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AD7889 Fiches technique(PDF) 17 Page - Analog Devices |
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AD7889 Fiches technique(HTML) 17 Page - Analog Devices |
17 / 40 page Data Sheet AD7879/AD7889 Rev. D | Page 17 of 40 MEDIAN AND AVERAGING FILTERS As explained in the Touch Screen Principles section, touch screens are composed of two resistive layers, normally placed over an LCD screen. Because these layers are in close proximity to the LCD screen, noise can be coupled from the screen onto these resistive layers, causing errors in the touch screen positional measurements. The AD7879/AD7889 contain a filtering block to process the data and discard the spurious noise before sending the information to the host. The purpose of this block is not only the suppression of noise; the on-chip filtering also greatly reduces the host processing loading. The processing function consists of two filters that are applied to the converted results: the median filter and the averaging filter. The median filter suppresses the isolated out of range noise and sets the number of measurements to be taken. These measurements are arranged in a temporary array, where the first value is the smallest measurement and the last value is the largest measurement. Bit 6 and Bit 5 in Control Register 2 (MED1 and MED0, respectively) set the window of the median filter and, therefore, the number of measurements taken. Table 8. Median Filter Size MED1 MED0 Number of Measurements 0 0 Median filter disabled 0 1 4 1 0 8 1 1 16 The averaging filter size determines the number of values to average. Bit 8 and Bit 7 in Control Register 2 (AVG1, AVG0) set the average to 2, 4, 8, or 16 samples. Only the final averaged result is written into the result register. Table 9. Averaging Filter Size AVG1 AVG0 Filter Size 0 0 Average of 2 middle samples 0 1 Average of 4 middle samples 1 0 Average of 8 middle samples 1 1 Average of 16 samples When both filter values are 00, only one measurement is transferred to the register map. The number specified with the MED1 and MED0 settings must be greater than or equal to the number specified with the AVG1 and AVG0 settings. If both settings specify the same number, the median filter is switched off. Table 10. Median Averaging Filters (MAVF) Settings Setting Function M = A Median filter is disabled; output is the average of A converted results M > A Output is the average of the middle A values from the array of M measurements M < A Not possible because the median filter size is always larger than the averaging window size Example In this example, MED1, MED0 = 11 and AVG1, AVG0 = 10; the median filter has a window size of 16. This means that 16 measurements are taken and arranged in descending order in a temporary array. The averaging window size in this example is 8. The output is the average of the middle eight values of the 16 measurements taken with the median filter. AVERAGING FILTER MEDIAN FILTER 12-BIT SAR ADC 6 2 13 4 16 5 15 10 9 3 11 8 1 12 14 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 M = 16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 A = 8 CONVERTED RESULTS 16 MEASUREMENTS ARRANGED AVERAGE OF MIDDLE 8 VALUES Figure 27. Median and Averaging Filter Example It takes approximately 2 μs to sort the data in the rank filter (tSORT in Figure 33); tSORT adds to the update rate of the AD7879. |
Numéro de pièce similaire - AD7889 |
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Description similaire - AD7889 |
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