Current Transducer

Abstract

An open-loop current transducer (1) for measuring a current flowing through a primary conductor (19) includes a magnetic circuit core (3) with an air gap (4) and a magnetic field sensing device (5) positioned at least partially within the air gap, the magnetic field sensing device including a circuit board, a first magnetic field detector (8) in the form of an ASIC mounted on the circuit board, and a second magnetic field detector (12) in the form of a conductive pickup coil formed on two or more layers in or on the circuit board. The outputs of the first and second magnetic field detectors are connected to a signal processing circuit (6) that generates output signals representative of the current, the output of the ASIC representing a low-frequency (LF) channel and the output of the pickup coil representing a high-frequency (HF) channel. The signal processing circuit includes a summing circuit configured to sum a signal output by the LF channel with a signal output by the HF channel, and one or more circuit components for adjusting the gain of the pickup coil output signal to the gain of the ASIC output signal.

Description

Disclosure Contents

[0001] The present invention relates to an open-loop current transducer for measuring current over a high frequency range.

[0002] Wide bandgap semiconductors have opened up the possibility of significantly faster switching frequencies and abrupt state-to-state transitions, resulting in higher di / dt switched currents and higher dv / dt switched voltages. Many applications use current measurements as input to their control loops, and implementing advanced control schemes requires fast and accurate current measurements, resulting in bandwidths one to two orders of magnitude higher than previous generation current converters (100 kHz → 5 MHz).

[0003] Open-loop current transducers are widely used to measure the current flowing through a primary conductor. Such sensors typically include a magnetic core surrounding the primary conductor, which includes an air gap, within which a magnetic field detector is positioned. The magnetic field detector can typically be a Hall effect sensor or a magnetoresistive magnetic field sensor. However, Hall sensors have limited bandwidth. While magnetoresistive sensors can have high bandwidth, they have limited offset stability and 1 / f noise, and are also subject to perming and hysteresis.

[0004] It is known to have current transducers without magnetic cores, which are advantageous for measuring large currents, but at low currents the magnetic signal is small and therefore such transducers are not as advantageous as current sensors with gapped magnetic cores in applications requiring accurate current measurement over a large current range.

[0005] An open-loop transducer for high-bandwidth, high-current-range measurements, such as that described in EP 1965217, combines the high-frequency measurement capabilities of a planar pickup coil with a Hall sensor. The pickup coil is implemented as a circuit track on a PCB, connected in series with the output of an amplifier connected to the Hall sensor. A drawback of this known transducer is that the crossover frequency at which the coil begins to contribute is an order of magnitude lower than the natural rolloff of the Hall sensor unit, resulting in a significant deviation from the ideal flat frequency response over a large current measurement range.

[0006] Other conventional current sensors with large operating frequency bandwidths typically suffer from a limited amplitude range for measurement with desired accuracy. Furthermore, the response time of conventional open-loop current sensors with large operating frequency bandwidths is typically less than 2 μs (2×10 -6 seconds), which may be too slow for certain applications.

[0007] In view of the above, it is an object of the present invention to provide an open-loop current transducer having a wide frequency bandwidth for measuring current with a short response time, high offset stability, and a large amplitude range with a frequency response that has low deviation from an ideal flat frequency response over the measurement range.

[0008] Specifically, the wide frequency bandwidth can be from 0 (DC) to approximately 1-5 MHz, and the large current amplitude range can be from approximately 0 A to a maximum value in the range of 50 A to 300 A.

[0009] It would be advantageous to provide a current transducer that is compact and economical to manufacture.

[0010] It would be advantageous to provide a current transducer that is compact and uses simple signal processing circuitry.

[0011] It would be further advantageous to provide a current converter that consumes less power, particularly for use in connection with autonomous devices.

[0012] The object of the present invention is achieved by providing an open loop current converter as claimed in claim 1.

