How to Use AD7745: Examples, Pinouts, and Specs

The AD7745 is a high-precision capacitive-to-digital converter (CDC) manufactured by Analog Devices. It is designed to measure small changes in capacitance with high accuracy and resolution. The device features a low-noise architecture and is ideal for applications requiring precise capacitive sensing.

• Touch sensing for user interfaces
• Liquid level detection in industrial and medical systems
• Proximity sensing for automation and robotics
• Pressure sensing and humidity measurement (when paired with external sensors)

The following table outlines the key technical details of the AD7745:

The AD7745 is housed in a 16-lead TSSOP package. Below is the pin configuration and description:

Note: Pins labeled as "NC" should not be connected to any signal or power line.

1. Power Supply: Connect the VDD pin to a stable power supply (2.7 V to 5.25 V) and the GND pin to the ground.
2. I²C Communication: Connect the SCL and SDA pins to the corresponding I²C lines of your microcontroller. Use pull-up resistors (typically 4.7 kΩ) on both lines.
3. Capacitive Sensing: Connect the capacitive sensor to the CAPDAC1 or CAPDAC2 pin. Use the EXC1 and EXC2 pins to provide the excitation signal for the sensor.
4. I²C Address: Configure the I²C address using the ADDR pin. Refer to the datasheet for address selection details.
5. Bypass Capacitor: Place a 0.1 µF ceramic capacitor close to the VDD pin for power supply decoupling.

• Ensure that the capacitive sensor is properly shielded to minimize noise and interference.
• Use short and low-impedance connections for the capacitive sensor to maintain measurement accuracy.
• Avoid placing the AD7745 near high-frequency switching components to reduce noise coupling.
• Configure the device registers via I²C to set the desired measurement mode and resolution.

Below is an example of how to interface the AD7745 with an Arduino UNO using the I²C protocol:

• VDD: Connect to the Arduino's 3.3 V pin.
• GND: Connect to the Arduino's GND pin.
• SCL: Connect to the Arduino's A5 pin (I²C clock).
• SDA: Connect to the Arduino's A4 pin (I²C data).
• CAPDAC1: Connect to the capacitive sensor.
• EXC1: Connect to the excitation electrode of the capacitive sensor.

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

#define AD7745_ADDRESS 0x48 // Default I²C address of the AD7745

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

  // Configure the AD7745
  Wire.beginTransmission(AD7745_ADDRESS);
  Wire.write(0x07); // Point to the configuration register
  Wire.write(0x30); // Set the device to continuous conversion mode
  Wire.endTransmission();

  Serial.println("AD7745 initialized.");
}

void loop() {
  // Read capacitance data from the AD7745
  Wire.beginTransmission(AD7745_ADDRESS);
  Wire.write(0x01); // Point to the capacitance data register
  Wire.endTransmission();

  Wire.requestFrom(AD7745_ADDRESS, 3); // Request 3 bytes of data
  if (Wire.available() == 3) {
    uint8_t msb = Wire.read(); // Most significant byte
    uint8_t lsb = Wire.read(); // Least significant byte
    uint8_t status = Wire.read(); // Status byte

    // Combine the MSB and LSB to form the 16-bit capacitance value
    int16_t capacitance = (msb << 8) | lsb;

    // Print the capacitance value
    Serial.print("Capacitance: ");
    Serial.print(capacitance);
    Serial.println(" (raw value)");
  }

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

Note: The above code assumes the AD7745 is configured with its default I²C address (0x48). Adjust the address if necessary.

1. No Response from the AD7745 on the I²C Bus:

   • Ensure the pull-up resistors (4.7 kΩ) are connected to the SCL and SDA lines.
   • Verify the I²C address matches the configuration of the ADDR pin.
   • Check the power supply voltage (VDD) and ensure it is within the specified range.

2. Inaccurate Capacitance Measurements:

   • Verify that the capacitive sensor is properly connected to the CAPDAC1 or CAPDAC2 pin.
   • Minimize noise by shielding the sensor and using short connections.
   • Ensure the excitation signal (EXC1/EXC2) is properly configured.

3. High Noise in Measurements:

   • Place a bypass capacitor (0.1 µF) close to the VDD pin.
   • Avoid placing the AD7745 near high-frequency switching components.

Q1: Can the AD7745 measure differential capacitance?
A1: Yes, the AD7745 can measure differential capacitance using its two input channels (CAPDAC1 and CAPDAC2).

Q2: What is the maximum capacitance range the AD7745 can measure?
A2: The AD7745 can measure capacitance changes within a range of ±4 pF with a resolution of 4 aF.

Q3: Can the AD7745 operate at 5 V?
A3: Yes, the AD7745 supports a supply voltage range of 2.7 V to 5.25 V.

Q4: Is the AD7745 suitable for battery-powered applications?
A4: Yes, the AD7745 has low power consumption (1 mA typical), making it suitable for battery-powered systems.