Time-of-flight ranging device suitable for indirect time-of-flight ranging

A time-of-flight ranging device suitable for indirect time-of-flight ranging is provided. The time-of-flight ranging device includes a light emitting module, a first sensing pixel, a second sensing pixel, a differential readout circuit, and a processing circuit. The light emitting module emits a light pulse to a sensing target, so that the sensing target reflects a reflected light pulse. The first sensing pixel generates a first sensing signal and a second sensing signal. The second sensing pixel generates a third sensing signal and a fourth sensing signal. The differential readout circuit generates first digital data according to the first sensing signal and the third sensing signal and generates second digital data according to the second sensing signal and the fourth sensing signal. The processing circuit calculates a distance between the time-of-flight ranging device and the sensing target according to the first digital data and the second digital data.

BACKGROUND

Technical Field

The disclosure relates to a ranging technology, and in particular to a time-of-flight ranging device.

Description of Related Art

During indirect time-of-flight ranging performed by a common time-of-flight ranging device, if background light is strong and is changed with time (with no fixed value), the time-of-flight ranging device may not be able to easily lessen or eliminate the impact of the background light in sensing results of the indirect time-of-flight ranging. Although the common time-of-flight ranging device can perform additional background light sensing to obtain background information for reducing or eliminating the impact of the background light in the sensing results of the indirect time-of-flight ranging, due to the long time interval between the background light sensing and the ranging sensing by the common time-of-flight ranging device, the obtained background information cannot be effectively used to lessen or eliminate the impact of the background light in the sensing results of the indirect time-of-flight ranging. In addition, in the event of the strong background light, the common time-of-flight ranging device has the defect of insufficient dynamic range. In view of the above, several solutions described in the embodiments below are proposed.

SUMMARY

The disclosure provides a time-of-flight ranging device suitable for indirect time-of-flight ranging, which may effectively sense a distance between the time-of-flight ranging device and a sensing target.

According to an embodiment of the disclosure, a time-of-flight ranging device is suitable for indirect time-of-flight ranging. The time-of-flight ranging device includes a light emitting module, a first sensing pixel, a second sensing pixel, a differential readout circuit, and a processing circuit. The light emitting module is configured to emit a light pulse to a sensing target, so that the sensing target reflects a reflected light pulse. The first sensing pixel is configured to respectively perform sensing in a first cycle in a first frame period and a second cycle in a second frame period to respectively generate a first sensing signal and a second sensing signal. The second sensing pixel is configured to respectively perform sensing in a third cycle in the first frame period and a fourth cycle in the second frame period to respectively generate a third sensing signal and a fourth sensing signal. The differential readout circuit is coupled to the first sensing pixel and the second sensing pixel. The differential readout circuit is configured to generate first digital data according to the first sensing signal and the third sensing signal in the first frame period and generate second digital data according to the second sensing signal and the fourth sensing signal in the second frame period. The processing circuit is coupled to the differential readout circuit. The processing circuit is configured to calculate a distance between the time-of-flight ranging device and the sensing target according to the first digital data and the second digital data.

According to an embodiment of the disclosure, a time-of-flight ranging device is suitable for indirect time-of-flight ranging. The time-of-flight ranging device includes a light emitting module, a first sensing pixel, a second sensing pixel, a third sensing pixel, a fourth sensing pixel, a differential readout circuit, and a processing circuit. The light emitting module is configured to emit a light pulse to a sensing target, so that the sensing target reflects a reflected light pulse. The first sensing pixel is configured to perform sensing in a first cycle in a frame period to generate a first sensing signal. The second sensing pixel is configured to perform sensing in a second cycle in the frame period to generate a second sensing signal. The third sensing pixel is configured to perform sensing in a third cycle in the frame period to generate a third sensing signal. The fourth sensing pixel is configured to perform sensing in the third cycle in the frame period to generate a fourth sensing signal. The differential readout circuit is coupled to the first sensing pixel to the fourth sensing pixel. The differential readout circuit is configured to generate first digital data and second digital data according to the first sensing signal to the fourth sensing signal in the frame period. The processing circuit is coupled to the differential readout circuit. The processing circuit is configured to calculate a distance between the time-of-flight ranging device and the sensing target according to the first digital data and the second digital data.

