Optical plethysmography front-end receiver

Abstract

The present disclosure relates to a photoplethysmogram front-end receiver. The receiver is capable of eliminating estimation errors caused by ambient light current. The receiver includes a current-to-voltage conversion circuit, an integration circuit, a switching circuit, and an analog-to-digital conversion circuit. The current-to-voltage conversion circuit converts an input current into a differential voltage signal. The integration circuit receives the differential voltage signal and outputs an analog output voltage in dependence thereon. The switching circuit is coupled between the current-to-voltage conversion circuit and the integration circuit, and forwards the differential voltage signal to the integration circuit for a first duration and forwards an inverted version of the differential voltage signal to the integration circuit for a second duration, the controllable light source being turned on during the first duration and turned off during the second duration, the second duration following the first duration or the second duration being followed by the first duration. The analog-to-digital conversion circuit generates a digital signal in dependence on the analog output voltage after the second duration for analysis.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a front-end receiver, and more particularly to a photoplethysmography (PPG) front-end receiver. BACKGROUND

[0002] Photoplethysmography (PPG) technology can achieve various applications (e.g., heart rate and blood oxygen measurement) by irradiating the skin with a controllable light source (e.g., a light-emitting diode) and measuring the amount of change in light absorption. However, there are usually other light sources (e.g., sunlight, indoor light) in the environment, and the influence of these ambient light sources must be eliminated to ensure the correctness of the aforementioned measurements. A general PPG front-end receiver includes a photo detector (PD) for detecting light energy to generate a current and a transimpedance amplifier (TIA) for converting the current into a voltage for subsequent processing and analysis. Some current PPG front-end receivers claim to eliminate the ambient current caused by ambient light sources, but do not take into account the influence that errors in the estimation of the ambient current can cause (e.g., error causes integral windup of the integrator of the PPG front-end receiver). The cause of the above-mentioned estimation error can be rapid changes in the intensity of the ambient light source. SUMMARY

[0003] One of the purposes of the present disclosure is to provide a photoplethysmography (PPG) front-end receiver to eliminate the estimation error of ambient photocurrent.

[0004] One embodiment of a PPG front-end receiver of the present disclosure includes a current-to-voltage conversion circuit, an integration circuit, a switching circuit, and an analog-to-digital conversion circuit. The current-to-voltage conversion circuit is configured to convert an input current into a differential voltage signal, the current-to-voltage conversion circuit including a positive output configured to output a positive signal of the differential voltage signal and a negative output configured to output a negative signal of the differential voltage signal, the positive signal and the negative signal being complementary signals. The integration circuit includes a positive input and a negative input and is configured to receive the differential voltage signal during a first time duration in which a controllable light source is turned on and receive an inverted version of the differential voltage signal during a second time duration in which the controllable light source is turned off, and then output an analog output voltage based thereon. The switching circuit is coupled between the current-to-voltage conversion circuit and the integration circuit and is configured to forward the positive signal and the negative signal to the positive input and the negative input, respectively, during the first time duration and forward the positive signal and the negative signal to the negative input and the positive input, respectively, during the second time duration, the second time duration being later than or earlier than the first time duration. The analog-to-digital conversion circuit is coupled to the integration circuit and is configured to generate a digital signal based on the analog output voltage during a later time duration, the later time duration being later than each of the second time duration and the first time duration.

[0005] The features, implementations, and effects of the present disclosure are described in detail below with reference to the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0006] Figure 1 An embodiment of a photoplethysmogram (PPG) front-end receiver of the present disclosure is shown;

[0007] Figure 2 An embodiment of a current-to-voltage conversion circuit of the present disclosure is shown; Figure 1

[0008] An embodiment of an ambient light estimation circuit of the present disclosure is shown; Figure 3 Figure 2 An embodiment of an integration circuit of the present disclosure is shown;

[0009] Figure 4 Figure 1 An embodiment of a switching circuit of the present disclosure is shown;

[0010] Figure 5 An embodiment of an input current I Figure 1 IN An example of an input current I

[0011] Figure 6 An example of an input current I Figure 1

[0012] ​​​​Figure 7 This invention discloses another embodiment of the PPG front-end receiver; and

[0013] Figure 8 This illustrates yet another embodiment of the PPG front-end receiver disclosed herein. Detailed Implementation

[0014] This disclosure proposes a photoplethysmography (PPG) front-end receiver that can eliminate estimation errors caused by ambient photocurrent.

