Dark offset cancellation arrangement and method for cancelling dark offset

Abstract

The invention relates to a dark offset cancellation arrangement, comprising a photosensitive device configured to provide a photosensitive device current value obtained in a measurement cycle of the dark offset cancellation arrangement, and a capacitor configured to be a compensation photo-sensitive device and configured to provide a dark offset current value obtained in another measurement cycle of the dark offset cancellation arrangement, and an operational amplifier configured to convert the photosensitive device current value to a photosensitive device voltage value and to convert the dark offset current value to a dark offset voltage value, and a dark offset cancellation circuit configured to provide a dark offset cancellation voltage value using the photosensitive device voltage value and the dark offset voltage value.

Description

[0001] 2023PF00789 - 1 - DARK OFFSET CANCELLATION ARRANGEMENT AND METHOD FOR CANCELLING DARK OFFSET DESCRIPTION TECHNICAL FIELD The present disclosure relates to a dark offset cancellation arrangement and a corresponding method. BACKGROUND Photodetectors, e.g. ambient light sensors (ALS), are com- monly used in portable electronic devices, such as cellular phones, tablets, and laptops with touchpads etc. Those de- vices commonly include displays that are in varying levels illuminated to facilitate an interaction with their user. Yet, they consume quite a bit of energy, and it is desirable to prolong their battery life. For this, it is necessary to save energy while being used, i.e., when the display is illu- minated. However, it is also desirable to provide sufficient illumina- tion so that the user can work, read, or use the device com- fortably. This is, of course, an important feature so that a user can operate the device in varying ambient light levels in varying different light conditions, each level adapted to a certain light condition. On top, it may also be highly desirable to illuminate the display only when a user is using the display and not when a user is placing the device, e.g., at his or her ear. That is, when the user is detected in a certain proximity to the de- vice. Such a photodetector, such as an ambient light sensor, com- prises at least one photosensitive device, e.g., a photodiode 2023PF00789 - 2 - which might be formed by a doped region within a semiconduc- tor substrate. In use, when the energy of incident photons removes electrons from the outer orbits of atoms within the photosensitive por- tion, a charge is generated. Further, in order to prevent re- combination and loss of the signal, the photodiode makes use of an electric field at a P-N junction to cause the photo generated electron to move away from the ion. Yet it is the case that said P-N junctions, typically in com- bination with the surrounding circuitry and other parasitic effects, have a bit of a leakage current. This current is called either “dark current” or “dark offset”. This is so, as this leakage current is present only when there is no light. Dark offset is a limiting factor especially in the perfor- mance, i.e., result characteristics of photodetectors, in particular of photosensitive devices, such as photodiodes. In order to address this disadvantage, it is highly desirable to provide a dark offset cancellation, which cancels the dark offset in the voltage output value. For this, it is known to use a covered photodiode as a dark channel, as the leakage current in the photodiode is here the dominant portion of the dark offset. Measuring the offset in the dark channel and subtracting it from the measurement in the light channel can- cels in this configuration much of the offset in the light channel. The disadvantage of the usage of a covered photodiode as a dark channel is the area, and thus cost and manufacturing complexity required by the covered photodiode that has no use other than measuring the dark offset. SUMMARY 2023PF00789 - 3 - The object of the present invention is therefore to provide a dark offset cancellation arrangement, comprising a photosen- sitive device configured to provide a photosensitive device current value obtained in a measurement cycle of the dark offset cancellation arrangement, a capacitor configured to be a compensation photosensitive device and configured to pro- vide a dark offset current value obtained in another measure- ment cycle of the dark offset cancellation arrangement, and an operational amplifier configured to convert the photosen- sitive device current value to a photosensitive device volt- age value and to convert the dark offset current value to a dark offset voltage value, and a dark offset cancellation circuit configured to provide a dark offset cancellation voltage value using the photosensitive device voltage value and the dark offset voltage value. One key parameter in a photodetector is the dark offset, that is, the light reading produced by the sensor when there isn't actually any light. This invention provides a low-cost way of measuring, and thus, cancelling, this offset. In this invention, a dark measurement is done by using a sim- ple capacitor that mimics the capacitive load of the photo- sensitive device without requiring the area of a true dark photodiode. The disadvantage of the dark photodiode is the area (and thus cost) required by the photodiode that has no use other than measuring offset. In this invention, a capacitor is substi- tuted for the dark photodiode, which means using a much smaller area. The result is that the dark offset cancellation arrangement is much more cost effective, less complex in its manufacturing and smaller than previously known dark offset cancellation arrangements. In an embodiment of the invention, the photosensitive device of the dark offset cancellation arrangement may comprise an island photodiode (IPD). 