FA-level precision current measuring device

Abstract

The invention discloses a current measuring device with fA-level precision. The current measuring device comprises a current sense circuit unit, a pre-voltage amplifier unit, a precision ADC circuit unit, an isolation interface circuit unit and a high-stability reference circuit unit, and the high-stability reference circuit unit provides high-stability reference voltage for ADC conversion and analog circuit comparison; the low-noise power supply unit is used for providing a stable low-noise power supply for any other units; the LO signal interface unit is connected to a voltage source or an analog ground according to a measurement mode; and the HI signal interface unit is directly connected to the equipment to be tested and is used for measuring the current passing through the DUT. The current sensing resistor is a core component of a current sense circuit and generally has a very high resistance value so as to ensure that a voltage signal which is strong enough is generated when small current is measured, so that amplification processing of a post-stage circuit is facilitated, and the signal-to-noise ratio is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of low current measurement, and in particular to a current measurement device with femtoampere (fA) level accuracy. Background Art

[0002] When measuring low currents, such as IV curves of semiconductor devices, dark current measurements in optical devices, insulation measurements of materials, and leakage current measurements of cable assemblies, these applications usually require measuring currents as low as 1pA or less.

[0003] In the existing technology, the instruments that can be used for low current measurement include electrometers, picoammeters, source meters, etc. These instruments usually have high indicators, and can measure currents below 1pA at the minimum, and currents at mA or even A at the maximum. Each measurement channel corresponds to a device (large size 104mm high × 238mm wide × 370mm deep, expensive 60,000 yuan per channel). For application scenarios that require multi-point measurement, the existing solutions have poor integration (such as semiconductor test benches, usually 1.5m*1.5m*1.5m in size), and are expensive (one device is integrated for each channel).

[0004] Other similar solutions use current collection resistors and differential amplifiers to measure current, such as insulation resistance testers. Such instruments are usually used for uA / mA level leakage current measurement and are not suitable for occasions that require fA level accuracy. Summary of the invention

[0005] The present invention aims to provide a current measuring device with femtoampere (fA) level accuracy to solve the above problems.

[0006] The present invention is achieved through the following technical solutions: A current measuring device with femtoampere (fA) level accuracy, comprising: A current sense circuit unit, wherein the current sense circuit unit converts the current of the device under test (DUT) into a voltage signal that can be processed by a post-amplifier. The current sense circuit unit is directly connected to the device under test and converts the current signal into a voltage signal for processing by a pre-voltage amplifier unit; A pre-voltage amplifier unit, which amplifies the voltage signal generated by the current sense circuit, converts the single-ended signal into a differential signal, and provides a low-pass filtering function, receives the voltage signal from the current sense circuit, and outputs the differential signal to the precision ADC circuit after amplification; A precision ADC circuit unit, wherein the precision ADC circuit unit receives a differential signal from a pre-voltage amplifier and performs analog-to-digital conversion; An isolation interface circuit unit, wherein the isolation interface circuit unit isolates the digital signal of the ADC and transmits it to achieve independent measurement of each channel; A highly stable reference circuit unit, which provides a highly stable reference voltage for ADC conversion and analog circuit comparison; A low-noise power supply unit, which provides a stable low-noise power supply for any other unit; An LO signal interface unit, wherein the LO signal interface unit is connected to a voltage source or an analog ground according to a measurement mode; HI signal interface unit, the HI signal interface unit is directly connected to the device under test to measure the current passing through the DUT.

[0007] Preferably, the current sense circuit unit includes a current sensing resistor and a preamplifier, the current sensing resistor is used to detect the current passing through the device under test, and the preamplifier amplifies the voltage signal on the sensing resistor; The input of the transimpedance amplifier is connected to the current sensing resistor, and the output provides a voltage signal that is proportional to the current passing through the current sensing resistor. The amplified and conditioned voltage signal is ultimately connected to the precision ADC circuit for analog-to-digital conversion.

[0008] The current sensing resistor of the present invention is the core component of the current sense circuit, which usually has a very high resistance value to ensure that a sufficiently strong voltage signal is generated when measuring a small current so that the subsequent circuit can amplify and process it to improve the signal-to-noise ratio.