[0013] Disclosed herein is an open-loop current transducer for measuring a current flowing through a primary conductor, the transducer comprising: a magnetic circuit core with an air gap; a circuit board; a first magnetic field detector in the form of an ASIC mounted on the circuit board; and a second magnetic field detector in the form of a conductive pickup coil formed below the ASIC on two or more layers in or above the circuit board and in an overlapping relationship with the ASIC, the ASIC and pickup coil being positioned within the air gap. The outputs of the first and second magnetic field detectors are connected to a signal processing circuit that generates output signals representative of the current, the ASIC output representing a low-frequency (LF) channel and the pickup coil output representing a high-frequency (HF) channel. The signal processing circuit includes a summing circuit configured to sum a signal output by the LF channel with a signal output by the HF channel, and one or more circuit components, including at least a gain adjustment resistor (Rac) connected in series with the pickup coil output to adjust the gain of the pickup coil output signal to the gain of the ASIC output signal.

[0014] In an advantageous embodiment, the summing circuit includes an operational amplifier, a resistor connected across the operational amplifier between the negative input and the output of the operational amplifier, and a capacitance connected across the operational amplifier between the negative input and the output of the operational amplifier, thus forming an inverting summing circuit with low pass filtering.

[0015] Converters according to embodiments of the present invention advantageously enable measurement of primary currents over a bandwidth from DC to several MHz, with frequency response flatness better than ±1 dB and highly stable offsets, typically less than 0.1% of full scale. Furthermore, open-loop response times achievable with converters according to embodiments of the present invention are approximately 10 ns (10×10 -9 seconds) or less.

[0016] Converters according to embodiments of the present invention also advantageously have a good signal-to-noise ratio (SNR) because the output noise from the ASIC is filtered by the low pass filter of the operational amplifier circuit.

[0017] In an advantageous embodiment, the positive input of the operational amplifier is connected to a reference voltage source of the signal processing circuit.

[0018] In an advantageous embodiment, the positive input of the operational amplifier is further connected via a resistor to an external voltage reference connection.

[0019] In an advantageous embodiment, the reference voltage source is connected to the positive input of the operational amplifier through a series resistor and a parallel capacitance, configured to match the impedance and reduce noise on the voltage reference input.

[0020] In advantageous embodiments, the gain adjustment resistor is adjustable: by laser trimming the gain adjustment resistor; by providing the gain adjustment resistor as a programmable resistor implemented on an integrated circuit; or by adding or removing one or more additional resistors connected in parallel to the gain adjustment resistor.

[0021] In an advantageous embodiment, the signal processing circuitry includes a capacitance connected in series with the output of the pick-up coil.

[0022] In an advantageous embodiment, the magnetic field sensing device includes electrostatic screens connected to ground and positioned on either side of the pickup coil, overlapping the pickup coil and configured to reduce capacitive coupling between the primary conductor and the pickup coil, the electrostatic screens including a plurality of conductive circuit traces on a circuit board separated by non-conductive gaps configured to reduce eddy currents.

[0023] In an advantageous embodiment, the pickup coil includes at least two coil portions on different layers embedded within a circuit board substrate, which are connected to each other in series by conductive connecting paths through the layers.

[0024] In an advantageous embodiment, the electrostatic screens are formed on opposite exterior surfaces of the circuit board.

[0025] Further objects and advantageous features of the present invention will be apparent from the claims, the detailed description, and the accompanying drawings. [Brief explanation of the drawings]

