LM3S308 Luminary Micro, Inc, LM3S308 Datasheet - Page 227

• Manufacturer Part Number: LM3S308
• Description: Lm3s308 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

June 04, 2008
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 227 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 227 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 228 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.
Figure 11-2. Differential Sampling Range, V
Figure 11-3. Differential Sampling Range, V
-1.5 V
-1.5 V
0 V
-0.75 V
0 V
ADC Conversion Result
0x3FF
0x1FF
ADC Conversion Result
0x3FF
0x1FF
+0.75 V
- Input Saturation
V
IN_ODD
- Input Saturation
1.5 V
0x0FF
0 V
= 1.5 V
Preliminary
IN_ODD
IN_ODD
+2.25 V
= 1.5 V
= 0.75 V
+1.5 V
3.0 V
1.5 V
V
V
IN_EVEN
V
V
IN_EVEN
LM3S308 Microcontroller
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