Light to Frequency Modulator

Abstract

A method of measuring light intensity, comprising exposing a photodiode to light so that the photodiode provides a current of a first polarity, providing the current to an integrator to integrate the current to provide an integrated output voltage, and comparing the output voltage to a threshold voltage. A charge packet of polarity opposite to the first polarity is applied to reset the integrated voltage before an integration time begins. At the end of the integration time, the photodiode is disconnected from the integrator and a reference voltage is coupled to the integrator input while a resistor is coupled into the circuit until a comparison signal switches. The comparison signal is monitored to measure the time between the end of the integration time and the switching of the comparison signal to provide a measure of the residual voltage.

Description

Technical Field

[0001] The present disclosure relates to light-to-frequency modulators. Background Art

[0002] The optical sensor device typically includes a photodiode as a light detector and measures the photocurrent flowing through the photodiode. The optical sensor device can convert the photocurrent into a digital signal. For example, the optical sensor device can be implemented as a light to frequency circuit, also known as a light to frequency machine or engine, abbreviated as an LTF engine. The LTF engine can generate a digital signal by integrating the photocurrent and comparing the integrated signal with a threshold using a comparator. The output of the device is a square wave or pulse stream whose frequency is linearly proportional to the light intensity. In a typical configuration, the processor 12 counts the number of pulses generated by the comparator over a fixed time period called the "integration time". The pulse count during the integration time is proportional to the light intensity. The accuracy of the detected digital signal depends on the reference value used during the integration and comparison.

[0003] In such Figure 1 In the typical LTF engine shown, when the photocurrent is integrated, each time the integrated signal crosses a threshold, a well-defined charge packet of opposite polarity compared to the photocurrent is provided to the input of the integrator for setting the integration process to a defined state. This process is repeated during a predefined integration time, and each crossing of the threshold is counted during the integration phase. The accuracy of the count is limited by the size of the well-defined charge packet. In particular, there will be a period of time between the final pulse within a given integration time and the end of that integration time, i.e., during which there is not enough time to generate another pulse.

[0004] WO2019011543 describes an improved concept that uses a second (not necessarily well-defined) charge packet to determine the residual count of such smaller charge packets between the charge state at the end of the integration phase and the corresponding threshold. In other words, small charge packets are incrementally added to the integrated charge at the end of the integration time to "count to" the switching threshold. By combining the primary count obtained at the end of the integration time with the residual count, an improved intensity measurement can be obtained. The method of WO2019011543 is Figure 2 , where the top trace shows the output voltage of the integrator and the lower trace shows the output of the comparator. Summary of the invention

[0005] According to a first aspect of the present invention, there is provided an optical sensor system for measuring light intensity, the optical sensor system comprising: a photodiode having a photodiode output for providing a current of a first polarity; an integrator having an integrator input selectively coupled to the photodiode output for receiving the current and an integrator output for providing an integrated output voltage; and a comparator having a comparator input coupled to the integrator output and configured to compare the integrated output voltage with a threshold voltage to provide a comparison signal at the comparator output. The system also comprises: a reference charge circuit coupled to the comparator output and the integrator for providing a charge packet to the integrator input, the charge packet having a polarity opposite to the first polarity; a reference voltage circuit having a reference voltage output for selectively providing at least a first defined voltage to the integrator input; and a resistor selectively coupled between the integrator input and a reference voltage node to provide a current path between the integrator input and the reference voltage node.

[0006] The system is provided with a control unit, which is used to:

[0007] (a) operating the integration unit within an integration time, during which the photodiode output is coupled to the integrator input and the reference voltage circuit does not provide the first defined voltage to the integrator input and the resistor does not provide the current path,

[0008] (b) at the end of the integration time, disconnecting the photodiode output from the integrator input and causing the reference voltage circuit to provide the first defined voltage to the integrator input while the resistor provides the current path until the comparison signal switches,

[0009] The system allows the monitoring unit to monitor the period between the end of the integration time and the switching of the comparison signal, which is indicative of the residual voltage on the integrator output at the end of the integration time.

[0010] The reference voltage circuit comprises a digital-to-analog converter and a digital controller for causing the converter to provide the first defined voltage to the integrator input.

[0011] The digital controller may cause the converter to provide an offset voltage different from the first predefined voltage to the integrator input during the integration time. The integrator comprises an operational amplifier circuit, and the digital-to-analog converter is integrated into the operational amplifier circuit.

[0012] The system may include a circuit or processor coupled to the output of the comparator for measuring a period of time between the end of the integration time and the switching of the comparison signal.

