QADC64E Enhanced Mode Operation
MPC561/MPC563 Reference Manual, Rev. 1.2
14-70
Freescale Semiconductor
The AltRef signal may be selected through the CCW as the high reference for a conversion. This allows
for the ability to “zoom” in on a portion of the convertible range with the full 10 bits. Refer to
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
RH
, AltRef,
and V
RL
, should be low-pass filtered from its source 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.
14.6.5
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 of capacitors that still have good high
frequency characteristics). This capacitor has two effects:
•
It helps attenuate any noise that may exist on the input.
•
It sources charge during the sample period when the analog signal source is a high-impedance
source.
Series resistance can be used with the capacitor on an input signal to implement a simple RC filter. The
maximum level of filtering at the input signals is application dependent and is based on the bandpass
characteristics required to accurately track the dynamic characteristics of an input. Simple RC filtering at
the signal may be limited by the source impedance of the transducer or circuit supplying the analog signal
to be measured. Refer to
Section 14.6.5.3, “Error Resulting from Leakage
” for more information. In some
cases, the size of the capacitor at the signal may be very small.
is a simplified model of an input channel. Refer to this model in the following discussion of
the interaction between the external circuitry and the circuitry inside the QADC64E.
Summary of Contents for MPC561
Page 84: ...MPC561 MPC563 Reference Manual Rev 1 2 lxxxiv Freescale Semiconductor...
Page 144: ...Signal Descriptions MPC561 MPC563 Reference Manual Rev 1 2 2 46 Freescale Semiconductor...
Page 206: ...Central Processing Unit MPC561 MPC563 Reference Manual Rev 1 2 3 62 Freescale Semiconductor...
Page 302: ...Reset MPC561 MPC563 Reference Manual Rev 1 2 7 14 Freescale Semiconductor...
Page 854: ...Time Processor Unit 3 MPC561 MPC563 Reference Manual Rev 1 2 19 24 Freescale Semiconductor...
Page 968: ...Development Support MPC561 MPC563 Reference Manual Rev 1 2 23 54 Freescale Semiconductor...
Page 1144: ...Internal Memory Map MPC561 MPC563 Reference Manual Rev 1 2 B 34 Freescale Semiconductor...
Page 1212: ...TPU3 ROM Functions MPC561 MPC563 Reference Manual Rev 1 2 D 60 Freescale Semiconductor...
Page 1216: ...Memory Access Timing MPC561 MPC563 Reference Manual Rev 1 2 E 4 Freescale Semiconductor...
