ELECTRIC CIRCUIT ARRANGEMENT TO DETERMINE A LEVEL OF AN EXCESS BIAS VOLTAGE OF A SINGLE PHOTON AVALANCHE DIODE

Technical Field

The disclosure relates to an electric circuit arrangement to determine a level of an excess bias voltage of a single photon avalanche diode.

Background

A single photon avalanche diode (SPAD) is a highly sensitive optical device mainly used to detect a moment when a photon hits an optical sensor of the avalanche diode. When the SPAD is reverse biased with a bias voltage VHV higher than a breakdown voltage VBD, an electron hole pair is generated, when a photon hits the SPAD. Due to the high electrical field, the SPAD generates a very short high peak current pulse .

The breakdown voltage is an intrinsic parameter of the SPAD. The performance of the SPAD depends on how much the bias voltage VHV is higher than the breakdown voltage VBD. The bias voltage VHV of the SPAD can be expressed as VHV = VBD- Vex, where Vex is the excess bias voltage. Since the

breakdown voltage VBD is a function of temperature, the excess bias voltage Vex will also change in response to a temperature change, if it is assumed that the (reverse) bias voltage VHV is kept constant. On the other hand, if the excess bias voltage Vex changes, the physical parameters of the SPAD, for example the DCR (Dark Count Rate) or PDP (Photon Detection Probability) , of the SPAD will also change. Therefore, if the bias voltage VHV would not be adapted in response to temperature changes, the breakdown voltage VBD and thus the excess bias voltage Vex as well as the physical parameters of the SPAD would also change, resulting in a systematic timing jitter, when the SPAD is used in an application of distance measurement, for example a time-of-flight application.

There is desire to provide an electric circuit arrangement to determine a level of an excess bias voltage of a single photon avalanche diode to adjust the bias voltage of the single photon avalanche diode so that the physical parameters of the single photon avalanche diode are nearly unaffected by environmental influences, for example temperature changes.

Summary

An embodiment of an electric circuit arrangement to determine a level of an excess bias voltage of a single photon

avalanche diode within a short time and nevertheless with high precision is specified in claim 1.

According to an embodiment of the electric circuit

arrangement to determine a level of an excess bias voltage of a single photon avalanche diode, the electric circuit

arrangement comprises a supply terminal to apply a supply potential, a reference terminal to apply a reference

potential, and a bias terminal to apply a bias potential to bias the single photon avalanche diode, and an output

terminal to provide an output signal of the single photon avalanche diode. The single photon avalanche diode is

connected between the bias terminal and the output terminal.

The electric circuit arrangement comprises a controllable switching circuit and an evaluation circuit to evaluate the output signal. The controllable switching circuit is

configured to couple the output terminal to the reference terminal in a first operational cycle of the circuit

arrangement so that a voltage jump to the level of the excess bias voltage occurs at the output terminal, when a photon hits a photosensitive area of the single photon avalanche diode. The controllable switching circuit is further

configured to couple the output terminal to the supply terminal in a subsequent second operational cycle. The evaluation circuit is configured to determine the level of the excess bias voltage in dependence on a signal course of the output signal.

The proposed solution enables the current level of the excess bias voltage to be found with a first detected photon event. For this purpose, the output terminal is coupled to the reference terminal to apply the reference potential, for example a ground potential, during the first operational cycle of the electric circuit arrangement. If a photon hits the photosensitive area of the single photon avalanche diode, the output signal at the output terminal shows a voltage jump/peak to the level of the excess bias voltage to be determined .

After the occurrence of the voltage jump/peak at the output terminal, the controllable switching circuit operates the electric circuit arrangement in the second operational cycle, in which the output terminal is coupled to the bias terminal to apply the bias potential so that the output terminal is charged. During the charging of the output terminal, the single photon avalanche diode stays quenched so that the SPAD does not conduct any current and no other photons can trigger the SPAD during the charging process. As a result, the output signal at the output terminal shows a linear increasing slope from the level of the excess bias voltage to be determined to the level of the supply potential.

