CN115900940A - Front end receiver for photo-volume change tracing method - Google Patents

Abstract

The present disclosure relates to photoplethysmography front end receivers. The receiver can eliminate estimation errors caused by ambient photocurrent. The receiver comprises 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 the input current into a differential voltage signal. The integrating circuit receives the differential voltage signal and outputs an analog output voltage accordingly. 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 the inverse of the differential voltage signal to the integration circuit for a second duration, wherein the controllable light source is turned on for the first duration, and turned off for the second duration, and the second duration follows the first duration or follows the second duration by the first duration. The analog-to-digital conversion circuit generates a digital signal for analysis according to the analog output voltage after the second duration.

Description

Front end receiver for photo-volume change tracing method

Technical Field

The present invention relates to front end receivers, and more particularly to photoplethysmography front end receivers.

Background

Photoplethysmography (PPG) techniques may be used for a variety of applications (e.g., heart rhythm and blood oxygenation) by illuminating 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 other sources of light in the environment (e.g., sunlight, room lights) whose effects must be eliminated to ensure the accuracy of the aforementioned measurements. The front end receiver of the PPG technique typically 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 ambient current (ambient current) caused by ambient light sources, but do not take into account the effects that errors in the estimation of the ambient current may cause (e.g., errors cause the integral saturation (integral windup) of the integrator of the PPG front end receiver). The estimation error may be caused by rapid ambient light intensity changes.

Disclosure of Invention

It is an objective of the present disclosure to provide a Photoplethysmography (PPG) front end receiver to eliminate the error in estimating the ambient photocurrent.

One embodiment of the 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 used for converting an input current into a differential voltage signal, and comprises a positive output end and a negative output end, wherein the positive output end is used for outputting a positive end signal of the differential voltage signal, the negative output end is used for outputting a negative end signal of the differential voltage signal, and the positive end signal and the negative end signal are complementary signals. The integrator circuit comprises a positive input terminal and a negative input terminal, and is configured to receive the differential voltage signal for a first duration in which the controllable light source is turned on and receive an inverse of the differential voltage signal for a second duration in which the controllable light source is turned off, and then output an analog output voltage accordingly. The switching circuit is coupled between the current-to-voltage conversion circuit and the integrating circuit, and is configured to forward the positive end signal and the negative end signal to the positive input terminal and the negative input terminal respectively in the first duration, and forward the positive end signal and the negative end signal to the negative input terminal and the positive input terminal respectively in the second duration, wherein the second duration is later or earlier than the first duration. The analog-to-digital conversion circuit is coupled to the integration circuit and configured to generate a digital signal for analysis according to the analog output voltage in a later duration that is later than each of the second duration and the first duration.

The features, operation and efficacy of the present invention will be described in detail with reference to the drawings.

Drawings

FIG. 1 illustrates one embodiment of a photoplethysmography (PPG) front end receiver of the present disclosure;

FIG. 2 illustrates one embodiment of the current-to-voltage conversion circuit of FIG. 1;

FIG. 3 shows one embodiment of the ambient light estimation circuit of FIG. 2;

FIG. 4 shows one embodiment of the integration circuit of FIG. 1;

FIG. 5 shows one embodiment of the switching circuit of FIG. 1;

FIG. 6 shows the input current I of FIG. 1 _IN An example of (a);

fig. 7 shows another embodiment of a PPG front end receiver of the present disclosure; and

fig. 8 shows yet another embodiment of the PPG front end receiver of the present disclosure.

Detailed Description

The disclosure provides a photo-plethysmography (PPG) front end receiver capable of eliminating estimation error caused by ambient photocurrent.

Fig. 1 shows an embodiment of a PPG front end receiver of the present disclosure. The PPG front end receiver 100 of fig. 1 comprises a current-to-voltage conversion circuit 110, an integration circuit 120, a switching circuit 130, and an analog-to-digital conversion circuit 140.

Please refer to fig. 1. The current-to-voltage conversion circuit 110 is used to convert the input current I _IN Conversion to differential voltage signal (V) ₊ 、V _- ). Electric currentThe voltage-to-voltage conversion circuit 110 comprises a positive output terminal OUT ₊ And a negative output terminal OUT _- The positive output terminal OUT ₊ A positive terminal signal V for outputting the differential voltage signal ₊ The negative output terminal OUT-is used for outputting a negative terminal signal V of the differential voltage signal _- . The positive terminal signal V ₊ And the negative side signal V-is a complementary signal (complementary signals).

