaneous conversion from the analog domain to the digital domain via analog-to-digital converters (IADC and VADC), which is particularly beneficial for applications requiring precise timing relationships of the corresponding sensor currents. In this configuration, the sensor voltage from the sensor element VS is ignored.

[0057] Figure 2 It shows the basis Figure 1 Another schematic diagram of the example signal processing circuit shown in the example implementation. Therefore, in Figure 2 The description has already combined Figure 1 The descriptions of functions and operating modes will be omitted, and the main focus will be on the differences or extensions.

[0058] For example, current ADCs (IADC) and voltage ADCs (VADC) include, for example, Δ-Σ modulators that allow noise shaping of the respective input signals. However, in alternative implementations, only one of the ADCs (IADC and VADC) can be implemented using an ADC based on a Δ-Σ modulator.

[0059] In addition, Figure 2In the example, the current-to-voltage converter TIA is implemented using a transimpedance amplifier. As schematically shown in the corresponding block, the transimpedance amplifier is implemented using an operational amplifier that has a resistive element in its feedback path for the actual current-to-voltage conversion. The feedback path is connected to the inverting input of the operational amplifier, which is also connected to the second current output of the first selection unit SEL1. The non-inverting input of the operational amplifier is connected to the reference potential terminal.

[0060] Without departing from the illustrative approach of the transimpedance amplifier shown in this example, additional details can be implemented using a transimpedance amplifier. The resistive elements in the feedback path can be adjustable, for example, to tune the precise ratio between the input current and output voltage of the transimpedance amplifier.

[0061] exist Figure 2 In the example implementation, a first offset current source OFF1 is connected to a first current output terminal, and a second offset current source OFF2 is connected to a second current output terminal. This allows, for example, compensation for the offset current included in the corresponding sensor signal, thereby improving signal quality.

[0062] The Digital Processing Block (DIG) may include buffer elements for buffering the corresponding digital values ​​provided by the ADC IADC and ADC VADC. For example, the buffer elements are implemented as FIFO buffers that allow, for example, asynchronous processing or forwarding of buffered values.

[0063] In addition, the Digital Processing Block (DIG) may include an interface block for providing buffered values ​​to another device, signal processing circuit, or signal processor. For example, the interface block may be configured to operate according to I2C and / or SPI transmission standards.

[0064] In an alternative implementation, the digital processing block may also include a digital signal processor for directly processing the buffered digital values.

[0065] exist Figure 2 In an example implementation, the sensor element VS includes at least two electrodes for ECG measurement, coupled to corresponding electrode terminals ECG_INP and ECG_INN. Furthermore, the sensor element VS is coupled to a voltage reference terminal VINREF. To process (and preprocess) the signals from the ECG electrodes, the signal processing device also includes a processing block VPROC connected between the voltage sensor input terminal VIN (corresponding electrode terminals ECG_INP and ECG_INN) and a first current input terminal of the second selection unit SEL2. For example, the processing block VPROC includes circuitry for leakage compensation and / or lead-off detection and / or amplification of the corresponding ECG signals. This may also include circuitry for providing a corresponding potential to the voltage reference terminal VINREF.

[0066] For example, at least two of the photosensitive elements connected to the current sensor input terminals PD1 to PD6 are configured for PPG measurement. For this purpose, the signal processing circuit may also include a circuit system for driving the respective LEDs as a light source for PPG measurement. However, for a better overview, in Figure 2 Such a circuit system is not shown in the current schematic representation.

[0067] Reference and combination Figure 1 The descriptions of different possible configurations, with corresponding photosensitive elements connected and configured for PPG measurement, are given. Figure 2 The signal processing circuitry allows for the selective parallel processing of two PPG channels, with one channel processed via a current ADC (IADC) and the other via a transimpedance amplifier and a voltage ADC (VADC). This, for example, enables multi-channel PPG processing that can improve PPG quality.

[0068] In another configuration, a PPG channel is processed in parallel with the ECG signal.

