AD7147 datasheet(12/68 Pages) AD

12 / 68 page

AD7147

Rev. 0 | Page 12 of 68

Registering a Sensor Activation

When a user approaches a sensor, the total capacitance associated

with that sensor changes and is measured by the AD7147. If

the change causes a set threshold to be exceeded, the AD7147

interprets this as a sensor activation.

On-chip threshold limits are used to determine when a sensor

activation occurs. Figure 20 shows the change in CDC_RESULT_Sx

when a user activates a sensor. The sensor is deemed to be active

only when the value of CDC_RESULT_Sx is either greater than the

value of STAGEx_HIGH_THRESHOLD or less than the value

of STAGEx_LOW_THRESHOLD.

STAGEx_HIGH_THRESHOLD

STAGEx_LOW_THRESHOLD

SENSOR ACTIVE (A)

SENSOR ACTIVE (B)

CDC_RESULT_Sx

AMBIENT OR

NO-TOUCH VALUE

Figure 20. Sensor Activation Thresholds

In Figure 20, two sensor activations are shown. Sensor Active A

occurs when a sensor is connected to the positive input of the

converter. In this case, when a user activates the sensor, there is an

increase in CDC code, and the value of CDC_RESULT_Sx exceeds

that of STAGEx_HIGH_THRESHOLD. Sensor Active B occurs

when the sensor is connected to the negative input of the converter.

In this case, when a user activates the sensor, there is a decrease

in CDC code, and the value of CDC_RESULT_Sx becomes less

than the value of STAGEx_LOW_THRESHOLD.

For each conversion stage, the STAGEx_HIGH_THRESHOLD

and STAGEx_LOW_THRESHOLD registers are in Register Bank

3. The values in these registers are updated automatically by the

AD7147 due to its environmental calibration and adaptive

threshold logic.

At power-up, the values in the STAGEx_HIGH_THRESHOLD

and STAGEx_LOW_THRESHOLD registers are the same as those

in the STAGEx_OFFSET_HIGH and STAGEx_OFFSET_LOW

registers in Bank 2. The user must program the STAGEx_OFFSET

_HIGH and STAGEx_OFFSET_LOW registers on device power-

up. See the Environmental Calibration section of the data sheet

for more information.

Complete Solution for Capacitance Sensing

Analog Devices, Inc., provides a complete solution for

capacitance sensing. The two main elements to the solution are

the sensor PCB and the AD7147.

If the application requires high resolution sensors such as scroll

bars or wheels, software is required that runs on the host

processor. The memory requirements for the host depend on

the sensor and are typically 10 kB of code and 600 bytes of data

memory, depending on the sensor type.

AD7147

SENSOR PCB

SPI OR I2C

HOST PROCESSOR

1 MIPS

10kB ROM

600 BYTES RAM

Figure 21. Three-Part Capacitance-Sensing Solution

Analog Devices supplies the sensor PCB footprint design

libraries to the customer and supplies any necessary software on

an open-source basis.

BIAS PIN

This pin is connected internally to a bias node of the AD7147.

To ensure correct operation of the AD7147 connect a 10 nF

capacitor between the BIAS pin and ground. The voltage seen at

the BIAS pin is VCC/2.

OPERATING MODES

The AD7147 has three operating modes. Full power mode, where

the device is always fully powered, is suited for applications where

power is not a concern (for example, game consoles that have an

ac power supply). Low power mode, where the part automatically

powers down when no senosr is active, is tailored to provide

significant power savings compared with full power mode and

is suited for mobile applications, where power must be

conserved. In shutdown mode, the part shuts down completely.

The POWER_MODE bits (Bit 0 and Bit 1) of the control

settings for each operating mode. To put the AD7147 into

shutdown mode, set the POWER_MODE bits to either 01 or 11.

Table 8. POWER_MODE Settings

POWER_MODE Bits

Operating Mode

Full power mode

Shutdown mode

Low power mode

Shutdown mode

The power-on default setting of the POWER_MODE bits is 00,

full power mode.

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