INA3221 from Texas Instruments measures current and voltage for three different channels concurrently. It supports both 3.3V and 5V environments.
- Load voltage must not exceed 26V.
- All loads and the chip must share a common GND.
With the typical R100 shunt resistor found on most breakout boards, the current must not exceed 1.638A.
The INA3221 is typically used in breakout boards that come with a R100 (0.1 Ohm) shunt resistor. Those boards are available from €1.00. The INA3221 chip can be used in own PCB designs as well, of course.
The commonly available purple and black PCBs are technically identical. On purple boards, the traces are easier to identify, helping to verify that the board you have is not affected by the design flaw of early versions.
Quick Overview
The INA3221 is a three-channel, high-side current and voltage monitor with an I2C- and SMBUS-compatible interface. Its default I2C address is 0x40, but three additional I2C addresses can be set.
| Parameter | Description |
|---|---|
| Supply Voltage | 2.7V - 5.5V |
| Current consumption | 350uA (typical) |
| Detected Voltage | 0-26V, 8mV increments |
| Maximum Detected Current | Depends on shunt resistor value, 4095 steps resolution |
| I2C Default Address | 0x40 (four addresses configurable) |
The device power supply (VC) and the three input channels are separated and can use different voltages (but must share a common GND). Bus voltages can be present with the supply voltage off and vice versa.
The chip has programmable conversion times and averaging modes and offers both critical and warning alerts to detect multiple programmable out-of-range conditions for each channel.
Wiring and Measuring
The current is measured as a voltage drop across the shunt resistor for a particular channel. Each channel uses its own shunt resistor.
Shunt resistors can have different values for each channel. However, identical values are required to use some of the built-in averaging functions.
The voltage is measured as a high-side measurement against GND, excluding any voltage drop at the shunt resistors. The channel bus voltage is measured between VNx- and GND.
Connecting Channels
Each channel has two connectors: VNx+ and VNx-.
- VNx+ must always be connected to the positive voltage (high side).
- VNx- connects to the positive side of the load you want to measure.
- All three channels must share the same GND.
This may sound confusing at first, but it is quite simple. Take a look at a practical example and its schematic (taken from here):
Breakout Boards
INA3221 breakout boards are available in two colors (both are functionally identical):
The purple version offers better visibility of PCB traces, which is helpful for understanding the breakout board design and identifying older PCB versions that have a design flaw:
Design Flaw in Old PCB Versions
The initial version of the breakout board had a serious design flaw: all three IN+ pins were connected.
This forced all three INA3221 channels to share the same positive voltage source, making it impossible to measure setups like solar panel appliances with different voltages at various circuitry locations. The flawed design required all three channels to use the same high side (positive input voltage).
How creative users fixed the flawed INA3221 breakout board
Users manually corrected the PCB design flaw by cutting and reworking tracks.
These fixes often worked, but the results were not always visually reliable or professional-looking.
New PCB Design Without Flaw
The issue has since been resolved, and the PCB layout has been completely redesigned. The updated design eliminates the flaw by separating the IN+ pins for all three channels.
Compare the new track routing to the earlier flawed design: each solder pad has its distinct trace leading directly to the shunt resistor, and each shunt resistor is connecting the two solder pads of any channel.
New PCBs can also be identified by the labels on the three large solder pads on the top side: VPU, GND, and GND. The old version lacked these labels and used the pins differently.
Since the initial breakout board worked completely different, when you are looking for examples in the Internet, you must pay attention which version of the board was used. You cannot use the newer breakout board in schematics that use the old one, and vice versa.
Shunt Value and Maximum Current
Most breakout boards come with an R100 shunt resistor (0.1 Ω), allowing them to measure a maximum current of 1.638 A per channel.
This current limit may not be sufficient for your project. Since the shunt resistor value and the maximum measurable current are inversely related, lowering the shunt resistor value increases the maximum current capacity.
To double the maximum current, you can solder another R100 resistor on top of the existing one, effectively halving the resistance.
Setting Maximum Current
The INA3221 measures current by detecting the voltage drop across a shunt resistor. It senses this voltage drop in 40 µV increments, up to a maximum of 163.8 mV. This provides a 12-bit resolution (4,095 steps, with one step reserved for internal use).