Fig. 2 shows an embodiment of the current-to-voltage conversion circuit 110 of fig. 1, which includes a transimpedance amplifier (TIA) 210 and an ambient light estimation circuit 220. The transimpedance amplifier 210 is used for determining the input current I _IN The differential voltage signal is generated. The ambient light estimation circuit 220 is used for generating a correction current I according to the differential voltage signal _CAL Wherein the correction current I _CAL Approximately equal to photocurrent I _PH Subtracting the input current I _IN Or said I _IN ≈I _PH -I _CAL (ii) a For example, the ambient light estimation circuit 220 determines the calibration current I by drawing (sink) current and/or supplying (source) current _CAL . The PPG front-end receiver 100 may further comprise a photodetector 102, as shown in fig. 2, for detecting optical energy (optical energy) to generate the photocurrent I _PH . Depending on implementation requirements, the photodetector 102 may be implemented independently from the PPG front end receiver 100.

Fig. 3 shows an embodiment of the ambient light estimation circuit 220 of fig. 2, which includes a voltage detector 310, an ambient light current estimation circuit 320, and an adjustable current source 330. The voltage detector 310 is used for detecting the differential voltage signal (V) ₊ 、V _- ) Generating a detection signal S _DET Dependent on (e.g., proportional to) the positive side signal V of the differential voltage signal ₊ And a negative terminal signal V _- The difference of (a). The ambient light current estimation circuit 320 is used for estimating the ambient light current according to the detection signal S _DET Generating an estimated signal S _EST To control the adjustable current source 330. The adjustable current source 330 is used for estimating the signal S according to the signal S _EST Generating the correction current I _CAL . It is noted that each of the voltage detector 310, the ambient light current estimation circuit 320 and the adjustable current source 330 may be implemented by conventional/self-developed methodsThe technology is realized; depending on the implementation requirement, the voltage detector 310 and the ambient photocurrent estimation circuit 320 may be integrated into a single circuit.

Please refer to fig. 1. The integrator circuit 120 includes a positive input IN ₊ And a negative input terminal IN _- The positive input terminal IN ₊ And the negative input terminal IN _- Respectively for a first duration T ₁ Receives the positive terminal signal V ₊ With the negative terminal signal V _- And respectively for a second duration T ₂ Receives the negative terminal signal V _- And the positive terminal signal V ₊ Then, the integration circuit 120 outputs an analog output voltage V according to the received signal _o+ 、V _o- . In one implementation example, the second duration T ₂ Later than the first duration T ₁ (ii) a For example, the second duration T ₂ Following the first duration T ₁ Or the second duration T ₂ Later than the first duration T ₁ For a predetermined time interval. In another example of implementation, the first duration T ₁ Later than the second duration T ₂ (ii) a For example, the first duration T ₁ Following the second duration T ₂ Or the first duration T ₁ Later than the second duration T ₂ For a predetermined time interval. The first duration T ₁ May be associated with a second duration T ₂ The same or different. Notably, during the first duration T ₁ A controllable light source (e.g., a light emitting diode) (not shown) is turned on, and thus, the differential voltage signal includes a signal derived from the energy of the controllable light source and a signal derived from the energy of the ambient light source; in the second duration T ₂ Whereby the differential voltage signal comprises a signal derived from the energy of the ambient light source but not from the energy of the controllable light source.

Fig. 4 shows an embodiment of the integration circuit 120 of fig. 1, which is a low pass filter including a resistor 410, a resistor 420 and a capacitor 430, wherein the value of each of the resistor 410, the resistor 420 and the capacitor 430 may be determined according to the implementation requirement. Since the low pass filter is a common technique in the art, its details are not described herein. It is noted that other known/self-developed integration circuits (e.g., other types of low-pass filters) may be used as the integration circuit 120 of fig. 1, where an implementation is feasible.

