MC912D60ACPVE8 Datasheet, PDF (353/460 Pages) Freescale Semiconductor, Inc – MC68HC912D60A MC68HC912D60C MC68HC912D60P Technical Data

Analog-to-Digital Converter
Functional Description
18.4.3 Sample and Hold Stage
A Sample and Hold (S/H) stage accepts the analog signal from the input
multiplexer and stores it as a capacitor charge on a storage node in the
module. The sample process uses a three stage approach:
1. The input signal is sampled onto a sample capacitor (for 2 module
clocks).
2. The sample amplifier quickly charges the storage node with a
copy of the sample capacitor potential (for 4 module clocks).
3. The input signal is connected directly to the storage node to
complete the sample for high accuracy (for 2, 4, 8 or 16 module
clocks). Longer sample times allow accurate measurement of
higher impedance sources.
This charge redistribution method eliminates the need for external
sample-and-hold circuitry.
18.4.4 Analog-to-Digital Converter Submodule
The Analog-to-Digital (A/D) Machine uses a successive approximation
A/D architecture to perform analog to digital conversions. The resolution
of the A/D converter is selectable at either 8 or 10 bits. It functions by
comparing the stored analog sample potential with a series of digitally
generated analog potentials (using CDAC & RDAC arrays). By following
a binary search algorithm, the converter quickly locates the
approximating potential that is nearest to the sampled potential. At the
end of the conversion process (10 module clocks for 8-bit, 12 module
clocks for 10-bit), the Successive Approximation Register (SAR)
contains the nearest approximation to the sampled signal, given the
resolution of the A/D converter, and is transferred to the appropriate
results register in the selected format.
18.4.5 Clock Prescaler Function
To keep the ATD module clock within the specified frequency range
(note: there is a minimum and maximum frequency), a clock prescaler
function is available. This function divides the system PCLK by a
MC68HC912D60A — Rev. 3.1
Freescale Semiconductor
Analog-to-Digital Converter
Technical Data
353