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AD7877

Data Sheet

Rev. E | Page 16 of 45

For greater accuracy, the ratiometric method has two significant

advantages:



The reference to the ADC is provided from the actual

voltage across the screen; therefore, when the voltage drops

across the switches, it has no effect.



Because the measurement is ratiometric, it does not matter

if the voltage across the screen varies in the long term.

However, it must not change after the signal has been

acquired.

The disadvantage of the ratiometric method is that the screen

must be powered up at all times because it provides the

reference voltage for the ADC.

TOUCH-PRESSURE MEASUREMENT

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

also be measured with the AD7877 using some simple calcula-

tions. 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).



Measurement of the X+ input with the excitation voltage

applied to Y+ and X− (Z2 measurement).

These three measurements are illustrated in Figure 31.

The AD7877 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 1010b, and the result is stored in

the register with Read Address 11010b. The Z2 measurement is

ADC Channel 0010b, and the result is stored in the register with

Read Address 10010b.

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 31. 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),

Y position (YPOSITION), and Z1 position.

The following equation also calculates the touch resistance:

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

− RYPlate × [1 − (YPOSITION/4096)]

(3)

STOPACQ PIN

As previously explained in the Touch Screen Principles section,

touch screens are composed of two resistive layers, normally

placed over an LCD screen. Because these layers are in close

proximity to the LCD screen, noise can be coupled from the

screen onto these resistive layers, causing errors in the touch

screen positional measurements.

For example, a jitter might be noticeable in the cursor on-screen.

In most LCD touch screen systems, a signal, such as an LCD

invert signal or other control signal, is present, and noise is

usually coupled onto the touch screen during the active period

of this signal (see Figure 32).

LCD SIGNAL

TOUCH SCREEN

SIGNAL

NOISY

PERIOD

NOISY

PERIOD

Figure 32. Effect of LCD Noise on Touch Screen Measurements

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