DRV401-EP_09 Datasheet, PDF (18/33 Pages) Texas Instruments – SENSOR SIGNAL CONDITIONING IC FOR SENSOR SIGNAL CONDITIONING IC FOR

DRV401-EP
SBVS104B – JANUARY 2008 – REVISED MARCH 2009 .................................................................................................................................................. www.ti.com
DRV401
I_COMP1
V+
External
Buffer
I_COMP2
V−
Compensation
Coil
R_SHUNT
Figure 4. DRV401 With External Compensation Coil Driver and R_SHUNT Connected to GND
SHUNT SENSE AMPLIFIER
The differential (H-bridge) driver arrangement for the compensation coil requires a differential sense amplifier for
the shunt voltage. This differential amplifier offers wide bandwidth and a high slew rate for fast current sensors.
Excellent dc stability and accuracy result from an auto-zero technique. The voltage gain is 4 V/V, set by precisely
matched and stable internal SiCr resistors.
Both inputs of the differential amplifier are normally connected to the current shunt resistor. This resistor adds to
the internal (10 kΩ) resistor, slightly reducing the gain in this leg. For best common-mode rejection (CMR), a
dummy shunt resistor (R₅) is placed in series with the REF_IN pin to restore matching of both resistor dividers, as
shown in Figure 5a.
For gains of 4 V/V:
4
+
R₂
R₁
+
R₄ ) R₅
R_SHUNT ) R₃
With R₂/R₁ = R₄/R₃ = 4; R₅ = R_SHUNT × 4
Typically, the gain error resulting from the resistance of R_SHUNT is negligible; for 70 dB of common-mode
rejection, however, the match of both divider ratios needs to be better than 1/3000.
The amplifier output can drive close to the supply rails, and is designed to drive the input of a SAR-type ADC;
adding an RC low-pass filter stage between the DRV401 and the ADC is recommended. This filter not only limits
the signal bandwidth but also decouples the high-frequency component of the converter input sampling noise
from the amplifier output. For R_F and C_F values, refer to the specific converter recommendations in the specific
product data sheet. Empirical evaluation may be necessary to obtain optimum results.
The output can drive 100 pF directly and shows 50% overshoot with approximately 1 nF capacitance. Adding R_F
allows much larger capacitive loads, as shown in Figure 5b and Figure 5c. Note that with R_F of only 20 Ω, the
load capacitor should be either smaller than 1 nF or larger than 33 nF to avoid overshoot; with R_F of 50 Ω this
transient area is avoided.
The reference input (REF_IN) is the reference node for the exact output signal (V_OUT). Connecting REF_IN to the
reference output (REF_OUT) results in a live zero reference voltage of 2.5 V. Using the same reference for REF_IN
and the ADC avoids mismatch errors that exist between two reference sources.
18
Submit Documentation Feedback
Copyright © 2008–2009, Texas Instruments Incorporated
Product Folder Link(s): DRV401-EP