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AD7142 Scheda tecnica(PDF) 25 Page - Analog Devices |
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AD7142 Scheda tecnica(HTML) 25 Page - Analog Devices |
25 / 70 page Data Sheet AD7142 Rev. B | Page 25 of 70 ADAPTIVE THRESHOLD AND SENSITIVITY The AD7142 provides an on-chip self-learning adaptive threshold and sensitivity algorithm. This algorithm continu- ously monitors the output levels of each sensor and automatically rescales the threshold levels proportionally to the sensor area covered by the user. As a result, the AD7142 maintains optimal threshold and sensitivity levels for all types of users regardless of their finger sizes. The threshold level is always referenced from the ambient level and is defined as the CDC converter output level that must be exceeded for a valid sensor contact. The sensitivity level is defined as how sensitive the sensor is before a valid contact is registered. Figure 36 provides an example of how the adaptive threshold and sensitivity algorithm works. The positive and negative sensor threshold levels are calculated as a percentage of the STAGE_ OFFSET_HIGH and STAGE_OFFSET_LOW values based on the threshold sensitivity settings and the ambient value. On configuration, initial estimates are supplied for both STAGE_OFFSET_HIGH and STAGE_OFFSET_LOW after which the calibration engine automatically adjusts the STAGE_HIGH_THRESHOLD and STAGE_LOW_ THRESHOLD values for sensor response. The AD7142 tracks the average maximum and minimum values measured from each sensor. These values give an indication of how the user is interacting with the sensor. A large finger gives a large average maximum or minimum value, and a small finger gives smaller values. When the average maximum or minimum value changes, the threshold levels are rescaled to ensure that the threshold levels are appropriate for the current user. Figure 37 shows how the minimum and maximum sensor responses are tracked by the on-chip logic. Reference A in Figure 36 shows an undersensitive threshold level for a small finger user, demonstrating the disadvantages of a fixed threshold level. By enabling the adaptive threshold and sensitivity algorithm, the positive and negative threshold levels are determined by the POS_THRESHOLD_SENSITIVITY and NEG_THRESHOLD_ SENSITIVITY register values and the most recent average maximum sensor output value. These registers can be used to select 16 different positive and negative sensitivity levels ranging between 25% and 95.32% of the most recent average maximum output level referenced from the ambient value. The smaller the sensitivity percentage setting, the easier it is to trigger a sensor activation. Reference B shows that the positive adaptive threshold level is set at almost mid-sensitivity with a 62.51% threshold level by setting POS_THRESHOLD_ SENSITIVITY = 1000. Figure 36 also provides a similar example for the negative threshold level with NEG_ THRESHOLD_SENSITIVITY = 0001. AMBIENT LEVEL AVERAGE MAX VALUE STAGE_OFFSET_HIGH 25% 95.32% 62.51% = POS ADAPTIVE THRESHOLD LEVEL 25% 62.51% = POS ADAPTIVE THRESHOLD LEVEL 95.32% NEG ADAPTIVE THRESHOLD LEVEL = 39.08% 25% 95.32% 25% 95.32% SENSOR CONTACTED BY SMALL FINGER AVERAGE MAX VALUE STAGE_OFFSET_LOW NEG ADAPTIVE THRESHOLD LEVEL = 39.08% SENSOR CONTACTED BY LARGE FINGER STAGE_OFFSET_HIGH IS UPDATED STAGE_OFFSET_HIGH IS UPDATED HERE STAGE_OFFSET_LOW IS UPDATED HERE STAGE_OFFSET_LOW IS UPDATED HERE A B Figure 36. Threshold Sensitivity Example with POS_THRESHOLD_SENSITIVITY = 1000 and NEG_THRESHOLD_SENSITIVITY = 0011 |
Codice articolo simile - AD7142_17 |
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Descrizione simile - AD7142_17 |
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