AD7877 Analog Devices, AD7877 Datasheet - Page 14

• Manufacturer Part Number: AD7877
• Description: Touch Screen Controller
• Manufacturer: Analog Devices

Specifications of AD7877

• Resolution (bits): 12bit
• # Chan: 9
• Sample Rate: 125kSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: CSP

AD7877
CIRCUIT INFORMATION
The AD7877 is a complete, 12-bit data acquisition system for
digitizing positional inputs from a touch screen in PDAs and
other devices. In addition, it can monitor two battery voltages,
ambient temperature, and three auxiliary analog voltages, with
high and low limit comparisons on three of the inputs, and has
up to four general-purpose logic I/O pins.
The core of the AD7877 is a high speed, low power, 12-bit
analog-to-digital converter (ADC) with input multiplexer,
on-chip track-and-hold, and on-chip clock. The results of
conversions are stored in 11 results registers, and the results
from one auxiliary input and two battery inputs can be com-
pared with high and low limits stored in limit registers to generate
an out-of-limit ALERT . The AD7877 also contains low resistance
analog switches to switch the X and Y excitation voltages to the
touch screen, a STOPACQ pin to control the ADC acquisition
period, 2.5 V reference, on-chip temperature sensor, and 8-bit
DAC to control LCD contrast. The high speed SPI serial bus
provides control of, and communication with, the device.
Operating from a single supply from 2.2 V to 5 V, the AD7877
offers throughput rates of up to 125 kHz. The device is available
in a 5 mm × 5 mm, 32-lead, lead frame chip scale package
(LFCSP), and in a 2.5 mm × 2.8 mm, wafer level chip scale
package (WLCSP), with a 5 × 5 ball grid array.
The data acquisition system of the AD7877 has a number of
advanced features:
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TOUCH SCREEN PRINCIPLES
A 4-wire touch screen consists of two flexible, transparent,
resistive-coated layers that are normally separated by a small air
gap. The X layer has conductive electrodes running down the
left and right edges, allowing the application of an excitation
voltage across the X layer from left to right.
Input channel sequenced automatically or selected by
the host.
STOPACQ feature to reduce noise from LCD.
Averaging of from 1 to 16 conversions for noise reduction.
Programmable acquisition time.
Power management.
Programmable ADC power-up delay before first
conversion.
Choice of internal or external reference.
Conversion at preprogrammed intervals.
Rev. D | Page 14 of 44
The Y layer has conductive electrodes running along the top
and bottom edges, allowing the application of an excitation
voltage down the Y layer from top to bottom.
Provided that the layers are of uniform resistivity, the voltage at
any point between the two electrodes is proportional to the
horizontal position for the X layer and the vertical position for
the Y layer.
When the screen is touched, the two layers make contact. If
only the X layer is excited, the voltage at the point of contact,
and therefore the horizontal position, can be sensed at one of
the Y layer electrodes. Similarly, if only the Y layer is excited,
the voltage, and therefore the vertical position, can be sensed at
one of the X layer electrodes. By switching alternately between
X and Y excitation and measuring the voltages, the X and
Y coordinates of the contact point can be found.
In addition to measuring the X and Y coordinates, it is also
possible to estimate the touch pressure by measuring the
contact resistance between the X and Y layers. The AD7877 is
designed to facilitate this measurement.
Figure 28 shows an equivalent circuit of the analog input
structure of the AD7877, showing the touch screen switches,
the main analog multiplexer, the ADC with analog and
differential reference inputs, and the dual 3-to-1 multiplexer
that selects the reference source for the ADC.
CONDUCTIVE ELECTRODE
CONDUCTIVE ELECTRODE
ON BOTTOM SIDE
ON TOP SIDE
Figure 27. Basic Construction of a Touch Screen
LCD SCREEN
Y+
X+
Y–
TRANSPARENT, RESISTIVE
COATING ON BOTTOM SIDE
TRANSPARENT, RESISTIVE
COATING ON TOP SIDE
PLASTIC FILM WITH
PLASTIC FILM WITH
Data Sheet
X–