Common-Mode Current Adjustment in the Receiver

Abstract

This text relates to common-mode current adjustment in a receiver, and the described example relates to a method and system for removing common-mode current. The receiver includes a photodetector, a common-mode adjustment circuit, an analog front end, an eye scan circuit, and a control unit. The photodetector generates an input photocurrent in response to a received optical signal. The common-mode adjustment circuit generates an adjusted input current based on the input photocurrent. The analog front end generates a differential voltage based on the adjusted input current. The eye scan circuit generates eye scan information based on the differential voltage, and the eye scan information defines a first outer eye and a second outer eye in an eye diagram. The control unit tunes the common-mode adjustment circuit based on a relative height metric of a first height of the first outer eye and a second height of the second outer eye to remove a portion of the common-mode current in the input photocurrent.

Description

Technical Field

[0001] The present disclosure generally relates to optical communication links. Background Art

[0002] Optical systems include optical devices that can generate, process, and / or transmit optical signals from one point to another. In some embodiments, compared with communication systems using electrical wires, optical systems such as optical communication systems can use a smaller cable width (or diameter) to facilitate data communication over longer distances at higher bandwidths. In an optical communication system, light can be generated by a light source such as a laser. The light can be modulated and / or encoded by a light transmitter, transmitted through an optical communication link, and demodulated and / or decoded by a photoelectric receiver including, for example, a photosensitive device. Transmitting a data signal via a communication link typically suffers from undesired signal impairments (e.g., attenuation, noise, etc.), which deteriorates the data signal quality. Summary of the Invention

[0003] A first aspect of the present disclosure relates to a method, including: determining, by a control unit, a first height of a first outer eye and a second height of a second outer eye based on eye scan information defining at least three eyes in an eye diagram, wherein the eye scan information is based on an input photocurrent generated by a photodetector in a photoelectric receiver based on a received multi-level pulse amplitude modulation (PAM) data signal, wherein the input photocurrent includes a common-mode current controlled by a common-mode adjustment circuit disposed in the photoelectric receiver and coupled to the photodetector and the control unit; generating a differential voltage based on the adjusted input current by: receiving the adjusted input current and generating a single-ended voltage; and receiving the single-ended voltage and generating a differential voltage by removing an average voltage of the single-ended voltage from the single-ended voltage; determining, by the control unit, whether a relative height metric based on the first height and the second height is greater than a threshold; in response to determining that the relative height metric is greater than the threshold: updating, by the control unit, a common-mode calibration value based on the first height and the second height; and tuning, by the control unit, the common-mode adjustment circuit to remove a portion of the common-mode current in the input photocurrent based on the common-mode calibration value.

[0004] The second aspect of the present disclosure relates to a receiver, comprising: a photodetector configured to receive a PAM4 optical signal and generate an input photocurrent; a common-mode adjustment circuit coupled to the photodetector to receive the input photocurrent and generate an adjusted input current by reducing a common-mode current in the input photocurrent; an analog front end (AFE) coupled to the common-mode adjustment circuit, wherein the AFE generates a differential voltage based on the adjusted input current; wherein the AFE includes: a current-to-voltage converter coupled to the common-mode adjustment circuit to receive the adjusted input current and generate a single-ended voltage; and a differential voltage generator coupled to the current-to-voltage converter to receive the single-ended voltage and generate the differential voltage by removing an average voltage of the single-ended voltage from the single-ended voltage; an eye scan circuit coupled to the AFE to generate eye scan information based on the differential voltage, wherein the eye scan information defines a first outer eye and a second outer eye in an eye diagram; and a control unit coupled to the eye scan circuit and the common-mode adjustment circuit, wherein the control unit tunes the common-mode adjustment circuit based on a height difference between a first height of the first outer eye and a second height of the second outer eye to remove a portion of the common-mode current in the input photocurrent.

[0005] A third aspect of the present disclosure relates to an electronic system, comprising: a circuit board; processing resources mounted on the circuit board; a storage medium mounted on the circuit board and communicatively coupled to the processing resources; and a receiver communicatively coupled to the processing resources and comprising: a photodetector configured to receive a PAM4 optical signal and generate an input photocurrent; a common-mode adjustment circuit coupled to the photodetector to receive the input photocurrent and generate an adjusted input current by reducing a common-mode current in the input photocurrent; an AFE coupled to the common-mode adjustment circuit, wherein the AFE generates a differential voltage based on the adjusted input current; wherein the AFE comprises: a current-to-voltage converter coupled to the common-mode adjustment circuit to receive the adjusted input current and generate a single-ended voltage; and a differential voltage generator coupled to the current-to-voltage converter to receive the single-ended voltage and generate the differential voltage by removing an average voltage of the single-ended voltage from the single-ended voltage; an eye scan circuit coupled to the AFE to generate eye scan information based on the differential voltage, wherein the eye scan information defines a first outer eye and a second outer eye in an eye diagram; and a control unit coupled to the eye scan circuit and the common-mode adjustment circuit, wherein the control unit tunes the common-mode adjustment circuit based on a height ratio of a first height of the first outer eye to a second height of the second outer eye to remove a portion of the common-mode current in the input photocurrent. Description of the Drawings

[0006] Various examples will be described below with reference to the following drawings.

[0007] Figure 1 is a block diagram of an example photoreceiver.

[0008] Figure 2A and Figure 2B is a graphical representation of an example eye diagram of a receiver depicting different tunings of a common-mode adjustment circuit.

[0009] Figure 3 is a block diagram of another example photoreceiver.

[0010] Figures 4A to 4D shows Figure 3 graphical representations of current and / or voltage signals at various stages within a photoreceiver.

[0011] Figure 5 is a flowchart of an example method for tuning a common-mode adjustment circuit of a photoreceiver.

[0012] Figure 6A flowchart of an example method for updating a common mode calibration (CMC) value for a common mode adjustment circuit in a photoreceiver.

[0013] Figure 7 A flowchart of another example method for tuning a common mode adjustment circuit in a photoreceiver.

[0014] Figure 8 A flowchart of yet another example method for tuning a common mode adjustment circuit in a photoreceiver.

[0015] Figure 9 A block diagram of an example electronic system including an example photoreceiver.

