Patent ID: 12193846

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present specification discloses an ambient light cancellation circuit functioning as a multi-order filter for filtering out an ambient light signal. The ambient light cancellation circuit is applicable to a Photoplethysmography (PPG) device, but the application of the present invention is not limited thereto.

FIG.1shows an embodiment of the ambient light cancellation circuit of the present disclosure. The ambient light cancellation circuit100ofFIG.1functions as a Kth-order filter to sample a detection signal SPD(K+1) times during a sampling period and thereby filter out an ambient-light signal of the detection signal SPD, wherein the K is a positive integer (e.g., an integer greater than one) and the detection signal SPDis generated by a photoelectric device (e.g., a photo diode (PD)).

Referring toFIG.1, the ambient light cancellation circuit100includes a capacitive transimpedance amplifying circuit110. The capacitive transimpedance amplifying circuit110includes an amplifier112, a capacitor circuit114, and a switch circuit116. The details of these circuits are described in the following paragraphs.

Referring toFIG.1, the amplifier112includes an input node (i.e., the symbol “+” of the amplifier112), an inverting input node (i.e., the symbol “−” of the amplifier112), and an output node. The input node is for receiving a reference voltage VREFand the inverting input node is for receiving the detection signal SPD, wherein the reference voltage VREFcan be determined according to the demand for implementation.

Referring toFIG.1, the capacitor circuit114includes a first capacitive path1142. The first capacitive path1142includes a first capacitor C1. The first capacitor C1includes a first electrode C11and a second electrode C12.

Referring toFIG.1, the switch circuit116includes a first switch SW1(+), a second switch SW2(+), a third switch SW3(−), and a fourth switch SW4(−). The first switch SW1(+) is set between the first electrode C11and the inverting input node; the second switch SW2(+) is set between the second electrode C12and the output node; the third switch SW3(−) is set between the second electrode C12and the inverting input node; and the fourth switch SW4(−) is set between the first electrode C11and the output node. The first switch SW1(+) and the second switch SW2(+) are scheduled to be turned on during N time slot(s) and to be turned off during M time slot(s) so as to couple the first electrode C11with the inverting input node and couple the second electrode C12with the output node during the N time slot(s) and thereby allow the first capacitor C1to sample the detection signal SPDduring the N time slot(s). The N time slot(s) and the M time slot(s) are included in the aforementioned sampling period, each of the N and the M is a positive integer, and the sum of the N and the M is not greater than (K+1) (e.g., (N+M)=(K+1)). The third switch SW3(−) and the fourth switch SW4(−) are scheduled to be turned on during the M time slot(s) and to be turned off during the N time slot(s) so as to couple the second electrode C12with the inverting input node and couple the first electrode C11with the output node during the M time slot(s) and thereby allow the first capacitor C1to sample the inversion of the detection signal SPDduring the M time slot(s). It is noted that the definition of the detection signal SPDand the definition of the inversion of the detection signal SPDare corresponding definitions, and thus are interchangeable.

Referring toFIG.1, the detection signal SPDincludes a controllable-light signal (e.g., a light signal originated from an LED) and the ambient-light signal during I time slot(s); the detection signal SPDincludes the ambient-light signal without including the controllable-light signal during J time slot(s); the I time slot(s) can be the N time slot(s) or the M time slot(s). When the I time slot(s) is/are the N time slot(s), the J time slot(s) is/are the M time slot(s); and when the I time slot(s) is/are the M time slot(s), the J time slot(s) is/are the N time slot(s). For example, the ambient light cancellation circuit100is applied to a PPG device, the controllable-light signal is originated from at least one controllable light source (e.g., at least one LED) of the PPG device, the at least one controllable light source is turned on to emit light during the N time slot(s), and is turned off to stop emitting light during the M time slot(s), and the ambient light signal varies with the variation in the intensity of the ambient light in the same space during the (N+M) time slots; on the basis of the above: the switch circuit116couples the amplifier112with the capacitor circuit114in a non-cross manner during the N time slot(s) to allow the capacitor circuit114to sample the detection signal SPDand generate N mixed-light sampling result(s); the switch circuit116couples the amplifier112with the capacitor circuit114in a cross manner during the M time slot(s) to allow the capacitor circuit114to sample the inversion of the detection signal SPDand generate M ambient-light sampling result(s); and the (N+M) sampling results are related with the variation in the charges stored in the capacitor circuit114and can be learned from the variation in the voltage at the output node of the amplifier112. It is noted that the control over the switch circuit116can be realized with known/self-developed technologies. It is also noted that the control over the switch circuit116is corresponding to the control over the aforementioned at least one controllable light source. The control over the at least one controllable light source falls beyond the scope of the present disclosure.

