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13.4.2
Channel Decode and Multiplexer
The internal multiplexer selects one of the 16 analog input signals for conversion. The
selected input is connected to the sample buffer amplifier. The multiplexer also includes
positive and negative stress protection circuitry, which prevents deselected channels from
affecting the selected channel when current is injected into the deselected channels. Refer
to Appendix E, "Electrical Characteristics" for specific current levels.
13.4.3
Sample Buffer Amplifier
The sample buffer is used to raise the effective input impedance of the A/D converter, so
that external components (higher bandwidth or higher impedance) are less critical to
accuracy. The input voltage is buffered onto the sample capacitor to reduce crosstalk
between channels.
13.4.4
Digital-to-Analog Converter (DAC) Array
The digital to analog converter (DAC) array consists of binary-weighted capacitors and a
resistor-divider chain. The reference voltages, V
perform ratiometric conversions. The DAC also converts the following three internal
channels:
• V
— Reference voltage high
RH
• V
— Reference voltage low
RL
• (V
– V
)/2 — Reference voltage
RH
RL
The DAC array serves to provide a mechanism for the successive approximation A/D
conversion.
Resolution begins with the most significant bit (MSB) and works down to the least
significant bit (LSB). The switching sequence is controlled by the comparator and
successive-approximation register (SAR) logic.
• Sample capacitor — The sample capacitor is employed to sample and hold the
voltage to be converted.
13.4.5
Comparator
The comparator is used during the approximation process to sense whether the digitally
selected arrangement of the DAC array produces a voltage level higher or lower than the
sampled input. The comparator output feeds into the SAR which accumulates the A/D
conversion result sequentially, beginning with the Msb.
MOTOROLA Chapter 13. Queued Analog-to-Digital Converter Legacy Mode Operation
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
and V
, are used by the DAC to
RH
RL
Analog Subsystem
13-35
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