AD7843 Analog Devices, AD7843 Datasheet - Page 14

• Manufacturer Part Number: AD7843
• Description: Touch Screen Digitizer
• Manufacturer: Analog Devices

Specifications of AD7843

• Resolution (bits): 12bit
• # Chan: 4
• Sample Rate: 125kSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: SOP

AD7843
CONTROL REGISTER
The control word provided to the ADC via the DIN pin is
shown in Table 6. This provides the conversion start, channel
addressing, ADC conversion resolution, configuration, and
power-down of the AD7843.
Table 6 provides detailed information 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 ignores 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 5
and Figure 19), touch screen drivers, and the reference inputs.
MODE
The MODE bit sets the resolution of the analog to digital
converter. With 0 in this bit, the following conversion has 12 bits
of resolution. With 1 in this bit, the following conversion has 8
bits of resolution.
SER/ DFR
The SER/ DFR bit controls the reference mode, which can be
either single-ended or differential if 1 or 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
Table 6. Control Register Bit Function Description
MSB
S
Bit
7
6–4
3
2
1, 0
Mnemonic
S
A2–A0
MODE
SER/DFR
PD1, PD0
A2
Comment
Start Bit. The control word starts with the first high bit on DIN. A new control word can start every 15th DCLK cycle
when in the 12-bit conversion mode, or every 11th 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 multiplexer input,
switches, and reference inputs, as described in Table 5.
12-Bit/8-Bit Conversion Select Bit. This bit controls the resolution of the following conversion. With 0 in this bit, the
conversion has a 12-bit resolution, or with 1 in this bit, the conversion has a 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 5.
Power Management Bits. These two bits decode the power-down mode of the AD7843, as shown in Table 7.
A1
A0
MODE
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derived from the voltage at the switch drivers, which is almost
the same as the voltage to the touch screen. In this case, a
separate reference voltage is not needed because the reference
voltage to the ADC is the voltage across the touch screen. In
single-ended mode, the reference voltage to the converter is
always the difference between the V
Table 5 and Figure 19 through Figure 22 for further
information.
Because the supply current required by the device is so low, a
precision 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 could require 5 mA to 10 mA. A
REF19x voltage reference can source up to 30 mA and, as such,
could supply both the ADC and the touch screen. Care must be
taken, however, to ensure that the input voltage applied to the
ADC does not exceed the reference voltage and therefore the
supply voltage. See the Absolute Maximum Ratings section.
Note that the differential mode can only be used for X-position
and Y-Position measurements. All other measurements require
single-ended mode.
PD0 and PD1
The power management options are selected by programming
the power management bits, PD0 and PD1, in the control
register. Table 7 summarizes the available options. On power-up,
PD0 defaults to 0, while PD1 defaults to 1..
SER/DFR
PD1
REF
and GND pins. See
LSB
PD0