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AD7151BRMZ Fiches technique(PDF) 11 Page - Analog Devices |
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AD7151BRMZ Fiches technique(HTML) 11 Page - Analog Devices |
11 / 28 page AD7151 Rev. 0 | Page 11 of 28 COMPARATOR AND THRESHOLD MODES The AD7151 comparator and its threshold can be programmed to operate in several different modes. In an adaptive mode, the threshold is dynamically adjusted and the comparator output indicates fast changes and ignores slow changes in the input (sensor) capacitance. Alternatively, the threshold can be programmed as a constant (fixed) value, and the output then indicates any change in the input capacitance that crosses the defined fixed threshold. The AD7151 logic output (active high) indicates either a positive or a negative change in the input capacitance, in both adaptive and fixed threshold modes (see Figure 23 and Figure 24). POSITIVE THRESHOLD INPUT CAPACITANCE OUTPUT OUTPUT ACTIVE TIME POSITIVE CHANGE Figure 23. Positive Threshold Mode Indicates Positive Change in Input Capacitance NEGATIVE THRESHOLD INPUT CAPACITANCE OUTPUT OUTPUT ACTIVE TIME NEGATIVE CHANGE Figure 24. Negative Threshold Mode Indicates Negative Change in Input Capacitance Additionally, for the adaptive mode only, the comparator can work as window comparator, indicating input either inside or outside a selected sensitivity band (see Figure 25 and Figure 26). POSITIVE THRESHOLD NEGATIVE THRESHOLD INPUT CAPACITANCE OUTPUT OUTPUT ACTIVE TIME INPUT INSIDE THRESHOLD WINDOW Figure 25. In-Window (Adaptive) Threshold Mode POSITIVE THRESHOLD NEGATIVE THRESHOLD INPUT CAPACITANCE OUTPUT OUTPUT ACTIVE TIME INPUT OUTSIDE THRESHOLD WINDOW Figure 26. Out-Window (Adaptive) Threshold Mode ADAPTIVE THRESHOLD In an adaptive mode, the thresholds are dynamically adjusted, ensuring indication of fast changes (for example an object moving close to a capacitive proximity sensor) and eliminating slow changes in the input (sensor) capacitance, usually caused by environment changes such as humidity or temperature or changes in the sensor dielectric material over time (see Figure 27). THRESHOLD INPUT CAPACITANCE OUTPUT OUTPUT ACTIVE TIME FAST CHANGE SLOW CHANGE Figure 27. Adaptive Threshold Indicates Fast Changes and Eliminates Slow Changes in Input Capacitance DATA AVERAGE The adaptive threshold algorithm is based on an average calculated from previous CDC output data. The response of the average to an input capacitance step change (more exactly, response to the change in the CDC output data) is an exponential settling curve, which can be characterized by the following equation: ) 1 ( ) 0 ( ) ( / TimeConst N e Change Average N Average − + = where: Average(N) is the value of average N complete CDC conversion cycles after a step change on the input. Average(0) is the value before the step change. TimeConst can be selected in the range between 2 and 65,536, in steps of power of 2, by programming the ThrSettling bits in the setup register. See Figure 28 and the Register Descriptions section. INPUT CAPACITANCE (CDC DATA) CHANGE DATA AVERAGE RESPONSE TIME Figure 28. Data Average Response to Data Step Change |
Numéro de pièce similaire - AD7151BRMZ |
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