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M68HC16 Z SERIES
ANALOG-TO-DIGITAL CONVERTER
MOTOROLA
USER’S MANUAL
8-21
8.8.6 Analog Input Pins
Analog inputs should have low AC impedance at the pins. Low AC impedance can be
realized by placing a capacitor with good high frequency characteristics at the input
pin of the part. Ideally, that capacitor should be as large as possible (within the practi-
cal range of capacitors that still have good high frequency characteristics). This capac-
itor has two effects. First, it helps attenuate any noise that may exist on the input.
Second, it sources charge during the sample period when the analog signal source is
a high-impedance source.
Series resistance can be used with the capacitor on an input pin to implement a simple
RC filter. The maximum level of filtering at the input pins is application dependent and
is based on the bandpass characteristics required to accurately track the dynamic
characteristics of an input. Simple RC filtering at the pin may be limited by the source
impedance of the transducer or circuit supplying the analog signal to be measured.
itor at the pin may be very small.
Figure 8-10 is a simplified model of an input channel. Refer to this model in the follow-
ing discussion of the interaction between the user's external circuitry and the circuitry
inside the ADC.
Figure 8-10 Electrical Model of an A/D Input Pin
ADC SAMPLE AMP MODEL
S1
S2
AMP
RF
S4
S3
CDAC
VI
CS
CF
VSRC
INTERNAL CIRCUIT MODEL
EXTERNAL CIRCUIT
= SOURCE VOLTAGE
= INTERNAL CAPACITANCE (FOR A BYPASSED CHANNEL, THIS IS THE CDAC CAPACITANCE)
VSRC
RF
CF
CS
CDAC
VI
= FILTER IMPEDANCE (SOURCE IMPEDANCE INCLUDED)
= FILTER CAPACITOR
= DAC CAPACITOR ARRAY
= INTERNAL VOLTAGE SOURCE FOR PRECHARGE (VDDA/2)