A differential-mode driving circuit for a face recognition VCSEL module and a driving method for eliminating interference voltage spikes

Abstract

The present invention provides a differential-mode driving circuit for a face recognition VCSEL module with a simple structure, convenient implementation, and capable of effectively avoiding voltage spikes in the output signal, as well as a driving method for eliminating interference voltage spikes of this circuit. The circuit of the present invention includes a face recognition VCSEL module, a pulsed constant current source, a transimpedance amplifier, and a differential-mode circuit, and the differential-mode circuit is used to eliminate the voltage spike output caused by the parasitic capacitance existing in the face recognition VCSEL module (VCSEL Module); the method of the present invention eliminates the changes caused by the interference of the parasitic capacitance existing in the face recognition VCSEL module itself in the TIA detection circuit through the provided differential-mode circuit, avoids the phenomenon of voltage spikes, and ensures that the detected waveform has no interference voltage spikes. The present invention can be applied to the field of face recognition technology.

Description

Technical Field

[0001] The present invention relates to the technical field of face recognition, and in particular to a differential-mode driving circuit for a face recognition VCSEL module and a driving method for eliminating interference voltage spikes. Background Art

[0002] In recent years, in the face recognition systems of smart phones and the spatial recognition systems of tablet computer terminals, VCSEL (Vertical Cavity Surface Emitting Laser) has been adopted as a laser light source, which has rapidly popularized the application of VCSEL. In addition, in many other fields, including the applications of AGV used in industries and inspection systems through gesture and shape recognition, are also becoming more and more popular, and it is expected that the demand for VCSEL will further increase in the future.

[0003] Currently, the VCSEL module generally adopts a single-ended driving mode of operation. As Figure 1 shown is the schematic diagram of the VCSEL single-ended mode driving circuit. As Figure 1 shown, a face recognition excitation and monitoring analog front-end module (VCSEL Module) is composed of a VCSEL and a PD (photodiode). Among them, the VCSEL emits laser light under the pulsed current drive of an external laser driver, while the PD is responsible for receiving the light emitted by the VCSEL, reflecting the light, and converting the light into current. Through an external transimpedance amplifier TIA, the current is converted into voltage (where RC is the transconductance resistance that converts the detected current into voltage, and CC is the transconductance capacitance that limits the bandwidth of the collected current). By analyzing the voltage, the distance between the VCSEL and the object to be measured can be calculated. Based on the distance between the two, a recognized model or a three-dimensional picture can be established as needed. However, when the existing VCSEL module is working, due to the parasitic capacitance between the input end and the output end of the VCSEL, between the output end and the output end of the VCSEL, and between the input end and the output end of the PD, voltage spikes will appear at the output end of the PD, which will cause interference signals in the output of the entire module. Its output voltage waveform is as Figure 2 shown, which will further cause errors in the calculated distance between the VCSEL and the object to be measured. Therefore, it is necessary to design a driving circuit that can avoid the above interference signals to ensure the authenticity of the output signal. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a differential-mode driving circuit for a face recognition VCSEL module with a simple structure, convenient implementation, and capable of effectively avoiding voltage spikes in the output signal, as well as a driving method for eliminating interference voltage spikes of this circuit.

[0005] The technical solution adopted by the differential-mode drive circuit of the face recognition VCSEL module of the present invention is: The drive circuit includes

[0006] A face recognition VCSEL module for emitting laser light and detecting a front-end analog signal,

[0007] A pulse constant current source for driving a light-emitting diode in the face recognition VCSEL module, and

[0008] A transimpedance amplifier for converting a current signal output by the face recognition VCSEL module into a voltage signal and outputting it,

[0009] The pulse constant current source is connected to the input end of the light-emitting diode, and the transimpedance amplifier is connected to the output end of the photodiode of the face recognition VCSEL module. The drive circuit further includes a differential-mode circuit, and the differential-mode circuit is connected across the two ends of the light-emitting diode. The differential-mode circuit is used to eliminate the voltage spike output caused by the parasitic capacitance existing in the face recognition VCSEL module.

[0010] The differential-mode circuit includes a buffer, a first resistor, and a second resistor connected in sequence. The resistance values of the first resistor and the second resistor are equal. An operational amplifier is connected in parallel across the two ends of the second resistor. The first resistor, the second resistor, and the operational amplifier together form an inverting amplifier.

