CN114877993A - Ambient light detection circuit, ambient light detection method and photoplethysmography measurement device - Google Patents
Ambient light detection circuit, ambient light detection method and photoplethysmography measurement device Download PDFInfo
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- G01J1/00—Photometry, e.g. photographic exposure meter
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Abstract
The embodiment of the application discloses an ambient light detection circuit and method and a photoplethysmography measuring device. The ambient light detection circuit comprises a current generation circuit, a current-voltage conversion circuit, an analog-to-digital conversion circuit and a code value processing circuit, wherein the current generation circuit is used for generating reduction current; the current-voltage conversion circuit is used for connecting the photoelectric sensor, receiving the photocurrent output by the photoelectric sensor, amplifying the difference value between the photocurrent and the reduced current and converting the difference value into a voltage signal; the analog-to-digital conversion circuit is used for periodically converting the voltage signal into a voltage code value; the code value processing circuit is used for calculating to obtain an ambient light pre-estimated value corresponding to each detection period according to at least one first ambient light voltage code value and at least one second ambient light voltage code value. The ambient light can be accurately estimated by adopting the ambient light detection circuit.
Description
Technical Field
And more particularly, to an ambient light detection circuit, method, and photoplethysmography apparatus.
Background
With the improvement of living standard, people are more concerned about their physical condition. The physical condition can be generally characterized by physiological parameters of the human body, and the physiological parameters can include parameters such as blood pressure, heart rate, pulse and the like.
Currently, pulse measurement is performed based on the principle that oxygenated hemoglobin and reduced hemoglobin affect the transmittance of a medium to be measured (such as a finger or a toe) to infrared light, and the specific detection process is to use an LED to emit infrared light with a certain light intensity onto the medium to be measured (such as a human body), and then detect the current change caused by the infrared light in the pulse beating process through a PD (Photo diode) to detect the pulse. In the related art, it is necessary to detect and eliminate the influence of ambient light on the current, and only the current increment caused by the reflected light of the LED is reserved. The current mainstream technology is to detect the current increment caused by the ambient light in the ambient light phase (i.e. when the LED does not emit light); a current of a similar magnitude is generated by a current source during the LED lighting phase to offset the current increase caused by ambient light.
However, since the ambient light usually changes with time and the ambient light phase and the LED light-emitting phase have a temporal difference, which causes a deviation in the ambient light estimation and thus a measurement deviation, how to accurately estimate the ambient light is an urgent technical problem to be solved.
Disclosure of Invention
The embodiment of the application provides an ambient light detection circuit and method and a photoplethysmography device, which can accurately estimate ambient light.
In a first aspect, an embodiment of the present application provides an ambient light detection circuit, which includes a current generation circuit, a current-voltage conversion circuit, an analog-to-digital conversion circuit, and a code value processing circuit. A current generating circuit for generating a clipping current; the current-voltage conversion circuit is used for connecting the photoelectric sensor, receiving the photocurrent output by the photoelectric sensor, and amplifying and converting the difference value of the photocurrent and the reduction current into a voltage signal; the analog-to-digital conversion circuit is used for periodically converting the voltage signal into voltage code values, wherein each detection period comprises a first ambient light phase before light source lighting and a second ambient light phase after the light source lighting, and the voltage code values obtained by conversion in each detection period comprise a first ambient light voltage code value corresponding to the first ambient light phase and a second ambient light voltage code value corresponding to the second ambient light phase; and the code value processing circuit is used for calculating an ambient light pre-estimated value corresponding to each detection period according to at least one first ambient light voltage code value and at least one second ambient light voltage code value.
In a second aspect, the embodiment of the present application further provides a photoplethysmography measurement apparatus, which includes a light source, a photoelectric sensor and the above-mentioned ambient light detection circuit, wherein the processor is connected to the light source and the ambient light detection circuit respectively, and the photoelectric sensor is configured to generate a photocurrent when detecting light emitted by the light source and transmit the photocurrent to the ambient light detection circuit.
In a third aspect, an embodiment of the present application further provides an ambient light detection method, including: acquiring a reduction current generated by a current generating circuit and a photocurrent output by a photoelectric sensor; amplifying the difference value of the photocurrent and the reduction current and converting the difference value into a voltage signal; periodically converting a voltage signal into voltage code values, wherein each detection period comprises a first ambient light phase before light source lighting and a second ambient light phase after light source lighting, and the voltage code values obtained by conversion in each detection period comprise a first ambient light voltage code value corresponding to the first ambient light phase and a second ambient light voltage code value corresponding to the second ambient light phase; and calculating to obtain an ambient light pre-estimation value corresponding to each detection period according to at least one first ambient light voltage code value and at least one second ambient light voltage code value.
