Analog-to-Digital Converter (ADC)
I/O Signals
3.6.4 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 19. Electrical Specifications.
.
NOTE:
In the 56-pin shrink dual in-line package (SDIP), VREFL and VSSAD are tied
together.
3.6.5 ADC Voltage In (ADVIN)
ADVIN is the input voltage signal from one of the 10 ADC channels to the ADC
module.
3.6.6 ADC External Connections
This section describes the ADC external connections: VREFH and VREFL, ANx, and
grounding.
3.6.6.1 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 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 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.
3.6.6.2 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 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 VSSAD pin. This should
be the only ground connection between these supplies if possible. The VSSA pin
makes a good single point ground location. Connect the VREFL pin to the same
potential as VSSAD at the single point ground location.
MC68HC908MR32 • MC68HC908MR16 — Rev. 6.0
MOTOROLA Analog-to-Digital Converter (ADC)
Data Sheet
53
FREESCALE [ Freescale ]
