LM3S1637 Datasheet, PDF (395/696 Pages) Texas Instruments – ARM and Thumb are registered trademarks and Cortex is a trademark

Stellaris^® LM3S1637 Microcontroller
11.3.4
11.3.5
Analog-to-Digital Converter
The converter itself generates a 10-bit output value for selected analog input. Special analog pads
are used to minimize the distortion on the input. An internal 3 V reference is used by the converter
resulting in sample values ranging from 0x000 at 0 V input to 0x3FF at 3 V input when in single-ended
input mode.
Differential Sampling
In addition to traditional single-ended sampling, the ADC module supports differential sampling of
two analog input channels. To enable differential sampling, software must set the Dn bit in the
ADCSSCTL0n register in a step's configuration nibble.
When a sequence step is configured for differential sampling, its corresponding value in the
ADCSSMUXn register must be set to one of the four differential pairs, numbered 0-3. Differential
pair 0 samples analog inputs 0 and 1; differential pair 1 samples analog inputs 2 and 3; and so on
(see Table 11-4 on page 395). The ADC does not support other differential pairings such as analog
input 0 with analog input 3. The number of differential pairs supported is dependent on the number
of analog inputs (see Table 11-4 on page 395).
Table 11-4. Differential Sampling Pairs
Differential Pair
0
1
Analog Inputs
0 and 1
2 and 3
The voltage sampled in differential mode is the difference between the odd and even channels:
∆V (differential voltage) = V_{IN_EVEN} (even channels) – V_{IN_ODD} (odd channels), therefore:
■ If ∆V = 0, then the conversion result = 0x1FF
■ If ∆V > 0, then the conversion result > 0x1FF (range is 0x1FF–0x3FF)
■ If ∆V < 0, then the conversion result < 0x1FF (range is 0–0x1FF)
The differential pairs assign polarities to the analog inputs: the even-numbered input is always
positive, and the odd-numbered input is always negative. In order for a valid conversion result to
appear, the negative input must be in the range of ± 1.5 V of the positive input. If an analog input
is greater than 3 V or less than 0 V (the valid range for analog inputs), the input voltage is clipped,
meaning it appears as either 3 V or 0 V, respectively, to the ADC.
Figure 11-2 on page 396 shows an example of the negative input centered at 1.5 V. In this
configuration, the differential range spans from -1.5 V to 1.5 V. Figure 11-3 on page 396 shows an
example where the negative input is centered at -0.75 V, meaning inputs on the positive input
saturate past a differential voltage of -0.75 V since the input voltage is less than 0 V. Figure
11-4 on page 397 shows an example of the negative input centered at 2.25 V, where inputs on the
positive channel saturate past a differential voltage of 0.75 V since the input voltage would be greater
than 3 V.
June 18, 2012
395
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