The ambient light detection circuit comprises a current generation circuit, a current-voltage conversion circuit, an analog-to-digital conversion circuit and a code value processing circuit, wherein the current generation circuit is used for generating reduction current; the current-voltage conversion circuit is used for connecting the photoelectric sensor, receiving the photocurrent output by the photoelectric sensor, amplifying the difference value between the photocurrent and the reduced current and converting the difference value into a voltage signal; the analog-to-digital conversion circuit is used for periodically converting a voltage signal into voltage code values, wherein each detection period comprises a first ambient light phase before light source light emitting and a second ambient light phase after light source light emitting, and the voltage code values obtained by conversion in each detection period comprise a first ambient light voltage code value corresponding to the first ambient light phase and a second ambient light voltage code value corresponding to the second ambient light phase; the code value processing circuit is used for calculating to obtain an ambient light pre-estimated value corresponding to each detection period according to at least one first ambient light voltage code value and at least one second ambient light voltage code value. By adopting the ambient light detection circuit, the deviation of ambient light estimation caused by the change of ambient light along with time can be effectively avoided, and the accuracy of ambient light estimation is effectively guaranteed.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
Fig. 1 shows a connection block diagram of an ambient light detection circuit provided in an embodiment of the present application;
FIG. 2 is a circuit diagram of an ambient light detection circuit according to an embodiment of the present disclosure;
FIG. 3 is a schematic phase diagram of two adjacent detection periods according to an embodiment of the present application;
FIG. 4 is a schematic phase diagram of two adjacent detection periods provided in the embodiment of the present application;
fig. 5 illustrates an ambient light prediction result deviation obtained by performing first-order prediction by using an ambient light detection circuit according to an embodiment of the present application;
fig. 6 illustrates an ambient light prediction result deviation obtained by performing second-order prediction by using an ambient light detection circuit according to an embodiment of the present application;
fig. 7 shows a schematic flowchart of an ambient light detection method provided in an embodiment of the present application.
Detailed Description
Reference will now be made in detail to embodiments of the present application, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the accompanying drawings are exemplary only for the purpose of explaining the present application and are not to be construed as limiting the present application.
In order to make the technical solutions of the present application better understood, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only a part of the embodiments of the present application, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.
The present application is described in detail below. Referring to fig. 1 and 2, an ambient light detection circuit 10 according to an embodiment of the present disclosure includes a current generation circuit 110, a current-to-voltage conversion circuit 120, an analog-to-digital conversion circuit 130, and a code value processing circuit 140.
A current generation circuit 110 for generating a clipping current; a current-voltage conversion circuit 120, configured to connect to the photo sensor 20, receive the photocurrent output by the photo sensor 20, and amplify and convert a difference between the photocurrent and the subtracted current into a voltage signal; the analog-to-digital conversion circuit 130 is configured to periodically convert the voltage signal into voltage code values, where each detection period includes a first ambient light phase before the light source emits light and a second ambient light phase after the light source emits light, and the voltage code value converted in each detection period includes a first ambient light voltage value corresponding to the first ambient light phase and a second ambient light voltage value corresponding to the second ambient light phase; and a code value processing circuit 140, configured to calculate an ambient light pre-estimated value corresponding to each detection period according to at least one of the first ambient light voltage code values and at least one of the second ambient light voltage code values.
In the embodiment of the present application, the time segment during which the light source does not emit light in each detection period is referred to as an ambient light phase, and the time segment during which the light source emits light is referred to as a measurement phase. The environment light phase comprises a first environment light phase before the light source emits light and a second environment light phase after the light source emits light. According to the first ambient light voltage code value obtained through the first ambient light phase conversion and the second ambient light voltage code value obtained through the second ambient light phase conversion, the change condition of ambient light along with time can be accurately obtained in real time, so that the ambient light estimated value in each detection period is calculated according to the change condition of the ambient light along with time, the accuracy of the ambient light estimated value is improved, the influence of the ambient light can be accurately eliminated from the measured value in the subsequent measurement, and the accuracy of the measurement result is improved.
In some embodiments, the current generating circuit 110 may be any circuit capable of generating a current, such as a current mode analog-to-digital converter, a current mirror, an op-amp current source, or a combination thereof.
In an embodiment of the present application, the current generating circuit 110 may employ a current analog-to-digital converter, which not only generates the trimming current, but also sets the value of the trimming current as required.
In an implementation manner of the present application, the current generating circuit 110 may adopt a combination of an operational amplifier current source and a current mirror, a reference current may be generated by the operational amplifier current source, and the reference current may be replicated by the current mirror, the current mirror may have a plurality of output branches, and the magnitude of the reduced current may be controlled by controlling the conducting number of the output branches.