[0069] As previously discussed, signal processing circuitry, together with corresponding sensors, can form a signal processing device. While the exact implementation of such a signal processing device should not be limited below, Figure 3A , Figure 3B and Figure 3C Various examples of such signal processing devices are shown. For example, Figure 3A and Figure 3B The signal processing device is shown to be implemented as a wearable device (e.g., Figure 3A Smartwatches and Figure 3B Examples include wristbands. For instance, a wristband may include a sensor region SAR carrying at least one of a photosensitive element and / or a voltage sensor element VS (e.g., an ECG electrode). Another implementation, not shown here, is as a signal processing device for eyeglasses, particularly smart glasses.

[0070] Figure 3C An implementation of a signal processing device as a sensor patch is shown, wherein the sensor region SAR is fixed to the skin of a person or animal by some kind of adhesive (e.g., a band-aid).

[0071] Despite the presence of a PPG, one or more photosensitive elements that can be connected to signal processing circuitry can be configured to detect oxygen saturation (e.g., peripheral oxygen saturation SpO2) and / or ambient light. For example, ambient light detection can be used to compensate for the influence of ambient light on the signal from the photosensitive element configured with the PPG.

[0072] In the context of this specification, a photosensitive element can be not only a single photodiode or other single device, but can also include groups of such devices, which, for example, provide their respective photocurrents in parallel to improve signal quality and / or intensity. Thus, several photodiodes or other devices can be arranged in groups, which together provide their respective photocurrents to one of the current sensor inputs PD1 to PD6.

[0073] Figure 4A and Figure 4B An example configuration of photosensitive elements, particularly photodiodes arranged in multiple groups, is shown.

[0074] For example, Figure 4A The first group of photodiodes GR1 and the second group of photodiodes GR2 are shown in a cross-shaped arrangement in this example. For example, each group of photodiodes is configured for PPG measurement, such that, using signal processing circuitry, either group with potentially higher signal quality or better signal strength can be selected when performing ECG measurements in parallel, or the signals from both groups of GR1 and GR2 can be processed in parallel.

[0075] Figure 4B A similar example configuration is shown, in which two sets of photodiodes are arranged as two groups, GR1 and GR2, and arranged in a cross shape. For example, one group of photodiodes is equipped with a green wavelength filter, while the other group of photodiodes is equipped with a wavelength filter for red / infrared light, making it possible to determine PPG and oxygen saturation (e.g., for peripheral oxygen saturation SpO2).

[0076] The second selection unit may include an additional voltage input terminal, which can be selected by a second selection signal as the output voltage to be supplied to the voltage ADC. For example, there may be an additional sensor input terminal and / or the internal voltage of the signal processing device may be measured.

[0077] It should be understood that this disclosure is not limited to the disclosed embodiments and those specifically shown and described above. Rather, features listed in the separate dependent claims or description may be advantageously combined. Furthermore, the scope of this disclosure includes variations and modifications that will be apparent to those skilled in the art and fall within the spirit of the appended claims. The term "comprising" as used in the claims or description does not exclude other elements or steps of the corresponding feature or process. When the terms "a" or "an" are used in conjunction with a feature, they do not exclude a plurality of such features. Moreover, any reference numerals in the claims should not be construed as limiting the scope.

[0078] List of reference numerals

[0079] PD1 to PD6 current sensor input terminals

[0080] PDREF Current Reference Terminal

[0081] VIN voltage sensor input terminal

[0082] VINREF reference voltage terminal

[0083] VS sensor components

[0084] SEL1, SEL2 Select Unit

[0085] SIN1, SIN2 Select input terminals

[0086] IADC Current ADC

[0087] VADC (Voltage ADC)

[0088] DIG digital processing block

[0089] TIA Current-to-Voltage Converter

[0090] OFF1, OFF2 offset current sources

[0091] FIFO buffer element

[0092] IFC Interface Block (IB)

