Datasheet

INA3221
www.ti.com SBOS576A –MAY 2012–REVISED JUNE 2013
MULTIPLE CHANNEL MONITORING
The INA3221 can be configured to monitor shunt and voltage measurements for up to three unique power-supply
rails. This configuration allows for a total of six different signals to be measured. The INA3221 can be configured
to adjust the number of channels and signals being measured through the Channel Enable and MODE bits in the
Configuration Register. This adjustment allows the device to be optimized based on application requirements
because the system is in use.
Channel Configuration
If an application requires that all three channels be monitored at power-up, but only one channel must be
monitored after the system has stabilized, the other two channels can be disabled after power-up. This process
allows the INA3221 to only monitor the power-supply rail of interest. Disabling unused channels helps improve
system response time by more quickly returning to sampling the channel of interest. The INA3221 linearly
monitors the enabled channels. This means that if all three channels are enabled for both shunt and bus voltage
measurements, it takes five more completed conversions after a signal is measured before the device returns to
that particular signal to begin another conversion. Changing the operating mode to monitor only the shunt voltage
reduces this requirement to two conversions before the device begins a new conversion on a particular channel
again.
There is also a timing aspect involved in reducing the signals being measured. The amount of time to complete
an all-channel, shunt and bus voltage sequence is equal to the sum of the shunt voltage conversion time and the
bus voltage conversion time (as programmed by the CT bits in the Configuration Register) multiplied by the three
channels. The conversion times for the shunt and bus voltage measurements are programmed independently,
however, the shunt and bus voltage conversion times selected apply to all channels.
Enabling a single channel with only one signal measured allows for that particular signal to be monitored solely.
This setting enables the fastest response over time to changes in that specific input signal because there is no
delay from the end of one conversion before the next conversion begins on that channel. Conversion time is not
affected by enabling or disabling other channels. Selecting both the shunt and bus voltage settings as well as
enabling additional channels extends the time from the end of one conversion on a signal before the beginning of
the next conversion of that signal.
Averaging and Conversion Time Considerations
The INA3221 has programmable conversion times for both the shunt and bus voltage measurements. The
conversion times for these measurements can be selected from 140 μs to 8.244 ms. The conversion time
settings, along with the programmable averaging mode, enable the INA3221 to be configured to optimize
available timing requirements in a given application. For example, if a system requires data to be read every 2
ms with all three channels monitored, the INA3221 can be configured with the conversion times for the shunt and
bus voltage measurements set to 332 μs. The INA3221 can also be configured with a different conversion time
setting for the shunt and bus voltage measurements. This approach is common in applications where the bus
voltage tends to be relatively stable. This situation allows for the time focused on the bus voltage measurement
to be reduced relative to the shunt voltage measurement. For example, the shunt voltage conversion time can be
set to 4.156 ms with the bus voltage conversion time set to 588 μs for a 5-ms update time.
There are trade-offs associated with conversion time settings and the averaging mode used. The averaging
feature can significantly improve the measurement accuracy by effectively filtering the signal. This approach
allows the INA3221 to reduce the amount of noise in the measurement that may be caused by noise coupling
into the signal. A greater number of averages allows the INA3221 to be more effective in reducing the
measurement noise component. The trade-off to this noise reduction is that the averaged value has a longer
response time to input signal changes. This aspect of the averaging feature is mitigated to some extent with the
critical alert feature that compares each single conversion to determine if a measured signal (with its noise
component) has exceeded the maximum acceptable level.
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