KR102189402B1 - a auto offset compensation device based binary searching of pulse oximeter - Google Patents

Abstract

The present invention is to obtain a high signal-to-noise ratio by removing an ambient signal from ambient light when amplifying and processing an input signal in the form of a pulse driven by an optical signal such as a pulse oximeter or oxygen saturation. It relates to a binary search-based automatic offset calibration device of an oxygen saturation meter, comprising: an LED driving unit that generates a current, a light emitting unit that emits light by receiving the current generated from the LED driving unit, and reflected from a blood vessel inside a finger approaching the light emitting unit. A detection unit that detects light, converts it into a current, and outputs it; a current-voltage conversion amplification unit that receives the converted current from the detection unit, converts it to a voltage, amplifies it, and outputs it; and the current-voltage conversion amplification unit converts and amplifies it. A sampling and filtering unit receiving a voltage, sampling and filtering, and outputting, a digital conversion unit receiving the filtered voltage from the sampling and filtering unit and converting it into a digital value, and an output signal of the current-voltage conversion amplifying unit A comparison unit that receives and compares whether it is a positive number or a negative number, and a binary search logic unit that searches for a value closest to 0 in which the output signal of the current-voltage conversion amplification unit uses a binary search logic according to the comparison result of the comparison unit, and the binary And a current conversion unit converting the value searched by the search logic unit into an analog value and outputting the converted value to the current-voltage conversion amplifying unit.

Description

{A auto offset compensation device based binary searching of pulse oximeter}

The present invention relates to an oxygen saturation meter, and in particular, to a binary search-based automatic offset calibration device of an oxygen saturation meter to obtain a high signal-to-noise ratio by automatically removing an offset component due to an ambient signal.

As personalization and aging progress, individuals are interested in their own health and undergo regular health check-ups. However, since there is an inconvenience of having to take extra time to do a health checkup, most of them are looking for a hospital when there is an abnormality in their health. However, in the case of the elderly or the handicapped with discomfort or chronic illness, it is inconvenient to go to the hospital, and it is possible to cope with a dangerous situation only when the state of the body (ie, a living body state) is checked regularly.

Therefore, oxygen saturation measurement is widely used as a measurement object for personal health. In the measurement of oxygen saturation, blood oxygen saturation (SpO2) is calculated non-invasively by measuring the light absorption by wavelength due to the pulsating component of arterial blood.

In general, in order to measure heart rate and oxygen saturation in blood vessels using an optical signal, a method of shining light from an LED onto a blood vessel and measuring light detected by a detector of a light receiving unit is used. At this time, in processing the light input signal of the light receiving unit, the quality of signal processing is often deteriorated due to ambient light that is much larger than the amount of light to be measured according to the LED on/off. In order to solve this problem, techniques for removing ambient light are applied when implementing a heart rate and oxygen saturation measurement system.

In the case of prior art US 5954644, in the case of PPG (photoplethysmography) measurement, when the LED is off, ambient light is sampled and held, and the PPG signal is sampled and held by turning on the LED to add two signals. At the differential amplification stage of, amplification is used to remove ambient light.

In the case of the prior art US 8315684, in order to remove ambient light from an oximeter for implementing PPG or oxygen saturation (SpO ₂ ), a light input signal when driving an infrared (IR) LED, when driving a red LED (Red) It converts the ambient light input signal (ambient) when the LED is off to digital using a sigma-delta interface and removes the ambient light signal from the microcontroller end. Apply the technique.

Among the prior art, in the case of a technique for removing ambient light using an additional differential amplifying unit used in US 5954644 or the like, an amplifying unit for removing ambient light is added, which is inefficient in terms of power consumption and size of the circuit. In addition, there is a limitation that only ambient light below the input range of the additional differential amplifier can be removed.

Among the prior art, in the case of the ambient light removal technique in the digital domain used in US 8315684, etc., since ambient light is removed through digital signal processing, when the signal of the ambient light is large, among the digital signals converted through an analog/digital converter There is a disadvantage in that the proportion occupied by ambient light is increased, so that the dynamic range of the signal to be actually acquired is reduced and the signal-to-noise ratio is decreased.

The present invention was devised to solve the above problems, and when amplifying and processing an input signal in the form of a pulse driven by an optical signal such as a pulse oximeter or oxygen saturation, a background signal caused by ambient light (ambient signal) to obtain a high signal-to-noise ratio, and to provide a binary search-based automatic offset calibration device of an oxygen saturation meter.

