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AD7879

Rev. 0 | Page 14 of 36

TOUCH-PRESSURE MEASUREMENT

The pressure applied to the touch screen by a pen or finger can

also be measured with the AD7879, using some simple

calculations. The contact resistance between the X and Y plates

is measured providing a good indication of the size of the

depressed area and, therefore, the applied pressure. The area of

the spot that is touched is proportional to the size of the object

touching it. The size of this resistance (RTOUCH) can be calculated

using two different methods.

First Method

The first method requires the user to know the total resistance

of the X-plate tablet (RX). Three touch screen conversions are

required: measurement of the X position, XPOSITION (Y+ input);

measurement of the Y− input with the excitation voltage applied

to Y+ and X− (Z1 measurement); and measurement of the X+

input with the excitation voltage applied to Y+ and X− (Z2

measurement).

These three measurements are illustrated in Figure 26.

The AD7879 has two special ADC channel settings that

configure the X and Y switches for Z1 and Z2 measurement

and store the results in the Z1 and Z2 results registers. The Z1

measurement is ADC Channel 101b, and the result is stored in

Channel 100b, and the result is stored in register with Read

Address 0x0B.

The touch resistance can then be calculated using the following

equation:

RTOUCH = (RXPLATE) × (XPOSITION /4096) × [(Z2/Z1) − 1]

(2)

Y–

Y+

X–

X+

TOUCH

RESISTANCE

MEASURE

Z1 POSITION

X–

X+

Y–

Y+

TOUCH

RESISTANCE

MEASURE

X POSITION

Y–

Y+

X–

X+

TOUCH

RESISTANCE

MEASURE

Z2 POSITION

Figure 26. Three Measurements Required for Touch Pressure

Second Method

The second method requires the user to know the resistance of

the X-plate and Y-plate tablets. Three touch screen conversions

are required: a measurement of the X position (XPOSITION), the

Y position (YPOSITION), and the Z1 position.

The following equation also calculates the RTOUCH:

RTOUCH = RXPLATE × (XPOSITION/4096) × [(4096/Z1) − 1] −

RYPLATE × [1 − (YPOSITION/4096)]

(3)

TEMPERATURE MEASUREMENT

A temperature measurement option called the single conversion

method is available on the AD7879. The conversion method

requires only a single measurement on ADC Channel 001b.

The results are stored in the results registers with Address 0x0D

(TEMP). The AD7879 does not provide an explicit output of

the temperature reading; the system must perform some

external calculations. This method is based on an on-chip

diode measurement.

The acquisition time is fixed at 16 ms for temperature

measurement.

Conversion Method

The conversion method makes use of the fact that the tem-

perature coefficient of a silicon diode is approximately

−2.1 mV/°C. However, this small change is superimposed

on the diode forward voltage, which can have a wide tolerance.

Therefore, it is necessary to calibrate by measuring the diode

voltage at a known temperature to provide a baseline from

which the change in forward voltage with temperature can be

measured. This method provides a resolution of approximately

0.3°C and a predicted accuracy of ±2°C.

The temperature limit comparison is performed on the result

in the TEMP results register, which is the measurement of the

diode forward voltage. The values programmed into the high

and low limits should be referenced to the calibrated diode

forward voltage to make accurate limit comparisons.

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