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LAYOUT

TSC2007

SBAS405 – MARCH 2007

The following layout suggestions should obtain optimum performance from the TSC2007. 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

situation is unique and the following suggestions should be reviewed carefully.

For optimum performance, care should be taken with the physical layout of the TSC2007 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 TSC2007 should be clean and well bypassed. A 0.1

µF ceramic bypass capacitor

should be placed as close to the device as possible. In addition, a 1

µF to 10µF capacitor may also be needed if

the impedance of the connection between VDD/REF and the power supply is high.

A bypass capacitor is generally not needed on the VDD/REF pin because the internal reference is buffered by

an internal op amp. 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 TSC2007 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. Any

noise and ripple from the supply will appear directly in the digital results. While high-frequency noise can be

filtered out, voltage variation due to line frequency (50Hz or 60Hz) can be difficult to remove. Some package

options have pins labeled as VOID. Avoid any active trace going under those pins marked as VOID unless they

are shielded by a ground or power plane.

The GND pin should be connected to a clean ground point. In many cases, this point will be 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. Since 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 ADC 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 will couple 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 a longer time for panel voltages to stabilize. The resistor value varies depending on the touch screen

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