How to Use AD7781: Examples, Pinouts, and Specs

The AD7781 is a low-power, 24-bit sigma-delta analog-to-digital converter (ADC) manufactured by Analog Devices. It is designed for high-precision measurement applications, offering exceptional performance in a compact package. The AD7781 features an integrated programmable gain amplifier (PGA) and operates via a simple serial interface, making it ideal for applications requiring accurate and reliable data acquisition.

• Weigh scales
• Industrial process control
• Temperature measurement systems
• Pressure sensors
• Medical instrumentation

The AD7781 is available in a 16-lead TSSOP package. Below is the pin configuration and description:

1. Power Supply: Connect the analog (AVDD) and digital (DVDD) power supplies to a voltage source between 2.7 V and 5.25 V. Ensure proper decoupling capacitors are placed close to the power pins.
2. Reference Voltage: Provide a stable reference voltage to the REFIN(+) and REFIN(−) pins. The reference voltage determines the input voltage range.
3. Analog Input: Connect the differential analog input signal to the AIN(+) and AIN(−) pins. Ensure the input voltage does not exceed the specified range (±VREF/Gain).
4. SPI Communication:
   • Connect the SCLK, DIN, DOUT/RDY, and CS pins to the SPI bus of your microcontroller or processor.
   • Configure the SPI interface for CPOL = 0 and CPHA = 1 (SPI Mode 1).
5. Initialization: After power-up, reset the AD7781 by toggling the PDRST pin or sending a reset command via SPI.
6. Data Acquisition:
   • Monitor the DOUT/RDY pin to detect when new data is available.
   • Read the 24-bit conversion result via the SPI interface.

• Use low-noise power supplies and proper grounding techniques to minimize noise and interference.
• Place decoupling capacitors (e.g., 0.1 µF and 10 µF) close to the AVDD and DVDD pins.
• Ensure the reference voltage is stable and within the specified range for accurate measurements.
• Avoid exceeding the absolute maximum ratings for any pin to prevent damage to the device.

Below is an example of how to interface the AD7781 with an Arduino UNO using SPI:

#include <SPI.h>

// Define AD7781 pins
const int CS_PIN = 10;   // Chip Select
const int RDY_PIN = 12;  // Data Ready (DOUT/RDY)
const int RESET_PIN = 9; // Power-down/Reset

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

  // Configure SPI settings
  SPI.begin();
  SPI.setDataMode(SPI_MODE1); // CPOL = 0, CPHA = 1
  SPI.setClockDivider(SPI_CLOCK_DIV16); // Set SPI clock speed
  SPI.setBitOrder(MSBFIRST); // Most significant bit first

  // Configure AD7781 pins
  pinMode(CS_PIN, OUTPUT);
  pinMode(RDY_PIN, INPUT);
  pinMode(RESET_PIN, OUTPUT);

  // Reset the AD7781
  digitalWrite(RESET_PIN, LOW);
  delay(1); // Hold reset low for at least 100 ns
  digitalWrite(RESET_PIN, HIGH);

  // Initialize the AD7781
  digitalWrite(CS_PIN, HIGH); // Deselect the chip
}

void loop() {
  // Wait for data ready signal
  if (digitalRead(RDY_PIN) == LOW) {
    // Select the AD7781
    digitalWrite(CS_PIN, LOW);

    // Read 24-bit data from the AD7781
    unsigned long data = 0;
    for (int i = 0; i < 3; i++) {
      data = (data << 8) | SPI.transfer(0x00);
    }

    // Deselect the AD7781
    digitalWrite(CS_PIN, HIGH);

    // Print the conversion result
    Serial.println(data, HEX);
  }
}

1. No Data Output:

   • Ensure the SPI connections are correct and the SPI settings match the AD7781 requirements.
   • Verify that the DOUT/RDY pin is being monitored correctly.

2. Incorrect Conversion Results:

   • Check the reference voltage and ensure it is stable.
   • Verify that the input signal is within the specified range.

3. Device Not Responding:

   • Confirm that the PDRST pin is properly toggled during initialization.
   • Ensure the power supply voltages (AVDD and DVDD) are within the specified range.

Q: Can the AD7781 operate with a single-ended input?
A: No, the AD7781 is designed for differential input signals. Single-ended inputs can be used with proper biasing, but this may reduce performance.

Q: What is the maximum SPI clock frequency supported by the AD7781?
A: The AD7781 supports SPI clock frequencies up to 5 MHz.

Q: How do I calculate the input voltage range for a given gain setting?
A: The input voltage range is ±VREF/Gain. For example, with a 2.5 V reference and a gain of 128, the range is ±19.53 mV.