[0016] It should be emphasized that in the drawings, the various features are not drawn to scale. In fact, in the drawings, for clarity of discussion, the dimensions of the various features have been arbitrarily increased or decreased. Detailed Description

[0017] The following detailed description refers to the accompanying drawings. Wherever possible, the same reference numerals are used in the drawings and in the following description to refer to the same or like parts. It should be clearly understood that the drawings are for illustrative and descriptive purposes only. Although several examples are described in this document, modifications, adaptations, and other implementations are possible. Therefore, the following detailed description does not limit the disclosed examples. Instead, the correct scope of the disclosed examples may be defined by the appended claims.

[0018] Data can be transmitted over an optical communication link. In a computer system, such a communication link can form part of the physical layer and can be implemented by physical layer electronics. The physical layer defines how bits are sent and received over an optical transmission medium such as an optical fiber. For optical communication, an optical transmitter can be configured to convert electrical pulses into optical signals, which can then be received by a photoreceiver over the optical communication link and converted back into electrical pulses for data sampling. An optical communication link can be used within a computing device, for example, to implement a system bus and / or to network different computing devices. Using serializer / deserializer (SERDES) circuits (e.g., a serializer circuit at the transmitter and a deserializer circuit at the receiver) in an optical communication system enables high-speed communication.

[0019] Due to signal impairments such as attenuation, distortion, and noise, the optical signals received at an optical receiver (also hereinafter referred to as "receiver") in a computing system are typically different from the transmitted signals. Such signal impairments may lead to various variations that degrade the quality of the optical signal and the overall performance of the receiver. The performance of a communication device such as a receiver is typically measured by the bit error rate (BER), which represents the ability of the communication device to transmit bits without error. For high-speed data communication systems, such as systems using multi-level pulse amplitude modulation (PAM) or multi-PAM formats (e.g., four-level pulse amplitude modulation (PAM4)), these signal impairments may be more pronounced due to the high data rate and result in a high BER.

[0020] Typically, a receiver (e.g., an optical SERDES receiver) includes a photodetector (e.g., a photodiode) that is configured to detect an incoming optical signal and generate a photocurrent (hereinafter referred to as the input photocurrent) in response to the optical power of the received optical signal. To correctly capture the data signal, the receiver includes a data sampler that is configured to compare the signal received through the data sampler with one or more sampling thresholds to interpret the symbol corresponding to the received signal. In a communication system using the PAM4 format, the data sampler interprets the signal received at its input as one of the digital symbols from 00, 01, 10, and 11 based on three sampling thresholds. Generally, the input photocurrent generated by the photodetector includes a common-mode current. The common-mode current is a direct current (DC) bias in the input photocurrent. The common-mode current is preferably maintained at a predetermined magnitude. The magnitude of the common-mode current affects the performance of the analog front end (AFE). The AFE is a circuit that processes the photocurrent to enhance the signal quality of the photocurrent and perform signal conversion (e.g., current-to-voltage conversion). If the magnitude of the common-mode current is too large, the AFE may distort the signal. If the signal is distorted, there is a risk that the BER of the receiver will also increase. Therefore, keeping the signal at a predetermined level may be useful for keeping the signal distortion at an acceptably low level.

[0021] According to one or more examples presented herein, methods and optical receivers are proposed for removing common-mode current from an input photocurrent. In some examples, the optical receiver includes a photodetector, a common-mode adjustment circuit, an AFE, an eye scan circuit, and a control unit communicatively coupled to the common-mode adjustment circuit, the AFE, and the eye scan circuit. The photodetector receives an optical signal (e.g., an optical signal carrying a PAM4-encoded data signal) and generates an input photocurrent. The common-mode adjustment circuit receives the input photocurrent from the photodetector and generates an adjusted input current by reducing the common-mode current in the input photocurrent. The AFE generates a differential voltage based on the adjusted input current.

[0022] The eye scan circuit performs an eye scan to generate eye scan information based on the differential voltage received from the AFE. The eye scan information represents the probability distribution of differential voltages with a given phase offset and given voltage values. The given phase offset refers to the time offset between the time when the signal is sampled and a predefined point of the unit interval (UI). The predefined point can be any one of the start time, middle time, or end time of the UI. The eye scan information can be represented in the form of an eye diagram. In particular, the eye diagram generated from the eye scan information obtained from a data signal encoded based on PAM4 includes three eyes: a first outer eye, a second outer eye, and a center eye formed between the first outer eye and the second outer eye.

[0023] The control unit determines a first height of the first outer eye and a second height of the second outer eye based on the corresponding signal voltage levels (as will be described in more detail later). In response to determining that the relative height metric (e.g., height difference or height ratio) between the first height and the second height is greater than a threshold, the control unit updates the common mode calibration (CMC) value based on the first height and the second height. The control unit controls the common mode adjustment circuit to remove a portion of the common mode current in the input photocurrent based on the CMC value. In one example, the control unit controls the common mode adjustment circuit to adjust the common mode current such that the level of the common mode current in the adjusted input current generated by the common mode adjustment circuit remains below or equal to a predetermined magnitude. The predetermined magnitude can be the level of the DC bias that can be processed by a downstream processing circuit such as the AFE without introducing signal distortion or limiting the signal distortion to an acceptable threshold.

[0024] In some examples, the control unit of the proposed receiver tunes the common mode adjustment circuit based on the eye scan information to remove the common mode current in the input photocurrent so that the adjusted input current is within the input operating range of the AFE. In particular, the control unit continues to tune the common mode adjustment circuit until the heights of the first outer eye and the second outer eye become similar or substantially similar to each other. When the heights of the first outer eye and the second outer eye become similar or substantially similar to each other, the level of the common mode current in the adjusted input current becomes below or equal to the predetermined magnitude, such that the adjusted input current remains within the input operating range of the AFE. Thus, the AFE can process the adjusted input current and thereby generate a differential voltage with enhanced signal quality. Since the common mode current is reduced to remain below or equal to the predetermined magnitude, the AFE generates a differential voltage with reduced distortion. Therefore, the eye scan circuit will generate eye scan information with reduced distortion, feedback the eye scan information to the control unit to further update the CMC value and tune the common mode adjustment circuit until the adjusted input is within the input operating range of the AFE. In particular, due to this adjustment of the common mode current and the reduction of distortion in signal processing, the BER of the receiver is also reduced.

