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參數(shù)資料
| 型號(hào)： | AD7877ACPZ-REEL7 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 消費(fèi)家電 |
| 英文描述： | 16-Bit, Pseudo Bipolar, Fully Diff Input, 250kSPS Serial Out, 2.7V to 5.5V Micro Power Sampling ADC 8-MSOP -40 to 85 |
| 中文描述： | SPECIALTY CONSUMER CIRCUIT, QCC32 |
| 封裝： | 5 X 5 MM, ROHS COMPLIANT, MO-220VHHD-2, LFCSP-32 |
| 文件頁數(shù)： | 16/44頁 |
| 文件大?。?/td> | 1002K |
| 代理商： | AD7877ACPZ-REEL7 |
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AD7877
For greater accuracy, the ratiometric method has two significant
advantages:
Rev. A | Page 16 of 44
The reference to the ADC is provided from the actual voltage
across the screen, so voltage drops across the switches have
no effect.
Because the measurement is ratiometric, it does not matter if
the voltage across the screen varies in the long term. However,
it must not change after the signal has been acquired.
The disadvantage of the ratiometric method is that the screen
must be powered up all the time, because it provides the
reference voltage for the ADC.
TOUCH-PRESSURE MEASUREMENT
The pressure applied to the touch screen via a pen or finger can
also be measured with the AD7877 using some simple calcula-
tions. The contact resistance between the X and Y plates is
measured. This provides a good indication of the size of the
depressed area and, therefore, the applied pressure. The area of
the spot touched is proportional to the size of the object
touching it. The size of this resistance (R
TOUCH
) 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 (R
X
). Three touch screen conversions are
required:
Measurement of the X position, X
POSITION
(Y+ input).
Measurement of the Y input with the excitation voltage
applied to Y+ and X (Z1 measurement).
Measurement of the X+ input with the excitation voltage
applied to Y+ and X (Z2 measurement).
These three measurements are illustrated in Figure 30.
The AD7877 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 1010b, and the result is stored in
the register with Read Address 11010b. The Z2 measurement is
ADC Channel 0010b, and the result is stored in the register with
Read Address 10010b.
The touch resistance can then be calculated using the following
equation:
R
TOUCH
= (
R
XPlate
) × (
X
POSITION
/4096 × [
Z2
/
Z1
)
1]
(2)
0
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 30. Three Measurements Required for Touch Pressure
Second Method
The second method requires that the resistance of the X-plate
and Y-plate tablets be known. Three touch screen conversions
again are required, a measurement of the X Position (X
POSITION
),
Y Position (Y
POSITION
), and Z1 position.
The following equation also calculates the touch resistance:
R
TOUCH
=
R
XPlate
× (
X
POSITION
/4096) × [(4096/
Z1
) 1]
R
YPlate
× [1 (
Y
POSITION
/4096)]
(3)
STOPACQ PIN
As explained previously, 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.
For example, a jitter might be noticeable in the cursor on-
screen. In most LCD touch screen systems, a signal, such as an
LCD invert signal or other control signal, is present, and noise is
usually coupled onto the touch screen during this signal’s active
period, as shown in Figure 31.
0
LCD SIGNAL
TOUCH SCREEN
SIGNAL
NOISY
PERIOD
NOISY
PERIOD
Figure 31. LCD Noise Affects Touch Screen Measurements
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