DRV425 Datasheet, PDF (19/37 Pages) Texas Instruments – DRV425 Fluxgate Magnetic-Field Sensor

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DRV425
SBOS729 – OCTOBER 2015
Feature Description (continued)
7.3.1.2 Bandwidth
The small-signal bandwidth of the DRV425 is determined by the behavior of the compensation loop versus
frequency. The implemented integrator limits the bandwidth of the loop to provide stable response. Use the
digital input pin BSEL to select the bandwidth. For a shunt resistor of 22 Ω and BSEL = 0, the bandwidth is
32 kHz; for BSEL = 1, the bandwidth is 47 kHz.
Bandwidth can be reduced by increasing the value of the shunt resistor because the shunt resistor and the
compensation coil resistance form a voltage divider. The reduced bandwidth (BW) can be calculated using
Equation 2:
BW
R_COIL  22 :
R_COIL  R_SHUNT
u BW_{22 :}
100
122 :
:  R_SHUNT
u BW₂₂
:
where
• R_COIL = internal compensation coil resistance (100 Ω),
• R_SHUNT = external shunt resistance, and
• BW_22Ω = sensor bandwidth with R_SHUNT = 22 Ω (depending on the BSEL setting)
(2)
The bandwidth for a given shunt resistor value can also be calculated using the DRV425 System Parameter
Calculator, SLOC331. For large magnetic fields (B > 500 μT), the effective bandwidth of the sensor is limited by
fluxgate saturation effects. For a magnetic signal with a 2-mT amplitude, the large-signal bandwidth is 10 kHz
with BSEL = 0 or 15 kHz with BSEL = 1.
Although the analog output responds slowly to large fields, a magnetic field with a magnitude of 1.6 mT (or
higher) beyond the measurement range of the DRV425 triggers the ERROR pin within 4 µs to 6 µs. See the
Magnetic Field Range, Overrange Indicator, and Error Flag section for more details.
7.3.1.3 Differential Driver for the Internal Compensation Coil
The differential compensation coil driver provides the current for the internal compensation coil at the DRV1 and
DRV2 pins. The driver is capable of sourcing up to ±250 mA with a 5-V supply or up to ±150 mA in 3.3-V mode.
The current capability is not internally limited. The actual value of the compensation coil current depends on the
magnetic field strength and is limited by the sum of the resistance of the internal compensation coil and the
external shunt resistor value. The internal compensation coil resistance depends on temperature (see Figure 17)
and must be taken into account when dimensioning the system. Select the value of the shunt resistor to avoid
OR pin trip levels in normal operation.
The common-mode voltage of the compensation coil driver outputs is set by the RSEL pins (see the Voltage
Reference section). Thus, the common-mode voltage of the shunt-sense amplifier is matched if the internal
reference is used.
Consider the polarity of the compensation coil connection to the output of the compensation coil driver. If the
polarity is incorrect, then the driver output drives to the power-supply rails, even at low primary-current levels. In
this case, interchange the connection of the DRV1 and DRV2 pins to the compensation coil.
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