MC68HC16Z1CPV16 Datasheet, PDF (207/500 Pages) Freescale Semiconductor, Inc – Symbols and Operators, CPU16 Register Mnemonics

Freescale Semiconductor, Inc.
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.
Refer to 8.8.6.2 Error Resulting from Leakage. In some cases, the size of the capac-
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.
EXTERNAL CIRCUIT
S1
R_F
INTERNAL CIRCUIT MODEL
S2
AMP
V_SRC
C_F
C_S
S3
S4
C_DAC
V_I
V_SRC = SOURCE VOLTAGE
R_F = FILTER IMPEDANCE (SOURCE IMPEDANCE INCLUDED)
C_F = FILTER CAPACITOR
C_S = INTERNAL CAPACITANCE (FOR A BYPASSED CHANNEL, THIS IS THE C_DAC CAPACITANCE)
C_DAC = DAC CAPACITOR ARRAY
V_I = INTERNAL VOLTAGE SOURCE FOR PRECHARGE (V_DDA/2)
Figure 8-10 Electrical Model of an A/D Input Pin
ADC SAMPLE AMP MODEL
M68HC16 Z SERIES
USER’S MANUAL
ANALOG-TO-DIGITAL CONVERTER
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