LM3S1621 Datasheet, PDF (604/947 Pages) Texas Instruments – ARM and Thumb are registered trademarks and Cortex is a trademark

Analog-to-Digital Converter (ADC)
The ADC operates from both the 3.3-V analog and 1.2-V digital power supplies. The ADC clock can
be configured to reduce power consumption when ADC conversions are not required (see “System
Control” on page 194). The analog inputs are connected to the ADC through specially balanced input
paths to minimize the distortion and cross-talk on the inputs. Detailed information on the ADC power
supplies and analog inputs can be found in “Analog-to-Digital Converter (ADC)” on page 902.
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. 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 13-5 on page 604 shows the ADC conversion function
of the analog inputs.
Figure 13-5. 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 21-26 on page 903.
Care must be taken to supply a reference voltage of acceptable quality.
604
January 21, 2012
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