The number of the photosensors 20 may be one or more. When the number of the photosensors 20 is plural, the plural photosensors 20 may be connected with the current-voltage conversion circuit 120 through a multiplexer to selectively connect at least one of the plural photosensors 20 with the current-voltage converter. The plurality of photoelectric sensors 20 may be connected in parallel and then connected to a current-voltage converter, and may be set according to actual requirements.
In one possible embodiment of the present application, the number of the photosensors 20 is plural, and the plural photosensors 20 are connected to the current-voltage conversion circuit 120 through a multiplexer.
The current-voltage conversion circuit 120 may include one or more of an amplifier, a resistor, a capacitor, an inductor, and the like, and the number of each of the electrical components may be one or more.
In one possible embodiment of the present application, as shown in fig. 2, the current-voltage conversion circuit 120 includes a transimpedance amplification circuit 122 and a voltage sampling circuit 124, an input terminal of the transimpedance amplification circuit is connected to the current generation circuit 110 and is used for connecting the photosensor 20, and an output terminal of the transimpedance amplification circuit 122 is connected to an input terminal of the voltage sampling circuit 124; the output terminal of the voltage sampling circuit 124 is connected to the input terminal of the analog-to-digital conversion circuit 130.
In an implementation manner of the present application, the transimpedance amplifier circuit in the current-voltage conversion circuit 120 specifically includes an operational amplifier CF, a first capacitor C1, a second capacitor C2, a first resistor R1, and a second resistor R2, the current generation circuit 110 includes a first current source 112 and a second current source 114, the first capacitor C1 is connected between a non-inverting input terminal and a first output terminal of the amplifier CF, a first resistor R1 is connected between the non-inverting input terminal and a first output terminal of the amplifier CF, the second capacitor C2 is connected between the inverting input terminal and a second output terminal of the amplifier CF, the second resistor R2 is connected between the inverting input terminal and a second output terminal of the amplifier CF, a first terminal of the first current analog-to-digital converter is connected to a power supply, a second terminal of the first current analog-to-digital converter is connected to the non-inverting input terminal of the amplifier CF, the first end of the second current analog-to-digital converter is connected with the inverting input end of the amplifier CF, and the second end is grounded. As an example, the first current source 112 and the second current source 114 may be current digital-to-analog converters.
Through the above circuit arrangement, when the two ports of the photosensor 20 respectively output photocurrents to the amplifier CF, that is, when the first end of the photosensor 20 inputs a first photocurrent to the non-inverting input terminal of the amplifier CF and the second end of the photosensor 20 inputs a second photocurrent to the inverting input terminal of the amplifier CF, the first current analog-to-digital converter may input a first subtraction current to the non-inverting input terminal of the amplifier CF and the second current analog-to-digital converter may input a second subtraction current to the inverting input terminal of the amplifier CF, so that the amplifier CF amplifies a difference value between the first photocurrent and the first subtraction current and converts the difference value into a first voltage signal and amplifies a difference value between the second photocurrent and the second subtraction current and converts the difference value into a second voltage signal. As an embodiment, the non-inverting input and the inverting input of the amplifier CF are further inputted with a common mode voltage VCM, so as to enable the light source in the device to achieve normal deflection (operation) when the ambient light detection circuit is applied to the device.
Alternatively, the voltage sampling circuit 124 may be a switched capacitor sampling circuit, or may be a sampling circuit composed of a voltage follower formed by an operational amplifier, a level-up circuit, a protection circuit, and the like, as long as the sampling circuit can sample the output voltage of the transimpedance amplifier circuit 122, which is not specifically limited in the embodiment of the present application. In one embodiment, the voltage sampling circuit 124 may further include a filter circuit, which may filter the sampled voltage signal.
In one embodiment of the present application, the current-voltage conversion circuit 120 further includes a buffer 126, and the buffer 126 is connected between the output terminal of the voltage sampling circuit 124 and the input terminal of the analog-to-digital conversion circuit 130. The buffer 126 has the characteristics of infinite input impedance and very small output impedance, so that a signal received by a subsequent stage circuit can be maximally close to a signal output by the voltage sampling circuit.
