Datasheet

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SBVS070B − JUNE 2006 − REVISED MAY 2009
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17
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 (R
5
) 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 +
R
2
R
1
+
R
4
) R
5
R
SHUNT
) R
3
With R
2
/R
1
= R
4
/R
3
= 4; R
5
= R
SHUNT
× 4
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.
a) Internal difference amplifier with an example of a decoupling filter.
10
µ
s/div
20mV/div
b) V
OUT
of Figure 5a with R
5
= 20Ω and C
D
= 100nF.
10
µ
s/div
20mV/div
c) V
OUT
of Figure 5a with R
5
= 50Ω and C
D
= 10nF.
R
2
40k
Ω
R
1
10k
Ω
R
4
40k
Ω
R
3
10k
Ω
R
5
Dummy
Shunt
Differential
Amplifier
R
F
50
Ω
Decoupling, Low−Pass Filter
REF
IN
Compensated
REF
IN
ADC
V
OUT
C
F
10nF
R
SHUNT
I
COMP2
K2
DRV401 Differential Amplifier Section
NOTE: R
5
is a dummy shunt resistor equal to 4x R
SHUNT
to compensate for R
SHUNT
and provide best CMR.
Figure 5. Internal Difference Amplifier with Example of a Decoupling Filter
(2)