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AD7148 データシート(PDF) 11 Page - Analog Devices |
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AD7148 データシート(HTML) 11 Page - Analog Devices |
11 / 56 page 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. |
同様の部品番号 - AD7148 |
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同様の説明 - AD7148 |
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