Analog front end receiving circuit, noise cancellation unit and method for active pen

Abstract

The invention provides an analog front-end receiving circuit for an active pen, a noise cancellation unit and a noise cancellation method. The circuit comprises a current transmitter, a band-pass filter and a comparator which are connected in sequence, the current transmitter receives an input signal and outputs a current signal; the band-pass filter receives the current signal, filters a noise signal and then outputs a first voltage signal; the comparator receives the first voltage signal and outputs a digital square wave signal. In a multi-channel parallel working mode, in-band common-mode interference with overlapped frequency and active pen signals is actively counteracted through current averaging and subtraction operation by utilizing spatial correlation of common-mode noise on each induction channel, and the signal-to-noise ratio and the anti-interference capability of the system are improved.

Description

Technical Field

[0001] This invention relates to the field of touch technology, and more specifically to an analog front-end receiving circuit, noise cancellation unit, and method for an active pen. Background Technology

[0002] In an active pen touch system, the pen tip interacts with the touchscreen by emitting high-frequency electrical signals to achieve precise writing and drawing functions. Typically, the pen tip emits a square wave signal with a relatively high amplitude (e.g., 10V). This signal is coupled to the touchscreen's self-capacitance (Cs, typically hundreds of pF, e.g., 600pF) through the parasitic capacitance (Cf, approximately 0.1-0.3pF) between the pen tip and the screen's sensing electrodes. Because the self-capacitance Cs is much larger than the coupling capacitance Cf, the signal undergoes severe attenuation during transmission, and the signal amplitude reaching the receiving end is only on the order of microvolts (μV), which is an extremely weak signal.

[0003] Meanwhile, touchscreens generate significant common-mode interference noise during normal operation. This is especially true for liquid crystal displays (LCDs), where the Vcom noise generated by the liquid crystal driving circuit can reach hundreds of millivolts. This noise, coupled with the weak pen signal, is coupled to the self-capacitance Cs, resulting in an extremely low signal-to-noise ratio (SNR), completely drowning out the useful signal. Traditional voltage-mode amplifiers (such as instrumentation amplifiers), due to their high input impedance, completely pick up this common-mode noise and, limited by a finite common-mode rejection ratio (CMRR), struggle to effectively extract the weak differential-mode pen signal. Therefore, how to faithfully recover the pen signal under conditions of large capacitive loads and strong common-mode noise is a pressing technical challenge in this field. Summary of the Invention

[0004] The purpose of this invention is to solve the signal attenuation problem caused by large capacitor loads in the prior art, and the technical problem that single-channel filters cannot suppress in-band common-mode noise in the same frequency band as the active pen signal.

[0005] A first aspect of the present invention provides an analog front-end receiving circuit for an active pen, comprising a current transmitter, a bandpass filter, and a comparator connected in sequence. The current transmitter receives an input signal and outputs a current signal; the bandpass filter receives the current signal, filters out noise signals, and outputs a first voltage signal; the comparator receives the first voltage signal and outputs a digital square wave signal.

[0006] Furthermore, the inverting input terminal of the current transmitter is connected to the output terminal of the current transmitter, a self-capacitance is formed between the inverting input terminal of the current transmitter and ground, and a parasitic capacitance is formed between the inverting input terminal of the current transmitter and the active probe.

[0007] Furthermore, the bandpass filter includes a transimpedance amplifier and a capacitively coupled amplifier connected in series. The inverting input terminal of the transimpedance amplifier is connected to the output terminal of the current transmitter, which converts the current signal into a voltage signal and performs low-pass filtering. The inverting input of the capacitively coupled amplifier is connected to the output of the transimpedance amplifier via a capacitor to block DC offset and perform high-pass filtering.

[0008] Furthermore, the cutoff frequency of the transimpedance amplifier is higher than the active pen signal frequency; the cutoff frequency of the capacitively coupled amplifier is lower than the active pen signal frequency.

[0009] Furthermore, the transimpedance amplifier is also connected to a first feedback network, which includes a first feedback resistor and a first feedback capacitor connected in parallel between the inverting input terminal and the output terminal of the transimpedance amplifier.

[0010] Furthermore, the capacitively coupled amplifier is also connected to a second feedback network, which includes a second feedback resistor and a second feedback capacitor connected in parallel between the inverting input terminal and the output terminal of the capacitively coupled amplifier.

