Data Sheet AD7873
Rev. F | Page 15 of 28
Acquisition Time
The track-and-hold amplifier enters tracking mode on the
falling edge of the fifth DCLK after the start bit is detected (see
Figure 35). The time required for the track-and-hold amplifier
to acquire an input signal depends on how quickly the 37 pF
input capacitance is charged. With zero source impedance on
the analog input, three DCLK cycles are always sufficient to
acquire the signal to the 12-bit level. With a source impedance
(R
IN
) on the analog input, the actual acquisition time required is
calculated using the formula:
( )
pF37Ω1004.8 ×+×=
IN
ACQ
Rt
where R
IN
is the source impedance of the input signal, and 100 Ω,
37 pF is the input RC. Depending on the frequency of DCLK
used, three DCLK cycles may or may not be sufficient to acquire
the analog input signal with various source impedance values.
Touch Screen Settling
In some applications, external capacitors could be required
across the touch screen to filter noise associated with it, for
example, noise generated by the LCD panel or backlight
circuitry. The value of these capacitors causes a settling time
requirement when the panel is touched. The settling time
typically appears as a gain error. There are several methods for
minimizing or eliminating this issue. The problem can be that
the input signal, reference, or both, have not settled to their
final value before the sampling instant of the ADC. Additionally,
the reference voltage could still be changing during the conversion
cycle. One option is to stop or slow down the DCLK for the
required touch screen settling time. This allows the input and
reference to stabilize for the acquisition time, resolving the issue
for both single-ended and differential modes.
The other option is to operate the AD7873 in differential mode
only for the touch screen, and program the AD7873 to keep
the touch screen drivers on and not go into power-down
(PD0 = PD1 = 1). Several conversions could be required,
depending on the settling time required and the AD7873 data
rate. Once the required number of conversions have been made,
the AD7873 can then be placed in a power-down state on the
last measurement. The last method is to use the 15-DCLK cycle
mode, maintaining the touch screen drivers on until it is
commanded by the processor to stop.
Internal Reference
The AD7873 has an internal reference voltage of 2.5 V. The
internal reference is available on the V
REF
pin for external use in
the system; however, it must be buffered before it is applied
elsewhere. The on-chip reference can be turned on or off with
the power-down address, PD1 = 1 (see Table 8 and Figure 27).
Typically, the reference voltage is only used in single-ended
mode for battery monitoring, temperature measurement, and
for using the auxiliary input. Optimal touch screen performance
is achieved when using the differential mode. The power-up
time of the 2.5 V reference is typically 10 µs without a load;
however, a 0.1 µF capacitor on the V
REF
pin is recommended for
optimum performance because it affects the power-up time (see
Figure 20).
Figure 27. On-Chip Reference Circuitry
Reference Input
The voltage difference between +REF and −REF (see Figure 26)
sets the analog input range. The AD7873 operates with a refer-
ence input in the range of 1 V to +V
CC
. Figure 27 shows the
on-chip reference circuitry on the AD7873. The internal
reference on the AD7873 can be overdriven with an external
reference; for best performance, however, the internal reference
should be disabled when an external reference is applied,
because SW1 in Figure 27 opens on the AD7873 when the
internal reference is disabled. The on-chip reference always is
available at the V
REF
pin as long as the reference is enabled. The
input impedance seen at the V
REF
pin is approximately 260 Ω
when the internal reference is enabled. When it is disabled, the
input impedance seen at the V
REF
pin is in the GΩ region.
When making touch screen measurements, conversions can be
made in differential (ratiometric) mode or single-ended mode.
If the SER/
DFR
bit is set to 1 in the control register, then a
single-ended conversion is performed. Figure 28 shows the
configuration for a single-ended Y coordinate measurement.
The X+ input is connected to the analog-to-digital converter,
the Y+ and Y− drivers are turned on, and the voltage on X+ is
digitized. The conversion is performed with the ADC referenced
from GND to V
REF
. This V
REF
is either the on-chip reference or
the voltage applied at the V
REF
pin externally, and is determined
by the setting of the power management Bit PD0 and Bit PD1
(see Table 7). The advantage of this mode is that the switches
that supply the external touch screen can be turned off once the
acquisition is complete, resulting in a power savings. However,
the on resistance of the Y drivers affects the input voltage that
can be acquired. The full touch screen resistance could be in the
order of 200 Ω to 900 Ω, depending on the manufacturer. Thus,
if the on resistance of the switches is approximately 6 Ω, true
full-scale and zero-scale voltages cannot be acquired, regardless
of where the pen/stylus is on the touch screen. Note that the
minimum touch screen resistance recommended for use with
X+
Y+
V
REF
260Ω
SW1
3-TO-1
MUX
2.5V
REF
ADC
BUF
02164-027
