Rainbow Electronics MAX9937 - Manual (Page 9)

In this case, R RS = R RSP = R RSN ,
ΔR
RS depends on the
tolerance of the R
RS
resistors used, and
ΔI
B
= input off-
set 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),
ΔI
B
=
±12% (max) of 5.6µA (max) = ±0.67µA (max), and R RS
= 500
Ω with ±1% tolerance (i.e., ΔR
RS = ±5
Ω 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 dif-
ferential 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 dis-
connected 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 common-
mode voltage and input differential-voltage transients
(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 .
Similarly, capacitor C OUT from OUT to ground helps fil-
ter the output voltage, thus providing not only differen-
tial filtering, but also filtering for input common-mode
transients 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.
f
R COUT
C OUT
OUT
−
=
×
1
2 π
f
R C
C IN
RSN IN
−
=
×
1
2
π
MAX9937
Automotive Current-Sense Amplifier
with Reverse-Battery Protection
_______________________________________________________________________________________ 9
LOAD
MICROCONTROLLER
ADC
R SENSE
GND
OUT
V CC
RS+ RS-
R
RSN
499 Ω
R
RSP
499 Ω
R OUT
10k Ω
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
Figure 1. Typical Application Circuit with Optional External Filtering