AD7843 datasheet(14/21 Pages) AD

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AD7843

Rev. B | Page 13 of 20

Touch Screen Settling

In some applications, external capacitors could be required

across the touch screen to filter noise associated with it, for

example, noise generated by the LCD panel or backlight

circuitry. The value of these capacitors causes a settling time

requirement when the panel is touched. The settling time

typically appears as a gain error. There are several methods for

minimizing or eliminating this issue. The problem could be that

the input signal, reference, or both have not settled to their final

value before the sampling instant of the ADC. Additionally, the

reference voltage could still be changing during the conversion

cycle. One option is to stop, or slow down the DCLK for the

required touch screen settling time. This allows the input and

reference to stabilize for the acquisition time, which resolves the

issue for both single-ended and differential modes.

The other option is to operate the AD7843 in differential mode

only for the touch screen and to program the AD7843 to keep

the touch screen drivers on and not go into power-down (PD0

= PD1 = 1). Several conversions might be required, depending

on the settling time required and the AD7843 data rate. Once

the required number of conversions are made, the AD7843 can

then be placed into a power-down state on the last

measurement. The last method is to use the 15 DCLK cycle

mode, which maintains the touch screen drivers on until it is

commanded to stop by the processor.

Reference Input

The voltage difference between +REF and −REF (see Figure 19)

sets the analog input range. The AD7843 operates with a refer-

ence input in the range of 1 V to VCC. The voltage into the VREF

input is not buffered and directly drives the capacitor DAC

portion of the AD7843. Figure 20 shows the reference input

circuitry. Typically, the input current is 8 µA with VREF = 2.5 V

and fSAMPLE = 125 kHz. This value varies by a few microamps,

depending on the result of the conversion. The reference current

diminishes directly with both conversion rate and reference

voltage. As the current from the reference is drawn on each bit

decision, clocking the converter more quickly during a given

conversion period does not reduce the overall current drain

from the reference.

X+

Y+

VREF

3-TO-1

MUX

ADC

Figure 20. Reference Input Circuitry

When making touch screen measurements, conversions can be

made in the differential (ratiometric) mode or the single-ended

mode. If the SER/DFR bit is set to 1 in the control register, a

single-ended conversion is performed. Figure 21 shows the

configuration for a single-ended Y-coordinate measurement.

The X+ input is connected to the analog to digital converter, the

Y+ and Y− drivers are turned on, and the voltage on X+ is

digitized. The conversion is performed with the ADC referenced

from GND to VREF. The advantage of this mode is that the

switches that supply the external touch screen can be turned off

once the acquisition is complete, resulting in a power saving.

However, the on resistance of the Y drivers affects the input

voltage that can be acquired. The full touch screen resistance

may be in the order of 200 Ω to 900 Ω, depending on the manu-

facturer. Therefore 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 AD7843 is approximately 70 Ω.

+VCC

VREF

GND

Y+

Y–

X+

IN+

IN–

REF+

ADC CORE

REF–

Figure 21. Single-Ended Reference Mode (SER/DFR = 1)

In this mode of operation, therefore, some voltage is likely to be

lost across the internal switches and, in addition to this, it is

unlikely that the internal switch resistance will track the resis-

tance of the touch screen over temperature and supply, providing

an additional source of error.

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

one again considers making a Y-coordinate measurement, 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 22 shows the configuration for a ratiometric Y-

coordinate measurement. It should be noted that the differential

reference mode can be used only with +VCC since the source of

the +REF voltage and cannot be used with VREF.

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.

+VCC

GND

Y+

Y–

X+

IN+

IN–

REF+

ADC CORE

REF–

Figure 22. Differential Reference Mode (SER/DFR = 0)

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