AD7873ACPZ datasheet(18/29 Pages) AD

18 / 29 page

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 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.

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 (RTOUCH) 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, Z1-position, and

Z2-position (see Figure 32). The following equation calculates

the touch resistance:

(

)

(

) (

)

[

4095

POSITION

XPLATE

TOUCH

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).

The following equation also calculates the touch resistance:

(

)

(

) (

)

[

]

{

}

(

)

[

]

4095

4096

4095

POSITION

YPLATE

POSITION

XPLATE

TOUCH

Figure 32. Pressure Measurement Block Diagram

BATTERY

0V TO 6V

DC/DC

CONVERTER

+VCC

VBAT

0V TO 1.5V

ADC CORE

7.5k

Ω

2.5k

Ω

–

X+

X–

X+

X–

Y+

Y–

TOUCH

Z2-POSITION

Z1-POSITION

TOUCH

X-POSITION

Y+

X+

X–

Y–

Y+

MEASURE X-POSITION

MEASURE Z1-POSITION

MEASURE Z2-POSITION

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