The linear increasing voltage slope of the output signal after the occurrence of the voltage jump/peak at the output terminal allows to determine the level of the excess bias voltage in a simple but precise manner. In particular, the level of the excess bias voltage is determined in dependence from a first time between the occurrence of the voltage jump/peak to the level of the excess bias voltage and the crossing of a first threshold value of the output signal at the output terminal, and in dependence from a second time between the crossing of the first threshold value of the output signal and the crossing of a second threshold value of the output signal.

A first timespan between the occurrence of the voltage jump/peak to the level of the excess bias voltage at the output terminal and the crossing of the first threshold value of the output signal, and a second timespan between the voltage jump/peak to the level of the excess bias voltage and the crossing of the second threshold value of the output signal can be determined in the digital domain by using a time-to-digital converter or a digital counter. The use of a time-to-digital converter (TDC) or a digital counter as a component of the evaluation circuit enables to calculate the level of the excess bias voltage in the digital domain. Thus, a dedicated reference circuit for different excess bias voltages is not needed. Moreover, the proposed solution of the electric circuit arrangement to determine the level of the excess bias voltage is expected to be small in area and to have low power consumption. Furthermore, the time

necessary to find the appropriate level of the excess bias voltage is expected to be small, because only one event of an impact of a photon onto the photosensitive area of the SPAD is needed to find the current level of the excess bias voltage .

The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate several embodiments of the electric circuit arrangement to determine a level of the excess bias voltage of a single photon

avalanche diode, and together with the description serve to explain principles and the operation of the various

embodiments of the electric circuit arrangement.

Brief Description of the Drawings

Figure 1 shows a first embodiment of an electric circuit arrangement to determine a level of an excess bias voltage of a SPAD;

Figure 2 shows a voltage slope of an output signal of the SPAD being used to determine a level of an excess bias voltage of the SPAD; Figure 3 shows a possible implementation of the first

embodiment of the electric circuit arrangement to determine a level of the excess bias voltage of the SPAD;

Figure 4 illustrates a time diagram of signals at internal nodes of the implementation of the first embodiment of the electric circuit arrangement to determine a level of the excess bias voltage of the SPAD;

Figure 5 illustrates a second embodiment of an electric circuit arrangement to determine a level of an excess bias voltage of a SPAD;

Figure 6 shows a possible implementation of the second embodiment of the electric circuit arrangement to determine a level of an excess bias voltage of a SPAD;

Figure 7 illustrates a time diagram of signals at internal nodes of the implementation of the second embodiment of the electric circuit arrangement to determine a level of the excess bias voltage of the SPAD;

Figure 8 shows an embodiment of a sensor device including an electric circuit arrangement to determine a level of an excess bias voltage of a SPAD; and

Figure 9 shows an embodiment of a communication device comprising a sensor device including the electric circuit arrangement to determine a level of an excess bias voltage of a SPAD.

Detailed Description Figure 1 shows a first embodiment of an electric circuit arrangement la to determine a level of an excess bias voltage Vex of a single photon avalanche diode. A second embodiment of an electric circuit arrangement lb to determine a level of an excess bias voltage of a single photon avalanche diode is shown in Figure 5. In the following, the common elements of the first and second embodiment of the electric circuit arrangement la and lb to determine a level of an excess bias voltage of a single photon avalanche diode are described.

The electric circuit arrangement la, lb comprises a supply terminal 10 to apply a supply potential AVDD and a reference terminal 20 to apply a reference potential GND, for example a ground potential. The supply potential AVDD may be generated by a main battery of the electric circuit arrangement or may be any other voltage generated, for example, by a voltage doubler, a charge pump, a DC-DC converter, etc.. The electric circuit arrangement la, lb further comprises a bias terminal 30 to apply a bias potential VHV to bias a single photon avalanche diode 100. The electric circuit arrangement la, lb comprises an output terminal 40 to provide an output signal Van of the single photon avalanche diode 100. The single photon avalanche diode 100 is connected between the bias terminal 30 and the output terminal 40.

The electric circuit arrangement la, lb comprises a

controllable switching circuit 200. The controllable

switching circuit 200 is configured to electrically couple the output terminal 40 to the reference terminal 20 in a first operational cycle of the circuit arrangement la, lb so that a voltage jump to the level of the excess bias voltage Vex occurs at the output terminal 40, when a photon hits a photosensitive area of the single photon avalanche diode 100. The controllable switching circuit 200 is further configured to electrically couple the output terminal 40 to the supply terminal to apply the supply potential AVDD in a subsequent second operational cycle of the circuit arrangement la, lb.

