SLAS379A− APRIL 2003 − REVISED JUNE 2004
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REGISTER 1DH: Audio Control 4
BIT NAME READ/
WRITE RESET VALUE FUNCTION
D15 Reserved R 0 Reserved. Write only 0 to this location.
D14 DASTPD R/W 0 DAC Soft-Stepping Control
0 => Soft-stepping is enabled
1 => Soft-stepping is disabled
D13−D2 Reserved R 000000000000 Reserved. Write only 000000000000 in this location.
D1−D0 Reserved R XX Reserved (write only 00)
LAYOUT
The following layout suggestions should provide optimum performance from the TSC2102. However, many portable
applications have conflicting requirements concerning power, cost, size, and weight. In general, most portable devices
have fairly clean power and grounds because most of the internal components are very low power. This situation means
less bypassing for the converter power and less concern regarding grounding. Still, each situation is unique and the
following suggestions should be reviewed carefully.
For optimum performance, care must be taken with the physical layout of the TSC2102 circuitry. The basic SAR architecture
is sensitive to glitches or sudden changes on the power supply, reference, ground connections, and digital inputs that occur
just prior to latching the output of the analog comparator. Therefore, during any single conversion for an n-bit SAR converter,
there are n windows in which large external transient voltages can easily affect the conversion result. Such glitches might
originate from switching power supplies, nearby digital logic, and high power devices. The degree of error in the digital
output depends on the reference voltage, layout, and the exact timing of the external event. The error can change if the
external event changes in time with respect to the timing of the critical n windows.
With this in mind, power to the TSC2102 must be clean and well bypassed. A 0.1-µF ceramic bypass capacitor must be
placed as close to the device as possible. A 1-µF to 10-µF capacitor may also be needed if the impedance between the
TSC2102 supply pins and the system power supply is high.
The VREF pin requires a minimum bypass capacitor of 0.1 µF, although a larger value can be used to reduce the reference
noise level. If an external reference voltage originates from an op-amp, make sure that it can drive any bypass capacitor
that is used without oscillation.
The TSC2102 architecture offers no inherent rejection of noise or voltage variation in regards to using an external reference
input. This is of particular concern when the reference input is tied to the power supply. Any noise and ripple from the supply
appears directly in the digital results. While high frequency noise can be filtered out, voltage variation due to line frequency
(50 Hz or 60 Hz) can be difficult to remove.
The ground pins must be connected to a clean ground point. In many cases, this is the analog ground. Avoid connections
that are too near the grounding point of a microcontroller or digital signal processor. If needed, run a ground trace directly
from the converter to the power supply entry or battery connection point. The ideal layout includes an analog ground plane
dedicated to the converter and associated analog circuitry.
In the specific case of use with a resistive touch screen, care must be taken with the connection between the converter
and the touch screen. Since resistive touch screens have fairly low resistance, the interconnection must be as short and
robust as possible. Loose connections can be a source of error when the contact resistance changes with flexing or
vibrations.
As indicated previously, noise can be a major source of error in touch-screen applications (e.g., applications that require
a back-lit LCD panel). This EMI noise can be coupled through the LCD panel to the touch screen and cause flickering of
the converted ADC data. Several things can be done to reduce this error , such as utilizing a touch screen with a bottom-side
metal layer connected to ground. This couples the majority of noise to ground. Additionally, filtering capacitors, from Y+,
Y–, X+, and X– to ground, can also help. Note, however, that the use of these capacitors increases screen settling time
and requires longer panel voltage stabilization times, as well as increased precharge and sense times for the PINTDAV
circuitry of the TSC2102.