Motorola MPC533 Reference Manual page 516

QADC64E Integration Requirements
Analog Power Supply
+5V
PCB
Figure 13-49. Star-Ground at the Point of Power Supply Origin
13.7.4 Analog Reference Signals
V and V are the dedicated input signals for the high and low reference
RH RL
voltages.Separating the reference inputs from the power supply signals allows for
additional external filtering, which increases reference voltage precision and stability, and
subsequently contributes to a higher degree of conversion accuracy.
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
QADC64E, supplied by signals V
to obtain a noise-free, clean signal. In many cases, simple capacitive bypassing may
sufficed. In extreme cases, inductors or ferrite beads may be necessary if noise or RF energy
is present. Series resistance is not advisable since there is an effective DC current
requirement from the reference voltage by the internal resistor string in the RC DAC array.
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.
13.7.5 Analog Input Signals
Analog inputs should have low AC impedance at the signals. Low AC impedance can be
realized by placing a capacitor with good high frequency characteristics at the input signal
of the part. Ideally, that capacitor should be as large as possible (within the practical range
13-72
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
AGND +5V
VSS
QADC64E
VDD
, and V
RH
MPC533 Reference Manual
Digital Power
PGND
, should be low-pass filtered from its source
RL
Supply
+5V
MOTOROLA