Using the AD7150 Capacitance-to-Digital Converter (CDC) for Proximity Sensing Applications
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.
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: Typical Proximity Performance of Sensor 1 on the Door Handle Demonstration Board
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.
|AD7150||Ultra-Low Power, 2-Channel, Capacitance Converter for Proximity Sensing||
|ADP1720||50 mA, High Voltage, Micropower Linear Regulator||