[0025] Now referring to the accompanying drawings, Figure 1illustrates an example optical receiver 100. The optical receiver 100 can be a stand-alone optical receiver or can be part of an optical transceiver module. In some examples, the optical receiver 100 can be a SERDES receiver. In this description, the terms "optical receiver" and "receiver" are used interchangeably. In Figure 1 the example of Figure 1 , the receiver 100 includes a photodetector 102, a common-mode adjustment circuit 104 (labeled CAC in

[0026] ), an AFE 106, a data sampling circuit (DSC) 107, an eye scan circuit 108, and a control unit 110. PD ) electrical signal. For a PAM4 data signal, the input photocurrent is expected to have one of four amplitude levels in each UI.

[0027] To reduce any adverse effects of signal distortion and common-mode current (i.e., DC bias) on the performance of the receiver 100, the input photocurrent is processed by circuits such as the common-mode adjustment circuit 104 and the AFE 106. For example, to reduce the BER of the receiver 100, it is useful to control the common-mode current from the input photocurrent. In particular, the common-mode adjustment circuit 104 is coupled to the photodetector 102 to receive the input photocurrent from the photodetector 102. The common-mode adjustment circuit 104 can include one or more electronic components that process the input photocurrent to remove at least a portion of the common-mode current from the input photocurrent and output a CM-adjusted current, hereinafter referred to as the adjusted input current (I A ). The common-mode adjustment circuit is controlled based on the CMC value received by the common-mode adjustment circuit 104 from the control unit 110.

[0028] In some examples, the common-mode adjustment circuit 104 can include a variable current source 105 that draws a portion of the input photocurrent according to the CMC value. The current source can be implemented using a digital-to-analog converter (DAC, not shown), and the CMC value can include a digital variable (e.g., an integer within the input range of the DAC). In this case, the CMC value can include a digital control value (e.g., a binary-encoded value) supplied to the DAC.

[0029] The amount of increment and / or decrement in the CMC value can be set as a configurable parameter of the receiver 100 and is hereinafter referred to as a predetermined value. In one case, the amount of increment and decrement can be one DAC integer step (e.g., +1 or -1). The common mode adjustment circuit 104 can be configured such that adjusting the variable current source 105 by updating the CMC value changes the current component removed from the input photocurrent I PD (e.g., increasing or decreasing a given negative bias). During initialization of the receiver 100, the control unit 110 can set the CMC value to a baseline amount, which in one example can be a value in the middle of the range of CMC values. For example, for a range of CMC values between 0 and 255, the control unit 110 can set the CMC baseline amount to 128. In some examples, the control unit 110 can algorithmically determine the CMC baseline amount as a non-limiting example.

[0030] The AFE 106 is coupled to the common mode adjustment circuit 104 to receive the adjusted current. The AFE 106 can include analog circuitry that conditions the adjusted input current by minimizing or removing distortion and amplifying the adjusted input current (e.g., enhancing the signal quality of the adjusted input current). In some examples, the AFE 106 can include circuitry such as one or more signal amplifiers, signal equalizers, current-to-voltage converters, single-ended to differential voltage converters, or combinations thereof. In particular, in Figure 1 the receiver 100, the AFE 106 processes the adjusted input current to generate a differential voltage (V D ).

[0031] In some examples, both the data sampling circuit 107 and the eye scan circuit 108 are coupled to the AFE 106 to receive the differential voltage. In some examples, the data sampling circuit 107 can be a single electronic circuit or a combination of multiple electronic circuits that can decode data symbols from the differential voltage of each UI (e.g., 00, 01, 10, 11 in the case of a PAM4 data signal). The eye scan circuit 108 can also include one or more circuits for generating eye scan information (ESI) that represents the probability distribution of a signal (e.g., differential voltage) having a given voltage value at a given phase offset. The given phase offset refers to the time offset between the time at which the signal is sampled and a predefined point of the UI. The predefined point can be any one of the start time, middle time, or end time of the UI. Details of generating the eye scan information are described in conjunction with Figure 3 .

[0032] The control unit 110 adjusts and removes the common-mode current in the input photocurrent through the common-mode adjustment circuit 104 based on the eye scan information. In one example, the control unit 110 may be implemented in the form of a microcontroller, which includes one or more processors and a machine-readable storage medium storing a processor-executable receiver of training program code. The receiver of the training program code may include instructions that, when executed by the microcontroller, cause the microcontroller to send a control signal (CS) to the common-mode adjustment circuit 104 to adjust the amount of common-mode current removed from the input photocurrent. In another example, the control unit 110 may be implemented through one or more processors and a machine-readable storage medium storing a receiver of training program code. In another example, the control unit 110 may be implemented through electronic circuits and devices (such as digital logic circuits, digital subtractors, comparators, counters, etc.). In yet another example, the control unit 110 may be implemented in the form of an application-specific integrated circuit (ASIC), a system-on-chip (SOC), a field-programmable gate array (FPGA), etc., to receive eye scan information from the eye scan circuit 108 and supply a CMC value to the common-mode adjustment circuit 104 to control the removal of common-mode current from the input photocurrent. In the following description, several operations described as being performed by the control unit 110 may be performed using any or a combination of the embodiments of the control unit 110 listed above.

[0033] Eye scan information that is useful for the control unit 110 to tune the common-mode adjustment circuit 104 may be represented in the form of an eye diagram (see Figure 2A and Figure 2B ). In particular, an eye diagram is a graphical representation of eye scan information. For descriptive purposes, Figure 2A and Figure 2B are referenced simultaneously with Figure 1 . In particular, Figure 2A and Figure 2B respectively depict for Figure 1Examples of eye diagrams 200A and 200B of receiver 100 for different tunings of the common - mode adjustment circuit 104. In eye diagrams 200A and 200B, the X - axis 202 represents time measured in UI of the sampling period, and the Y - axis 204 represents voltage levels. The waveform lines 206, 208, and 210 (although there are many waveform lines, only three are labeled for illustrative purposes) show how the PAM4 data signal is received. For example, due to repeated sampling, the received PAM4 data signal includes a random sequence of symbols 00, 01, 10, and 11. As can be seen, the original rectangular waveform is "smeared" by channel effects and becomes more jagged. In some examples, when an eye scan is performed by the eye scan circuit 108, data called eye scan information can be generated, which indicates at which voltage levels the waveform lines 206, 208, and 210 cross or overlap with each other during the UI.

