AD7873 Analog Devices, AD7873 Datasheet - Page 17

• Manufacturer Part Number: AD7873
• Description: Touch Screen Digitizer
• Manufacturer: Analog Devices

Specifications of AD7873

• Resolution (bits): 12bit
• # Chan: 6
• Sample Rate: 125kSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: CSP,SOP

Battery Measurement
The AD7873 can monitor a battery voltage from 0 V to 6 V.
Figure 31 shows a block diagram of a battery voltage monitored
through the V
maintained at the desired supply voltage via the dc-to-dc
regulator while the input to the regulator is monitored. This
voltage on V
presented to the ADC as 1.5 V. To conserve power, the divider is
on only during the sampling of a voltage on V
the control bit settings required to perform a battery
measurement.
BATTERY
0V TO 6V
Figure 31. Block Diagram of Battery Measurement Circuit
BAT
BAT
is divided by 4 so that a 6 V battery voltage is
pin. The voltage to the +V
+
7.5kΩ
2.5kΩ
CONVERTER
V
BAT
DC/DC
0V TO 1.5V
X+
Z
X–
2
-POSITION
TOUCH
+V
CC
ADC CORE
CC
BAT
of the AD7873 is
Y+
Y–
. Table 6 shows
Figure 32. Pressure Measurement Block Diagram
+
–
+
–
MEASURE Z
X+
X-POSITION
X–
MEASURE Z
TOUCH
Rev. E | Page 17 of 28
2
-POSITION
1
-POSITION
Y+
Y–
MEASURE X-POSITION
Pressure Measurement
The pressure applied to the touch screen via a pen or finger
can also be measured with the AD7873 with some simple
calculations. The 8-bit resolution mode would be sufficient for
this measurement, but the following calculations are shown
with the 12-bit resolution mode. The contact resistance between
the X and Y plates is measured, providing a good indication of
the size of the depressed area and 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
using two different methods.
The first method requires the user to know the total resistance
of the X-plate tablet. Three touch screen conversions are
required, a measurement of the X-position, Z
Z
the touch resistance:
The second method requires that the resistance of both the
X-plate and Y-plate tablets are known. Again three touch screen
conversions are required, a measurement of the X-position,
Y-position, and Z
The following equation also calculates the touch resistance:
R
2
TOUCH
-position (see Figure 32). The following equation calculates
R
TOUCH
=
X+
Z
X–
1
{
-POSITION
(
R
TOUCH
=
XPLATE
(
R
1
XPLATE
-position (see Figure 32).
/
Z
Y+
Y–
1
) (
) (
×
×
−
+
–
[
X
R
X
YPLATE
POSITION
POSITION
×
/
/
(
Y
4095
4095
TOUCH
POSITION
)
)
1
) can be calculated
×
-position, and
×
[
[
(
(
/
Z
4096
4095
2
AD7873
/
Z
/
)
]
1
Z
)
1
−
)
1
−
]
1
]
}