Please refer to fig. 1-3. During the first duration T ₁ The controllable light source is turned on, so that the photo-detector 102 generates the photocurrent I _PH Comprising a controllable light source current I _LED With the actual ambient photocurrent I _AMB (I _{PH_T1} ＝I _LED +I _AMB ). During the second duration, the controllable light source is turned off, so that the photo-detector 102 generates the photocurrent I _PH Including the actual ambient photocurrent I _AMB But does not contain the controllable light source current I _LED (I _{PH_T2} ＝I _AMB ). The correction current I _CAL During the first duration T ₁ And the second duration T ₂ Is constant and equal to the actual ambient photocurrent I _AMB Minus an error current I _ERR (I _CAL ＝I _AMB -I _ERR ) Wherein the value of the error current may be positive or negative (by supplying/drawing current). At the first duration T ₁ In the input current I _IN Is equal to the photocurrent I _PH Subtracting the correction current I _CAL (I _IN ＝I _{PH_T1} -I _CAL ) I.e. the input current I _IN Equal to the controllable light source current I _LED Plus the error current I _ERR (I _IN ＝I _{PH_T1} -I _CAL ＝(I _LED +I _AMB )-(I _AMB -I _ERR )＝I _LED +I _ERR ) This makes the differential voltage signal dependent on the controllable light source current I as shown in FIG. 6 _LED With the error current I _ERR And (I) _LED +I _ERR ). In the second duration, the input current I _IN Is equal to the photocurrent I _PH Subtracting the correction current I _CAL (I _IN ＝I _{PH_T2} -I _CAL ) I.e. 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 ) This makes the differential voltage signal dependent on the error current I as shown in FIG. 6 _ERR . The integration circuit 120 is in the first duration T ₁ Receives the differential voltage signal (dependent on I) from the switching circuit 130 _LED +I _ERR ) And during the second duration T ₂ Receives an inverted signal (dependent on-I) of the differential voltage signal from the switching circuit 130 _ERR ) To eliminate the error current I _ERR The resulting effect.

The above description is provided. In one implementation, the first duration T ₁ And the second duration T ₂ Are all later than the previous duration T ₀ (ii) a In the preceding time period T ₀ Wherein the controllable light source is not turned on and the correction current I _CAL Is not provided to the current-to-voltage conversion circuit 110; therefore, in the preceding duration T ₀ In the photocurrent I _PH Including the actual ambient photocurrent I _AMB But does not contain the controllable light source current I _LED The input current I _IN Is equal to the photocurrent I _PH The differential voltage signal comprises a signal derived from the energy of the ambient light source. The current-to-voltage conversion circuit 110 is in the preceding duration T ₀ Wherein the correction current I is updated according to the differential voltage signal _CAL So as to correct the current I _CAL Is equal to the actual ambient photocurrent I _AMB Subtracting the error current I _ERR (I _CAL ＝I _AMB -I _ERR ). The current-to-voltage conversion circuit 110 is in the first duration T ₁ And the second duration T ₂ In which the correction current I is provided _CAL Without updating the correction current I _CAL So that the correction current I _CAL During the first duration T ₁ And the second duration T ₂ Is kept constant. It is noted that the current-to-voltage conversion circuit 110 may be only during the preceding duration T ₀ In updating the correction current I _CAL However, this is not a limitation of the present invention.

Please refer to fig. 1. Switching circuit 130Connected between the current-to-voltage conversion circuit 110 and the integration circuit 120 for the first duration T ₁ The positive terminal signal V ₊ With the negative terminal signal V _- Respectively forwarded to the positive input terminal IN ₊ And the negative input terminal IN _- And in the second duration T ₂ The positive terminal signal V ₊ With the negative end signal V _- Respectively forwarded to the negative input IN _- And the positive input terminal IN ₊ (ii) a In other words, the switching circuit 130 is used for the first duration T ₁ Forwards the differential voltage signal to the integrating circuit 120, and for the second duration T ₂ The inverse of the differential voltage signal is forwarded to the integrating circuit 120.

FIG. 5 shows an embodiment of the switching circuit 130 of FIG. 1, including a first positive side switch S _P1 A first negative terminal switch S _N1 A second positive terminal switch S _P2 And a second negative side switch S _N2 . Referring to fig. 1 and 5, the first positive terminal switch S _P1 The positive output terminal OUT coupled to the current-to-voltage conversion circuit 110 ₊ And the positive input terminal IN of the integrating circuit 120 ₊ To (c) to (d); first negative side switch S _N1 The negative output terminal OUT coupled to the current-to-voltage conversion circuit 110 _- And the negative input terminal IN _ of the integrating circuit 120; second positive side switch S _P2 The positive output terminal OUT coupled to the current-to-voltage conversion circuit 110 ₊ And the negative input terminal IN of the integrating circuit 120 _- To (c) to (d); second negative side switch S _N2 The negative output terminal OUT coupled to the current-to-voltage conversion circuit 110 _- And the positive input terminal IN of the integrating circuit 120 ₊ In the meantime. At the first duration T ₁ The first positive terminal switch S _P1 And the first negative terminal switch S _N1 Is turned on and the second positive side switch S _P2 And the second negative terminal switch S _N2 Is non-conductive and thus forwards the differential voltage signal to the integrating circuit 120. In the second duration T ₂ The first positive terminal switch S _P1 And the first negative terminal switch S _N1 Is not conducted, and the second positive side switch S _P2 And the second negative terminal switch S _N2 Conducting and forwarding the inverse of the differential voltage signal to the integrating circuit 120.

