AD7889 datasheet(17/40 Pages) AD

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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

Median filter disabled

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

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 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

M = 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.

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