AD7889

The AD7889 is a 12-bit successive approximation analog-to-digital converter (SAR ADCs) with a synchronous serial interface and low on-resistance switches for driving 4-wire resistive touch screens. The AD7889 works with a very low power supply-a single 1.6 V to 3.6 V supply-and feature throughput rates of 105 kSPS. The devices include a shutdown mode that reduces current consumption to less than 6 A.

Features of the AD7889 are:(1)4-wire touch screen interface; (2)1.6 V to 3.6 V operation; (3)median and averaging filter to reduce noise; (4)automatic conversion sequencer and timer; (5)user-programmable conversion parameters; (6)auxiliary analog input/battery monitor (0.5 V to 5 V); (7)touch-pressure measurement; (8)wake-up on touch function.To reduce the effects of noise from LCDs and other sources, the AD7889 contains a preprocessing block. The preprocessing function consists of a median filter and an averaging filter. The combination of these two filters provides a more robust solution, discarding the spurious noise in the signal and keeping only the data of interest. The size of both filters is programmable.Other user-programmable conversion controls include variable acquisition time and first conversion delay; up to 16 averages can be taken per conversion. The AD7889 can run in slave mode or standalone (master) mode, using an automatic conversion sequencer and timer.

The absolute maximum ratings of the AD7889 can be summarized as:(1)VCC to GND:-0.3V to +3.6V;(2)storage temperature range:-65 to +150;(3)lead temperature:300;(4)junction temperature:150;(5)analog input voltage to AGND:-0.3 V to Vcc+0.3V;(6)input current to any pin except supplies:10 mAA.Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.For greater accuracy, the ratiometric method has two significant advantages. One is that the reference to the ADC is provided from the actual voltage across the screen; therefore, any voltage dropped across the switches has no effect. The other advantage is that because the measurement is ratiometric, it does not matter if the voltage across the screen varies in the long term.However, it must not change after the signal has been acquired.