INA3221

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SBOS576 – MAY 2012

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.

Copyright © 2012, Texas Instruments Incorporated

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