AD7873ACP datasheet(16/28 Pages) AD

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AD7873

Rev. D | Page 16 of 28

supply the external touch screen can be turned off once the

acquisition is complete, resulting in a power savings. However,

the on-resistance of the Y drivers affects the input voltage that

can be acquired. The full touch screen resistance could be in the

order of 200 Ω to 900 Ω, depending on the manufacturer. Thus,

if the on-resistance of the switches is approximately 6 Ω, true

full-scale and zero-scale voltages cannot be acquired, regardless

of where the pen/stylus is on the touch screen. Note that the

minimum touch screen resistance recommended for use with

the AD7873 is approximately 70 Ω. In this mode of operation,

therefore, some voltage is likely to be lost across the internal

switches, and it is unlikely that the internal switch resistance

will track the resistance of the touch screen over temperature

and supply, providing an additional source of error.

+VCC

VREF

GND

Y+

Y–

X+

IN+

IN–

REF+

ADC CORE

REF–

Figure 28. Single-Ended Reference Mode (SER/DFR = 1)

The alternative to this situation is to set the SER/DFR bit low.

Again, making a Y coordinate measurement is considered, but

now the +REF and –REF nodes of the ADC are connected

directly to the Y+ and Y– pins. This means the analog-to-digital

conversion is ratiometric. The result of the conversion is always

a percentage of the external resistance, independent of how it

could change with respect to the on resistance of the internal

switches. Figure 29 shows the configuration for a ratiometric Y

coordinate measurement.

+VCC

GND

Y+

Y–

X+

IN+

IN–

REF+

ADC CORE

REF–

Figure 29. Differential Reference Mode (SER/DFR = 0)

The disadvantage of this mode of operation is that during both

the acquisition phase and conversion process, the external touch

screen must remain powered. This results in additional supply

current for the duration of the conversion.

MEASUREMENTS

Temperature Measurement

Two temperature measurement options are available on the

AD7873, the single conversion method and the differential

conversion method. Both methods are based on an on-chip

diode measurement.

In the single conversion method, a diode voltage is digitized and

recorded at a fixed calibration temperature. Any subsequent

polling of the diode provides an estimate of the ambient tem-

perature through extrapolation from the calibration temperature

diode result. This assumes a diode temperature drift of

approximately –2.1 mV/°C. This method provides a resolution

of approximately 0.3°C and a predicted accuracy of ±3°C.

The differential conversion method is a 2-point measurement.

The first measurement is performed with a fixed bias current

into a diode, and the second measurement is performed with a

fixed multiple of the bias current into the same diode. The

voltage difference in the diode readings is proportional to

absolute temperature and is given by the following formula:

(

)

( )

VBE

∆

where VBE represents the diode voltage, N is the bias current

multiple, k is Boltzmann’s constant and q is the electron charge.

This method provides more accurate absolute temperature

measurement of ± 2°C. However, the resolution is reduced to

approximately 1.6°C. Assuming a current multiple of 105

(which is typical for the AD7873) taking Boltzmann’s constant,

k = 1.38054 ×10–23 electrons volts/degrees Kelvin, the electron

charge q = 1.602189 × 10–19, then T, the ambient temperature in

degrees centigrade, would be calculated as follows:

(

)

( )

()

273

−

∆

∆VBE is calculated from the difference in readings from the first

conversion and second conversion.

Figure 30 shows a block diagram of the temperature

measurement mode.

TEMP0

TEMP1

105*I

MUX

ADC

Figure 30. Block Diagram of Temperature Measurement Circuit

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