[0026] [Figure 1a] FIG. 2 is a perspective view of a portion of a current transformer (with the housing removed) according to an embodiment of the present invention. [Figure 1b] FIG. 1b is a side view of a portion of the current converter of FIG. 1a. [Figure 1c] FIG. 1c is a cross-sectional view taken along line 1c-1c of FIG. [Figure 2] 1 is a schematic perspective view of a pickup coil of a current transducer according to an embodiment of the present invention; [Figure 3] FIG. 2 is a block diagram of a signal processing circuit of a current transducer according to an embodiment of the present invention. [Figure 4] FIG. 2 is a circuit diagram of a signal processing circuit of a current converter according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring to the drawings, a current transducer 1 for measuring current flowing through a primary conductor 19 across a central opening 2 of the transducer includes a magnetic circuit 3 with an air gap 4 and a magnetic field sensing device 5 positioned at least partially within the air gap. The magnetic circuit 3 may be made of a stack of thin films or a solid core of different shapes and sizes and of different magnetically conductive materials, as known in the art. The magnetic circuit may also be split into two parts, as known in the art, to allow assembly of a sensor around the primary conductor. Furthermore, a portion of the primary conductor 19 may be integrated into the sensor and have terminals for connection to the primary conductor, as known in the art.

[0028] In a preferred embodiment, the magnetic core is made of a ferrite material to better support high frequency primary currents compared to a soft iron core.

[0029] The current sensor may include a second air gap and a second magnetic field sensing device positioned at least partially within the second air gap.

[0030] The magnetic field sensing device includes a circuit board 7, a first magnetic field detector 8 and a second magnetic field detector 12 mounted on the circuit board, and a signal processing circuit 6.

[0031] The signal processing circuit 6 is provided on a circuit board 7 and is connected to the magnetic field detectors 8 and 12 .

[0032] Connection terminals 20 connected to the circuit board allow connection to a circuit (not shown) external to the device (e.g., electric motor) in which the transducer is implemented for measuring primary currents (e.g., phase currents of an electric motor) and typically controlling the function of the device (e.g., operation of the motor).

[0033] The first magnetic field detector 8 is in the form of a Hall effect sensor included in an integrated circuit (ASIC) in a preferred embodiment, with connection terminals 10 connected, for example by soldering, to circuit traces on the circuit board 7. However, the first magnetic field detector may also comprise other types of known magnetic field sensors, for example a magnetoresistive magnetic field sensor or a fluxgate magnetic field sensor. In a preferred embodiment, the Hall effect sensor may be operated using current spinning techniques known per se to reduce offset and 1 / f noise.

[0034] The second magnetic field detector 12 of the magnetic sensing device includes a conductive coil including at least two coil portions 12a, 12b formed by circuit traces on different layers on or in a circuit board. The coil portions can be connected to each other in series to form a single pickup coil. The coil portions can be connected to each other by conductive plated-through holes 14 or interconnection pins or vias through the board to interconnect the coil portions. Thus, the coil can be formed on the circuit board by conventional technology for fabricating conductive tracks on a circuit board, thereby benefiting from the presence of the circuit board for connecting the first magnetic field detector and for signal processing or pre-processing circuitry 6. The coil is preferably connected to the ASIC reference voltage VREF rather than ground to reduce noise in the coil's output.

[0035] The circuit board may advantageously further include electrostatic screens 13a, 13b on the top and bottom layers, with the pickup coil 12 sandwiched between these screens to form an electrical shielding layer around the pickup coil. The shielding layer reduces capacitive coupling between the pickup coils 12a, 12b and the primary conductor 19, either directly or through the magnetic core 3, thereby providing protection against electrical disturbances, particularly those caused by primary currents with high voltage change rates (high dV / dt). The electrostatic screens may be formed on the circuit board using conventional technology for fabricating conductive tracks on a circuit board and may be connected to ground or another reference voltage connection. The electrostatic screens may be formed on opposing outer surfaces of the circuit board, and the pickup coil may be formed on an inner embedded layer, or the electrostatic screens may be arranged on layers embedded within the circuit board substrate. The electrostatic screen may advantageously have a comb-like structure with multiple conductive tracks separated by non-conductive tracks to reduce the formation of eddy currents in the shielding layer. The conductive tracks are preferably electrically interconnected to each other at only one end of the tracks to prevent eddy currents from circulating in the loops that would otherwise be formed by the tracks if they were connected to each other at both ends.

