Motorola MC68HC908MR16 用户手册
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 (V
REFL
)
V
REFL
is the lower reference supply for the ADC. Connect the V
REFL
pin
to the same voltage potential as V
SSAD
. A finite current will be
associated with V
REFL
. See
.
NOTE:
In the 56-pin shrink dual in-line package (SDIP), V
REFL
and V
SSAD
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.
the ADC module.
19.7.6 ADC External Connections
This section describes the ADC external connections: V
REFH
and V
REFL
,
ANx, and grounding.
19.7.6.1 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
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
µ
F to 1
µ
F 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.
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 V
REFL
improve ADC performance. 0.01-
µ
F and 0.1-
µ
F capacitors with good
high-frequency characteristics are sufficient. These capacitors must be
placed as close as possible to the package pins.
placed as close as possible to the package pins.