Based on the above, the time-of-flight ranging device provided in one or more embodiments of the disclosure is suitable for indirect time-of-flight ranging and may respectively sense the reflected light pulse and the background light through different sensing pixels, so as to obtain the distance between the time-of-flight ranging device and the sensing target through indirect time-of-flight ranging calculation.

DESCRIPTION OF THE EMBODIMENTS

In order to make the content of the disclosure easier to understand, the following specific embodiments are provided as to how the disclosure can be implemented. In addition, wherever possible, the same reference numbers of components/elements/steps are used in the drawings and embodiments to represent the same or similar components/elements/steps.

FIG.1is a schematic diagram of a time-of-flight ranging device according to an embodiment of the disclosure. With reference toFIG.1, the time-of-flight ranging device100includes a first sensing pixel110, a second sensing pixel120, a differential readout circuit130, a light emitting module140and a processing circuit150. The differential readout circuit130is coupled to the first sensing pixel110, the second sensing pixel120and the processing circuit150. The processing circuit150is coupled to the light emitting module140. In the present embodiment, the processing circuit150may include, for example, a digital signal processor (DSP), a driver, a controller and other functional circuits. The processing circuit150may output a pulse signal PS to the light emitting module140to drive the light emitting module140to emit a light pulse LP to a sensing target200, so that the sensing target200reflects a reflected light pulse RLP. The first sensing pixel110is configured to perform indirect time-of-flight ranging to generate a first sensing signal S1and a second sensing signal S2. The second sensing pixel120performs sensing after the first sensing pixel110obtains the first sensing signal S1and the second sensing signal S2so as to generate a third sensing signal S3and a fourth sensing signal S4according to background light BL.

In the present embodiment, the differential readout circuit130subtracts background noise from the first sensing signal S1by the third sensing signal S3, and subtracts background noise from the second sensing signal S2by the fourth sensing signal S4. The differential readout circuit130generates first digital data D1according to the first sensing signal S1and the third sensing signal S3, and generates second digital data D2according to the second sensing signal S2and the fourth sensing signal S4. The processing circuit150may perform indirect time-of-flight ranging calculation according to the first digital data D1and the second digital data D2to obtain a distance between the time-of-flight ranging device100and the sensing target200.

In the present embodiment, the light emitting module140may include one or a plurality of laser light sources, and the one or plurality of laser light sources may be, for example, a pulsed light emitter or a laser diode. The laser light source120may, for example, be configured to emit a light pulse of infrared radiation (IR) to the sensing target200. In the present embodiment, the time-of-flight ranging device100may include a complementary metal oxide semiconductor image sensor (CMOS image sensor, CIS), and the image sensor includes a pixel array. The pixel array may include a plurality of first sensing pixels110and a plurality of second sensing pixels120. In the present embodiment, the first sensing pixel110and the second sensing pixel120may include a photodiode, and the photodiode is configured to receive or sense a reflected light pulse of the infrared radiation reflected by the sensing target200.