Illustratively, since the environment detection circuit is usually applied in a detection device, such as a pulse wave measurement device, such a device is usually additionally provided with a light source, accordingly, the photocurrent output by the ambient light photoelectric sensor 20 is usually reflected by the ambient light IAMB and the LEDILED. The subtraction current generated by the current analog-to-digital converter is denoted as Iof, the common-mode voltage of the amplifier CF is denoted as VCM, and the first resistor R1 and the second resistor R2 are assumed to have the same resistance value and are denoted as R F Then, the voltage OUTP output by the first output terminal of the amplifier CF and the voltage OUTN output by the second output terminal are respectively:
OUTP＝VCM+(I AMB +I LED -I of )*R F ；
OUTN＝VCM-(I AMB +I LED -I of )*R F 。
the voltage OUTP and the voltage OUTN are a pair of differential signals, the voltage sampling circuit samples the differential signals, namely, the difference value of the voltage OUTP and the voltage OUTN is sampled, the analog-to-digital conversion circuit 130 performs analog-to-digital conversion on the voltage sampled by the voltage sampling circuit to obtain the voltage V which is equal to the input voltage V ADC Corresponding voltage code value for subsequent processing. Wherein, V ADC Is the input voltage of the analog-to-digital converter, i.e. the voltage sampled by the voltage sampling circuit, V ADC The following conditions are satisfied:
V ADC ＝OUTP-OUTN＝2*(I AMB +I LED -I of )*R F
the analog-to-digital conversion circuit 130 may include a sampling circuit, that is, the voltage sampling circuit 124 may be integrated in the analog-to-digital conversion circuit 130, or may be separately disposed outside the analog-to-digital conversion circuit 130, which is not limited in this application.
The analog-to-digital conversion circuit 130 may periodically convert the voltage signal into a voltage code value that reflects the magnitude of the photocurrent generated by the PD. When the LED emits light, the voltage code value corresponds to the sum of the current generated by ambient light and the current generated by light reflected by the LED; the voltage code value corresponds to the current generated by ambient light when the LED is not emitting light.
In the phase of ambient light, the LED does not emit light, I LED When 0, the photocurrent generated by the PD is induced by ambient light, I AMB . As an embodiment, the current generating circuit 110 is an IDAC (current mode digital-to-analog converter), and the IDAC may be further adjusted by a successive approximation method, so that the current I output by the IDAC is equal to the current I of Is close to I AMB At this time, the input voltage V of the analog-to-digital conversion circuit 130 ADC_AMB ＝2*(I AMB -I of )*R F 0. Accordingly, the ambient light current I can be determined according to the voltage code value output by the analog-to-digital conversion circuit 130 AMB Of (c) is used. When the voltage code value output by the analog-to-digital conversion circuit 130 is close to 0, it represents the current I of the IDAC output of Is close to I AMB According to I of The corresponding current value can determine the ambient light current I AMB The size of (2).
It should be understood that when the ambient light detection circuit 10 is applied to an apparatus having a light source, each detection period further includes a measurement phase when the light source emits light, and the voltage code value converted in each detection period further includes a first measurement code value corresponding to the measurement phase, where the first measurement code value includes a normal measurement code value generated due to the light source emitting light and an ambient light code value due to the influence of ambient light. In order to eliminate the influence of the ambient light, the code value processing circuit 140 is further configured to calculate a second measured code value in the detection period according to the ambient light pre-estimation value and the first measured code value in the same detection period, that is, subtract the ambient light pre-estimation value from the first measured code value, and the obtained second measured code value may represent a normal measured code value generated due to the light source being lighted.
That is, in one possible embodiment of the present application, in order to effectively eliminate the influence of ambient light on the measurement result of the device during the measurement phase, the current generation circuit 110 is further configured to adjust the subtraction current according to the ambient light estimation value. Therefore, the reduction current approaches to the current generated by the ambient light, and the influence of the ambient light on the measurement result of the equipment is further reduced.
Each detection period comprises a first ambient light phase before the light source emits light and a second ambient light phase after the light source emits light, and correspondingly, the voltage code value obtained by conversion in each detection period comprises a first ambient light voltage code value corresponding to the first ambient light phase and a second ambient light voltage code value corresponding to the second ambient light phase. Accordingly, the code value processing circuit may calculate an ambient light estimated value corresponding to each detection period according to the at least one first ambient light voltage code value and the at least one second ambient light voltage code value.
Optionally, the code value processing circuit may calculate an ambient light pre-estimated value of the current detection period according to the first ambient light voltage code value and the second ambient light voltage code value in the previous detection period; the code value processing circuit can calculate the ambient light estimated value corresponding to the current detection period according to the first ambient light voltage code value and the second ambient light voltage code value in the previous multiple detection periods. Specifically, the code value processing circuit may select a corresponding ambient light estimation method according to a change condition of the ambient light. For example, when the ambient light changes linearly, a first-order estimation mode may be adopted, and an ambient light estimated value of a current detection period is obtained by calculating according to a first ambient light voltage code value and a second ambient light voltage code value in a previous detection period; when the ambient light changes nonlinearly, a second-order or higher-order estimation mode can be adopted to obtain the ambient light pre-estimation value of the current detection period according to the first ambient light voltage code value and the second ambient light voltage code value in the previous detection periods. When the ambient light is not changed within a period of time, a zero-order estimation mode can be adopted, that is, any ambient light voltage code value is adopted as an ambient light estimated value.
