Analog front-end circuit structure for magnetic encoder

Abstract

The invention relates to an analog front-end circuit structure for a magnetic encoder, comprising an instrument amplifier, a filter driver and a gating unit. The input end of the instrument amplifier is connected to a sensor head and is used to amplify a weak voltage signal generated by the sensor head and output a differential signal to the filter driver. The filter driver also receives Vrefi and Vcmi as output common mode point adjustment and input common mode point adjustment, and is used to further amplify the signal, filter and adjust the output common mode point. The gating unit is connected to the instrument amplifier and the filter driver and receives the differential signal output by the instrument amplifier and the differential signal output by the filter driver. The output end of the gating unit is connected to an analog-to-digital converter. The analog front-end circuit structure for a magnetic encoder of the invention can be applied not only to the analog front end of a magnetic encoder, but also to the front-end analog interface circuit application of all four-resistance bridges, thereby reducing the complexity of hardware design.

Description

Technical Field

[0001] The present invention relates to the field of sensors, in particular to the field of signal conditioning circuits in the field of angle sensors, and specifically refers to an analog front-end circuit structure for a magnetic encoder. Background Art

[0002] The signals generated by sensor detection elements are generally weak, usually in the millivolt or even microvolt level. This requires the conditioning circuit used for sensor signals to amplify the detected weak sensor signals and remove the offset voltage. Moreover, according to the current requirements for sensor application control accuracy, more and more designs require high precision of more than 14 bits. Therefore, the system design requirements also need to ensure a high signal-to-noise ratio of more than 90db for the output signal. Most sensors have very high requirements for chip power consumption, so under the premise of meeting performance, the power consumption is also required to be as low as possible.

[0003] The magnetic encoder is actually a special coded angle sensor, which is often used in the field of motor servo control. The magnetic encoder detects the angle information of the motor by detecting the change of the rotating magnetic field. The magnetic sensitive element usually used to detect the magnetic field is magnetoresistance (xMR). There are different types of magnetoresistance, including anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR). These magnetoresistances can be equivalent to two pairs of four-resistance bridges. The resistance of the bridge changes with the change of the direction of the magnetic field, and the change in resistance causes the output voltage to change, thereby generating a weak signal output representing the angle of the magnetic field. Usually the two pairs of four-resistance bridges are processed separately by two sets of analog front-end circuits. As the direction of the rotating magnetic field changes, the two pairs of bridges generate orthogonal sine and cos signals, such as Figure 1 If there is a common-mode mismatch between these two orthogonal signals, the integral nonlinear error of the subsequent demodulation angle will increase. Figure 2 As shown in the figure, the actual output angle is different from the ideal output angle due to the presence of a certain offset deviation between the orthogonal signals. Therefore, the common mode point of the differential signal output by the analog front-end circuit must also be adjustable.

[0004] There is a mismatch between bridge resistors, and the conditioning circuit itself also has an offset voltage caused by device mismatch. The weak signal generated by the magnetic resistance is usually relatively small. Taking AMR as an example, it is about millivolt level. This requires an amplifier gain of dozens or even hundreds of times. If the offset voltage at the input is amplified year-on-year, it will seriously affect the dynamic range of the signal. A chopper circuit is usually used to eliminate the offset voltage at the input. However, the chopper circuit means that the chopping frequency is much higher than the signal frequency, which requires a higher bandwidth for the amplifier and consumes more current. At the same time, the chopper circuit will also cause a lot of high-frequency interference and distortion.

[0005] In addition, most sensor conditioning circuits contain relatively complex algorithm operations. For example, magnetic encoders contain angle operations, encoding and other algorithms. These need to be processed in the digital part, so the signal amplified by the analog front-end circuit must be converted into a digital signal by an analog-to-digital converter before digital signal processing. The input interface of the analog-to-digital converter is mostly a sampling and holding circuit that works with a high-frequency clock. A driving circuit is required to ensure its sampling and settling accuracy. At the same time, it must meet high signal-to-noise ratio and minimum design power consumption.

[0006] In summary, it is necessary to design an analog front-end circuit (AFE) for the readout, amplification, filtering and other processing of the weak signals of the two groups of four-resistance bridges of the magnetic encoder chip. The analog front-end circuit also needs to eliminate the offset voltage at the input end to ensure a high signal-to-noise ratio of more than 90db. The output common-mode voltage is adjustable to drive the high-speed and high-precision switched capacitor circuit at the front end of the analog-to-digital converter. At the same time, the hardware design and power consumption need to be minimized. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and provide an analog front-end circuit structure for a magnetic encoder that meets the requirements of high signal-to-noise ratio, low power consumption and a wide range of applications.

[0008] In order to achieve the above object, the analog front-end circuit structure of the magnetic encoder of the present invention is as follows:

[0009] The analog front-end circuit structure for a magnetic encoder has the following main features: the circuit structure includes an instrument amplifier, a filter driver and a gating unit; the input end of the instrument amplifier is connected to the sensor head, and is used to amplify the weak voltage signal generated by the sensor head and output a differential signal to the filter driver; the filter driver also receives external input voltage signals Vrefi and Vcmi as output common mode point adjustment and input common mode point adjustment, and is used to further amplify the signal, filter and adjust the output common mode point; the gating unit is connected to the instrument amplifier and the filter driver, and receives the differential signals Vop1 and Von1 output by the instrument amplifier and the differential signals Vop2 and Von2 output by the filter driver; and the output end of the gating unit is connected to the analog-to-digital converter.