FIG.2is a schematic diagram of a first sensing pixel, a second sensing pixel, and a differential readout circuit according to an embodiment depicted inFIG.1. The first sensing pixel110, the second sensing pixel120and the differential readout circuit130depicted inFIG.1above may be a first sensing pixel310, a second sensing pixel320and a differential readout circuit330depicted inFIG.2. With reference toFIG.2, the first sensing pixel310includes a photodiode311, a reset switch312, a readout switch313, a storage capacitor314and a reset switch315. A first terminal of the photodiode311is coupled to a first reference voltage Vf1. A first terminal of the reset switch312is coupled to a second reference voltage Vf2, and a second terminal of the reset switch312is coupled to a second terminal of the photodiode311. A first terminal of the first readout switch313is coupled to the second terminal of the photodiode311. A first terminal of the storage capacitor314is coupled to the first reference voltage Vf1, and a second terminal of the storage capacitor314is coupled to a second terminal of the first readout switch313and the differential readout circuit330. A first terminal of the reset switch315is coupled to the second reference voltage Vf2, and a second terminal of the reset switch315is coupled to the second terminal of the storage capacitor314. In the present embodiment, a full well of the storage capacitor314is greater than a full well of the photodiode311. The full well of the photodiode311may be, for example, FW1, and the full well of the storage capacitor314may be, for example, FW2. In the present embodiment, FW2>N×FW1. N is a positive integer. That is, the full well of the storage capacitor314is greater than the full well of the photodiode311, so that more exposures are performed to collect more charges. In other words, since the first sensing pixel310of the present embodiment may equivalently use N times the full well of the photodiode311, the time-of-flight ranging device of the present embodiment may obtain a more accurate distance measurement result.

The second sensing pixel320includes a photodiode321, a reset switch322, a first readout switch323, a storage capacitor324and a reset switch325. A first terminal of the photodiode321is coupled to the first reference voltage Vf1. A first terminal of the reset switch322is coupled to the second reference voltage Vf2, and a second terminal of the reset switch322is coupled to a second terminal of the photodiode321. A first terminal of the first readout switch323is coupled to the second terminal of the photodiode321. A first terminal of the storage capacitor324is coupled to the first reference voltage Vf1, and a second terminal of the storage capacitor324is coupled to a second terminal of the first readout switch323and the differential readout circuit330. A first terminal of the reset switch325is coupled to the second reference voltage Vf2, and a second terminal of the reset switch325is coupled to the second terminal of the storage capacitor324. In the present embodiment, a full well of the storage capacitor324is greater than a full well of the photodiode321. The full well of the photodiode321may be, for example, FW1, and the full well of the storage capacitor324may be, for example, FW2. In the present embodiment, FW2>N×FW1. N is a positive integer. That is, the full well of the storage capacitor324is greater than the full well of the photodiode321, so that more exposures are performed to collect more charges. In other words, since the second sensing pixel320of the present embodiment may equivalently use N times the full well of the photodiode321, the time-of-flight ranging device of the present embodiment may obtain a more accurate distance measurement result.

The differential readout circuit330includes a differential operation circuit331and an analog to digital (A/D) conversion circuit332. A first input terminal of the differential operation circuit331is coupled to the second terminal of the storage capacitor314of the first sensing pixel310. A second input terminal of the differential operation circuit331is coupled to the second terminal of the storage capacitor324of the second sensing pixel320. The analog to digital conversion circuit332is coupled to the differential operation circuit331. In the present embodiment, the differential operation circuit331may perform signal integration K times, and may equivalently use a full well of the photodiode311of K×N×FW1. Therefore, the time-of-flight ranging device of the present embodiment may obtain a more accurate distance measurement result. It should be particularly noted that for architecture of the traditional single-ended input operation circuit, when a signal received by the traditional operation circuit contains most of the background noise, a swing of an actual signal of the traditional operation circuit will be limited, thereby limiting a dynamic range of the operation circuit. In contrast, for the differential architecture of the disclosure, the differential operation circuit of the disclosure only performs operations (for example, integration, amplification and the like) on the difference of input signals, so the differential operation circuit of the disclosure may have a larger signal swing and may obtain a higher dynamic range.