[0009] Preferably, in the pre-voltage amplifier unit, the amplification circuit includes a voltage input signal (VS), a load (VL), a sense resistor (Rsense), a transimpedance amplifier, a non-inverting input terminal (+) of the amplifier, an inverting input terminal (-) of the amplifier, a feedback loop, and an output voltage (VC); The voltage input signal (VS) is connected to the series combination of the load (VL) and the sense resistor (Rsense). The voltage drop generated when the current flows through the sense resistor (Rsense) is detected by the transimpedance amplifier, and a differential voltage is generated between the amplifier's non-inverting input (+) and inverting input (-). The transimpedance amplifier amplifies this differential voltage through a feedback loop and generates a voltage signal at the output (VC) that is proportional to the input current.

[0010] The transimpedance amplification scheme of the present invention makes the output impedance of the transimpedance scheme lower than that of the shunt scheme, and will not affect the accuracy of the voltage output to the load. The measurement principle of the transimpedance amplifier is that the input current of the amplifier can be ignored, so the current flowing through the load is equal to the current flowing through Rsense. As long as the current on the sense resistor is measured, the load current can be measured.

[0011] Preferably, in the pre-voltage amplifier unit, the amplification circuit specifically includes a voltage input signal (VS), an output impedance of a voltage source (RS), a floating power supply reference ground (FGND), a leakage resistor (RG), and a parasitic capacitor (CG), wherein the voltage input signal (VS) is the input signal of the amplifier, which is used to set the voltage of the device under test (DUT), the output impedance of the voltage source (RS) is the internal resistance of the voltage source, the floating power supply reference ground (FGND) and the signal source reference ground (AGND) are connected through a leakage resistor (RG), the leakage resistor (RG) is connected between the signal source reference ground (AGND) and the floating power supply reference ground (FGND), and its resistance is much larger than the output impedance of the voltage source (RS), the parasitic capacitor (CG) is between the floating power supply reference ground (FGND) and the signal source reference ground (AGND), and it and the leakage resistor (RG) together form a low-pass filter circuit, and the signal amplitude (VS') transmitted to the current conversion device is a signal processed by the amplification circuit, so that the signal amplitude (VS') transmitted to the current conversion device is close to the original voltage input signal (VS).

[0012] The main function of the pre-voltage amplifier of the present invention is to amplify the positive and negative 1V voltage signals generated by the current sense circuit, improve the signal-to-noise ratio of the acquisition result of the analog-to-digital converter (ADC), and is also responsible for converting the single-ended signal into a differential signal to enhance the ability to suppress interference noise, and provide a low-pass filtering function to suppress redundant noise signals. The pre-voltage amplifier converts the single-ended signal into a differential signal. The differential signal has better anti-interference ability and higher signal integrity than the single-ended signal. Through this design, even in the case of a high voltage input signal (VS), the voltage difference between the power supply voltage (FVCC) and the reference ground (FGND) of the amplifier can be kept constant, thereby achieving low-noise amplification. This floating ground power supply design scheme helps to maintain the accuracy and stability of the measurement in a high voltage measurement environment.

[0013] Preferably, in the pre-voltage amplifier unit, the leakage control circuit includes a GUARD protection ring and a PCB slot, wherein the GUARD protection ring is an equipotential conductor wrapped around a sensitive network (a network with a small leakage current to be controlled), and the GUARD protection ring is a conductive ring surrounding a main circuit path, and the voltage of the ring is equal to that of the sensitive network; PCB grooving refers to the process of removing PCB materials near sensitive networks, leaving air as a conductive medium, thereby reducing leakage current by increasing the physical distance of sensitive networks and utilizing the high resistivity of air.

[0014] The present invention can reduce the leakage current caused by insufficient insulation resistance or pollution of PCB board material by setting up GUARD protection ring. The voltage of GUARD protection ring and sensitive network are equal, so as to reduce the influence of leakage current. The present invention sets PCB slotting, which is a technology of digging out PCB material near sensitive network and leaving air as conductive medium. Since the resistivity of air is much higher than that of conventional FR4 board material, it can significantly improve insulation resistance and reduce leakage. In fA level current measurement, since there are many factors affecting the measurement results, it is necessary to carefully control leakage current. Leakage current control involves factors such as insulation resistance of PCB board material, reduced insulation due to pollution, and AC current coupling leakage caused by parasitic capacitance. These measures work together to ensure that the pre-voltage amplifier can work reliably under high voltage and low noise conditions and maintain the accuracy and stability of measurement. Through leakage control, the performance of current measurement device can be significantly improved, especially in applications requiring high-precision measurement.

[0015] Preferably, in the low-noise power supply unit, the power supply circuit receives input from the device bus power supply. As the starting point of the entire power supply circuit, the input power is first connected to the isolation power supply module, the output of the isolation power supply module is connected to the DCDC module, the output of the DCDC module is then connected to the LC filter, the LC filter is connected to the LDO voltage regulator to adjust the voltage fluctuation within 100KHz, and the output of the LDO module is then connected to the RC filter. Finally, the power supply after multi-stage filtering is supplied to the amplifier circuit unit.

