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TC1784
Analog to Digital Converter (ADC)
User´s Manual
23-77
V1.1, 2011-05
ADC, V1.3
23.2.13.2 Reference Selection
The conversion result of the ADC is always referring to a reference voltage. The
maximum digital result value (full scale) is obtained if the analog input voltage equals the
reference voltage. In order to support more than one measurement range with full scale
digital representation, the user can select between the standard reference input V
AREF
and an alternative reference input at the analog input channel CH0 for each ADC kernel.
The reference selection can be individually programmed for each input channel.
This feature can be used to connect 5 V based sensors and 3.3 V based sensors to the
same ADC kernel. In this case, one set of input channels refers to the standard reference
input, whereas the other one refers to the voltage level at input CH0.
Please note that the smallest granularity 1 LSB
n
for n bit resolution refers to the selected
reference voltage. The granularity becomes very small if a low reference voltage is
applied, and as a consequence, the resulting TUE increases due to noise effects.
Therefore it is recommended to avoid small reference voltages.
23.2.13.3 Alias Feature
The ADC kernel provides an alias feature, allowing a re-direction of conversion requests
for channels CH0 or CH1 to other channel numbers. This feature can be used to
measure the same input channel and to store the conversion results in two different
result registers.
•
The same signal can be measured twice without the need to read out the conversion
result to avoid data loss. This allows triggering both conversions quickly one after the
other and being independent from CPU interrupt latency.
•
The sensor signal is connected to only one input channel (instead of two analog
inputs). This saves input pins in low-cost applications and only the leakage of one
input has to be considered in the error calculation.
•
Even if the analog input CH0 is used as alternative reference (see
), the
internal trigger and data handling features for channel CH0 can be used.
•
The channel settings for both conversions can be different (boundary values,
interrupts, etc.).
•
If a sequential conversion request source has been set up, a conversion request for
channels CH0 or CH1 can be easily directed to other input channels without flushing
the queue.
In typical low-cost AC-drive applications, only one common current sensor is used to
determine the phase currents. Depending on the applied PWM pattern, the measured
value has different meanings and the sample points have to be precisely located in the
PWM period.
shows an example where the sensor signal is connected to
one input channel (CHx) but two conversions are triggered for two different channels
(CHx and CH0). With the alias feature, a conversion request for CH0 leads to a
conversion of the analog input CHx instead of CH0, but taking into account the settings
for CH0. Although the same analog input (CHx) has been measured, the conversion
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