Data Sheet
AD7873
Battery Measurement
Pressure 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 VBAT pin. The voltage to the +VCC 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 VBAT 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 VBAT. Table 6 shows
the control bit settings required to perform a battery
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 (RTOUCH) can be calculated
using two different methods.
DC/DC
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, Z1-position, and
Z2-position (see Figure 32). The following equation calculates
the touch resistance:
CONVERTER
+
BATTERY
0V TO 6V
+V
CC
V
BAT
0V TO 1.5V
ADC CORE
7.5kΩ
2.5kΩ
RTOUCH
=
RXPLATE
×
XPOSITION / 4095
×
[(
Z2 / Z1
)
/ 1
]
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 Z1-position (see Figure 32).
Figure 31. Block Diagram of Battery Measurement Circuit
The following equation also calculates the touch resistance:
RTOUCH
=
{(
RXPLATE / Z1
)
×
/
(
XPOSITION / 4095
)
×
[
(
4096 / Z1
)
/ 1
]}
[
RYPLATE YPOSITION / 4095
×
(
)
]
MEASURE X-POSITION
Y+
X+
TOUCH
+
–
X-POSITION
X–
Y–
MEASURE Z -POSITION
1
Y+
Y–
Y+
X+
X+
TOUCH
TOUCH
+
+
–
–
Z -POSITION
2
Z -POSITION
1
X–
Y–
X–
MEASURE Z -POSITION
2
Figure 32. Pressure Measurement Block Diagram
Rev. F | Page 17 of 28
ADI [ ADI ]
