AD7843 datasheet(12/21 Pages) AD

12 / 21 page

AD7843

Rev. B | Page 11 of 20

CIRCUIT INFORMATION

The AD7843 is a fast, low-power, 12-bit, single-supply, A/D

converter. The AD7843 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 AD7843 is capable of throughput rates of 125 kSPS when

provided with a 2 MHz clock.

The AD7843 provides the user with an on-chip track-and-hold,

multiplexer, ADC, and serial interface housed in tiny 16-lead

QSOP or TSSOP packages, which offer 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 analog input to the ADC is provided via an on-chip

multiplexer. This analog input can be any one of the X and Y

panel coordinates. 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 AD7843 is straight binary. The

designed code transitions occur at successive integer LSB values

(that is, 1 LSB, 2 LSBs, and so forth.). The LSB size equals

VREF/4096. The ideal transfer characteristic for the AD7843 is

shown in Figure 17.

ANALOG INPUT

1LSB

+VREF–1LSB

1LSB = VREF/4096

111...111

111...110

111...000

011...111

000...010

000...001

000...000

Figure 17. AD7843 Transfer Characteristic

TYPICAL CONNECTION DIAGRAM

Figure 18 shows a typical connection diagram for the AD7843

in a touch screen control application. The AD7843 requires an

external reference and an external clock. The external reference

can be any voltage 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 the available power management

options.

TOUCH

SCREEN

µF TO 10µF

(OPTIONAL)

2.2V TO 5V

0.1

µF

0.1

µF

AUXILIARY INPUTS

AD7843

+VCC

X+

Y+

X–

Y–

GND

IN3

IN4

DCLK

DIN

BUSY

DOUT

PENIRQ

+VCC

VREF

SERIAL/CONVERSION CLOCK

CHIP SELECT

SERIAL DATA IN

CONVERTER STATUS

SERIAL DATA OUT

PEN INTERRUPT

100k

Ω

(OPTIONAL)

Figure 18. Typical Application Circuit

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