Datasheet

®
12
ADS7845
large external transient voltages can easily affect the conver-
sion result. Such glitches might originate from switching
power supplies, nearby digital logic, and high power de-
vices. The degree of error in the digital output depends on
the reference voltage, layout, and the exact timing of the
external event. The error can change if the external event
changes in time with respect to the DCLK input.
With this in mind, power to the ADS7845 should be clean
and well bypassed. A 0.1µF ceramic bypass capacitor should
be placed as close to the device as possible. A 1µF to 10µF
capacitor may also be needed if the impedance of the
connection between +V
CC
and the power supply is high.
The reference should be similarly bypassed with a 0.1µF
capacitor. If the reference voltage originates from an op
amp, make sure that it can drive the bypass capacitor without
oscillation. The ADS7845 draws very little current from the
reference on average, but it does place larger demands on the
reference circuitry over short periods of time (on each rising
edge of DCLK during a conversion).
The ADS7845 architecture offers no inherent rejection of
noise or voltage variation in regards to the reference input.
This is of particular concern when the reference input is tied
to the power supply. Any noise and ripple from the supply
will appear directly in the digital results. While high fre-
quency noise can be filtered out, voltage variation due to line
frequency (50Hz or 60Hz) can be difficult to remove.
The GND pin should be connected to a clean ground point.
In many cases, this will be the “analog” ground. Avoid
connections which are too near the grounding point of a
microcontroller or digital signal processor. If needed, run a
ground trace directly from the converter to the power supply
entry or battery connection point. The ideal layout will
include an analog ground plane dedicated to the converter
and associated analog circuitry.
In the specific case of use with a resistive touch screen, care
should be taken with the connection between the converter
and the touch screen. Since resistive touch screens have
fairly low resistance, the interconnection should be as short
and robust as possible. Longer connections will be a source
of error, much like the on-resistance of the internal switches.
Likewise, loose connections can be a source of error when
the contact resistance changes with flexing or vibrations.
PENIRQ Output
The pen interrupt output function is detailed in Figure 11. By
connecting a pull-up resistor to V
CC
(typically 100kΩ), the
PENIRQ output is HIGH. While in the power-down mode,
with PD0 = PD1 = 0, the lower-right panel corner is
connected to GND and the PENIRQ output is connected to
the WIPER input. When the panel is touched, the PENIRQ
output goes LOW, due to the current path through the panel
to GND, initiating an interrupt to the processor. During the
measurement cycles for X and Y position, the PENIRQ
output diode will be internally connected to GND and the
WIPER disconnected from the PENIRQ diode to eliminate
any leakage current from the pull-up resistor to flow through
the WIPER, thus causing no errors.
In addition, when the DIN has selected A2 = 1, A1 = 1,
A0 = 0, and the ADS7845 is commanded into the power-
down mode (PD0 and PD1 = 0) and CS is LOW (when CS
is HI, the DOUT line is high impedance), the DOUT will be
LOW (all “0”s) during no touch and HI (all “1”s) when the
panel is touched. This feature eliminates the need for an
additional port to detect panel touch. Since all panels have
end resistance, all “0”s and all “1”s are an unused set of
codes.
FIGURE 11. PENIRQ Functional Block Diagram.
+V
CC
100kΩ
UR
OFFOFF
OFF
LR
ON
WIPER
UR, LL
Driver
PENIRQ
UL
LL