[0013] The control unit may be configured to apply the charge packet to the integrator input when the photodiode is disconnected from the integrator input, to couple the resistor between the integrator input and a reference voltage node while the photodiode remains disconnected, to cause the reference voltage circuit to provide the first defined voltage to the integrator input, and to determine a time to switch the output of the comparator. The determined time provides a calibration time that can be used to determine a residual voltage using the period between the end of the integration time and the switching of the comparison signal.

[0014] According to a second aspect of the present invention, a method for measuring light intensity is provided. The method comprises the following steps: exposing a photodiode to light so that the photodiode provides a current of a first polarity, providing the current to an input of an integrator and integrating the current over an integration time using an integrator to provide an integrated output voltage, and comparing the integrated output voltage with a threshold voltage to provide a comparison signal. The method comprises applying a charge packet of opposite polarity to the first polarity to reset the integrated voltage before the integration time begins. At the end of the integration time, the photodiode is disconnected from the integrator and a reference voltage is coupled to the integrator input, while a resistor is coupled between the integrator input and the reference voltage node until the comparison signal switches. The comparison signal is monitored to measure the time between the end of the integration time and the switching of the comparison signal, thereby providing a measurement of the residual voltage at the end of the integration time. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A prior art light-to-frequency modulator circuit is schematically shown;

[0016] Figure 2 shows traces associated with the operation of a light-to-frequency modulator circuit that uses known techniques to determine a residual count at the end of an integration time;

[0017] Figure 3 An improved light-to-frequency modulator circuit is schematically shown;

[0018] Figure 4 Shown with Figure 3 traces associated with the operation of the circuit; and

[0019] Figure 5 is a flow chart illustrating a method of operating a light-to-frequency modulator circuit to determine a residual voltage. DETAILED DESCRIPTION

[0020] As referenced above Figure 1As explained, a mechanism is needed to obtain a residual voltage measurement of the light-to-frequency modulator circuit at the end of each integration time. While mechanisms for this purpose are known, it is desirable to provide a mechanism that reuses as many existing circuit components and functions as possible.

[0021] Figure 3 A light to frequency modulator circuit is schematically shown comprising a photodiode 1, which is configured to provide a current of a first polarity when exposed to light. A switch S1 is operable to connect the photodiode 1 to a negative input of an amplifier 2. A capacitor 3 is coupled between the output of the amplifier 2 and the negative input, so that the amplifier acts as an integrator. Another capacitor 4 is coupled between the negative input of the amplifier 2 and a reference voltage Vref via switches S3, S4. The operation of these switches is further explained below. Another switch S2 is configured to connect the negative input of the amplifier 2 to a reference voltage, such as ground potential, via a resistor 11.

[0022] The output of amplifier 2 is coupled to the negative input of comparator 5. The positive input of the comparator is coupled to another reference voltage (which may be the same as or different from the above Vref). The output of comparator 5 provides a signal ltf_clk, which is used to control switches S3 and S4.

[0023] The amplifier 2 comprises an auto-zero digital to analog converter (AZ DAC) 6. This is a known amplifier function and is commonly used to correct offset in amplifiers. Figure 3 A digital controller 7 is shown comprising functionality 8 for determining an offset to be applied. This offset value is signaled via switch S5 to the AZ DAC 6. The digital controller 7 also comprises functionality 9 for applying a reference voltage to the AZ DAC, also via switch S5.

[0024] During normal modulator operation, the digital controller 7 selects the appropriate offset voltage and applies it to the AZ DAC 6 of the amplifier 2 via switch S5. At the beginning of each integration period, switch S1 is closed and the photodiode current is integrated into the modulator capacitor 3 for the duration of the integration time. When the comparator reference voltage vref is reached, a predefined charge is applied to the modulator input, causing a voltage step drop vstep. This is achieved via the control circuit 10, which opens and closes switches S3 and S4 to move charge to and from the capacitor 4. The result is a pulse train in the output signal ltf_clk of the comparator 5. Figure 4 The outputs of amplifier 2 (modulator output) and comparator 5 during the integration time are shown in FIG.

[0025] At the end of the integration time, the number of pulses detected in the output of the comparator is a measure of the intensity of light to which the comparator has been exposed. However, at this point, the output voltage of the amplifier will likely be at some voltage between zero and Vref (the switching voltage of the comparator). In order to determine the light intensity more accurately, it is desirable to determine this residual voltage. Therefore, at the end of the integration time, switch S1 is opened to prevent the photodiode from providing further current to the amplifier. Substantially simultaneously, switch S2 is closed to connect the negative input of amplifier 2 to the reference voltage via resistor 11. Similarly, and again substantially simultaneously, the digital controller 7 operates switch S5 to cause the AZ DAC to generate an "artificial" voltage and apply it to the negative input of the amplifier. The artificial voltage is a fixed voltage. Switch S2 is also closed, causing current to flow through the resistor into the negative input of the amplifier.

