AD7877(RevA) 查看數據表（PDF） - Analog Devices

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AD7877
(Rev.:RevA)

Touch Screen Controller

Analog Devices 

VCC

X+

X–

Y+

Y–

AUX1/GPIO2

AUX2/GPIO3

AUX3/GPIO4

BAT1

BAT2

TEMPERATURE

SENSOR

REF

INT/EXT

9 TO 1

I/P

MUX

IN+

X– Y– GND X+ Y+ VREF

DUAL 3-1

MUX

REF– REF+

12-BIT SUCCESSIVE

APPROXIMATION ADC

WITH TRACK-AND-HOLD

Figure 27. Analog Input Structure

The AD7877 can be set up to convert specific input channels or

to convert a sequence of channels automatically. 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

The single-ended method is illustrated for the Y position in

Figure 28. For the X position, the excitation voltage would be

applied to X+ and X− and the voltage measured at Y+.

VCC

Y+

VREF

X+

INPUT

(VIA MUX)

TOUCH

SCREEN

Y–

REF+

ADC

REF–

GND

Figure 28. Single-Ended Conversion of Touch Screen Inputs

AD7877

The voltage seen at the input to the ADC in Figure 28 is

VIN = VCC ×

RY −

(1)

RYTOTAL

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

screen excitation voltage can be switched off once the signal has

been 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 utilized.

• 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 of from 200 Ω

to 900 Ω. Taking the lowest screen resistance of 200 Ω and a

typical switch resistance of 14 Ω, this could reduce the appar-

ent 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, which means that it can never reach zero.

The single-ended method is adequate for applications in which

the input device is a fairly blunt and imprecise instrument such

as a finger.

Ratiometric Method

The ratiometric method is illustrated in Figure 29. Here, the

negative input of the ADC reference is tied to Y− and the

positive input is connected to Y+, so the screen excitation

voltage provides the reference for the ADC. The input of the

ADC is connected to X+ to determine the Y position.

VCC

Y+

INPUT

X+ (VIA MUX)

TOUCH

SCREEN

Y–

REF+

ADC

REF–

GND

Figure 29. Ratiometric Conversion of Touch Screen Inputs

Rev. A | Page 15 of 44

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