LM3S9B81 Datasheet, PDF (504/1155 Pages) Texas Instruments – Stellaris® LM3S9B81 Microcontroller

Analog-to-Digital Converter (ADC)
to minimize the distortion on the inputs. Detailed information on the ADC power supplies and analog
inputs can be found in “Analog-to-Digital Converter” on page 1095.
13.3.4.1
Internal Voltage Reference
The band-gap circuitry generates an internal 3.0 V reference that can be used by the ADC to produce
a conversion value from the selected analog input. The range of this conversion value is from 0x000
to 0x3FF. In single-ended-input mode, the 0x000 value corresponds to an analog input voltage of
0.0 V; the 0x3FF value corresponds to an analog input voltage of 3.0 V. This configuration results
in a resolution of approximately 2.9 mV per ADC code. While the analog input pads can handle
voltages beyond this range, the ADC conversions saturate in under-voltage and over-voltage cases.
Figure 13-6 on page 504 shows the ADC conversion function of the analog inputs.
Figure 13-6. 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 external voltage reference when using the 3.0-V setting and three times the external
voltage reference when using the 1.0-V setting, resulting in a smaller voltage resolution per ADC
code. Analog input voltages above the external voltage reference saturate to 0x3FF while those
below 0.0 V continue to saturate at 0x000. Figure 13-7 on page 505 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.
504
June 29, 2010
Texas Instruments-Advance Information