The electric circuit arrangement la, lb further comprises an evaluation circuit 300 to evaluate the output signal Van. The evaluation circuit 300 is configured to determine the level of the excess bias voltage Vex in dependence on a signal course of the output signal Van. For this purpose, the evaluation circuit 300 monitors the signal course of the output signal Van at the output terminal 40.

The single photon avalanche diode 100 of the electric circuit arrangement la, la is quenched, i.e. operated in a non- conductive state, when the voltage jump/peak of the output signal Van to the level of the excess bias voltage Vex is generated at the output terminal 40. As a consequence, the level of the output signal Van rises with a linear increasing slope. This configuration advantageously enables that the evaluation circuit 300 has to evaluate a voltage course of the output signal with a linear gradient to determine the level of the excess bias voltage Vex. The level of the excess bias voltage can be determined by elementary mathematical functions as explained later so that the evaluation circuit can be implemented with reduced complexity.

According to the electric circuit arrangement la, lb, a control circuit 400 is provided to control the controllable switching circuit 200. The control circuit 400 is configured to control the controllable switching circuit 200 in

dependence on a value of the output signal Van at the output terminal 40. As an advantageous consequence, a reference circuit for evaluating different Vex bias voltages is not needed .

The controllable switching circuit 200 is configured to selectively electrically couple the output terminal 40 to the supply terminal 10 or the reference terminal 20 in dependence on the value of the output signal Van at the output terminal 40. This configuration of the controllable switching circuit 200 allows to provide a voltage jump/peak of the output signal Van at the output terminal 40, when a photon hits the photosensitive area of the single photon avalanche diode 100 for the first time and the controllable switching circuit 200 electrically couples the output terminal 40 to the reference potential at the reference terminal 20.

The control circuit 400 is configured to control the

controllable switching circuit 200 so that the output

terminal 40 is electrically coupled by the controllable switching circuit 200 to the supply terminal 10, when the output terminal 40 is previously electrically coupled to the reference terminal 20 and the control circuit 40 detects the occurrence of the voltage jump/peak of the output signal Van at the output terminal 40. This configuration of the control circuit 400 or the controllable switching circuit 200 enables that the voltage at the output terminal 40 continuously increases during the second operational cycle, when a photon has hit the photosensitive area of the single photon

avalanche diode 100 for the first time and the single photon avalanche diode is quenched. Thus, the output signal Van at the output terminal 40 shows a linear increasing voltage slope during the second operational cycle. According to the embodiment of the electric circuit arrangement la, lb, the controllable switching circuit 200 comprises a first current path 201 being arranged between the output terminal 40 and the reference terminal 20, and a second current path 202 being arranged between the output terminal 40 and the supply terminal 10. The first current path 201 comprises a first controllable switch 210 and a first current generator 220. The second current path 202 comprises a second controllable switch 230 and a second current generator 240.

According to the embodiment of the electric circuit

arrangement la and lb, the controllable switching circuit 200 comprises a quenching circuit which is realized by the first controllable switch 210 and the first current source 220.

This configuration of the controllable switching circuit 200 advantageously enables that a voltage jump/peak of the output signal Van occurs at the output terminal 40, when a photon hits the photosensitive area of the single photon avalanche diode 100 for the first time, and the output terminal 40 is connected to the reference terminal 20 via the first current path 201, i.e. via the first controllable switch 210 and the first current generator 220. Furthermore, the configuration of the controllable switching circuit 200 enables that the output signal Van rises at the output terminal 40 from the level of the excess bias voltage Vex to a value of the supply voltage AVDD with a linear increasing slope, when the output terminal 40 is electrically coupled to the supply terminal 10 via the second current path 202, i.e. via the second

controllable switch 230 and the second current generator 240. According to the embodiments of the electric circuit

arrangement la and lb, the first controllable switch 210 and the first current generator 220 are connected in series in the current path 201 between the output terminal 40 and the reference terminal 20. The controllable switch 230 and the current generator 240 are connected in series in the current path 202 between the output terminal 40 and the supply terminal 10.

