LM3S5G31 Datasheet, PDF (631/1223 Pages) Texas Instruments – Stellaris LM3S5G31 Microcontroller

Stellaris^® LM3S5G31 Microcontroller
Figure 13-8. Internal Voltage Conversion Result
12-bit/
10-bit
0xFFF/
0x3FF
0xBFF/
0x2FF
0x7FF/
0x1FF
0x3FF/
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 configuring the VREF field in the ADC Control (ADCCTL) register. The VREF field
specifies whether to use the internal, an external reference in the 3.0 V range, or an external reference
in the 1.0 V range. While the range of the conversion value remains the same (0x000 to 0xFFF or
0x3FF), the analog voltage associated with the 0xFFF or 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. While the analog input pads can handle voltages beyond
this range, the analog input voltages must remain within the limits prescribed by “Electrical
Characteristics” on page 1145 to produce accurate results. The V_REFA specification defines the useful
range for the external voltage reference, see Table 25-27 on page 1163. Care must be taken to supply
a reference voltage of acceptable quality.
Figure 13-9 on page 632 shows the ADC conversion function of the analog inputs when using anthe
3.0-V setting on the external voltage reference. Figure 13-10 on page 632 shows the ADC conversion
function when using the 1.0-V setting on the 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.
July 03, 2014
631
Texas Instruments-Production Data