Datasheet

Error_V =
OS
? 100
V
V
OS
SENSE
INA216
SBOS503C JUNE 2010REVISED NOVEMBER 2011
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These comments have framed the decision on what Calculating Total Error
the shunt resistor value should be, based on the
The electrical specifications for the INA216 include
full-scale value; but many applications require
the typical individual errors terms such as gain error,
accurate measurements at levels as low as 10% of
offset error, and nonlinearity error. Total error
the full-scale value. At this level, the input offset
including all of these individual error components is
voltage of the current shunt monitor becomes a larger
not specified in the Electrical Characteristics table. To
percentage of the shunt voltage, and thus contributes
accurately calculate the error that can be expected
a larger error to the output. The percentage of error
from the device, we must first know the operating
created by the input offset voltage relative to the
conditions to which the device is subjected. Some
shunt voltage is shown in Equation 1.
current shunt monitors specify a total error in the
product data sheet. However, this total error term is
accurate under only one particular set of operating
(1)
conditions. Specifying the total error at this one point
Ideally, the differential input voltage at 10% would be
has little practical value, though, because any
increased to minimize the effects of the input offset
deviation from these specific operating conditions no
voltage; however, we are bound by the full-scale
longer yields the same total error value. This section
value. The full-scale output voltage on the INA216 is
discusses the individual error sources, with
limited to 200mV below the supply voltage (IN+).
information on how to apply them in order to calculate
Selecting a shunt resistor to increase the shunt
the total error value for the device under normal
voltage at the low operating range of the load current
operating conditions.
could easily saturate the output of the current shunt
The typical error sources that have the largest impact
monitor at the full-scale load current. For applications
on the total error of the device are input offset
where accuracy over a larger range is needed, a
voltage, common-mode voltage rejection, gain error,
lower gain option (and therefore, a larger differential
and nonlinearity error.
input voltage) is selected. For applications where a
minimal voltage drop on the line that powers the load
The nonlinearity error of the INA216 is relatively low
is required, a higher gain option (and so, a smaller
compared to the gain error specification, which
differential input voltage) is selected.
results in a gain error that can be expected to be
relatively constant throughout the linear input range of
For example, consider a design that requires a
the device. While the gain error remains constant
full-scale output voltage of 4V, a maximum load
across the linear input range of the device, the error
current of 10A, and a maximum voltage drop on the
associated with the input offset voltage does not. As
common-mode line of 25mV. The 25mV maximum
the differential input voltage developed across a
voltage drop requirement and a 4V full-scale output
shunt resistor at the input of the INA216 decreases,
limits the gain option to the 200V/V device. A 100V/V
the inherent input offset voltage of the device
setting would require a maximum voltage drop of
becomes a larger percentage of the measured input
40mV with the other two lower gain versions creating
signal, resulting in an increase in measurement error.
larger voltage drops. Based on the gain of 200 on a
This varying error is present among all current shunt
4V full-scale output, the maximum differential input
monitors, given the input offset voltage ratio to the
voltage would be 20mV. The shunt resistor needed to
voltage being sensed by the device. The low input
create a 20mV drop with a 10A load current is 2mΩ.
offset voltages present in the INA216 devices,
When choosing the proper shunt resistor, it is also
however, limit the amount of contribution the offset
important to consider that at higher currents, the
voltage has on the total error term.
power dissipation in the shunt resistor becomes
Two examples are provided that detail how different
greater. Therefore, it is important to evaluate the drift
operating conditions can affect the total error
of the sense resistor as a result of power dissipation,
calculations. Typical and maximum calculations are
and choose an appropriate resistor based on its
shown as well to provide the user more information
power wattage rating.
on how much error variance could be present from
device to device.
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