[0036] A multi-layer pickup coil has increased gain over a single layer coil and advantageously allows the cutoff frequency of the low pass filter to be reduced to obtain a lower noise output for the signal from the pickup coil.

[0037] The signal processing circuit 6 according to an embodiment of the present invention includes circuitry for combining the output signal from the first magnetic field detector 8, which represents the low frequency (LF) channel, and the output signal from the pickup coil 12, which represents the high frequency (HF) channel. The two output signals are advantageously combined by summing the LF and HF output signals using a summing circuit 24, which may advantageously include a low pass filter 25.

[0038] In a preferred embodiment, the summing circuit 24 with low-pass filter 25 includes an operational amplifier 26 and a resistor R1 and a capacitance C1 connected across the operational amplifier between the first input (negative input) and the output of the operational amplifier, thus forming an inverting summing circuit with low-pass filtering. The operational amplifier 26, resistor R1, and capacitance C1 form an active first-order low-pass filter. The second input (positive input) of the operational amplifier is connected to a reference voltage.

[0039] The reference voltage input can be connected to a reference voltage source VREF (e.g., a 2.5 V voltage source) of the signal processing circuit, optionally via a series resistor Rr2 and a parallel capacitance Cr, to adapt the impedance and reduce noise. The level of the input reference voltage can optionally be adjusted by an external connection VREF_IN, which is connected to the second input of the operational amplifier via a resistor Rr1. This allows the user of the current converter to adapt the voltage level of the converter output measurement signal Uout to their requirements, in particular the requirements of the external circuit to which the converter is connected via the connection terminal 20. This allows, for example, the user to change the voltage level of the output measurement signal to maintain a positive current throughout the entire measurement range of the converter, which is advantageous for controlling the device in many applications.

[0040] The gain of the first magnetic field sensor included in the ASIC can be adjusted by a resistor Rdc connected to the ASIC output OUT and / or by adjusting registers in the ASIC integrated circuit to set the current output level of the ASIC.

[0041] In the HF channel connected to the pickup coil 12, a high-pass filter 21 may be formed by a capacitance Cac, which removes any DC or low-frequency signals, preferably signals with frequencies below 10 Hz. The capacitance Cac serves to reduce the direct current (DC) or low-frequency gain for the HF path and avoid amplifying operational amplifier offset. The pickup coil gain may be advantageously adjusted to match the gain of the LF channel, so that the sum of the two signals has a frequency response close to an ideal flat frequency response, preferably better than + / - 1 dB, while benefiting from a highly stable offset. The stable offset is achieved by a combination of providing a Hall-effect sensor within the ASIC, driven by high-frequency current spinning technology but operating at low-frequency primary current measurement, and a multilayer pickup coil formed on a circuit board that operates at a higher frequency (thus allowing for a larger magnetic field pickup surface area than the ASIC), resulting in a pickup coil that has essentially no offset drift. Compared to a series connection as in the prior art, the summation of signals according to the present invention makes it possible to flatten the frequency response; however, in a series connection, the crossover frequency at which the coil begins to contribute is an order of magnitude lower than the natural roll-off of the Hall sensor unit, resulting in a significant deviation from the ideal flat frequency response over a large current measurement range.

[0042] Gain adjustment of the HF channel may advantageously be performed by a resistor Rac connected in series with the output of the pickup coil. The resistance value of the HF channel gain adjustment resistor Rac may be adjusted by various means, including by laser trimming resistor Rac; by providing Rac as a programmable resistor implemented on an integrated circuit; or by adding or removing one or more additional resistors connected in parallel with Rac.

[0043] The LF channel further includes a sensitivity adjustment component 23, for example to allow calibration of the transducer, for example to compensate for variations in the length of the magnetic gap 4. This sensitivity adjustment component may be implemented as a programmable element within the ASIC to adjust the output level OUT, or by using a programmable gain amplifier (PGA) (not shown) at the output of the ASIC.

