Converter

+IN

+REF

Y+

+V

DD

V

REF

X+

Y–

GND

–REF

–IN

Converter

+IN

+REF

Y+

+V

DD

X+

Y–

GND

–REF

–IN

TSC2003-Q1

SBAS454 – DECEMBER 2008 ........................................................................................................................................................................................... www.ti.com

Figure 6. Simplified Diagram of Single-Ended Reference

This situation is remedied, as shown in Figure 7, by using the differential mode: the +REF and –REF inputs are

connected directly to Y+ and Y–, respectively. This makes the ADC 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 7. Simplified Diagram of Differential Reference (Y Switches Enabled, X+ is Analog Input)

Differential reference mode always uses the supply voltage, through the drivers, as the reference voltage for the

ADC. VREF cannot be used as the reference voltage in differential mode.

It is possible to use a high-precision reference on VREF in single-ended reference mode for measurements which

do not need to be ratiometric (i.e., battery voltage, temperature measurement, etc.). In some cases, it could be

possible to power the converter directly from a precision reference. Most references can provide enough power

for the TSC2003, but they might not be able to supply enough current for the external load, such as a resistive

touch screen.

14

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