TSC2200_16 Datasheet, PDF (10/45 Pages) Texas Instruments – PDA ANALOG INTERFACE CIRCUIT

In some applications, external capacitors may be required
across the touch screen for filtering noise picked up by the
touch screen; i.e., noise generated by the LCD panel or
back-light circuitry. The value of these capacitors will provide
a low-pass filter to reduce the noise, but will cause an
additional settling time requirement when the panel is touched.
Several solutions to this problem are available in the TSC2200.
A programmable delay time is available that sets the delay
between turning the drivers on and making a conversion.
This is referred to as the Panel Voltage Stabilization time,
and is used in some of the modes available in the TSC2200.
In other modes, the TSC2200 can be commanded to turn on
the drivers only without performing a conversion. Time can
then be allowed before a conversion is started.
The TSC2200 touch screen interface can measure position (X
and Y) and pressure (Z). Determination of these coordinates
is possible under three different modes of the A/D converter:
conversion controlled by the TSC2200, initiated by detection
of a touch; conversion controlled by the TSC2200, initiated by
the host responding to the PENIRQ signal; or conversion
completely controlled by the host processor.
A/D CONVERTER
The analog inputs of the TSC2200 are shown in Figure 4. The
analog inputs (X, Y, and Z touch panel coordinates, battery
voltage monitors, chip temperature, and auxiliary inputs) are
provided via a multiplexer to the Successive Approximation
Register (SAR) A/D converter. The A/D converter architecture
is based on capacitive redistribution architecture that inher-
ently includes a sample-and-hold function.
TEMP1 TEMP0
+V_DD
V_REF
X+
X–
Y+
Y–
V_BAT1
V_BAT2
AUX1
AUX2
GND
7.5kΩ
7.5kΩ
2.5kΩ
Battery
On
2.5kΩ
Battery
On
2.5V
Reference
Ref ON/OFF
+REF
+IN
Converter
–IN
–REF
FIGURE 4. Simplified Diagram of the Analog Input Section.
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