27.9.1 Analog Reference Signals
No A/D converter can be more accurate than its analog reference. Any noise in the
reference can result in at least that much error in a conversion. The reference for the QADC,
supplied by signals V
noise-free, clean signal. In many cases, simple capacitive bypassing may suffice. In
extreme cases, inductors or ferrite beads may be necessary if noise or RF energy is present.
External resistance may introduce error in this architecture under certain conditions. Any
series devices in the filter network should contain a minimum amount of DC resistance.
For accurate conversion results, the analog reference voltages must be within the limits
defined by V
and V
DDA
27.9.2 Analog Power Signals
The analog supply signals (V
voltages (V
and V
RH
the analog input circuitry.
16 Channels Total
Because the sample amplifier is powered by V
input signal levels up to but not exceeding V
input signal is outside of this range, the output from the sample amplifier is clipped.
In addition, V
and V
RH
V
is less than or equal to V
RH
amplifier has accurately transferred the input signal, resolution is ratiometric within the
limits defined by V
RL
MOTOROLA
and V
, should be low-pass filtered from its source to obtain a
RH
RL
, as explained in this subsection.
SSA
and V
DDA
) and of the analog multiplexer inputs. Figure 27-44 is a diagram of
RL
Figure 27-44. Equivalent Analog Input Circuitry
must be within the range defined by V
RL
, and V
DDA
RL
and V
. If V
RH
RH
Chapter 27. Queued Analog-to-Digital Converter (QADC)
) define the limits of the analog reference
SSA
V
DDA
Sample
AMP
S/H
Comparator
C
V
SSA
and V
DDA
SSA
and down to but not below V
DDA
is greater than or equal to V
is greater than V
DDA
Signal Connection Considerations
V
RH
V
RL
, it can accurately transfer
SSA.
and V
. As long as
DDA
SSA
, and the sample
SSA
, the sample amplifier can
If the
27-63