[0026] The current now flowing into the negative input of amplifier 2 is integrated by the modulator. Figure 4 The states of switches S1 and S2 are shown, both changing state at the end of the integration time, and the step change in voltage provided by the AZ DAC. It can be seen that the trace (bottom) showing the amplifier output increases linearly at a first rate just before the end of the integration time due to the current provided by the photodiode. At the end of the integration time, the rate of increase changes to a second rate that is less than the first rate due to the photodiode being disconnected and the AZ DAC applying an artificial voltage. The time t required for the output of the amplifier to reach vref can be measured by monitoring the comparator output ltf-clk.

[0027] Before the first operation of the circuit, a calibration step is performed to determine the time tcal taken by the artificial voltage to charge the integrator from zero to vref. The residual voltage at the end of a given integration period is therefore: vstep*(1-t / tcal).

[0028] exist Figure 5 The method of operation of the circuit is further illustrated in the flow chart of FIG.

[0029] The embodiments presented here have the following advantages over known methods and circuits:

[0030] - Improved accuracy at low signal counts;

[0031] - No need to use very high gain factors with attendant disadvantages (e.g. noise)

[0032] -Possibility to use reduced integration time.

[0033] Those skilled in the art will appreciate that various modifications may be made to the above-described embodiments without departing from the scope of the present invention.

Claims

1. An optical sensor system for measuring light intensity, comprising: a photodiode having a photodiode output for providing a current of a first polarity; an integrator having an integrator input selectively coupled to the photodiode output for receiving the current and an integrator output for providing an integrated output voltage; a comparator having a comparator input coupled to the integrator output and configured to compare the integrated output voltage to a threshold voltage to provide a comparison signal at a comparator output; a reference charge circuit coupled to the comparator output and the integrator for providing a charge packet to the integrator input, the charge packet having a polarity opposite to the first polarity; a reference voltage circuit having a reference voltage output for selectively providing at least a first defined voltage to said integrator input; a resistor selectively coupled between the integrator input and a reference voltage node to provide a current path between the integrator input and the reference voltage node; A control unit, the control unit is used for: (a) operating the integrator within an integration time, during which the photodiode output is coupled to the integrator input and the reference voltage circuit does not provide the first defined voltage to the integrator input and the resistor does not provide the current path, (b) at the end of the integration time, disconnecting the photodiode output from the integrator input and causing the reference voltage circuit to provide the first defined voltage to the integrator input while the resistor provides the current path until the comparison signal switches, Wherein the period between the end of the integration time and the switching of the comparison signal is indicative of the residual voltage on the integrator output at the end of the integration time.

2. The system of claim 1, wherein the reference voltage circuit comprises a digital-to-analog converter and a digital controller, the digital controller being configured to cause the converter to provide the first defined voltage to the integrator input.

3. The system of claim 2, wherein the digital controller causes the converter to provide an offset voltage to the integrator input during the integration time that is different from the first defined voltage. 4 . The system of claim 3 , wherein the integrator comprises an operational amplifier circuit and the digital-to-analog converter is integrated into the operational amplifier circuit.

5. A system according to any one of claims 1 to 4, comprising a circuit or processor coupled to the output of the comparator for measuring the time period between the end of the integration time and the switching of the comparison signal.

6. The system according to any one of claims 1 to 4, wherein the control unit is configured to: applying a charge packet to the integrator input when the photodiode is disconnected from the integrator input; coupling the resistor between the integrator input and a reference voltage node while the photodiode remains disconnected; causing the reference voltage circuit to provide the first defined voltage to the integrator input; as well as determining a time to switch the output of the comparator, Thereby, the determined time provides a calibration time which can be used to determine a residual voltage using the period between the end of the integration time and the switching of the comparison signal.

7. A method for measuring light intensity, comprising: exposing a photodiode to light so that the photodiode provides a current of a first polarity; providing the current to an input of an integrator and integrating the current over an integration time using the integrator to provide an integrated output voltage; comparing the integrated output voltage with a threshold voltage to provide a comparison signal; applying a charge packet of opposite polarity to the first polarity to reset the integrated output voltage before the integration time begins; at the end of the integration time, disconnecting the photodiode from the integrator and coupling a reference voltage to the integrator input while coupling a resistor between the integrator input and a reference voltage node until the comparison signal switches, The comparison signal is monitored to measure the time between the end of the integration time and the switching of the comparison signal to provide a measure of the residual voltage at the end of the integration time.

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