[0015] Figure 1 This invention illustrates one embodiment of the PPG front-end receiver disclosed herein. Figure 1 The PPG front-end receiver 100 includes a current-to-voltage conversion circuit 110, an integration circuit 120, a switching circuit 130, and an analog-to-digital conversion circuit 140.

[0016] Please see Figure 1 The current-to-voltage conversion circuit 110 is used to convert the input current I... IN Converted to differential voltage signal (V + V - The current-to-voltage conversion circuit 110 includes a positive output terminal OUT. + With negative output terminal OUT - The positive output terminal OUT + The positive terminal signal V used to output the differential voltage signal + The negative output terminal OUT- is used to output the negative signal V of the differential voltage signal. - The positive terminal signal V + The signal V- at the negative terminal is a complementary signal.

[0017] Figure 2 show Figure 1 One embodiment of the current-to-voltage conversion circuit 110 includes a transimpedance amplifier (TIA) 210 and an ambient light estimation circuit 220. The transimpedance amplifier 210 is used to estimate the ambient light based on the input current I. IN This differential voltage signal is generated. The ambient light estimation circuit 220 uses this differential voltage signal to generate a correction current I. CAL The correction current I CAL Approximately equal to photocurrent I PH Subtract the input current I IN Or I IN ≈I PH -I CALFor example, the ambient light estimation circuit 220 determines the correction current I by sinking current and / or supplying current. CAL The PPG front-end receiver 100 may further include a photodetector 102, such as... Figure 2 As shown, it is used to detect optical energy to generate the photocurrent I. PH Depending on the implementation requirements, the photodetector 102 can be implemented independently of the PPG front-end receiver 100.

[0018] Figure 3 show Figure 2 One embodiment of the ambient light estimation circuit 220 includes a voltage detector 310, an ambient light current estimation circuit 320, and an adjustable current source 330. The voltage detector 310 is used to estimate the differential voltage signal (V) based on the ambient light current estimation circuit 320. + V - Generate detection signal S DET It depends on (e.g., proportional to) the positive terminal signal V of the differential voltage signal. + With negative terminal signal V - The difference. The ambient photocurrent estimation circuit 320 is used to estimate the difference based on the detection signal S. DET Generate estimation signal S EST This is to control the adjustable current source 330. The adjustable current source 330 is used to control the estimated signal S. EST This corrective current I is generated CAL It is worth noting that each of the voltage detector 310, the ambient photocurrent estimation circuit 320, and the adjustable current source 330 can be implemented using known / self-developed technologies; depending on implementation requirements, the voltage detector 310 and the ambient photocurrent estimation circuit 320 can be integrated into a single circuit.

[0019] Please see Figure 1 The integrator circuit 120 includes the positive input terminal IN. + With negative input IN - The positive input terminal IN + With the negative input terminal IN - These are used to receive the positive terminal signal V during the first duration T1. + With the negative terminal signal V - and respectively used to receive the negative terminal signal V during the second duration T2. - With the positive terminal signal V + Then, the integrator circuit 120 outputs an analog output voltage V based on the received signal. o+ V o-In one implementation example, the second time duration T2 is later than the first time duration Tl; for example, the second time duration T2 follows the first time duration Tl, or the second time duration T2 is later than the first time duration Tl by a predetermined time interval. In another implementation example, the first time duration Tl is later than the second time duration T2; for example, the first time duration Tl follows the second time duration T2, or the first time duration Tl is later than the second time duration T2 by a predetermined time interval. The first time duration Tl can be the same as or different from the second time duration T2. It is noted that in the first time duration Tl, the controllable light source (not shown in the figure) is turned on, and thus the differential voltage signal contains the signal originated from the energy of the controllable light source and the signal originated from the energy of the ambient light source; in the second time duration T2, the controllable light source is turned off, and thus the differential voltage signal contains the signal originated from the energy of the ambient light source but does not contain the signal originated from the energy of the controllable light source.