[0093] ECG_INP, ECG_INN electrode terminals

[0094] VPROC processing block

[0095] SAR sensor area

[0096] GR1 and GR2 groups

Claims

1. A signal processing circuit, comprising: - First current sensor input terminal (PD1), which is used to receive the first sensor current; - The second current sensor input terminal (PD2) is used to receive the second sensor current; - Voltage sensor input (VIN), which is used to receive the sensor voltage; - A first selection unit (SEL1) includes a first current input terminal coupled to the first current sensor input terminal (PD1) and a second current input terminal coupled to the second current sensor input terminal (PD2), and is configured to: select one of the first current input terminal and the second current input terminal as a first selected input terminal to be connected to the first current output terminal based on a first selection signal, and select one of the first current input terminal and the second current input terminal as a second selected input terminal to be connected to the second current output terminal; - A second selection unit (SEL2) includes: a first voltage input terminal coupled to the voltage sensor input terminal (VIN); and a second voltage input terminal, and is configured to connect one of the first voltage input terminal and the second voltage input terminal to a voltage output terminal based on a second selection signal; - A current analog-to-digital converter (IADC) coupled to the first current output terminal; - A voltage analog-to-digital converter (VADC) coupled to the voltage output terminal; - A digital processing block (DIG) coupled to the respective outputs of the current analog-to-digital converter (IADC) and the voltage analog-to-digital converter (VADC); and - A current-to-voltage converter (TIA) coupled between the second current output and the second voltage input.

2. The signal processing circuit according to claim 1, wherein, The first selection unit (SEL1) is configured to simultaneously provide corresponding current from the first selected input terminal to the first current output terminal and from the second selected input terminal to the second current output terminal.

3. The signal processing circuit according to claim 1, wherein, At least one of the current analog-to-digital converter (IADC) and the voltage analog-to-digital converter (VADC) includes a Δ-Σ modulator.

4. The signal processing circuit according to claim 1, wherein, The current-to-voltage converter (TIA) includes a transimpedance amplifier.

5. The signal processing circuit according to claim 1 further includes at least one offset current source (OFF1, OFF2) connected to the first current output terminal or the second current output terminal.

6. The signal processing circuit according to claim 1, wherein, The digital processing block (DIG) includes a buffer element (FIFO) for buffering digital values ​​provided by the current analog-to-digital converter (IADC) and the voltage analog-to-digital converter (VADC).

7. The signal processing circuit according to claim 1 further includes at least one additional current sensor input terminal (PD3, ..., PD6) for receiving at least one additional sensor current, wherein, The first selection unit (SEL1) further includes at least one additional current input terminal coupled to the at least one additional current sensor input terminal (PD3, ..., PD6), and is configured to: select one of the first current input terminal, the second current input terminal, and the at least one additional current input terminal as the first selected input terminal based on the first selection signal, and select one of the first current input terminal, the second current input terminal, and the at least one additional current input terminal as the second selected input terminal.

8. The signal processing circuit according to claim 1 or 7, wherein, The first selected input terminal and the second selected input terminal are different from each other.

9. A signal processing apparatus, comprising: - The signal processing circuit according to any one of claims 1 to 8; - A first photosensitive element, which is connected to the input terminal (PD1) of the first current sensor. - A second photosensitive element, which is connected to the input terminal (PD2) of the second current sensor; and - A sensor element (VS) connected to the voltage sensor input (VIN) for providing at least one sensor voltage.

10. The signal processing apparatus according to claim 9, wherein, The sensor element (VS) includes at least two electrodes for electrocardiogram (ECG) measurement, and the signal processing device further includes a processing block (VPROC) connected between the voltage sensor input (VIN) and the first voltage input.

11. The signal processing apparatus according to claim 9, wherein, The first photosensitive element and the second photosensitive element are suitable for at least one of the following configurations: - The first photosensitive element and the second photosensitive element are configured for photoplethysmography (PPG); - The first photosensitive element is configured for PPG, and the second photosensitive element is configured for detecting oxygen saturation; - The first photosensitive element is configured for PPG, and the second photosensitive element is configured for ambient light detection.

12. The signal processing apparatus according to claim 11, wherein, The oxygen saturation is the peripheral oxygen saturation SpO2.

13. The signal processing apparatus according to claim 9, wherein, The signal processing device is implemented as a wearable device.

14. The signal processing apparatus according to claim 13, wherein, The wearable device is a smartwatch, wristband, or glasses.

15. The signal processing apparatus according to claim 9, wherein, The signal processing device is implemented as a sensor patch.

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