The control circuit 400 is arranged in a feedback path between the output terminal 40 and the controllable switching circuit 200. In particular, the control circuit 400 generates a control signal to control the controllable switches 210 and 230 in an opposed way. If the controllable switch 210 is switched in a conductive state, the controllable switch 230 is operated in a non-conductive state and vice versa.

The first embodiment of the electric circuit arrangement la is described in the following with reference to Figures 1 to

4.

According to the embodiment of the electric circuit

arrangement la, the evaluation circuit 300 comprises a comparator circuit 310 having a first input terminal 1310a coupled to the output terminal 40, and a second input

terminal 1310b to apply a first threshold value VREFL. The comparator circuit 310 further comprises a first comparator output terminal 0310 to output a first comparison signal OUTL. The first comparator circuit 310 advantageously enables to detect the instant of time when the level of the output signal Van reaches the first threshold value VREFL. The evaluation circuit 300 further comprises a second

comparator circuit 320 having a first input terminal 1320a coupled to the output terminal 40, and a second input

terminal 1320b to apply a second threshold value VREFH. The second comparator circuit 320 further comprises a second comparator output terminal 0320 to output a second comparison signal OUTH. The second comparator circuit 320 advantageously enables to detect the instant of time when the output signal Van reaches the second threshold value VREFH.

The electric circuit arrangement la comprises a time-to- digital converter or a digital counter 330 being coupled to the first and second comparator output terminal 0310, 0320 and to the output terminal 40. The time-to-digital

converter/digital counter 330 advantageously enables to execute the calculation of the level of the excess bias voltage Vex in the digital domain.

As shown in Figure 1, the evaluation circuit 300 of the electric circuit arrangement la comprises a calculation circuit 360 being coupled to the time-to-digital

converter/digital counter 330. The calculation circuit 360 may be configured as a microprocessor to calculate the level of the excess bias voltage Vex in response to an output signal of the time-to-digital converter/digital counter 330.

Figure 2 illustrates the measurement/calculation principle of the electric circuit arrangement la to determine the level of the excess bias voltage Vex of the single photon avalanche diode 100. The approach is based on the calculation of the level of the excess bias voltage Vex from the voltage slope of the output signal Van. As explained above, in the first operational cycle of the circuit arrangement la, the output terminal 40 is

electrically coupled to the reference terminal 20 by the controllable switching circuit 200. In particular, the control circuit 400 controls the controllable switching circuit 200 such that the first controllable switch 210 is operated in a conductive state and the second controllable switch 230 is operated in a non-conductive state. As a result, the level of the output signal Van is at a level of the reference potential, for example the ground potential

GND .

If a photon hits the single photon avalanche diode 100, the voltage level of the output signal Van rises up to the level of the excess bias voltage Vex. A voltage jump/peak of the output signal Van to the level of the excess bias voltage Vex occurs at the output terminal 40. After the firing of the SPAD, i.e. the operation of the SPAD in the conductive state, the control circuit 400 in the feedback loop is activated to operate the first controllable switch 210 in a non-conductive state and to operate the second controllable switch 230 in a conductive state. The second current source 240 starts to charge the output terminal 40 so that the output signal Van rises at the output terminal 40 from the level of the excess bias voltage Vex to a value of the supply voltage AVDD.

During the charging process, the single photon avalanche diode 100 stays quenched, i.e. the SPAD 100 is operated in the non-conductive state, because by charging the output terminal 40/anode node of the SPAD 100 from the level of the excess bias voltage Vex up to the level of the supply voltage AVDD, the voltage at the SPAD, which means the voltage between the cathode and the anode of the SPAD, rolls off below breakdown voltage VBD up to VBD- (AVDD-Vex) . This means that other photons cannot trigger the SPAD 100 during the charging process.

If the voltage values of the output signal Van are known at certain time points of the slope of the output signal Van, the level of the excess bias voltage Vex can be calculated. The capacitance of the output terminal 40/anode node of the SPAD 100 or the amount of constant current is not an issue for this solution.