[0010] Preferably, the instrument amplifier includes a current-type offset elimination module, a first operational amplifier, a second operational amplifier, an input resistor, a first feedback resistor and a second feedback resistor. The input ends of the first operational amplifier and the second operational amplifier are respectively connected to the sensor head, and the input ends of the first operational amplifier and the second operational amplifier are also respectively connected to the current-type offset elimination module. The input resistor is connected between the first operational amplifier and the second operational amplifier, the first feedback resistor is connected between the input and output ends of the first operational amplifier, and the second feedback resistor is connected between the input and output ends of the second operational amplifier. The first operational amplifier and the second operational amplifier are used to amplify the weak voltage signal generated by the sensor head and output a differential signal.

[0011] Preferably, the filter driver comprises an input common-mode feedback module, a third operational amplifier, a first resistor, a second resistor, a third resistor, a fourth resistor, a third feedback resistor, a fourth feedback resistor, a first capacitor and a second capacitor;

[0012] The input common mode feedback module receives external input voltage signals Vrefi and Vcmi, the output end of the input common mode feedback module is connected to the input end of the third operational amplifier, the first resistor and the third feedback resistor are connected between the output end of the first operational amplifier and the output end of the third operational amplifier, the second resistor and the fourth feedback resistor are connected between the output end of the second operational amplifier and the output end of the third operational amplifier, the third resistor and the fourth resistor are both connected between the DC input end and the input end of the third operational amplifier, the first capacitor is connected to both ends of the third feedback resistor, the second capacitor is connected to both ends of the fourth feedback resistor, the first capacitor and the second capacitor, the third feedback resistor, and the fourth feedback resistor form a filter network, the third operational amplifier is used to achieve high gain and provide low noise amplification and drive, the first resistor, the second resistor, the third feedback resistor, and the fourth feedback resistor are used to determine the amplification factor of the filter driver, and the third resistor, the fourth resistor, the third feedback resistor, and the fourth feedback resistor are used to achieve a suitable output common mode point.

[0013] Preferably, the input common-mode feedback module includes a first-stage telescope structure and a common-mode input feedback module, wherein the first-stage telescope structure includes a PMOS current source, a first PMOS tube, a second PMOS tube, a third PMOS tube, a fourth PMOS tube, a fourth NMOS tube, a fifth NMOS tube, a sixth NMOS tube, and a seventh NMOS tube; the common-mode input feedback module includes a first NMOS tube, a second NMOS tube, a third NMOS tube, a first NMOS current source, a second NMOS current source, a third NMOS current source, a load unit, a common-mode detection and comparator unit;

[0014] The first NMOS tube, the second NMOS tube and the third NMOS tube are completely identical, the first NMOS current source, the second NMOS current source and the third NMOS current source are completely identical, the first NMOS tube, the second NMOS tube and the third NMOS tube are all connected to the load unit, the first NMOS tube, the second NMOS tube and the third NMOS tube are respectively connected to the first NMOS current source, the second NMOS current source and the third NMOS current source, the gate of the first NMOS tube is connected to the first PMOS tube, the gate of the second NMOS tube is connected to the second PMOS tube, the gate of the third NMOS tube is connected to the external input voltage signal Vcmi, the source ends of the first NMOS tube, the second NMOS tube and the third NMOS tube are respectively connected to the input end of the common mode detection and comparator unit, and the output end of the common mode detection and comparator unit is connected to the sixth NMOS tube and the seventh NMOS tube.

[0015] Preferably, the input resistor and the first feedback resistor and the second feedback resistor are resistors with opposite resistance coefficients, and are used to compensate for the temperature coefficient of the sensor head.

[0016] Preferably, the current-type offset elimination module includes a current source group, a first CMOS switch and a second CMOS switch, the current source group is an NMOS current source, one end of the first CMOS switch and the second CMOS switch are both connected to the NMOS current source, the other end of the first CMOS switch is connected to the input end of the first operational amplifier, and the other end of the second CMOS switch is connected to the input end of the second operational amplifier.

[0017] Preferably, if the analog-to-digital converter detects that the differential output voltage signal Vop1 of the first operational amplifier is greater than the differential output voltage signal Von1 of the second operational amplifier, the second CMOS switch is closed, and the NMOS current source is connected to the signal Von1 through the second CMOS switch and the second feedback resistor Rf2, so that Von1 is increased, and a plurality of NMOS current sources are connected until the signal Von1 is equal to the signal Vop1; if the analog-to-digital converter detects that the signal Von1 is greater than the signal Vop1, the first CMOS switch is closed, and the NMOS current source is connected to the signal Vop1 through the first CMOS switch and the first feedback resistor.

[0018] Preferably, the current source group is a PMOS current source.

