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AD7879 データシート(PDF) 15 Page - Analog Devices

部品番号 AD7879
部品情報  Low Voltage Controller for Touch Screens
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メーカー  AD [Analog Devices]
ホームページ  http://www.analog.com
Logo AD - Analog Devices

AD7879 データシート(HTML) 15 Page - Analog Devices

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Data Sheet
AD7879/AD7889
Rev. D | Page 15 of 40
TOUCH PRESSURE MEASUREMENT
The pressure applied to the touch screen by a pen or a finger can
also be measured with the AD7879/AD7889 by using simple
calculations. The contact resistance between the X and Y plates
is measured, providing a good indication of the size of the
depressed area and, therefore, the applied pressure. The area of
the spot that is touched is proportional to the size of the object
touching it. The size of this resistance (RTOUCH) can be calculated
using two different methods.
First Method for Calculating the Size of RTOUCH
The first method for calculating RTOUCH requires the user to know
the total resistance of the X-plate tablet (RX). Three touch screen
conversions are required: the measurement of the X position,
XPOSITION (Y+ input); the measurement of the X+ input with the
excitation voltage applied to Y+ and X− (Z1 measurement); and
the measurement of the Y− input with the excitation voltage
applied to Y+ and X− (Z2 measurement). These three measure-
ments are shown in Figure 26.
The AD7879/AD7889 has two special ADC channel settings that
configure the X and Y switches for the Z1 and Z2 measurements
and store the results in the Z1 and Z2 result registers. The Z1
measurement is selected by setting the CHNL ADD[2:0] bits to
101 in Control Register 1 (Address 0x01); the result is stored in
the X+ (Z1) result register (Address 0x0A). The Z2 measurement is
selected by setting the CHNL ADD[2:0] bits to 100 in Control
Register 1 (Address 0x01); the result is stored in the Y− (Z2)
result register (Address 0x0B).
The touch resistance (RTOUCH) can then be calculated using
Equation 2:
RTOUCH = (RXPLATE) × (XPOSITION/4096) × ((Z2/Z1) − 1)
(2)
Y–
Y+
X–
X+
TOUCH
RESISTANCE
MEASURE
Z1 POSITION
X–
X+
Y–
Y+
TOUCH
RESISTANCE
MEASURE
X POSITION
Y–
Y+
X–
X+
TOUCH
RESISTANCE
MEASURE
Z2 POSITION
Figure 26. Three Measurements Required for Touch Pressure
Second Method for Calculating the Size of RTOUCH
The second method for calculating RTOUCH requires the user to
know the resistance of the X-plate and Y-plate tablets. Three
touch screen conversions are required: a measurement of the
X position (XPOSITION), the Y position (YPOSITION), and the Z1
position.
Equation 3 also calculates the touch resistance (RTOUCH):
RTOUCH = RXPLATE × (XPOSITION/4096) × ((4096/Z1) − 1) −
RYPLATE × (1 − (YPOSITION/4096))
(3)
TEMPERATURE MEASUREMENT
A temperature measurement option called the single-conversion
method is available on the AD7879/AD7889. The conversion
method requires only a single measurement on ADC Channel
001. The results are stored in the temperature conversion result
register (Address 0x0D). The AD7879/AD7889 do not provide
an explicit output of the temperature reading; the system must
perform some external calculations. This method is based on
an on-chip diode measurement.
The acquisition time is fixed at 16 ms for temperature
measurement.
Conversion Method
The conversion method makes use of the fact that the tempera-
ture coefficient of a silicon diode is approximately −2.1 mV/°C.
However, this small change is superimposed on the diode forward
voltage, which can have a wide tolerance. Therefore, it is necessary
to calibrate by measuring the diode voltage at a known temperature
to provide a baseline from which the change in forward voltage
with temperature can be measured. This method provides a
resolution of approximately 0.3°C and a predicted accuracy
of ±2°C.
The temperature limit comparison is performed on the result
in the temperature conversion result register (Address 0x0D),
which is the measurement of the diode forward voltage. The
values programmed into the high and low limits should be
referenced to the calibrated diode forward voltage to make
accurate limit comparisons.


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