The AD7877 is a 12-bit successive approximation ADC with a synchronous serial interface and low on resistance switches for driving touch screens. The AD7877 features direct battery measurement on two inputs, temperature and touch-pressure measurement. The AD7877 has many user-programmable conversion controls, which include variable acquisition time, first conversion delay, and averaging. It is ideal for battery-powered systems such as personal digital assistants, smart phones, and other portable equipment with resistive touch screens. One of the AD7877’s key features is the STOPACQ input pin, which can be used to reduce the effects of noise on the touch screen measurements. This application note explains in detail how noise can affect the touch screen measurements, where this noise comes from, and how to use the AD7877’s STOPACQ feature to reduce or eliminate this noise in an application.
The document describes the AD7877, a 12-bit successive approximation ADC designed for resistive touch screens. It details how external noise, particularly from nearby LCD displays during refresh cycles, can distort positional measurements.
The note focuses on the STOPACQ input pin feature, which allows users to disable acquisitions when an active noise signal (such as HSYNC or Vcom) is present. The text includes timing details for acquisition and conversion phases, explains how to configure the polarity bit in Control Register 2, and presents experimental results demonstrating a significant reduction in measurement error when using this feature.
The STOPACQ pin allows the device to monitor an external signal and disable input acquisitions during noisy periods, such as when an LCD refresh signal is active.
If the STOPACQ signal becomes active during an acquisition cycle, the current acquisition is aborted and restarted from the beginning once the signal deasserts.
The user can program the acquisition time to be 2 us, 4 us, 8 us, or 16 us.