[0011] The transimpedance amplifier includes a transconductance capacitor, a transconductance resistor, and a low-noise amplifier connected in parallel with each other.

[0012] The above-mentioned driving method for eliminating interference voltage spikes of the differential-mode drive circuit of the face recognition VCSEL module. In this method, the parasitic capacitance between the input end of the light-emitting diode and the output end of the photodiode is set as CXA, the parasitic capacitance between the output end of the light-emitting diode and the output end of the photodiode is set as CXC, and the parasitic capacitance between the input end of the photodiode and the output end of the photodiode is set as CXP. This method includes the following steps:

[0013] Step a: Assume that the current input to the positive terminal of the buffer is 0 and the voltage VLDA at the input end of the light-emitting diode is equal to the voltage VBUFF at the input end of the first resistor, then there is

[0014] VBUFF = VLDA ………………………………………………………………………(1);

[0015] Step b: The first resistor, the second resistor, and the operational amplifier form an inverting amplifier. Since R1 = R2, assume that the voltage at the output end of the light-emitting diode is VLDC, then there is

[0016] VBUFF = -VLDC …………………………………………………………………… (2);

[0017] Step c, it can be obtained from Equation (1) and Equation (2) that:

[0018] VLDA = -VLDC ………………………………………………………………………(3);

[0019] Step d, define the voltage change at the input end of the light-emitting diode caused when current enters the VCSEL as 1 / 2*ΔV,

[0020] Let the voltage change at the output end of the photodiode caused by the parasitic capacitance CXA be ΔVX 1 , then there is

[0021] ΔVX 1 = 1 / 2*ΔV * CXA / (CXC + CXP)……………………………………………(4);

[0022] Step e, define the voltage change at the output end of the light-emitting diode caused when current enters the VCSEL as -1 / 2*ΔV;

[0023] Let the voltage change at the output end of the photodiode caused by the parasitic capacitance CXA be ΔVX 2 , then there is

[0024] ΔVX 2 = -1 / 2*ΔV * CXC / (CXA + CXP)……………………………………………(5);

[0025] Step f, then the total voltage change ΔVX at the output end (PDC) of the photodiode caused by the parasitic capacitance CXA and the parasitic capacitance CXC is

[0026] ΔVX = ΔVX 1 +ΔVX 2 = 0 …………………………………………………………(6);

[0027] Step g, the interference voltage spike of the output signal of the face recognition VCSEL module is eliminated.

[0028] The beneficial effects of the present invention are as follows: By setting a differential-mode circuit, the present invention successfully eliminates the influence of the voltage spikes in the output caused by the parasitic capacitance existing in the face recognition VCSEL module itself on the output signal, avoids the influence of interference signals on the output voltage signal, and ensures the authenticity and reliability of the output signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a simplified schematic diagram of a prior art VCSEL single-ended mode driving circuit;

[0030] Figure 2 is a voltage waveform diagram of the output of a prior art VCSEL single-ended mode driving circuit;

[0031] Figure 3 is a simplified schematic diagram of the differential-mode driving circuit of the VCSEL module of the present invention;

[0032] Figure 4 is a waveform diagram without voltage spikes of the output of the differential-mode driving circuit of the VCSEL module of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] The embodiments of the present invention are specifically as follows.

[0034] As Figure 3 and Figure 4 shown, the driving circuit of the present invention includes a face recognition VCSEL module VCSEL Module for emitting laser and detecting a front-end analog signal,

[0035] a pulse constant current source LDD for driving the light-emitting diode VCSEL in the face recognition VCSEL module VCSEL Module, and

[0036] a transimpedance amplifier TIA for converting the current signal output by the face recognition VCSEL module VCSEL Module into a voltage signal and outputting it,

