voltage, more care must be taken to provide a clean layout
including adequate bypassing, a clean (low-noise, low-ripple)
power supply, a low-noise reference (if an external reference
is used), and a low-noise input signal.
The voltage into the V
REF
input directly drives the capacitor
digital-to-analog converter (CDAC) portion of the ADS7846.
Therefore, the input current is very low (typically < 13µA).
There is also a critical item regarding the reference when
making measurements where the switch drivers are on. For
this discussion, it is useful to consider the basic operation of
the ADS7846 (see Figure 1). This particular application
shows the device being used to digitize a resistive touch
screen. A measurement of the current Y position of the
pointing device is made by connecting the X+ input to the
ADC, turning on the Y+ and Y– drivers, and digitizing the
voltage on X+ (Figure 4 shows a block diagram). For this
measurement, the resistance in the X+ lead does not affect
the conversion (it does affect the settling time, but the
resistance is usually small enough that this is not a concern).
However, since the resistance between Y+ and Y– is fairly
low, the on-resistance of the Y drivers does make a small
difference. Under the situation outlined so far, it is not
possible to achieve a 0V input or a full-scale input regardless
of where the pointing device is on the touch screen, because
some voltage is lost across the internal switches. In addition,
the internal switch resistance is unlikely to track the resis-
tance of the touch screen, providing an additional source of error.
+V
CC
V
REF
+V
CC
Y+
X+
+IN
–IN
+REF
Converter
–REF
Y–
GND
FIGURE 5. Simplified Diagram of Differential Reference
(SER/DFR Low, Y Switches Enabled, X+ is
Analog Input).
As a final note about the differential reference mode, it must
be used with +V
CC
as the source of the +REF voltage and
cannot be used with V
REF
. It is possible to use a high
precision reference on V
REF
and single-ended reference
mode for measurements which do not need to be ratiometric.
In some cases, it is possible to power the converter directly
from a precision reference. Most references can provide
enough power for the ADS7846, but might not be able to
supply enough current for the external load (such as a
resistive touch screen).
TOUCH SCREEN SETTLING
Y+
X+
+IN
–IN
+REF
Converter
–REF
Y–
GND
FIGURE 4. Simplified Diagram of Single-Ended Reference
(SER/DFR High, Y Switches Enabled, X+ is
Analog Input).
This situation can be remedied as shown in Figure 5. By
setting the
SER/DFR
bit low, the +REF and –REF inputs are
connected directly to Y+ and Y–, respectively, which makes
the analog-to-digital conversion ratiometric. The result of the
conversion is always a percentage of the external resistance,
regardless of how it changes in relation to the on-resistance of
the internal switches. Note that there is an important consid-
eration regarding power dissipation when using the ratiometric
mode of operation (see the Power Dissipation section for
more details).
In some applications, external capacitors may be required
across the touch screen for filtering noise picked up by the
touch screen (for example, noise generated by the LCD panel
or backlight circuitry). These capacitors provide a low-pass
filter to reduce the noise, but cause a settling time requirement
when the panel is touched that typically shows up as a gain
error. The problem is that the input and/or reference has not
settled to the final steady-state value prior to the ADC sam-
pling the input(s) and providing the digital output. Additionally,
the reference voltage may still be changing during the mea-
surement cycle. There are several methods for minimizing or
eliminating this issue. Option 1 is to stop or slow down the
ADS7846 DCLK for the required touch screen settling time.
This allows the input and reference to have stable values for
the Acquire period (3 clock cycles of the ADS7846; see Figure
9). This works for both the single-ended and the differential
modes. Option 2 is to operate the ADS7846 in the differential
mode only for the touch screen measurements and command
the ADS7846 to remain on (touch screen drivers on) and not
go into power-down (PD0 = 1). Several conversions are made
depending on the settling time required and the ADS7846 data
rate. Once the required number of conversions have been
made, the processor commands the ADS7846 to go into the
power-down state on the last measurement. This process is
10
ADS7846
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