[0044] The operating principle features of the illustrated embodiment of FIG. 4 are further described below: LF Path: Below the crossover frequency, defined by the time constant R1 x C1, an operational amplifier circuit amplifies the ASIC output voltage by a factor R1 / Rdc. Without other components, the circuit is a simple active first order lowpass, with the corner frequency as mentioned above. HF Path: The idealized output voltage of the pickup coil rises in proportion to the frequency of the current to be measured, leading in phase by 90°. In the frequency range where the signal is used as an image of the primary current, it must pass through an integrator. This is the function of the circuit shown, at frequencies above the crossover frequency, where the pickup coil voltage is higher than the ASIC output voltage. To obtain a smooth transition between the two frequency ranges, the HF gain must be adjusted so that at the crossover frequency, the pickup coil output voltage has the same amplitude as that from the ASIC. In practice (due to parasitics, nonlinearities, etc. not shown in the schematic), a slightly better response in the time domain may be obtained by adjusting the HF gain slightly higher than the LF gain.

[0045] Preferred values ​​for components: The capacitance Cac should preferably have a high capacitance value of 5-20 μF, for example about 10 μF for a good price / performance compromise.

[0046] Resistor R1 and capacitance C1 values: The lower the cutoff frequency, the lower the output noise (limited by the practical size of the pickup coil; a small pickup coil with fewer turns will provide a high enough voltage at higher frequencies). C1: A capacitance of 1nF to 10nF is preferably used; R1: The resistor value can vary from 30 kΩ to 200 kΩ depending on the desired gain; A crossover frequency of about 1 kHz is preferred; for a given crossover frequency, increasing C1 improves the dv / dt perturbation effect (by a factor of 10 from 1 nF to 10 nF).

[0047] However, as the value of capacitance C1 increases, the gain adjustment resistor Rac must be reduced to keep the crossover frequency at the desired value, e.g., 1 kHz, and the di / dt performance deteriorates, so a compromise must be found depending on the desired converter sensitivity.

[0048] The gain adjustment resistor Rac should preferably be higher than 1 kΩ, and the higher the better (1 kΩ to 30 kΩ; depending on the converter gain).

[0049] Advantageous, but non-limiting, examples of approximations of other component values ​​may include, for example: Rdc=R1 Rr2=R1 / 2 Rr1=R1 Cr≧(=>) approx. 10μF (as large as possible)

[0050] [Embodiment] (1) An open-loop current transducer (1) for measuring the current flowing through a primary conductor (19), a magnetic circuit core (3) having an air gap (4); a magnetic field sensing device including a circuit board, a first magnetic field detector (8) in the form of an ASIC mounted on said circuit board, and a second magnetic field detector (12) in the form of a conductive pick-up coil formed on two or more layers in or on said circuit board below said ASIC and in overlapping relationship with said ASIC; Including, the ASIC and the pickup coil are positioned within the air gap, and outputs of the first magnetic field detector and the second magnetic field detector are connected to a signal processing circuit (6) that generates output signals representative of the current, the output of the ASIC representing a low frequency (LF) channel and the output of the pickup coil representing a high frequency (HF) channel; 1. The open-loop current converter, wherein the signal processing circuitry includes: a summing circuit configured to sum the signal output by the LF channel with the signal output by the HF channel; and one or more circuit components including at least a gain adjustment resistor (Rac) connected in series with the output of the pickup coil to adjust the gain of the output signal of the pickup coil to the gain of the output signal of the ASIC. (2) The converter of claim 1, wherein the summing circuit includes an operational amplifier (26), a resistor (R1) connected across the operational amplifier between the negative input and the output of the operational amplifier, and a capacitance (C1) connected across the operational amplifier between the negative input and the output of the operational amplifier. (3) The converter of embodiment 2, wherein the positive input of the operational amplifier is connected to a reference voltage source (VREF) of the signal processing circuit. (4) The converter of embodiment 3, wherein the positive input of the operational amplifier is further connected to an external connection (VREF_IN) via a resistor (Rr1). (5) The converter of embodiment 3, wherein the reference voltage source (VREF) is connected to the positive input of the operational amplifier via a series resistor (Rr2) and a parallel capacitance (Cr) to match impedance and reduce noise on the voltage reference input.