[0011] A second aspect of the present invention provides a noise cancellation unit comprising a plurality of analog front-end receiving circuits for an active pen as described in any of the preceding claims, wherein the plurality of analog front-end receiving circuits operate in parallel and correspond to a plurality of sensing channels of a touch screen, wherein a current transmitter in each of the analog front-end receiving circuits receives an input signal and outputs a net current signal, and after processing, outputs a corresponding digital square wave signal.

[0012] Furthermore, the gates of the current mirrors in the current transmitters of the multiple analog front-end receiving circuits are connected in sequence to form a common gate node.

[0013] Furthermore, the common gate node generates an average current, which is the average value of the current signals output by the plurality of current transmitters; Each of the analog front-end receiving circuits receives the net current signal, which is the difference between the current signal and the average current.

[0014] The third invention provides a noise cancellation method, comprising: Obtain the output current of the current transmitter in multiple analog front-end receiving circuits; Calculate the average current of the plurality of said current transmitters; The net current signal is obtained by subtracting the average current from the output current of each current transmitter. The net current signal is processed sequentially through a transimpedance amplifier and a capacitively coupled amplifier. The processed signal is converted into a digital square wave signal using a comparator.

[0015] Compared to existing technologies, this invention offers at least the following advantages: It converts the weak charge signal coupled by capacitance into a current signal via a current transmitter, and employs current-domain processing to reduce the attenuation effect of large capacitive loads on the signal, thereby improving receiver sensitivity; it utilizes a bandpass filter to selectively filter out out-of-band noise, improving the signal-to-noise ratio. In multi-channel parallel operation mode, it actively cancels in-band common-mode interference (such as liquid crystal driving noise) whose frequency overlaps with the active pen signal by using current averaging and subtraction operations and leveraging the spatial correlation of common-mode noise in each sensing channel, thus improving the system's signal-to-noise ratio and anti-interference capability. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained as provided without creative effort.

[0017] Figure 1 This is a schematic diagram of the simulated front-end receiving circuit in Embodiment 1 of the present invention; Figure 2 This is a circuit diagram of the noise cancellation unit in Embodiment 2 of the present invention; Figure 3 To adopt Figure 2 The waveform obtained from the noise cancellation unit in the image; Figure 4 This is a schematic diagram of the steps of the noise cancellation method in Embodiment 3 of the present invention.

[0018] Where Cs is the self-capacitance; Cf is the parasitic capacitance; C1 is the integrating capacitor; C2 is the first feedback capacitor; C3 is the second feedback capacitor; C4 is the capacitor; R1 is the first feedback resistor; R2 is the second feedback resistor. A1 is the current transmitter; A2 is the transimpedance amplifier; A3 is the capacitively coupled amplifier; A4 is the comparator. Detailed Implementation

[0019] The present invention will now be described in more detail with reference to the accompanying drawings, which illustrate preferred embodiments of the invention. It should be understood that those skilled in the art can modify the invention described herein while still achieving its advantageous effects. Therefore, the following description should be understood as being broadly known to those skilled in the art and is not intended to limit the invention.

[0020] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0021] The invention is described more specifically by way of example in the following paragraphs with reference to the accompanying drawings. The advantages and features of the invention will become clearer as explained below. It should be noted that the drawings are in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of the invention.

[0022] Example 1 This embodiment provides an analog front-end receiving circuit for an active pen. Please refer to [reference needed]. Figure 1 It includes a current transmitter A1, a bandpass filter, and a comparator A4 connected in sequence.

[0023] The current transmitter A1 receives the input signal and outputs a current signal; the bandpass filter receives the current signal, filters out noise signals, and outputs a first voltage signal; the comparator A4 receives the first voltage signal and outputs a digital square wave signal VOUT.

[0024] Specifically, the input signal is first fed into the current transmitter A1 for preliminary processing, and its output is then fed into a bandpass filter for noise suppression. Finally, the filtered signal is sent to the comparator A4 to be converted into digital form. This series connection method allows the signal to be processed in an orderly manner, thereby gradually improving the signal quality.

[0025] In this circuit, current transmitter A1 receives the input signal from the active pen and converts it into a current signal, during which common-mode noise is present. One implementation approach is to use an operational amplifier with a feedback capacitor to convert the input charge into a voltage, which is then output as a current signal via a voltage-to-current converter. Alternatively, current transmitter A1 can be designed as a direct current output, directly converting the input charge into a current signal. The generation of this current signal aims to provide a more easily processed current-domain signal for subsequent filtering.

[0026] In the single-channel operating mode of this embodiment, the current signal (active probe signal + common-mode noise) output by the current transmitter A1 is directly fed into the bandpass filter for processing. The bandpass filter improves signal quality by selectively filtering out out-of-band noise, including low-frequency and high-frequency interference.