[0016] The power supply circuit design of the present invention is a key part to ensure that the entire current measurement device can work in a low-noise environment. It provides a stable and clean power supply for the device through carefully designed power management and filtering technology. This multi-stage filtering circuit design strategy processes the noise in the low-frequency band, medium-frequency band, and high-frequency band separately, and different modules are responsible for it, thereby achieving an overall small size while also achieving low output noise, meeting the requirements of high integration and high performance.

[0017] Preferably, in the high-stability reference circuit unit, a cavity shielding structure is provided, including a sensitive circuit, a soft magnetic alloy (internal shielding material), and tinplate (external shielding material). The shielding structure adopts a double-layer design, with the outer layer being tinplate and the inner layer being soft magnetic alloy. The sensitive circuit is located inside the shielding structure, wherein the sensitive circuit is the circuit part that needs to be protected from magnetic field interference. The soft magnetic alloy has a high magnetic permeability and is used for internal shielding to attenuate magnetic field interference. Tinplate is used as an external shielding material, and its magnetic permeability is relatively low, but it is not easy to saturate. It is used to construct the outer layer of the shielding structure, which first contacts the external magnetic field and attenuates the magnetic field strength.

[0018] The present invention can achieve high magnetic field interference suppression capability in a limited space through the double-layer shielding design, and at the same time, does not need to increase the thickness of the shielding structure, meeting the requirements of small space volume and high integration. This shielding interference solution design effectively solves the electromagnetic interference problem in a high-integration current measurement device.

[0019] Preferably, a decentralized layout is adopted for the amplifier and the power supply in the low-noise power supply unit, with the amplifier on one side and the power supply on the other side. An open radiation heat dissipation design is adopted for the isolated DCDC. The chassis cover is designed with heat dissipation ribs to increase the heat exchange area, and high thermal conductivity materials or thermal interface materials are used to improve the heat conduction efficiency, so that the heat is conducted from the heat source to the chassis or the heat sink, and then the heat is dissipated to the environment through radiation and natural convection.

[0020] The purpose of the heat dissipation design of the present invention is to ensure that the device can work stably under the conditions of high power density and high integration, and the accuracy of the measurement results will not be affected by excessive temperature. Through the above measures, effective thermal management can be achieved to maintain the long-term stability and reliability of the device.

[0021] Preferably, the current sense circuit unit, pre-voltage amplifier unit, precision ADC circuit unit, isolation interface circuit unit, high-stability reference circuit unit, low-noise power supply unit, LO signal interface unit, and HI signal interface unit are all integrated in a 19-inch 1U chassis to form 8 channels that can independently measure current.

[0022] Preferably, the precision ADC circuit unit in the present application has a resolution of 24 bits. The precision ADC circuit performs an analog-to-digital conversion (ADC) process to convert an analog voltage signal into a digital value. The precision ADC circuit unit includes a calibration and compensation mechanism to correct system errors and temperature drifts.

[0023] Compared with the prior art, the present invention has the following advantages and beneficial effects: The present invention adopts low leakage amplifier chip and board hollowing protection circuit design, which effectively reduces leakage current and improves measurement accuracy. The circuit topology of transimpedance amplification and floating ground measurement structure, combined with the low noise power supply provided by LDO and LC combination circuit, as well as the partition layout and sensitive area shielding, ensures low noise performance under high voltage resistance and high precision requirements. The application of cavity shielding structure design and thin shielding structure materials reduces the module volume and improves the integration while improving the shielding effectiveness. The 24-bit resolution of the precision ADC circuit ensures the acquisition accuracy, while the isolation interface circuit and the high stability reference circuit further improve the accuracy and stability of the measurement. The low noise power supply part realizes a small size and low output noise power supply through a multi-stage filtering circuit design. In addition, the heat dissipation design effectively manages the heat consumption under high power density and maintains the stability of the circuit through radiation and conduction cooling. Overall, these technical solutions jointly realize a device that can accurately measure current in the range of -100V to +100V, while meeting the needs of high-performance current measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings: Figure 1 It is a schematic diagram of unit connection of the present invention; Figure 2 Schematic diagram of the amplifier circuit of the present invention Figure 1 ; Figure 3 Schematic diagram of the amplifier circuit of the present invention Figure 2 ; Figure 4 This is a schematic diagram of the sub-cavity shielding structure of the invention; Figure 5 Schematic diagram of the invented power supply circuit. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments and drawings. The schematic implementation modes and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention. It should be noted that the present invention is already in the actual development and use stage.