The binary search-based automatic offset calibration device of the oxygen saturation meter according to the present invention for achieving the above object includes an LED driving unit that generates a current, a light emitting unit that emits light by receiving the current generated from the LED driving unit, and the light emitting unit. A detection unit that detects light reflected from an internal blood vessel of the approached finger, converts it into a current, and outputs a current, a current-voltage conversion amplification unit that receives the converted current and converts it into a voltage and amplifies it for output, and the current-voltage A sampling and filtering unit receiving the converted and amplified voltage from the conversion amplifying unit and sampling and filtering the output, a digital conversion unit receiving the filtered voltage from the sampling and filtering unit and converting it into a digital value, and outputting the current -A comparison unit that receives the output signal of the voltage conversion amplification unit and compares whether it is positive or negative, and a binary search logic that searches for a value closest to zero of the output signal of the current-voltage conversion amplification unit according to the comparison result of the comparison unit. It characterized in that it comprises a binary search logic unit and a current conversion unit for converting the value searched by the binary search logic unit into an analog value and outputting the converted value to the current-voltage conversion amplifying unit.

The binary search-based automatic offset calibration apparatus of the oxygen saturation meter according to an embodiment of the present invention has the following effects.

That is, when amplifying and processing an input signal in the form of a pulse driven by an optical signal such as a pulse oximeter or oxygen saturation, a high signal-to-noise ratio can be obtained by removing the ambient signal from ambient light. have.

1 is a block diagram schematically showing a binary search-based automatic offset calibration apparatus of an oxygen saturation meter according to the present invention
FIG. 2 is a flowchart illustrating an operation of the turnover search logic unit of FIG. 1
3 is a circuit diagram showing a binary search logic unit of FIG. 1
4 is an operation timing diagram of the binary logic operation unit of 3

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily implement the present invention. However, the present invention may be implemented in various forms and is not limited to the embodiments described herein. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and similar reference numerals are assigned to similar parts throughout the specification.

Throughout the specification, when a part is said to be "connected" to another part, this includes not only "directly connected" but also "electrically connected" with another element interposed therebetween. . In addition, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless specifically stated to the contrary.

An oxygen saturation meter according to an embodiment of the present invention measures oxygen saturation using a PPG (Photoplethysmogram) sensor. Oxygen saturation measurement using a photovoltaic pulse wave sensor uses a method of acquiring a signal by using a linear relationship between the amount of light absorbed by hemoglobin in the blood and the volume of blood that changes due to contraction and relaxation of the heart. This is to measure the change in intensity of infrared rays using, and is generally measured on fingers, wrists, toes, and earlobe.

In the following embodiments, a case of measuring oxygen saturation from a finger will be described as an example. In addition, the oxygen saturation meter according to an embodiment of the present invention uses a Zigbee wireless communication module that uses a 2.4 GHz ISM band region to prevent the patient and doctor from being spatially constrained, and remotely transmits the oxygen saturation measurement result. Transfer to.

1 is a block diagram schematically showing a binary search-based automatic offset calibration apparatus of an oxygen saturation meter according to the present invention.

The binary search-based automatic offset calibration apparatus of the oxygen saturation meter according to the present invention includes an LED driving circuit 110 that generates a current and a current generated from the LED driving unit 110 as shown in FIG. 1. A light-emitting unit (LED) 120 that receives and emits light, a detection unit 130 that detects light reflected from an internal blood vessel of the finger (A) approaching the light-emitting unit 120, converts it into a current, and outputs it, and the detection unit A current-voltage conversion amplifying unit 140 that receives the converted current from 130, converts it into a voltage, amplifies and outputs it, and a voltage converted and amplified by the current-voltage conversion amplifying unit 140 is received and sampled. A sampling and filtering unit (Sampling & Flitering) 150 for filtering and outputting, and a digital conversion unit (ADC) 160 receiving the filtered voltage from the sampling and filtering unit 150 and converting it into a digital value for output. , A comparison unit 170 for comparing positive or negative numbers by receiving the output signal Vout of the current-voltage conversion amplifying unit 140, and the binary search logic according to the comparison result of the comparison unit 170 The output signal Vout of the current-voltage conversion amplifying unit 140 is a binary search logic unit 180 that searches for a value closest to 0, and a value searched by the binary search logic unit 180 It is configured to include a current converter (Current DAC) 190 converting the analog value and outputting (iDAC) to the current-voltage conversion amplifier 140.

The binary search-based automatic offset calibration device of the oxygen saturation meter according to the present invention irradiates light (eg, LED light) to the user's finger (A) and receives the light transmitted through the finger (A) through a PD (detector), and , Measure the oxygen saturation using the received light.

The binary search-based automatic offset calibration device of the oxygen saturation meter according to the present invention configured as described above generates a required current from the LED driving unit 110 and supplies it to the light emitting unit 120, and the light emitting unit 120 is the LED Light is emitted through the current supplied from the driving unit 110 to illuminate the blood vessel inside the finger A.

When the blood vessel inside the finger is illuminated through the light emitting unit 120, the light reflected from the detection unit 130 made of a photodiode is detected, converted into a current, and output. At this time, the current output from the detection unit 130 is generally driven in the form of a pulse. Accordingly, the current-voltage conversion amplifying unit 140 converts into a voltage, amplifies it, and outputs it.