[0027] Furthermore, the inverting input terminal of the current transmitter A1 is connected to the output terminal of the current transmitter A1, a self-capacitance Cs is formed between the inverting input terminal of the current transmitter A1 and ground, and a parasitic capacitance Cf is formed between the inverting input terminal of the current transmitter A1 and the active probe.

[0028] Specifically, the voltage signal induced by the self-capacitor Cs (containing μV-level pen signal and mV-level common-mode noise) is input to the current transmitter A1. The current transmitter A1 has low input impedance characteristics, enabling it to quickly respond to charge changes on the self-capacitor Cs and output a proportional current signal Iout. In this embodiment, one end of an integrating capacitor C1 is connected between the output terminal of the current transmitter A1 and the bandpass filter, and the other end of the integrating capacitor C1 is grounded. The current signal Iout is injected into the integrating capacitor C1, completing the first current-to-voltage conversion and preliminary low-pass filtering. In this embodiment, the current transmitter A1 is preferably a second-generation current transmitter CCII.

[0029] Furthermore, the bandpass filter includes a transimpedance amplifier A2 and a capacitively coupled amplifier A3 connected together.

[0030] The inverting input of the transimpedance amplifier A2 is connected to the output of the current transmitter A1, converting the current signal Iout into a voltage signal and performing low-pass filtering.

[0031] The inverting input of the capacitively coupled amplifier A3 is connected to the output of the transimpedance amplifier A2 via capacitor C4, which is used to block DC offset and perform high-pass filtering.

[0032] Furthermore, the cutoff frequency of the transimpedance amplifier A2 is higher than the active pen signal frequency, while the cutoff frequency of the capacitively coupled amplifier A3 is lower than the active pen signal frequency.

[0033] Furthermore, the transimpedance amplifier A2 is also connected to a first feedback network, which includes a first feedback resistor R1 and a first feedback capacitor C2 connected in parallel between the inverting input terminal and the output terminal of the transimpedance amplifier A2.

[0034] Furthermore, the capacitively coupled amplifier A3 is also connected to a second feedback network, which includes a second feedback resistor R2 and a second feedback capacitor C3 connected in parallel between the inverting input terminal and the output terminal of the capacitively coupled amplifier A3.

[0035] The non-inverting input of the current transmitter A1, the non-inverting input of the transimpedance amplifier A2, the non-inverting input of the capacitively coupled amplifier A3, and the non-inverting input of the comparator A4 are all input to the reference voltage VCM.

[0036] A bandpass filter is designed to allow signals within a specific frequency range to pass through while attenuating or blocking signals at frequencies outside that range. The inverting input of transimpedance amplifier A2 is connected to the output of current transmitter A1, receiving the output signal from current transmitter A1. The inverting input of capacitively coupled amplifier A3 is connected to the output of transimpedance amplifier A2, receiving the voltage signal processed by transimpedance amplifier A2. This sequential connection means that the output of transimpedance amplifier A2 directly serves as the input of capacitively coupled amplifier A3, forming a cascaded signal processing chain. This cascaded structure allows each module to focus on specific filtering and amplification tasks, thereby achieving more refined signal processing.

[0037] In this embodiment, the transimpedance amplifier A2 converts the signal from the current transmitter A1 into a voltage signal, and performs low-pass filtering in the process. Its cutoff frequency refers to the highest frequency at which the amplifier can effectively transmit the signal. When this cutoff frequency is set higher than the active pen signal frequency, the transimpedance amplifier A2 allows the active pen signal to pass completely, avoiding attenuation or distortion of the useful signal. Simultaneously, it can still effectively suppress high-frequency noise above its cutoff frequency. The cutoff frequency of the transimpedance amplifier A2 is typically determined by the values ​​of the resistors and capacitors in its feedback network. For example, if the transimpedance amplifier A2 uses a first feedback network, which includes a first feedback resistor R1 and a first feedback capacitor C2 connected in parallel between the inverting input and output terminals of the transimpedance amplifier A2, its cutoff frequency can be precisely set by adjusting the values ​​of the first feedback resistor R1 and the first feedback capacitor C2. Specifically, the cutoff frequency can be increased to be higher than the active pen signal frequency by decreasing the capacitance of the first feedback capacitor C2 or decreasing the resistance of the first feedback resistor R1.

