LM3S2793 Datasheet, PDF (628/1194 Pages) Texas Instruments – Stellaris® LM3S2793 Microcontroller

Analog-to-Digital Converter (ADC)
Figure 13-8. Internal Voltage Conversion Result
0x3FF
0x2FF
0x1FF
0x0FF
0.00 V
0.75 V
1.50 V
2.25 V
3.00 V
V_IN
- Input Saturation
13.3.4.2
External Voltage Reference
The ADC can use an external voltage reference to produce the conversion value from the selected
analog input by setting the VREF bit in the ADC Control (ADCCTL) register. The VREF bit specifies
whether to use the internal or external reference. While the range of the conversion value remains
the same (0x000 to 0x3FF), the analog voltage associated with the 0x3FF value corresponds to the
value of the voltage when using the 3.0-V setting and three times the voltage when using the 1.0-V
setting, resulting in a smaller voltage resolution per ADC code. Ground is always used as the
reference level for the minimum conversion value. Analog input voltages above the external voltage
reference saturate to 0x3FF while those below 0.0 V continue to saturate at 0x000. The V_REFA
specification defines the useful range for the external voltage reference, see Table 25-26 on page 1140.
Care must be taken to supply a reference voltage of acceptable quality.
Figure 13-9 on page 629 shows the ADC conversion function of the analog inputs when using an
external voltage reference.
The external voltage reference can be more accurate than the internal reference by using a
high-precision source or trimming the source.
628
January 20, 2012
Texas Instruments-Production Data