The voltage converted and amplified through the current-voltage conversion amplifying unit 140 is received from the sampling and filtering unit 150, sampled and filtered at an appropriate timing, and the voltage is converted back to digital using the digital conversion unit 160. Convert to value.

Here, the current-voltage conversion amplification unit 140 is configured in a completely differential type, and the comparison unit 170 compares whether the output signal Vout of the current-voltage conversion amplification unit 140 is positive or negative. And, based on the result transmitted through the comparison unit 170, the binary search logic unit 180 searches for a digital value in which the output signal of the current-voltage conversion amplifying unit 140 is close to 0, and the binary The current conversion unit 190 converts the digital current value searched through the search logic unit 180 into an analog value, and outputs the converted current to the current-voltage conversion amplification unit 140.

The binary search-based automatic offset calibration apparatus of the oxygen saturation meter according to the present invention will be described in more detail as follows.

A control signal for supplying a current required for driving the light emitting unit 120 by driving the LED driving unit 110 is referred to as LED_ON. The total current flowing through the detection unit 130 is iPD, the ambient light current component flowing through the detection unit 130 due to ambient light is iAMB, and the reflected light current flowing through the detection unit 130 due to blood vessel reflection for measuring a photoplethysmogram Assuming that the component is iPPG, the output current of the current conversion unit 190 is iDAC, and the current flowing through the feedback impedance Zf of the current-voltage conversion amplifying unit 140 is iOUT, it is summarized by the following equation.

That is, iPD is the sum of iAMP and iPPG, and iOUT is the sum of -iPD and iDAC.

When LED_ON, which is the control signal of the light-emitting unit 120, is 0, no current flows through the light-emitting unit 120, and the reflected light current component iPPG flowing through the detection unit 130 becomes 0, and the current flowing through the detection unit 130 iPD is as shown in Equation 2 below.

Therefore, through the binary search logic unit 180, the output current iDAC of the current mode DAC of the current converter 190 is adjusted so that Vout becomes the closest value to 0. Assuming that the output signal Vout = 0 of the current-voltage conversion amplifying unit 140 by the binary search, iDAC = iAMB can be written.

Thereafter, when LED_ON = H, which is the control signal of the light emitting unit 120, is set, both the ambient light current iAMB and the reflected light current iPPG exist in the total current iPD flowing through the detection unit 130. At this time, since the current conversion unit 190 generates a current corresponding to the ambient light current iAMB, the output current iDAC of the current conversion unit 190 is equal to the ambient light current iAMB.

The output voltage Vout of the current-voltage conversion amplifier 140 when the control signal LED_ON = H of the light-emitting unit 120 is as shown in Equation 3 below.

Therefore, only the signal corresponding to the reflected light current iPPG to be measured without components due to the ambient light current iAMB can be measured at Vout, and the desired signal is extracted from the sampling and filtering unit 150 at an appropriate timing with the LED_ON signal, and the digital conversion unit ( 160) to digitally convert only the desired reflected light current iPPG component and output it.

The current-voltage conversion amplification unit 140 may be driven in the form of various timing signals, and an appropriate timing sets the LED_ON signal, which is a driving signal of the light-emitting unit 120, to H, and the current-voltage conversion amplification unit ( After the stabilization time of 140), the sampling and filtering unit 150 samples and outputs the signal.

2 is a flowchart illustrating an operation of the turnover search logic unit of FIG. 1.

As shown in FIG. 2, the binary search logic unit 180 assumes that the current mode DAC of the current converter 180 has a resolution of N bits.

First, in the initialization state, when the SC signal (start of conversion) rises, the initialization mode is entered.At this time, the most significant bit (MSB) is set to H, and the least significant bits (LSBs) are set to L. Set. And the counter variable k is set to 0 (S110).

Subsequently, after some time in which the current-voltage conversion amplification unit 140 is stabilized, the fraud comparison unit 170 determines whether the sign of the positive or negative output signal Vout of the current-voltage conversion amplification unit 140 is Thus, the output signal of the comparator 170 is outputted (S120).

When the output signal COMP of the comparison unit 170 is H, the MSB-k-th bit is set to H (S130), and when the output signal COMP of the comparison unit 170 is L, the MSB-k-th bit is set to L. Set (S140).

At this time, the magnitudes of k and N are compared (S150), and if k <N as a result of the comparison, the counter variable k is increased by 1 (S160), MSB-kth bit is set to H (S170), and the comparison operation is repeated. do.

In this process, through a binary search process, the output signal Vout of the current-voltage conversion amplifying unit 140 is searched for a value closest to 0. Thereafter, if k> N, EOC (End of Conversion) is generated as H and is terminated (S180).