The evaluation circuit 300 is configured to determine a first time T1 during which the output signal Van rises at the output terminal 40 from the level of the excess bias voltage Vex to the first threshold value VREFL. Furthermore, the evaluation circuit 300 is configured to determine a second time T2 during which the output signal Van rises at the output terminal 40 from the first threshold value VREFL to a second threshold value VREFH. This configuration of the evaluation circuit advantageously allows to calculate the level of the excess bias voltage Vex from the voltage slope of the output signal Van shown in Figure 2 according to the formula

The instant of times when the voltage slope of the output signal Van crosses the first threshold value VREFL and the second threshold value VREFH can be detected with the first comparator circuit 310 and the second comparator circuit 320. However, the comparator circuits 310, 320 introduce offset times from the input to the output, due to offset voltage and internal parasitic nodes of the comparator circuits. Because of these offset times, the formula should be changed to

The offset time T _{0ffset low} is the offset time of the first comparator circuit 310, and the offset time T _{offSet high} is the offset time of the second comparator circuit 320. The offset times can be both positive or negative, meaning that the first and second comparator circuit can change their state sometime before or sometime after the voltage slope of the output signal Van crosses the first and second threshold values VREFL, VREFH. If T1 is very large against T _{0ffset low} (T1 » I T _offSet _i _ow I ) , and T2 is very large against T _0ffSet-high (T2 >> I T _{offset high} I ) , the measurement is less affected by

comparator offsets and internal parasitics.

Figure 3 shows a possible implementation to realize the electric circuit arrangement la of Figure 1 to determine the level of the excess bias voltage Vex of the single photon avalanche diode 100. The electric circuit arrangement la comprises the single photon avalanche diode 100, the

controllable switching circuit 200, the evaluation circuit 300 and the controller 400. The single photon avalanche diode 100 is arranged between the bias terminal 30 to apply the bias potential VHV and the output terminal 40 to provide the output signal Van.

The controllable switching circuit 200 comprises the first controllable switch 210, the first current generator 220 and the second controllable switch 230, the second current generator 240. The first current generator 230 is realized by a current mirror comprising transistors 221 and 222. The current mirror is coupled to a constant current source 260 to provide the constant current Ibias. The second current generator 240 is realized by a first current mirror including transistors 221 and 243 and a second current mirror including transistors 241 and 242. Furthermore, a controllable switch 250 is provided to enable/activate the electric circuit arrangement la to determine the level of the excess bias voltage Vex.

The evaluation circuit 300 comprises the first comparator circuit 310 and the second comparator circuit 320, the time- to-digital converter/digital counter 330 and the calculation circuit 360. The evaluation circuit 300 further comprises in-series connected inverters 370 to electrically couple the time-to-digital converter/digital counter 330 to the output terminal 40.

The control circuit 400 is coupled with its input side to the output terminal 40 and to a reset terminal 50 to apply a reset signal RST. The control circuit 400 may be embodied as a NAND gate 401 having the reset terminal 50. Another input terminal of the NAND gate 401 is coupled to the output terminal 40.

The functioning of the electric circuit arrangement la of Figure 3 is explained in the following with reference to the time diagram shown in Figure 4. When the control signal EN to enable/activate the electric circuit arrangement la is low, the output terminal 40 is connected to the supply terminal 10 via the controllable switch 250 which is operated in a conductive state. As a consequence, the single photon

avalanche diode 100 is quenched, i.e. operated in a non- conductive state. If the electric circuit arrangement la is activated/enabled to determine the level of the excess bias voltage Vex of the single photon avalanche diode 100 the control signal EN and the reset signal RST are set to a high level. As a result, the controllable switch 230 is operated in the conductive state and the controllable switch 210 is operated in the non-conductive state. The output terminal 40 stays connected to the supply potential AVDD via the

controllable switch 230 and transistor 241.

The control circuit 200 is configured to electrically couple the output terminal 40 to the reference terminal 20, when the control circuit 400 detects a pulse of the reset signal RST applied to the reset terminal 50. Thus, when the reset signal RST changes from the high state to a low state (reset phase) , the first controllable switch 210 is switched in the

conductive state and the second controllable switch 230 is switched in the non-conductive state. As a result, the output terminal 40 is discharged to ground via current regulated quenching transistor 222.

