AD7879 datasheet(13/36 Pages) AD

13 / 36 page

AD7879

Rev. 0 | Page 13 of 36

Figure 23 shows an equivalent circuit of the analog input

structure of the AD7879, showing the touch screen switches,

the main analog multiplexer, the ADC, and the dual 3-to-1

multiplexer that selects the reference source for the ADC.

AUX/VBAT/GPIO

12-BIT SUCCESSIVE

APPROXIMATION ADC

WITH TRACK-AND-HOLD

INPUT

MUX

TEMPERATURE

SENSOR

Y–

Y+

X–

X+

VCC

REF–

IN+

REF+

DUAL 3-TO-1 MUX

X– Y– GND X+ Y+ VCC

Figure 23. Analog Input Structure

The AD7879 can be set up to automatically convert either

specific input channels or a sequence of channels. The results

of the ADC conversions are stored in the results registers.

When measuring the ancillary analog inputs (AUX, TEMP, or

VBAT), the ADC uses a VCC reference and the measurement is

referred to GND.

MEASURING TOUCH SCREEN INPUTS

When measuring the touch screen inputs, it is possible to

measure using VCC as a reference, or to use the touch screen

excitation voltage as the reference and to perform a ratiometric,

differential measurement. The differential method is the default

method and is selected by clearing the SER/DFR bit (Bit 9 in

Control Register 2) to 0. The single-ended method is selected

by setting this bit to 1.

Single-Ended Method

Figure 24 illustrates the single-ended method for the Y position.

For the X position, the excitation voltage is applied to X+ and

X− and the voltage is measured at Y+.

ADC

REF+

INPUT

(VIA MUX)

X+

REF–

TOUCH

SCREEN

Y+

Y–

GND

VREF

VCC

Figure 24. Single-Ended Conversion of Touch Screen Inputs

The voltage seen at the input to the ADC in Figure 24 is

YTOTAL

−

(1)

The advantage of the single-ended method is that the touch

screen excitation voltage is switched off once the signal is

acquired. Because a screen can draw over 1 mA, this is a

significant consideration for a battery-powered system.

The disadvantage of the single-ended method is that voltage

drops across the switches can introduce errors. Touch screens

can have a total end-to-end resistance ranging from 200 Ω to

900 Ω. By taking the lowest screen resistance of 200 Ω and a

typical switch resistance of 14 Ω, the user can reduce the apparent

excitation voltage to 200/228 × 100 = 87% of its actual value.

In addition, the voltage drop across the low-side switch adds to

the ADC input voltage. This introduces an offset into the input

voltage; thus, it can never reach zero.

Ratiometric Method

The ratiometric method illustrated in Figure 25 shows the negative

input of the ADC reference tied to Y− and the positive input

connected to Y+. Thus, the screen excitation voltage provides

the reference for the ADC. The input of the ADC is connected

to X+ to determine the Y position.

ADC

REF+

INPUT

(VIA MUX)

REF–

VCC

X+

TOUCH

SCREEN

Y+

Y–

GND

Figure 25. Ratiometric Conversion of Touch Screen Inputs

For greater accuracy, the ratiometric method has two significant

advantages. One is that the reference to the ADC is provided

from the actual voltage across the screen; therefore, any voltage

dropped across the switches has no effect. The other advantage

is that 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.

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