參數(shù)資料
型號(hào)： EVAL-AD7877EB
廠商： Analog Devices, Inc.
英文描述： Touch Screen Controller
中文描述： 觸摸屏控制器
文件頁(yè)數(shù)： 15/44頁(yè)
文件大?。?/td> 1002K
代理商： EVAL-AD7877EB
AD7877
Rev. A | Page 15 of 44
AUX3/GPIO4
BAT1
BAT2
AUX2/GPIO3
AUX1/GPIO2
12-BIT SUCCESSIVE
APPROXIMATION ADC
9 TO 1
I/P
MUX
TEMPERATURE
SENSOR
Y–
Y+
X–
X+
V
CC
REF–
IN+
REF
INT/EXT
REF+
DUAL 3-1
MUX
X– Y– GND X+ Y+ V
REF
0
Figure 27. Analog Input Structure
The AD7877 can be set up to convert specific input channels or
to convert a sequence of channels automatically. The results of
the ADC conversions are stored in the results registers. See the
Serial Interface section for details.
When measuring the ancillary analog inputs (AUX1 to AUX3,
BAT1 and BAT2), the ADC uses the internal reference, or an
external reference applied to the V
REF
pin, and the measurement
is referred to GND.
MEASURING TOUCH SCREEN INPUTS
When measuring the touch screen inputs, it is possible to
measure using the internal (or external) reference, or to use the
touch screen excitation voltage as the reference and perform a
ratiometric, differential measurement. The differential method
is the default and is selected by clearing the SER/DFR bit
(Bit 11) in Control Register 1. The single-ended method is
selected by setting this bit.
Single-Ended Method
The single-ended method is illustrated for the Y position in
Figure 28. For the X position, the excitation voltage would be
applied to X+ and X and the voltage measured at Y+.
0
ADC
REF+
(INPUT
X+
REF–
TOUCH
SCREEN
Y+
Y–
GND
V
REF
V
CC
Figure 28. Single-Ended Conversion of Touch Screen Inputs
The voltage seen at the input to the ADC in Figure 28 is
V
IN
=
V
CC
×
YTOTAL
R
Y
R
(1)
The advantage of the single-ended method is that the touch
screen excitation voltage can be switched off once the signal has
been acquired. Because a screen can draw over 1 mA, this is a
significant consideration for a battery-powered system.
The disadvantages of the single-ended method are as follows:
It can be used only if V
CC
is close to V
REF
. If V
CC
is greater than
V
REF
, some positions on the screen are outside the range of
the ADC. If V
CC
is less than V
REF
, the full range of the ADC is
not utilized.
The ratio of V
CC
to V
REF
must be known. If V
REF
and/or V
CC
vary relative to one another, this can introduce errors.
Voltage drops across the switches can introduce errors. Touch
screens can have a total end-to-end resistance of from 200
to 900 . Taking the lowest screen resistance of 200 and a
typical switch resistance of 14 , this could reduce the appar-
ent excitation voltage to 200/228 × 100 = 87% of its actual
value. In addition, the voltage drop across the low-side switch
adds to the ADC input voltage. This introduces an offset into
the input voltage, which means that it can never reach zero.
The single-ended method is adequate for applications in which
the input device is a fairly blunt and imprecise instrument such
as a finger.
Ratiometric Method
The ratiometric method is illustrated in Figure 29. Here, the
negative input of the ADC reference is tied to Y and the
positive input is connected to Y+, so the screen excitation
voltage provides the reference for the ADC. The input of the
ADC is connected to X+ to determine the Y position.
0
ADC
REF+
INPUT
(VIA MUX)
REF–
V
CC
X+
TOUCH
SCREEN
Y+
Y–
GND
Figure 29. Ratiometric Conversion of Touch Screen Inputs
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