When a photon hits the SPAD 100, the potential of the output signal Van at the output terminal 40 rises up to the level of the excess bias voltage Vex. As shown in the time diagram of Figure 2, a voltage jump/peak of the output signal Van to the level of the excess bias voltage Vex occurs at the output terminal 40. At this moment, the SPAD 100 is quenched. If the potential of the output signal Van at the output terminal 40 is higher than a threshold voltage Vth inside one of the inverters 370 and the NAND gate 401 of the control circuit 400, a feedback signal FS generated at the output side of the in-series coupled inverters 370 becomes high and the NAND gate 401 switches the controllable switch 210 in the non- conductive state and the second controllable switch 230 in the conductive state.

As a result, the current path 202 with the transistor 241 starts to charge the output terminal 40 up to the supply potential AVDD. The output signal Van rises at the output terminal 40 from the level of the excess bias voltage Vex to a value of the supply voltage AVDD. The rising voltage shows a linear increasing course. In particular, the voltage slope of the output signal Van shows a linear gradient.

During this charging process the potential of the output signal at the output terminal 40 crosses the first threshold value VREFL and the second threshold value VREFH. The

evaluation circuit 300 determines the first time T1 during which the output signal Van rises at the output terminal 40 from the level of the excess bias voltage Vex to the first threshold value VREFL. Furthermore, the evaluation circuit 300 determines the second time T2 during which the output signal Van rises at the output terminal 40 from the first threshold value VREFL to the second threshold value VREFH.

For this purpose, the first comparator circuit 310 is

configured to output a pulse, for example a rising edge from the low to the high state, of the first comparison signal OUTL, when the output signal Van exceeds the first threshold value VREFL. Furthermore, the second comparator circuit 320 is configured to output a pulse, for example a rising edge from the low to the high state, of the second comparison signal OUTH, when the output signal Van exceeds the second threshold value VREFH. The first and second comparison signal VREFL and VREFH of the comparator circuits 310 and 320 thus define start and stop points of T1 and T2 time periods from Figure 2.

The time-to-digital converter/digital counter 330 is

configured to determine a first timespan TS1 between the occurrence of the voltage jump/peak of the output signal Van to the level of the excess bias voltage Vex and the

occurrence of the pulse of the first comparison signal OUTL. Furthermore, the time-to-digital converter/digital counter 330 is configured to determine a second timespan TS2 between the occurrence of the voltage jump/peak of the output signal Van and the occurrence of the pulse of the second comparison signal OUTH.

The calculation circuit 360 is configured to calculate the first and second time Tl, T2 in dependence on the first and second timespan TS1, TS2. Moreover, the calculation circuit 360 is configured to determine the level of the excess bias voltage Vex in dependence on the first and the second time Tl, T2 and in dependence on the first and second threshold value VREFL, VREFH. The level of the excess bias voltage Vex can be calculated from the calculation circuit 360 by evaluating the formula

Vex = VREFL - ^T1+T °ff ^set - ^low ( VREFH - VREFL

D+1 offset_high )

Figure 5 shows a second embodiment of the electric circuit arrangement lb to determine a level of the excess bias voltage Vex of the single photon avalanche diode 100.

The electric circuit arrangement lb comprises the single photon avalanche diode 100, the controllable switching circuit 200 and the control circuit 400 being embodied as explained above with reference to Figures 1 and 3. The electric circuit arrangement lb further comprises an

evaluation circuit 300 to evaluate the output signal Van at the output terminal 40. The evaluation circuit 300 is

configured to determine the level of the excess bias voltage Vex in dependence on the signal course of the output signal Van .

According to the embodiment of the electric circuit

arrangement lb, the evaluation circuit 300 comprises a comparator circuit 340 having a first input terminal 1340a coupled to the output terminal 40, and a second input

terminal 1340b to apply the first and second threshold value VREFL, VREFH, and a comparator output terminal 0340 to output a comparison signal OUT_COMP .

The evaluation circuit 300 further comprises a selection circuit 350 to select one of the first or second threshold value VREFL, VREFH to be applied to the second input terminal 1340b of the comparator circuit 340. In particular, the selection circuit 350 comprises a control circuit 355 to generate a control signal to control a controllable switch 351 to apply the first threshold value VREFL to the second input terminal 1340b of the comparator circuit 340.

Furthermore, the control circuit 355 of the selection circuit 350 generates a control signal to control a controllable switch 352 to apply the second threshold value VREFH to the second input terminal 1340b of the comparator circuit 340.

