LM8500-REF-01 National Semiconductor, LM8500-REF-01 Datasheet - Page 21

• Manufacturer Part Number: LM8500-REF-01
• Description: BOARD EVALUATION LM8500
• Manufacturer: National Semiconductor

Specifications of LM8500-REF-01

• Main Purpose: Interface, Touch Screen Controller
• Utilized Ic / Part: LM8500
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -

Touch Screen Connection
The device was designed to drive a 4 wire resistive touchscreen directly without the need of
external components. Care should be exercised when the touchscreen is used in a noisy
environment. Noise can be picked up from sources such as back-light for a LCD display or from
external EMI/RFI sources. The simplest method is to connect a capacitor from the touchscreen
lines to ground, as shown in fig. 1.
As an ESD precaution, each touch screen line is also connected to a pair of clamping diodes.
These diode pairs ensure that the voltage level going into the device is no greater than analog
V
(AVCC) and no less than analog ground (AGND).
CC
Fig. 1
External filtering
With the use of the diode clamp and the bypass capacitor, the amount of noise being filtered is
already very good. To further eliminate noise, it is possible to connect a filter stage before the
A/D conversion is done on the touchscreen. The filter can either be a passive or active filter.
The passive filter has the advantage of being lower in component count while the active filter has
the advantage of being closer to the ideal “brick wall” for frequency filtering.
The active filter shown in Fig. 2 is op-amp base low pass filter and the passive filter is a low pass
RC filter. The corner frequency for both filters is 3 kHz. J4 and J5 are jumper blocks that are
used to switch between the passive and active filter.