FIG.3Ais a timing diagram in a first frame period according to the embodiment depicted inFIG.2.FIG.3Bis a timing diagram in a second frame period according to the embodiment depicted inFIG.2. With reference toFIG.2andFIG.3A, when the light emitting module emits a light pulse LP to the sensing target so that the sensing target reflects a reflected light pulse RLP, in the first frame period, the reset switch312, the readout switch313and the reset switch315may be switched, so that the photodiode311stores charges stored by a part of the received reflected light pulse RLP to the storage capacitor314, and provides a first sensing signal S1to the first input terminal of the differential operation circuit331via the storage capacitor314. The disclosure does not limit the switching timing of the reset switch312, the readout switch313and the reset switch315. Besides, the reset switch322, the readout switch323and the reset switch325may be switched, so that the photodiode321stores charges stored by the received background light to the storage capacitor324, and provides a third sensing signal S3to the second input terminal of the differential operation circuit331via the storage capacitor324. The disclosure does not limit the switching timing of the reset switch322, the readout switch323and the reset switch325.

Specifically, the photodiode311of the first sensing pixel310performs image integration in a first cycle P1in the first frame period. The first cycle P1is synchronized with a first pulse cycle PW1of the light pulse LP. As shown inFIG.3A, the photodiode311may sense the background light and a part of the reflected light pulse RLP (for example, the shaded area of S1) in the first cycle P1. Therefore, the first sensing pixel310may provide the first sensing signal S1to the first input terminal of the differential operation circuit331. Next, the photodiode321of the second sensing pixel320performs image integration in a third cycle P3in the first frame period. The first cycle P1and the third cycle P3have a same cycle length, and the third cycle P3does not overlap and is adjacent to a second pulse cycle PW2of the reflected light pulse RLP, so the photodiode311may sense the same or similar background light in the first cycle P1and the third cycle P3. Therefore, the second sensing pixel320may provide the third sensing signal S3to the second input terminal of the differential operation circuit331. The third sensing signal S3is a pure background signal.

In the present embodiment, the differential operation circuit331may perform subtraction (voltage subtraction) on the first sensing signal S1and the third sensing signal S3to generate a first differential signal. The differential operation circuit331provides the first differential signal to the analog to digital conversion circuit332to output a first digital signal D1. In other words, the differential readout circuit330of the present embodiment may provide a sensing value of a part of the reflected light pulse RLP with the background noise removed in the first frame period.

With reference toFIG.2andFIG.3B, when the light emitting module emits a light pulse LP to the sensing target so that the sensing target reflects a reflected light pulse RLP, in the second frame period, the reset switch312, the readout switch313and the reset switch315may be switched, so that the photodiode311stores charges stored by another part of the received reflected light pulse RLP to the storage capacitor314, and provides a second sensing signal S2to the first input terminal of the differential operation circuit331via the storage capacitor314. The disclosure does not limit the switching timing of the reset switch312, the readout switch313and the reset switch315. Besides, the reset switch322, the readout switch323and the reset switch325may be switched, so that the photodiode321stores charges stored by the received background light to the storage capacitor324, and provides a fourth sensing signal S4to the second input terminal of the differential operation circuit331via the storage capacitor324. The disclosure does not limit the switching timing of the reset switch322, the readout switch323and the reset switch325.

Specifically, the photodiode311of the first sensing pixel310performs image integration in a second cycle P2in the second frame period. A rising edge of the second cycle P2follows a falling edge of the light pulse LP. As shown inFIG.3B, the photodiode311may sense the background light and another part of the reflected light pulse RLP (for example, the shaded area of S2) in the second cycle P2. Therefore, the first sensing pixel310may provide the second sensing signal S2to the first input terminal of the differential operation circuit331. Next, the photodiode321of the second sensing pixel320performs image integration in a fourth cycle P4in the second frame period. The second cycle P2and the fourth cycle P4have a same cycle length, the fourth cycle P4does not overlap and is adjacent to a second pulse cycle PW2of the reflected light pulse RLP, and the fourth cycle P4even does not overlap the second cycle P2, so the photodiode311may sense the same or similar background light in the second cycle P2and the fourth cycle P4. Therefore, the second sensing pixel320may provide the fourth sensing signal S4to the second input terminal of the differential operation circuit331. The fourth sensing signal S4is a pure background signal.