[0034] The region between two or more of the waveform lines 206, 208, and 210 in the UI is called an eye. In eye diagrams 200A and 200B, three eyes can be seen, such as the first outer eye 212, the center eye 214, and the second outer eye 216, because the data signal carried by the incoming optical signal is a PAM4 - encoded data signal. The maximum vertical opening of eyes 212 to 216 (measured as the voltage range on the Y - axis 204) is hereinafter referred to as the eye height. During the training phase, for a given eye, when the signal probability at the voltage sampling is greater than a predetermined value, the control unit 110 can identify the voltage levels above and below the center voltage (e.g., the sampling threshold) of the given eye. The control unit 110 can determine the height of the given eye as the difference between these two voltages. For example, for the first outer eye 212, the control unit 110 can identify the first outer - eye high voltage (V C1 ) at the voltage level above and below the center voltage of the first eye of the first outer eye 212 (V U1 ) and the first outer - eye low voltage (V L1 ). In particular, the voltage level above the center voltage of the first eye is identified as the first outer - eye high voltage, at which the probability of the data signal is greater than a predetermined threshold. Similarly, the voltage level below the center voltage of the first eye is identified as the first outer - eye low voltage, at which the probability of the data signal is greater than a predetermined threshold. The center voltage of the first eye can be equal to the sampling threshold corresponding to the first outer eye 212. The control unit 110 can determine the height of the first outer eye 212 (hereinafter referred to as the first height H EYE1 ) as the difference between the voltages V U1 and V L1 .

[0035] Similarly, for the second outer eye 216, the control unit 110 may identify a second outer eye high voltage (V C2 ) at a voltage level above and below the second eye center voltage (V U2 ) of the second outer eye 216, and a second outer eye low voltage (V L2 ). The second eye center voltage may be equal to the sampling threshold corresponding to the second outer eye 216. In particular, a voltage level above the second eye center voltage is identified as the second outer eye high voltage, at which the probability of the sampled data signal is greater than a predetermined threshold. Similarly, a voltage level below the second eye center voltage is identified as the second outer eye low voltage (V L1 ), at which the probability of the sampled data signal is greater than a predetermined threshold. The control unit 110 may determine the height of the second outer eye 216 (hereinafter referred to as the second height H EYE2 ) as the difference between the voltage V U2 and V L2 .

[0036] As can be seen in eye diagrams 200A and 200B, the eye heights of the first outer eyes 212 are different from each other. In particular, in Figure 2A , the first height is less than the second height, which indicates that the common mode adjustment circuit 104 is poorly tuned. In some examples, the control unit 110 tunes the common mode adjustment circuit 104 based on the eye height until the outermost eyes in the eye diagram of the receiver 100 have substantially similar or equal eye heights. For example, eye diagram 200B can be achieved when the control unit 110 tunes the common mode adjustment circuit 104 to constrain the level of the common mode current in the adjusted input current below a predetermined magnitude.

[0037] The control unit 110 determines a relative height metric based on the first height and the second height. In one example, the relative height metric may be the height difference between the first height and the second height. In another example, the relative height metric may be the height ratio of the first height to the second height. Once the relative height metric is determined, the control unit 110 performs an inspection to determine whether the relative height metric is greater than a threshold. In response to determining that the relative height metric is greater than the threshold, the control unit 110 may update the CMC value based on the first height and the second height. Further, the control unit 110 tunes the common mode adjustment circuit 104 to remove the common mode current in the input optical current based on the CMC value. Additional details of the operation performed by the control unit 110 to tune the common mode adjustment circuit 104 are described in connection with the method of Figures 5 to 8 .

[0038] Now referring to Figure 3 , a block diagram of another example receiver 300 is depicted. Additionally, Figures 4A to 4D depicts a diagram showing Figure 3Graphical representations of the current and / or voltage signals at various stages within the receiver 300. In the following description, Figures 4A to 4D is referenced in conjunction with Figure 3 simultaneously. Figure 3 The receiver 300 of Figure 1 can be an example representation of the receiver 100 of Figure 1 and includes one or more components similar to those described in connection with the receiver 100 of Figure 1 . The description of those components will not be repeated herein. For example, the receiver 300 includes a photodetector 302, a common-mode adjustment circuit 304, an AFE 306, a data sampling circuit 307, an eye scan circuit 308, and a control unit 310, which are Figure 3 example representations of the photodetector 102, the common-mode adjustment circuit 104, the AFE 106, the data sampling circuit 107, the eye scan circuit 108, and the control unit 110 of Figure 3 . In particular, Figure 3 depicts certain circuits that form the AFE 306 and the eye scan circuit 308. For example, the AFE 306 can include a current-to-voltage converter 312 (labeled "I-V converter" in Figure 3 ) and a differential voltage generator 314. Additionally, Figure 3 also depicts certain circuits, such as an offset sampling circuit (OSC) 318 and an eye scan logic circuit 320 that form the eye scan circuit 308.

[0039] The photodetector 302 can generate an input photocurrent in response to an optical signal impinging on the photodetector 302 (see Figure 4A ). Similarly, the common-mode adjustment circuit 304 generates an adjusted input current by removing a portion of the common-mode current from the input photocurrent (see Figure 4B ). Figure 4A and Figure 4B depict graphical representations 400A and 400B, which respectively show the input photocurrent and the adjusted input current. In the graphical representations 400A and 400B, the X-axes 402 and 404 represent time, and the Y-axes 406 and 408 represent current amplitude in mA. Additionally, the current amplitudes marked by lines 410 and 412 define the input operating range (IO RANGE ) of the current-to-voltage converter 312. As depicted in the graphical representation 400A, the input photocurrent is outside the input operating range due to the presence of excessive common-mode current in the input photocurrent. The control unit 110 tunes the common-mode adjustment circuit 304 (see Figures 5 to 8method) to remove at least a portion of the common-mode current from the input photocurrent until the adjusted input current remains within the input operating range of the current-to-voltage converter 312. Once the relative height metric (e.g., height difference or height ratio) of the first height and the second height is determined to be less than or equal to a threshold (e.g., threshold height difference or threshold ratio), the adjusted input current is limited within the input operating range.

[0040] The current-to-voltage converter 312 is an electronic circuit that receives a current signal (e.g., I A ) as an input and generates a voltage signal as an output. The current-to-voltage converter 312 can be implemented using an operational amplifier (OPAMP)-based current-to-voltage converter circuit, a resistor-based network, or a combination thereof. During operation of the receiver 100, the current-to-voltage converter 312 receives the adjusted input current from the common-mode adjustment circuit 304 and generates a single-ended voltage (V S ), which is depicted in Figure 4C graphical representation 400C. In graphical representation 400C, the X-axis 414 represents time, and the Y-axis 416 represents the voltage amplitude in mV. The line 418 represents the average voltage (V S_AVG ) of the single-ended voltage.