[0019] Preferably, the first resistor and the second resistor have the same resistance value, the third feedback resistor and the fourth feedback resistor have the same resistance value, and the first capacitor and the second capacitor have the same capacitance value.

[0020] Preferably, the first operational amplifier and the second operational amplifier are two-stage operational amplifiers, wherein the first stage is a telescopic structure.

[0021] Preferably, the third operational amplifier is a two-stage operational amplifier, wherein the first stage is a telescope structure and the second stage is an AB type output structure.

[0022] The analog front-end circuit structure for magnetic encoders of the present invention can be applied not only to the analog front-end of magnetic encoders, but also to the front-end analog interface circuit applications of all four-resistance bridges. The analog front-end circuit amplifies the weak sensor head signal in two stages and improves the linearity and noise characteristics. The imbalance caused by the sensor head and the integrated circuit device is eliminated without using the traditional chopping circuit, reducing the complexity of the hardware design. At the same time, it has an extremely high output drive capability, which ensures the sampling and establishment accuracy of the subsequent high-speed switched capacitor analog-to-digital converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The invention is a two-way orthogonal output of the magnetic encoder analog front end of the prior art.

[0024] Figure 2 It is the actual angle output deviation caused by the orthogonal misalignment deviation in the prior art.

[0025] Figure 3 The figure is a circuit block diagram of the analog front-end circuit structure for a magnetic encoder of the present invention.

[0026] Figure 4 The present invention is a schematic diagram of an instrument amplifier with current-type offset cancellation for an analog front-end circuit structure of a magnetic encoder.

[0027] Figure 5 The present invention is a schematic diagram of the input common-mode feedback and first-stage telescope structure of the operational amplifier of the analog front-end circuit structure for the magnetic encoder. DETAILED DESCRIPTION

[0028] In order to more clearly describe the technical content of the present invention, further description is given below in conjunction with specific embodiments.

[0029] The analog front-end circuit structure for a magnetic encoder of the present invention includes an instrument amplifier, a filter driver and a gating unit. The input end of the instrument amplifier is connected to the sensor head and is used to amplify the weak voltage signal generated by the sensor head and output a differential signal to the filter driver. The filter driver also receives external input voltage signals Vrefi and Vcmi as output common mode point adjustment and input common mode point adjustment, and is used to further amplify the signal, filter and adjust the output common mode point. The gating unit is connected to the instrument amplifier and the filter driver, and receives the differential signals Vop1 and Von1 output by the instrument amplifier and the differential signals Vop2 and Von2 output by the filter driver. The output end of the gating unit is connected to the analog-to-digital converter.

[0030] As a preferred embodiment of the present invention, the instrument amplifier includes a current-type offset elimination module, a first operational amplifier, a second operational amplifier, an input resistor Ri, a first feedback resistor Rf1 and a second feedback resistor Rf2. The input ends of the first operational amplifier and the second operational amplifier are respectively connected to the sensor head, and the input ends of the first operational amplifier and the second operational amplifier are also respectively connected to the current-type offset elimination module. The input resistor Ri is connected between the first operational amplifier and the second operational amplifier, the first feedback resistor Rf1 is connected between the input and output ends of the first operational amplifier, and the second feedback resistor Rf2 is connected between the input and output ends of the second operational amplifier. The first operational amplifier and the second operational amplifier are used to amplify the weak voltage signal generated by the sensor head and output a differential signal.

[0031] As a preferred embodiment of the present invention, the filter driver includes an input common-mode feedback module, a third operational amplifier, a first resistor Ri1, a second resistor Ri2, a third resistor Ri3, a fourth resistor Ri4, a third feedback resistor Rf3, a fourth feedback resistor Rf4, a first capacitor Cf1 and a second capacitor Cf2,

[0032] The input common-mode feedback module receives external input voltage signals Vrefi and Vcmi, the output end of the input common-mode feedback module is connected to the input end of the third operational amplifier, the first resistor Ri1 and the third feedback resistor Rf3 are connected between the output end of the first operational amplifier and the output end of the third operational amplifier, the second resistor Ri2 and the fourth feedback resistor Rf4 are connected between the output end of the second operational amplifier and the output end of the third operational amplifier, the third resistor Ri3 and the fourth resistor Ri4 are both connected between the DC input end and the input end of the third operational amplifier, and the first capacitor Cf1 is connected between the third The two ends of the feedback resistor Rf3, the second capacitor Cf2 is connected to the two ends of the fourth feedback resistor Rf4, the first capacitor Cf1 and the second capacitor Cf2, the third feedback resistor Rf3, and the fourth feedback resistor Rf4 form a filtering network, the third operational amplifier is used to achieve high gain and provide low noise amplification and drive, the first resistor Ri1, the second resistor Ri2, the third feedback resistor Rf3, and the fourth feedback resistor Rf4 are used to determine the amplification factor of the filter driver, and the third resistor Ri3, the fourth resistor Ri4, the third feedback resistor Rf3, and the fourth feedback resistor Rf4 are used to achieve a suitable output common mode point.