[0051] (6) The converter of embodiment 1, wherein the gain adjustment resistor (Rac) is adjustable by laser trimming the gain adjustment resistor Rac; by providing the gain adjustment resistor as a programmable resistor implemented on an integrated circuit; or by adding or removing one or more additional resistors connected in parallel to the gain adjustment resistor. (7) The converter of claim 1, wherein the signal processing circuit includes a capacitance (Cac) connected in series with the output of the pickup coil. (8) The transducer of embodiment 1, wherein the magnetic field sensing device includes electrostatic screens (13a, 13b) connected to ground, positioned on either side of the pickup coil, overlapping the pickup coil and configured to reduce capacitive coupling between the primary conductor and the pickup coil, and the electrostatic screens include a plurality of conductive circuit traces on the circuit board separated by non-conductive gaps configured to reduce eddy currents. (9) A transducer as described in embodiment 8, wherein the pickup coil includes at least two coil portions (12a, 12b) on different layers embedded within the circuit board substrate, the coil portions being connected to each other in series by a conductive connection path through the layers. (10) A converter as described in embodiment 9, wherein the electrostatic screens are formed on opposite outer surfaces of the circuit board.

Claims

1. An open-loop current sensor (1) for measuring a current flowing through a primary conductor (19), comprising: a magnetic circuit core (3) having an air gap (4); a magnetic field sensing device including a circuit board, a first magnetic field detector (8) in the form of an ASIC mounted on said circuit board, and a second magnetic field detector (12) in the form of a conductive pick-up coil formed on two or more layers in or on said circuit board below said ASIC and in overlapping relationship with said ASIC; Including, the ASIC and the pickup coil are positioned within the air gap, and outputs of the first magnetic field detector and the second magnetic field detector are connected to a signal processing circuit (6) that generates output signals representative of the current, the output of the ASIC representing a low frequency (LF) channel and the output of the pickup coil representing a high frequency (HF) channel; the signal processing circuitry includes a summing circuit configured to sum the signal output by the LF channel with the signal output by the HF channel, and one or more circuit components including at least a gain adjustment resistor (Rac) connected in series with the output of the pickup coil to adjust the gain of the pickup coil output signal to the gain of the ASIC output signal; 10. An open-loop current sensor, wherein the signal processing circuit includes a capacitance (Cac) connected in series with the output of the pickup coil.

2. An open-loop current sensor (1) for measuring a current flowing through a primary conductor (19), comprising: a magnetic circuit core (3) having an air gap (4); a magnetic field sensing device including a circuit board, a first magnetic field detector (8) in the form of an ASIC mounted on said circuit board, and a second magnetic field detector (12) in the form of a conductive pick-up coil formed on two or more layers in or on said circuit board below said ASIC and in overlapping relationship with said ASIC; Including, the ASIC and the pickup coil are positioned within the air gap, and outputs of the first magnetic field detector and the second magnetic field detector are connected to a signal processing circuit (6) that generates output signals representative of the current, the output of the ASIC representing a low frequency (LF) channel and the output of the pickup coil representing a high frequency (HF) channel; the signal processing circuitry includes a summing circuit configured to sum the signal output by the LF channel with the signal output by the HF channel, and one or more circuit components including at least a gain adjustment resistor (Rac) connected in series with the output of the pickup coil to adjust the gain of the pickup coil output signal to the gain of the ASIC output signal; the summing circuit includes an operational amplifier (26), a resistor (R1) connected across the operational amplifier between the negative input and the output of the operational amplifier, and a capacitance (C1) connected across the operational amplifier between the negative input and the output of the operational amplifier; The positive input of the operational amplifier is connected to a reference voltage source (VREF) of the signal processing circuit; The positive input of the operational amplifier is further connected to an external connection (VREF_IN) through a resistor (Rr1), an open loop current sensor.