[0041] The differential voltage generator 314 is an electronic circuit coupled to the current-to-voltage converter 312 to receive the single-ended voltage and generate a differential voltage based on the single-ended voltage. The differential voltage generator 314 can generate the differential voltage by subtracting the average voltage from the single-ended voltage. The differential voltage generator 314 can be implemented using an OPAMP-based inverting buffer, a resistor-capacitor low-pass filter network, or a combination thereof. The differential voltage is depicted in Figure 4D graphical representation 400D. In graphical representation 400D, the X-axis 420 represents time, and the Y-axis 422 represents the voltage in mV. The differential voltage can be received by the data sampling circuit 307 and the eye scan circuit 308.

[0042] In one example embodiment, the data sampling circuit 307 is configured to apply decision logic to the differential voltage to output a first data stream (D DS)。For example, the data sampling circuit 307 can compare the received differential voltage with a configurable voltage threshold at a configurable sampling time. The sampling time can be set based on a clock signal (not shown). The first data stream is a symbol stream (e.g., 00, 01, 10, or 11 in the case of a PAM4 data signal), and the symbol stream represents the data received by the photodetector 302 through the optical communication channel. The first data stream can be output by the receiver 300 in the order of the received bits. In some examples, the first data stream can be supplied to a deserialization circuit (not shown) for deserializing the data symbols into multiple data streams.

[0043] The eye scan logic circuit 320 generates eye scan information using the data sampling circuit 307 and the offset sampling circuit 318 to tune the common mode adjustment circuit 304. The offset sampling circuit 318 of the eye scan circuit 308 is coupled in parallel with the data sampling circuit 307 to receive the differential voltage from the differential voltage generator 314. The offset sampling circuit 318 is configured to output a second data stream (D OS )。The offset sampling circuit 318 can be controlled to use the differential voltage threshold from the data sampling circuit 307. For example, the offset sampling circuit 318 can use a common sampling time but apply a positive or negative voltage offset.

[0044] In Figure 3 example, the voltage threshold used by the data sampling circuit 307 can be fixed to a predetermined value, e.g., fixed to the voltage threshold of the initial calibration, while the voltage threshold for the offset sampling circuit 318 is variable. In this example, the eye scan performed by the eye scan logic circuit 320 includes applying a positive or negative voltage offset (VO) to the voltage threshold of the offset sampling circuit 318. After a specific voltage offset is applied by the offset sampling circuit 318, the eye scan logic circuit 320 compares the second data stream obtained using the offset sampling circuit 318 with the first data stream obtained using the data sampling circuit 307.

[0045] The eye scan logic circuit 320 is coupled to both the data sampling circuit 307 and the offset sampling circuit 318, and receives a first data stream and a second data stream. Based on the first data stream, the second data stream, and / or internal signals generated within the data sampling circuit 107 and the offset sampling circuit 318, the eye scan logic circuit 320 can generate eye scan information representing the probability distribution of a signal (e.g., differential voltage) having a given voltage value with a given phase offset. As previously noted, the given phase offset is the time offset between the time at which the signal is sampled and a predefined point of the UI (e.g., any one of the start time, the middle time, or the end time). In some embodiments, the receiver 300 may include a clock data recovery circuit (not shown) that generates a periodic "clock" signal having the same period as the received data signal period and calibrated to the middle of the UI. The data sampling circuit 307 may be configured to sample the incoming signal (e.g., the differential voltage received from the AFE 306) at the edges of the clock signal (also referred to as the recovered clock edges). To perform an eye scan, the eye scan logic circuit 320 may configure the offset sampling circuit 318 to sample the input signal at a time up to half of the UI before or after the recovered clock edge. Specifically, the phase offset may refer to the difference between the recovered clock edge and the time at which the signal is sampled.

[0046] The eye scan information can be used by the control unit 310 to tune the common mode adjustment circuit 304 to remove the common mode current in the input photocurrent until it is determined that the relative height metric (e.g., height difference or height ratio) between the first height and the second height is less than or equal to a threshold (e.g., threshold height difference or threshold ratio). Once the relative height metric is set to a value less than or equal to the threshold, the adjusted input current is limited within the input operating range of the current-to-voltage converter 312, which results in reduced signal distortion and a lower BER of the receiver 300.

[0047] In the following description, several operations performed by the control unit 110 will be described with the aid of Figures 5 to 8 the flowcharts depicted in Figures 5 to 8 For illustrative purposes, the flowcharts depicted in Figure 1 are described in conjunction with the receiver 100 of Figures 2A to 2B and the eye diagram of Figures 5 to 8 However, the Figures 5 to 8 method should not be construed as being limited to the example configuration of the receiver 100. For example, Figures 5 to 8 the Figure 1 method can also be applied to the receiver 300 and can be performed by the control unit 310. During the description of Figures 2A to 2B reference is made simultaneously to Figures 5 to 8The method described can represent an example logical flow of some operations performed by control unit 110. However, in some other examples, Figures 5 to 8 the order of execution of the boxes depicted in Figure 5 can be different from the order shown. The operations at each box can be performed in serial, parallel, or a combination of serial and parallel manners. For example, Figure 7 method boxes 502, 504 in Figure 7 , method boxes 702, 704 in

[0048] or method boxes 802, 804 in Figure 5 can be performed in parallel during the execution of the corresponding methods.

[0049] Now referring to Figures 2A to 2B , there is a flowchart of an example method 500 for tuning a common mode adjustment circuit (such as common mode adjustment circuit 104) to control the removal of common mode current in the input photocurrent of receiver 100. Method 500 can be performed during the training phase of receiver 100 (e.g., at initialization). In some examples, to perform method 500, a training data signal can be supplied to receiver 100 at a predetermined data rate of a PAM4 signal (e.g., 28Ghz to 56Ghz), and an eye scan (e.g., using eye scan circuit 108) can be performed to generate eye scan information, which can represent the probability distribution of signals with a given phase offset and given voltage values.

[0050] Method 500 can start at box 501, where control unit 110 receives the eye scan information. In some examples, control unit 110 continuously receives the eye scan information during the training phase. In some examples, at box 501, control unit 110 instructs eye scan circuit 108 to perform an eye scan and return the eye scan information to control unit 110. As previously noted, the eye scan information can be graphically represented in the form of an eye diagram, which includes three eyes 212, 214, and 216 for the PAM4 data signal (see EYE1 Figures 2A to 2B )).

