Motorola ColdFire MCF5281 - Manual (Page 595)

Queued Analog-to-Digital Converter (QADC)
Freescale Semiconductor 28-57
Figure 28-44. Equivalent Analog Input Circuitry
Because the sample amplifier is powered by V
DDA
and V
SSA
, it can accurately transfer input signal levels
up to but not exceeding V
DDA
and down to but not below V
SSA.
If the input signal is outside of this range,
the output from the sample amplifier is clipped.
In addition, V
RH
and V
RL
must be within the range defined by V
DDA
and V
SSA
. As long as V
RH
is less
than or equal to V
DDA
, and V
RL
is greater than or equal to V
SSA
, and the sample amplifier has accurately
transferred the input signal, resolution is ratiometric within the limits defined by V
RL
and V
RH
. If V
RH
is
greater than V
DDA
, the sample amplifier can never transfer a full-scale value. If V
RL
is less than V
SSA
, the
sample amplifier can never transfer a 0 value.
Figure 28-45 shows the results of reference voltages outside the range defined by V
DDA
and V
SSA
. At the
top of the input signal range, V
DDA
is 10 mV lower than V
RH
. This results in a maximum obtainable 10-bit
conversion value 0x03fe. At the bottom of the signal range, V
SSA
is 15 mV higher than V
RL
, resulting in
a minimum obtainable 10-bit conversion value of 0x0003.
Sample
AMP
16 Channels Total
V
RL
V
DDA V
RH
S/H
Comparator
C
V
SSA
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3