10.1 Layout Guidelines

The following layout suggestions may allow optimum performance from the TSC2007-Q1. Keep in mind that 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. However, each situation is unique and the following suggestions must be reviewed carefully.

For optimum performance, take care of the physical layout of the TSC2007-Q1 circuitry. The basic SAR architecture is sensitive to glitches or sudden changes on the power supply, reference, ground connections, and digital inputs that occur immediately before 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 consideration in mind, power to the TSC2007-Q1 must be clean and well bypassed. A 0.1-µF ceramic bypass capacitor must be placed as close to the device as possible. In addition, a 1-µF to 10-µF capacitor may also be required if the impedance of the connection between VDD/REF and the power supply is high.

A bypass capacitor is generally not required 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-Q1 architecture offers no inherent rejection of noise or voltage variation with regard 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 appears directly in the digital results. While high-frequency noise can be filtered out, voltage variation because of line frequency (50 Hz or 60 Hz) can be difficult to remove. Some package options have pins labeled as VOID. Avoid any active trace going under any pin marked as VOID unless it is shielded by a ground or power plane.

The GND pin must 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 required, 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, take care with the connection between the converter and the touch screen. Resistive touch screens have fairly low resistance; therefore, 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 (for example, applications that require a back-lit LCD panel). This electromagnetic interference (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. Additionally, filtering capacitors, from Y+, Y–, X+, and X– to ground, can also help. However, 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 sensor used. The PENIRQ pullup resistor (R_IRQ) may be adequate for most of sensors.

10.2 Layout Example

Figure 38. Example Layout for TSC2007-Q1