Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
348 Analog-to-Digital Converter (ADC) MOTOROLA
Analog-to-Digital Converter (ADC)
19.7.4 ADC Voltage Reference Low Pin (VREFL)
VREFL is the lower reference supply for the ADC. Connect the VREFL pin
to the same voltage potential as VSSAD. A finite current will be
associated with VREFL. See Section 22. Electrical Specifications.
NOTE: In the 56-pin shrink dual in-line package (SDIP), VREFL and VSSAD are
tied together.
19.7.5 ADC Voltage In (ADVIN)
ADVIN is the input voltage signal from one of the 10 ADC channels to
the ADC module.
19.7.6 ADC External Connections
This section describes the ADC external connections: VREFH and VREFL,
ANx, and grounding.
19.7.6.1 VREFH and VREFL
Both ac and dc current are drawn through the VREFH and VREFL loop.
The AC current is in the form of current spikes r equired to sup ply charge
to the capacitor array at each successive approximation step. The
current flows through the internal resistor string. The best external
component to meet both these current demands is a capacitor in the
0.01 µF to 1 µF range with good high frequency characteristics. This
capacitor is connected between VREFH and VREFL and must be placed
as close as possible to the package pins. Resistance in the path is not
recommended because the dc current will cause a voltage drop which
could result in conversion errors.
19.7.6.2 ANx
Empirical data shows that capacitors from the analog inputs to VREFL
improve ADC performance. 0.01-µF and 0.1-µF capacitors with good
high-frequency characteristics are sufficient. These capacitors must be
placed as close as possible to th e package pins.