Data Sheet
AD7873
CONTROL REGISTER
The control word provided to the ADC via the DIN pin is
shown in Table 7. This provides the conversion start, channel
addressing, ADC conversion resolution, configuration, and
power-down of the AD7873. Table 7 provides detailed
information on the order and description of these control bits
within the control word.
converter is always the difference between the VREF and GND
pins. See Table 6 and Figure 26 through Figure 29 for further
information.
If X-position, Y-position, and pressure touch are measured in
single-ended mode, an external reference voltage or +VCC is
required for maximum dynamic range. The internal reference
can be used for these single-ended measurements; however, a
loss in dynamic range is incurred. If an external reference is
used, the AD7873 should also be powered from the external
reference. Because the supply current required by the device is
so low, a precision reference can be used as the supply source to
the AD7873. It might also be necessary to power the touch
screen from the reference, which can 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.
Initiate START
The first bit, the S bit, must always be set to 1 to initiate the start
of the control word. The AD7873 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 6
and Figure 26), touch screen drivers, and the reference inputs.
Mode
The MODE bit sets the resolution of the analog-to-digital
converter. With a 0 in this bit, the following conversion has
12 bits of resolution. With a 1 in this bit, the following
conversion has eight bits of resolution.
Note that the differential mode can only be used for X-position,
Y-position, and pressure touch measurements. All other
measurements require single-ended mode.
SER/
DFR
PD0 and PD1
The SER/
bit controls the reference mode, set to either
DFR
The power management options are selected by programming
the power management bits, PD0 and PD1, in the control
register. Table 8 summarizes the options available and the
internal reference voltage configurations. The internal reference
can be turned on or off independent of the analog-to-digital
converter, allowing power saving between conversions using the
power management options. On power-up, PD0 defaults to 0,
while PD1 defaults to 1.
single-ended or differential when 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, Y-position, and pressure-touch 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 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
MSB
LSB
S
A2
A1
A0
MODE
SER/DFR
PD1
PD0
Table 7. Control Register Bit Function Description
Bit No.
Mnemonic Comment
7
S
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.
± to ꢀ
3
A2 to A0
MODE
SER/DFR
PD1, PD0
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 detailed in Table ±.
12-Bit/8-Bit Conversion Select Bit. This bit controls the resolution of the following conversion. With a 0 in this bit,
the conversion has 12-bit resolution or, with a 1 in this bit, 8-bit resolution.
Single-Ended/Differential Reference Select Bit. Together with Bit A2 to Bit A0, this bit controls the setting of the
multiplexer input, switches, and reference inputs as described in Table ±.
Power Management Bits. These two bits decode the power-down mode of the AD7873 as shown in Table 8.
2
1, 0
Rev. F | Page 19 of 28
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