[0050] At box 502, control unit 110 determines the first height of the first outer eye 212. In some examples, control unit 110 can determine the height H EYE1 based on the first outer eye high voltage and the first outer eye low voltage (e.g., marked in eye diagrams 200A and 200B),

[0051] where the first outer eye high voltage and the first outer eye low voltage are obtained by control unit 110 based on the eye scan information received from eye scan circuit 108. In one example, the first height can be determined according to the relationship of equation (1).

[0051] H EYE1 = V U1 - V L1 …(1)

[0052] In another example, the first height can be determined based on the standard deviation (σ) of the voltage values at each of the locations in V U1 and V L1 in accordance with the relationship of Equation (2).

[0053] H EYE1 =(V U1 - 3σ)-(V L1 + 3σ)...(2)

[0054] Further, at block 504, the control unit 110 determines the second height of the second outer eye 216 based on the second outer eye high voltage and the second outer eye low voltage (e.g., marked in eye diagrams 200A and 200B), where the second outer eye high voltage and the second outer eye low voltage are obtained by the control unit based on the eye scan information received from the eye scan circuit 108. In one example, the second height can be determined in accordance with the relationship of Equation (3).

[0055] H EYE2 =V U2 -V L2 ...(3)

[0056] In another example, the first height can be determined based on the standard deviation (σ) of the voltage values at each of the locations in V U1 and V L1 in accordance with the relationship of Equation (2).

[0057] H EYE2 =(V U2 - 3σ)-(V L2 + 3σ)...(4)

[0058] At block 506, the control unit 110 can determine a relative height metric (H REL ) based on the first height and the second height. The relative height metric can be determined as a function of the first height and the second height as represented in Equation 5.

[0059] H REL =f(H EYE1 , H EYE2 )...(5)

[0060] In one example, the relative height metric can be the difference between the first height and the second height (see Figure 7 ). In another example, the relative height metric can be the ratio of the first height to the second height (see Figure 8 ). It should be noted that in other examples, the relative height metric can be any other function that does not limit the scope of the present disclosure.

[0061] Further, at block 508, the control unit 110 may compare the relative height metric with a threshold (TH) to determine whether the relative height metric is greater than the threshold. The threshold may be a customizable boundary value, and the customizable boundary value may be set based on the allowable tolerance in the performance of the receiver 100. For example, the threshold may be set based on the receiver-allowable BER value for the acceptable performance of the receiver 100. At block 508, if it is determined that the relative height metric is greater than the threshold, then at block 510, the control unit 110 may update the CMC value based on the first height and the second height. In particular, the CMC value may be incremented or decremented based on the first height and the second height. Details of updating the CMC value are described in conjunction with Figure 6 the method of. Further, at block 512, the control unit 110 tunes the common-mode adjustment circuit 104 to remove the common-mode current from the input photocurrent based on the common-mode calibration value. In particular, the control unit 110 sends a command or control signal that indicates the common-mode adjustment circuit 104 to increase or decrease the amount of common-mode current removed from the input photocurrent. In particular, in one example, the command or control signal may include an updated CMC value (e.g., a binary-coded value), and the common-mode adjustment circuit increases or decreases the amount of current drawn from the input photocurrent based on the updated CMC value, thereby changing the level of the common-mode current. After operating on block 512, control may pass back to block 501.

[0062] In some examples, when the relative height metric becomes less than or equal to the threshold, the first height and the second height become equal to or substantially similar to each other, indicating that the common-mode adjustment circuit 104 is being properly tuned. Accordingly, at block 508, if it is determined that the relative height metric is not greater than the threshold (i.e., H REL ≤TH), then the control unit 110 may terminate method 500.

[0063] Figure 6 is a flowchart of an example method 600 for updating the CMC value for the common-mode adjustment circuit 104 in the receiver 100. In particular, method 600 includes example steps to be implemented to update the Figure 5 CMC value indicated at block 510. In some examples, prior to the training phase and updating the CMC value, the control unit 110 may have initialized the CMC value to a CMC baseline amount.

[0064] At block 602, the control unit 110 may perform an inspection to determine whether the first height is greater than the second height. At block 602, if it is determined that the first height is greater than the second height, then at block 604, the control unit 110 may decrease the CMC value. In some examples, the control unit 110 may decrease the CMC value by a predetermined amount. However, at block 602, if it is determined that the first height is not greater than the second height, then at block 606, the control unit 110 may increase the CMC value. In some examples, the control unit 110 may increase the CMC value by a predetermined amount. In particular, at blocks 604 and 606, the amount of the previously set CMC value (e.g., the baseline CMC value for the first instance of updating the CMC value) may be decreased or increased by a predetermined amount.

[0065] Figure 7 is a flowchart of another example method 700 for tuning the common-mode adjustment circuit 104 to remove the common-mode current in the input photocurrent. In particular, method 700 is Figure 5 an example representation of method 500, and includes certain steps similar to those Figure 5 described in, and the description of those steps will not be repeated herein. For example, Figure 7 blocks 701, 702, 704, 710, and 712 of Figure 5 are similar to blocks 501, 502, 504, 510, and 512 of EYE1 and H EYE2 . In method 700, after determining the first height and the second height, at block 706, the control unit 110 may determine the height difference (ΔH) between the first height and the second height. In one example, the height difference may be the height H

[0066] ΔH = |H EYE1 - H EYE2 |...(6)

[0067] Further, at block 708, the control unit 110 may compare the height difference with a threshold height difference (TH ΔH) are compared to determine whether the relative height difference is greater than a threshold height difference. The threshold height difference can be a customizable boundary value, and the customizable boundary value can be set based on the allowable tolerance in the performance of the receiver 100. For example, the threshold height difference can be set based on the allowable BER value of a receiver with acceptable performance. In one example, the value of the threshold height difference can be set to zero (0). In another example, the threshold height difference can be set to a value equal to a predetermined percentage of the first height (e.g., 5% to 10%). In yet another example, the threshold height difference can be set to a value equal to a predetermined percentage of the second height (e.g., 5% to 10%). In some examples, at block 708, if it is determined that the height difference is greater than the threshold height difference (i.e., ΔH > TH ΔH ), then at block 710, the control unit 110 can update the CMC value based on the first height and the second height in a manner similar to the manner described in conjunction with Figure 6 .