[0026] Example like Figures 1 to 5 As shown, this embodiment includes.

[0027] Disadvantages of existing technologies 1: Simple solutions cannot meet high performance requirements.

[0028] There are many challenges in measuring picoampere current, such as dielectric absorption, triboelectricity, radiated noise, RF noise, power line noise, ground noise, leakage current, surface contamination, stability, humidity, electrostatic coupling, board materials, solder paste materials, device noise and DC errors. Each of these factors will affect the current measurement results and need to be carefully controlled. The above factors can be classified into two categories: leakage current control and noise control.

[0029] In fA-level current measurement, since there are many factors that affect the measurement results, a simple solution based on conventional sampling resistors and amplifiers cannot meet the measurement accuracy (high performance) requirements.

[0030] Disadvantage 2 of the existing technology: the solution of commercial instruments cannot meet the requirements of high integration.

[0031] In many cases, users want to measure the operating current at multiple points and judge the working state of the circuit through the operating current. For example, the volt-ampere characteristic measurement of a single MOSFET transistor requires simultaneous measurement of the gate, source, and drain currents to draw an accurate response curve. The number of measurements required for multi-bit quantum measurement and control increases exponentially with the number of bits.

[0032] The common commercial instruments on the market are usually designed with the dimensions of 104mm high × 238mm wide × 370mm deep. If you need to measure the current of 8 channels, the minimum dimensions need to be 416mm high × 476mm wide × 370mm deep. If you include the rack and cables, this will be a very large volume, which is not friendly to space, noise, and heat.

[0033] Disadvantage 3 of the existing technology: The commercial instrument solution cannot meet the requirement of moderate cost.

[0034] The common commercial instruments on the market are priced at RMB 60,000 per channel, and the cost will increase exponentially for multi-channel measurement needs. This is too expensive for users who need multi-channel measurement.

[0035] Based on the above shortcomings, the high performance described in this application refers to high relative accuracy of current measurement (low noise) and high absolute accuracy of current measurement (small leakage current, good linearity, and good stability). These two requirements will introduce four engineering challenges, namely low-noise amplifier circuit design, low-noise power supply design, shielding of external noise interference, and leakage current control.

[0036] High integration means placing more channels in a limited device, which introduces two problems. First, the circuit has to have fewer and smaller devices, which makes it difficult to implement the solution. Second, the close proximity of the devices leads to high power density and thus heat dissipation difficulties.

[0037] Ease of use refers to a wide operating voltage range (-100V to +100V), multiple measurement modes (supporting high-side current measurement and low-side current measurement), and user-friendly software (python is used as the control software, and the user learning cost is low). This will introduce a problem, the compatibility of high-voltage devices and low-noise devices.

[0038] Therefore, this embodiment proposes a current measurement device with femtoampere (fA) level accuracy, including: A current sense circuit unit, wherein the current sense circuit unit converts the current of the device under test (DUT) into a voltage signal that can be processed by a post-amplifier. The current sense circuit unit is directly connected to the device under test and converts the current signal into a voltage signal for processing by a pre-voltage amplifier unit; A pre-voltage amplifier unit, which amplifies the voltage signal generated by the current sense circuit, converts the single-ended signal into a differential signal, and provides a low-pass filtering function, receives the voltage signal from the current sense circuit, and outputs the differential signal to the precision ADC circuit after amplification; A precision ADC circuit unit, wherein the precision ADC circuit unit receives a differential signal from a pre-voltage amplifier and performs analog-to-digital conversion; An isolation interface circuit unit, wherein the isolation interface circuit unit isolates the digital signal of the ADC and transmits it to achieve independent measurement of each channel; A highly stable reference circuit unit, which provides a highly stable reference voltage for ADC conversion and analog circuit comparison; A low-noise power supply unit, which provides a stable low-noise power supply for any other unit; An LO signal interface unit, wherein the LO signal interface unit is connected to a voltage source or an analog ground according to a measurement mode; HI signal interface unit, the HI signal interface unit is directly connected to the device under test to measure the current passing through the DUT.

[0039] Preferably, the current sense circuit unit includes a current sensing resistor and a preamplifier, the current sensing resistor is used to detect the current passing through the device under test, and the preamplifier amplifies the voltage signal on the sensing resistor; The input of the transimpedance amplifier is connected to the current sensing resistor, and the output provides a voltage signal that is proportional to the current passing through the current sensing resistor. The amplified and conditioned voltage signal is ultimately connected to the precision ADC circuit for analog-to-digital conversion.