[0038] In this embodiment, the capacitively coupled amplifier A3 blocks DC offset and performs high-pass filtering. Its cutoff frequency refers to the lowest frequency at which the amplifier can effectively transmit signals. When this cutoff frequency is set below the active pen signal frequency, the capacitively coupled amplifier A3 allows the active pen signal to pass completely, avoiding attenuation or distortion of the useful signal. The cutoff frequency of the capacitively coupled amplifier A3 is typically determined by its input coupling capacitor and input resistor, or by components in its feedback network. In this embodiment, the capacitively coupled amplifier A3 employs a second feedback network, which includes a second feedback resistor R2 and a second feedback capacitor C3 connected in parallel between the inverting input and output terminals of the capacitively coupled amplifier A3. Therefore, its cutoff frequency can be precisely set by adjusting the values ​​of the second feedback resistor R2 and the second feedback capacitor C3. Specifically, the cutoff frequency can be lowered to be below the active pen signal frequency by increasing the capacitance of the second feedback capacitor C3 or increasing the resistance of the second feedback resistor R2.

[0039] Example 2 For common-mode interference (such as LCD driving noise) whose frequency overlaps with the active pen signal, a single channel cannot acquire spatial information of multiple sensing points and therefore lacks the ability to distinguish and actively cancel such common-mode noise.

[0040] To address the aforementioned issues, this embodiment provides a noise cancellation unit. Please refer to [link / reference]. Figure 2 It includes multiple analog front-end receiving circuits for active pen as described in Embodiment 1. The multiple analog front-end receiving circuits operate in parallel and correspond to multiple sensing channels of the touch screen. In each of the analog front-end receiving circuits, the current transmitter A1 receives the input signal and outputs the signal, and after processing, outputs the corresponding digital square wave signal.

[0041] In touch chips, multiple (e.g., N) sensing channels are typically processed in parallel. Each channel's current transmitter A1 outputs its own current signal Iout. Due to the high spatial correlation of noise such as liquid crystal interference, it manifests as a common-mode signal across multiple adjacent channels. The calculation module calculates the arithmetic mean Ioutavg of the output currents of these N channels in real time. This average Ioutavg primarily reflects the current component of the common-mode noise, while the pen signal, as a differential-mode signal, may have different polarities / phases in each channel, and its influence is significantly weakened during the averaging process. Subsequently, this calculated average current Ioutavg is subtracted from the current signal Iout output by each channel. That is, the net output current of each channel is: Ioutnet = Iout - Ioutavg.

[0042] In the specific circuit implementation, the gates of the current mirrors in the current transmitters A1 of the multiple analog front-end receiving circuits are connected sequentially to form a common gate node. This common gate node generates an average current, which is the average value Ioutavg of the output currents of the multiple current transmitters A1. The bandpass filter in each analog front-end receiving circuit receives the net current signal Ioutnet, which is the difference between the current signal Iout and the average current Ioutavg.

[0043] Through the aforementioned connection method, the voltage on the common gate node reflects the average state of common-mode noise. This results in the net current signal, obtained by subtracting the average current from the output current of each current transmitter, suppressing common-mode noise components while preserving the differential-mode characteristics of the pen signal. The strong common-mode interference noise generated by the touchscreen is effectively canceled, while the pen signal, as a differential-mode signal, is preserved and enhanced, thereby improving the signal-to-noise ratio. Ultimately, the digital square wave signal output by each analog front-end receiving circuit accurately reflects the pen's writing state, achieving high-fidelity recovery and digital processing of the active pen signal under conditions of large capacitive load and strong common-mode noise.

[0044] Please refer to Figure 3 The waveform diagram shows the original input signal VIN_PEN coupled to the signal VIN on the self-capacitance Cs before processing. <19> The signal was completely drowned out by noise. After the above noise cancellation, the common-mode noise was greatly suppressed, allowing the net current signal, which contained only the pen signal and residual noise, to stand out. After subsequent transimpedance amplification, capacitive coupling amplification, and comparison shaping, a clear pen signal square wave was recovered.

[0045] Example 3 The third invention provides a noise cancellation method using a noise cancellation unit as described in Embodiment 2. Please refer to [reference needed]. Figures 2-4 ,include: Obtain the current signal Iout output by the current transmitter A1 in multiple analog front-end receiving circuits.

[0046] Calculate the average current Ioutavg of the plurality of current transmitters A1.

[0047] The net current signal Ioutnet is obtained by subtracting the average current Ioutavg from the current signal Iout output by each current transmitter A1.

