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 AD7745 features a low-noise architecture and is ideal for applications requiring precise capacitive sensing.

• Touch Sensing: Used in touch-sensitive interfaces for consumer electronics.
• Liquid Level Detection: Ideal for non-contact liquid level measurement in industrial and medical applications.
• Proximity Sensing: Used in proximity detection systems for automation and security.
• Environmental Monitoring: Suitable for humidity and pressure sensing when paired with appropriate sensors.

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

The AD7745 is available in a 16-lead TSSOP package. The pin configuration and descriptions are as follows:

1. Power Supply: Connect the VDD pin to a regulated power supply (2.7 V to 5.25 V) and the GND pin to the ground.
2. Capacitive Sensor Connection: Attach the capacitive sensor to the CAPDAC1 and CAPDAC2 pins. Use the EXC1 and EXC2 pins to provide excitation signals to the sensor.
3. 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.
4. I²C Address Selection: Configure the I²C address using the ADDR pin. This allows multiple AD7745 devices to share the same I²C bus.
5. Bypass Capacitor: Place a 0.1 µF ceramic capacitor close to the VDD pin for power supply decoupling.

• Capacitive Sensor Design: Ensure the sensor is designed to operate within the ±4 pF range of the AD7745 for optimal performance.
• PCB Layout: Minimize noise by keeping the capacitive sensor traces short and away from high-frequency signals.
• I²C Pull-Up Resistors: Use appropriate pull-up resistor values for the I²C lines based on the bus capacitance and operating speed.

The following example demonstrates how to interface the AD7745 with an Arduino UNO using the I²C protocol:

#include <Wire.h>

// AD7745 I²C address (default is 0x48, depending on ADDR pin configuration)
#define AD7745_ADDRESS 0x48

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

  // Configure AD7745 (example: set to continuous conversion mode)
  Wire.beginTransmission(AD7745_ADDRESS);
  Wire.write(0x07); // Write to the configuration register
  Wire.write(0x30); // Continuous conversion mode, default settings
  Wire.endTransmission();

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

void loop() {
  // Read capacitance data from 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 (optional)

    // Combine MSB and LSB to get the raw capacitance value
    int16_t capacitance = (msb << 8) | lsb;

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

  delay(1000); // Wait 1 second before the next reading
}

1. No Response from AD7745 on I²C Bus

   • Cause: Incorrect I²C address or wiring.
   • Solution: Verify the ADDR pin configuration and ensure proper connections to the SCL and SDA lines.

2. Inaccurate Capacitance Measurements

   • Cause: Noise or improper sensor design.
   • Solution: Minimize noise by improving PCB layout and shielding the sensor. Ensure the sensor operates within the ±4 pF range.

3. Device Overheating

   • Cause: Excessive supply voltage or incorrect connections.
   • Solution: Ensure the supply voltage is within the 2.7 V to 5.25 V range and check all connections.

1. Can the AD7745 measure differential capacitance?

   • Yes, the AD7745 can measure differential capacitance using its CAPDAC1 and CAPDAC2 inputs.

2. What is the maximum I²C speed supported by the AD7745?

   • The AD7745 supports I²C speeds up to 400 kHz (Fast Mode).

3. Can I use the AD7745 with a 5 V microcontroller?

   • Yes, the AD7745 is compatible with 5 V systems as long as the supply voltage (VDD) is within the specified range.

4. How do I calibrate the AD7745?

   • Calibration can be performed by adjusting the CAPDAC settings to offset any baseline capacitance.

By following this documentation, users can effectively integrate the AD7745 into their projects for high-precision capacitive sensing applications.