TECHNICAL FIELD

Embodiments of the present application relate to the field of electronic technologies, and more particularly, to a current cancellation circuit, a heart rate detection device and a wearable device.

BACKGROUND

Photoplethysmograph (PPG) technology is a technology for human exercise heart rate detection. Specifically, a photoelectric sensor may be configured to detect an intensity of reflected light after absorption by human blood and tissues, then, trace a change in blood vessel volume during a cardiac cycle, and a human heart rate was calculated according to the traced pulse waveform.

In PPG detection, if there is too much background light, a channel of a PPD detection circuit is saturated, and then the detection fails. In related technologies, an additional background light cancellation (BGC) circuit is configured to cancel the background light to ensure the normal operation of the channel. However, the use of BGC will increase noise of the channel, and the use of the BGC circuit requires participation of an external algorithm circuit to cancel the background light, which increases complexity and cost of the detection circuit.

SUMMARY

Embodiments of the present application provide a current cancellation circuit, a heart rate detection device and a wearable device, which could reduce complexity and cost of a PPG detection circuit.

A current-voltage conversion circuit and a successive approximation SAR analog-to-digital converter ADC, where the SAR ADC includes a digital-to-analog converter DAC, an SAR logic circuit and a comparator;

the current-voltage conversion circuit is configured to receive an analog current output by the DAC and an interference current output by a photoelectric sensor, calculate a difference between the analog current and the interference current, and output an analog voltage, where the interference current is obtained by photoelectric conversion of an interference light signal by the photoelectric sensor;

the comparator is configured to receive the analog voltage output by the current-voltage conversion circuit, and output a comparison result according to the analog voltage; and

the DAC is configured to output the analog current according to a digital signal corresponding to the comparison result that is output by the SAR logic circuit, and the analog current is used to cancel the interference current output by the photoelectric sensor.

In some possible implementation manners, the SAR logic circuit is particularly configured to:

determine the digital signal, and control, according to the digital signal, the DAC to output a next analog current so that the next analog current is more approximate to the interference current.

In some possible implementation manners, the SAR logic circuit determines the digital signal according to a dichotomy.

In some possible implementation manners, the DAC is particularly configured to:

output, according to a current analog current and the digital signal, a next analog current, and input the next analog current to the current-voltage conversion circuit.

In some possible implementation manners, the DAC is a resistive DAC, the resistive DAC includes N first resistors, the current cancellation circuit further includes N first switches, and the N first resistors are in one-to-one correspondence to the N first switches, where N is a number of bits of the digital signal output by the SAR ADC; and the SAR logic circuit is further configured to:

control, according to the digital signal, a corresponding first switch so that a first resistor corresponding to the first switch is connected to a first voltage or a second voltage.

In some possible implementation manners, the SAR logic circuit is particularly configured to:

control, if the digital signal is 1, the first switch so that the first resistor corresponding to the first switch is connected to the first voltage; or

control, if the digital signal is 0, the first switch so that the first resistor corresponding to the first switch is connected to the second voltage.

In some possible implementation manners, the DAC is a single-ended resistive DAC, and the current-voltage conversion circuit is a transimpedance amplifier TIA, where the TIA includes a first input end, a common mode input end, a first output end and a second output end, an output end of the single-ended resistive DAC is connected to the first input end of the TIA, an output end of the photoelectric sensor is also connected to the first input end of the TIA, the common mode input end of the TIA is configured to input a common mode voltage, the first output end of the TIA is connected to a first input end of the comparator, the second output end of the TIA is connected to a second input end of the comparator, an output end of the comparator is connected to an input end of the SAR logic circuit, and an output end of the SAR logic circuit is connected to an input end of the single-ended resistive DAC.

In some possible implementation manners, the DAC is a differential resistive DAC, the differential resistive DAC further includes N second resistors, the current cancellation circuit further includes N second switches, the N second resistors are in one-to-one correspondence to the N second switches, and the SAR logic circuit is further configured to:

control, according to an inverted signal of the digital signal, a corresponding second switch so that a second resistor corresponding to the second switch is connected to the second voltage or the first voltage.

