AD7889 Analog Devices, AD7889 Datasheet - Page 14

• Manufacturer Part Number: AD7889
• Description: Low Voltage Controller for Touch Screens
• Manufacturer: Analog Devices

Specifications of AD7889

• Resolution (bits): 12bit
• # Chan: 6
• Sample Rate: 105kSPS
• Interface: I²C/Ser 2-Wire,Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni Vdd
• Adc Architecture: SAR
• Pkg Type: CSP

AD7879/AD7889
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.
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 V
referred to GND.
MEASURING TOUCH SCREEN INPUTS
When measuring the touch screen inputs, it is possible to use
V
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+.
AUX/VBAT/GPIO
CC
as a reference or instead to use the touch screen excitation
Figure 24. Single-Ended Conversion of Touch Screen Inputs
X+
X–
Y+
Y–
TEMPERATURE
V
SENSOR
SCREEN
TOUCH
CC
Figure 23. Analog Input Structure
GND
Y+
X+
Y–
CC
V
reference and the measurement is
CC
(VIA MUX)
INPUT
INPUT
MUX
IN+
V
ADC
REF
X– Y– GND X+ Y+ V
REF+
REF–
WITH TRACK-AND-HOLD
APPROXIMATION ADC
DUAL 3-TO-1 MUX
12-BIT SUCCESSIVE
REF–
REF+
CC
Rev. C | Page 14 of 40
The voltage seen at the input to the ADC in Figure 24 is
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 Ω to
900 Ω. By taking the lowest screen resistance of 200 Ω and a
typical switch resistance of 14 Ω, 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.
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.
V
IN
Figure 25. Ratiometric Conversion of Touch Screen Inputs
=
V
CC
×
SCREEN
TOUCH
R
YTOTAL
R
Y
−
GND
Y+
X+
Y–
(VIA MUX)
INPUT
V
CC
ADC
REF+
REF–
(1)