[0020] Figure 4 The display Figure 1 One embodiment of the integrating circuit 120, which is a low-pass filter, contains a resistor 410, a resistor 420 and a capacitor 430, each of which can have a value determined according to the implementation requirement. Since the low-pass filter is a common technique in the art, the details thereof are not described herein. It is noted that other known / internally developed integrating circuits (e.g., other types of low-pass filters) can also be used as the integrating circuit 120 of the present application, provided that they are implemented. Figure 1

[0021] Referring to Figures 1-3 In the first time duration Tl, the aforementioned controllable light source is turned on, and thus the photodetector 102 generates the photocurrent I PH containing the controllable light source current I LED and the actual ambient light current I AMB (I PH_T1 = I LED + I AMB ). In the second time duration, the controllable light source is turned off, and thus the photodetector 102 generates the photocurrent I PH containing the actual ambient light current I AMB but not the controllable light source current I LED (I PH_T2 = I AMB ). The correction current I CAL remains unchanged in the first time duration Tl and the second time duration T2, and is equal to the actual ambient light current I AMB minus the error current I ERR (I CAL ​=I AMB -I ERR The error current can be positive or negative (by supplying / drawing current). During the first duration T1, the input current I... IN Equal to the photocurrent I PH Subtract the correction current I CAL (I IN =I PH_T1 -I CAL That is, the input current I IN Equal to the controllable light source current I LED Add this error current I ERR (I IN =I PH_T1 -I CAL =(I LED +I AMB )-(I AMB -I ERR ) = I LED +I ERR )like Figure 6 As shown, this makes the differential voltage signal dependent on the controllable light source current I. LED With the error current I ERR The sum (I) LED +I ERR During this second duration, the input current I IN Equal to the photocurrent I PH Subtract the correction current I CAL (I IN =I PH_T2 -I CAL That is, the input current is equal to the error current I. ERR (I IN =I PH_T2 -I CAL =(I AMB )-(I AMB -I ERR ) = I ERR )like Figure 6 As shown, this makes the differential voltage signal dependent on the error current I. ERR The integrating circuit 120 receives the differential voltage signal (dependent on I) from the switching circuit 130 during the first duration T1. LED +I ERR During the second duration T2, the switching circuit 130 receives the inverted signal of the differential voltage signal (dependent on -I). ERR To eliminate the error current I ERR The impact caused.

[0022] As stated above. In one practical example, both the first duration T1 and the second duration T2 are later than the preceding duration T0; during the preceding duration T0, the controllable light source is not turned on and the correction current I... CAL No current-to-voltage conversion circuit 110 is supplied; therefore, the photocurrent I is not supplied during the preceding duration T0. PH Includes the actual ambient photocurrent I AMB But it does not include the controllable light source current I. LED The input current I IN Equal to the photocurrent I PH The differential voltage signal contains a signal of energy originating from the ambient light source. The current-to-voltage conversion circuit 110 updates the correction current I based on the differential voltage signal during the preceding duration T0. CAL This makes the correction current I CAL Equal to the actual ambient photocurrent I AMB Subtract the error current I ERR (I CAL =I AMB -I ERR The current-to-voltage conversion circuit 110 provides the correction current I during the first duration T1 and the second duration T2. CAL And the correction current I is not updated. CAL This makes the correction current I CAL The current remains constant during the first duration T1 and the second duration T2. ​​It is worth noting that the current-to-voltage conversion circuit 110 can update the correction current I only during the first duration T0. CAL However, this is not a limitation on the implementation of this invention.