[0068] In some examples, when the height difference becomes less than or equal to the threshold height difference, the first height and the second height become equal to or substantially similar to each other, indicating that the common-mode adjustment circuit is being properly tuned to limit the common-mode current level in the adjusted input current to below a predetermined magnitude. Thus, at block 708, if it is determined that the height difference is not greater than the threshold height difference (i.e., ΔH ≤ TH ΔH ), then the control unit 110 can terminate method 700.

[0069] Now returning to Figure 8 , a flowchart of another example method 800 for tuning the common-mode adjustment circuit 104 is presented. In particular, method 800 is an example representation of method 500 of Figure 5 and includes certain steps similar to the steps described in Figure 5 , and the description of those steps will not be repeated herein. For example, Figure 8 blocks 801, 802, 804, 810, and 812 of Figure 5 are similar to blocks 501, 502, 504, 510, and 512 of RATIO . In method 800, after determining the first height and the second height, at block 806, the control unit 110 can determine the height ratio (H EYE1 ) between the first height and the second height. In one example, the height ratio can be the ratio of the maximum value of the heights H EYE2 to the minimum value of the heights H EYE1 , H EYE2 , as represented by the example relationship of Equation 7.

[0070]

[0071] Further, at block 808, the control unit 110 may compare the height ratio with a threshold ratio (TH RATIO ) to determine whether the height ratio is greater than the threshold ratio (TH RATIO ). The threshold ratio may be a customizable boundary value, and the customizable boundary value may be set based on an allowable tolerance in the performance of the receiver 100. For example, the threshold ratio may be set based on the allowable BER value of a receiver with acceptable performance. In one example, the value of the threshold ratio may be set to one (1). At block 708, if it is determined that the height ratio is not greater than the threshold ratio (i.e., H RATIO ≤TH RATIO ), the control unit 110 may terminate method 800. However, in some examples, at block 808, if it is determined that the height ratio is greater than the threshold (i.e., H RATIO >TH RATIO ), then at block 810, the control unit 110 may update the CMC value based on the first height and the second height in a manner similar to that described in conjunction with Figure 6 .

[0072] Figure 9 is a block diagram of an example electronic system 900 that includes an example receiver 901. Examples of the electronic system 900 may include, but are not limited to, a computer (a stationary computer or a portable computer), a server, a storage system, a wireless access point, a network switch, a router, a docking station, a printer, or a scanner. The electronic system 900 may be provided as a stand-alone product, a packaged solution, and may be utilized by purchasing the complete product / solution at once or on a pay-per-use basis. The electronic system 900 may include one or more multi-chip modules (e.g., multi-chip module (MCM) 902) to process and / or store data. In some examples, the MCM 902 may include processing resources 904 and a storage medium 906 mounted on a circuit board 908. Also, in some examples, the MCM 902 may host the receiver 901 on the circuit board 908. In some other examples, one or more of the processing resources 904, the storage medium 906, and the receiver 901 may be hosted on separate MCMs (not shown). The circuit board 908 may be a printed circuit board (PCB) that includes a number of conductive traces (not shown) to interconnect the processing resources 904, the storage medium 906, and the receiver 901 with each other and / or with other components disposed on or external to the PCB.

[0073] The processing resource 904 may be a physical device, e.g., one or more central processing units (CPUs), one or more semiconductor-based microprocessors, microcontrollers, one or more graphics processing units (GPUs), ASICs, FPGAs, other hardware devices capable of fetching and executing instructions stored in the storage medium 906, or a combination thereof. The processing resource 904 may fetch, decode, and execute the instructions stored in the storage medium 906. As an alternative or additional solution to executing instructions, the processing resource 904 may include at least one integrated circuit (IC), control logic, electronic circuitry, or a combination thereof including multiple electronic components. The storage medium 906 may be any electronic, magnetic, optical, or any other physical storage device that contains or stores instructions readable and executable by the processing resource 904. Thus, the storage medium 906 may be, for example, random access memory (RAM), non-volatile RAM (NVRAM), electrically erasable programmable read-only memory (EEPROM), a storage device, an optical disc, etc. In some embodiments, the storage medium 906 may be a non-transitory storage medium, where the term "non-transitory" does not cover transitory propagated signals. Further, in some examples, the receiver 901 may be any one of the receivers 100 or 300.

[0074] The terms used herein are for the purpose of describing example embodiments only and are not intended to be limiting. Unless the context indicates otherwise, the singular forms "a", "an", and "the" as used herein are also intended to include the plural forms. As used herein, the term "plurality" is defined as two or more. As used herein, unless otherwise indicated, the term "coupled" is defined as connected, either directly without any intermediate element or indirectly through at least one intermediate element. The term "and / or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. In some cases, the presence of broad words and phrases (such as "one or more", "at least", "but not limited to", or other similar phrases) should not be construed to imply that a narrower case is intended or required in situations where such broad phrases may not be present.

[0075] For purposes of illustration and description, the foregoing description of the various examples has been presented. The foregoing description is not intended to be exhaustive or limited to the disclosed examples, and modifications and variations are possible or can be derived from the practice of the various examples in light of the above teachings. The examples discussed herein are chosen and described in order to explain the principles and nature of the various examples of the present disclosure and their practical application so that those skilled in the art can utilize the present disclosure in various examples and in various modifications suitable for the particular purposes contemplated. The features of the examples described herein can be combined in all possible combinations of methods, apparatus, modules, systems, and computer program products. Further, in the foregoing description, numerous details are set forth to provide an understanding of the subject matter disclosed herein. However, embodiments may be practiced without some or all of these details. Other embodiments may include modifications, combinations, and variations of the details discussed above. The appended claims are intended to cover such modifications and variations.