The first threshold value VREFL is generated as a voltage level by a voltage source 353, and the second threshold value VREFH is generated as a voltage level by a voltage source 354. The evaluation circuit 300 of the electric circuit arrangement lb further comprises a time-to-digital converter/ digital counter 330 being coupled to the comparator output terminal 0340 and to the output terminal 40. Furthermore, the evaluation circuit 300 comprises a calculation circuit 360 being coupled to the time-to-digital converter/digital counter 330.

Figure 6 shows a possible implementation of the electric circuit arrangement lb of Figure 5. The electric circuit arrangement lb of Figure 6 comprises the single photon avalanche diode 100, the controllable switching circuit 200 and the control circuit 400 as explained with reference to Figure 3.

The evaluation circuit 300 of the electric circuit

arrangement lb differs from the evaluation circuit of the electric circuit arrangement la shown in Figure 3. In

particular, the evaluation circuit 300 of the electric circuit arrangement lb only comprises one comparator circuit 340 instead of the two comparator circuits 310, 320 of the evaluation circuit of the electric circuit arrangement la.

The use of only one comparator circuit 340 has advantages regarding power consumption and measurement errors due to offset voltages and internal parasitics in comparison to the two comparator approach of Figure 3.

As shown in Figure 6, the control circuit 355 of the

selection circuit 350 may be embodied as a flip-flop, for example a D-flip-flop. An input side of the flip-flop 355 is connected to the comparator output terminal 0340 and to a terminal to apply a supply voltage DVDD. The supply voltage DVDD may be lower than the supply voltage AVDD. An output side of the control circuit 355 may be coupled via an

optional level shifter 357 to a control terminal of the controllable switch 351 and via an optional level shifter 356 to a control terminal of the controllable switch 352.

The time-to-digital converter/digital counter 330 is coupled to the output terminal 40 via a feedback path including the in-series connected inverters 370.

The functioning of the electric circuit arrangement lb shown in Figure 6 is described in the following with reference to the time diagram of Figure 7. The course of the control signal EN, the reset signal RST and the output signal Van is similar to the courses of the respective signals shown in, and described with reference to, Figure 4. However, due to the different configuration of the evaluation circuit 300, the course of the comparison signal OUT_COMP generated by the comparator circuit 340 at the comparator output terminal 0340 differs from the first and second comparison signals OUTL and OUTH of the first and second comparator circuits 310, 320 shown in Figure 4.

The comparator circuit 340 is configured to output a first change of state/pulse of the comparison signal OUT_COMP at the comparator output terminal 0340, when the output signal Van exceeds the first threshold value VREFL. Furthermore, the comparator circuit 340 is configured to output a second change of state/pulse of the comparison signal OUT_COMP at the comparator output terminal 0340, when the output signal Van exceeds the second threshold value VREFH. Figure 7 shows an exemplified course of the comparison signal OUT_COMP . The time-to-digital converter/digital counter 330 may be configured to determine the first timespan TS1 between the occurrence of the voltage jump/peak of the output signal Van and the occurrence of the first change of state/pulse of the comparison signal OUT_COMP . Furthermore, the time-to-digital converter/digital counter 330 may be configured to determine the second timespan TS2 between the occurrence of the voltage jump/peak of the output signal Van and the occurrence of the second change of state/pulse of the comparison signal

OUT COMP.

The calculation circuit 360 may be configured to calculate the first and second time Tl, T2 in dependence on the

determined first and second timespan TS1, TS2. Furthermore, the calculation circuit 360 may be configured to determine the level of the excess bias voltage Vex in dependence on the first and second time Tl, T2 and in dependence on the first and second threshold value VREFL and VREFH.

