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AN-577
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • 781/329-4700 • www.analog.com
© Analog Devices, Inc., 2001
INTRODUCTION
This application note provides layout suggestions to op-
timize the performance of the AD7843/AD7873. The
AD7843/AD7873 are 12-bit successive-approximation
ADCs with a synchronous serial interface and low on-
resistance switches for driving touch screens. The
AD7843/AD7873 operate from a single 2.2 V to 5.25 V
power supply and feature throughput rates greater
than 125 kSPS.
GROUNDING AND LAYOUT
Both supplies to each of the AD7843 and AD7873 are
analog supplies and should be connected directly
together. Although various portable applications may
have conflicting requirements with respect to power
consumption, cost, size or weight, most should have
relatively clean power supplies and ground as the power
supply is from a battery and most of the internal compo-
nents are very low power. However care should still be
taken with regard to grounding and layout. It should
also be noted that each situation is unique and the fol-
lowing suggestions should be carefully reviewed.
The printed circuit board that houses the AD7843 or the
AD7873 should be designed so the analog and digital
sections are separated and confined to certain areas of
the board. This facilitates the use of ground planes that
can easily be separated. A minimum etch technique is
generally best for ground planes as it gives the best
shielding. The GND pin of the AD7843/AD7873 should
be sunk in the analog ground plane. The AGND plane
Layout and Grounding Recommendations for Touch Screen Digitizers
by Nicola O’Byrne
FUNCTIONAL BLOCK DIAGRAMS
PEN
INTERRUPT
DCLK
GND
+V
CC
BUSY
PENIRQ
SAR + ADC
CONTROL LOGIC
SPORT
CS
DOUTDIN
4-TO-1
I/P
MUX
AD7843
+V
CC
IN3
V
REF
IN4
Y–
Y+
X–
X+
T/H
CHARGE
REDISTRIBUTION
DAC
COMP
DCLK
GND
+V
CC
BUSY
PENIRQ
PEN
INTERRUPT
SAR + ADC
CONTROL LOGIC
SPORT
CS
DOUTDIN
6-TO-1
I/P
MUX
AD7873
+V
CC
AUX
V
REF
Y–
Y+
X–
X+
V
BAT
TEMP
SENSOR
CHARGE
REDISTRIBUTION
DAC
2.5V
REF
BUF
BATTERY
MONITOR
T/H
COMP
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