TC1796
Peripheral Units (Vol. 2 of 2)
Analog-to-Digital Converter (ADC)
User’s Manual
25-42
V2.0, 2007-07
ADC, V2.0
The external multiplexer controls (bit field CHCONn.EMUX) from the channel-specific
control registers are not taken into account for sequential sources.
25.1.8.1 Inverse Current Injection (Overload) Behavior
An overload condition occurs when the analog input voltage is above or below the supply
range. An overload condition at a channel connected to an external multiplexer (such as
AN-0 in
) can affect the conversion of another channel connected to the
same external multiplexer (such as AN-1 to AN-3), depending on the overload capability
of the external multiplexer. In case of an overload condition at one channel while another
channel of the same external multiplexer is sampled by the A/D Converter, an even
higher conversion error must be expected.
Note: The overload behavior of every channel that is directly connected to the internal
multiplexer or through another external multiplexer does not change.
25.1.8.2 On Resistance of the External Multiplexer
If an external multiplexer is connected to an analog input channel, a typical application
might add an RC filter before the external multiplexer to each additional external analog
inputs. For example, each of the external analog inputs AN-0 to AN-3 in
is
adapted by an RC filter. In this case, the resistance of the external multiplexer reduces
the efficiency of the external capacitors of the RC filter. An additional blocking capacitor
between the external multiplexer and the analog input line could improve the noise
suppression capability. However, in this case, the capacitance that must be charged,
would be increased by the size of the blocking capacitor.
25.1.8.3 Timing of the External Multiplexer
An analog input channel of an external analog multiplexer is selected after the arbitration
round is finished. Therefore, the information to drive an external multiplexer is available
before the sample time begins.
25.1.8.4 Load Capacitance
Because each analog input of the external multiplexer might be applied by different
analog voltages, the total input capacitance of the A/D Converter must be recharged
within the sample time, each time an analog input channel of an external multiplexer is
measured.
For analog input channels that are directly applied to the analog input pin of the A/D
Converter, the input capacitance does not change. The analog voltage source of such
channels must solely recharge the switched input capacitance of the A/D Converter.