In the present embodiment, the differential operation circuit331may perform subtraction (voltage subtraction) on the second sensing signal S2and the fourth sensing signal S4to generate a second differential signal. The differential operation circuit331provides the second differential signal to the analog to digital conversion circuit332to output a second digital signal D2. In other words, the differential readout circuit330of the present embodiment may provide a sensing value of another part of the reflected light pulse RLP with the background noise removed in the second frame period.

In the present embodiment, the analog to digital conversion circuit332may provide a first digital signal D1and a second digital signal D2to a back-end digital signal processing circuit (for example, the processing circuit150depicted inFIG.1as described above), so that the digital signal processing circuit may perform indirect time-of-flight ranging calculation according to the following formula (1) based on the first digital signal D1and the second digital signal D2. Here, d is a distance, c is a speed of light, and t is a pulse width of the first pulse cycle PW1. However, the calculation manner of the indirect time-of-flight ranging of the disclosure is not limited to this.

Therefore, the differential readout circuit330of the present embodiment may effectively provide the sensing result of the reflected light pulse RLP with the background value removed, so that the back-end digital signal processing circuit may effectively calculate the accurate distance. In addition, a full well of the storage capacitors314,324of the present embodiment is greater than a full well of the photodiodes311,321. Therefore, compared with the traditional design in which the full well of the storage capacitor is equal to the full well of the photodiode, the storage capacitors314,324of the disclosure may store the sensing results of more reflected light pulses. In addition, the differential operation circuit331may further perform signal integration to provide a sensing result with a large dynamic range. For example, the sensing result of M pulses or background light may correspond to the charge amount FW1, and the storage capacitors314,324may store a charge amount N×FW1corresponding to the sensing result of M×N pulses or M×N times of background light in one frame period.

FIG.4is a schematic diagram of a first sensing pixel, a second sensing pixel, and a differential readout circuit according to another embodiment depicted inFIG.1. The first sensing pixel110, the second sensing pixel120, and the differential readout circuit130depicted inFIG.1above may be a first sensing pixel410, a second sensing pixel420and a differential readout circuit430depicted inFIG.4. With reference toFIG.4, the first sensing pixel410includes a photodiode411, a reset switch412, a readout switch413, a storage capacitor414, a reset switch415, a diode416and a readout switch417. A first terminal of the photodiode411is coupled to a first reference voltage Vf1. A first terminal of the reset switch412is coupled to a second reference voltage Vf2, and a first terminal of the readout switch417is coupled to a second terminal of the photodiode411. A first terminal of the diode416is coupled to the first reference voltage Vf1, and a second terminal of the diode416is coupled to a first terminal of the readout switch413. A first terminal of the storage capacitor414is coupled to the first reference voltage Vf1, and a second terminal of the storage capacitor414is coupled to a second terminal of the readout switch413and the differential readout circuit430. A first terminal of the reset switch415is coupled to the second reference voltage Vf2, and a second terminal of the reset switch415is coupled to the second terminal of the storage capacitor414. In the present embodiment, the diode416may be used as a storage node, and a full well of the diode416is greater than a full well of the photodiode411. A full well of the storage capacitor414is greater than a full well of the photodiode411. The full well of the photodiode411may be, for example, FW, and the full wells of the storage capacitor414and the diode416may respectively be, for example, FW2and FW3. In the present embodiment, FW2>N×FW1, and FW3>N×FW1. N is a positive integer. That is, more exposures may be performed on the first sensing pixel410to collect more charges. In other words, since the first sensing pixel410of the present embodiment may equivalently use N times the full well of the photodiode411, the time-of-flight ranging device of the present embodiment may obtain a more accurate distance measurement result. It should be particularly noted that when charges are stored in the diode416, the storage capacitor414may be reset without affecting the diode416. Therefore, the first sensing pixel410of the present embodiment may perform a true correlated double sampling (true CDS) or true double delta sampling (true DDS) operation.

