MC908MR32CFUE Datasheet, PDF (51/282 Pages) Freescale Semiconductor, Inc – Microcontrollers

3.6.5 ADC Voltage In (ADVIN)
ADVIN is the input voltage signal from one of the 10 ADC channels to the ADC module.
I/O Registers
3.6.6 ADC External Connections
This section describes the ADC external connections: V_REFH and V_REFL, ANx, and grounding.
3.6.6.1 V_REFH and V_REFL
Both ac and dc current are drawn through the V_REFH and V_REFL loop. The AC current is in the form of
current spikes required to supply 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 V_REFH and V_REFL 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.
3.6.6.2 ANx
Empirical data shows that capacitors from the analog inputs to V_REFL 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.
3.6.6.3 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_SSAD pin. This should be the only ground connection between
these supplies if possible. The V_SSA pin makes a good single point ground location. Connect the V_REFL
pin to the same potential as V_SSAD at the single point ground location.
3.7 I/O Registers
These I/O registers control and monitor operation of the ADC:
• ADC status and control register, ADSCR
• ADC data registers, ADRH and ARDL
• ADC clock register, ADCLK
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
Freescale Semiconductor
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