AD7843ARU(Rev0) データシートの表示（PDF） - Analog Devices

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AD7843ARU
(Rev.:Rev0)

Touch Screen Digitizer

Analog Devices 

AD7843

reference mode can only be used with +VCC as the source of the

+REF voltage and cannot be used with VREF.

The disadvantage of this mode of operation is that during both

the acquisition phase and conversion process, the external touch

screen must remain powered. This will result in additional sup-

ply current for the duration of the conversion.

+VCC

Y+

X+

IN+

REF+

IN+ ADC CORE

IN– REF–

Y–

GND

Figure 7. Differential Reference Mode (SER/DFR = 0)

CONTROL REGISTER

The control word provided to the ADC via the DIN pin is

shown in Table II. This provides the conversion start, channel

addressing, ADC conversion resolution, configuration and

power-down of the AD7843. Table II provides detailed infor-

mation on the order and description of these control bits within

the control word.

Initiate START

The first bit, the “S” bit, must always be set to 1 to initiate the

start of the control word. The AD7843 will ignore any inputs on

the DIN line until the start bit is detected.

Channel Addressing

The next three bits in the control register, A2, A1 and A0, select

the active input channel(s) of the input multiplexer (see Table I

and Figure 4), touch screen drivers, and the reference inputs.

MODE

The MODE bit sets the resolution of the analog to digital con-

verter. With a 0 in this bit the following conversion will have

12 bits of resolution. With a 1 in this bit the following conver-

sion will have 8 bits of resolution.

SER/DFR

The SER/DFR bit controls the reference mode which can be

either single ended or differential if a 1 or a 0 is written to this

bit respectively. The differential mode is also referred to as

the ratiometric conversion mode. This mode is optimum for

X-Position and Y-Position measurements. The reference is

derived from the voltage at the switch drivers, which is almost

the same as the voltage to the touch screen. In this case a sepa-

rate reference voltage is not needed as the reference voltage to

the analog to digital converter is the voltage across the touch

screen. In the single-ended mode, the reference voltage to the

converter is always the difference between the VREF and GND pins.

See Table I and Figures 4 through 7 for further information.

As the supply current required by the device is so low, a preci-

sion reference can be used as the supply source to the AD7843.

It may also be necessary to power the touch screen from the

reference, which may require 5 mA to 10 mA. A REF19x volt-

age reference can source up to 30 mA and, as such, could supply

both the ADC and the touch screen. Care must be taken, how-

ever, to ensure that the input voltage applied to the ADC does

not exceed the reference voltage and hence the supply voltage.

See Maximum Ratings section.

NOTE: The differential mode can only be used for X-Position

and Y-Position measurements All other measurements require

the single-ended mode.

PD0 and PD1

The power management options are selected by programming

the power management bits, PD0 and PD1, in the control regis-

ter. Table III summarizes the available options.

MSB

S

A2

Table II. Control Register Bit Function Description

A1

A0

MODE SER/DFR PD1

LSB

PD0

Bit Mnemonic

7S

6–4 A2–A0

3 MODE

2 SER/DFR

1, 0 PD1, PD0

Comment

Start Bit. The Control word starts with the first high bit on DIN. A new control word can start every fifteenth

DCLK cycle when in the 12-bit conversion mode or every eleventh DCLK cycle when in 8-bit conversion mode.

Channel Select Bits. These three address bits along with the SER/DFR bit control the setting of the multi-

plexer input, switches, and reference inputs, as detailed in Table I.

12-Bit/8-Bit Conversion Select Bit. This bit controls the resolution of the following conversion. With a 0 in

this bit the conversion will have 12-bit resolution or with a 1 in this bit, 8-bit resolution.

Single-Ended/Differential Reference Select Bit. Along with bits A2–A0, this bit controls the setting of the

multiplexer input, switches, and reference inputs as described in Table I.

Power Management Bits. These two bits decode the power-down mode of the AD7843 as shown in Table III.

REV. 0

–11–

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