By varying the shunt resistor, you can either increase precision (for small currents) or increase maximum current (at the expense of precision):
The maximum measurable current is determined by the shunt resistor value:
| Shunt Resistor | Resistance (Ω) | Maximum Current (A) | Remark |
|---|---|---|---|
| R1 | 1 Ω | 0.1638A | precision measuring for small currents, i.e. microelectronics power consumption |
| R100 | 0.1 Ω | 1.638 | default |
| 2x R100 in parallel | 0.05 Ω | 3.276 | simple hack; soldering another R100 on top of the existing |
| R020 | 0.01 Ω | 8.19 | high currents, breakout board traces may not handle this current |
Ensure that the shunt resistor value produces a voltage drop of no more than ±163.8mV for the highest expected current.
Changing the shunt resistor may allow for higher loads, however always make sure the rest of your circuitry can handle these currents.
1 mm traces (as used in typical INA3221 breakout boards) can handle 2–3 A. Beyond that, they can heat up and eventually get destroyed.
For high currents, consider wiring the resistors directly (bypassig the traces), or using an external shunt resistor.
For currents beyond a few Amperes, you may also want to look into Hall Sensors.
Calculating Shunt Resistor Values
Texas Instruments provides a free Power Monitor Tool, a Microsoft Excel sheet with embedded formulas. This tool helps calculate the appropriate shunt resistor values for your specific use case.
Pin Layout
| Pin Label | Description |
|---|---|
| IN1+ | Positive pin of power supply for load 1 |
| IN1- | Positive side of load 1 |
| IN2+ | Positive pin of power supply for load 2 |
| IN2- | Positive side of load 2 |
| IN3+ | Positive pin of power supply for load 3 |
| IN3- | Positive side of load 3 |
| VS | INA3221 power supply, 2.7-5.5V |
| GND | Common ground (must be shared with all connected loads) |
| SCL | I2C clock line (SCL) |
| SDA | I2C data line (SDA) |
| PV | Power valid alert, open-drain output |
| CRI | Critical alert, open-drain output, conversion-triggered |
| WAR | Warning alert, open-drain output, average-triggered |
| TC | Timing control alert, open-drain output |
| VPU | Pull-up supply voltage used to bias power valid output circuitry (on top side of board) |
| A0 | Defines the I2C address. Default address is 0x40 (on top side of board) |
I2C Address
By default, the I2C address is 0x40. This can be modified via pin A0 as follows:
| I2C Address | Pin A0 connected to |
|---|---|
| 0x40 | Not connected (n/c) |
| 0x40 | GND |
| 0x41 | VS |
| 0x42 | SDA |
| 0x43 | SCL |
On most breakout boards, pin A0 is exposed on the front side, along with solder pads for each of the four possible connections. Add a solder bridge to one of these pads to manually change the I2C address.
Schematics
The INA3221 operates as a high-side device with the following rules:
- Common Ground: All loads, channels, and the INA3221 power supply must share the same GND.
- High Side: The positive pin of each channel connects to the positive power supply. The negative pin connects to the positive side of the load, with the negative side of the load connecting to the common GND.
Depending on how you wire the board up, the user experience can be anywhere from simple to horrible complex. Here are a few examples and suggestions:
Simple Design, Horrible Experience
Here is an example how you can connect a microcontroller with the least effort and least number of wires to a INA3221 breakout board. The result works, but a user would have a hard time figuring out how to measure currents and voltages with it:
That’s because this example is shifting complexity to the user, and the user would need to know the rules for connecting:
- The positive power source connects to VNx+.
- The positive side of the load connects to VNx-.
- Both the negative power source and load connect to GND.
Huh? Right.
If you consider the two solder pads per channel as the two sides of a shunt resistor (which they are), the connection logic becomes a bit clearer:
- Negative Pole: The load is directly connected to the negative pole of the power supply.
- Positive Pole: The load receives its positive voltage from IN-, while the power supply provides positive voltage to IN+. The breakout board’s shunt resistor bridges IN- and IN+, and the INA3221 measures the voltage drop across it.
Yet even now, things are still not working: that’s because the INA3221 must also share ground with the circuit it measures:
If you are put off by this bad example, you might be inclined to invest a bit more thought on the schematics side to improve the user experience drastically (see next).