In one embodiment, in two adjacent detection periods, the second ambient light phase of the previous detection period is the same as the first ambient light phase of the next detection period. Specifically, referring to fig. 3, in two adjacent detection periods, when the second ambient light phase of the previous detection period is the same as the first ambient light phase of the next detection period, the previous detection period includes the first ambient light phase AMBR0, the measurement phase LED0, and the second ambient light phase AMBR0, and the next detection period includes the first ambient light phase AMBR0, the measurement phase LED1, and the second ambient light phase AMBR1, that is, the first ambient light phase of the next detection period and the second ambient light phase AMBR0 of the previous detection period are in the same time segment. Therefore, every two adjacent detection periods can share one ambient light detection phase, the number of times of ambient light detection is reduced, and the measurement efficiency is improved. In this embodiment, according to the change condition of the ambient light, a zero-order, a first-order, or a second-order estimation manner may be adopted, and the obtained ambient light estimation values are respectively:
zeroth order predictive estimate est (0) ═ AMBL0
First order estimate
Second order pre-estimation
In the above formula, AMBL0, AMBR0, and AMBR1 respectively represent ADC code values corresponding to ambient light phases (i.e. voltage code values output by the analog-to-digital converter), and LED0 represents ADC code values corresponding to LED0 phases. Any one of the above-mentioned zeroth order, first order and second order pre-estimates may be used as the ambient light pre-estimate, or the ambient light pre-estimate may be calculated according to at least two of the zeroth order, first order and second order pre-estimates, for example, the three may be superimposed to be used as the ambient light pre-estimate.
In another embodiment, in two adjacent detection periods, the second ambient light phase of the previous detection period is before the first ambient light phase of the next detection period. Specifically, referring to fig. 4, in two adjacent detection periods, the previous detection period includes the first ambient light phase AMBL0, the measurement phase LED0, and the second ambient light phase AMBR0, the next detection period includes the first ambient light phase AMBL1, the measurement phase LED1, and the second ambient light phase AMBR1, and the first ambient light phase AMBL1 of the next detection period is after the second ambient light phase AMBR0 of the previous detection period, and the two detection periods are in different time slices. I.e. between two adjacent measurement phase LEDs 1 and 0, two detections of ambient light are made. Therefore, the ambient light detection is separately carried out before and after the measurement of each detection period, the times of the ambient light detection are increased, the real-time performance is higher, and the estimation result is more accurate. In this embodiment, according to the change condition of the ambient light, a zero-order, a first-order, or a second-order estimation manner may be adopted, and the obtained ambient light estimation values are respectively:
zeroth order predictive estimate est (0) ═ AMBL0
First order estimate
Second order pre-estimation
In the above equations, AMBL0, AMBR0, AMBL0, and AMBR1 represent ADC code values corresponding to the phases of ambient light, respectively, and LED0 represents ADC code values corresponding to the phases of LED 0. Any one of the above-mentioned zeroth order, first order and second order pre-estimates may be used as the ambient light pre-estimate, or the ambient light pre-estimate may be calculated according to at least two of the zeroth order, first order and second order pre-estimates, for example, the three may be superimposed to be used as the ambient light pre-estimate.
Specifically, the code value processing circuit 140 may be a device having data calculation and processing functions, such as a controller or a processor, and may include modules, such as an adder and a shift register. This embodiment is not particularly limited.
In an implementation manner of the present application, the code value processing circuit 140 includes a shift register and an adder, the shift register is connected to the analog-to-digital conversion circuit 130 and is configured to shift the voltage code value obtained by the conversion of the analog-to-digital conversion circuit 130 to obtain a staged estimation result, and the adder is connected to the shift register and is configured to accumulate the staged estimation result to obtain the ambient light estimation result.
When the shift register performs shift operation on the voltage code value obtained by the conversion of the analog-to-digital conversion circuit 130 to obtain the staged estimation result, multiple stages of estimation may be specifically performed, so as to obtain the ambient light estimation value corresponding to each detection period by using the estimation results of at least two stages of the multiple stages of estimation results. For example, the performing of the prediction of the plurality of stages may include performing zeroth order prediction, first order prediction, second order prediction, and the like.
In one possible embodiment of the present application, the ambient light estimate corresponding to each detection period comprises a first ambient light estimate; the code value processing circuit 140 is configured to perform first-order calculation on the first ambient light voltage code value and the second ambient light voltage code value in the same detection period to obtain a first ambient light estimated value.
In another possible embodiment of the present application, the ambient light estimate corresponding to each detection period comprises a second ambient light estimate; the code value processing circuit 140 is configured to perform second-order calculation on the first ambient light voltage code value and the second ambient light voltage code value in two adjacent detection periods to obtain a second ambient light estimated value.
