Reference Mode

Converter

+IN

+REF

Y+

VDD/REF

X+

Y

-

GND

-REF

-IN

Converter

+IN

+REF

Y+

VDD/REF

X+

Y

-

GND

-REF

-IN

TSC2008

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 TSC2008 (see Figure 20). 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 A/D converter, turning on the Y+ and Y–

drivers, and digitizing the voltage on X+, as shown in Figure 25. 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, 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.

Figure 25. Simplified Diagram of Single-Ended Reference

This situation is resolved, as shown in Figure 26, 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. Note that there is an important consideration regarding power dissipation

when using the ratiometric mode of operation (see the Power Dissipation section for more details).

Figure 26. Simplified Diagram of Differential Reference

(Both Y Switches are Enabled, and X+ is the Analog Input)

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