[0023] Please see Figure 1 The switching circuit 130 is coupled between the current-to-voltage conversion circuit 110 and the integrating circuit 120, and is used to switch the positive terminal signal V during the first duration T1. + With the negative terminal signal V - Forward separately to the positive input terminal IN + With the negative input terminal IN - And during the second duration T2, the positive terminal signal V + With the negative terminal signal V - Forward separately to the negative input terminal IN - With the positive input terminal IN + In other words, the switching circuit 130 is used to forward the differential voltage signal to the integrating circuit 120 during the first duration T1, and to forward the differential voltage signal inverted to the integrating circuit 120 during the second duration T2.

[0024] Figure 5Display Figure 1 One embodiment of the switching circuit 130 includes a first positive terminal switch S P1 , a first negative terminal switch S N1 , a second positive terminal switch S P2 , and a second negative terminal switch S N2 . Referring to Figure 1 and Figure 5 , the first positive terminal switch S P1 is coupled between the positive output terminal OUT + of the current-to-voltage conversion circuit 110 and the positive input terminal IN + of the integration circuit 120; the first negative terminal switch S N1 is coupled between the negative output terminal OUT - of the current-to-voltage conversion circuit 110 and the negative input terminal IN P2 of the integration circuit 120; the second positive terminal switch S + is coupled between the positive output terminal OUT - of the current-to-voltage conversion circuit 110 and the negative input terminal IN N2 of the integration circuit 120; and the second negative terminal switch S - is coupled between the negative output terminal OUT + of the current-to-voltage conversion circuit 110 and the positive input terminal IN P1 of the integration circuit 120. In the first time duration T1, the first positive terminal switch S N1 and the first negative terminal switch S P2 are turned on, while the second positive terminal switch S N2 and the second negative terminal switch S P1 are turned off, so as to forward the differential voltage signal to the integration circuit 120. In the second time duration T2, the first positive terminal switch S N1 and the first negative terminal switch S P2 are turned off, while the second positive terminal switch S N2 and the second negative terminal switch S O+ are turned on, so as to forward the inverted differential voltage signal to the integration circuit 120.

[0025] Referring to Figure 1 , the analog-to-digital conversion circuit 140 is used to generate a digital signal D O- in accordance with the analog output voltage V OUTFor analysis purposes. The subsequent duration T3 is later than each of the second duration T2 and the first duration T1; for example, the subsequent duration T3 follows the second duration T2 or the first duration T1, or the subsequent duration T3 is later than the second duration T2 or the first duration T1 by a predetermined time interval. It is worth noting that in the aforementioned preceding duration T0 and the subsequent duration T3, the first positive terminal switch S of the switching circuit 130... P1 First negative terminal switch S N1 Second positive terminal switch S P2 With the second negative terminal switch S N2 None of them conduct; however, this is not a limitation of the invention, provided it is feasible. It is also worth noting that, depending on implementation requirements, the preceding duration T0, the first duration T1, the second duration T2, and the subsequent duration T3 can be repeated periodically or non-periodically. These durations, these switches, and the input current I... IN An example of a relationship Figure 6 As shown, LED OFF This indicates that the aforementioned controllable light source is off, and the LED... ON This indicates that the controllable light source is turned on, S P1_OFF S N1_OFF S P2_OFF and S N2_OFF These represent the first positive terminal switch S. P1 Non-conducting, first negative terminal switch S N1 Non-conducting, second positive terminal switch S P2 Non-conducting and the second negative terminal switch S N2 No conduction, S P1_ON S N1_ON S P2_ON and S N2_ON These represent the first positive terminal switch S. P1 On, first negative terminal switch S N1 On, second positive terminal switch S P2 On and the second negative terminal switch S N2 Conduction.

