AD7889 Analog Devices, AD7889 Datasheet - Page 15

• Manufacturer Part Number: AD7889
• Description: Low Voltage Controller for Touch Screens
• Manufacturer: Analog Devices

Specifications of AD7889

• 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

TOUCH-PRESSURE MEASUREMENT
The pressure applied to the touch screen by a pen or finger can
also be measured with the AD7879/AD7889 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 X+ input with the excitation voltage applied
to Y+ and X− (Z1 measurement); and measurement of the Y−
input with the excitation voltage applied to Y+ and X− (Z2
measurement). These three measurements are illustrated in
Figure 26.
The AD7879/AD7889 have two special ADC channel settings
that configure the X and Y switches for the Z1 and Z2 measure-
ments 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 (R
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
TOUCH
). Three touch screen conversions are
POSITION
) can then be calculated using the
/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);
Rev. C | Page 15 of 40
(2)
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 touch resistance
(R
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.
TOUCH
R
R
TOUCH
YPLATE
):
= R
× [1 − (Y
POSITION
XPLATE
), and the Z1 position.
× (X
POSITION
POSITION
/4096)]
/4096) × [(4096/Z1) − 1] −
AD7879/AD7889
POSITION
), the
(3)