Datasheet

Reference
Reference Mode
Converter
+IN
+REF
Y+
VDD/REF
X+
Y-
GND
-REF
-IN
Converter
+IN
+REF
Y+
VDD/REF
X+
Y-
GND
-REF
-IN
TSC2007
www.ti.com ...................................................................................................................................................... SBAS405A – MARCH 2007 – REVISED MARCH 2009
The TSC2007 uses an external voltage reference that is applied to the VDD/REF pin. The upper reference
voltage range is the same as the supply voltage range, which allows for simple, 1.2V to 3.6V, single-supply
operation of the chip.
There is a critical item regarding the reference when making measurements while the switch drivers are on. For
this discussion, it is useful to consider the basic operation of the TSC2007 (see Figure 19 ). The application used
in the following example shows the device being used to digitize a resistive touch screen. If the touch screen
controller uses a single-ended reference mode, as shown in Figure 24 , a measurement of the current Y position
of the pointing device is made by connecting the X+ input to the A/D converter, turning on the Y+ and Y – drivers,
and digitizing the voltage on X+. 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 timing is not a
concern. However, because 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 would not be 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 resistance of the
touch screen, providing an additional source of error. Therefore, the TSC2007 does not support single-ended
reference mode.
Figure 24. Simplified Diagram of Single-Ended Reference
This situation is resolved, as shown in Figure 25 , by using the differential mode; the +REF and – REF inputs are
connected directly to Y+ and Y – , respectively. This mode makes the A/D converter ratiometric. The result of the
conversion is always a percentage of the external reference, regardless of how it changes in relation to the
on-resistance of the internal switches.
Figure 25. Simplified Diagram of Differential Reference
(Both Y Switches Enabled, X+ is Analog Input)
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