Patent Publication Number: US-10760963-B2

Title: Method for electromagnetic energy sensing and a circuit arrangement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of priority under 35 U.S.C. § 119 from U.S. Provisional Patent Application No. 62/534,034 filed on Jul. 18, 2017, and claims priority to European Patent Application 17184308.9 filed on Aug. 1, 2017, both disclosures of which are hereby incorporated by reference in their entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     The invention relates to a method for electromagnetic energy sensing, e.g. light sensing, and a circuit arrangement therefor. 
     BACKGROUND 
     Electromagnetic energy sensing may include electromagnetic energy to frequency conversion. This type of electromagnetic energy sensing may be used for light sensing. Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum; light includes ultraviolet light, visible light and infrared light. 
     A light-to-frequency circuit arrangement, which may also be called light-to-frequency engine, used in a light sensor may have two main components. There is an analog circuit comprising a photo diode and an integrating feedback loop that converts light into a series of pulses. This set of pulses is counted over a fixed period of time by a digital circuit. The number of pulses generated within the fixed period indicates the light intensity. Light-to-frequency converters may be used in applications such as ambient light measurement, light absorption/reflection in products such as white goods, photographic equipment, colorimetry, chemical analyzers and display contrast controls. 
     Improving the effective resolution of light-to-frequency converters for low light levels for the above-mentioned architecture may be achieved by increasing analog gain and/or increasing the diode area. These approaches may also be used for other architectures. Increasing the low light sensitivity may be achieved by increasing the analog front end gain. This has the side effect of saturation at a lower light level. 
     It is an aim of the invention to improve the effective resolution of a light-to-frequency converter for low light levels in an alternative way. This is especially important in spectroscopy applications that use a relatively low intensity narrow band source. In these applications, a small spectral response is expected in a different narrow band. 
     SUMMARY 
     The method for electromagnetic energy sensing, wherein light sensing may be one form of electromagnetic energy sensing, according to independent claim  1  comprises detecting electromagnetic energy, generating a sequence of events wherein an event interval of the sequence depends on the detected electromagnetic energy, measuring a time period until a given number of events has been generated and providing the time period as an output value. 
     The interval is the time space between two successive events. The way of counting according to the above-mentioned method differs from conventional approaches; the method allows higher accuracy for low light conditions. The events are counted until a given number of events has been generated and the time period until this given number has been generated is measured, which differs from the conventional counting approach wherein the number of events during a given time period is determined. The inventive method allows a finer resolution since the time period ends immediately after the last one of the given number of events has been generated. Contrary to the inventive approach, the conventional approach has an imminent uncertainty since the number of events within a given interval is associated with larger quantization than determining and measuring a time period per se. 
     The method uses a non-fixed time integrating feedback loop, which allows improvement of the dynamic range of a light-to-frequency converter. Light-to-frequency converters may be used in applications such as ambient light measurement, light absorption/reflection in products such as white goods, photographic equipment, colorimetry, chemical analyzers and display contrast controls. 
     Detecting electromagnetic energy may comprise converting the electromagnetic energy into a current, which may be achieved by means of a photodiode. The current depends on the electromagnetic energy. 
     Generating the sequence of events may comprise integrating the current by charging an integrating capacitor of an integrator amplifier. If the integrated current reaches or exceeds a given threshold, one event will be generated and the capacitor will be discharged. The events, which may be signal pulses, can be generated by means of a flip-flop that samples the output values of an analog front-end of the circuit arrangement; the analog front-end includes the integrator amplifier. 
     Measuring the time period usually means measuring clock cycles. It may comprise incrementing a time value in dependence of a clock signal, which may be achieved by a clocked incrementing counter serving as a timer. The time value may be a counting up integer value. In other words, the time value is counted up with every cycle of the clock signal, whereby the time value is suitable for indicating a time interval. 
     While incrementing the time value, the events are counted and the count value is compared with the given number. When the count value has reached or exceeded the given number, the time value is evaluated and provided as an output value, which may be achieved by triggering an enabling input of the incrementing counter. 
     A circuit arrangement operating according to the above-mentioned method comprises a photo detector for detecting electromagnetic energy and a signal generating means being suitable for generating a sequence of events wherein an event interval of the sequence depends on the detected electromagnetic energy. The signal generating means is coupled downstream of the photo detector. A counting means for measuring a time period until a given number of events has been generated is coupled downstream of the signal generating means. It has an output for providing the measured time period as an output value. 