[0026] Figure 7 This illustrates another embodiment of the PPG front-end receiver disclosed herein. Compared to Figure 1 , Figure 7The PPG front-end receiver 700 further comprises a timing control circuit 710 for controlling the cooperation of all involved circuits in each time duration according to a timing signal (not shown in the figure), e.g. a clock signal. For example, the timing control circuit 710 is used for controlling the operation of the switching circuit 130 in the first time duration Tl and the second time duration T2. For another example, the timing control circuit 710 is further used for enabling the current-to-voltage conversion circuit 110 to update the correction current I CAL For yet another example, the timing control circuit 710 is further used for enabling the analog-to-digital conversion circuit 140 in the subsequent time duration T3, and disabling the analog-to-digital conversion circuit 140 in the preceding time duration TO, the first time duration Tl and the second time duration T2. Since the timing control circuit 710 is a common technique in the art, the details thereof are omitted here.

[0027] Figure 8 Another embodiment of the PPG front-end receiver of the present disclosure is shown. Compared with the embodiment shown in Fig. 7, the PPG front-end receiver 800 further comprises a light source driving circuit 810 for driving the controllable light source. Figure 7 Figure 8 The timing control circuit 710 of the PPG front-end receiver 700 can be used for turning on the light source driving circuit 810 to turn on the controllable light source in the first time duration Tl, and for turning off the light source driving circuit 810 to turn off the controllable light source in the second time duration T2. Since the light source driving circuit 810 is a common technique in the art, the details thereof are omitted here. Figure 8 Since those skilled in the art can understand the implementation details and variations of the present method invention by referring to the disclosure of the aforementioned device invention, i.e. the technical features of the aforementioned device invention can be reasonably applied to the present method invention, therefore, the repeated and redundant descriptions are omitted here without affecting the disclosure requirements and feasibility of the present method invention.

[0028] Please note that, in the implementation as possible, those skilled in the art can selectively implement part or all of the technical features in any of the aforementioned embodiments, or selectively implement a combination of part or all of the technical features in the aforementioned multiple embodiments, thereby increasing the flexibility of the implementation of the present invention.

[0029] In summary, the present invention can eliminate the estimation error of the ambient light current in a simple and effective manner.

[0030]

[0031] ​​Although the present application has been described in terms of the embodiments set forth above, these embodiments are non-limiting and should not be construed to limit the scope of the present application, and changes can be made by those skilled in the art to the technical features of the present application according to the explicit or implicit content of the present application, and any such changes are possible within the scope of the present application, in other words, the scope of the patent protection of the present application should be defined by the patentable scope defined by the patent application of the present specification.