The second sensing pixel420includes a photodiode421, a reset switch422, a readout switch423, a storage capacitor424, a reset switch425, a diode426, and a readout switch427. A first terminal of the photodiode421is coupled to the first reference voltage Vf1. A first terminal of the reset switch422is coupled to the second reference voltage Vf2, and a first terminal of the readout switch427is coupled to a second terminal of the photodiode421. A first terminal of the diode426is coupled to the first reference voltage Vf1, and a second terminal of the diode426is coupled to a first terminal of the readout switch423. A first terminal of the storage capacitor424is coupled to the first reference voltage Vf1, and a second terminal of the storage capacitor424is coupled to a second terminal of the readout switch423and the differential readout circuit430. A first terminal of the reset switch425is coupled to the second reference voltage Vf2, and a second terminal of the reset switch425is coupled to the second terminal of the storage capacitor424. In the present embodiment, the diode426may be used as a storage node, and a full well of the diode426is greater than a full well of the photodiode421. A full well of the storage capacitor424is greater than a full well of the photodiode421. The full well of the photodiode421may be, for example, FW, and the full wells of the storage capacitor424and the diode426may respectively be, for example, FW2and FW3. In the present embodiment, FW2>N×FW1, and FW3>N×FW1. N is a positive integer. That is, more exposures may be performed on the second sensing pixel420to collect more charges. In other words, since the second sensing pixel420of the present embodiment may equivalently use N times the full well of the photodiode421, the time-of-flight ranging device of the present embodiment may obtain a more accurate distance measurement result. It should be particularly noted that when charges are stored in the diode426, the storage capacitor424may be reset without affecting the diode426. Therefore, the second sensing pixel420of the present embodiment may perform a true correlated double sampling (true CDS) or true double delta sampling (true DDS) operation.

The differential readout circuit430includes a differential operation circuit431and an analog to digital conversion circuit432. A first input terminal of the differential operation circuit431is coupled to a second terminal of the storage capacitor414of the first sensing pixel410. A second input terminal of the differential operation circuit431is coupled to the second terminal of the storage capacitor424of the second sensing pixel420. The analog to digital conversion circuit432is coupled to the differential operation circuit431. In the present embodiment, the first sensing pixel410, the second sensing pixel420and the differential readout circuit430may perform the image integration depicted inFIG.3AandFIG.3Bdescribed above to generate the first sensing signal S1to the fourth sensing signal S4to the differential operation circuit431of the differential readout circuit430. The differential operation circuit431may output the first differential signal and the second differential signal to the analog to digital conversion circuit432according to the first sensing signal S1to the fourth sensing signal S4, so that the analog to digital conversion circuit432outputs a first digital signal D1and a second digital signal D1.

Compared with the embodiment depicted inFIG.2above, the first sensing pixel410and the second sensing pixel420provided in the present embodiment may accumulate and store real sensing results of the photodiodes411,421through the diodes416,426, and provide charge accumulation results to the storage capacitors414,424. It should be particularly noted that when the storage capacitors414,424are reset, the diodes416,426may not be affected by the reset. Therefore, the first sensing pixel410and the second sensing pixel420may perform true correlated double sampling (true CDS) or true double delta sampling (true DDS). Besides, since multiple exposures may be continuously performed on the first sensing pixel410and the second sensing pixel420and the differential operation circuit331may perform signal integration, a sensing result with a larger dynamic range may be provided. For example, the sensing result of M pulses or background light may correspond to the charge amount FW1, and the diodes416,426and the storage capacitors414,424may store a charge amount N×FW1corresponding to the sensing result of N×M pulses or N×M times of background light in one frame period. Besides, the differential operation circuit331may perform image integration K times, and the charge amount corresponding to the image integration result may be K×N×FW1. K is a positive integer. In other words, compared with the traditional architecture, the equivalent FW1of the disclosure becomes K×N times.