     The counting means is a digital circuit. This approach is a digital solution rather than an analog one, which really distinguishes it from the conventional approaches for improving a light-to-frequency converter. The circuit arrangement increases the effective resolution without increasing the die area, which the conventional analog approaches do. 
     This invention has the advantage that it does not decrease the maximum light the system can digitalize before the saturating effect would occur caused by analog changes. 
     In one embodiment, the photo detector comprises a photodiode, which converts electromagnetic energy, e.g. light, into a current. The signal generating means may comprise an integrator amplifier, a comparator having a first input coupled to the integrator amplifier and a second input for applying a reference voltage. The signal generating means further comprises a signal event generator coupled downstream of an output of the comparator; the signal event generator is suitable for generating an event when triggered by the comparator. The integral amplifier comprises an operational amplifier and an integrating capacitor coupled between an input and an output of the operational amplifier. The integrating capacitor is charged by the current of the photodiode. The comparator may comprise a differential amplifier which detects whether a voltage provided at an output of the operational amplifier and dependent on the integrated current has already reached or exceeded a reference voltage. In this case, the comparator triggers the signal event generator. The triggering signal may be a signal transfer from one voltage level to another voltage level. The signal event generator may be embodied as a flip-flop where an input signal propagates through the flip-flop circuit when triggered by a clock signal. 
     Resetting means for resetting the integrator amplifier may be coupled to the signal event generator in such a way that the integrator amplifier is reset when one event is generated. Resetting causes discharging of the integrating capacitor; thereby the output of the comparator goes back to its original voltage level again, which causes the signal event generator to provide a pulse-shaped event. In other words, one event is a signal transfer between two logical levels forming a signal pulse. The signal goes up when reaching or exceeding the reference voltage and then goes back to its initial logical level or vice versa. 
     In one embodiment the counting means comprises an event counter having an input coupled to the signal event generator and a further comparator having a first input coupled to an output of the event counter and a second input coupled to a register suitable for providing the given number of events. The event generator counts the number of events that have been generated by the signal event generator. The further comparator detects whether the counted pulses have already reached or exceeded the given number stored in the register. If so, the comparator provides a signal which may be a logical signal transfer, e.g. from LOW to HIGH or vice versa. 
     The counting means preferably comprises an incrementing counter having an enable input coupled to an output of the further comparator; the latter is able to generate a trigger signal when the counted pulses have reached or exceeded the given number. The incrementing counter is suitable for counting time pulses of a clock signal applied to a clock input and evaluate and provide the amount of counted time pulses when triggered at the enable input. The incrementing counter serves as a timer that counts up with every cycle of the clock signal. Once triggered at the enable input, the evaluated value indicates the time period until the given number of events has been generated. 
     The event counter, the incrementing counter and the further comparator are embodied as digital electronic circuits. A clock generator connected to the clock input of the incrementing counter as well as to the event counter, the further comparator and the flip-flop synchronises operation of the digital circuit components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting, exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  shows a block diagram of an embodiment of a conventional circuit arrangement for light sensing; 
         FIG. 2  shows a block diagram of an embodiment of a circuit arrangement for electromagnetic energy sensing, e.g. light sensing, according to the invention; 
         FIG. 3  illustrates a method for electromagnetic energy sensing. 
     
    
    
     REFERENCE NUMERALS 
     
         
           2  photo detector 
           4  capacitor 
           6  operational amplifier 
           8 ,  14  voltage source 
           10  switch 
           12  differential amplifier 
           16  flip-flop 
           18 ,  24 ,  30 ,  36  counter 
           20 ,  32  register 
           22 ,  34  comparator 
           25 ,  37 , Q output 
           26  clock generator 
           51 ,  53 ,  55 ,  61 ,  63 , 
           65 ,  67  step 
         CLK, D INC, EN input 
         FB, REF voltage 
       
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of an embodiment of a conventional circuit arrangement for light sensing. 
     The circuit arrangement comprises a photo detector  2  that may be embodied as a photodiode. The photo detector  2  is a sensor of light or other electromagnetic energy which provides a current in dependence of the light intensity or the electromagnetic energy. The photo detector  2  may be considered as a light-to-current converter. The circuit arrangement further comprises an operational amplifier  6  and an integrating capacitor  4 ; the latter is coupled between an input and an output of the operational amplifier  6 . Another input of the operational amplifier  6  is connected to ground GND. The photo detector  2  is connected to the same input as the integrating capacitor  4 . The operational amplifier  6  and the integrating capacitor  4  form an integrator amplifier, where the integrating capacitor  4  may be charged by the current of the photo detector  2 . 
