CN0095: Using the AD7150 Capacitance-to-Digital Converter (CDC) for Proximity Sensing Applications

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OVERVIEW

Circuit Note PDF, 11/2010 (pdf, 147 kB)
Benefits & Features
  • Capacitance-to-Digital Converter for Proximity Sensing Applications
  • Designed for Automotive Door Handles
  • Optimized for High Resolution
Products Used
    Applications: 
  • Sensors and Sensor Interface
  • White Goods & Vending
  • Access Control
  • Asset Tracking
Design Resources
Device Drivers
Software, such as C code and/or FPGA code, used to communicate with a component's digital interface.

CIRCUIT FUNCTION AND BENEFITS

The circuit described in this document provides the basis for developing a proximity sensing application using the AD7150 capacitance-to-digital converter (CDC).

The AD7150 CDC measures the capacitance between two electrodes and compares its measurement result with a threshold value, which can be either fixed or dynamically adjusted by the on-chip adaptive threshold algorithm engine.

If the input capacitance is altered, for example, by the presence of a hand, an output flag is set to indicate that a threshold has been exceeded, thus indicating proximity.

This on-chip adaptive threshold algorithm engine also enables the AD7150 to adapt to slow changes in the sensing capacitance, which may be caused by environmental changes, such as humidity or temperature, without losing the capability of proximity sensing.

Figure 1: AD7150 as a Proximity Detector in the Standalone Operation

CIRCUIT DESCRIPTION

A proximity sensing application using AD7150 in the standalone operation requires very few peripheral components, as shown in Figure 1. The curcuit needs a supply voltage (Battery B1), some filtering of the supply voltage (R1, C1), and weak pull-ups (R2, R3) on the I2C® compatible I/O pins. The red LED, D1, provides a visual indicator that the AD7150 has detected the proximity, for example, of a hand. The circuit requires a capacitive sensing element (SENS1), which can simply consist of two tracks on an FR4 PCB, as shown in Figure 2.

Figure 2: AD7150 Proximity Detector Demonstration Board

COMMON VARIATIONS

Variations in the AD7150 proximity circuit depend on the environment and the targeted application. For example, automotive applications must withstand a high level of EMC noise and transient pulses at the system level. Therefore, this type of application requires a design suitable for harsh electrical and physical environments.

The AD7150’s unique design for measuring floating capacitive sensors allows placing a filter structure in the capacitive front-end. The filter structure (R1 to R6, C1 to C6), as shown in Figure 3, filters noise coupled into the electrodes of the sensor. The optional network consisting of R7, R8, C7, and C8 prevents noise from the external I2C-compatible interface from coupling back into the circuit.

Figure 3: AD7150 in an Automotive Door Handle Application in the Standalone Operation

Substantial EMC testing has been performed on the AD7150. The results of the AD7150 EMC performance can be found in the AN-1011 Application Note, EMC Protection of the AD7150.

The excitation voltages (EXC1, EXC2), which drive the capactive sensors, are generated by circuits within the AD7150. These circuits are powered from VDD. Therefore, a noisy supply voltage can result in unwanted noise signals on the capacitive input.

The voltage supply circuit shown in Figure 3. uses the ADP1720 LDO (used in the 3.3 V mode) to filter battery noise and to suppress transient pulses in automotive applications.

If the outputs of the AD7150 are not connected directly to a microcontroller, they may require conditioning to translate the voltage level and/or signal polarity. Typical conditioning circuits for OUT1 and OUT2 are shown in Figure 3. DMOS FETs (Q1 and Q2) act as open drain output drivers, and the 27 V varistors (V1 and V2) protect the circuitry from large external transients.

When connected to a microcontroller, some of the AD7150 registers used by the on-chip adaptive threshold algorithm engine can be programmed to settings other than the power-up default settings. This is done via the I2C compatible interface and enables the AD7150 to be used for different applications with different requirements. See the AD7150 data sheet for more details.

Table 1 and Table 2 show a typical proximity performance of the door handle demonstration with different sensitivity and capacitive input range settings.

Table 1

Table 1: Typical Proximity Performance of Sensor 1 on the Door Handle Demonstration Board

Table 2

Table 2: Typical Proximity Performance of Sensor 2 on the Door Handle Demonstration Board

Figure 4: AD7150 Door Handle Demonstration Board

The AD7150’s unique design for measuring floating capacitive sensors makes the AD7150 tolerant of parasitic capacitances to ground. This allows the use of ground planes to either shield the capacitive front-end signals from other analog or digital signals on the board or to shield them from each other. Figure 4 shows the AD7150 door handle demonstration board where Sensor 2 on the door handle demonstration board has a ground plane on the entire top layer to prevent proximity detection when a person leans against the door handle of a car. The sensor electrodes are placed on the bottom layer in the same way as shown for Sensor 1. Therefore, Sensor 2 detects proximity only when a hand reaches behind the door handle.

SAMPLE PRODUCTS USED IN THIS CIRCUIT

Product Description Available Product Models to Sample
AD7150 Ultra-Low Power, 2-Channel, Capacitance Converter for Proximity Sensing AD7150BRMZ
ADP1720 50 mA, High Voltage, Micropower Linear Regulator ADP1720ARMZ-3.3-R7 ADP1720ARMZ-5-R7 ADP1720ARMZ-R7
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