It should be understood that, in the above two manners, specifically, when the ambient light changes linearly with time, the code value processing circuit 140 is configured to perform a first-order calculation on the first ambient light voltage code value and the second ambient light voltage code value in the same detection period to obtain a first ambient light pre-estimated value according to a first-order calculation result, and when the ambient light changes non-linearly with time, the code value processing circuit 140 is configured to perform a second-order calculation on the first ambient light voltage code value and the second ambient light voltage code value in two adjacent detection periods to obtain a second ambient light pre-estimated value according to a second-order calculation result.
The following is a description of a specific example:
ideally (without ambient light changing), from the AMBL0 phase to the LED0 phase, the ADC code value changes by:
wherein, the LED0 and the AMBL0 respectively indicate ADC code values of corresponding phases.
The change in ambient photocurrent from AMBL0 phase to LED0 phase is noted as Δ I LAL The change from the phase of LED0 to the phase of AMBR0 is denoted as Δ I LRL 。
When a zero-order estimation mode is adopted, the deviation of the actual variation of the ADC code value relative to the ideal condition is as follows:
Δ I when the ambient light does not change with time LAL ＝0，Δ(LED0-AMBL0) est0 Therefore, the accurate estimation of the ambient light can be realized by adopting a zero-order estimation mode.
When the first-order estimation method is adopted, the deviation of the actual variation of the ADC code value relative to the ideal case is as follows:
Δ I when the ambient light varies linearly with time LRL ＝-ΔI LAL ，Δ(LED0-AMBL0) est1 Therefore, accurate estimation of the ambient light can be realized by adopting a first-order estimation mode.
When the ambient light changes nonlinearly, a more accurate estimation mode needs to be adopted to reduce the estimation deviation. The ambient light changes mostly at low frequency, and when the system sampling frequency (the single phase time length corresponds to the system sampling period) is much larger than the ambient light change frequency, the polynomial can be used to pre-estimate the ambient light. Therefore, in one possible implementation of the present application, the first 3 terms can be obtained by using time as a variable according to the Talor expansion formula, where t represents the time of a single phase and is also the time interval of adjacent sampling points. f' (t), f "(t) denote the first and second derivatives of f (t), respectively.
f(0)＝I AMBL0
Multiplication of ambient photocurrent to ADC code value
And obtaining ADC code values which are respectively a zeroth-order estimated value est (0), a first-order estimated value est (1) and a second-order estimated value est (2). The zero-order estimated value est (0), the first-order estimated value est (1) and the second-order estimated value est (2) can be stored in a shift register, and the estimated values are accumulated by an adder to obtain an ambient light estimated result.
Specifically, in an implementation manner of the present application, the shift register may specifically adopt a staged estimation manner when performing zeroth order estimation, first order estimation and second order estimation, and may obtain a zeroth order estimation result based on AMBL0 when performing zeroth order estimation; when the first-order prediction is carried out, AMBL0-AMBR0 can be calculated and stored firstly and shifted to obtain the first-order prediction based on AMBL0 and AMBR0
And using it as a first-order estimation result; when second-order estimation is performed, based on AMBL1 and AMBR1, AMBL1-AMBR1 are calculated and stored first, and based on obtaining a storage constant 1/12, where the storage constant 1/12 occupies 7 effective bits, AMBL0-AMBR0 and AMBL1-AMBR1 are circularly shifted for 7 times and summed to obtain
And uses it as a second order estimate.
Taking a sine wave ambient light with a frequency of 4Hz and an amplitude of 50uA as an example, as shown in fig. 3, the deviation of the ambient light estimated value obtained based on the zeroth order estimated result and the first order estimated result is shown, and as shown in fig. 4, the deviation of the ambient light estimated value obtained based on the zeroth order estimated result, the first order estimated result and the second order estimated result is shown. In fig. 5 and 6, the abscissa is the number of phases, and the ordinate is the multiple of the estimated deviation corresponding to the IDAC minimum resolution current, and it can be seen that the estimated deviation of the ambient light obtained by the second-order estimation method is about 10% of the estimated deviation of the ambient light obtained by the first-order estimation, and the measurement accuracy is greatly improved when the ambient light changes nonlinearly. It should be understood that the more the orders, the more accurate the corresponding estimation result may be, that is, the third order calculation or the fourth order calculation may be performed.
By adopting the ambient light detection circuit 10, multi-order estimation of ambient light can be realized. When the ambient light changes nonlinearly, the estimated value of the ambient light is determined according to the multi-order estimation result, so that the estimation deviation of the ambient light is effectively reduced, and the pulse wave measurement accuracy is improved. In addition, when the ambient light detection circuit 10 is applied to a detection device with an LED, no additional circuit is added, so that the ambient light detection circuit 10 is simple and cost-controllable in application.