[0037] The pulse constant current source LDD is connected to the input end of the light-emitting diode VCSEL, and the transimpedance amplifier TIA is connected to the output end of the photodiode PD of the face recognition VCSEL module VCSEL Module. The face recognition VCSEL module VCSEL Module is a face recognition excitation and detection analog front-end module, where VCSEL is a vertical cavity surface-emitting laser, and it emits laser under the drive of a pulsed current by an external laser driver. By changing the period, current duty cycle, and current rise and fall of the pulse constant current source LDD, different current values are output to meet the usage requirements of different scenarios. The photodiode PD is responsible for receiving the light emitted by the VCSEL and converting the light into current, and then converting the current into a voltage signal through the transimpedance amplifier TIA. The transimpedance amplifier TIA includes a transconductance capacitor CC, a transconductance resistor RC, and a low-noise amplifier LNA connected in parallel. Among them, the transconductance resistor RC converts the detected current into voltage, and the transconductance capacitor CC is used to limit the bandwidth of the collected current. By analyzing the time difference and algorithm of the voltage, the distance between the VCSEL and the object to be measured is calculated. In addition, the LNA is a low-noise amplifier, and the voltage at the PDC end of the photodiode PD is clamped to the VBIAS voltage due to feedback, and the user can adjust the VBIAS voltage to control different bias voltages of the photodiode PD.

[0038] The drive circuit further includes a differential mode circuit, and the differential mode circuit is connected across the light-emitting diode VCSEL, and the differential mode circuit is used to eliminate the voltage spike output caused by the parasitic capacitance existing in the face recognition VCSEL module VCSEL Module. The differential mode circuit includes a buffer OP0, a first resistor R1, and a second resistor R2 connected in sequence. The resistance values of the first resistor R1 and the second resistor R2 are equal, and an operational amplifier OP1 is connected in parallel across the second resistor R2. The first resistor R1, the second resistor R2, and the operational amplifier OP1 together form an inverting amplifier.

[0039] In the above method for driving the differential mode of the face recognition VCSEL module to eliminate the interference voltage spike, it is assumed that the parasitic capacitance between the input end LDA of the light-emitting diode VCSEL and the output end PDC of the photodiode PD is CXA, the parasitic capacitance between the output end LDC of the light-emitting diode VCSEL and the output end PDC of the photodiode PD is CXC, and the parasitic capacitance between the input end PDA of the photodiode PD and the output end PDC of the photodiode PD is CXP. The method includes the following steps:

[0040] Step a: Assume that the current input to the positive terminal of the buffer OP0 is 0 and the voltage VLDA at the input terminal LDA of the light-emitting diode VCSEL is equal to the voltage VBUFF at the input terminal of the first resistor R1. Then, we have

[0041] VBUFF = VLDA ………………………………………………………………………(1);

[0042] Step b: The first resistor R1, the second resistor R2, and the operational amplifier OP1 form an inverting amplifier. Since R1 = R2, assume that the voltage at the output terminal LDC of the light-emitting diode VCSEL is VLDC. Then, we have

[0043] VBUFF = -VLDC …………………………………………………………………… (2);

[0044] Step c: From equations (1) and (2), we can obtain:

[0045] VLDA = -VLDC ………………………………………………………………………(3);

[0046] Step d: Define the voltage change at the input terminal LDA of the light-emitting diode VCSEL caused by the current entering the VCSEL as 1 / 2*ΔV.

[0047] Assume that the voltage change at the output terminal PDC of the photodiode PD caused by the parasitic capacitance CXA is ΔVX 1 , then we have

[0048] ΔVX 1 = 1 / 2*ΔV * CXA / (CXC + CXP)……………………………………………(4);

[0049] Step e: Define the voltage change at the output terminal LDC of the light-emitting diode VCSEL caused by the current entering the VCSEL as -1 / 2*ΔV;

[0050] Assume that the voltage change at the output terminal PDC of the photodiode PD caused by the parasitic capacitance CXA is ΔVX 2 , then we have

[0051] ΔVX 2 = -1 / 2*ΔV * CXC / (CXA + CXP)……………………………………………(5);

[0052] Step f: Then, the total voltage change ΔVX at the output terminal PDC of the photodiode PD caused by the parasitic capacitance CXA and the parasitic capacitance CXC is

[0053] ΔVX = ΔVX 1 +ΔVX 2 = 0 …………………………………………………………(6);

[0054] Step g, the interference voltage spikes of the output signal of the facial recognition VCSEL module are eliminated.

[0055] Thus, the changes caused by the interference caused by the parasitic capacitance existing in the VCSEL driving circuit itself in the TIA detection circuit are successfully eliminated, ensuring the authenticity and reliability of the detection results.