However, for the actuating relationship between the components of the present embodiment, reference may be made to the description of the embodiments depicted inFIG.1toFIG.3above to obtain sufficient teachings, suggestions and implementation description, so details will not be repeated here.

FIG.5is a schematic diagram of a time-of-flight ranging device according to an embodiment of the disclosure. With reference toFIG.5, the time-of-flight ranging device500includes a first sensing pixel511, a second sensing pixel512, a third sensing pixel521, a fourth sensing pixel522, a differential readout circuit530, a light emitting module540and a processing circuit550. The differential readout circuit530includes a first differential operation circuit531, a second differential operation circuit532and an analog to digital conversion circuit533. The differential readout circuit530is coupled to the first sensing pixel511, the second sensing pixel512, the third sensing pixel521, the fourth sensing pixel522and the processing circuit550. The processing circuit550is coupled to the light emitting module540. The first sensing pixel511and the third sensing pixel521are coupled to a first input terminal and a second input terminal of the first differential operation circuit531. An output terminal of the first differential operation circuit531is coupled to the analog to digital conversion circuit533. The second sensing pixel512and the fourth sensing pixel522are coupled to a first input terminal and a second input terminal of the second differential operation circuit532. An output terminal of the second differential operation circuit532is coupled to the analog to digital conversion circuit533.

It is worth noting that the second sensing pixel512, the fourth sensing pixel522and the second differential operation circuit532may correspond to circuits of the first sensing pixel310, the second sensing pixel320, and the differential operation circuit331in the embodiment depicted inFIG.2as described above, or correspond to circuit forms of the first sensing pixel410, the second sensing pixel420, and the differential operation circuit431in the embodiment depicted inFIG.4as described above. The first sensing pixel511, the third sensing pixel521and the first differential operation circuit531may correspond to circuits of the first sensing pixel310, the second sensing pixel320, and the differential operation circuit331in the embodiment depicted inFIG.2as described above, or correspond to circuit forms of the first sensing pixel410, the second sensing pixel420, and the differential operation circuit431in the embodiment depicted inFIG.4as described above.

In the present embodiment, the processing circuit550may include, for example, a digital signal processor, a driver, a controller and other functional circuits. The processing circuit550may output a pulse signal PS to the light emitting module540to drive the light emitting module540to emit a light pulse LP to a sensing target600, so that the sensing target600reflects a reflected light pulse RLP. The first sensing pixel511and the second sensing pixel512are configured to perform indirect time-of-flight ranging to generate a first sensing signal S1′ and a second sensing signal S2′. The third sensing pixel521and the fourth sensing pixel522perform sensing respectively after the first sensing pixel110and the second sensing pixel512obtain the first sensing signal S1′ and the second sensing signal S2′ so as to generate a third sensing signal S3′ and a fourth sensing signal S4′ according to background light BL.

In the present embodiment, the first differential operation circuit531subtracts background noise from the first sensing signal S1′ by the third sensing signal S3′, and the second differential operation circuit532subtracts background noise from the second sensing signal S2′ by the fourth sensing signal S4′. The first differential operation circuit531generates first digital data D1′ according to the first sensing signal S1′ and the third sensing signal S3′, and the second differential operation circuit532generates second digital data D2′ according to the second sensing signal S2′ and the fourth sensing signal S4′. The processing circuit550may perform indirect time-of-flight ranging calculation according to the first digital data D1′ and the second digital data D2′ to obtain a distance between the time-of-flight ranging device500and the sensing target600.

However, for the circuit features and the actuating relationship between the components of the present embodiment, reference may be made to the description of the embodiments depicted inFIG.1toFIG.4above to obtain sufficient teachings, suggestions and implementation description, and thus so details will not be repeated here.

