EVAL-AD7843CB3 AD [Analog Devices], EVAL-AD7843CB3 Datasheet - Page 11

• Manufacturer Part Number: EVAL-AD7843CB3
• Description: Touch Screen Digitizer
• Manufacturer: AD [Analog Devices]

reference mode can only be used with +V
+REF voltage and cannot be used with V
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.
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.
Bit
7
6–4
3
2
1, 0
MSB
S
Mnemonic
S
A2–A0
MODE
SER/DFR
PD1, PD0
GND
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.
A2
Y–
Y+
X+
+V
CC
IN+
IN+ ADC CORE
Table II. Control Register Bit Function Description
IN– REF–
REF
A1
CC
REF+
as the source of the
.
DFR
A0
MODE
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 V
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.
SER/DFR
PD1
LSB
PD0
REF
AD7843
and GND pins.