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AD7147 Datasheet(PDF) 23 Page - Analog Devices |
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AD7147 Datasheet(HTML) 23 Page - Analog Devices |
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23 / 68 page  AD7147 Rev. 0 | Page 23 of 68 ENVIRONMENTAL CALIBRATION The AD7147 provides on-chip capacitance sensor calibration to automatically adjust for environmental conditions that have an effect on the ambient levels of the capacitance sensor. The output levels of the capacitance sensor are sensitive to temperature, humidity, and, in some cases, dirt. The AD7147 achieves optimal and reliable sensor performance by continuously monitoring the CDC ambient levels and compensating for any environmental changes by adjusting the values of the STAGEx_HIGH_THRESHOLD and STAGEx_LOW_THRESHOLD registers as described in Equation 1 and Equation 2. The CDC ambient level is defined as the output level of the capacitance sensor during periods when the user is not approaching or in contact with the sensor. After the AD7147 is configured, the compensation logic runs automatically with each conversion when the AD7147 is not being touched. This allows the AD7147 to compensate for rapidly changing environmental conditions. The ambient compensation control registers provide the host with access to general setup and controls for the compensation algorithm. On-chip RAM stores the compensation data for each conversion stage, as well as setup information specific for each stage. Figure 34 shows an example of the ideal behavior of a capacitance sensor, where the CDC ambient level remains constant regardless of the environmental conditions. The CDC output shown is for a pair of differential button sensors, where one sensor caused an increase and the other caused a decrease in measured capacitance when activated. The positive and negative sensor threshold levels are calculated as a percentage of the STAGEx_OFFSET_HIGH and STAGEx_OFFSET_LOW values and are based on the threshold sensitivity settings and the ambient value. These values are sufficient to detect a sensor contact and result in the AD7147 asserting the INT output when the threshold levels are exceeded. t STAGEx_LOW_THRESHOLD STAGEx_HIGH_THRESHOLD CDC AMBIENT VALUE CHANGING ENVIRONMENTAL CONDITIONS SENSOR 1 INT ASSERTED SENSOR 2 INT ASSERTED Figure 34. Ideal Sensor Behavior with a Constant Ambient Level CAPACITANCE SENSOR BEHAVIOR WITHOUT CALIBRATION Figure 35 shows the typical behavior of a capacitance sensor when calibration is not applied. This figure shows ambient levels drifting over time as environmental conditions change. As a result of the initial threshold levels remaining constant while the ambient levels drift upward, Sensor 2 fails to detect a user contact in this example. The Capacitance Sensor Behavior with Calibration section describes how the AD7147 adaptive calibration algorithm prevents such errors from occurring. t CDC AMBIENT VALUE DRIFTING CHANGING ENVIRONMENTAL CONDITIONS STAGEx_HIGH_THRESHOLD STAGEx_LOW_THRESHOLD SENSOR 1 INT ASSERTED SENSOR 2 INT NOT ASSERTED Figure 35. Typical Sensor Behavior Without Calibration Y SENSITIVIT THRESHOLD POS HIGH OFFSET STAGEx HIGH OFFSET STAGEx HIGH OFFSET STAGEx AMBIENT SF STAGEx THRESHOLD HIGH STAGEx _ _ 16 4 _ _ _ _ 4 _ _ _ _ _ _ × − + + = ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ Equation 1. On-Chip Logic Stage High Threshold Calculation Y SENSITIVIT THRESHOLD NEG LOW OFFSET STAGEx LOW OFFSET STAGEx LOW OFFSET STAGEx AMBIENT SF STAGEx THRESHOLD LOW STAGEx _ _ 16 4 _ _ _ _ 4 _ _ _ _ _ _ × − + + = ⎟ ⎟ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ Equation 2. On-Chip Logic Stage Low Threshold Calculation |
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