FIG.6is a timing diagram in a frame period according to the embodiment depicted inFIG.5. With reference toFIG.5andFIG.6, when the light emitting module emits a light pulse LP to the sensing target so that the sensing target reflects a reflected light pulse RLP, the photodiode of the first sensing pixel511performs image integration in a first cycle P1′ in a frame period. The first cycle P1′ is synchronized with a first pulse cycle PW1′ of the light pulse LP. As shown inFIG.6, the photodiode of the first sensing pixel511may sense the background light and a part of the reflected light pulse RLP (for example, the shaded area of S1′) in the first cycle P1′. Therefore, the first sensing pixel511may provide the first sensing signal S1′ to the first input terminal of the first differential operation circuit531. Next, the photodiode of the second sensing pixel512performs image integration in a second cycle P2′ in the same frame period. A rising edge of the second cycle P2′ follows a falling edge of the first cycle P1′. As shown inFIG.6, the photodiode of the second sensing pixel512may sense the background light and another part of the reflected light pulse RLP (for example, the shaded area of S2′) in the second cycle P2′. Therefore, the second sensing pixel512may provide the second sensing signal S2′ to the first input terminal of the second differential operation circuit532. Next, the photodiodes of the third sensing pixel521and the fourth sensing pixel522may respectively perform image integration in a third cycle P3′ in the same frame period to respectively provide the third sensing signal S3′ and the fourth sensing signal S4′ to the second input terminal of the first differential operation circuit531and the second input terminal of the second differential operation circuit532.

In the present embodiment, the first cycle P1′, the second cycle P2′ and the third cycle P3′ have a same cycle length. The third cycle P3′ does not overlap and is adjacent to a second pulse cycle PW2′ of the reflected light pulse RLP, so the photodiodes of the third sensing pixel521and the fourth sensing pixel522may sense in the third cycle P3′ the background light that is the same as or similar to the background light respectively sensed by the first sensing pixel511and the second sensing pixel512in the first cycle P1′ and the second cycle P2′. Therefore, the third sensing pixel521and the fourth sensing pixel522may provide the third sensing signal S3′ and the fourth sensing signal S4′ to the second input terminal of the first differential operation circuit531and the second input terminal of the second differential operation circuit532. The third sensing signal S3′ and the fourth sensing signal S4′ are respectively a pure background signal.

In the present embodiment, the first differential operation circuit531may perform subtraction (voltage subtraction) on the first sensing signal S1′ and the third sensing signal S3′ to generate a first differential signal. The second differential operation circuit532may perform subtraction (voltage subtraction) on the second sensing signal S2′ and the fourth sensing signal S4′ to generate a second differential signal. The first differential operation circuit531may first provide the first differential signal to the analog to digital conversion circuit533, so that the analog to digital conversion circuit533outputs a first digital signal D1′. Next, the second differential operation circuit532provides the second differential signal to the analog to digital conversion circuit533, so that the analog to digital conversion circuit533outputs a second digital signal D2′. In other words, the time-of-flight ranging device500of the present embodiment may simultaneously perform ranging sensing and background light sensing in one frame period, so that a sensing frame rate of indirect time-of-flight ranging of the time-of-flight ranging device500may be improved.

Therefore, the differential readout circuit530of the present embodiment can complete the indirect time-of-flight ranging sensing once within one frame period, and can effectively remove or reduce the effect of the background noise on the sensing result, so that the back-end digital signal processing circuit can effectively calculate the accurate distance. However, for the calculation manner of the distance in the present embodiment, reference may be made to the illustration of formula (1) in the embodiment above to obtain sufficient teachings, suggestions and implementation description, so details will not be repeated here.

To sum up, the time-of-flight ranging device provided in one or more embodiments of the disclosure may effectively perform indirect time-of-flight ranging and may effectively eliminate the impact of the background noise, so as to accurately obtain the distance between the time-of-flight ranging device and the sensing target after calculation based on the sensing signal after the background is removed.