Datasheet

´
Power Register
Current Register
I C
2
Interface
Voltage Register
GND
V
BUS
ADC
V
I
A0
Alert
A1
SDA
SCL
C
0.1 F
BYPASS
m
Load
Alert Register
V
(Supply Voltage)
S
Power Supply
(0V to 36V)
R
10
FILTER
W£
R
10
FILTER
W£
C
0.1 F to 1 F
Ceramic
Capacitor
FILTER
m m
V
IN-
V
IN+
INA226
SBOS547 –JUNE 2011 www.ti.com
Filtering and Input Considerations
Measuring current is often noisy, and such noise can be difficult to define. The INA226 offers several options for
filtering by allowing the conversion times and number of averages to be selected independently in the
Configuration Register. The conversion times can be set independently for the shunt voltage and bus voltage
measurements to allow added flexibility in configuring the monitoring of the power-supply bus.
The internal ADC is based on a delta-sigma (ΔΣ) front-end with a 500kHz (±30%) typical sampling rate. This
architecture has good inherent noise rejection; however, transients that occur at or very close to the sampling
rate harmonics can cause problems. Because these signals are at 1MHz and higher, they can be managed by
incorporating filtering at the input of the INA226. The high frequency enables the use of low-value series resistors
on the filter with negligible effects on measurement accuracy. In general, filtering the INA226 input is only
necessary if there are transients at exact harmonics of the 500kHz (±30%) sampling rate (greater than 1MHz).
Filter using the lowest possible series resistance (typically 10Ω or less) and a ceramic capacitor. Recommended
values for this capacitor are 0.1μF to 1.0μF. Figure 22 shows the INA226 with an additional filter added at the
input.
Overload conditions are another consideration for the INA226 inputs. The INA226 inputs are specified to tolerate
40V across the inputs. A large differential scenario might be a short to ground on the load side of the shunt. This
type of event can result in full power-supply voltage across the shunt (as long the power supply or energy
storage capacitors support it). Keep in mind that removing a short to ground can result in inductive kickbacks that
could exceed the 40V differential and common-mode rating of the INA226. Inductive kickback voltages are best
controlled by zener-type transient-absorbing devices (commonly called transzorbs) combined with sufficient
energy storage capacitance.
In applications that do not have large energy storage electrolytics on one or both sides of the shunt, an input
overstress condition may result from an excessive dV/dt of the voltage applied to the input. A hard physical short
is the most likely cause of this event, particularly in applications with no large electrolytics present. This problem
occurs because an excessive dV/dt can activate the ESD protection in the INA226 in systems where large
currents are available. Testing has demonstrated that the addition of 10Ω resistors in series with each input of
the INA226 sufficiently protect the inputs against this dV/dt failure up to the 40V rating of the INA226. Selecting
these resistors in the range noted has minimal effect on accuracy.
Figure 22. INA226 with Input Filtering
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