Datasheet

In this case, R
RS
= R
RSP
= R
RSN
, ∆R
RS
depends on the
tolerance of the RRS resistors used, and ∆I
B
= input offset
current of the amplifier.
The temperature drift of these parameters similarly add
up to give a final result.
Shown below is an example calculation of V
OS
at +25°C:
With V
OS
= ±1.2mV (max), I
B
= 5.6µA (max), ∆I
B
= ±12%
(max) of 5.6µA (max) = ±0.67µA (max), and R
RS
= 500Ω
with ±1% tolerance (i.e., ∆R
RS
= ±5Ω max)
V
OS
-FINAL = 1.25mV (max).
Flexible EMI Filtering
Real-world applications of current-sense amplifiers need
to measure currents precisely in the presence of a wide
variety of input transients. For example, fast load-current
transients when measuring at the input of a switching
buck regulator can cause high-frequency differential
sense voltages to occur at inputs of the MAX9937,
although the signal of interest is the average DC value.
Alternately, parasitic voltage pickup on a disconnected or
long cable can cause common-mode voltage transients to
occur at inputs of the MAX9937, which are required to be
rejected effectively.
The MAX9937 allows two methods of filtering to help
improve performance in the presence of input com-
mon-mode voltage and input differential-voltage tran-
sients (see Figure 1).
The capacitor C
IN
between RS+ and RSN helps filter
against input differential voltages, and prevents them from
reaching the MAX9937. The corner frequency of this filter
is determined by the choice of R
RSN
and C
IN
.
C IN
RSN IN
1
f
2R C
−
=
π×
Similarly, capacitor C
OUT
from OUT to ground helps filter
the output voltage, thus providing not only differential
filtering, but also filtering for input common-mode tran-
sients that have made it past the MAX9937. The corner
frequency of this filter is similarly determined by choice of
R
OUT
and C
OUT
. Note: The MAX9937 is a current-output
device, and has the ability to drive an infinite amount of
load capacitance.
C OUT
OUT
1
f
2 R COUT
−
=
π×
Figure 1. Typical Application Circuit with Optional External Filtering
LOAD
MICROCONTROLLER
ADC
R
SENSE
GND
OUT
V
CC
RS+
RS-
R
RSN
499Ω
R
RSP
499Ω
R
OUT
10kΩ
5V
RSP
RSN
BIAS
BLOCK
V
BAT
= 4V
TO 28V
MAX9937
C
OUT
*
1nF
C
IN
*
220nF
*FILTER CAPACITORS ARE OPTIONAL.
GAIN =
V
OUT
V
SENSE
=
R
OUT
R
RSP
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Maxim Integrated │ 9
MAX9937 Current-Sense Amplier with
Reverse-Battery Protection