Please refer to fig. 1. The analog-to-digital conversion circuit 140 is used for the following duration T ₃ According to the analog output voltage V _O+ 、V _O- Generating a digital signal D _OUT For analysis. The post-duration T ₃ Later than the second duration T ₂ And the first duration T ₁ Each of (a); for example, the post-duration T ₃ Following the second duration T ₂ Or the first duration T ₁ Or the post-duration T ₃ Later than the second duration T ₂ Or the first duration T ₁ For a predetermined time interval. It is noted that the preceding duration T is ₀ And the post-duration T ₃ In, the first positive side switch S of the switching circuit 130 _P1 A first negative terminal switch S _N1 A second positive terminal switch S _P2 And a second negative terminal switch S _N2 Are not conducted; however, this is not a practical limitation of the present invention, provided that implementation is possible. It is also noted that the preceding duration T may be varied depending on implementation requirements ₀ The first duration T ₁ The second duration T ₂ And the following duration T ₃ May be repeated periodically or aperiodically. The durations, the switches and the input current I _IN An example of the relationship therebetween is shown in FIG. 6, in which the LEDs _OFF Indicating that the controllable light source is off, LED _ON Indicating that the controllable light source is on, S _{P1_OFF} 、S _{N1_OFF} 、S _{P2_OFF} And S _{N2_OFF} Respectively, a first positive side switch S _P1 Non-conductive, first negative side switch S _N1 Non-conductive, second positive terminal switch S _P2 Non-conducting and second negative side switch S _N2 Is not conducted, S _{P1_ON} 、S _{N1_ON} 、S _{P2_ON} And S _{N2_ON} Respectively, a first positive side switch S _P1 Conducting, first negative terminal switch S _N1 Conducting second positive terminal switch S _P2 Conducting and a second negative side switch S _N2 And conducting.

Fig. 7 shows another embodiment of the PPG front end receiver of the present disclosure. Compared to fig. 1, the PPG front end receiver 700 of fig. 7 further includes a timing control circuit 710 for controlling the cooperation of the correlation circuits (clock circuits) in each duration according to a timing signal (not shown), such as a clock signal. For example, the timing control circuit 710 is used to control the switching circuit 130 for the first duration T ₁ And the second duration T ₂ The operation of (1). For another example, the timing control circuit 710 is further configured to determine the previous duration T ₀ The intermediate current-to-voltage conversion circuit 110 updates the correction current I _CAL . For another example, the timing control circuit 710 is further configured to determine the post-duration T ₃ Enables the analog-to-digital conversion circuit 140 and for the preceding duration T ₀ The first duration T ₁ And the second duration T ₂ The analog-to-digital conversion circuit 140 is disabled. Since the timing control circuit 710 alone is a common technique in the art, the details thereof are omitted here.

Fig. 8 shows yet another embodiment of the PPG front end receiver of the present disclosure. Compared to fig. 7, the PPG front end receiver 800 of fig. 8 further comprises a light source driving circuit 810 for driving the aforementioned controllable light source. The timing control circuit 710 of FIG. 8 can be used for the first duration T ₁ The light source driving circuit 810 is enabled to turn on the controllable light source for the second duration T ₂ The light source driving circuit 810 turns off the controllable light source. Since the light source driving circuit 810 alone is a common technique in the art, the details thereof are omitted herein.

Since the detailed implementation and variations of the present invention can be understood by those skilled in the art with reference to the disclosure of the device invention disclosed in the foregoing, the technical features of the device invention can be reasonably applied to the present invention, and therefore, the repeated and redundant descriptions will be omitted herein without affecting the disclosure requirement and the feasibility of the present invention.

It should be noted that, when the implementation is possible, a person skilled in the art can selectively implement some or all of the technical features of any of the above embodiments, or selectively implement a combination of some or all of the technical features of the above embodiments, thereby increasing the flexibility in implementing the invention.

In summary, the present invention can eliminate the estimation error of the ambient photocurrent in a simple and effective manner.

Although the embodiments of the present invention have been described above, these embodiments are not intended to limit the present invention, and those skilled in the art can make variations on the technical features of the present invention according to the explicit or implicit contents of the present invention, and all such variations may fall within the scope of the patent protection sought by the present invention.

