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| 型号： | EVAL-AD7879EBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 7/41页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR AD7879 |
| 标准包装： | 1 |
| 主要目的： | 接口，触摸屏控制器 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD7879 |
| 主要属性： | 4 线电阻式触控屏控制器，I²C 和 SPI 接口 |
| 次要属性： | 图形用户界面，USB 接口 |
| 已供物品： | 板，线缆，CD，触控屏 |
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AD7879/AD7889
Rev. C | Page 14 of 40
Figure 23 shows an equivalent circuit of the analog input structure
of the AD7879/AD7889, showing the touch screen switches, the
main analog multiplexer, the ADC, and the dual 3-to-1 multi-
plexer that selects the reference source for the ADC.
AUX/VBAT/GPIO
12-BIT SUCCESSIVE
APPROXIMATION ADC
WITH TRACK-AND-HOLD
INPUT
MUX
TEMPERATURE
SENSOR
Y–
Y+
X–
X+
VCC
REF–
IN+
REF+
DUAL 3-TO-1 MUX
X– Y– GND X+ Y+ VCC
07667-
023
Figure 23. Analog Input Structure
The AD7879/AD7889 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 result registers.
When measuring the ancillary analog inputs (AUX, TEMP, or
VBAT), the ADC uses a VCC reference and the measurement is
referred to GND.
MEASURING TOUCH SCREEN INPUTS
When measuring the touch screen inputs, it is possible to use
VCC as a reference or instead to use the touch screen excitation
voltage as the reference and to perform a ratiometric, differential
measurement. The differential method is the default method
and is selected by clearing the SER/DFR bit (Bit 9 in Control
Register 2) to 0. The single-ended method is selected by setting
this bit to 1.
Single-Ended Method
Figure 24 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 is measured at Y+.
ADC
REF+
INPUT
(VIA MUX)
X+
REF–
TOUCH
SCREEN
Y+
Y–
GND
VREF
VCC
07667-
024
Figure 24. Single-Ended Conversion of Touch Screen Inputs
The voltage seen at the input to the ADC in Figure 24 is
YTOTAL
Y
CC
IN
R
V
×
=
(1)
The advantage of the single-ended method is that the touch
screen excitation voltage is switched off when the signal is
acquired. Because a screen can draw over 1 mA, this is a
significant consideration for a battery-powered system.
The disadvantage of the single-ended method is that voltage
drops across the switches can introduce errors. Touch screens
can have a total end-to-end resistance ranging from 200 to
900 . By taking the lowest screen resistance of 200 and a
typical switch resistance of 14 , the user 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 0.
Ratiometric Method
The ratiometric method illustrated in Figure 25 shows the
negative input of the ADC reference connected 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.
ADC
REF+
INPUT
(VIA MUX)
REF–
VCC
X+
TOUCH
SCREEN
Y+
Y–
GND
07667-
025
Figure 25. Ratiometric Conversion of Touch Screen Inputs
For greater accuracy, the ratiometric method has two significant
advantages. One is that the reference to the ADC is provided
from the actual voltage across the screen; therefore, any voltage
dropped across the switches has no effect. The other advantage
is that 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 at all times because it provides the reference
voltage for the ADC.
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相关代理商/技术参数 |
参数描述 |
|---|---|
| EVAL-AD7887CB | 制造商:Analog Devices 功能描述:EVAL BD FOR 2.7 V TO 5.25 V, MICROPWR, 2CH, 125 KSPS, 12-BIT - Bulk |
| EVAL-AD7887CB4 | 制造商:AD 制造商全称:Analog Devices 功能描述:2.7 V to 5.25 V, Micropower, 2-Channel, 125 kSPS, 12-Bit ADC in 8-Lead MSOP |
| EVAL-AD7887CBZ | 功能描述:BOARD EVAL FOR AD7887 RoHS:是 类别:编程器，开发系统 >> 评估板 - 模数转换器 (ADC) 系列:* 产品培训模块:Obsolescence Mitigation Program 标准包装:1 系列:- ADC 的数量:1 位数:12 采样率（每秒）:94.4k 数据接口:USB 输入范围:±VREF/2 在以下条件下的电源（标准）:- 工作温度:-40°C ~ 85°C 已用 IC / 零件:MAX11645 已供物品:板，软件 |
| EVAL-AD7888CB | 制造商:Analog Devices 功能描述:EVAL KIT FOR 2.7 V TO 5.25 V, MICROPWR, 8CH, 125 KSPS, 12-BI - Bulk |
| EVAL-AD7890-2 | 制造商:Analog Devices 功能描述:AD7890 EVAL BOARD |
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