DRV425 Datasheet, PDF (22/37 Pages) Texas Instruments – DRV425 Fluxgate Magnetic-Field Sensor

DRV425
SBOS729 – OCTOBER 2015
www.ti.com
Feature Description (continued)
7.3.2 Shunt-Sense Amplifier
The compensation coil current creates a voltage drop across the external shunt resistor, R_SHUNT. The internal
differential amplifier senses this voltage drop. This differential amplifier offers wide bandwidth and a high slew
rate. Excellent dc stability and accuracy result from a chopping technique. The voltage gain is 4 V/V, set by
precisely-matched and thermally-stable internal resistors.
Both the AINN and AINP differential amplifier inputs are connected to the external shunt resistor. This shunt
resistor, in series with the internal 10-kΩ input resistors of the shunt sense amplifier, causes an additional gain
error. Therefore, for best common-mode rejection performance, place a dummy shunt resistor (R₅) with a value
higher than the shunt resistor in series with the REFIN pin to restore the matching of both resistor dividers, as
shown in Figure 65.
DRV2
DRV1
Device
AINP
R1
10 k 
R2
40 k 
RSHUNT
AINN
R3
10 k 
_
Shunt-Sense
Amplifier
+
R4
40 k 
ICOMP1
ICOMP2
VOUT
RF
500 
Optional
CF
10 nF
ADC
REFIN
R5
(Dummy Shunt)
REFIN (Compensated)
Figure 65. Internal Difference Amplifier with an Example of a Decoupling Filter
For an overall gain of 4 V/V, calculate the value of R₅ using Equation 6:
4 = ^R2 = ^{R4 + R5}
R₁ R_SHUNT + R₃
where:
• R₂ / R₁ = R₄ / R₃ = 4,
• R₅ = R_SHUNT × 4
(6)
If the input signal is large, the amplifier output drives close to the supply rails. The amplifier output is able to drive
the input of a successive approximation register (SAR) analog-to-digital converter (ADC). For best performance,
add an RC low-pass filter stage between the shunt-sense amplifier output and the ADC input. This filter limits the
noise bandwidth and decouples the high-frequency sampling noise of the ADC input from the amplifier output.
For filter resistor R_F and filter capacitor C_F values, see the specific converter recommendations in the respective
product data sheet.
The shunt-sense amplifier output drives 100 pF directly and shows a 50% overshoot with a 1-nF capacitance.
Filter resistor R_F extends the capacitive load range. Note that with an R_F of only 20 Ω, the load capacitor must be
either less than 1 nF or more than 33 nF to avoid overshoot; with an R_F of 50 Ω, this transient area is avoided.
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