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| 型号： | AD7879WARUZ-RL7 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 8/41页 |
| 文件大小： | 0K |
| 描述： | IC ADC 12BIT CTLR TOUCH 16TSSOP |
| 标准包装： | 1,000 |
| 类型： | 电阻 |
| 触摸面板接口： | 4 线 |
| 输入数/键： | 1 TSC |
| 分辨率（位）： | 12 b |
| 评估套件： | * |
| 数据接口： | 串行，SPI? |
| 数据速率/采样率 (SPS，BPS)： | 105k |
| 电压基准： | 内部 |
| 电源电压： | 1.6 V ~ 3.6 V |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 16-TSSOP（0.173"，4.40mm 宽） |
| 供应商设备封装： | 16-TSSOP |
| 包装： | 带卷 (TR) |
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AD7879/AD7889
Rev. C | Page 15 of 40
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 (RTOUCH) can be calculated
using two different methods.
First Method
The first method requires the user to know the total resistance
of the X-plate tablet (RX). Three touch screen conversions are
required: measurement of the X position, XPOSITION (Y+ input);
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
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 (RTOUCH) can then be calculated using the
following equation:
RTOUCH = (RXPLATE) × (XPOSITION/4096) × [(Z2/Z1) 1]
(2)
07667-
026
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
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 (XPOSITION), the
Y position (YPOSITION), and the Z1 position.
The following equation 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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