DRV401 Datasheet, PDF (17/28 Pages) Burr-Brown (TI) – Sensor Signal Conditioning IC for Closed-Loop Magnetic Current Sensor

DRV401
www.ti.com
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 4V/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 (10kΩ) 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 4V/V:
4
+
R₂
R₁
+
R₄ ) R₅
R_SHUNT ) R₃
(2)
With R₂/R₁ = R₄/R₃ = 4; R₅ = R_SHUNT × 4
SBVS070A − JUNE 2006 − REVISED OCTOBER 2006
Typically, the gain error resulting from the resistance of
R_SHUNT is negligible; for 70dB 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 100pF directly and shows 50%
overshoot with approximately 1nF capacitance. Adding R_F
allows much larger capacitive loads, as shown in
Figure 5b and Figure 5c. Note that with R_F of only 20Ω, the
load capacitor should be either smaller than 1nF or larger
than 33nF to avoid overshoot; with R_F of 50Ω this transient
area is avoided.
I_COMP2
DRV401 Differential Amplifier Section
R₁
10kΩ
R₂
40kΩ
Decoupling, Low−Pass Filter
R_SHUNT
R₃
10kΩ
Differential
Amplifier
R₄
40kΩ
V_OUT
R_F
50Ω
REF_IN
R₅
Dummy
Shunt
C_F
10nF
ADC
Compensated
REF_IN
K2 ^{NOTE: R5 is a dummy shunt resistor equal to 4x RSHUNT to compensate for RSHUNT and provide best CMR.}
a) Internal difference amplifier with an example of a decoupling filter.
10µs/div
b) V_OUT of Figure 5a with R₅ = 20Ω and C_D = 100nF.
10µs/div
c) V_OUT of Figure 5a with R₅ = 50Ω and C_D = 10nF.
Figure 5. Internal Difference Amplifier with Example of a Decoupling Filter
17