Current & Power Monitor: INA219 & INA226

Working Principle

The INA219 and INA226 from Texas Instruments are high-side current and power monitors with an integrated 12-bit (INA219) or 16-bit (INA226) ADC. They measure the voltage drop across a small shunt resistor (typically 0.1 Ω) placed in the high-side (V+ rail) of the circuit. Ohm’s law converts this voltage to current: I = V_shunt / R_shunt. The chip also measures bus voltage, computing power (P = V × I) internally.

Why high-side? Measuring on the positive rail avoids disturbing the ground reference, allowing accurate monitoring of power consumption without affecting the circuit’s ground plane.

Electrical Characteristics

Parameter	INA219	INA226
ADC Resolution	12-bit	16-bit
Bus Voltage Range	0–26 V	0–36 V
Shunt Voltage Range	±320 mV (default)	±81.92 mV
Current Resolution	0.1 mA (0.1 Ω shunt)	~1.25 µA (0.1 Ω)
Supply Voltage	3–5.5 V	2.7–5.5 V
Interface	I2C (4 address options)	I2C (16 address options)
Power Calculation	On-chip (V × I)	On-chip (V × I)
Alert Pin	No	Yes (configurable threshold)

Interfacing with an MCU

Connect VCC, GND, SDA, SCL for I2C communication. Wire the shunt resistor in series with the load on the V+ rail, then connect Vin+ and Vin- to each side of the shunt. The INA219 default I2C address is 0x40; address pins A0/A1 select from 4 (INA219) or 16 (INA226) options.

Calibration

• Calibration register: Set based on shunt resistance and max expected current. Libraries calculate this automatically.
• Shunt resistor: Choose R_shunt so max current produces <320 mV drop (INA219) or <81.92 mV (INA226)
• Averaging: INA226 supports hardware averaging (1–1024 samples) for noise-free readings
• Zero offset: Read with no load to measure offset; subtract from all measurements

Code Example

/*
 * INA219 Current & Power Monitor — Arduino
 * Library: Adafruit INA219 (install via Library Manager)
 * Wiring: VCC → 5V, GND → GND, SDA → A4, SCL → A5
 *         Vin+ → V+ rail, Vin- → Load (0.1Ω shunt between)
 */
#include <Wire.h>
#include <Adafruit_INA219.h>

Adafruit_INA219 ina219;

void setup() {
    Serial.begin(9600);
    if (!ina219.begin()) {
        Serial.println("INA219 not found!");
        while (1);
    }
    ina219.setCalibration_32V_1A();  /* 0.1 Ω shunt, 1A max */
    Serial.println("INA219 Ready");
}

void loop() {
    float busV    = ina219.getBusVoltage_V();
    float shuntmV = ina219.getShuntVoltage_mV();
    float current  = ina219.getCurrent_mA();
    float power    = ina219.getPower_mW();

    Serial.print("Bus: ");
    Serial.print(busV, 2);
    Serial.print(" V  Current: ");
    Serial.print(current, 1);
    Serial.print(" mA  Power: ");
    Serial.print(power, 1);
    Serial.println(" mW");

    delay(1000);
}

Real-World Applications

Limitations

• Shunt resistor heat: At high currents, the shunt dissipates P = I²R power; size the resistor for thermal handling.
• Voltage drop: The shunt introduces a voltage drop in the supply rail; minimize R_shunt for low-voltage circuits.
• No galvanic isolation: The sensor shares ground with the measured circuit; not suitable for mains AC monitoring.
• Sample rate: I2C read latency limits effective sample rate to ~1 kHz; not suitable for fast transient analysis.