[0033] As a preferred embodiment of the present invention, the input common-mode feedback module includes a first-stage telescope structure and a common-mode input feedback module, the first-stage telescope structure includes a PMOS current source, a first PMOS tube, a second PMOS tube, a third PMOS tube, a fourth PMOS tube, a fourth NMOS tube, a fifth NMOS tube, a sixth NMOS tube, and a seventh NMOS tube; the common-mode input feedback module includes a first NMOS tube, a second NMOS tube, a third NMOS tube, a first NMOS current source, a second NMOS current source, a third NMOS current source, a load unit, a common-mode detection and comparator unit;

[0034] The first NMOS tube, the second NMOS tube and the third NMOS tube are completely identical, the first NMOS current source, the second NMOS current source and the third NMOS current source are completely identical, the first NMOS tube, the second NMOS tube and the third NMOS tube are all connected to the load unit, the first NMOS tube, the second NMOS tube and the third NMOS tube are respectively connected to the first NMOS current source, the second NMOS current source and the third NMOS current source, the gate of the first NMOS tube is connected to the first PMOS tube, the gate of the second NMOS tube is connected to the second PMOS tube, the gate of the third NMOS tube is connected to the external input voltage signal Vcmi, the source ends of the first NMOS tube, the second NMOS tube and the third NMOS tube are respectively connected to the input end of the common mode detection and comparator unit, and the output end of the common mode detection and comparator unit is connected to the sixth NMOS tube and the seventh NMOS tube.

[0035] As a preferred embodiment of the present invention, the input resistor Ri and the first feedback resistor Rf1 and the second feedback resistor Rf2 are resistors with opposite resistance coefficients, and are used to compensate for the temperature coefficient of the sensor head.

[0036] As a preferred embodiment of the present invention, the current-type offset elimination module includes a current source group, a first CMOS switch and a second CMOS switch, the current source group is an NMOS current source, one end of the first CMOS switch and the second CMOS switch are both connected to the NMOS current source, the other end of the first CMOS switch is connected to the input end of the first operational amplifier, and the other end of the second CMOS switch is connected to the input end of the second operational amplifier.

[0037] As a preferred embodiment of the present invention, if the analog-to-digital converter detects that the differential output voltage signal Vop1 of the first operational amplifier is greater than the differential output voltage signal Von1 of the second operational amplifier, the second CMOS switch is closed, and the NMOS current source is connected to the signal Von1 through the second CMOS switch and the second feedback resistor Rf2, so that Von1 is increased, and a plurality of NMOS current sources are connected until the signal Von1 is equal to the signal Vop1; if the analog-to-digital converter detects that the signal Von1 is greater than the signal Vop1, the first CMOS switch is closed, and the NMOS current source is connected to the signal Vop1 through the first CMOS switch and the first feedback resistor Rf1.

[0038] As a preferred embodiment of the present invention, the current source group is a PMOS current source.

[0039] As a preferred embodiment of the present invention, the first resistor Ri1 and the second resistor Ri2 have the same resistance value, the third feedback resistor Rf3 and the fourth feedback resistor Rf4 have the same resistance value, and the first capacitor Cf1 and the second capacitor Cf2 have the same capacitance value.

[0040] As a preferred embodiment of the present invention, the first operational amplifier and the second operational amplifier are two-stage operational amplifiers, wherein the first stage is a telescopic structure.

[0041] As a preferred embodiment of the present invention, the third operational amplifier adopts a two-stage operational amplifier, wherein the first stage is a telescope structure and the second stage is an AB type output structure.

[0042] In a specific embodiment of the present invention, the present invention discloses an analog front-end circuit for a magnetic encoder. Under the premise of ensuring optimal power consumption and hardware design, many functions are realized, including input offset voltage elimination; adjustable output common-mode voltage; amplification and filtering; and as a driver of an analog-to-digital converter to ensure sampling and settling accuracy.

[0043] The analog front-end circuit for the magnetic encoder comprises three parts: an instrument amplifier, a filter driver and a gating unit.

[0044] The instrument amplifier is composed of a current offset elimination module, two operational amplifiers A1, and three resistors including an input resistor Ri and two feedback resistors Rf1 and Rf2. Its two input terminals are connected to the sensor head to amplify the weak voltage signals Vip and Vin generated by the sensor head, and output two differential signals Vop1 and Von1 after amplification.

[0045] The current-type offset elimination module is composed of an NMOS current source and a CMOS switch, and is used to eliminate the common-mode voltage offset of the outputs Vop1 and Von1.

[0046] The two operational amplifiers A1 are two-stage operational amplifiers for achieving high gain and low noise amplification.

[0047] Among the three resistors, Ri, Rf1 and Rf2 are resistors with opposite resistance coefficients and are used to compensate for the temperature coefficient of the sensor head.

[0048] The filter driver includes an input common-mode feedback module, an operational amplifier A2, six resistors Ri1, Ri2, Ri3, Ri4, Rf3, Rf4, and two capacitors Cf1 and Cf2.

[0049] The input of the filter driver is the differential output signal Vop1 and Von1 of the instrument amplifier, and the other two inputs are Vrefi and Vcmi, which are used for output common mode point adjustment and input common mode point adjustment respectively.