[0048] The net current signal Ioutnet is processed sequentially through transimpedance amplifier A2 and capacitive coupling amplifier A3.

[0049] The processed signal is converted into a digital square wave signal by comparator A4.

[0050] By differentially combining the output currents of multiple current transmitters A1 with the calculated average current Ioutavg, the strong common-mode noise generated by the touchscreen is effectively canceled. Since the common-mode noise in multiple sensing channels is highly correlated, while the pen signal, as a differential-mode signal, is independent between channels, the average current primarily reflects the common-mode noise component. By subtracting this average current from the output current of each channel, common-mode noise can be suppressed, improving the signal-to-noise ratio of the net current signal. Based on this, a bandpass filter removes noise from the net current signal. Transimpedance amplifier A2 performs low-pass filtering, and capacitively coupled amplifier A3 performs high-pass filtering, together forming a bandpass characteristic to match the active pen signal frequency. The cutoff frequency of transimpedance amplifier A2 is higher than the active pen signal frequency, while the cutoff frequency of capacitively coupled amplifier A3 is lower than the active pen signal frequency, preserving the effective signal bandwidth while blocking DC offset.

[0051] The above examples illustrate the present invention only to aid in understanding it and are not intended to limit the scope of the invention. Those skilled in the art can make various simple deductions, modifications, or substitutions based on the principles of this invention.

Claims

1. An analog front-end receiving circuit for an active pen, characterized in that, It includes a current transmitter, a bandpass filter, and a comparator connected in sequence; The current transmitter receives an input signal and outputs a current signal; the bandpass filter receives the current signal, filters out noise signals, and outputs a first voltage signal; the comparator receives the first voltage signal and outputs a digital square wave signal.

2. The analog front-end receiving circuit as described in claim 1, characterized in that, The inverting input terminal of the current transmitter is connected to the output terminal of the current transmitter. A self-capacitance is formed between the inverting input terminal of the current transmitter and ground, and a parasitic capacitance is formed between the inverting input terminal of the current transmitter and the active probe.

3. The analog front-end receiving circuit as described in claim 2, characterized in that, The bandpass filter includes a transimpedance amplifier and a capacitively coupled amplifier connected in series. The inverting input terminal of the transimpedance amplifier is connected to the output terminal of the current transmitter, which converts the current signal into a voltage signal and performs low-pass filtering. The inverting input of the capacitively coupled amplifier is connected to the output of the transimpedance amplifier via a capacitor to block DC offset and perform high-pass filtering.

4. The analog front-end receiving circuit as described in claim 3, characterized in that, The cutoff frequency of the transimpedance amplifier is higher than the active pen signal frequency; the cutoff frequency of the capacitively coupled amplifier is lower than the active pen signal frequency.

5. The analog front-end receiving circuit as described in claim 3, characterized in that, The transimpedance amplifier is also connected to a first feedback network, which includes a first feedback resistor and a first feedback capacitor connected in parallel between the inverting input terminal and the output terminal of the transimpedance amplifier.

6. The analog front-end receiving circuit as described in claim 3, characterized in that, The capacitively coupled amplifier is also connected to a second feedback network, which includes a second feedback resistor and a second feedback capacitor connected in parallel between the inverting input terminal and the output terminal of the capacitively coupled amplifier.

7. A noise cancellation unit, comprising a plurality of analog front-end receiving circuits as described in any one of claims 1-6, characterized in that, The multiple analog front-end receiving circuits operate in parallel, each corresponding to a multiple sensing channel of the touch screen. In each analog front-end receiving circuit, the current transmitter receives the input signal and outputs a net current signal, which is then processed and outputs a corresponding digital square wave signal.

8. The noise cancellation unit as described in claim 7, characterized in that, The gates of the current mirrors in the current transmitters of the multiple analog front-end receiving circuits are connected in sequence to form a common gate node.

9. The noise cancellation unit as described in claim 8, characterized in that, The common gate node generates an average current, which is the average value of the current signals output by the plurality of current transmitters; Each of the analog front-end receiving circuits receives the net current signal, which is the difference between the current signal and the average current.

10. A noise cancellation method, employing the noise cancellation unit as described in any one of claims 7-9, characterized in that, include: Acquire the current signals output by the current transmitters in multiple analog front-end receiving circuits; Calculate the average current of the plurality of said current transmitters; The net current signal is obtained by subtracting the average current from the current signal output by each current transmitter. The net current signal is processed sequentially through a transimpedance amplifier and a capacitively coupled amplifier. The processed signal is converted into a digital square wave signal using a comparator.

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