LM3S1651 Datasheet, PDF (543/1033 Pages) Texas Instruments – ARM and Thumb are registered trademarks and Cortex is a trademark

Figure 12-8. Internal Voltage Conversion Result
0x3FF
0x2FF
0x1FF
0x0FF
Stellaris^® LM3S1651 Microcontroller
0.00 V
0.75 V
1.50 V
2.25 V
3.00 V
V_IN
- Input Saturation
12.3.4.2
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 23-22 on page 983.
Care must be taken to supply a reference voltage of acceptable quality.
Figure 12-9 on page 544 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.
January 21, 2012
543
Texas Instruments-Production Data