2023PF00789 - 4 - A photodiode is a type of photodetector used in optoelectron- ics to convert light signals into electrical signals. In a traditional photodiode, the light is collected by a single PN junction that covers the entire light sensitive area of the photodiode. In an island photodiode (IPD), the PN junction is broken up into small pieces (or islands) surrounded by in- trinsic space, that is, space without PN junctions. These is- lands are connected in parallel. Carriers that are generated in the intrinsic space are gathered by the PN islands. The main advantage of the island photodiode over a traditional photodiode is the reduced capacitance and leakage due to the reduced PN junction area. This structure allows for more efficient detection and exami- nation of specific areas of the light spectrum. Island photo- diodes are often used in applications where precise spectral analysis or enhanced sensitivity to different wavelength ranges is needed, such as in spectroscopy, imaging, and opti- cal communication technologies. The segmentation enables more accurate analysis and adaptation to different light sources and application requirements. In the latest generations of photodiodes, such as the IPDs, the photodiode leakage current is very small. This means that the dominant portions of offset voltage are not related to the photodiode. Because of this, a simple capacitor can be chosen to mimic the load and leakage current of the IPD used in the dark offset cancellation arrangement. This allows the measurement of offset voltage using a much smaller (and thus less costly and complex in the manufacturing) device. By this, the whole arrangement and thus the sensor in which it is installed, will be cheaper and less complex to manufac- ture. Furthermore, the dark offset cancellation circuit of the dark offset cancellation arrangement may be configured to subtract 2023PF00789 - 5 - said dark offset voltage value from said photosensitive de- vice voltage value to obtain said dark offset cancellation voltage value. This is a very easy and cost-effective way of determining the dark offset and of cancelling it in the value of the output voltage. Furthermore, the dark offset cancellation circuit may be con- figured to store said dark offset cancellation voltage value and / or said photosensitive device voltage value and / or said dark offset voltage value. This has the advantage that for the further procurement the data can be retrieved when (once again) needed. In another embodiment of the invention, the dark offset can- cellation arrangement may be a part of a photodetector. Photodetectors are used, for example, in portable electronic devices, such as cellular phones and personal digital assis- tants. Those devices commonly include displays that are illu- minated to facilitate interaction with a user. Yet, they consume quite a bit of energy, and it is desirable to prolong their battery life. For this, it is necessary to save energy while being used, i.e., when the display is illu- minated. However, it is also desirable to provide sufficient illumina- tion so that the user can work, read, or use the device com- fortably. This is, of course, an important feature so that a user can operate the device in varying ambient light levels in varying different light conditions, each level adapted to a certain light condition. Furthermore, the photodetector may be an ambient light sen- sor. 2023PF00789 - 6 - Furthermore, a method for cancelling dark offset using the dark offset cancellation arrangement is disclosed, which com- prises the following steps: performing a light measurement cycle to obtain the photosensitive device voltage value, per- forming a dark offset measurement cycle in order to obtain the dark offset voltage value, and providing the dark offset cancellation voltage value by subtracting the dark offset voltage value from the photosensitive device voltage value. The method is related for a cancellation of the dark offset in the value obtained at the voltage output in order to re- ceive a voltage value without the dark offset. What has been said with respect to the device may analogously be applied to the method and therefore need not be repeated there. Device embodiments and details have a counterpart in the method and vice versa. In a further embodiment, the step of performing the light measurement cycle may be performed prior to the step of per- forming the dark offset measurement cycle. With this, when applying the measurement there is more free- dom in the performance of said method, which leaves one able to choose the more efficient order for the application case needed. BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in further detail with reference to the accompanying drawings, wherein: Fig. 1 depicts a photodetector with an embodiment of a dark offset cancellation arrangement. DETAILED DESCRIPTION 2023PF00789 - 7 - In Fig. 1 an embodiment of the invention is shown. More spe- cifically, the figure shows a dark offset cancellation ar- rangement 1 in a photodetector, which in this embodiment is an ambient light sensor, but could be any other suitable pho- todetector as well. In this arrangement, a photosensitive device is depicted as 4. In the arrangement of Fig. 1, the photosensitive device 4 is modelled as current produced by a photodiode 4a and an in- trinsic capacitance 4b. In this embodiment the photodiode 4a is an island photodiode (IPD), the leakage current