Data Sheet
AD7873
CIRCUIT INFORMATION
The AD7873 is a fast, low power, 12-bit, single-supply analog-
to-digital converter (ADC). The AD7873 can be operated from
a 2.2 V to 5.25 V supply. When operated from either a 5 V
supply or a 3 V supply, the AD7873 is capable of throughput
rates of 125 kSPS when provided with a 2 MHz clock.
111...111
111...110
111...000
011...111
1LSB = V
/4096
REF
The AD7873 provides the user with on-chip track-and-hold,
multiplexer, ADC, reference, temperature sensor, and serial
interface, housed in a tiny 16-lead QSOP, TSSOP, or LFCSP
package, offering the user considerable space-saving advantages
over alternative solutions. The serial clock input (DCLK)
accesses data from the part and also provides the clock source
for the successive approximation ADC. The analog input range
is 0 V to VREF (where the externally applied VREF can be between
1 V and +VCC). The AD7873 has a 2.5 V reference on-board
with this reference voltage available for use externally if buffered.
000...010
000...001
000...000
+V
– 1LSB
1LSB
REF
0V
ANALOG INPUT
Figure 24. Transfer Characteristic
TYPICAL CONNECTION DIAGRAM
Figure 25 shows a typical connection diagram for the AD7873
in a touch screen control application. The AD7873 features an
internal reference, but this can be overdriven with an external
low impedance source between 1 V and +VCC. The value of the
reference voltage sets the input range of the converter. The
conversion result is output MSB first, followed by the remaining
11 bits and three trailing zeros, depending on the number of
clocks used per conversion (see the Serial Interface section). For
applications where power consumption is a concern, the power
management option should be used to improve power perform-
ance. See Table 8 for available power management options.
The analog input to the ADC is provided via an on-chip
multiplexer. This analog input can be any one of the X, Y, and Z
panel coordinates, the battery voltage, or the chip temperature.
The multiplexer is configured with low resistance switches that
allow an unselected ADC input channel to provide power and
an accompanying pin to provide ground for an external device.
For some measurements, the on resistance of the switches could
present a source of error. However, with a differential input to
the converter and a differential reference architecture, this error
can be negated.
ADC TRANSFER FUNCTION
The output coding of the AD7873 is straight binary. The
designed code transitions occur at successive integer LSB values
(that is, 1 LSB, 2 LSBs, and so on). The LSB size is VREF/4096.
The ideal transfer characteristic for the AD7873 is shown in
Figure 24.
2.2V TO 5V
+
+
1µF TO 10µF
(OPTIONAL)
0.1µF
+V
DCLK
1
2
3
4
5
6
7
8
16
15
14
SERIAL/CONVERSION CLOCK
CHIP SELECT
CC
X+
Y+
X–
CS
AD7873
SERIAL DATA IN
DIN
CONVERTER STATUS
SERIAL DATA OUT
BUSY 13
TOUCH
SCREEN
12
Y–
DOUT
TO BATTERY
11
GND
PEN INTERRUPT
PENIRQ
+V
V
V
10
9
CC
BAT
AUXILIARY
INPUT
50kΩ
AUX
REF
+
0.1µF
VOLTAGE
REGULATOR
Figure 25. Typical Application Circuit
Rev. F | Page 13 of 28
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