3. An open-loop current sensor (1) for measuring a current flowing through a primary conductor (19), comprising: a magnetic circuit core (3) having an air gap (4); a magnetic field sensing device including a circuit board, a first magnetic field detector (8) in the form of an ASIC mounted on said circuit board, and a second magnetic field detector (12) in the form of a conductive pick-up coil formed on two or more layers in or on said circuit board below said ASIC and in overlapping relationship with said ASIC; Including, the ASIC and the pickup coil are positioned within the air gap, and outputs of the first magnetic field detector and the second magnetic field detector are connected to a signal processing circuit (6) that generates output signals representative of the current, the output of the ASIC representing a low frequency (LF) channel and the output of the pickup coil representing a high frequency (HF) channel; the signal processing circuitry includes a summing circuit configured to sum the signal output by the LF channel with the signal output by the HF channel, and one or more circuit components including at least a gain adjustment resistor (Rac) connected in series with the output of the pickup coil to adjust the gain of the pickup coil output signal to the gain of the ASIC output signal; the summing circuit includes an operational amplifier (26), a resistor (R1) connected across the operational amplifier between the negative input and the output of the operational amplifier, and a capacitance (C1) connected across the operational amplifier between the negative input and the output of the operational amplifier; The positive input of the operational amplifier is connected to a reference voltage source (VREF) of the signal processing circuit; The open-loop current sensor, wherein the reference voltage source (VREF) is connected to the positive input of the operational amplifier through a series resistor (Rr2) and a parallel capacitance (Cr) configured to match impedance and reduce noise on the voltage reference input.

4. 2. The open-loop current sensor of claim 1, wherein the gain adjustment resistor (Rac) is adjustable by laser trimming the gain adjustment resistor Rac; by providing the gain adjustment resistor as a programmable resistor realized on an integrated circuit; or by adding or removing one or more additional resistors connected in parallel to the gain adjustment resistor.

5. An open-loop current sensor (1) for measuring a current flowing through a primary conductor (19), comprising: a magnetic circuit core (3) having an air gap (4); a magnetic field sensing device including a circuit board, a first magnetic field detector (8) in the form of an ASIC mounted on said circuit board, and a second magnetic field detector (12) in the form of a conductive pick-up coil formed on two or more layers in or on said circuit board below said ASIC and in overlapping relationship with said ASIC; Including, the ASIC and the pickup coil are positioned within the air gap, and outputs of the first magnetic field detector and the second magnetic field detector are connected to a signal processing circuit (6) that generates output signals representative of the current, the output of the ASIC representing a low frequency (LF) channel and the output of the pickup coil representing a high frequency (HF) channel; the signal processing circuitry includes a summing circuit configured to sum the signal output by the LF channel with the signal output by the HF channel, and one or more circuit components including at least a gain adjustment resistor (Rac) connected in series with the output of the pickup coil to adjust the gain of the pickup coil output signal to the gain of the ASIC output signal; the magnetic field sensing device includes electrostatic screens (13a, 13b) connected to ground, disposed on either side of the pickup coil, overlapping the pickup coil and configured to reduce capacitive coupling between the primary conductor and the pickup coil, the electrostatic screens including a plurality of conductive circuit traces on the circuit board separated by non-conductive gaps configured to reduce eddy currents.

6. 6. The open-loop current sensor of claim 5, wherein the pickup coil includes at least two coil portions (12a, 12b) on different layers embedded within the circuit board, the coil portions being connected to each other in series by conductive connection paths through the layers.

7. The open-loop current sensor of claim 6 , wherein the electrostatic screens are formed on opposite outer surfaces of the circuit board.

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