Referring toFIG.1, the switch circuit116controls the coupling between the amplifier112and the capacitor circuit114to make the capacitive transimpedance amplifying circuit110perform time domain sampling as in Z-domain (i.e., Z-transform) as shown inFIG.2and obtain the aforementioned N mixed-light sampling result(s) and the M ambient-light sampling result(s), wherein the Z-domain sampling and the time domain are known in this technical field. Providing (M+N)=(K+1), the Kth-order filter (i.e., the ambient light cancellation circuit100) is characterized by the conversion equation “c0z0+c1z−1+c2z−2+ . . . +cKz−K”. Regarding the above equation, c0˜cKare filter coefficients of the Kth-order filter and can be determined and/or adjusted according to the demand for implementation, and z0˜z−Kare (K+1) sampling results (i.e., the N mixed-light sampling result(s) and the M ambient-light sampling result(s)) obtained in sampling time sequence (i.e., the sequence of T0, T1, T2, T3, . . . , and TKinFIG.2). For example, if (M+N)=(K+1) and N=M=2, the Kth-order filter is a third-order filter; when the (M+N) sampling results are “a mixed-light sampling result obtained at T0, an ambient-light sampling result obtained at T1, an ambient-light sampling result obtained at T2, and a mixed-light sampling result obtained at T3” in sequence, the conversion equation of the third-order filter can be expressed as “1z0−z1−z−2+z−3”, which means the filtration result of the third-order filter is the sum of z0, −z−1, −z−2, and z−3. In the above example, after obtaining the four sampling results z0, −z−1, −z−2, and z−3, the third-order filter outputs the sum of the sampling results (i.e., the sum of z0, −z−1, −z−2, and z−3) to a back-end circuit (e.g., an analog-to-digital converter).

The filter coefficients c0˜cKof the conversion equation “c0z0+c1z−1+c2z−2+ . . . +cKz−K” can be determined and/or dynamically adjusted according to the demand for implementation. For example, as shown inFIGS.3a˜3b, the capacitor circuit114further includes a second capacitive path1144. The second capacitive path1144is coupled with the first capacitive path1142in parallel, and includes a second capacitive-path switch SWC2and a second capacitor C2. InFIGS.3a-3b, the capacitance of the second capacitor C2is equal to the capacitance of the first capacitor C1, and each of the two capacitances is

C2;
accordingly, the equivalent capacitance of the parallel-connected capacitors C1and C2is

C2+C2=C.
Referring toFIG.2andFIG.3a, during the even time slots T0and T2, the switch circuit116couples the amplifier112with the capacitor114in a non-cross manner while the second capacitive-path switch SWC2is turned on, and thus the first capacitive path1142and the second capacitive path1144sample the detection signal SPDsimultaneously (i.e., both the first capacitor C1and the second capacitor C2are charged/discharged); providing the charge/discharge amount in a unit of time ΔT is Q1, the voltage difference between the inverting input node and the output node of the amplifier112is

V1=Q1C.
Referring toFIG.2andFIG.3b, during the odd time slot T1, the switch circuit116couples the amplifier112with the capacitor114in a cross manner while the second capacitive-path switch SWC2is turned off, and thus the first capacitive path1142samples the inversion of the detection signal SPD(i.e., only the first capacitor C1is discharged/charged); providing the discharge/charge amount in the unit of time ΔT is Q2, the voltage difference between the inverting input node and the output node of the amplifier112is

V2=-Q2C2=-2⁢Q2C;
as a result, if Q2≈Q1, V2≈−2V1. In light of the above, the ambient-light cancellation100functions as a second-order filter and generates a filtration result “1z0−2z−1+z−2” which can be scaled up/down with a known/self-developed analog/digital manner. It is noted that the K is a positive integer in the example ofFIGS.3a˜′3b.

