DRV425 Datasheet, PDF (28/37 Pages) Texas Instruments – DRV425 Fluxgate Magnetic-Field Sensor

DRV425
SBOS729 – OCTOBER 2015
www.ti.com
In Figure 73, the feedback loops of both DRV425 sensors are combined to directly produce a differential output
V_DIFF that is proportional to the sensed magnetic field difference inside the busbar hole. Both compensation coils
are connected in series and are driven from a single side of the compensation coil driver (the DRV1 pins of each
DRV425). Therefore, both driver stages ensure that a current proportional to the magnetic fields B_R and B_L is
driven through the respective compensation coil. The difference in current through both compensation coils, and
thus the difference field between the sensors, flows through resistor R₃ and is sensed by the shunt-sense
amplifier of U2. The current proportional to the common-mode field inside the busbar hole flows through R₁ and
R₂ and is sensed by the shunt-sense amplifier of U1.
Use the output V_CM to verify that the sensors are correctly positioned in the busbar hole with the following steps:
1. Measure V_CM with no current flow through the busbar and the PCB in the middle of the busbar hole. This
value is the offset voltage V_OFFSET. The value of V_OFFSET only depends on stray fields and varies little with the
absolute position of the sensors.
2. Apply current through the busbar and move the PCB along the y-axis in the busbar hole, as shown in
Figure 72. The PCB is in the center of the hole if V_CM = V_OFFSET.
The sensitivity drift performance of the circuit shown in Figure 73 is dominated by the temperature coefficient of
the external resistors R₁, R₂, and R₃. Select low-drift resistors for best sensor performance. For overall system
error calculation, also consider the affect of thermal expansion on the PCB and busbar.
The internal voltage reference of the DRV425 cannot be used in this application because of its limited driver
capability. The OPA320 (U3) is a low-noise operational amplifier with a short-circuit current capability of ±65 mA
and is used to support the required compensation current.
The advantage of this solution is its simplicity: the currents are subtracted by the two DRV425 devices without
additional components. The series connection of the compensation coils halves the voltage swing and reduces
the measurement range of the sensors also by 50%. If a larger sensing range is required, operate the two
sensors independently and use a differential amplifier or ADC to subtract both voltage outputs (VOUT).
Use the ERROR outputs for fast overcurrent detection on the system level.
8.2.2.3 Application Curves
Figure 74 and Figure 75 show the measurement results on a 16-mm wide and 6-mm thick copper busbar with a
12-mm hole diameter using the circuit shown in Figure 73. The two DRV425 devices are placed at a distance of
1 mm from each other on opposite sides of the PCB. The measurement range is ±500 A; measurement results
are limited by test setup. Independent operation of the two DRV425 sensors increases the measurement range
to ±1000 A with the same busbar geometry.
400
350
300
250
200
150
100
50
0
0
20 40 60 80 100 120 140 160 180 200
Busbar Current (A)
D066
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
0
20 40 60 80 100 120 140 160 180 200
Busbar Current (A)
D067
Figure 74. Analog Output Voltage vs
Busbar Current
Figure 75. Linearity Error vs Busbar Current
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