Converter

+IN

+REF

Y+

VDD/REF

X+

Y

-

GND

-REF

-IN

Converter

+IN

+REF

Y+

VDD/REF

X+

Y

-

GND

-REF

-IN

TSC2017

www.ti.com

SBAS472 – DECEMBER 2009

Reference

The TSC2017 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, 1.6V to 3.6V, single-supply

operation of the chip.

Reference Mode

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 TSC2017 (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 TSC2017 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)

Copyright © 2009, Texas Instruments Incorporated

Submit Documentation Feedback

17

Product Folder Link(s): TSC2017