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Data Sheet

AD7877

AUX3/GPIO4

BAT1

BAT2

AUX2/GPIO3

AUX1/GPIO2

12-BIT SUCCESSIVE

APPROXIMATION ADC

WITH TRACK-AND-HOLD

9 TO 1

I/P

MUX

TEMPERATURE

SENSOR

Y–

Y+

X–

X+

VCC

REF–

IN+

REF

INT/EXT

REF+

DUAL 3-1

MUX

X– Y– GND X+ Y+ VREF

Figure 28. Analog Input Structure

The AD7877 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. See the

Serial Interface section for details.

When measuring the ancillary analog inputs (AUX1 to AUX3,

BAT1 and BAT2), the ADC uses the internal reference, or an

external reference applied to the VREF pin, and the measurement

is referred to GND.

MEASURING TOUCH SCREEN INPUTS

When measuring the touch screen inputs, it is possible to

measure using the internal (or external) reference, or to use the

touch screen excitation voltage as the reference and perform a

ratiometric, differential measurement. The differential method

is the default and is selected by clearing the SER/DFR bit

(Bit 11) in Control Register 1. The single-ended method is

selected by setting this bit.

Single-Ended Method

Figure 29 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 measured at Y+.

ADC

REF+

INPUT

(VIA MUX)

X+

REF–

TOUCH

SCREEN

Y+

Y–

GND

VREF

VCC

Figure 29. Single-Ended Conversion of Touch Screen Inputs

The voltage seen at the input to the ADC in Figure 29 is

YTOTAL

−

(1)

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

screen excitation voltage can be 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 disadvantages of the single-ended method are as follows:

•

It can be used only if VCC is close to VREF. If VCC is greater

than VREF, some positions on the screen are outside the

range of the ADC. If VCC is less than VREF, the full range of

the ADC is not used.

•

The ratio of VCC to VREF must be known. If VREF and/or VCC

vary relative to one another, this can introduce errors.

•

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. Taking the lowest screen

resistance of 200 Ω and a typical switch resistance of 14 Ω

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.

The single-ended method is adequate for applications that use a

fairly blunt and imprecise instrument for an input device, such

as a finger.

Ratiometric Method

The ratiometric method illustrated in Figure 30 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 30. Ratiometric Conversion of Touch Screen Inputs

Rev. E | Page 15 of 45

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