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AD7147 Datenblatt(PDF) 20 Page - Analog Devices |
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AD7147 Datenblatt(HTML) 20 Page - Analog Devices |
20 / 71 page Data Sheet AD7147 NONCONTACT PROXIMITY DETECTION The AD7147 internal signal processing continuously monitors all capacitance sensors for noncontact proximity detection. This feature provides the ability to detect when a user is approaching a sensor, at which time all internal calibration is immediately disabled while the AD7147 is automatically configured to detect a valid contact. The proximity control register bits are described in Table 12. The FP_PROXIMITY_CNT and LP_PROXIMITY_CNT register bits control the length of the calibration disable period after the user stops touching the sensor and is not in close proximity to the sensor during full or low power mode. The calibration is disabled during this period and then enabled again. Figure 30 and Figure 31 show examples of how these registers are used to set the calibration disable periods for the full and low power modes. The calibration disable period in full power mode is the value of the FP_PROXIMITY_CNT multiplied by 16 multiplied by the time for one conversion sequence in full power mode. The calibration disable period in low power mode is the value of the LP_PROXIMITY_CNT multiplied by 4 multiplied by the time for one conversion sequence in low power mode. RECALIBRATION In certain situations (for example, when a user hovers over a sensor for a long time), the proximity flag can be set for a long period. The environmental calibration on the AD7147 is sus- pended while proximity is detected, but changes may occur to the ambient capacitance level during the proximity event. This means that the ambient value stored on the AD7147 no longer represents the actual ambient value. In this case, even when the user is not in close proximity to the sensor, the prox- imity flag may still be set. This situation can occur if the user interaction creates some moisture on the sensor, causing the new sensor ambient value to be different from the expected value. In this situation, the AD7147 automatically forces a recalibration internally. This ensures that the ambient values are recalibrated, regardless of how long the user hovers over the sensor. A recalibration ensures maximum AD7147 sensor performance. The AD7147 recalibrates automatically when the measured CDC value exceeds the stored ambient value by an amount determined by the PROXIMITY_RECAL_LVL bits for a set period of time known as the recalibration timeout. In full power mode, the recali- bration timeout is controlled by FP_PROXIMITY_RECAL; in low power mode, by LP_PROXMTY_RECAL. The recalibration timeout in full power mode is the value of the FP_PROXIMITY_RECAL multiplied by the time for one conversion sequence in full power mode. The recalibration time- out in low power mode is the value of the LP_PROXIMITY_ RECAL multiplied by the time for one conversion sequence in low power mode. Figure 32 and Figure 33 show examples of how the FP_PROXIMITY_RECAL and LP_PROXIMITY_RECAL register bits control the timeout period before a recalibration while operating in the full and low power modes. In these examples, a user approaches a sensor and then leaves, but the proximity detection remains active. The measured CDC value exceeds the stored ambient value by the amount set in the PROXIMITY_RECAL_LVL bits for the entire timeout period. The sensor is automatically recalibrated at the end of the timeout period. PROXIMITY SENSITIVITY The fast filter in Figure 34 is used to detect when someone is close to the sensor (proximity). Two conditions, detected by Compa- rator 1 and Comparator 2, set the internal proximity detection signal: Comparator 1 detects when a user is approaching or leaving a sensor, and Comparator 2 detects when a user hovers over a sensor or approaches a sensor very slowly. The sensitivity of Comparator 1 is controlled by the PROXIMITY_DETECTION_RATE bits. For example, if PROXIMITY_DETECTION_RATE is set to 4, the Proximity 1 signal is set when the absolute difference between WORD1 and WORD3 exceeds (4 × 16) LSB codes. The PROXIMITY_RECAL_LVL bits (Address 0x003) control the sensitivity of Comparator 2. For example, if PROXIMITY_ RECAL_LVL is set to 75, the Proximity 2 signal is set when the absolute difference between the fast filter average value and the ambient value exceeds (75 × 16) LSB codes. Rev. E | Page 19 of 70 |
Ähnliche Teilenummer - AD7147_17 |
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Ähnliche Beschreibung - AD7147_17 |
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