[0050] The filter driver is used to further amplify and filter the signal and adjust the output common mode point of the channel.

[0051] The input common mode feedback module is composed of two input differential biases, one common mode input bias, a common mode sampling, a comparator and a feedback control current source circuit, which is used to ensure the correct common mode input and provide the most suitable DC working level for the operational amplifier.

[0052] The six resistors are resistors of the same type, among which Ri1, Ri2, Rf3, and Rf4 are used to determine the amplification factor of the filter driver. Ri3, Ri4, Rf3, and Rf4 are used to achieve a suitable output common mode point. The third feedback resistor Rf3 and the fourth feedback resistor Rf4 are usually equal. These two feedback resistors not only determine the amplification factor of the filter, but also realize the adjustment of the output common mode point. Ri1 and Ri2 are equal, and the ratio of Ri1, Ri2 to Rf3 and Rf4 determines the amplification factor of the filter; Ri3 and Ri4 are equal, and their ratio to Rf3 and Rf4 is used to achieve a suitable output common mode point.

[0053] The two capacitors Cf1 and Cf2 and the resistors Rf3 and Rf4 form a filtering network.

[0054] The A2 is a two-stage operational amplifier used to achieve high gain and provide low noise amplification and driving.

[0055] The gating unit is used to select the outputs Vop1 and Von1 of the instrumentation amplifier or the outputs Vop2 and Von2 of the filter driver as input signals of a subsequent analog-to-digital converter.

[0056] The present invention implements an analog front-end circuit for a magnetic encoder. Such a sensor head is usually composed of two pairs of four-resistance bridges, which are processed by two sets of analog front-end circuits respectively. Figure 1 As shown. As the direction of the rotating magnetic field changes, the two pairs of bridges generate orthogonal sine and cos signals respectively. First, the output of a single-channel sine or cos signal is a differential signal. The common-mode offset of the differential signal itself will affect the dynamic range of the subsequent circuit; at the same time, if there is a common-mode mismatch between the two orthogonal signals, it will also cause the integral nonlinear error of the subsequent demodulation angle to increase, such as Figure 2 The core of the present invention is to provide an analog front-end circuit for a magnetic encoder, which realizes the common-mode offset adjustment of a single-channel differential signal, the common-mode offset adjustment between two channels, the low-noise and low-temperature drift amplification and filtering of the weak signal of the sensor head, and the driving of the subsequent analog-to-digital converter, so as to ensure the sampling accuracy of the analog-to-digital converter for the front-end signal.

[0057] The instrument amplifier is composed of a current offset elimination module, three resistors Ri, Rf1, Rf2, and two identical operational amplifiers A1. It has two inputs Vip and Vin, which are connected to the two outputs of the sensor head respectively. It has two outputs Vop1 and Von1, which serve as differential inputs of the subsequent filter driver. At the same time, Vop1 and Von1 are connected to the gating unit module, and when necessary, the analog-to-digital converter detects the values ​​of Vop1 and Von1.

[0058] Among them, Ri and Rf1, Rf2 are different types of resistors with opposite temperature coefficients, which are used to compensate for the temperature coefficient of the voltage signal generated by the sensor head. Take Rf1 = Rf2, at this time, the instrument amplifier gain is equal to Rf1 / Ri. At this time, if the sensor head has an offset or the op amp A1 has an offset voltage, it will be amplified to Vop1 and Von1 year on year, causing the dynamic range of the subsequent processing circuit to be greatly reduced.

[0059] Instrumentation amplifiers with current-mode offset cancellation such as Figure 4 As shown. The current-type offset cancellation consists of a group of NMOS current sources and two CMOS switches SW1 and SW2. When there is no external magnetic field, the analog-to-digital converter detects that Vop1 is greater than Von1, the switch SW2 is closed, and the NMOS current source is connected to Von1 through SW2 and Rf2, causing Von1 to increase. Select a suitable number of NMOS current sources to connect until Von1 is equal to Vop1. Conversely, when Von1 is greater than Vop1, SW1 is closed, and the NMOS current source is connected to Vop1 through SW1 and Rf1. The NMOS current source is a group of binary unit current sources that can be finely adjusted and gated. This method achieves precise low-noise regulation.

[0060] Optionally, the current source group may also be composed of a group of equal current sources or current sources of other units. PMOS current sources may also be used to replace NMOS current sources.

[0061] The two identical operational amplifiers A1 use a two-stage operational amplifier, wherein the first stage is a sleeve structure, which is used to achieve low noise contribution while providing high gain. Alternatively, other types of operational amplifiers such as a sleeve structure with gain enhancement may also be used.

[0062] The filter driver includes an input common mode feedback module, an operational amplifier A2, six resistors Ri1, Ri2, Ri3, Ri4, Rf3 and Rf4, and two capacitors Cf1 and Cf2. It has two inputs Vop1 and Von1, which come from the differential outputs of the instrumentation amplifier respectively. It has two differential outputs Vop2 and Von2. It also has two DC voltage inputs, Vrefi and Vcmi. It is used to achieve output regulation; further low-noise amplification and filtering of sensor signals and driving of subsequent analog-to-digital converters.

