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| 型号： | AD7877ACPZ-REEL7 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 8/45页 |
| 文件大小： | 0K |
| 描述： | IC ADC 12BIT TOUCHSCREEN 32LFCSP |
| 标准包装： | 1,500 |
| 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： | 可供 |
| 数据接口： | 串行，SPI? |
| 数据速率/采样率 (SPS，BPS)： | 125k |
| 电压基准： | 外部，内部 |
| 电源电压： | 2.7 V ~ 5.25 V |
| 电流 - 电源： | 1µA |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-VFQFN 裸露焊盘，CSP |
| 供应商设备封装： | 32-LFCSP-VQ |
| 包装： | 带卷 (TR) |
| 配用： | EVAL-AD7877EBZ-ND - BOARD EVALUATION FOR AD7877 |
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Data Sheet
AD7877
Rev. D | Page 15 of 44
AUX3/GPIO4
BAT1
BAT2
AUX2/GPIO3
AUX1/GPIO2
12-BIT SUCCESSIVE
APPROXIMATION ADC
WITH TRACK-AND-HOLD
9 TO 1
I/P
MUX
TEMPERATURE
SENSOR
Y–
Y+
X–
X+
VCC
REF–
IN+
REF
INT/EXT
REF+
DUAL 3-1
MUX
X– Y– GND X+ Y+ VREF
03796-006
Figure 28. Analog Input Structure
The AD7877 can be set up to automatically convert either
specific input channels or a sequence of channels. 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 VREF 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
Figure 29 illustrates the single-ended method for the Y position.
For the X position, the excitation voltage is applied to X+ and
X and the voltage measured at Y+.
03796-007
ADC
REF+
INPUT
(VIA MUX)
X+
REF–
TOUCH
SCREEN
Y+
Y–
GND
VREF
VCC
Figure 29. Single-Ended Conversion of Touch Screen Inputs
The voltage seen at the input to the ADC in Figure 29 is
YTOTAL
Y
CC
IN
R
V
×
=
(1)
The advantage of the single-ended method is that the touch
screen excitation voltage can be switched off once the signal is
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 VCC is close to VREF. If VCC is greater
than VREF, some positions on the screen are outside the
range of the ADC. If VCC is less than VREF, the full range of
the ADC is not used.
The ratio of VCC to VREF must be known. If VREF and/or VCC
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
ranging from 200 to 900 . Taking the lowest screen
resistance of 200 and a typical switch resistance of 14
can reduce the apparent 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, thus, it can
never reach zero.
The single-ended method is adequate for applications that use a
fairly blunt and imprecise instrument for an input device, such
as a finger.
Ratiometric Method
The ratiometric method illustrated in Figure 30 shows the
negative input of the ADC reference tied to Y and the positive
input connected to Y+. Thus, the screen excitation voltage
provides the reference for the ADC. The input of the ADC is
connected to X+ to determine the Y position.
03796-008
ADC
REF+
INPUT
(VIA MUX)
REF–
VCC
X+
TOUCH
SCREEN
Y+
Y–
GND
Figure 30. Ratiometric Conversion of Touch Screen Inputs
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