of which is greatly reduced, leav- ing the offsets in the rest of the signal chain as the main contributors to the dark offset. A further photosensitive device is used in the invention as a compensation photosensitive device 6 or as “dark photodiode”. As the offsets of the photodiode 4a are in the rest of the signal chain as the main contributors to the dark offset, it is possible to use a simple capacitor 6a as a substitute for the load of the dark photodiode. Since the capacitance den- sity per unit area of a capacitor 6a is much higher than of an IPD, the area consumed by the compensation photosensitive device 6 or “dark capacitor” 6 is greatly reduced. Both, the photosensitive device 4 and the dark capacitor 6 have a grounding 4c and 6b. The dark offset cancellation arrangement 1 in an ambient light sensor further comprises an operational amplifier 8 and a capacitor 10, which acts as a feedback capacitor. The IPD 4a and the capacitor 6a provide respective inverting 8a and non-inverting 8b inputs to the operational amplifier 8. 2023PF00789 - 8 - The IPD 4a is, of course, light sensitive in a normal fashion while the dark capacitor 6a is not light sensitive. The out- put from the dark capacitor 6 therefore represents a dark offset. Each of the IPD 4a and the dark capacitor 6a is connected to a voltage source vcmo 12, which also acts as a reset voltage. A dark offset cancellation circuit 18 is provided at a volt- age output vout 20. Switches 22a, 22b, 22c, 22d, 22e, 22f and 22g are located in the dark offset cancellation arrangement 1 as depicted in Fig. 1. For the IPD 4a the current is Ipd: Ipd= Ipdpd+ Ipddcwhere Ipdis the photo current and Ipddcis the dark current / dark offset through IPD. For the dark capacitor 6a the cur- rent is only Icdc, i.e., the dark current / dark offset through the dark capacitor 6a. To determine and cancel the dark offset, a method for cancel- ling dark offset using the dark offset cancellation arrange- ment 1 in the ambient light sensor of this embodiment is used. In general, there are three steps to be performed, whereas the first two steps, the light measurement cycle and the dark offset measurement cycle, can be performed in a variable or- der. In this embodiment, the light measurement cycle is performed primarily. As a preparation, switches 22a and 22c, 22e, and 22f are closed to precharge a voltage on the capacitor 10 to vcmo 12 2023PF00789 - 9 - and the voltage must be held across the IPD 4a and the dark capacitor 6a to zero. The amplifier 8 is placed in a low gain state, and the switch 22e holds its input nodes to zero. In said light measurement cycle, only the switches 22b and 22g are closed. This causes the photodiode current Ipdto be integrated on to the capacitor 10, and the voltage VvoutPDat the voltage output 20 vout will be: where Vvcmois the voltage at vcmo 12, Tintis the integration time, Cpdis the capacitance of 4b and Voffsetis the voltage caused by a charge injection from the switches, non-ideali- ties in the integrating amplifier, and leakage current Ipddcin IPD 4a. The output voltage Vvoutis then sampled and converted to a digital value by any of several well-known analog-to-digital conversion techniques in the dark offset cancellation circuit 18. Then, a dark offset measurement cycle is performed. If needed, a preparation cycle is done once again, where switches 22a and 22c, 22e, and 22f are closed to precharge a voltage on the capacitor 10 to a voltage source vcmo 12 and the voltage must be held across the IPD 4a and the dark ca- pacitor 6a to zero. The amplifier 8 is placed in a low gain state, and switch 22e holds its input nodes to zero. In said dark offset measurement cycle, a second path with the dark capacitor 6a is used to mimic the load and leakage cur- rent of the IPD 4a. In the latest generations of photodiodes – as the IPD 4a used in this embodiment of the invention - the photodiode leakage current is very small. This means that the dominant portions of Voffsetare not related to the photodiode, here the IPD 4a. 2023PF00789 - 10 - Because of this, a dark capacitor 6a can be chosen to mimic the load and leakage current of the IPD 4a. This allows the measurement of Voffsetusing a much smaller (and thus less costly and complex in the manufacturing) device. In this second path, only the switches 22d and 22g are closed and the voltage VvoutDCat the voltage output vout 20 will be: VvoutDC= Vvcmo+ Voffsetwhere Voffsetis caused by a charge injection from the switches, non-idealities in the integrating amplifier, and leakage current Idcin the dark capacitor 6a. The output voltage Vvoutis then sampled and converted to a digital value by any of several well-known analog-to-digital conversion techniques in the dark offset cancellation circuit 18. As the next step in the method for cancelling dark offset us- ing the dark offset cancellation arrangement 1 in the ambient light sensor of this embodiment, the output voltage VvoutPDcancelwith dark offset cancellation is determined. This is done by subtracting the output voltage VvoutDCfrom the output voltage VvoutPD(see equation 1): VvoutPDcancel= VvoutPD– VvoutDC(1) VvoutPD– VvoutDC =(Vvcmo+ Ipd* Tint / Cpd+ Voffset) – (Vvcmo+ Voffset) (2) VvoutPDcancel= Ipd* Tint / Cpd(3) That is, if the leakage current of the dark capacitor 6a matches or is near to that of the IPD 4a, which is in this embodiment assumed, the value of VvoutDCwill be Vvcmo+ Voffset, and subtracting this value of VvoutDC, which is Vvcmo+ Ipd* Tint 2023PF00789 - 11 - / Cpd+ Voffset, from the light measurement cycle VvoutPD(see eu- ation 2), leaves just the desired value of VvoutPDcancelwhich is Ipd* Tint / Cpd(see equation 3).

