AD7873ACP datasheet(13/28 Pages) AD

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AD7873

Rev. D | Page 13 of 28

CIRCUIT INFORMATION

The AD7873 is a fast, low-power, 12-bit, single-supply A/D

converter. 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 an on-chip track-and-hold,

multiplexer, A/D converter, reference, temperature sensor, and

serial interface housed in a tiny 16-lead QSOP, TSSOP, or

LFCSP package, which offers 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.

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, battery voltage, or 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.

ANALOG INPUT

1LSB

+VREF–1LSB

1LSB = VREF/4096

111...111

111...110

111...000

011...111

000...010

000...001

000...000

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 zeroes, 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

performance. See Table 7 for available power management

options.

TOUCH

SCREEN

µF TO 10µF

(OPTIONAL)

2.2V TO 5V

0.1

µF

0.1

µF

AUXILIARY

INPUT

AD7873

+VCC

X+

Y+

X–

Y–

GND

VBAT

AUX

DCLK

DIN

BUSY

DOUT

PENIRQ

+VCC

VREF

SERIAL/CONVERSION CLOCK

CHIP SELECT

SERIAL DATA IN

CONVERTER STATUS

SERIAL DATA OUT

PEN INTERRUPT

50k

Ω

TO BATTERY

VOLTAGE

REGULATOR

Figure 25. Typical Application Circuit

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