     A feedback mechanism comprises a voltage source  8  and a switch  10  coupled in series between ground GND and the input of the operational amplifier  6  to which the integrating capacitor  4  is connected. The feedback mechanism allows discharging the integrating capacitor  4  if the switch  10  is in a conducting state; thereby a given feedback voltage VB provided by a voltage source  8  is applied to the integrating capacitor  4 . If the switch  10  is in a non-conducting state, the integrating capacitor  4  may be charged by the current of the photo detector  2 . 
     A comparator  12  is connected downstream of the integrator amplifier  4 ,  6 . The comparator  12 , which may be embodied as a differential amplifier, has a first input coupled to the output of the operational amplifier  6  and a second input for applying a reference voltage REF which is provided by a reference voltage source  14 . The voltage at the first input depends on the voltage across the integrating capacitor  4 . When the voltage reaches or exceeds a given threshold value, which is the reference voltage REF, there is a signal transition at the comparator&#39;s output. This may be a signal transition from a low voltage level to a high voltage level. 
     A flip-flop  16 , preferably a D flip-flop, is connected downstream of the comparator  12 . The flip-flop  16  serves as a signal generating means and allows clock-controlled direct transfer of data from an input D to an output Q of the flip-flop  16 . The output Q provides either a first state LOW, which may indicate a low voltage level, or a second state HIGH, which may indicate a high voltage level. A clock generator  26  applies a clock signal to the flip-flop  16 . The clock frequency may be 730 kHz, for example. 
     Once the signal at the comparator&#39;s output has changed, the state at the output Q of the flip-flop  16  also changes. Since the output Q is connected to the switch  10 , the latter also switches, thereby discharging the capacitor  4 . Then the voltage at the output of the operator amplifier  6  changes as well as the voltage at the output of the comparator  12 , which causes again a state transition at the output Q of the flip-flop  16 . Thus a pulse at the flip-flop&#39;s output Q is generated when a certain amount of light has been detected. During light detection a sequence of pulses is generated; the frequency of the sequence depends on the intensity of the detected light. The higher the intensity, the higher is the frequency. In other words, the lower the intensity, the larger is the interval between the pulses. 
     The number of pulses within a given time interval is detected by the following digital circuit arrangement comprising a first counter  18 , a second counter  24 , a further comparator  22  and an integer time register  20 . The first counter  18  and the integer time register  20  are connected to inputs of the further comparator  22 . The first counter  18  is an incrementing counter that counts up with every cycle of the clock signal applied to a clock input CLK of the first counter  18 . 
     The output of the further comparator  22  is connected to an enable input EN of the second counter  24  serving as an event counter. The output Q of the flip-flop  16  is connected to an increment input INC of the second counter  24 . The clock signal is applied to a clock input CLK of the second counter  24 . The second counter  24  counts up when triggered at the increment input INC and stops and evaluates the count value when triggered at the enable input EN. 
     The first counter  18  counts up in dependence of the clock signal during a given time interval; thus it serves as a timer. The further comparator  22  compares whether the first counter  18  has reached a given time value provided by the integer time register  20 . If this happens, the further comparator  22  triggers the enable input EN of the second counter  24 . While the first counter  18  counts up, the second counter  24  counts the number of pulses generated by the flip-flop  16 . When the second counter  24  is triggered at the enable input EN at the end of the given time interval, the number of pulses counted so far, which is provided at the output  25  of the second counter  24 , indicates the intensity of the detected light. The higher the light intensity, the higher is the count. 
       FIG. 2  shows a block diagram of an embodiment of a circuit arrangement for electromagnetic energy or light sensing according to the invention. 
     Pulse generation by means of the flip-flop  16  and its upstream components forming the analog front-end is performed in the same manner as described in connection with  FIG. 1 . To avoid duplication of information, coupling and functions of the photo detector  2 , which may detect electromagnetic energy, of the integrator amplifier  4 ,  6 , the comparator  12 , the feedback mechanism  8 ,  10  and the flip-flop  16  will not be described again. 
