MC912D60AVPVE8 Datasheet, PDF (358/460 Pages) Freescale Semiconductor, Inc – HC12 Microcontrollers

Analog-to-Digital Converter
18.8 Application Considerations
Note that the A/D converter’s accuracy is limited by the accuracy of the
reference potentials. Noise on the reference potentials will result in noise
on the digital output data stream: the reference potential lines do not
reject reference noise.
The reference potential pins must have a low AC impedance path back
to the source. A large bypass capacitor (100nF or larger) will suffice in
most cases. In extreme cases, inductors and/or ferrite beads may be
necessary if high frequency noise is present. Series resistance is not
advisable since the ATD module draws current from the reference. A
potential drop across any series resistance would result in gain and
offset errors in the digital data output stream unless the reference
potential was sensed at the reference input pin and any potential drop
compensated for.
For best performance, the analog inputs should have a low AC
impedance at the input pins to shunt noise current coupled onto the input
node away from the A/D input. This can be accomplished by placing a
capacitor with good high frequency characteristics between the input pin
and V_SSA. The size of this capacitor is application dependent; larger
capacitors will lower the AC impedance and be more effective at
shunting away noise current. However, the input analog signal has its
own dynamic characteristics which the A/D converter is being used to
track. These, along with the source impedance of the signal driver, must
also be considered when choosing the capacitor size to avoid rolling off
any high frequency components of interest.
If the input signal contains excessive high frequency conducted noise,
then a series resistance may be used with the capacitor to generate a
one pole, low pass anti-aliasing filter.
18.9 ATD Registers
Control and data registers for the ATD modules are described below.
Both ATDs have identical control registers mapped in two blocks of 16
bytes.
Technical Data
358
Analog-to-Digital Converter
MC68HC912D60A — Rev. 3.1
Freescale Semiconductor