[0063] Vrefi and Vcmi are both external input voltage signals, which come from the voltage reference source circuit. Vcmi determines the input common mode point of the op amp, and Vcmi and Vrefi jointly determine the output common mode point of the op amp. Vcmi is a fixed voltage, and the op amp output common mode point can be adjusted by adjusting Vrefi.

[0064] Optionally, Ri1=Ri2, Rf3=Rf4, Cf1=Cf2. The amplification gain of the filter driver is Rf3 / Ri1. Cf1 and Rf3 form a low-pass filter.

[0065] Op amp A2 uses a two-stage op amp, where the first stage is a telescope structure and the second stage is an AB type output structure. This ensures the op amp's high DC gain and low noise, while achieving high drive capability at a lower quiescent current. However, the telescope structure has very high requirements for the quiescent DC operating point of the input end in low-voltage design, usually within a very narrow range. Therefore, an input common-mode feedback structure is used here.

[0066] According to the power supply voltage and the op amp design, the most reasonable input common mode voltage Vcmi is taken as a DC input of the op amp, which is connected to the input common mode feedback module. The output of the input common mode feedback is used to control the tail current source of the op amp A2. Figure 5 The figure shows the first-stage telescope structure and common-mode input feedback diagram of the op amp. The first stage of the op amp consists of a PMOS current source Ip0, four other PMOS tubes PM1 to PM4, and four NMOS tubes NM4 to NM7. PM1 and PM2 are used as differential input pairs of the op amp. Ip0 is used as the tail current source of the op amp.

[0067] The input common-mode feedback is composed of three identical NMOS tubes NM1~NM3, three identical NMOS current sources In1~In3, a load unit load, and a common-mode detection and comparator unit. NM1~NM3, In1~In3, and the load unit load form two input differential biases and one common-mode input bias. The gate of NM1 is connected to PM1, the gate of NM2 is connected to PM2, and the gate of NM3 is connected to the DC input Vcmi. The source terminals of NM1~NM3 are respectively used as the inputs of the common-mode detection and comparator modules. By tracking the values ​​of the input VIP and VIN, the source terminal signals of NM1 and NM2 are added, averaged, filtered, and input into the comparator together with the source terminal signal of NM3. The output signal Vcmfb of the comparator is used to control the current of NM6 and NM7 through feedback, thereby adjusting the input DC level of the op amp to work at the value of Vcmi. This ensures that no matter how the output signal from the instrumentation amplifier changes, the input DC operating point of op amp A2 always remains at the Vcmi value. Thereby ensuring the working performance of this clock structure op amp.

[0068] Optionally, the output Vcmfb of the common mode detection and comparator module can also adjust the size of the micro current source Ipo.

[0069] Optionally, NM1 to NM3 used to track VIP and VIN values ​​may also be PMOS tubes.

[0070] Another DC input Vrefi of the filter driver is a configurable DC voltage input. Take Ri3 = Ri4, through the proportional relationship between Ri3 and Rf3, combined with the output common mode level of the first stage instrumentation amplifier, the value of Vcmi, the output common mode level of a single channel can be adjusted, so as to achieve the consistency of the common mode level of the sine and cos signals of the above two orthogonal channels. It works together with the input common mode feedback, not only realizing the adjustable output common mode, but also making the input common mode of the op amp always work at the optimal DC operating point.

[0071] In summary, the present invention relates to an analog front-end circuit for a magnetic encoder. The analog front-end circuit (AFE) comprises an instrumentation amplifier, a filter driver and a gating unit.

[0072] The instrument amplifier includes two operational amplifiers, three resistors and a current-type offset elimination circuit. The instrument amplifier is used to receive the weak voltage signal generated by the sensor head, pre-amplify the weak voltage signal by dozens of times, and achieve a signal-to-noise ratio of more than 90db. The offset elimination circuit is used to eliminate the front-end offset voltage introduced by the sensor head and the offset voltage brought by the two operational amplifiers A1 of the instrument amplifier, so that the outputs of the two outputs Vop1 and Von1 of the instrument amplifier are at the same common mode point.

[0073] The filter driver is composed of an operational amplifier, six resistors, two capacitors and an input common mode feedback. The filter driver realizes three functions: first, it performs secondary amplification on the signal of the instrument amplifier so that the output signal is close to the range of the analog-to-digital converter behind; second, it is used to filter out the high-frequency unit to realize the function of the anti-aliasing filter; third, it is used as the driver of the front-end sampling and holding circuit of the analog-to-digital converter to ensure the sampling and establishment accuracy of the switch capacitor circuit. The filter driver uses the output Vop1 and Von1 of the instrument amplifier as its pair of differential input signals; another adjustable DC input signal Vrefi is used as another input signal, and the output common mode level of the output Vop2 and Von2 of the filter driver is jointly determined by the proportional relationship of Ri3, Ri4, and Rf3 and the DC input signal Vcmi of the input common mode feedback. The input of the input common mode feedback module is a DC level Vcmi, and its output signal controls the tail current source inside the operational amplifier, so that the input differential DC operating point of the operational amplifier is fixed at Vcmi.

