mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 230

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

10.8.6 Analog Input Pins
10-22
MOTOROLA
C
V
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 10.8.6.2 Error Resulting from Leakage. In some cases, the size of the capac-
itor at the pin may be very small.
Figure 10-10 is a simplified model of an input channel. Refer to this model in the fol-
lowing discussion of the interaction between the user's external circuitry and the cir-
cuitry inside the ADC.
SRC
DAC
R
C
C
V
F
F
S
I
= SOURCE VOLTAGE
= FILTER IMPEDANCE (SOURCE IMPEDANCE INCLUDED)
= FILTER CAPACITOR
= INTERNAL CAPACITANCE (FOR A BYPASSED CHANNEL, THIS IS THE C
= DAC CAPACITOR ARRAY
= INTERNAL VOLTAGE SOURCE FOR PRECHARGE (V
V
SRC
S1
EXTERNAL CIRCUIT
Figure 10-10 Electrical Model of an A/D Input Pin
R
F
C
Freescale Semiconductor, Inc.
F
For More Information On This Product,
ANALOG-TO-DIGITAL CONVERTER
INTERNAL CIRCUIT MODEL
Go to: www.freescale.com
S2
C
DDA
S
/2)
AMP
DAC
CAPACITANCE)
S3
C
DAC
MC68HC16Y3/916Y3
S4
USER’S MANUAL
ADC SAMPLE AMP MODEL
V
I