In some possible implementation manners, the SAR logic circuit is particularly configured to:

control, if the digital signal is 1, the second switch so that the second resistor corresponding to the second switch is connected to the second voltage; or

control, if the digital signal is 0, the second switch so that the second resistor corresponding to the second switch is connected to the first voltage.

In some possible implementation manners, the current-voltage conversion circuit is a differential TIA, the differential TIA includes a first input end, a second input end, a common mode input end, a first output end and a second output end, a first output end and a second output end of the differential resistive DAC are respectively connected to the first input end and the second input end of the differential TIA, a first end and a second end of the photoelectric sensor are respectively connected to the first input end and the second input end of the differential TIA, the common mode input end of the differential TIA is configured to input a common mode voltage, the first output end and the second output end of the differential TIA are respectively connected to a first input end and a second input end of the comparator, an output end of the comparator is connected to an input end of the SAR logic circuit, and an output end of the SAR logic circuit is connected to an input end of the differential resistive DAC.

In some possible implementation manners, the first voltage is a reference voltage, and the second voltage is a ground voltage or a common mode voltage.

In some possible implementation manners, an output end of the DAC is connected to an input end of the current-voltage conversion circuit, the input end of the current-voltage conversion circuit is simultaneously connected to an output end of the photoelectric sensor, output ends of the current-voltage conversion circuit is connected to input ends of the comparator, an output end of the comparator is connected to an input end of the SAR logic circuit, and an output end of the SAR logic circuit is connected to an input end of the DAC.

In some possible implementation manners, the photoelectric sensor is a photodiode.

In a second aspect, a heart rate detection device is provided, including:

the current cancellation circuit in the first aspect or any one of possible implementation manners of the first aspect.

In some possible implementation manners, the heart rate detection device further includes:

a transmitting circuit configured to transmit a light signal for heart rate detection; and

a receiving circuit including a photoelectric sensor and an analog-to-digital converter ADC;

where the photoelectric sensor is connected to an input end of a current-voltage conversion circuit in the current cancellation circuit, and configured to receive the light signal, and perform photoelectric conversion on the received light signal to obtain a current signal; the current signal is input to the input end of the current-voltage conversion circuit; and

the ADC is connected to an output end of the current-voltage conversion circuit, and configured to receive an analog voltage output by the current-voltage conversion circuit.

In some possible implementation manners, in a first stage, the transmitting circuit does not transmit a light signal, the photoelectric sensor receives an interference light signal and performs photoelectric conversion on the interference light signal to obtain an interference current, the current cancellation circuit determines a digital signal output by a successive approximation SAR analog-to-digital converter ADC in the current cancellation circuit according to the interference current and controls a digital-to-analog converter DAC in the current cancellation circuit to output a corresponding analog current according to the digital signal, and the analog current is used to cancel the interference current; and in a second stage, the transmitting circuit transmits a light signal for heart rate detection, the photoelectric sensor receives the interference light signal and the light signal for heart rate monitoring, and performs photoelectric conversion on the interference light signal and the light signal for heart rate monitoring to obtain a mixed current, where the mixed current includes the interference current and a signal current generated from the light signal for heart rate monitoring, the DAC outputs the analog current, and the current-voltage conversion circuit receives the mixed current and the analog current, calculates a difference between the mixed current and the analog current, and outputs the analog voltage to the ADC.

In a third aspect, a wearable device is provided, the heart rate detection device in the second aspect or any one of possible implementation manners of the second aspect.

Based on the foregoing technical solutions, a current cancellation circuit of the embodiments of the present application controls an analog current output by a DAC through SAR logic inside an SAR ADC so that the analog current is successively approximate to an interference current to be cancelled out without participation of an additional algorithm circuit, which is beneficial for simplifying a circuit structure and reducing circuit cost.