[0074] The gating unit is used to select the differential outputs Vop1 and Von1 of the instrumentation amplifier and the differential outputs Vop2 and Von2 of the filter driver. The output signal of the gating unit is used as the input signal of the subsequent analog-to-digital converter.

[0075] The specific implementation scheme of this embodiment can refer to the relevant description in the above embodiment, which will not be repeated here.

[0076] It can be understood that the same or similar parts of the above embodiments can be referenced to each other, and the contents not described in detail in some embodiments can refer to the same or similar contents in other embodiments.

[0077] It should be noted that, in the description of the present invention, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. In addition, in the description of the present invention, unless otherwise specified, the meaning of "plurality" refers to at least two.

[0078] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, segment or portion of code that includes one or more executable instructions for implementing the steps of a specific logical function or process, and the scope of the preferred embodiments of the present invention includes alternative implementations in which functions may not be performed in the order shown or discussed, including performing functions in a substantially simultaneous manner or in the reverse order depending on the functions involved, which should be understood by those skilled in the art to which the embodiments of the present invention belong.

[0079] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, a plurality of steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution device. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0080] A person skilled in the art may understand that all or part of the steps in the method for implementing the above-mentioned embodiment may be completed by instructing related hardware through a program, and the corresponding program may be stored in a computer-readable storage medium, which, when executed, includes one of the steps of the method embodiment or a combination thereof.

[0081] In addition, each functional unit in each embodiment of the present invention may be integrated into a processing module, or each unit may exist physically separately, or two or more units may be integrated into one module. The above-mentioned integrated module may be implemented in the form of hardware or in the form of a software functional module. If the integrated module is implemented in the form of a software functional module and sold or used as an independent product, it may also be stored in a computer-readable storage medium.

[0082] The storage medium mentioned above can be a read-only memory, a magnetic disk or an optical disk, etc.

[0083] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0084] The analog front-end circuit structure for magnetic encoders of the present invention can be applied not only to the analog front-end of magnetic encoders, but also to the front-end analog interface circuit applications of all four-resistance bridges. The analog front-end circuit amplifies the weak sensor head signal in two stages and improves the linearity and noise characteristics. The imbalance caused by the sensor head and the integrated circuit device is eliminated without using the traditional chopping circuit, reducing the complexity of the hardware design. At the same time, it has an extremely high output drive capability, which ensures the sampling and establishment accuracy of the subsequent high-speed switched capacitor analog-to-digital converter.

[0085] In this specification, the present invention has been described with reference to specific embodiments thereof. However, it is apparent that various modifications and variations may be made without departing from the spirit and scope of the present invention. Therefore, the specification and drawings should be regarded as illustrative rather than restrictive.

Claims

1. An analog front-end circuit structure for a magnetic encoder, characterized in that: The circuit structure includes an instrument amplifier, a filter driver and a gating unit. The input end of the instrument amplifier is connected to the sensor head, and is used to amplify the weak voltage signal generated by the sensor head and output a differential signal to the filter driver. The filter driver also receives external input voltage signals Vrefi and Vcmi as output common mode point adjustment and input common mode point adjustment, and is used to further amplify the signal, filter and adjust the output common mode point. The gating unit is connected to the instrument amplifier and the filter driver, and receives the differential signal output by the instrument amplifier and the differential signal output by the filter driver. The output end of the gating unit is connected to the analog-to-digital converter. The instrument amplifier comprises a current-type offset cancellation module, a first operational amplifier, a second operational amplifier, an input resistor (Ri), a first feedback resistor (Rf1) and a second feedback resistor (Rf2); The filter driver comprises an input common mode feedback module, a third operational amplifier, a first resistor (Ri1), a second resistor (Ri2), a third resistor (Ri3), a fourth resistor (Ri4), a third feedback resistor (Rf3), a fourth feedback resistor (Rf4), a first capacitor (Cf1) and a second capacitor (Cf2); The input common-mode feedback module receives external input voltage signals Vrefi and Vcmi, the output end of the input common-mode feedback module is connected to the input end of the third operational amplifier, the first resistor (Ri1) and the third feedback resistor (Rf3) are connected between the output end of the first operational amplifier and the output end of the third operational amplifier, the second resistor (Ri2) and the fourth feedback resistor (Rf4) are connected between the output end of the second operational amplifier and the output end of the third operational amplifier, the third resistor (Ri3) and the fourth resistor (Ri4) are both connected between the DC input end and the input end of the third operational amplifier, and the first capacitor (Cf1) is connected to the third feedback resistor (Rf3). ), the second capacitor (Cf2) is connected to the two ends of the fourth feedback resistor (Rf4), the first capacitor (Cf1) and the second capacitor (Cf2), the third feedback resistor (Rf3), and the fourth feedback resistor (Rf4) form a filtering network, the third operational amplifier is used to achieve high gain, provide low noise amplification and drive, the first resistor (Ri1), the second resistor (Ri2), the third feedback resistor (Rf3) and the fourth feedback resistor (Rf4) are used to determine the amplification factor of the filter driver, and the third resistor (Ri3), the fourth resistor (Ri4), the third feedback resistor (Rf3) and the fourth feedback resistor (Rf4) are used to achieve a suitable output common mode point.