3 is a circuit diagram illustrating a binary search logic unit of FIG. 1.

The binary search logic of the binary search logic unit 180 may be implemented in various forms, and a suitable implementation example is, as shown in FIG. 3, a plurality of D-flip-flops (DFF) in series. It can be connected and implemented. In this case, the DFF array of two rows connected in series is used, and the DFF of the first row serves as a counter k in the form of a shift register.

Here, each of the DFFs is composed of a DFF having an asynchronous SET (SET) and an asynchronous reset (RST) signal. When the SC signal becomes H, the output Q of the first DFF in the first row becomes H, and the outputs of all other DFFs become L.

On the other hand, DFF in line 2 plays a role of updating the COMP value, and when COMP = H, the COMP value is updated to DFF, and when COMP = L, the COMP value is not updated and L is maintained.

When the last COMP signal comes in, the EOC signal is finally generated as H, indicating that the binary search has ended.

4 is an operation timing diagram of the binary logic operation unit of 3;

As shown in FIG. 4, the clock signal CLK is input to the binary search logic, and the light emitting unit 120 is in an OFF state when LED_ON = L. When the SC signal is input, a binary search operation to remove ambient current is initiated. After the ambient signal is removed by an N-bit clock operation, an EOC signal is generated to signal the end of the binary search operation, and then set LED_ON = H to amplify only the PPG signal.

During the period in which LED_ON = H, the digital conversion unit 160 samples and filters Vout, which is the output signal of the current-voltage conversion amplifying unit 140, after a stabilization time of the current-voltage conversion amplifying unit 140 is passed, and after an appropriate time delay. ) To convert to digital.

Accordingly, the binary search-based automatic offset calibration apparatus of the oxygen saturation meter according to the present invention can automatically remove the ambient light current included in the current of the detection unit 130 using a simple binary search-based circuit. Compared to the existing invention, the circuit structure and algorithm are simpler, and since the output signal of the current-voltage conversion amplifying unit is directly used, a digital converter is used to convert it to digital or the ambient light within a shorter time than the existing technology that requires communication with the host processor. Current can be removed.

For this reason, the oxygen saturation meter is used for elderly people over 60 years of age with respiratory disorders, consciousness disorders, shock, or heart failure, etc., and also for people who need long-term concentration, long-distance drivers or people who are tired from chronic fatigue, sports. It will be used for people with lung disease or chronic obstructive pulmonary disease, etc.

On the other hand, the above description of the present invention is for illustration only, and those of ordinary skill in the art to which the present invention pertains will understand that it is possible to easily transform it into other specific forms without changing the technical spirit or essential features of the present invention. I will be able to. Therefore, it should be understood that the embodiments described above are illustrative and non-limiting in all respects. For example, each component described as a single type may be implemented in a distributed manner, and similarly, components described as being distributed may also be implemented in a combined form.

The scope of the present invention is indicated by the claims to be described later rather than the detailed description, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be interpreted as being included in the scope of the present invention. do.

110: LED driving unit 120: light emitting unit
130: detection unit 140: current-voltage conversion amplification unit
150: sampling and filtering unit 160: digital conversion unit
170: comparison unit 180: binary search logic unit
190: current converter

Claims (5)

An LED driver that generates current,
A light emitting unit that emits light by receiving the current generated from the LED driving unit,
A detection unit that detects the light reflected from the blood vessel inside the finger approaching the light emitting unit, converts it into a current, and outputs it,
A current-voltage conversion amplifying unit that receives the converted current from the detection unit, converts it into a voltage, amplifies and outputs it,
A sampling and filtering unit receiving the converted and amplified voltage from the current-voltage conversion amplifying unit, sampling and filtering, and outputting the sample,
A digital conversion unit for receiving the filtered voltage from the sampling and filtering unit, converting it into a digital value, and outputting a digital value;
A comparison unit that receives the output signal of the current-voltage conversion amplifying unit and compares whether it is positive or negative,
A binary search logic unit for searching a binary search logic for a value closest to 0 of the output signal of the current-voltage conversion amplifying unit according to the comparison result of the comparison unit;
And a current conversion unit converting the value searched by the binary search logic unit into an analog value and outputting the converted value to the current-voltage conversion amplifying unit. 15. An automatic offset calibration device based on binary search for an oxygen saturation meter. [2] The apparatus of claim 1, wherein the binary search logic unit is configured by connecting a plurality of D-flip-flops in series. [3] The apparatus of claim 2, wherein the binary search logic unit is composed of a two-row DFF array in which the plurality of D-flip-flops are connected in series. [4] The apparatus of claim 3, wherein the DFF of the first row of the two rows of DFF arrays connected in series serves as a counter in the form of a shift register. [4] The apparatus of claim 3, wherein each of the DFFs is composed of a DFF having an asynchronous set and an asynchronous reset signal.

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