[0002] 2023PF00789 - 12 - LIST OF REFERENCE SIGNS Dark offset cancellation arrangement 1 Photosensitive device 4 Photodiode / Island Photodiode (IPD) 4a Intrinsic capacitance 4b Grounding 4c Compensation photosensitive device / 6 Dark capacitor Grounding 6b Operational amplifier 8 Inverting input 8a Non-inverting inputs 8b Capacitor / Feedback capacitor 10 Voltage source 12 Capacitor 14 Grounding 16 Dark offset cancellation circuit 18 Voltage output 20 Switch 22a Switch 22b Switch 22c Switch 22d Switch 22e Switch 22f Switch 22g List of abbreviations: Photodiode PD Ambient Light Sensor ALS Island Photodiode IPD

Claims

2023PF00789 - 13 - CLAIMS 1. Dark offset cancellation arrangement, comprising: - a photosensitive device configured to provide a pho- tosensitive device current value obtained in a meas- urement cycle of the dark offset cancellation ar- rangement, - a capacitor configured to be a compensation photosen- sitive device and configured to provide a dark offset current value obtained in another measurement cycle of the dark offset cancellation arrangement, - an operational amplifier configured to convert the photosensitive device current value to a photosensi- tive device voltage value and to convert the dark offset current value to a dark offset voltage value, and - a dark offset cancellation circuit configured to pro- vide a dark offset cancellation voltage value using the photosensitive device voltage value and the dark offset voltage value.

2. Dark offset cancellation arrangement according to claim 1, wherein the photosensitive device comprises an island photodiode.

3. Dark offset cancellation arrangement according to any one of the preceding claims, wherein said dark offset cancel- lation circuit is further configured to subtract said dark offset voltage value from said photosensitive device voltage value to obtain said dark offset cancellation voltage value.2023PF00789 - 14 - 4. Dark offset cancellation arrangement according to any one of the preceding claims, wherein said dark offset cancel- lation circuit is configured to store said dark offset cancellation voltage value and / or said photosensitive device voltage value and / or said dark offset voltage value. 5 Dark offset cancellation arrangement according to any one of the preceding claims, wherein the dark offset cancel- lation arrangement is part of a photodetector.

6. Dark offset cancellation arrangement according to claim 5, wherein the photodetector is an ambient light sensor.

7. Method for cancelling dark offset using the dark offset cancellation arrangement of any one of the claims 1 to 6, comprising the following steps: - performing a light measurement cycle in order to ob- tain the photosensitive device voltage value, - performing a dark offset measurement cycle in order to obtain the dark offset voltage value, and - providing the dark offset cancellation voltage value by subtracting the dark offset voltage value from the photosensitive device voltage value.

8. Method for cancelling dark offset according to claim 7, wherein the step of performing the light measurement cy- cle is performed prior to the step of performing the dark offset measurement cycle.

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