[0040] The current sensing resistor of the present invention is the core component of the current sense circuit, which usually has a very high resistance value to ensure that a sufficiently strong voltage signal is generated when measuring a small current so that the subsequent circuit can amplify and process it to improve the signal-to-noise ratio.

[0041] In this embodiment, the amplifier is TI's LMC662, the input current of which is only 2fA and the current noise is only 0.2fA / sqrt(Hz). Its low noise and low leakage current characteristics can ensure the measurement accuracy of this device. (For example, in the 1nA range, 20fA leakage current and noise only contribute to a total error of 20ppm) In this embodiment, the sense resistor is another device that significantly affects the accuracy besides the amplifier. For example, a low-noise resistor can reduce the output noise of the link, thereby improving the measurement accuracy (relative accuracy). A resistor with a low voltage coefficient can reduce the nonlinearity of the measurement circuit and provide assistance for subsequent linear calibration. Therefore, choosing a resistor with low noise and low voltage coefficient is very important to achieve the performance of the device.

[0042] Thick film resistors are selected for this solution, which have very low current noise and a voltage coefficient not exceeding 5ppm.

[0043] In this embodiment, the design of the amplifier circuit needs to simultaneously meet the characteristics of "high withstand voltage", "low background noise (relative accuracy)" and "small DC error (leakage current, linearity, stability)".

[0044] It is difficult for commercial ICs to simultaneously meet the two requirements of "high withstand voltage" and "low background noise (relative accuracy)". Taking the chips of ADI and TI as an example, amplifier chips that can achieve high operating voltage usually have high background noise, reaching tens or hundreds of , the total noise integrated in the 10KHz bandwidth reaches 66pApp = 6.6×10pArms = 6.6 * 100 For a 1nA measurement range, the 66pA noise results in a measurement accuracy of only 6.6%. High-precision, low-noise amplifier chips can achieve a noise level below 0.01%, but their operating voltage is less than 36V. Therefore, they cannot meet the requirements of the device's measurement range of + / - 100V.

[0045] Based on this problem, the solution is selected as follows: the amplifier circuit includes a voltage input signal (VS), an output impedance of a voltage source (RS), a floating power supply reference ground (FGND), a leakage resistor (RG), and a parasitic capacitor (CG), wherein the voltage input signal (VS) is the input signal of the amplifier, which is used to set the voltage of the device under test (DUT). The output impedance (RS) of the voltage source is the internal resistance of the voltage source. The floating power supply reference ground (FGND) and the signal source reference ground (AGND) are connected through a leakage resistor (RG). The leakage resistor (RG) is connected between the signal source reference ground (AGND) and the floating power supply reference ground (FGND), and its resistance is much larger than the output impedance (RS) of the voltage source. The parasitic capacitor (CG) is between the floating power supply reference ground (FGND) and the signal source reference ground (AGND), and it and the leakage resistor (RG) together form a low-pass filter circuit. The signal amplitude (VS') transmitted to the current conversion device is the signal processed by the amplifier circuit, so that the signal amplitude (VS') transmitted to the current conversion device is close to the original voltage input signal (VS).

[0046] The main function of the pre-voltage amplifier of the present invention is to amplify the positive and negative 1V voltage signals generated by the current sense circuit, improve the signal-to-noise ratio of the acquisition result of the analog-to-digital converter (ADC), and is also responsible for converting the single-ended signal into a differential signal to enhance the ability to suppress interference noise, and provide a low-pass filtering function to suppress redundant noise signals. The pre-voltage amplifier converts the single-ended signal into a differential signal. The differential signal has better anti-interference ability and higher signal integrity than the single-ended signal. Through this design, even in the case of a high voltage input signal (VS), the voltage difference between the power supply voltage (FVCC) and the reference ground (FGND) of the amplifier can be kept constant, thereby achieving low-noise amplification. This floating ground power supply design scheme helps to maintain the accuracy and stability of the measurement in a high voltage measurement environment.

[0047] The transimpedance amplification scheme of the present invention makes the output impedance of the transimpedance scheme lower than that of the shunt scheme, and will not affect the accuracy of the voltage output to the load. The measurement principle of the transimpedance amplifier is that the input current of the amplifier can be ignored, so the current flowing through the load is equal to the current flowing through Rsense. As long as the current on the sense resistor is measured, the load current can be measured.