Claims

1. A differential-mode driving circuit for a face recognition VCSEL module, comprising a face recognition VCSEL module (VCSEL Module) for emitting laser light and detecting a front-end analog signal, a pulse constant current source (LDD) for driving a light-emitting diode (VCSEL) in the face recognition VCSEL module (VCSEL Module), and a transimpedance amplifier (TIA) for converting a current signal output by the face recognition VCSEL module (VCSEL Module) into a voltage signal and outputting it, the pulse constant current source (LDD) is connected to an input end of the light-emitting diode (VCSEL), and the transimpedance amplifier (TIA) is connected to an output end of a photodiode (PD) of the face recognition VCSEL module (VCSEL Module), characterized in that: the driving circuit further includes a differential-mode circuit, the differential-mode circuit is connected across the two ends of the light-emitting diode (VCSEL), and the differential-mode circuit is used to eliminate a voltage spike output caused by a parasitic capacitance existing in the face recognition VCSEL module (VCSEL Module); the differential-mode circuit includes a buffer (OP0), a first resistor (R1) and a second resistor (R2) connected in sequence, the resistance values of the first resistor (R1) and the second resistor (R2) are equal, and an operational amplifier (OP1) is connected in parallel across the two ends of the second resistor (R2), and the first resistor (R1), the second resistor (R2) and the operational amplifier (OP1) together form an inverting amplifier.

2. A differential-mode driving circuit for a face recognition VCSEL module according to claim 1, characterized in that: the transimpedance amplifier (TIA) includes a transconductance capacitor (CC), a transconductance resistor (RC) and a low-noise amplifier (LNA) connected in parallel with each other.

3. A driving method for eliminating interference voltage spikes of a differential-mode driving circuit for a face recognition VCSEL module as described in claim 2. In this method, the parasitic capacitance between the input end (LDA) of the light-emitting diode (VCSEL) and the output end (PDC) of the photodiode (PD) is set as CXA, the parasitic capacitance between the output end (LDC) of the light-emitting diode (VCSEL) and the output end (PDC) of the photodiode (PD) is set as CXC, and the parasitic capacitance between the input end (PDA) of the photodiode (PD) and the output end (PDC) of the photodiode (PD) is set as CXP, characterized in that, the method includes the following steps: Step a, assume that the current input to the positive terminal of the buffer (OP0) is 0 and the voltage VLDA at the input end (LDA) of the light-emitting diode (VCSEL) is equal to the voltage VBUFF at the input end of the first resistor (R1), then there is VBUFF = VLDA …………………………………………………………………………(1); Step b: A reverse amplifier is composed of the first resistor (R1), the second resistor (R2), and the operational amplifier (OP1). Since R1 = R2, assuming the voltage at the output terminal (LDC) of the light-emitting diode (VCSEL) is VLDC, we have VBUFF = -VLDC ……………………………………………………………………… (2); Step c: From equations (1) and (2), it can be obtained that VLDA = -VLDC …………………………………………………………………………(3); Step d: Define the voltage change at the input terminal (LDA) of the light-emitting diode (VCSEL) caused by the current entering the VCSEL as 1 / 2*ΔV, Let the voltage change at the output terminal (PDC) of the photodiode (PD) caused by the parasitic capacitance CXA be ΔVX 1 , then there is ΔVX 1 = 1 / 2*ΔV * CXA / (CXC+CXP)………………………………………………(4); Step e: Define the voltage change at the output terminal (LDC) of the light-emitting diode (VCSEL) caused by the current entering the VCSEL as -1 / 2*ΔV; Let the voltage change at the output terminal (PDC) of the photodiode (PD) caused by the parasitic capacitance CXC be ΔVX 2 , then we have ΔVX 2 = -1 / 2*ΔV * CXC / (CXA+CXP)………………………………………………(5); Step f: Then the total voltage change ΔVX at the output terminal (PDC) of the photodiode (PD) caused by the parasitic capacitance CXA and the parasitic capacitance CXC is ΔVX = ΔVX 1 +ΔVX 2 = 0 ……………………………………………………………(6); The interference voltage spike of the output signal of the face recognition VCSEL module is eliminated.

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