     However, the digital circuit is different. The invention changes the digital circuit to count the amount of time needed to produce a fixed number of pulses. 
     The digital circuit arrangement serving as a counting means comprises a first counter  30  that serves as an event counter, a terminal count register  32 , a further comparator  34  and a second counter  36  which is an incrementing counter serving for simple time keeping. 
     The amount of circuitry added by this additional digital configuration is minimal because there already is a counter for the count number in the conventional approach. A comparator  34  needs to be added. A register  32  also needs to either be added or repurposed for the terminal count. 
     An increment input INC of the first counter  30  is connected downstream of the output Q of the flip-flop  16 . An output of the first counter  30  and the terminal count register  32  are connected to inputs of the further comparator  34 . An output of the further comparator  34  is connected to an enable input EN of the second counter  36 . The clock signal provided by the clock generator  26 , which has already been described in connection with  FIG. 1 , is applied to clock inputs CLK of the first and second counters  30 ,  36 . 
     The second counter  36  counts up in dependence of the clock signal; thereby it serves as a time keeper measuring the time until the enable input is triggered. 
     Once a triggering event, i.e. a pulse, is applied to the increment input INC of the first counter  30 , it counts up; thereby the pulses generated by the flip-flip  16  are counted. The further comparator  34  compares whether the first counter  30  has reached a given number of pulses that is provided by the terminal count register  32 . If this happens, the further comparator  34  triggers the enable input EN of the second counter  36 . Its current value is provided as an output value at the output  37 . The output value indicates the time period until the given number of events has been generated. The higher the intensity, the lower the output value. 
     The circuit arrangement in  FIG. 2  is suitable for performing the following method for electromagnetic energy sensing as illustrated in  FIG. 3 . The method comprises the following steps: Detecting electromagnetic energy by means of the photodetector  2 , which is illustrated by step  51 , generating a sequence of events, which is illustrated by step  53 , measuring a time period until a given number of events has been generated and providing the time period as an output value, which is illustrated by step  55 . 
     Detecting electromagnetic energy comprises converting the electromagnetic energy into a current. Generating the sequence of events comprises integrating the current by charging the capacitor  4  of the integrator amplifier  4 ,  6  and generating one event and discharging the capacitor  4  if the integrated current reaches or exceeds a given threshold. When generating a sequence of events by means of the integrator amplifier  4 ,  6 , the comparator  12  and the signal event generator  16 , an event interval of the sequence depends on the detected electromagnetic energy. 
     Measuring the time period comprises the following steps. 
     A time value is incremented in dependence of the clock signal, as illustrated in step  61 , which is performed by the up-counting second counter  36 . While incrementing the time value, the events are counted and the count value is compared with the given number, as illustrated in step  63 , which is performed by the first counter  30 , the register  32  and the further comparator  34 . The time value is evaluated when the count value has reached or exceeded the given number, as illustrated in step  65 , which is performed by the second counter  36  when triggered by the comparator  34 . The evaluated time value is provided as an output value at the second counter&#39;s output  37 , as illustrated in step  67 . 
     The following calculations show the benefit of the above-mentioned way of electromagnetic energy or light sensing. 
     Considering the conventional measurement, for an exemplary programmed integration time of 179.3 μS the integration counter will count for (2{circumflex over ( )}16−1) or 65536 clock cycles. An average of 648.87 to 655.36 clocks per feedback event will produce a pulse count of 100. An average of less than 648.87 would produce a final pulse count of 101 and an average of greater than 655.36 would produce a final pulse count of 99. The point at which the count reaches 100 could be 64887 and 65536 or in between, given a constant diode current. The difference in average diode current to produce a final count of 100 being reached at 64887 and 65536 clocks is (65536−64887)/65536 or 1.0%. 
     Coming to the inventive measurement, if we know the number of clock cycles at which 100 counts is achieved, we can realize an increase in resolution of 1/649 (65536−64887) since the exact time value of the integration counter is taken into account. It is important to note that the increased resolution is not linear. As the average current increases, the resolution gain decreases. 
     In the case of multiple channels additional logic would have to be added if the light had to be sampled by all sensors over the same period. Alternatively, light sampling for multiple channels could be done sequentially. 
     The scope of protection is not limited to the examples given herein above. The invention is embodied in each novel characteristic and each combination of characteristics, which particularly includes every combination of any features which are stated in the claims, even if this feature or this combination of features is not explicitly stated in the claims or in the examples.