[0048] Preferably, in the pre-voltage amplifier unit, the leakage control circuit includes a GUARD protection ring and a PCB slot, wherein the GUARD protection ring is an equipotential conductor wrapped around a sensitive network (a network with a small leakage current to be controlled), and the GUARD protection ring is a conductive ring surrounding a main circuit path, and the voltage of the ring is equal to that of the sensitive network; PCB grooving refers to the process of removing PCB materials near sensitive networks, leaving air as a conductive medium, thereby reducing leakage current by increasing the physical distance of sensitive networks and utilizing the high resistivity of air.

[0049] The present invention can reduce the leakage current caused by insufficient insulation resistance or pollution of PCB board material by setting up GUARD protection ring. The voltage of GUARD protection ring and sensitive network are equal, so as to reduce the influence of leakage current. The present invention sets PCB slotting, which is a technology of digging out PCB material near sensitive network and leaving air as conductive medium. Since the resistivity of air is much higher than that of conventional FR4 board material, it can significantly improve insulation resistance and reduce leakage. In fA level current measurement, since there are many factors affecting the measurement results, it is necessary to carefully control leakage current. Leakage current control involves factors such as insulation resistance of PCB board material, reduced insulation due to pollution, and AC current coupling leakage caused by parasitic capacitance. These measures work together to ensure that the pre-voltage amplifier can work reliably under high voltage and low noise conditions and maintain the accuracy and stability of measurement. Through leakage control, the performance of current measurement device can be significantly improved, especially in applications requiring high-precision measurement.

[0050] In this embodiment, Preferably, in the low-noise power supply unit, the power supply circuit receives input from the device bus power supply. As the starting point of the entire power supply circuit, the input power is first connected to the isolation power supply module, the output of the isolation power supply module is connected to the DCDC module, the output of the DCDC module is then connected to the LC filter, the LC filter is connected to the LDO voltage regulator to adjust the voltage fluctuation within 100KHz, and the output of the LDO module is then connected to the RC filter. Finally, the power supply after multi-stage filtering is supplied to the amplifier circuit unit.

[0051] The power supply circuit design of the present invention is a key part to ensure that the entire current measurement device can work in a low-noise environment. It provides a stable and clean power supply for the device through carefully designed power management and filtering technology. This multi-stage filtering circuit design strategy processes the noise in the low-frequency band, medium-frequency band, and high-frequency band separately, and different modules are responsible for it, thereby achieving an overall small size while also achieving low output noise, meeting the requirements of high integration and high performance.

[0052] Preferably, in the high-stability reference circuit unit, a cavity shielding structure is provided, including a sensitive circuit, a soft magnetic alloy (internal shielding material), and tinplate (external shielding material). The shielding structure adopts a double-layer design, with the outer layer being tinplate and the inner layer being soft magnetic alloy. The sensitive circuit is located inside the shielding structure, wherein the sensitive circuit is the circuit part that needs to be protected from magnetic field interference. The soft magnetic alloy has a high magnetic permeability and is used for internal shielding to attenuate magnetic field interference. Tinplate is used as an external shielding material, and its magnetic permeability is relatively low, but it is not easy to saturate. It is used to construct the outer layer of the shielding structure, which first contacts the external magnetic field and attenuates the magnetic field strength.

[0053] As the magnetic field strength increases, there is an upper limit to the magnetic flux density. At this upper limit, μ is 0, which means that increasing the magnetic field strength will not lead to an increase in the magnetic induction intensity B, which means that the newly added magnetic field cannot be conducted away by the material and will penetrate the shielding material into the sensitive circuit to cause interference, so it is called saturation magnetic flux density.

[0054] Common magnetic shielding materials include tinplate and soft magnetic alloy. The magnetic permeability of tinplate is 1000 and the saturation magnetic flux density is 2.2T. The magnetic permeability of soft magnetic alloy is 500,000 and the saturation magnetic flux density is 0.7T. It is impossible to find a material that can simultaneously meet the requirements of high magnetic permeability (to ensure shielding effect) and high saturation magnetic flux density (to withstand a sufficiently strong external magnetic field).

[0055] When the interfering magnetic field reaches the outer shielding shell (tinplate), the tinplate will first attenuate this part of the magnetic field. Assuming its attenuation capacity is -20dB and non-saturated, the magnetic field can be attenuated to one tenth of the original value. That is to say, the internal shielding material (soft magnetic alloy) only needs to handle a magnetic flux density of 0.22T (one tenth of the saturation magnetic flux density of tinplate, 2.2T), which is less than the saturation magnetic flux density of 0.7T of the soft magnetic alloy.

