AD7879 datasheet(16/36 Pages) AD

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AD7879

Rev. 0 | Page 16 of 36

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 posi-

tional measurements.

The AD7879 contains a filtering block to process the data and

discard the spurious noise before sending the information to

the host. The goal of this block is not just the suppression of

noise; the on-chip filtering also remarkably 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 measure-

ments 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 (M1, M0)

set the window of the median filter, and therefore, the number

of measurements taken.

Table 6. Median Filter Size

Function

Median filter does not operate

4 measurements

8 measurements

16 measurements

The averaging filter size determines the number of values to

average. Bit 8 and Bit 7 in Control Register 2 (A1, A0) allow the

average of 2, 4, 8, or 16 samples. Only the final averaged result is

written into the results register.

Table 7. Averaging filter Size

Function

Average of 2 middle samples

Average of 4 middle samples

Average of 8 middle samples

Average of 16 samples

When both filter values are 00, only one measurement is

transferred to the register map.

The number chosen with the M1 and M0 settings must be

equal to or larger than the number chosen with the A1 and

A0 settings. If both settings select the same number, the median

filter is switched off.

Table 8. Median Averaging Filters (MAVF) Settings

Function

M = A

Median filter does not operate; output is the

average of A converted results

M < A

Not possible because the median filter size is

always bigger than the averaging window size

M > A

Output is the average of the middle A values

from the array of M measurements

Example

M1, M0 = 11, A1, A0 = 10; in this example 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 case is 8. The output is an

average of the middle 8 values of the 16 measurements taken

with the median filter.

AVERAGING

FILTER

MEDIAN

FILTER

12-BIT SAR

ADC

M = 16

A = 8

CONVERTED

RESULTS

16 MEASUREMENTS

ARRANGED

AVERAGE OF

MIDDLE 8 VALUES

Figure 27. Median and Averaging Filter Example

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