Thoughtful Design, Intuitive Use
You can easily incorporate the connection rules in your schematics. This way, the complexity is moved back inside the measuring device, and the user simply plugs in the power supply and the load that needs to be measured:
Essentially, the design takes care that all the ground lines connect properly. Now the user can inutitively work with the device:
Anything inside the dotted line is shielded from the end user.
How INA3221 Works Inside
At its core, the INA3221 is an analog-to-digital converter (ADC) that measures analog voltages. It performs two key measurements on up to three power supplies:
- Current: Measured indirectly as the voltage drop across the shunt resistor (between INx+ and INx-).
- Voltage: The bus voltage is measured between INx- and GND.
The bus voltage approximates the power supply voltage, as the voltage drop across the shunt resistor is negligible. For precise values, account for the voltage drop derived from the current measurement.
Operating Modes
Normal Mode
In this mode, the chip continuously measures:
- Shunt voltage for one channel.
- Bus voltage for the same channel.
- Proceeds sequentially to the next channel.
Channels can be independently enabled or disabled. Disabled channels are bypassed.
Single Shot Mode
In this mode:
- The chip measures all enabled channels sequentially once.
- Afterward, it enters power-down mode. Measurements can still be read and represent the last cycle.
Power-Down Mode
In power-down mode:
- Quiescent current is minimized.
- Registers remain functional, but current flowing into the INA3221 inputs is cut off.
Wake-up time from this mode is 40µs.
Alerts
INA3221 supports four different alerts with programmable thresholds per channel. Alert states are available at designated pins. Breakout boards also connect these pins to built-in LED to indicate the alert state of the device:
- VS: power supply to the board is present
- PV: all enabled channels have a valid voltage (power valid)
- C: at least one channel has exceeded the critical current
- W: at least one channel has exceeded the warning current
- TC: the power rails on channel 1 and channel 2 did not start up quickly enough (timing control)
- Critical: very fast-acting alert that triggers whenever any enabled channel current exceeds a programmed threshold. Pin goes low in alert state. Can optionally use the sum of all enabled channels instead (provided all channels use the same shunt resistor value).
- Warning: less quickly-acting but more fault-tolerant alert that triggers when the average current exceeds a programmed threshold. Pin goes low in alert state.
- Power Valid: changes from low to high once all enabled channels reach a predefined voltage (by default 10V). Goes back to low once any channel voltage drops below a second threshold (by default 9V). As long as this pin is high, the device power supply is in expected shape. All thresholds can be reprogrammed to a minimum voltage of 2.7V.
- Timing-Control: This is a very specific alert that can be used to ensure proper power-supply sequencing but requires appropriate assignment of channels to the power supplies in question. This alert is not used for most typical application. Here is how it works: At power-up only (or after a reset), the chip measures the bus voltage at channel 1, and once it exceeds 1.2V, it continues to monitor the bus voltage at channel 2. If this voltage does not also exceed 1.2V after 28.6ms (four complete measurement cycles at default values), this pin pulls low, indicating that no valid power rail was present on channel 2.
Conversion Time And Averaging
Measurement errors can occur: current is measured via very low voltages. Such voltages can be subject to trace inductance and other parasitic impedances between the shunt resistor and the chips input pins.
When using breakout boards, such interferences are minimized by the close proximity of the shunt resistors. When you are required to use external shunt resistors to measure large currents, measurement errors may become a problem.
Noise And Errors
INA3221 can limit the impact of noise on single measurements by two separate types of averaging:
- Conversion Time: this is the time the internal ADC can use to perform a single measurement. The more conversion time it is granted, the more accurate the results are: internally, the ADC uses its conversion time to take more samples, effectively averaging its single measurement.
- Averaging: INA3221 can also average. It can take multiple ADC measurements and average them once more.
Both techniques significantly decrease the effect of noise and increase the accuracy of measurements. The price you pay for both optimizations is a lower response times.
This graph illustrates the effect of averaging:
In a nutshell, you can optimize the accuracy in a number of ways:
- Hardware Design: move the shunt resistor as close to the INA3221 input pins to minimize noise effects in the first place.
- Conversion Time: give more time to the ADC for better sampling.
- Average: average the raw ADC samples, effectively filtering out single bad measurements.
Good hardware design is most important: it minimizes noise, so expensive conversion time and post-averaging is less needed, effectively improving response time.
Datasheet
INA3221 (Texas Instruments)
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(content created Jul 07, 2024 - last updated Jan 07, 2025)