[0056] The magnetic permeability of the inner shielding material (soft magnetic alloy) is 500 times that of the outer shielding material (tinplate), and the attenuation capability is 500 times stronger (-56dB), so the total magnetic field attenuation capability reaches -76dB.

[0057] If only tinplate is used, the magnetic field attenuation is only -20dB due to its limited magnetic permeability, which cannot effectively shield magnetic field interference.

[0058] If only soft magnetic alloys are used, they are prone to saturation failure due to their limited saturation flux density. Once saturated, the shielding ability of the magnetic field is zero.

[0059] In summary, this embodiment achieves high magnetic field interference suppression capability in a high magnetic flux density environment by combining different shielding materials, and does not require a thick shell to enhance magnetic field suppression, which can meet the requirements of small space volume and high integration.

[0060] The present invention can achieve high magnetic field interference suppression capability in a limited space through the double-layer shielding design, and at the same time, does not need to increase the thickness of the shielding structure, meeting the requirements of small space volume and high integration. This shielding interference solution design effectively solves the electromagnetic interference problem in a high-integration current measurement device.

[0061] Preferably, a decentralized layout is adopted for the amplifier and the power supply in the low-noise power supply unit, with the amplifier on one side and the power supply on the other side. An open radiation heat dissipation design is adopted for the isolated DCDC. The chassis cover is designed with heat dissipation ribs to increase the heat exchange area, and high thermal conductivity materials or thermal interface materials are used to improve the heat conduction efficiency, so that the heat is conducted from the heat source to the chassis or the heat sink, and then the heat is dissipated to the environment through radiation and natural convection.

[0062] The purpose of the heat dissipation design of the present invention is to ensure that the device can work stably under the conditions of high power density and high integration, and the accuracy of the measurement results will not be affected by excessive temperature. Through the above measures, effective thermal management can be achieved to maintain the long-term stability and reliability of the device.

[0063] Preferably, the current sense circuit unit, pre-voltage amplifier unit, precision ADC circuit unit, isolation interface circuit unit, high-stability reference circuit unit, low-noise power supply unit, LO signal interface unit, and HI signal interface unit are all integrated in a 19-inch 1U chassis to form 8 channels that can independently measure current.

[0064] Preferably, the precision ADC circuit unit in the present application has a resolution of 24 bits. The precision ADC circuit performs an analog-to-digital conversion (ADC) process to convert an analog voltage signal into a digital value. The precision ADC circuit unit includes a calibration and compensation mechanism to correct system errors and temperature drifts.

[0065] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A current measuring device with femtoampere (fA) level accuracy, characterized in that: include: A current sense circuit unit, wherein the current sense circuit unit converts the current of the device under test (DUT) into a voltage signal that can be processed by a post-amplifier. The current sense circuit unit is directly connected to the device under test and converts the current signal into a voltage signal for processing by a pre-voltage amplifier unit; A pre-voltage amplifier unit, which amplifies the voltage signal generated by the current sense circuit, converts the single-ended signal into a differential signal, and provides a low-pass filtering function, receives the voltage signal from the current sense circuit, and outputs the differential signal to the precision ADC circuit after amplification; A precision ADC circuit unit, wherein the precision ADC circuit unit receives a differential signal from a pre-voltage amplifier and performs analog-to-digital conversion; An isolation interface circuit unit, wherein the isolation interface circuit unit isolates the digital signal of the ADC and transmits it; A highly stable reference circuit unit, which provides a highly stable reference voltage for ADC conversion and analog circuit comparison; A low-noise power supply unit, which provides a stable low-noise power supply for any other unit; An LO signal interface unit, wherein the LO signal interface unit is connected to a voltage source or an analog ground according to a measurement mode; HI signal interface unit, the HI signal interface unit is directly connected to the device under test to measure the current passing through the DUT.

2. A current measuring device with femtoampere (fA) level accuracy according to claim 1, characterized in that: The current sense circuit unit includes a current sensing resistor and a preamplifier. The current sensing resistor is used to detect the current passing through the device under test, while the preamplifier amplifies the voltage signal on the sensing resistor. The input of the transimpedance amplifier is connected to the current sensing resistor, and the output provides a voltage signal that is proportional to the current passing through the current sensing resistor. The amplified and conditioned voltage signal is ultimately connected to the precision ADC circuit for analog-to-digital conversion.