The embodiment of the electric circuit arrangement lb shown in Figures 5 and 6 is believed to be an improved solution in comparison to the embodiment of the electric circuit

arrangement la shown in Figures 1 and 3. The embodiment of the electric circuit arrangement la uses two comparator circuits 310 and 320 in comparison to the use of only one comparator circuit 340 of the electric circuit arrangement lb. The two comparator circuits 310, 320 have different offset voltages and internal parasitics which introduce a measurement error by offset times. Also, the power

consumption is doubled in the case of using two comparator circuits 310, 320 instead of only one comparator circuit 340. With the approach of the electric circuit arrangement lb, the offset voltages and parasitics have less influence on

measurement results. In comparison to the formula to

calculate the level of the excess bias voltage Vex for the electric circuit arrangement la, the formula to calculate the level of the excess bias voltage Vex by the calculation circuit 360 of the electric circuit arrangement lb is given by

The formula which may be evaluated by the calculation circuit 360 shows that the measurement/calculation of the level of the excess bias voltage Vex is less influenced by the offset voltage and parasitics. For example, assuming T1 = T2 = 10 ys and T _0ffset _i _ow ^{— —} T _0ffset-high ^— ^ _offset ^— 0.1 ys, the equations become, for the two comparator case of the electric circuit arrangement la:

T1 + T _{offset low =} lOys + O.lys ₌ lO.lys

T2 + T _offsetJligh lOys— O.lys 9.9 ys and for the one comparator case of the electric circuit arrangement lb:

T1 + T _offset _ lOys + O.lys _ lO.lys

T2 + T ₀ ff _set 10ys + 0.1ys lO.lys

The time T _0ffset is a function of the bias conditions.

As shown in Figure 7, after the reset phase (RST = 0), the first threshold value VREFL is connected via the controllable switch 351, realized for example by a PMOS transistor, to the second comparator input terminal 1340b, because the

controllable switch 351 is operated in a conductive state.

The controllable switch 352 that may also be realized by a PMOS transistor, is operated in a non-conductive state.

When the voltage slope of the output signal Van crosses the first threshold value VREFL, the comparison signal OUT_COMP has a change of state, for example a positive pulse, which is used by the calculation circuit 360 to determine the first time Tl. The comparison signal OUT_COMP is also applied to the control circuit 355 and changes the state of the flip- flop 353 so that the controllable switch 351 is operated in a non-conductive state and the controllable switch 352 is operated in a conductive state. As a consequence, the second threshold value VREFH is forwarded to the second input terminal 1340b of the comparator circuit 340.

Figure 8 illustrates a possible application in which the electric circuit arrangement la, lb to determine a level of an excess bias voltage Vex of a single photon avalanche diode may be used. The electric circuit arrangement la, lb may be included in a sensor device 2. The sensor device 2 may comprise a bias voltage generator 500 to provide the bias potential VHV. The bias voltage generator 500 may be

controlled by the calculation circuit 360 which may be embodied as a microprocessor.

In dependence on the calculated level of the excess bias voltage Vex a control signal, for example trimming bits, are generated by the calculation circuit 360 and received by the bias voltage generator 500. The bias voltage generator 500 may be configured to regulate the bias potential VHV in dependence on the level of the excess bias voltage Vex determined by the electric circuit arrangement la, lb. The bias potential VHV may then be applied to the bias terminal 30 of the electric circuit arrangement la, lb and to a sensor matrix 600 which may comprise optical sensors 610.

As shown in Figure 9, the sensor device 2 may be included in a communication device 3. The communication device 3 may be embodied, for example, as one of a time-of-flight measurement device, a LIDAR measurement device, a time correlated single photon counting device, a tomograph, etc. .

The single photon avalanche diode 100 is shown in the

embodiments of the electric circuit arrangements la and lb as being connected to the bias terminal 30 to apply a positive bias potential. Thus, the single photon avalanche diode 100 is connected with its cathode to the bias terminal 30 and with its anode to the output terminal 40. It is mentioned for the sake of completeness that a person skilled in the art can easily modify the electric circuit arrangements la, lb to use a negative bias potential instead of a positive bias

potential. In this case, the single photon avalanche diode 100 is connected with its anode side to the bias terminal 30 and with its cathode side to the output terminal 40. However, the basic principle of the electric circuit arrangements la and lb stays unchanged.

List of Reference Signs la, lb electric circuit arrangement

10 supply terminal

20 reference terminal

30 bias terminal

40 output terminal

100 single photon avalanche diode

200 controllable switching circuit

201, 202 current path

210, 230 controllable switch

220, 240 current generator

300 evaluation circuit

310, 320 comparator circuit

330 time-to-digital converter/digital counter 340 comparator circuit

350 selection circuit

360 calculation circuit

400 control circuit

500 bias voltage generator

600 sensor matrix