AD7148 datasheet(11/56 Pages) AD

部品番号	AD7148

部品情報	Programmable Touch Controller for Single Electrode Capacitance Sensors

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AD7148

Rev. 0 | Page 11 of 56

Registering a Sensor Activation

When a user approaches a sensor, the total capacitance associated

with that sensor changes and is measured by the AD7148. When

the capacitance changes to such an extent that a set threshold

is exceeded, the AD7148 registers this as a sensor activation.

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

vation occurs. Figure 17 shows the change in CDC_RESULT_Sx

that occurs 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

AMBIENT OR

NO-TOUCH VALUE

SENSOR ACTIVE (A)

CDC_RESULT_Sx

SENSOR ACTIVE (B)

Figure 17. Sensor Activation Thresholds

In Figure 17, two different sensor activations are shown. Sensor

Activate (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 the CDC code, and the value of

CDC_RESULT_Sx exceeds the value 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 the 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 the STAGEx_LOW_THRESHOLD registers are in Register

Bank 3. The values in these registers are updated automatically

by the AD7148 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 Register Bank 2. The user must program the STAGEx_

OFFSET_HIGH and STAGEx_OFFSET_LOW registers on device

power-up. See the Environmental Calibration section for more

information.

Complete Solution for Capacitance Sensing

Analog Devices, Inc., provides a complete solution for capacitance

sensing. The two main elements of the solution are the sensor PCB

and the AD7148.

If the application requires high resolution sensors such as scroll

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

No position algorithm is required for button sensors.

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.

AD7148

SPI OR I2C

SENSOR PCB

HOST PROCESSOR

1 MIPS

10kB ROM

600 BYTES RAM

Figure 18. 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

The BIAS pin (Pin 6) is connected internally to the bias node in

the AD7148. To ensure correct operation of the AD7148, connect

a 10 nF capacitor between the BIAS pin and ground. The voltage at

the BIAS pin is VCC/2.

OPERATING MODES

The AD7148 has three operating modes. Full power mode, in

which 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, in which the part

automatically powers down, is tailored to give significant power

savings over 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 of the PWR_CONTROL register

(Address 0x000[1:0]) set the operating mode on the AD7148.

Table 7 shows the POWER_MODE settings for each operating

mode. To put the AD7148 into shutdown mode, set the

POWER_MODE bits to either 01 or 11.

Table 7. POWER_MODE Settings

POWER_MODE Bits

Operating Mode

00

Full power mode

01

Shutdown mode

10

Low power mode

11

Shutdown mode

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

full power mode.

Full Power Mode

In full power mode, all sections of the AD7148 remain fully

powered and converting at all times. While a sensor is being

touched, the AD7148 processes the sensor data. If no sensor is

touched, the AD7148 measures the ambient capacitance level

and uses this data for the on-chip compensation routines. In full

power mode, the AD7148 converts at a constant rate. See the CDC

Conversion Sequence Time section for more information.

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