MC68HC908MR8 Datasheet, PDF (318/372 Pages) Motorola, Inc – Microcontrollers

Analog-to-Digital Converter (ADC)
18.8.2 ADC Voltage In (ADVIN)
ADVIN is the input voltage signal from one of the seven ADC channels
to the ADC module.
18.8.3 ADC External Connection
18.8.3.1 V_REFH
Both ac and dc current are drawn through V_REFH. The ac current is in the
form of current spikes required to supply charge to the capacitor array at
each successive approximation step. The dc 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
V_REFH and V_SS 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.
18.8.3.2 ANx
Empirical data shows that capacitors from the analog inputs to V_RL
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 the package pins.
18.8.3.3 Grounding
In cases where separate power supplies are used for analog and digital
power, the ground connection between these supplies should be at the
V_SS pin. This should be the only ground connection between these
supplies if possible. The V_SS pin makes a good single-point ground
location.
Technical Data
318
Analog-to-Digital Converter (ADC)
MC68HC908MR8 — Rev 4.1
Freescale Semiconductor