[0032] Symbol explanation

[0033] 100: PPG front-end receiver

[0034] 110: Current-to-voltage conversion circuit

[0035] 120: Integration circuit

[0036] 130: Switching circuit

[0037] 140: Analog-to-digital conversion circuit

[0038] I IN : Input current

[0039] OUT + : Positive output terminal

[0040] OUT - : Negative output terminal

[0041] V + : Positive terminal signal

[0042] V - : Negative terminal signal

[0043] IN + : Positive input terminal

[0044] IN - : Negative input terminal

[0045] V o+ , V o- : Analog output voltage

[0046] D OUT : Digital signal

[0047] 102: Photodetector

[0048] 210: Transimpedance amplifier

[0049] 220: Ambient light estimation circuit

[0050] I CAL : Correction current

[0051] I PH : Photocurrent

[0052] 310: voltage detector

[0053] 320: ambient light current estimation circuit

[0054] 330: adjustable current source

[0055] S DET : detection signal

[0056] S EST : estimation signal

[0057] 410: resistor

[0058] 420: resistor

[0059] 430: capacitor

[0060] S P1 : first positive end switch

[0061] S N1 : first negative end switch

[0062] S P2 : second positive end switch

[0063] S N2 : second negative end switch

[0064] I LED : controllable light source current

[0065] I AMB : actual ambient light current

[0066] I ERR : error current

[0067] LED OFF : controllable light source off

[0068] LED ON : controllable light source on

[0069] S P1_OFF : first positive end switch not conducting

[0070] S N1_OFF : first negative end switch not conducting

[0071] S P2_OFF : second positive end switch not conducting

[0072] S N2_OFF : second negative end switch not conducting

[0073] S P1_ON : first positive end switch conducting

[0074] S N1_ON : first negative end switch conducting

[0075] S P2_ON : second positive end switch on

[0076] S N2_ON : second negative end switch on

[0077] T0: pre-time duration

[0078] T1: first time duration

[0079] T2: second time duration

[0080] T3: post-time duration

[0081] 700: PPG front-end receiver

[0082] 710: timing control circuit

[0083] 800: PPG front-end receiver

[0084] 810: light source driving circuit

Claims

1. A photoplethysmography (PPG) front-end receiver, comprising: a current-to-voltage conversion circuit to convert an input current to a differential voltage signal, the current-to-voltage conversion circuit comprising a positive output to output a positive signal of the differential voltage signal and a negative output to output a negative signal of the differential voltage signal, the positive signal and the negative signal being complementary signals; an integration circuit to receive the differential voltage signal in a first time duration and to receive an inverted version of the differential voltage signal in a second time duration, to output an analog output voltage in response thereto, the integration circuit comprising a positive input and a negative input; a switching circuit coupled between the current-to-voltage conversion circuit and the integration circuit to forward the positive signal and the negative signal to the positive input and the negative input, respectively, in the first time duration and to forward the positive signal and the negative signal to the negative input and the positive input, respectively, in the second time duration, wherein the second time duration is later than or earlier than the first time duration; and an analog-to-digital conversion circuit coupled to the integration circuit to generate a digital signal in response to the analog output voltage in a later time duration, the later time duration being later than each of the second time duration and the first time duration.

2. The PPG front-end receiver of claim 1, wherein the current-to-voltage conversion circuit comprises a transimpedance amplifier to generate the differential voltage signal in response to the input current and an ambient light estimation circuit to generate a correction current in response to the differential voltage signal, the correction current being equal to a photo current minus the input current.

3. The PPG front-end receiver of claim 2, wherein the ambient light estimation circuit comprises a voltage detector to generate a detection signal in response to the differential voltage signal, an ambient light current estimation circuit to generate an estimation signal in response to the detection signal, and an adjustable current source to generate the correction current in response to the estimation signal.

4. The PPG front-end receiver of claim 2, wherein the first time duration and the second time duration are later than a preceding time duration, the current-to-voltage conversion circuit updates the correction current in the preceding time duration, and the current-to-voltage conversion circuit does not update the correction current in the first time duration and the second time duration, such that the correction current remains unchanged in the first time duration and the second time duration.

5. The PPG front-end receiver of claim 4, wherein the switching circuit is not enabled in the preceding time duration.

6. The PPG front-end receiver of claim 2, further comprising a light detector to detect light energy to generate the photo current. a first negative switch coupled between the negative output of the current-to-voltage conversion circuit and the negative input of the integration circuit; 7. The PPG front-end receiver of claim 1, wherein the switching circuit comprises: a first positive terminal switch coupled between the positive output terminal of the current-to-voltage conversion circuit and the positive input terminal of the integration circuit. a second positive switch coupled between the positive output of the current-to-voltage conversion circuit and the negative input of the integration circuit; and a second negative switch coupled between the negative output of the current-to-voltage conversion circuit and the positive input of the integration circuit. ​ 8. The PPG front-end receiver of claim 7, wherein in the first time duration, the first positive switch and the first negative switch are on, and the second positive switch and the second negative switch are off; in the second time duration, the first positive switch and the first negative switch are off, and the second positive switch and the second negative switch are on; and in the later time duration, the first positive switch and the first negative switch are off, and the second positive switch and the second negative switch are off.

9. The PPG front-end receiver of claim 2, further comprising a timing control circuit for controlling the operation of the switching circuit, and for enabling the analog-to-digital conversion circuit in the later time duration.

10. The PPG front-end receiver of claim 9, wherein the timing control circuit is further for causing the current-to-voltage conversion circuit to update the correction current in a preceding time duration, the preceding time duration being earlier than each of the first time duration and the second time duration.

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