Z-World eDisplay OP7200, User Manual

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User’s Manual 27
3.4.4 Touchscreen
The OP7200 analog touchscreen provides a high-resolution matrix of 4096 × 4096
elements. The touchscreen is mounted to the front of and is the same size as the LCD mod-
ule. A four-conductor flex cable connects the touchscreen to the OP7200 at connector J13.
The inputs from the touchscreen are protected from ESD by ferrite beads, capacitors, and
shunt diodes. The ferrite beads and capacitors also serve to eliminate EMI radiating from
the cable. Ferrite beads rather than resistors are used in series with the inputs to maintain
the most accurate measurement of the touchscreen x,y position.
A reference voltage is applied across the touchscreen. When the touchscreen is touched,
resistances that represent the x,y position are presented at the input circuit. The touch-
screen controller chip U9 converts these resistances into digital form for use by the soft-
ware.
3.4.5 Analog Supply Voltage
The analog section is isolated from digital noise generated by other components by way of a
low-pass filter composed of L2, C31, and C32 as shown in the left side of Figure 13. The +V
analog power supply powers the A/D converter chip.
Figure 13. Analog Supply and Voltage Reference Circuits
3.4.6 A/D Converter Reference Voltage (+V)
A reference voltage of 2.048 V is generated by the A/D converter chip. The reference volt-
age is used by the touchscreen controller chip, and may also be used to bias the input
attenuator circuits when bipolar inputs are to be measured. As shown in Figure 13, the
factory default is for a surface-mounted 0
Ω
resistor to connect pins 1–2 on header JP8.
This enables the internal reference voltage of 2.048 V generated by the A/D converter chip.
By connecting pins 2–3 on header JP8 instead, a ratiometric reference can be provided by
the divider consisting of R148 and R150. A fixed reference can be configured by remov-
ing R150 and installing a zener diode at U12. The zener diode will then set the reference
voltage. C91 would be always installed, and provides filtering. None of these components
(R148, R150, C91, or U12) is factory-installed.
C91
100 nF
R148
100
W
+V
R150
453
W
1 3 2
JP8
ADC Chip
Internal
Reference
Voltage
U12
To
A/D Converter
+V
Vcc
C31
100 nF
C32
100 nF
L2