Claims

1. A method for adjusting common-mode current, comprising: Determining, by a control unit, a first height of a first outer eye and a second height of a second outer eye based on eye scan information of at least three eyes in a defined eye diagram, wherein the eye scan information is based on an input optical current, the input optical current is generated by a photodetector in a photoreceiver based on a received multi-level pulse amplitude modulation (PAM) data signal, wherein the input optical current includes a common-mode current, the common-mode current is controlled by a common-mode adjustment circuit provided in the photoreceiver, and the common-mode adjustment circuit is coupled to the photodetector and the control unit; Generating a differential voltage based on the adjusted input current by: Receiving the adjusted input current and generating a single-ended voltage; and Receiving the single-ended voltage and generating a differential voltage by removing an average voltage of the single-ended voltage from the single-ended voltage; wherein an analog front end (AFE) generates the differential voltage based on the adjusted input current, and the AFE is coupled to the common-mode adjustment circuit; wherein the AFE includes: A current-to-voltage converter coupled to the common-mode adjustment circuit to receive the adjusted input current and generate a single-ended voltage; and A differential voltage generator coupled to the current-to-voltage converter to receive the single-ended voltage and generate the differential voltage by removing the average voltage of the single-ended voltage from the single-ended voltage; Determining, by the control unit, whether a relative height metric based on the first height and the second height is greater than a threshold; In response to determining that the relative height metric is greater than the threshold: Updating, by the control unit, a common-mode calibration value based on the first height and the second height; and Tuning, by the control unit, the common-mode adjustment circuit to remove a portion of the common-mode current from the input optical current based on the common-mode calibration value.

2. The method according to claim 1, further comprising repeating the steps of: determining the first height and the second height, determining whether the relative height metric is greater than the threshold, updating the common-mode calibration value, and tuning the common-mode adjustment circuit until the relative height metric becomes less than or equal to the threshold.

3. The method according to claim 1, wherein The multi-level PAM data signal is a four-level pulse amplitude modulation (PAM4) optical signal.

4. The method according to claim 1, wherein, The first outer eye and the second outer eye are the outermost eyes in the eye diagram.

5. The method according to claim 1, wherein The eye scan information is generated based on the differential voltage, wherein the differential voltage is generated based on the adjusted input current.

6. The method according to claim 1, wherein, Updating the common-mode calibration value includes: increasing the common-mode calibration value by a predetermined amount in response to determining that the first height is less than the second height.

7. The method according to claim 1, further comprising determining whether the first height is greater than the second height, wherein, Updating the common-mode calibration value includes decreasing the common-mode calibration value by a predetermined amount in response to determining that the first height is greater than the second height.

8. The method according to claim 1, wherein The relative height metric is a difference between the first height and the second height, and wherein the threshold is a height difference threshold.

9. The method according to claim 1, wherein, The relative height metric is a ratio of the first height to the second height, and wherein the threshold is a height ratio threshold.

10. The method according to claim 1, wherein, Removing the portion of the common-mode current in the input photocurrent reduces the bit error rate of the photoreceiver.

11. A receiver, comprising: a photodetector configured to receive a PAM4 optical signal and generate an input photocurrent; a common-mode adjustment circuit coupled to the photodetector to receive the input photocurrent and generate an adjusted input current by reducing the common-mode current in the input photocurrent; an analog front end (AFE) coupled to the common-mode adjustment circuit, wherein the AFE generates a differential voltage based on the adjusted input current; wherein the AFE includes: a current-to-voltage converter coupled to the common-mode adjustment circuit to receive the adjusted input current and generate a single-ended voltage; and a differential voltage generator coupled to the current-to-voltage converter to receive the single-ended voltage and generate the differential voltage by removing an average voltage of the single-ended voltage from the single-ended voltage; an eye scan circuit coupled to the AFE to generate eye scan information based on the differential voltage, wherein the eye scan information defines a first outer eye and a second outer eye in an eye diagram; and a control unit coupled to the eye scan circuit and the common-mode adjustment circuit, wherein the control unit tunes the common-mode adjustment circuit based on a height difference between a first height of the first outer eye and a second height of the second outer eye to remove a portion of the common-mode current in the input photocurrent.

12. The receiver according to claim 11, wherein, The control unit: determines whether the height difference is greater than a height difference threshold; and in response to determining that the height difference is greater than the height difference threshold, updates a common-mode calibration value based on the first height and the second height, wherein the control unit tunes the common-mode adjustment circuit to remove the common-mode current in the input photocurrent based on the common-mode calibration value.

13. The receiver according to claim 12, wherein, The control unit: determines whether the first height is less than or equal to the second height; and in response to determining that the first height is less than or equal to the second height, increases the common-mode calibration value by a predetermined amount.

14. The receiver according to claim 12, wherein, The control unit: determines whether the first height is greater than the second height; and in response to determining that the first height is greater than the second height, decreases the common-mode calibration value by a predetermined amount.

15. The receiver according to claim 11, wherein, The control unit tunes the common-mode adjustment circuit to remove the common-mode current such that the adjusted input current remains within an input operating range of the current-to-voltage converter.

16. An electronic system, comprising: a circuit board: processing resources mounted on the circuit board; a storage medium mounted on the circuit board and communicatively coupled to the processing resources; and a receiver communicatively coupled to the processing resources and including: a photodetector configured to receive a PAM4 optical signal and generate an input photocurrent; A common-mode adjustment circuit, the common-mode adjustment circuit being coupled to the photodetector to receive the input photocurrent and generating an adjusted input current by reducing the common-mode current in the input photocurrent; An AFE, the AFE being coupled to the common-mode adjustment circuit, wherein the AFE generates a differential voltage based on the adjusted input current; Wherein, the AFE includes: A current-to-voltage converter, the current-to-voltage converter being coupled to the common-mode adjustment circuit to receive the adjusted input current and generating a single-ended voltage; and A differential voltage generator, the differential voltage generator being coupled to the current-to-voltage converter to receive the single-ended voltage and generating the differential voltage by removing the average voltage of the single-ended voltage from the single-ended voltage; An eye scan circuit, the eye scan circuit being coupled to the AFE to generate eye scan information based on the differential voltage, wherein the eye scan information defines a first outer eye and a second outer eye in an eye diagram; and A control unit, the control unit being coupled to the eye scan circuit and the common-mode adjustment circuit, wherein the control unit tunes the common-mode adjustment circuit based on a height ratio of a first height of the first outer eye to a second height of the second outer eye to remove a portion of the common-mode current in the input photocurrent.

17. The electronic system according to claim 16, wherein, The control unit: Determines whether the height ratio is greater than a height ratio threshold; And In response to determining that the height ratio is greater than the height ratio threshold, updates a common-mode calibration value based on the first height and the second height, wherein the control unit tunes the common-mode adjustment circuit to remove the common-mode current in the input photocurrent based on the common-mode calibration value.

18. The electronic system according to claim 17, wherein, The control unit: Determines whether the first height is less than or equal to the second height; and In response to determining that the first height is less than or equal to the second height, increases the common-mode calibration value by a predetermined amount, or in response to determining that the first height is greater than the second height, decreases the common-mode calibration value by a predetermined amount.

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