It should be understood that, when the ambient light detection circuit 10 is applied to a device having a light source, in order to accurately obtain a light source signal estimated value corresponding to the light source (LED) in the device, a calculation formula ADC (2) ═ LED0- (est (0) + est (1) + est (2)) may be used to perform calculation, so as to obtain a light source signal estimated value of a final LED light source, and perform subsequent calculation, such as pulse wave calculation, using the light source signal estimated value.
Another embodiment of the present application provides a photoplethysmography measurement apparatus, including a light source, a photoelectric sensor 20 and the ambient light detection circuit 10 in the above embodiments, the processor is connected to the light source and the ambient light detection circuit 10 respectively, the photoelectric sensor 20 is configured to generate a photocurrent when detecting light emitted by the light source and transmit the photocurrent to the ambient light detection circuit 10.
The light source can be an LED light source or an O-LED light source, and can also be any light source which can be used for pulse wave detection and can be set according to actual requirements. The number of the light sources may be one or more, and if the number of the light sources is more than one, each light source is correspondingly connected to one of the photosensors 20. The photosensors 20 may be connected in parallel to the ambient light detection circuit 10. Each photosensor 20 may also be connected to the ambient light detection circuit 10 via a multiplexer switch. The embodiment of the present application is not particularly limited, and may be set according to actual requirements.
In an implementation manner of the present application, the light sources are LED light sources, the number of the light sources is 3 or 4, each light source corresponds to one photo sensor 20, so that each photo sensor 20 can collect light emitted by the corresponding light source and convert the light into photocurrent, and the photo sensors 20 are connected to the ambient light detection circuit 10 through a multi-way selector switch.
It should be understood that, since the photoplethysmography measurement apparatus includes the ambient light detection circuit 10, the photoplethysmography measurement apparatus has the same or corresponding technical features as the ambient light detection circuit 10, and details are not repeated in this embodiment.
It should be noted that, when the photoplethysmography apparatus includes the ambient light detection circuit 10, the ADC code value of the light source may be used to subtract the ambient light estimated value, so as to obtain the final accurate pulse wave measurement value.
It should be understood that the photoplethysmography device may further include one or more of a display, a processor, a memory, and a communication device, among others.
Referring to fig. 7, the present application further provides a method for detecting ambient light, the method comprising:
step S210: the subtracted current generated by the current generation circuit 110 and the photocurrent output by the photosensor 20 are obtained.
Step S220: and amplifying the difference value of the photocurrent and the reduction current and converting the difference value into a voltage signal.
The steps S210 and S220 may be executed by the current-voltage conversion circuit 120, and for the specific structure and the operation principle of the current-voltage conversion circuit 120, reference may be made to the detailed description of the foregoing embodiments, which is not repeated in this embodiment.
Step S230: the method comprises the steps of periodically converting a voltage signal into voltage code values, wherein each detection period comprises a first ambient light phase before light source emitting and a second ambient light phase after light source emitting, and the converted voltage code values of each detection period comprise a first ambient light voltage value corresponding to the first ambient light phase and a second ambient light voltage value corresponding to the second ambient light phase.
The step S230 may be executed by the analog-to-digital conversion circuit 130, and for the specific structure and the operation principle of the analog-to-digital conversion circuit 130, reference may be made to the detailed description of the foregoing embodiments, which is not repeated in this embodiment.
Step S240: and calculating to obtain an ambient light pre-estimation value corresponding to each detection period according to at least one first ambient light voltage code value and at least one second ambient light voltage code value.
The step S230 may be executed by the code value processing circuit 140, and specific structures and working principles of the code value processing circuit 140 may refer to the specific description of the foregoing embodiments, which is not repeated in this embodiment.
In summary, the present application provides an ambient light detection circuit 10, a method and a photoplethysmography apparatus. The ambient light detection circuit 10 includes a current generation circuit 110, a current-voltage conversion circuit 120, an analog-to-digital conversion circuit 130, and a code value processing circuit 140, the current generation circuit 110 being configured to generate a trim current; the current-voltage conversion circuit 120 is configured to be connected to the photo sensor 20, receive a photocurrent output by the photo sensor 20, and amplify and convert a difference between the photocurrent and the subtracted current into a voltage signal; the analog-to-digital conversion circuit 130 is used for periodically converting the voltage signal into a voltage code value; the code value processing circuit 140 is configured to calculate an ambient light estimated value corresponding to each detection period according to at least one first ambient light voltage code value and at least one second ambient light voltage code value. In addition, when the ambient light threshold corresponding to each detection period is obtained through calculation according to at least one first ambient light code value and at least one second ambient light code value, multi-order estimation (zero order estimation, first order estimation and second order estimation) can be performed, so that an ambient light estimated value can be accurately obtained according to multi-order estimation results.