2. The analog front-end circuit structure for a magnetic encoder according to claim 1, characterized in that: The input ends of the first operational amplifier and the second operational amplifier are respectively connected to the sensor head, and the input ends of the first operational amplifier and the second operational amplifier are also respectively connected to the current-type offset elimination module. The input resistor (Ri) is connected between the input end of the first operational amplifier and the input end of the second operational amplifier. The first feedback resistor (Rf1) is connected between the input end and the output end of the first operational amplifier, and the second feedback resistor (Rf2) is connected between the input end and the output end of the second operational amplifier. The first operational amplifier and the second operational amplifier are used to amplify the weak voltage signal generated by the sensor head and output a differential signal.

3. The analog front-end circuit structure for a magnetic encoder according to claim 1, characterized in that: The input common-mode feedback module includes a first-stage telescope structure and a common-mode input feedback module, wherein the first-stage telescope structure includes a PMOS current source, a first PMOS tube, a second PMOS tube, a third PMOS tube, a fourth PMOS tube, a fourth NMOS tube, a fifth NMOS tube, a sixth NMOS tube and a seventh NMOS tube; the common-mode input feedback module includes a first NMOS tube, a second NMOS tube, a third NMOS tube, a first NMOS current source, a second NMOS current source, a third NMOS current source, a load unit, a common-mode detection and comparator unit; The first NMOS tube, the second NMOS tube and the third NMOS tube are completely identical, the first NMOS current source, the second NMOS current source and the third NMOS current source are completely identical, the first NMOS tube, the second NMOS tube and the third NMOS tube are all connected to the load unit, the first NMOS tube, the second NMOS tube and the third NMOS tube are respectively connected to the first NMOS current source, the second NMOS current source and the third NMOS current source, the gate of the first NMOS tube is connected to the first PMOS tube, the gate of the second NMOS tube is connected to the second PMOS tube, the gate of the third NMOS tube is connected to the external input voltage signal Vcmi, the source ends of the first NMOS tube, the second NMOS tube and the third NMOS tube are respectively connected to the input end of the common mode detection and comparator unit, and the output end of the common mode detection and comparator unit is connected to the sixth NMOS tube and the seventh NMOS tube.

4. The analog front-end circuit structure for a magnetic encoder according to claim 1, characterized in that: The input resistor (Ri), the first feedback resistor (Rf1) and the second feedback resistor (Rf2) are resistors with opposite resistance coefficients and are used to compensate for the temperature coefficient of the sensor head.

5. The analog front-end circuit structure for a magnetic encoder according to claim 2, characterized in that: The current-type offset elimination module includes a current source group, a first CMOS switch and a second CMOS switch. The current source group is an NMOS current source. One end of the first CMOS switch and the second CMOS switch are both connected to the NMOS current source. The other end of the first CMOS switch is connected to the input end of the first operational amplifier. The other end of the second CMOS switch is connected to the input end of the second operational amplifier.

6. The analog front-end circuit structure for a magnetic encoder according to claim 5, characterized in that: If the analog-to-digital converter detects that the differential output voltage signal Vop1 of the first operational amplifier is greater than the differential output voltage signal Von1 of the second operational amplifier, the second CMOS switch is closed, and the NMOS current source is connected to the differential output voltage signal Von1 of the second operational amplifier through the second CMOS switch and the second feedback resistor (Rf2), so that the differential output voltage signal Von1 of the second operational amplifier is increased, and a plurality of NMOS current sources are connected until the differential output voltage signal Von1 of the second operational amplifier is equal to the differential output voltage signal Vop1 of the first operational amplifier; if the analog-to-digital converter detects that the differential output voltage signal Von1 of the second operational amplifier is greater than the differential output voltage signal Vop1 of the first operational amplifier, the first CMOS switch is closed, and the NMOS current source is connected to the differential output voltage signal Vop1 of the first operational amplifier through the first CMOS switch and the first feedback resistor (Rf1).

7. The analog front-end circuit structure for a magnetic encoder according to claim 5, characterized in that: The current source group is a PMOS current source.

8. The analog front-end circuit structure for a magnetic encoder according to claim 1, characterized in that: The first resistor (Ri1) and the second resistor (Ri2) have the same resistance value, the third feedback resistor (Rf3) and the fourth feedback resistor (Rf4) have the same resistance value, and the first capacitor (Cf1) and the second capacitor (Cf2) have the same capacitance value.

9. The analog front-end circuit structure for a magnetic encoder according to claim 1, characterized in that: The first operational amplifier and the second operational amplifier are two-stage operational amplifiers, wherein the first stage is a telescopic structure.

10. The analog front-end circuit structure for a magnetic encoder according to claim 1, characterized in that: The third operational amplifier adopts a two-stage operational amplifier, wherein the first stage is a telescope structure and the second stage is an AB type output structure.

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