Data Sheet AD7873
Rev. F | Page 13 of 28
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.
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 V
REF
(where the externally applied V
REF
can be between
1 V and +V
CC
). The AD7873 has a 2.5 V reference on-board
with this reference voltage available for use externally if buffered.
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 V
REF
/4096.
The ideal transfer characteristic for the AD7873 is shown in
Figure 24.
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 +V
CC
. 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.
Figure 25. Typical Application Circuit
ADC CODE
ANALOG INPUT
1LSB
0V
+V
REF
– 1LSB
1LSB = V
REF
/4096
111...111
111...110
111...000
011...111
000...010
000...001
000...000
02164-024
16
15
14
13
12
11
9
8
1
2
3
4
7
6
5
10
TOUCH
SCREEN
1µF TO 10µF
(OPTIONAL)
2.2V TO 5V
0.1µF
0.1µF
AUXILIARY
INPUT
AD7873
+V
CC
X+
Y+
X–
Y–
GND
V
BAT
AUX
DCLK
CS
DIN
BUSY
DOUT
PENIRQ
+V
CC
V
REF
SERIAL/CONVERSION CLOCK
CHIP SELECT
SERIAL DATA IN
CONVERTER STATUS
SERIAL DATA OUT
PEN INTERRUPT
50kΩ
TO BATTERY
VOLTAGE
REGULATOR
+ +
+
02164-025
