AD7879 Analog Devices, AD7879 Datasheet - Page 17

• Manufacturer Part Number: AD7879
• Description: Low Voltage Controller for Touch Screens
• Manufacturer: Analog Devices

Specifications of AD7879

• Resolution (bits): 12bit
• # Chan: 6
• Sample Rate: 105kSPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni Vdd
• Adc Architecture: SAR
• Pkg Type: CSP,WLCSP

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 infor-
mation 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 measure-
ment. Bit 6 and Bit 5 in Control Register 2 (MED1, MED0) set
the window of the median filter and, therefore, the number of
measurements taken.
Table 8. Median Filter Size
MED1
0
0
1
1
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
0
0
1
1
MED0
0
1
0
1
AVG0
0
1
0
1
Number of Measurements
Median filter disabled
4
8
16
Filter Size
Average of 2 middle samples
Average of 4 middle samples
Average of 8 middle samples
Average of 16 samples
Rev. C | Page 17 of 40
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
M = A
M > A
M < A
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.
It takes approximately 2 μs to sort the data in the rank filter
(t
SORT
CONVERTED
in Figure 34); t
12-BIT SAR
RESULTS
ADC
13
16
15
10
12
14
11
6
2
4
5
9
3
8
1
7
Figure 27. Median and Averaging Filter Example
Function
Median filter is disabled; output is the average of
A converted results
Output is the average of the middle A values from
the array of M measurements
Not possible because the median filter size is always
larger than the averaging window size
16 MEASUREMENTS
SORT
ARRANGED
adds to the update rate of the AD7879.
MEDIAN
FILTER
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
M = 16
AD7879/AD7889
MIDDLE 8 VALUES
AVERAGE OF
AVERAGING
FILTER
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
A = 8