Datasheet

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LAYOUT
TSC2004
SBAS408E – JUNE 2007 – REVISED MARCH 2008
The following layout suggestions should obtain optimum performance from the TSC2004. However, many
portable applications have conflicting requirements for 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 would mean less bypassing for the converter power and less concern regarding grounding. Still, each
application is unique and the following suggestions should be reviewed carefully.
For optimum performance, care should be taken with the physical layout of the TSC2004 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 SCL input.
With this in mind, power to the TSC2004 should be clean and well-bypassed. A 0.1 µ F ceramic bypass capacitor
should be added between (SNSVDD to AGND and SNSGND) or (I/OVDD to DGND). A 0.1 µ F decoupling
capacitor between VREF to AGND is also needed unless the SNSVDD is used as a reference input and is
connected to VREF. These capacitors must be placed as close to the device as possible. A 1 µ F to 10 µ F
capacitor may also be needed if the impedance of the connection between SNSVDD and the power supply is
high. The I/OVDD needs to be shorted to the same supply plane as the SNSVDD. Short both SNSVDD and
I/OVDD to the analog VDD plane.
The A/D converter architecture offers no inherent rejection of noise or voltage variation in regards to using an
external reference input, which is of particular concern when the reference input is tied to the power supply for
auxiliary input and temperature measurements. Any noise and ripple from the supply appears directly in the
digital results. While high-frequency noise can be filtered out by the built-in MAV filter, voltage variation as a
result of line frequency (50Hz or 60Hz) can be difficult to remove. Some package options have pins labeled as
NC (no connection). It is recommended that these NC pins be connected to the ground plane. Avoid any active
trace going under the analog pins listed in the Pin Assignments table, unless they are shielded by a ground or
power plane.
All GND (AGND, DGND, SUBGND and SNSGND) pins should be connected to a clean ground point. In many
cases, this point 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 should be taken with the connection between the
converter and the touch screen. Because resistive touch screens have fairly low resistance, the interconnection
should 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 (for example,
applications that require a back-lit LCD panel). This electromagnetic interfence (EMI) noise can be coupled
through the LCD panel to the touch screen and cause flickering of the converted A/D converter data. Several
things can be done to reduce this error, such as using a touch screen with a bottom-side metal layer connected
to ground, which couples the majority of noise to ground. Another way to filter out this type of noise is by using
the TSC2004 built-in MAV filter (see the Preprocessing section). 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, and also increases precharge and sense times for the PINTADV
circuitry of the TSC2004. The resistor value varies depending on the touch screen sensor used. The internal
50k Ω pull-up resistor (R
IRQ
) may be adequate for most of sensors.
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Product Folder Link(s): TSC2004