AD7877 datasheet(16/44 Pages) AD

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AD7877

Rev. A | Page 16 of 44

For greater accuracy, the ratiometric method has two significant

advantages:

• The reference to the ADC is provided from the actual voltage

across the screen, so voltage drops across the switches have

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 all the time, 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. This provides a good indication of the size of the

depressed area and, therefore, 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.

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, X

POSITION

(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 30.

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 30. Three Measurements Required for Touch Pressure

Second Method

The second method requires that the resistance of the X-plate

and Y-plate tablets be known. Three touch screen conversions

again 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 explained previously, 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 this signal’s active

period, as shown in Figure 31.

LCD SIGNAL

TOUCH SCREEN

SIGNAL

NOISY

PERIOD

NOISY

PERIOD

Figure 31. LCD Noise Affects Touch Screen Measurements

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