ad7879-1acbz-rl Analog Devices, Inc., ad7879-1acbz-rl Datasheet - Page 14

• Manufacturer Part Number: ad7879-1acbz-rl
• Description: Low Voltage Controller For Touch Screens
• Manufacturer: Analog Devices, Inc.

AD7879
TOUCH-PRESSURE MEASUREMENT
The pressure applied to the touch screen by a pen or finger can
also be measured with the AD7879, using some 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 (R
using two different methods.
First Method
The first method requires the user to know the total resistance
of the X-plate tablet (R
required: measurement of the X position, X
measurement of the Y− input with the excitation voltage applied
to Y+ and X− (Z1 measurement); and measurement of the X+
input with the excitation voltage applied to Y+ and X− (Z2
measurement).
These three measurements are illustrated in Figure 26.
The AD7879 has two special ADC channel settings that
configure the X and Y switches for Z1 and Z2 measurement
and store the results in the Z1 and Z2 results registers. The Z1
measurement is ADC Channel 101b, and the result is stored in
register with Read Address 0x0A. The Z2 measurement is ADC
Channel 100b, and the result is stored in register with Read
Address 0x0B.
The touch resistance can then be calculated using the following
equation:
R
TOUCH
Figure 26. Three Measurements Required for Touch Pressure
= (R
XPLATE
Y–
X+
X–
Y+
Y–
Y+
RESISTANCE
RESISTANCE
RESISTANCE
) × (X
X
TOUCH
TOUCH
TOUCH
). Three touch screen conversions are
POSITION
/4096) × [(Z2/Z1) − 1]
Y+
Y–
X+
X–
X+
X–
MEASURE
X POSITION
MEASURE
Z1 POSITION
MEASURE
Z2 POSITION
TOUCH
POSITION
) can be calculated
(Y+ input);
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Second Method
The second method 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 (X
Y position (Y
The following equation also calculates the R
TEMPERATURE MEASUREMENT
A temperature measurement option called the single conversion
method is available on the AD7879. The conversion method
requires only a single measurement on ADC Channel 001b.
The results are stored in the results registers with Address 0x0D
(TEMP). The AD7879 does 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 tem-
perature 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 TEMP results register, 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.
R
R
TOUCH
YPLATE
= R
× [1 − (Y
POSITION
XPLATE
), and the Z1 position.
× (X
POSITION
POSITION
/4096)]
/4096) × [(4096/Z1) − 1] −
TOUCH
POSITION
:
), the
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