Description of the symbols

100: PPG front end receiver

110: current-to-voltage conversion circuit

120: integrating circuit

130: switching circuit

140: analog-to-digital conversion circuit

I _IN : input current

OUT ₊ : positive output terminal

OUT _- : negative output end

V ₊ : positive terminal signal

V _- : negative side signal

IN ₊ : positive input end

IN _- : negative input terminal

V _o+ 、V _o- : analog output voltage

D _OUT : digital signal

102: photodetector

210: trans-impedance amplifier

220: ambient light estimation circuit

I _CAL : correcting current

I _PH : photocurrent of light

310: voltage detector

320: ambient photocurrent estimation circuit

330: adjustable current source

S _DET : detecting the signal

S _EST : estimating a signal

410: electric resistance

420: electric resistance

430: capacitor with a capacitor element

S _P1 : first positive terminal switch

S _N1 : first negative terminal switch

S _P2 : second positive side switch

S _N2 : second negative terminal switch

I _LED : controllable light source current

I _AMB : actual ambient photocurrent

I _ERR : error current

LED _OFF : controllable light source turn-off

LED _ON : controllable light source turn-on

S _{P1_OFF} : the first positive side switch is non-conductive

S _{N1_OFF} : the first negative side switch is not conductive

S _{P2_OFF} : the second positive side switch is non-conductive

S _{N2_OFF} : the second negative side switch is not conducted

S _{P1_ON} : the first positive side switch is turned on

S _{N1_ON} : first negative side switch conducting

S _{P2_ON} : the second positive side switch is turned on

S _{N2_ON} : second negative side switch conducting

T ₀ : preceding duration

T ₁ : first duration

T ₂ : for a second duration

T ₃ : at a later time

700: PPG front end receiver

710: sequential control circuit

800: PPG front end receiver

810: light source driving circuit

Claims (10)

1. A photoplethysmography PPG front end receiver, comprising:

a current-to-voltage conversion circuit for converting an input current into a differential voltage signal, the current-to-voltage conversion circuit comprising a positive output terminal for outputting a positive end signal of the differential voltage signal and a negative output terminal for outputting a negative end signal of the differential voltage signal, the positive end signal and the negative end signal being complementary signals;

an integrating circuit for receiving the differential voltage signal for a first duration and receiving an inverted version of the differential voltage signal for a second duration to output an analog output voltage, the integrating circuit comprising a positive input terminal and a negative input terminal;

a switching circuit, coupled between the current-to-voltage conversion circuit and the integrating circuit, for forwarding the positive side signal and the negative side signal to the positive input terminal and the negative input terminal, respectively, during the first duration, and forwarding the positive side signal and the negative side signal to the negative input terminal and the positive input terminal, respectively, during the second duration, wherein the second duration is later or earlier than the first duration; and

an analog-to-digital conversion circuit coupled to the integration circuit for generating a digital signal according to the analog output voltage in a later duration that is later than each of the second duration and the first duration.

2. The PPG front-end receiver of claim 1, wherein the current-to-voltage conversion circuit comprises: a trans-impedance amplifier for generating the differential voltage signal according to the input current; and an ambient light estimation circuit for generating a correction current according to the differential voltage signal, wherein the correction current is equal to a photocurrent minus the input current.

3. The PPG front end receiver of claim 2, wherein the ambient light estimation circuit comprises: a voltage detector for generating a detection signal according to the differential voltage signal; an ambient photocurrent estimation circuit for generating an estimation signal according to the detection signal; and an adjustable current source for generating the correction current according to the estimation signal.

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

5. The PPG front end receiver of claim 2, wherein the switching circuit is non-conductive for the preceding time duration.

6. The PPG front end receiver of claim 2, further comprising: a photo detector for detecting light energy to generate the photo current.

7. The PPG front-end receiver of claim 1, wherein the switching circuit comprises: a first positive side switch coupled between the positive output terminal of the current-to-voltage conversion circuit and the positive input terminal of the integration circuit; a first negative side switch coupled between the negative output terminal of the current-to-voltage conversion circuit and the negative input terminal of the integration circuit; a second positive side switch coupled between the positive output terminal of the current-to-voltage conversion circuit and the negative input terminal of the integration circuit; and a second negative side switch coupled between the negative output terminal of the current-to-voltage conversion circuit and the positive input terminal of the integration circuit.

8. The PPG front end receiver of claim 7, wherein in the first duration, the first positive side switch and the first negative side switch are conductive and the second positive side switch and the second negative side switch are non-conductive; during the second duration, the first positive side switch is not in conduction with the first negative side switch, and the second positive side switch is in conduction with the second negative side switch; during the post-duration, the first positive side switch is not conductive with the first negative side switch, and the second positive side switch is not conductive with the second negative side switch.

9. The PPG front-end receiver of claim 1, further comprising: and a timing control circuit for controlling the operation of the switching circuit and enabling the analog-to-digital conversion circuit for the later duration.

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

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