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit the same; although the present application has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; such modifications and substitutions do not depart from the spirit and scope of the corresponding technical solutions in the embodiments of the present application.
Claims (12)
1. An ambient light detection circuit, comprising:
a current generating circuit for generating a clipping current;
the current-voltage conversion circuit is used for connecting the photoelectric sensor, receiving the photocurrent output by the photoelectric sensor, and amplifying and converting the difference value of the photocurrent and the reduction current into a voltage signal;
the analog-to-digital conversion circuit is used for periodically converting the voltage signal into voltage code values, wherein each detection period comprises a first ambient light phase before light source lighting and a second ambient light phase after the light source lighting, and the voltage code values obtained by conversion in each detection period comprise a first ambient light voltage code value corresponding to the first ambient light phase and a second ambient light voltage code value corresponding to the second ambient light phase;
and the code value processing circuit is used for calculating an ambient light pre-estimated value corresponding to each detection period according to at least one first ambient light voltage code value and at least one second ambient light voltage code value.
2. The ambient light detection circuit of claim 1 wherein the ambient light estimate corresponding to each detection period comprises a first ambient light estimate;
the code value processing circuit is used for performing first-order calculation on the first ambient light voltage code value and the second ambient light voltage code value in the same detection period to obtain a first ambient light estimated value.
3. The ambient light detection circuit of claim 1 wherein the ambient light estimate corresponding to each detection period comprises a second ambient light estimate;
the code value processing circuit is used for performing second-order calculation on the first ambient light voltage code value and the second ambient light voltage code value in two adjacent detection periods to obtain a second ambient light estimated value.
4. The ambient light detection circuit of claim 1 wherein the current generation circuit is further configured to adjust the trim current based on the ambient light estimate.
5. The ambient light detection circuit according to any one of claims 1 to 4, wherein each detection cycle further includes a measurement phase at which the light source emits light, the voltage code value converted by each detection cycle further includes a first measurement code value corresponding to the measurement phase,
and the code value processing circuit is also used for calculating a second measuring code value in the detection period according to the ambient light estimated value and the first measuring code value in the same detection period.
6. The ambient light detection circuit according to any one of claims 1-4,
in two adjacent detection periods, the second ambient light phase of the previous detection period precedes the first ambient light phase of the subsequent detection period.
7. The ambient light detection circuit according to any one of claims 1 to 4,
in two adjacent detection periods, the second ambient light phase of the previous detection period and the first ambient light phase of the next detection period are the same phase.
8. The ambient light detection circuit according to claim 1, wherein the code value processing circuit includes a shift register and an adder, the shift register is connected to the analog-to-digital conversion circuit and configured to shift the voltage code value converted by the analog-to-digital conversion circuit to obtain a staged estimation result, and the adder is connected to the shift register and configured to accumulate the staged estimation result to obtain the ambient light estimation result.
9. The ambient light detection circuit according to claim 1, wherein the current-voltage conversion circuit comprises a transimpedance amplifier circuit and a voltage sampling circuit, an input terminal of the transimpedance amplifier circuit is connected to the current generation circuit and is used for connecting the photosensor, and an output terminal of the transimpedance amplifier circuit is connected to an input terminal of the voltage sampling circuit; and the output end of the voltage sampling circuit is connected with the input end of the analog-to-digital conversion circuit.
10. The ambient light detection circuit of claim 9 wherein the current-to-voltage conversion circuit further comprises a buffer connected between the output of the voltage sampling circuit and the input of the analog-to-digital conversion circuit.
11. A photoplethysmography apparatus, comprising a light source, a photosensor and the ambient light detection circuit of any one of claims 1 to 9, the processor being connected to the light source and the ambient light detection circuit, respectively, the photosensor being configured to generate a photocurrent when light from the light source is detected and to transmit the photocurrent to the ambient light detection circuit.
12. An ambient light detection method, comprising:
acquiring a reduction current generated by a current generation circuit and a photocurrent output by a photoelectric sensor;
amplifying the difference value of the photocurrent and the reduction current and converting the difference value into a voltage signal;
periodically converting a voltage signal into voltage code values, wherein each detection period comprises a first ambient light phase before light source lighting and a second ambient light phase after light source lighting, and the voltage code values obtained by conversion in each detection period comprise a first ambient light voltage code value corresponding to the first ambient light phase and a second ambient light voltage code value corresponding to the second ambient light phase;
and calculating to obtain an ambient light pre-estimation value corresponding to each detection period according to at least one first ambient light voltage code value and at least one second ambient light voltage code value.
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