Datasheet
Data Sheet AD7873
Rev. F | Page 17 of 28
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
BAT
pin. The voltage to the +V
CC
of the AD7873 is
maintained at the desired supply voltage via the dc-to-dc
regulator while the input to the regulator is monitored. This
voltage on V
BAT
is divided by 4 so that a 6 V battery voltage is
presented to the ADC as 1.5 V. To conserve power, the divider is
on only during the sampling of a voltage on V
BAT
. Table 6 shows
the control bit settings required to perform a battery
measurement.
Figure 31. Block Diagram of Battery Measurement Circuit
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
TOUCH
) can be calculated
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
1
-position, and
Z
2
-position (see Figure 32). The following equation calculates
the touch resistance:
( )
( )
( )
[ ]
1/4095/ /××=
12
POSITIONXPLATETOUCH
ZZXRR
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
1
-position (see Figure 32).
The following equation also calculates the touch resistance:
( )
( )
( )
[ ]
{ }
( )
[ ]
4095/
1/40964095//
POSITIONYPLATE
1
POSITION
1
XPLATETOUCH
YR
ZXZRR
×/
/××=
Figure 32. Pressure Measurement Block Diagram
+
BATTERY
0V TO 6V
DC/DC
CONVERTER
+V
CC
V
BAT
0V TO 1.5V
ADC CORE
7.5kΩ
2.5kΩ
02164-031
+
–
+
–
+
–
X+
X–
X+
X–
Y+
Y–
Y–
TOUCH TOUCH
Z
2
-POSITION Z
1
-POSITION
TOUCH
X-POSITION
Y+
X+
X–
Y–
Y+
MEASURE X-POSITION
MEASURE Z
1
-POSITION
MEASURE Z
2
-POSITION
02164-032