The example ofFIGS.3a˜3bcan be modified as shown inFIGS.4a˜4c. Referring toFIGS.4a˜4c, the capacitive path114includes j capacitive paths from the 1stcapacitive path (which includes a capacitor n1×C) to the jthcapacitive path (which includes a capacitor n1×C), wherein each capacitive path includes a capacitor, the jthcapacitive path further includes a jthcapacitive-path switch SWCj, each of n1˜njis a scaling factor and can be determined according to the demand for implementation, C denotes a unit of capacitance, and the j is an integer greater than two.FIGS.4a˜4cshow the sampling operation during three consecutive time slots (i.e., the three consecutive time slots T0, T1, and T2inFIG.2). Regarding the example ofFIGS.4a-4c, the ambient-light cancellation circuit100functions as a second-order filter and generates a filtration result

z0-∑(n1,…,nj)∑(n1,…,nm)⁢z-1+∑(n1,…,nj)∑(n1,…,nk)⁢z-2
which can be scaled up/down with a known/self-developed analog/digital manner. Those having ordinary skill in the art can derive more examples from the examples ofFIGS.3a-4c.

The embodiments of the present disclosure could be modified to include at least one of the following features:(1) Compared with the example ofFIGS.3a˜3b, the second capacitive-path switch SWC2is set at the other side of the second capacitor C2as shown inFIG.5; and compared with the example ofFIGS.4a˜4c, the jthcapacitive-path switch SW0is set at the other side of the jthcapacitor nj×C as shown inFIG.6.(2) Compared with the example ofFIGS.3a˜3b, during the even time slots T0and T2as shown inFIG.2, the switch circuit116couples the amplifier112with the capacitor circuit114in a non-cross manner while the second capacitive-path switch SWC2is turned off; during the odd time slot T1, the switch circuit116couples the amplifier112with the capacitor circuit114in a cross manner while the second capacitive-path switch SWC2is turned on; and accordingly, the ambient light cancellation circuit100generates a filtration result as follows: 2×(1z0−½z−1+z−2).(3) Referring toFIGS.3a˜′3b, when the switch circuit116couples the amplifier112with the capacitor circuit114in a non-cross manner during a certain time slot, the second capacitive-path switch SWC2can optionally be turned on or turned off, wherein the certain time slot is or is not one of the aforementioned N time slot(s); and when the switch circuit116couples the amplifier112with the capacitor circuit114in a cross manner during a certain time slot, the second capacitive-path switch SWC2can optionally be turned on or turned off, wherein the certain time slot is or is not one of the aforementioned M time slot(s).(4) Referring toFIGS.1˜4c, any capacitor of the capacitor circuit114is a capacitor of fixed capacitance or a capacitor of adjustable capacitance. For example, the first capacitor C1of the first capacitive path1142is an adjustable capacitor; and when the capacitive transimpedance amplifying circuit110performs sampling during each time slot, the capacitance of the adjustable capacitor can be determined according to the required filtration result.(5) When the N is greater than one, any two of the N time slots are equal in time length (e.g., microseconds) or unequal in time length; and when the M is greater than one, any two of the M time slots are equal in time length (e.g., microseconds) or unequal in time length.(6) Any of the N time slot(s) is equal to any of the M time slot(s) in time length (e.g., two microseconds).

The example ofFIGS.3a˜3bcan be modified as shown inFIG.7. As shown inFIG.7, the capacitor circuit114includes a plurality of capacitive paths coupled in parallel. The plurality of capacitive paths includes the first capacitive path1142. Each of the plurality of capacitive paths except the first capacitive path1142includes a switch and a capacitor, wherein the conducting state of the said switch (i.e., the switch being turned on or turned off) and the capacitance of the said capacitor are determined according to the demand for implementation to determine the filter coefficient(s) of the Kth-order filter (i.e., the ambient light cancellation circuit100) and realize a required filtration effect.

It is noted that people having ordinary skill in the art can selectively use some or all of the features of any embodiment in this specification or selectively use some or all of the features of multiple embodiments in this specification to implement the present invention as long as such implementation is practicable; in other words, the way to implement the present invention is flexible based on the present disclosure.

To sum up, the ambient light cancellation circuit of the present disclosure functions as a multi-order filter to filter out an ambient-light signal. Compared with the prior art, the ambient light cancellation circuit of the present disclosure can be implemented flexibly to realize many kinds of filtration effects.

The aforementioned descriptions represent merely the preferred embodiments of the present invention, without any intention to limit the scope of the present invention thereto. Various equivalent changes, alterations, or modifications based on the claims of the present invention are all consequently viewed as being embraced by the scope of the present invention.