• Single-ended mode (CHNCFG bit=0). The input multiplex of the ADC selects one of
the 8 analog inputs and directs it to the plus terminal of the A/D core. The minus
terminal of the A/D core is connected to the V
REFL
reference. The ADC measures the
voltage of the selected analog input and compares it against the (V
REFH
- V
REFL
)
reference voltage range.
• Differential mode (CHNCFG bit=1). The ADC measures the voltage difference
between two analog inputs and compares that value against the (V
REFH
–V
REFL
)
voltage range. The input is selected as an input pair: ANA0/1, ANA2/3, ANA4/5,
ANA6/7, ANB0/1, ANB2/3, ANB4/5, or ANB6/7. The plus terminal of the A/D core
is connected to the even analog input, and the minus terminal is connected to the odd
analog input.
A mix and match combination of single-ended and differential configurations may exist.
For example:
• ANA[0:1] differential; ANA[2:3] single-ended
• ANA[4:5] differential; ANA[6:7] single-ended
• ANB[0:1] differential; ANB[2:3] single-ended
• ANB[4:5] differential; ANB[6:7] single-ended
34.5.2.1.1 Single-Ended Samples
The ADC module performs a ratio metric conversion. For single-ended measurements,
the digital result is proportional to the ratio of the analog input to the reference voltage:
SingleEndedValue = round(((V
IN
- V
REFL
)/(V
REFH
- V
REFL
)) × 4096) × 8
V
IN
is the applied voltage at the input pin.
V
REFH
and V
REFL
are the voltages at the external reference pins on the device (typically V
REFH
=V
DDA
and V
REFL
=V
SSA
).
Note: The 12-bit result is rounded to the nearest LSB.
Note: The ADC is a 12-bit function with 4096 possible states. However, the 12 bits have been left shifted by 3 bits on the
16-bit data bus. As a result, the magnitude of this function, as read from the data bus, is now 32760.
34.5.2.1.2 Differential Samples
For differential measurements, the digital result is proportional to the ratio of the
difference in the inputs to the difference in the reference voltages (V
REFH
and V
REFL
).
When differential measurements are converted, the following formula is useful:
Chapter 34 12-bit Cyclic Analog-to-Digital Converter (ADC)
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
711
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