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 built-in programmable gain amplifier (PGA), low noise, and high resolution, making it ideal for applications requiring accurate weight or force measurements. The NAU7802 is commonly used in digital weighing scales, industrial force measurement systems, and other applications where precise load cell data acquisition is critical.

• Digital weighing scales
• Force measurement systems
• Industrial automation
• IoT-based weight monitoring
• Laboratory equipment

The NAU7802 offers a range of features that make it suitable for high-precision measurements. Below are its key technical specifications:

• Supply Voltage: 2.7V to 5.5V
• Input Channels: 2 differential inputs
• Resolution: 24-bit ADC
• Programmable Gain Amplifier (PGA): Gain settings from 1 to 128
• Data Rate: Configurable from 10 SPS to 320 SPS
• Operating Temperature Range: -40°C to +85°C
• Communication Interface: I²C (up to 400 kHz)
• Low Noise: 50 nV RMS noise at gain = 128
• Integrated Oscillator: No external clock required
• Power Consumption: Ultra-low power in standby mode

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

The NAU7802 is straightforward to use in load cell applications. Below are the steps and considerations for integrating it into a circuit:

1. Power Supply:

   • Connect AVDD and DVDD to a 3.3V or 5V power supply.
   • Connect AVSS and DVSS to ground.

2. Load Cell Connection:

   • Connect the load cell's positive and negative signal outputs to VIN1P and VIN1N, respectively.
   • If using a second load cell, connect it to VIN2P and VIN2N.

3. Reference Voltage:

   • Use an external reference voltage (e.g., 3.3V) connected to VREFP and VREFN.
   • Alternatively, use the internal reference voltage if supported.

4. I²C Communication:

   • Connect the SCL and SDA pins to the corresponding I²C pins on your microcontroller.
   • Use pull-up resistors (typically 4.7kΩ) on the SCL and SDA lines.

5. Reset and Data Ready:

   • Connect the RST pin to the microcontroller for resetting the ADC.
   • Use the DRDY pin to monitor when new data is available.

Below is an example of how to interface the NAU7802 with an Arduino UNO to read load cell data:

#include <Wire.h> // Include the Wire library for I²C communication

#define NAU7802_ADDRESS 0x2A // Default I²C address of the NAU7802
#define REG_PU_CTRL 0x00     // Power-up control register
#define REG_CTRL1 0x01       // Control register 1
#define REG_CTRL2 0x02       // Control register 2
#define REG_ADCO_B2 0x12     // ADC output MSB register

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

  // Initialize the NAU7802
  nau7802Init();
}

void loop() {
  long adcValue = readADC(); // Read ADC value
  Serial.println(adcValue); // Print ADC value to the serial monitor
  delay(500); // Delay for readability
}

void nau7802Init() {
  // Reset the NAU7802
  writeRegister(REG_PU_CTRL, 0x01); // Set reset bit
  delay(10); // Wait for reset to complete
  writeRegister(REG_PU_CTRL, 0x02); // Power up the ADC
  delay(10); // Wait for power-up to complete

  // Configure the ADC
  writeRegister(REG_CTRL1, 0x30); // Set gain to 128
  writeRegister(REG_CTRL2, 0x00); // Set default settings
}

long readADC() {
  // Wait for data ready
  while ((readRegister(REG_PU_CTRL) & 0x20) == 0);

  // Read ADC output registers
  long value = (long)readRegister(REG_ADCO_B2) << 16;
  value |= (long)readRegister(REG_ADCO_B2 + 1) << 8;
  value |= (long)readRegister(REG_ADCO_B2 + 2);

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

  return value;
}

void writeRegister(byte reg, byte value) {
  Wire.beginTransmission(NAU7802_ADDRESS);
  Wire.write(reg);
  Wire.write(value);
  Wire.endTransmission();
}

byte readRegister(byte reg) {
  Wire.beginTransmission(NAU7802_ADDRESS);
  Wire.write(reg);
  Wire.endTransmission(false);
  Wire.requestFrom(NAU7802_ADDRESS, (byte)1);
  return Wire.read();
}

• Use a stable power supply to minimize noise.
• Shield the load cell wires to reduce electromagnetic interference.
• Calibrate the load cell and ADC for accurate measurements.
• Use appropriate pull-up resistors for the I²C lines.

1. No Data Output:

   • Ensure the NAU7802 is powered correctly.
   • Verify the I²C connections and address.

2. Inconsistent Readings:

   • Check for noise in the power supply.
   • Ensure proper shielding of load cell wires.

3. I²C Communication Failure:

   • Verify the pull-up resistors on the SCL and SDA lines.
   • Check the I²C address of the NAU7802.

Q: Can I use the NAU7802 with a 5V microcontroller?
A: Yes, the NAU7802 supports a supply voltage of up to 5.5V, making it compatible with 5V systems.

Q: How do I calibrate the load cell with the NAU7802?
A: Perform a two-point calibration using known weights to calculate the scale factor and offset.

Q: What is the maximum load cell resistance supported?
A: The NAU7802 can support load cells with resistance values typically in the range of 350Ω to 1kΩ.

By following this documentation, you can effectively integrate the NAU7802 into your load cell applications for precise and reliable measurements.