How to Use NAU7802 Load Cell ADC: Examples, Pinouts, and Specs

The NAU7802 is a high-precision analog-to-digital converter (ADC) designed specifically for load cell applications. It features a 24-bit resolution and a built-in programmable gain amplifier (PGA), making it ideal for applications requiring accurate weight measurements. The device communicates via the I2C protocol, ensuring seamless integration with microcontroller-based systems. Its low power consumption and high accuracy make it suitable for battery-powered devices, industrial scales, and IoT-based weighing systems.

• Digital weighing scales
• Industrial load measurement systems
• IoT-based weight monitoring
• Portable weighing devices
• Force measurement in robotics

The NAU7802 is typically available in a 16-pin QFN package. Below is the pin configuration:

1. Power Supply: Connect AVDD and DVDD to a stable power source (2.7V to 5.5V). Connect AVSS and DVSS to ground.
2. Load Cell Connection: Connect the load cell's differential output to VINP and VINN. Ensure proper shielding to minimize noise.
3. Reference Voltage: Provide a stable reference voltage to VREFP and VREFN. Typically, AVDD and AVSS can be used as references.
4. I2C Communication: Connect the SDA and SCL pins to the corresponding I2C pins on your microcontroller. Use pull-up resistors (typically 4.7kΩ) on both lines.
5. External Capacitor: Connect a capacitor (e.g., 1µF) to the CAP pin for the internal voltage regulator.
6. Crystal Oscillator: If required, connect a crystal oscillator to XTAL1 and XTAL2 for clock generation.

• Use decoupling capacitors (e.g., 0.1µF) near the power supply pins to reduce noise.
• Ensure proper grounding to avoid measurement errors.
• Configure the programmable gain amplifier (PGA) based on the load cell's output range.
• Use shielded cables for the load cell to minimize electromagnetic interference (EMI).

Below is an example of how to interface the NAU7802 with an Arduino UNO using the I2C protocol:

#include <Wire.h>

// NAU7802 I2C address
#define NAU7802_ADDRESS 0x2A

// Register addresses
#define NAU7802_PU_CTRL 0x00
#define NAU7802_CTRL1   0x01
#define NAU7802_CTRL2   0x02
#define NAU7802_ADCO_B2 0x12 // ADC output MSB

void setup() {
  Wire.begin(); // Initialize I2C communication
  Serial.begin(9600); // Initialize serial communication for debugging

  // Power up the NAU7802
  Wire.beginTransmission(NAU7802_ADDRESS);
  Wire.write(NAU7802_PU_CTRL);
  Wire.write(0x30); // Enable power-up and oscillator
  Wire.endTransmission();
  delay(10); // Wait for the device to stabilize

  // Configure the gain (e.g., gain = 128)
  Wire.beginTransmission(NAU7802_ADDRESS);
  Wire.write(NAU7802_CTRL1);
  Wire.write(0x07); // Set gain to 128
  Wire.endTransmission();
}

void loop() {
  // Read ADC data
  Wire.beginTransmission(NAU7802_ADDRESS);
  Wire.write(NAU7802_ADCO_B2); // Start reading from ADC output MSB
  Wire.endTransmission(false);
  Wire.requestFrom(NAU7802_ADDRESS, 3); // Request 3 bytes (MSB, Mid, LSB)

  if (Wire.available() == 3) {
    long adcValue = 0;
    adcValue |= (long)Wire.read() << 16; // MSB
    adcValue |= (long)Wire.read() << 8;  // Mid
    adcValue |= (long)Wire.read();       // LSB

    // Convert to signed 24-bit value
    if (adcValue & 0x800000) {
      adcValue |= 0xFF000000; // Sign extend if negative
    }

    Serial.println(adcValue); // Print ADC value
  }

  delay(500); // Wait before the next reading
}

1. No Communication with the NAU7802

   • Ensure the I2C pull-up resistors (4.7kΩ) are connected to SDA and SCL.
   • Verify the I2C address (default is 0x2A) and ensure no address conflicts on the bus.
   • Check the wiring for loose or incorrect connections.

2. Unstable or Noisy Readings

   • Use shielded cables for the load cell to reduce EMI.
   • Add decoupling capacitors near the power supply pins.
   • Ensure the reference voltage is stable and noise-free.

3. Incorrect ADC Values

   • Verify the load cell connections (VINP and VINN).
   • Check the PGA gain setting and ensure it matches the load cell's output range.
   • Confirm that the load cell is properly calibrated.

Q: Can the NAU7802 be used with a 3.3V microcontroller?
A: Yes, the NAU7802 operates with a supply voltage range of 2.7V to 5.5V, making it compatible with 3.3V systems.

Q: What is the maximum load cell output voltage the NAU7802 can handle?
A: The NAU7802 supports a differential input voltage range of ±0.5V.

Q: How do I calibrate the NAU7802 for accurate weight measurements?
A: Perform a two-point calibration using a known zero weight and a known reference weight. Use these values to calculate the scale factor.