3. The current measuring device with femtoampere (fA) level accuracy according to claim 1, characterized in that: In the pre-voltage amplifier unit, the amplification circuit includes a voltage input signal, a load, a sense resistor, a transimpedance amplifier, a non-inverting input terminal of the amplifier, an inverting input terminal of the amplifier, a feedback loop, and an output voltage; The voltage input signal is connected to the series combination of the load and the Sense resistor. The voltage drop generated when the current flows through the Sense resistor is detected by the transimpedance amplifier, and a differential voltage is generated between the non-inverting input and the inverting input of the amplifier. The transimpedance amplifier amplifies this differential voltage through a feedback loop and generates a voltage signal proportional to the input current at the output.

4. The current measuring device with femtoampere (fA) level accuracy according to claim 3, characterized in that: In the pre-voltage amplifier unit, the amplification circuit specifically includes a voltage input signal, an output impedance of a voltage source, a floating power supply reference ground, a leakage resistor, and a parasitic capacitor, wherein the voltage input signal is the input signal of the amplifier, which is used to set the voltage of the device under test. The output impedance of the voltage source is the internal resistance of the voltage source. The floating power supply reference ground and the signal source reference ground are connected through a leakage resistor. The leakage resistor is connected between the signal source reference ground and the floating power supply reference ground, and its resistance is much larger than the output impedance of the voltage source. The parasitic capacitor is between the floating power supply reference ground and the signal source reference ground, and it and the leakage resistor together form a low-pass filter circuit. The signal amplitude transmitted to the current conversion device is the signal processed by the amplification circuit, so that the signal amplitude transmitted to the current conversion device is close to the original voltage input signal.

5. The current measuring device with femtoampere (fA) level accuracy according to claim 4, characterized in that: In the pre-voltage amplifier unit, the leakage control circuit includes a GUARD protection ring and a PCB slot, wherein the GUARD protection ring is an equipotential conductor wrapped around the sensitive network. The GUARD protection ring is a conductive ring around the main circuit path, and the voltage of this ring is equal to that of the sensitive network. PCB grooving refers to the process of removing PCB materials near sensitive networks, leaving air as a conductive medium, thereby reducing leakage current by increasing the physical distance of sensitive networks and utilizing the high resistivity of air.

6. The current measuring device with femtoampere (fA) level accuracy according to claim 1, characterized in that: In the low-noise power supply unit, the power supply circuit receives input from the device bus power supply. As the starting point of the entire power supply circuit, the input power is first connected to the isolation power module, and the output of the isolation power module is connected to the DCDC module. The output of the DCDC module is then connected to the LC filter. The LC filter is connected to the LDO voltage regulator to adjust the voltage fluctuation within 100KHz. The output of the LDO module is then connected to the RC filter. Finally, the power supply after multi-stage filtering is supplied to the amplifier circuit unit.

7. The current measuring device with femtoampere (fA) level accuracy according to claim 1, characterized in that: In the high-stability reference circuit unit, a cavity shielding structure is provided, including a sensitive circuit, internal shielding materials, and external shielding materials. The shielding structure adopts a double-layer design, with an outer layer of tinplate and an inner layer of a soft magnetic alloy. The sensitive circuit is located inside the shielding structure, and the sensitive circuit is the circuit part that needs to be protected to prevent magnetic field interference.

8. The current measuring device with femtoampere (fA) level accuracy according to claim 1, characterized in that: For the amplifier and the power supply in the low-noise power supply unit, a decentralized layout is adopted, with the amplifier on one side and the power supply on the other side. For the isolated DCDC, an open radiation heat dissipation design is adopted. The chassis cover is designed with heat dissipation ribs to increase the heat exchange area, and high thermal conductivity materials or thermal interface materials are used to improve the heat conduction efficiency, conduct the heat from the heat source to the chassis or heat sink, and then dissipate the heat to the environment through radiation and natural convection.

9. The current measuring device with femtoampere (fA) level accuracy according to claim 1, characterized in that: The current sense circuit unit, pre-voltage amplifier unit, precision ADC circuit unit, isolation interface circuit unit, high stability reference circuit unit, low noise power supply unit, LO signal interface unit, and HI signal interface unit are all integrated in a 19-inch 1U chassis to form 8 channels that can independently measure current.

10. The current measuring device with femtoampere (fA) level accuracy according to claim 1, characterized in that: The precision ADC circuit unit in the present application has a resolution of 24 bits. The precision ADC circuit performs an analog-to-digital conversion (ADC) process to convert an analog voltage signal into a digital value. The precision ADC circuit unit includes a calibration and compensation mechanism to correct system errors and temperature drifts.

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